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New materials for organic light emitting diodes
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Content
NEW MATERIALS FOR ORGANIC LIGHT EMITTING DIODES.
by
Carsten Borek
____________________________________________________________
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSTIY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)
August 2008
Copyright 2008 Carsten Borek
ii
Epigraph
“La Clairvoyance” (René Magritte 1937)
c. Estate of Rene Magritte/ Artists Rights Society, N.Y
“Research is to see what everybody else has seen, and think of what
nobody else has thought.”
Albert Szent-Györgyi de Nagyrápolt
(1937 Nobel Prize in Medicine.)
iii
Dedication
Diese Dissertation widme ich meiner Familie, Klaus und Christel Borek,
Alexandra, Bernd und Fabian Clement, die mich in jeder noch so aussichtslosen
Lage immer unterstützt haben.
Zusätzlich möchte ich diese Dissertation meiner Freundin Nicole widmen. Durch
Ihre Liebe und Zuneigung hat Sie es mir ermöglicht die eine oder andere Nacht um
die Ohren zu schlagen, um diese Arbeit fertig stellen zu können. Ich liebe Dich.
iv
Acknowledgements
In Germany, the in official name for the graduate advisor is “Doktorvater”,
which translated word for word means “doctoral father”. Prof. Mark E. Thompson
embodied each and every aspect of this role, diligently teaching me not only the
mysterious ways of inorganic and photophysical chemistry, but also providing me
with insight into managerial decision making. For that I would like to thank him.
Further I would like to thank Prof. James Haw, Prof. Chongwu Zhou, Prof.
Stephen E. Bradforth, Prof. Amy M. Barrios, Prof. Florian Mansfeld and Prof. Roy
A. Periana for being on my screening, qualification and dissertation committees.
Just as much thanks is in order for the entire chemistry department staff,
especially Heather Meunier Connor, Michele Dea and Dr. Bruno Herreros. Special
thanks to Judy Hom, a college and great friend, who always went the extra mile if I
had any problems on or off campus or if it suddenly rained in sunny southern
California. Thanks for being a great friend away from home.
Naturally I would like to thank all the present and past Thompson group
members, especially Dr. Peter Djurovich, who always knew which paper had the
information I was looking for. Cody Schlenker, thanks to you for always having
the group fridge full of beer and more beer. Thanks man, you know where to find
me, if you are thirsty and want to discuss science! Thanks M. Dolores Perez for
making cool solar cells with my molecules and for teaching me how to correctly
say “zumodenaaaahhhhhhh” and to Marco Curreli, the only TRUE Italian, for
v
many discussions and his European cleanliness and thoroughness, which somewhat
rubbed off on the lab.
Thanks to Dr. Laurent Griffe and Dr. Seogshin Kang…..the green room will
be for ever in my memories and sublimation of Pt(TPBP) for ever my nightmare.
Also, I would like to thank Matthew Jurow, who was my “apprentice” for 2
year and help me out with a lot of synthesis during that time. Good Luck in
Graduate School.
vi
Table of Contents
Epigraph.................................................................................................................ii
Dedication .............................................................................................................iii
Acknowledgements................................................................................................iv
List of Figures........................................................................................................ix
List of Tables.....................................................................................................xviii
List of Schemes ...................................................................................................xvi
Abstract ...............................................................................................................xix
Chapter 1 Organic Light-Emitting Diodes (OLEDs)................................................1
General Introduction to Organic Light-Emitting Diodes (OLEDs).......................1
OLED Architecture.........................................................................................2
Energy Transfer in OLEDs..............................................................................7
OLED Containing Phosphorescent Dopants ....................................................9
Current – Voltage Behavior of OLEDs..........................................................11
External Quantum Efficiency of OLEDs .......................................................12
vii
Chapter 2 Cross-linkable Hole Transporting Layer (HTL) for the use in Solution
Processed Organic Light Emitting Diodes (OLED)...............................................13
Abstract ............................................................................................................13
Introduction to Photoinitiated Cationic Radical Polymerization.........................14
Experimental.....................................................................................................19
Synthesis and Characterization......................................................................19
Thin Film Fabrication ...................................................................................28
Results and Discussion......................................................................................29
Introduction Thermally Cross-linkable Polymers ..............................................38
Experimental.....................................................................................................40
Synthesis and Characterization......................................................................40
Cross-linking of the TPD and TPD Copolymers............................................46
OLED Fabrication and Testing......................................................................50
Results and Discussion......................................................................................51
Conclusion........................................................................................................56
Chapter 3 Near-Infrared Electrophosphorescence from
Pt-Metalloporphyrin Complexes ...........................................................................58
Abstract ............................................................................................................58
Introduction ......................................................................................................59
Introduction to Metalloporphyrins.................................................................62
Experimental.....................................................................................................66
Synthesis and Characterization......................................................................67
X-ray Crystallographic Procedure .................................................................78
ORTEP Representation of the X-ray Structure of Pt(TPBP) ..........................79
Summary of Structure Determination of Pt(TPBP)........................................79
Normal Coordinate Structural Decomposition (NSD) Analysis of Pt(TPBP) .81
Electrochemistry ...........................................................................................82
Density Functional Calculations....................................................................82
Spectroscopic Measurements.........................................................................83
Quantum Yield Measurement........................................................................84
Device Fabrication and Testing.....................................................................84
Results and Discussion......................................................................................86
Conclusion...................................................................................................... 111
viii
Chapter 4 Near-Infrared Phosphorescence Emission from
Asymmetrical Benzo-/Naphtho Pt-Metalloporphyrin Complexes ........................ 112
Abstract .......................................................................................................... 112
Introduction .................................................................................................... 113
Experimental................................................................................................... 115
Synthesis and Characterization.................................................................... 115
MALDI Mass Spectroscopy ........................................................................ 124
HPLC Purification Parameters .................................................................... 125
Results and Discussion.................................................................................... 125
Conclusion...................................................................................................... 170
Chapter 5 Octasubstituted Cyclooctatetraene as Host for Blue, Green,
and Red Electrophosphorescence OLED............................................................. 172
Abstract .......................................................................................................... 172
Introduction .................................................................................................... 173
Experimental................................................................................................... 177
Synthesis and Characterization.................................................................... 177
Density Functional Calculations.................................................................. 185
Normal coordinate structural decomposition (NSD) analysis....................... 186
Spectroscopic Measurements....................................................................... 187
Results and Discussion.................................................................................... 188
Conclusion...................................................................................................... 194
References .......................................................................................................... 195
ix
List of Figures
Figure 1 Typical OLED architecture showing transparent metal anode, hole
transporting layer (HTL), emissive layer (EML), electron transporting
layer (ETL), spacer and metal cathode.............................................................2
Figure 2 The energy diagram of a double heterostructure device. Holes
(recircles) and electron (blue circles) are injected from their respective
electrodes into the HOMO and LUMO of the organic molecules.
The holes and electrons migrate along the interface, forming an exciton
at that interface and radiative decaying through the emission of a photon. .......5
Figure 3 Energy diagram of an OLED with HTL, EML (containing host
(orange) and dopant (green) energy levels) and ETL. ......................................6
Figure 4 Dexter energy transfer from host to dopant...............................................7
Figure 5 Förster energy transfer from host to dopant. .............................................8
Figure 6 Log – log plot of current versus voltage (I - V) showing the typical
charge transport behavior of an OLED. .........................................................11
Figure 7 General structure of diaryliodinium (1) and tryarylsulfonium (2) salts....17
Figure 8 NMR Spectra of compound mixture 6 and 7...........................................22
Figure 9 a) FTIR spectrum of 9-(1-propenyl)carbazole and IOC 10.
b) Enlarged region from 1800 - 800 cm
-1
shows diagnostic IR-band
frequency for a 1-propenyl group at 1670 cm
-1
(black circle).........................32
x
Figure 10a) FTIR spectrum of 9-(1-propenyl)carbazole, DV-NPD and
IOC 10. b) Enlarged region from 1800 - 800 cm
-1
shows diagnostic
IR-band frequency for a 1-propenyl group at 1670 cm
-1
and out
of plane deformation frequency at 900 cm
-1
(blue and black circles
respectively)..................................................................................................34
Figure 11 Cartoon of heterogeneous film of DV-NPD (green film),
9-(1-propenyl)carbazole (orange bars) and IOC 10 (blue circles)
on a substrate (gray) before UV irradiation....................................................36
Figure 12 Cartoon of heterogeneous film of DV-NPD (green film),
9-(1-propenyl)carbazole (orange bars) and IOC 10 (blue circles)
on a substrate (gray) after UV irradiation. The black squares represent a
random copolymer / oligomeric species of DV-NPD and
9-(1-propenyl)carbazole. ...............................................................................37
Figure 13 SEC traces of the sol fraction of thin films extracted from THF............47
Figure 14 a) Quantum efficiency versus current density for TPA
homopolymer, non-cross-linked, cross-linked polymer.
b) Quantum efficiency versus current density for TPD homopolymer,
non-cross-linked, cross-linked polymer and small molecule vacuum
deposited TPD...............................................................................................53
Figure 15 a)Brightness versus voltage for homopolymer, non-cross-linked
and cross-linked polymer based on TPA. b) Brightness versus voltage
for TPD homopolymer, non-cross-linked and cross-linked polymer and
small molecule vacuum deposited TPD.........................................................55
Figure 16 Typical absorption spectrum of a metallated porphyrin. Right:
Calculated energy levels of LUMO+1, LUMO, HOMO, HOMO-1 for
Pt(TPBP).......................................................................................................63
Figure 17 Pt(II)-tetraphenyltetrabenzoporphyrin. .................................................65
xi
Figure 18 Pt(II) tetraphenyltetranaphthoporphyrin. ..............................................66
Figure 19 ORTEP structure of Pt(II)-tetraphenyltetrabenzoporphyrin...................79
Figure 20 NSD analysis with edge view of Pt(TPBP) (meso-phenyl
substituents where removed for clarity).........................................................86
Figure 21 B3LYP / LACVP** calculation of
Pt(II)tetraphenyltetrabenzoporphyrin.............................................................88
Figure 22 Room temperature absorption spectra (black squares), and
normalized emission spectra at room temperature (λ=765nm
(green triangles)) and 77K (λ=751nm (red circles)) of Pt(TPBP) in
2-methyl-THF...............................................................................................89
Figure 23 a) Absorption measurement of Pt(TPBP). b) Lifetime of bubble
degassed solutions with know absorbance from Figure 23a. ..........................91
Figure 24 Quantum yield analysis of a bubble degassed sample of Pt(TPBP).......92
Figure 25 Absorption measurement of Pt(TPBP). b) Lifetime of freeze-
pump-thaw degassed solutions with know absorbance from Figure 25a.........93
Figure 26 Quantum yield analysis of a freeze-pump-thaw degassed sample of
Pt(TPBP).......................................................................................................94
Figure 27 Normalized EL spectra for device architecture ITO/400Å NPD/
400Å Alq
3
with 6% Pt(TPBP)/10Å LiF/1100Å Al showing nIR
emission at 765 nm of Pt(TPBP) and Alq
3
host emission at 520 nm...............97
xii
Figure 28 Normalized EL spectra for device architecture ITO/(400Å) NPD/
(400Å) Alq
3
with 6% Pt(TPBP)/ (500Å) Alq
3
/(10Å) LiF/(1100Å) Al
showing nIR emission at 765 nm of Pt(TPBP) and no Alq
3
host emission at
520 nm..........................................................................................................98
Figure 29 External quantum efficiency versus current density. Device 1:
ITO/(400 Å) NPD/(400 Å) Alq
3
+ 6wt% [Pt(TPBP)]/(500 Å) Alq
3
/
(10 Å) LiF/(1100 Å) Al; device 2: ITO/(400 Å) NPD/(300 Å)
Alq
3
+ 6wt% [Pt(TPBP)]/(400 Å) BAlq/(10 Å) LiF/(1000 Å) Al. Inset:
Intensity and current density versus voltage for device 1 and 2......................99
Figure 30 External quantum efficiency versus current density. Inset: device
architecture. ................................................................................................ 100
Figure 31 Intensity versus operation time, normalized to initial intensity
(closed squares) and voltage (open circles). The initial intensity was
740 mW cm
-2
. The insert shows the time versus radiance data plotted
on a semilogarithmic scale with extrapolation to 1000 hours. ...................... 101
Figure 32 Absorption spectra of M(II) tetraphenyltetrabenzoporphyrin.
M = Pt, Pd, Ni, Zn....................................................................................... 102
Figure 33 Absorption measurement of Pt(TPNP). .............................................. 104
Figure 34 Quantum yield analysis of a bubble degassed sample of Pt(TPNP)...... 105
Figure 35 Lifetime measurement of Pt(TPNP) in a degassed toluene solution. ... 106
Figure 36 Room temperature absorption spectra (black open squares) and
normalized emission spectra at room temperature (λ=923nm (red open
circles)) and 77K (λ=887nm (green open triangles)) of Pt(TPNP) in
2-methyl-THF............................................................................................. 107
Figure 37 Normalized electroluminescence spectrum with λ
max
= 900nm at
current density of 20 mA/cm
2
(red) and 100 mA/cm
2
(blue). ....................... 109
xiii
Figure 38 Quantum efficiency versus current density (black) and current
density versus lumens (red) of a Pt(TPNP) device. ...................................... 110
Figure 39 Absorption spectrum of Pt(TP-t-BNP) red circles, Pt(TPBP) black
squares and Pt(TPNP) blue triangles............................................................ 126
Figure 40 MALDI mass spectrum of Pt(TP-t-BNP). Red numbers identify
the porphyrin............................................................................................... 127
Figure 41 Porphyrin molecules identified by MALDI MS.................................. 128
Figure 42 a) Chromatography result of the separation of Pt(TP-t-BNP).
b) UV-VIS absorption spectrum of the above fractions................................ 129
Figure 43 MALDI Spectra of the a) green fraction, b) purple fraction and
c) the yellow fraction. Red numbers identify the porphyrin moiety.............. 130
Figure 44 a) MALDI spectrum of crudely purified statistical synthetic
procedure. Red numbers identify the porphyrin. b) structures of
compounds analyzed by MALDI MS from the statistical synthetic
procedure. ................................................................................................... 135
Figure 45 Absorption spectrum of second oxidation step (black squares),
before the second oxidation (red circles) of the statistically synthesized
asymmetric porphyrin; For comparison symmetric porphyrins Pt(TPBP)
(blue triangles) and Pt(TPNP) (green stars). ................................................ 138
Figure 46 MALDI MS spectrum of reaction mixture after the second
oxidation step using DDQ. Red numbers identify the porphyrin. ................. 139
Figure 47 a) Methanol:Toluene gradients used for the HPLC of second
oxidation reaction mixture (First run (blue trace) and second run
(red trace)). b) HPLC chromatogram corresponding to the blue trace.
c) HPLC chromatorgam corresponding to the red trace. .............................. 140
xiv
Figure 48 MALDI analysis of Peaks A - Peak G. Red numbers identify the
porphyrin. b) Relative intensity plot versus type of porphyrin versus
elution peak................................................................................................. 143
Figure 49 a) MALDI spectrum of elution peak C. Inset: structure of
porphyrins 18 and 21. b) MALDI spectrum of elution peak E. Inset:
structure of porphyrins 20'', 21 and 27......................................................... 154
Figure 50 Excitation and emission spectra of peak C (black circles),
peak E (red triangles) and Pt(TPNP) (blue squares) at 77K in 2Me-THF. .... 156
Figure 51 a) Methanol:Toluene gradients used for the HPLC of the second
oxidation reaction mixture (previous conditions (red trace) and current
run (green trace)). b) HPLC chromatogram corresponding to the green
trace............................................................................................................ 157
Figure 52 MALDI analysis of peaks A - K1. Red numbers identify the
porphyrin. b) Relative intensity plot versus type of porphyrin versus
elution peak................................................................................................. 159
Figure 53 a) MALDI spectrum of elution peak F3. Inset: structure of
porphyrin 21. b) Excitation and emission spectra of peak F3 (red
squares) and Pt(TPNP) (black circles) at 77K in 2Me-THF. ........................ 167
Figure 54 a) Excitation and emission spectra of peak F3 (black squares),
peak C (blue circles) and peak E (red triangles) at 77K in 2Me-THF.
b) Emission spectra of peak F3 (black squares), peak C (blue circles),
peak E (red triangles) and Pt(TPNP) (green stars) at 77K in 2Me-THF........ 169
Figure 55 DFT calculation of the triplet excited state showing the
planarization. .............................................................................................. 175
Figure 56 DFT calculation of the triplet excited state with frozen geometry. ..... 176
xv
Figure 57 Possible isomeric structures of asymmetric COT................................ 188
Figure 58 NSD analysis of a) COT core, b) NP-COT and c) BPP-COT. The
substituents have been removed for clarity. B
2u
= saddling, B
1u
= ruffling,
A
2u
= doming, E
g(x)
= wave in x axis, E
g(y)
= wave in y axis, A
1u
=
propellering................................................................................................. 190
Figure 59 Absorption (black open squares) and emission (blue open
triangle) spectrum of BPP-COT at 77K in 2-Me-THF with a
phosphorescence emission (red open circles) at 525 nm. ............................. 192
Figure 60 Fluorescence emission of impurity (brown open diamond) and
enlarged view (green open stars). ................................................................ 193
xvi
List of Schemes
Scheme 1 Generalized mechanism for the electron - transfer
photosensitization of aryliodonium salts........................................................18
Scheme 2 Synthesis of 9-allyl-carbazole (5) from carbazole (3) and allyl
bromide (4). ..................................................................................................20
Scheme 3 Base catalyzed isomerization of 9-allyl-carbazole (5)...........................20
Scheme 4 Synthesis of (4-n-decyloxyphenyl)phenyliodonium
hexafluoroantimonate (12). ...........................................................................23
Scheme 5 Vilsmeier-Haack Reaction on NPD......................................................26
Scheme 6 Wittig reaction yielding DV-NPD........................................................26
Scheme 7 Thermal cross-linking of the reactive benzocyclobutene to the
dibenzocyclooctadiene. .................................................................................39
Scheme 8 Polymerization of the cross-linkable monomer 4-vinyl-
benzocyclobutene 17 and the hole transporters 16 and 23 with nitroxide
initiator. ........................................................................................................42
Scheme 9 Monomer synthesis of MV-TPD derivative 23.....................................43
Scheme 10 Synthesis of Pt(II)-tetraphenyltetrabenzoporphyrin. ...........................68
Scheme 11 Synthesis of Pt(II)-tetraphenyltetranaphthoporphyrin.........................73
Scheme 12 Retrosynthetic analysis of asymmetric porphyrins............................ 114
xvii
Scheme 13 One pot asymmetric benzo-/naphthoporphyrin synthesis.................. 116
Scheme 14 Synthetic scheme of the precursor isoindole (9) for the rational
design of trans-benzonaphthoporphyrin. ..................................................... 117
Scheme 15 Synthetic scheme for the precursor (9) for the synthesis of
Pt(II)-tetraphenyl-trans-benzonaphthoporphyrin. ........................................ 120
Scheme 16 Synthetic scheme for the synthesis of Pt(II)-tetraphenyl-trans-
benzonaphthoporphyrin (18). ...................................................................... 123
Scheme 17 Random synthetic approach to asymmetric porphyrins..................... 133
Scheme 18 Synthesis of octa-p-tolyl-cyclooctatetraene (OT-COT) (4). .............. 178
Scheme 19 Synthesis of Tetra-biphenyl-tetra-phenyl-cyclooctatetraene
(BPP-COT) (8)............................................................................................ 180
Scheme 20 Synthesis of Tetra-m-tolyl-tetraphenyl-cycooctatetraene
(mTP-COT) (11). ........................................................................................ 182
Scheme 21 Synthesis of Tetra-1-naphthyl-tetraphenyl-cycooctatetraene
(NP-COT) (14)............................................................................................ 184
xviii
List of Tables
Table 1 Film thickness of ally-carbazole / IOC 10 as a function of
solvent and rpm.............................................................................................30
Table 2 Results of statistical copolymerization between hole transporter
and cross-linker.............................................................................................46
Table 3 Radiative and nonradiative rates for bubble and freeze-
pump-thaw degassed samples of Pt(TPBP)....................................................95
Table 4 Radiative and nonradiative rates for a bubble degassed sample of
Pt(TPNP). ................................................................................................... 106
Table 5 NSD results of COT, OP-COT and BPP-COT. D
oop
= total
out of plane distortion, B
2u
= saddling, B
1u
= ruffling, A
2u
= doming,
A
1u
= propellering. ...................................................................................... 191
xix
Abstract
This dissertation describes the development of interesting materials for
different organic layers, which compose the standard organic light emitting diode
(OLED) architecture.
Chapter one introduces the OLEDs architecture, energy transfer
mechanisms within the OLED and current – voltage behavior.
The first part of chapter two deals with the photoinitiated cationic radical
polymerization of cross-linkable monomeric compounds for solution processable
hole-transporting layers. These solutions where subjected to UV irradiation and the
polymerization process monitored by the disappearance of characteristic IR-Bands
using FTIR spectroscopy. The second part of chapter two deals with thermally
cross-linkable hole-transporting polymers based on 4-vinyl-benzocyclobutene and,
4-[N-(4-vinylphenyl)-N-(4-methylphenyl)amino]-4´-[N-phenyl-N-(4
methylphenyl)-amino]-biphenyl or 3-vinyl-triphenylamine. These polymer films
were thermally annealed for 2 h at 170 °C followed by cross-linking at 210 °C for 5
h. OLED devices were made by vacuum deposition of aluminum tris(8-
hydroxyquinoline) acting as both the emitting- and electron injection layer on top
of spin-coated cross-linked, non-cross-linked, and homopolymer hole-transporting
layers. The results obtained were compared with small molecule vacuum deposited
TPD, a common hole transporting material used in devices today.
xx
Chapter three deals with phosphorescent complexes employed as dopants in
nIR emitting OLEDs. A family of metal complexes that have shown intense
absorption and emission in the red-to-near infrared region of the electromagnetic
spectrum are metalloporphyrins. In this chapter we studied the photophysical and
electroluminescence properties of two Pt-metalloporphyrins; Pt(II)-
tetraphenyltetrabenzoporphyrin and Pt(II)-tetraphenyltetranaphthoporphyrin.
Chapter four describes the rational large scale synthesis of asymmetric
porphyrins to provide a phosphorescence dopant for the application in organic light
emitting diode. These asymmetric porphyrins are expected to have a
phosphorescence emission maximum between those reported for
Pt(II)(tetraphenylbenzoporphyrin) and Pt(II)(tetraphenylnaphthoporphyrin). Do to
scrambling during the porphyrin formation reaction, systematic synthesis of
different asymmetric porphyrins was not possible on large scale without extensive
work on improving the reaction conditions in order to minimize scrambling and
product purification efforts. Thus a statistical synthetic approach was chosen along
with HPLC purification to identify specific asymmetric compounds and to
investigate their corresponding phosphorescence emission energies.
Chapter 5 describes host materials for OLEDs. The most common design
for phosphorescence-based OLEDs involves a doped emissive region, where the
emissive dopant is either an Ir or a Pt complex. While high-efficiency green and
red emitting colors could be
obtained readily by doping in the commonly used host
materials,
such as tris(8-hydroxyquinolinato)aluminum (Alq
3
), a wider band gap
xxi
host is
essential for the efficient generation of blue dopant emission.
Cyclooctatetraene (COT) is a highly interesting class of organic molecules and
despite having the same (CH)
n
formulation, benzene and COT have strikingly
different properties for example, COT is a non-planar tub-shaped molecule. But,
the reduction of COT is accompanied by a structural change of the tub shaped
neutral molecule to a planar ring, which has been both studied computationally and
spectroscopically. For this reason, the geometry of the COT has to be locked into
place in order to keep the high triplet energy need for efficient energy transfer to
the blue phosphorescent dopant. This may be accomplished through the addition of
bulky substituents to the COT scaffold.
1
Chapter 1 Organic Light-Emitting Diodes (OLEDs)
General Introduction to Organic Light-Emitting Diodes (OLEDs)
Organic Light Emitting Diodes (OLEDs)(Tang and VanSlyke 1987) are
electroluminescent display devices made out of stacked organic thin films
sandwiched between two metal electrodes. The total thickness of an OLED is less
than 500 nm (for a typical OLED architecture) making it the most compact flat
panel display on the market. Unlike LCD displays, OLED displays do not require
any back lighting and can operate in broader temperature ranges. OLED displays
are lighter, brighter, have wider viewing angles (160º), faster response time, and
low turn on voltages compared to the LCD displays.(Tullo 2006) The operation of
OLEDs relies on molecular excitation and radiative relaxation (or decay) of
conjugated organic molecules (chromophores). When a voltage is applied across
the OLED, the luminescent (fluorescent or phosphorescent chromophores) within
the emissive layer get promoted to the excited state of the specific chromophore by
recombination of opposite charge carriers. Radiative relaxation of the excited
chromophore back to the ground state results in the generation of photons with
energies defined by the chromophores electronic structure. An efficient pathway of
obtaining pure colored emission in OLEDs, is accomplished by choosing the
correct host – dopant system, where a proper energy match between the host and
2
dopant allows for energy transfer to the emissive species thus allowing for pure
RGB, white or IR emission.
OLED Architecture
The simplest form of an OLED (Figure 1) includes a hole transporting layer
(HTL)(Tokito, Tanaka, Okada and Taga 1996; Giebeler, Antoniadis, Bradley and
Shirota 1999; Ren, Alleyne, Djurovich, Adachi, Tsyba, Bau and Thompson 2004),
which is a p-type semiconductor, an electron transporting layer (ETL)(Shen, Hill,
Kahn and Schwartz 2000; Aziz and Popovic 2002; Ichikawa, Kawaguchi,
Kobayashi, Miki, Furukawa, Koyama and Taniguchi 2006), which is an n-type
semiconductor, and a spacer layer, all sandwiched between an anode and a
cathode.(Adamovich, Cordero, Djurovich, Tamayo, Thompson, D'Andrade and
Forrest 2003; Yoon, Yang, Choo, Kim and Oh 2005)
Figure 1 Typical OLED architecture showing transparent metal anode, hole
transporting layer (HTL), emissive layer (EML), electron transporting layer
(ETL), spacer and metal cathode.
3
In conventional OLEDs, the metal cathode is opaque and the metal anode
transparent, therefore the photos created within the device are allowed to exit the
structure via this transparent electrode. Indium Tin Oxide (ITO) (In
2
O
3
: SnO
2
) is
the most widely used material for anode in OLEDs.(Tak, Kim, Park, Lee and Lee
2002)
OLEDs are fabricated by vapor depositing (10
-6
torr) or spin-coating of
organic layers on an ITO coated glass substrate, followed by vapor deposition of
the metal cathode. Most commonly used HTL and ETL materials used in OLEDs
are N,N’-di(naphthalene-1-yl)-N,N’-diphenyl-benzidine (α-NPD)(Giebeler,
Antoniadis, Bradley and Shirota 1999)
and aluminum tris(8-hydroxyquinoline)
(Alq
3
)(Shen, Hill, Kahn and Schwartz 2000; Aziz and Popovic 2002)
respectively.
In addition, HTL and ETL materials need to form thermally stable glasses, which
requires high glass transition temperatures (Tg) and low crystal-growth velocity to
prevent recrystallization upon heating due to the operation of OLEDs under an
applied bias. Recrystallization can expand the materials, thus disrupting the multi-
layer structure, finally leading to the irreversible failure of devices.(Fenter,
Schreiber, Bulovic and Forrest 1997)
Some devices also incorporate copper phthalocyanines (CuPc) as a hole
injecting layer (HIL) between the anode and the HTL layers for balanced charge
recombination.(Vestweber and Riess 1997; Nuesch, Carrara, Schaer, Romero and
Zuppiroli 2001; Tadayyon, Grandin, Griffiths, Norton, Aziz and Popovic 2004)
Most commonly used cathodes in OLEDs are Ca,(Choong, Shi, Curless and So
4
2000)
Mg,(Turak, Grozea, Feng, Lu, Aziz and Hor 2002)
or Al.(Hung, Tang and
Mason 1997)
The use of low work function metals like Ca and Mg in OLEDs
improves the electron injection from the cathode to the ETL. However, OLEDs
fabricated with these electrodes actually give decreased device efficiency due to
highly reactive nature of these metals. On the other hand Aluminum, due to its
comparatively higher work function decreases device efficiency by creating a
barrier for electron injection.(Choong, Shi, Curless and So 2000)
Insertion of a
spacer layer between the ETL and the cathode has been demonstrated to
significantly increase device performance. The presence of LiF as the spacer layer
at the Al-Alq
3
interface lowers the electron injection barrier height due to band
bending of the Alq
3
electronic structure.(Hung, Tang and Mason 1997)
The first classic double heterostructure device was demonstrated by Tang
and Van Slyke in 1987.(Tang and VanSlyke 1987) A diammine (acting as the
HTL)/Alq
3
(acting as the EML and ETL) device (Figure 2) can achieve an external
quantum efficiency (photons/electron) of ~1%, brightness >1000 Cd/m
2
and low
turn-on voltage (ca. 5V at 1 Cd/m
2
).
5
Figure 2 The energy diagram of a double heterostructure device. Holes (red
circles) and electron (blue circles) are injected from their respective electrodes
into the HOMO and LUMO of the organic molecules. The holes and electrons
migrate along the interface, forming an exciton at that interface and radiative
decaying through the emission of a photon.
Hole injection occurs when one electron is removed from the HOMO of
hole-transporters, resulting in a positively charged radical cation. Similarly, the
electron transport layer is expected to transport electrons from the cathode. The
electron transport process adds one electron into the LUMO of electron-
transporters, making the ETL molecule a negatively charged radical anion (Figure
2). The emission from this OLED device comes from the green emitter, Alq
3
. This
is due to the fact that Alq
3
has a smaller HOMO-LUMO energy gap compared with
diammine, resulting in preferential energy transfer from diammine to Alq
3
.
Additionally, diammine transports holes faster than Alq3 transports electrons, so
the excitons (a loosely bound hole and electron pair) are generated within the Alq
3
6
layer, near the diammine/Alq
3
interface. One of the drawbacks of using Alq
3
neat
film as emissive layer, is the severe self-quenching of excitons in the neat film.
This problem can be overcome by doping an emissive dye into the ETL, thus
creating a new layer called the light emitting layer (EML).(Zhang, Wei, Liu, Zhu,
Jiang and Zhang 2007)
Figure 3 Energy diagram of an OLED with HTL, EML (containing host
(orange) and dopant (green) energy levels) and ETL.
Inside of the EML, energy will transfer readily from the host to a dopant
with a smaller optical gap resulting in efficient emission from the dopant (Figure
3). The self-quenching of excitons is also suppressed by lowering the concentration
of excitons and thus device efficiency is improved. An additional benefit of doping
is to control emission colors of OLEDs by doping different emissive
dyes.(Shoustikov, Yujian and Thompson 1998)
7
Energy Transfer in OLEDs
There are two distinct mechanisms for the energy transfer from host to
dopant: Förster mechanism or Dexter electron exchange. In the Dexter electron
exchange (Figure 4), the exciton hops directly between molecules in a concerted
energy transfer (ET).(Baldo, Thompson and Forrest 2000)
Figure 4 Dexter energy transfer from host to dopant.
This is a short-range process dependent on the overlap of molecular orbitals
of neighboring molecules. It also preserves the symmetry of the host and dopant
pair. Thus, a triplet–singlet energy transfer is not possible by a Dexter mechanism.
A change in spin-symmetry is possible if the host exciton breaks up and reforms on
the dopant by incoherent electron exchange.(Klessinger and Michl 1995) However,
this process is considered to be relatively unlikely as it requires the dissociation of
the host exciton, which in most molecular systems has a binding energy of 1
eV.(Baldo, Thompson and Forrest 2000)
8
The Förster energy transfer process relies on the electrostatic interaction
between a host and dopent molecule (Figure 5).
Figure 5 Förster energy transfer from host to dopant.
In this mechanism, an excited host molecule with oscillating dipoles creates
an electromagnetic disturbance. When a dopant molecule in the ground state
comes in the vicinity of this electromagnetic disturbance, due to coulombic
interaction, the dopant starts oscillating at the same frequency as the oscillating
dipole of the host. This energy transfer from the host to the dopant occurs through
dipole-dipole coupling between the transition dipole moments of the excited host
and the ground state dopant. In the process, as the excited host relaxes to the
ground state, it transfers the energy to the dopant via coulombic interaction. The
Förster energy transfer process may occurs in long distances (up to 100Å) and does
not require any physical contact between the orbitals of the host and the dopant
molecules.(Turro 1991; Shoustikov, Yujian and Thompson 1998)
9
The energy transfer in a doped double heterostructured OLED (where the
dopant level in the EML may be as high as 10% for certain emissive dopants in a
host matrix) follows the following pathway: 1) Opposite charges are injected into
HTL and ETL layers from the two electrodes. 2) Holes and electrons migrate at
different velocities through their respective organic layers towards the interface of
the EML. 3) Charge recombination and exciton formation occur on the host in the
following way. The radical cation HTL and the radical anion ETL come in contact
with the host, forming on opposite sides of the EML the radical cation and radical
anion host species and the ground state molecules of the HTL and ETL. These two
opposite charged host moieties combine within the EML to from an exciton and a
ground state host molecule. 4) Finally this host exciton gets transferred to the
dopant molecule forming a dopant exciton and a ground state host molecule. 5)
Than exciton dopant radiatively relaxes to the ground state via production of a
photon.
OLED Containing Phosphorescent Dopants
The hole and electron in OLEDs are odd electron species with an equal
distribution of ms = ± ½. Thus, when the hole and electron recombine to form an
exciton, a statistical mixture of singlet and triplet excitons are generated.(Baldo,
O’Brien, Thompson and Forrest 1999; Segal, Baldo, Holmes, Forrest and Soos
2003) This leads to a population of excitons that is 25% singlet and 75% triplet
10
and has a profound effect on OLED efficiency. Most of the earlier emitting
dopants emitted from fluorescent states, which only utilize the singlet fraction of
formed excitons.(Shoustikov, Yujian and Thompson 1998) This limits the internal
quantum efficiency of fluorescence based devices to 25%. Later, a new class of
emissive dopants was introduced that gave marked increases in OLED efficiency,
which was accomplished by collecting the triplet excitons as well as the singlet
ones.
Efficient harvesting of triplet excitons requires a phosphorescent dopant,
which will trap both singlet and triplet excitons. The best way to achieve both high
phosphorescence efficiency and a radiative lifetime on the order of microseconds,
is to incorporate a heavy metal atom into the dopant. The most commonly used
metal for this purpose is Pt and Ir, however, efficient phosphorescent dopants have
been also prepared with other heavy metals as well, including Ru, Re, Au and Os.
The emission energy for organometallic phosphors is closely related to the
structure of organic ligands, making it possible to fine tune the emission
wavelength as a function of ligand substitution.(Brooks, Babayan, Lamansky,
Djurovich, Tsyba, Bau and Thompson 2002) As mentioned above, heavy atoms
can promote intersystem crossing by a mechanism know as spin-orbit coupling.
Strong spin-orbit-coupling mixes singlet and triplet metal-to-ligand charge transfer
(MLCT) states. Mixing of
1
MLCT and
3
MLCT states with the triplet ligand-
centered state (
3
LC) creates a hybrid
3
(LCMLCT). This mixing removes the spin-
forbidden nature of the radiative relaxation of the triplet state thus leading to high
11
phosphorescence efficiencies.(Sprouse, King, Spellane and Watts 1984)
Current – Voltage Behavior of OLEDs
The current-voltage (I-V) characteristic of an OLED follows the typical
non-liner pattern of a p-n junction diode (Figure 6).
Figure 6 Log – log plot of current versus voltage (I - V) showing the typical
charge transport behavior of an OLED.
At low voltages, electrons and holes are injected through the electrode-
organic interface as thermally generated free charge, and the current is proportional
to the square of the applied voltage (I ∝ V
2
). This is considered a space charge
limited (SCL) conduction and is used to describe the conduction of insulating
solids. At a higher potential, the IV characteristics are dominated by trap charge
limited (TCL) conduction. TCL conduction occurs when the applied potential is
great enough to inject holes or electrons within the HOMO-LUMO energy gap of
the charge carriers. As charge is trapped by (i.e. injected into) the HTL and ETL
12
carriers, a dramatic increase in the amount of current passed at a given voltage is
observed (I = ~V
8±1
).(Burrows and Forrest 1994)
External Quantum Efficiency of OLEDs
A direct and accurate means for measuring the external quantum efficiency
(EQE) of the OLED employs a calibrated photodetector used in a measurement set-
up requiring a minimal amount of correction for losses due to lenses, overfilling of
the detector active area with light, conversion between light out put of the OLED
(usually measured in lumens) to percent etc.(S.R. Forrest, Bradley and Thompson
2003) The EQE of a device is defined as the ratio of the number of photons
emitted by the OLED into the viewing direction to the number of injected
electrons.
13
Chapter 2 Cross-linkable Hole Transporting Layer (HTL)
for the use in Solution Processed Organic Light Emitting
Diodes (OLED)
Abstract
In the first part of this chapter, the photoinitiated cationic radical
polymerization of cross-linkable monomeric compounds was investigated as a
solution processable monomeric hole-transporting layers (HTL) for use in organic
light emitting diodes (OLED). 9-(1-propenyl)carbazole, N,N'-di-1-naphthyl-N,N'-
(4,4'-divinyl)diphenyl-1,1'-biphenyl-4,4'diamine (DV-NPD) and photoinitiator (4-
n-decyloxyphenyl)phenyliodonium hexafluoroantimonate (IOC 10) where
synthesized. Solutions containing only 9-(1-propenyl)carbazole / IOC 10 and
solutions containing 9-(1-propenyl)carbazole / DV-NPD / IOC 10 where subjected
to UV irradiation and the polymerization process monitored by the disappearance
of characteristic IR-Bands using FTIR spectroscopy. Since these films of cross-
linkable monomeric species did not render an insoluble layer after UV irradiation,
thermally cross-linkable hole-transporting polymers based on 4-vinyl-
benzocyclobutene and, 4-[N-(4-vinylphenyl)-N-(4-methylphenyl)amino]-4´-[N-
phenyl-N-(4-methylphenyl)-amino]-biphenyl or 3-vinyl-triphenylamine were
investigated. The polymerizations were carried out with a nitroxide initiator
yielding polymers with predictable molecular weights and low polydispersities.
Polymer films were thermally annealed for 2 h at 170 °C followed by cross-linking
14
at 210 °C for 5 h. Properties of the films were investigated by ellipsometry and
atomic force microscopy (AFM). OLED devices were made by vacuum deposition
of aluminum tris(8-hydroxyquinoline) (Alq
3
) as both the emitting- and electron
injection layer on top of spin-coated cross-linked, non-cross-linked, and
homopolymer hole-transporting layers. The results obtained were compared with
small a molecule vacuum deposited TPD device, a common hole transporting
material used in devices today.
Introduction to Photoinitiated Cationic Radical Polymerization
Different fabrication methods are used to manufacture Organic light
emitting diodes (OLEDs), but the most frequently applied method is a high vacuum
deposition process. Here the organic compounds, as well as the cathode metals, are
sublimed onto a substrate under high vacuum conditions (10
-6
torr). Due to the fact
that the organic substances are sublimed, films of superb quality (e.g. homogeneity)
can be achieved. On the other hand, this deposition process also limits the choice
of possible organic materials to those with high thermal stability suitable for
sublimation. This aspect introduces a confining factor towards the diversity of
compounds applicable for OLED manufacturing.
15
Alternatively these multilayer structures can be realized via a solution
process, for example spin casting. Here a solution of a specific organic molecule is
spun onto the anode, during which the solvent evaporates and a thin film is formed.
One significant disadvantage of this type of process is that each layer has to be
coated onto the previous layer without altering the one below it. One way this
might be achieved is through the use of orthogonal solvent systems, but this once
again introduces a limiting factor to the diversity of compounds that may dissolve
in these solvents. Another useful method to avoid redissolution is to crosslink the
coated layer before applying the next one.(Allen, Khan, Edge, Billings and Veres
1998) A lot of research has been done on how to initiate the cross linking reaction.
James Crivello et al(Yujing Hua 2000; Zaza Gomurashvili, Yujing Hua James V.
Crivello 2001) demonstrated the use of photosensitizers and photoacids to
successfully crosslink 9-(1-propenyl)carbazole moieties, but the produced films
were on the order of one magnitude larger in thickness than typically found in an
OLED. Other ways of polymerization initiation, thermal polymerization of
functional groups, has also been investigated.(Li, Ding, Day, Tao, Lu and D'Iorio
2003) To achieve homogeneous and crack-free layers, the polymerizable group
used for this cross-linking should show only little volume shrinkage, short
conversion time, and nearly quantitative yield.(Hreha, Zhang, Domercq, Larribeau,
Haddock, Kippelen and Marder 2002; Zhang, Ya-Dong , Hreha, Richard D. ,
Jabbour, Ghassan E., Kippelen, Bernard, Peyghambarian, N. and Marder, Seth R.
2002; Domercq, Hreha, Zhang, Larribeau, Haddock, Schultz, Marder and Kippelen
16
2003) This is very important since the average thickness of an organic film in an
OLED is only about 250 – 500 Å. Any imperfections or low local cross-linking
density will render the produced film soluble in common organic solvents. A
molecule with high functionality provides locally- as well as a globally high cross-
linking density, if this monomer is statistically distributed throughout the film.
Taking this approach one step further and incorporating lithographical methods,
production of multicolor OLEDs can be realized. Since the technology of
photolithography has been thoroughly developed, the challenge lies within the
chemical derivatization of existing molecules (without altering their electronical
properties), which emit in the spectrum of the primary colors. In addition, those
molecules having to perform hole transporting and electron injecting functions, also
need adequate cross-linkable functional groups for efficient OLED operation.
Over the past decade, onium salt photoinitiated cationic polymerizations
have become established as both processes of commercial importance as well as
reactions of academic interest.(Crivello and Jang 2003) Most onium salts possess
absorption bands that are confined to the short wavelength region of the UV
spectrum. For this reason, much of the energy emitted by broadband light sources,
such as commonly used mercury arc lamps, is wasted due to the inferior absorption
cross section at lower energies of the electromagnetic spectrum. The use of long
wavelength absorbing photosensitizers permits the capture of a higher fraction of
the available emitted light from these commonly applied sources. This contributes
to an overall more efficient photolysis of the photoinitiator and consequently, the
17
generation of a larger number of initiating species than when no photosensitizer is
present.(Crivello and Jang 2003) Currently, the most commonly used
photoinitiators for photoinduced cationic vinyl and ring-opening polymerizations
are diaryliodonium (1) and triarylsulfonium salts (2) Figure 7.(Zaza Gomurashvili,
James V. Crivello 2001)
Ar I Ar
MtX
n
-
1
Ar S Ar
Ar
MtX
n
-
2
Figure 7 General structure of diaryliodinium (1) and tryarylsulfonium (2)
salts.
Electron-transfer photosensitization involves firstly, absorption of light by the
photosensitizer, PS, to give the corresponding excited species [PS]* (Scheme 1).
18
PS
h!
PS
*
PS
*
+ Ar
2
I
+
MtX
n
-
PS Ar
2
I
+
MtX
n
-
*
exciplex
PS Ar
2
I
+
MtX
n
-
*
PS MtX
n
-
+ Ar
2
I
Ar
2
I ArI + Ar
PS MtX
n
-
+ mM Polymer
Scheme 1 Generalized mechanism for the electron - transfer
photosensitization of aryliodonium salts.
A heterodimeric excited state complex (exciplex) is often formed as an
intermediate between the onium salt and the excited photosensitizer. Subsequently,
the onium salt is reduced by a formal electron transfer between the photosensitizer
and the diaryliodonium salt. The rapid decomposition of the resulting unstable
diaryliodine free radical prevents back electron-transfer and renders the overall
process irreversible. Finally the photosensitizer radical cation reacts with a
monomeric species mM to form the polymer.(Zaza Gomurashvili, Yujing Hua
James V. Crivello 2001) Due to their lower reduction potentials, diaryliodonium
salts are more easily photosensitized by electron-transfer process than
triarylsulfonium salts.(Crivello 1978) Chen et al.(Yaohong Chen 2000) and
Crivello et. al.(Yujing Hua 2000) have described studies of photosensitized
photolysis of onium salts by various compounds containing the carbazole moiety.
19
Experimental
Synthesis and Characterization
Unless otherwise noted, all reagents and solvents where obtained from
Sigma-Aldrich and used without any further purification. 9-Allyl-carbazole (5), 9-
(1-propenyl)carbazole (6 and 7) (Zaza Gomurashvili, Yujing Hua James V.
Crivello 2001) and (4-n-decyloxyphenyl)phenyliodonium hexafluoroantimonate
(Crivello and Lee 1989) where prepared following literature procedures. Extracts
were dried over Na
2
SO
4
and solvents were removed with a rotary evaporator at
aspirator pressure. NMR spectra were recorded on Bruker AM 250 instrument with
TMS as standard signal.
Synthesis of 9-(1-propenyl)carbazole
A wide variety of 9-substituted (N-substituted) carbazole compounds can be
readily prepared by alkylation, acylation or arylation of carbazole under strongly
basic conditions.(Joule 1984) 9-Allyl-carbazole (5) was synthesized by
condensation of commercially available carbazole (3) in dry THF with allyl
bromide (4) in the presence of potassium tert.-butoxide as described in Scheme 2.
20
N
H
3
+
Br
4
THF
Base
N
5
Scheme 2 Synthesis of 9-allyl-carbazole (5) from carbazole (3) and allyl
bromide (4).
The base catalyzed isomerization of compound (5) yielded 9-(1-
propenyl)carbazole (trans (6) / cis (7) ratio of 3:1 by NMR) in 90% yield as
described in Scheme 3.
Base
N
6
N
5
N
7
+
Scheme 3 Base catalyzed isomerization of 9-allyl-carbazole (5).
9-Allyl-carbazole (5). In a round bottom flask with addition funnel was
placed carbazole (3) (6 mmol) and in the addition funnel potassium tert.-butoxide
(6 mmol). After repeatedly purging the flak with N
2
, dry THF (60 ml) was added
to the addition funnel suspending the potassium salt, which was slowly added to the
21
carbazole and stirred for 1h at room temperature. Allyl bromide (4) (6.5 mmol)
was added through a septum and the mixture was stirred at room temperature for 30
minutes. Tetrabutylammonium bromide (6 mmol) was added and the reaction
mixture stirred over night. The resulting solution was filtered, the solvent
evaporated leaving behind a yellow oil, which was recrystallized once from
methanol affording the product 5 in 49 % yield. Mp (Lit): 52 – 53 °C, Mp
(compound): 53 – 54 °C. MS (expected) m/z: 207.27, MS (found) m/z: 207
9-(1-propenyl)carbazole (6 and 7). Ally-carbazole (5) (5 mmol),
potassium tert.-butoxide (5 mmol), and 10 ml of DMSO were placed in a flask and
stirred for 20 minutes at room temperature. The temperature was raised to 120 °C
and kept for 2 hours. After cooling the reaction mixture to room temperature, 20 ml
of distilled water was added and the aqueous phase extracted with diethyl ether.
The organic phase was washed with water, dried with Na
2
SO
4
, filtered and the
solvent evaporated under reduced pressure affording (6 and 7) in 90 % yield (trans
(6) / cis (7) ratio of 3:1 by NMR)(Figure 8).
1
H-NMR (250 MHz, d
6
-DMSO): δ =
8.19-8.14 (m, 1H), 7.74-7.70 (m, 1H), 7.51-7.42 (m, 1H), 7.28-7.20 (m, 1H), 6.90-
6.82 (m, 1H), 6.62-6.59 (m, 1H), 6.22-6.09 (m, 1H), 6.06-5.94 (m, 1H), 1.96-1.90
(dd, 1H, J = 6.8, 1.6 Hz), 1.57-1.50 (dd, 1H, J = 6.9, 1.7 Hz). MS (expected) m/z:
207.27, MS (found) m/z: 207
22
Figure 8 NMR Spectra of compound mixture 6 and 7.
Synthesis of (4-n-decyloxyphenyl)phenyliodonium
hexafluoroantimonate (IOC 10) (12)
The ability of these compound to generate very strong protonic acids on
photolysis, makes this onium salt derivative very attractive for large scale cross-
linked surface coatings also termed as “UV curing”.(Crivello and Lee 1989)
Eckberg and LaRochelle (Eckberg and LaRochelle 1981; Eckberg and LaRochelle
1983) have developed photoinitiators with long alkyl pendant groups attached to
N
Hc
CH
3
(a)
Hb
Hd
He
Hf
Hg
N
CH
3
(a')
Hc'
Hb'
Hd
He
Hf
Hg
Ha’
Ha
Hb’
Hb
Hc’
Hc
Hd
Hg
Hf
He
6 7
23
the phenyls rings, which greatly enhances the solubility of these onium salts in both
polar / non-polar solvents and or in the monomers themselves. The alkoxy
substituted diaryliodonium salt was prepared as shown in Scheme 4.
OH
C
10
H
21
Br +
KOH
(n-Bu)
4
NBr
O
C
10
H
21
8 9
O
C
10
H
21
9
+
I
HO O S
O
O
CH
3
10
O
C
10
H
21
I
11
O
C
10
H
21
I
11
OTs
OTs
NaSbF
6
+
O
C
10
H
21
I
12
SbF
6
Scheme 4 Synthesis of (4-n-decyloxyphenyl)phenyliodonium
hexafluoroantimonate (12).
n-Decylphenyl ether (9). Phenol (8) (10.6 mmol), 1-bromodecane
(9) (3.5 mmol), (n-Bu)
4
NBr (0.4 mmol), KOH (10.6 mmol), H
2
O (10 ml) and
toluene (10 ml) where refluxed for 20 hours. After the reaction mixture was cooled
to room temperature, it was transferred to a separatory funnel. The organic layer
24
was washed with 0.5 N NaOH to remove excess phenol. The toluene layer was
washed with water, dried with Na
2
SO
4
and the solvent removed under vacuum to
afford a colored oil 9 in 30 % yield.
1
H-NMR (250 MHz, d
6
-DMSO): δ = 7.50-
7.44 (m, 2H), 7,14-7.06 (m, 3H), 4,13(t, 2H, J = 6.5 Hz), 1.95-1.84 (m, 2H), 1.53
(s, 14H), 1.06 (t, 3H, J = 6.6 Hz). MS (expected) m/z: 234.38, MS (found) m/z:
234.
(4-n-decyloxyphenyl)phenyliodonium tosylate (11). To n-decylphenyl
ether (9) (25.5 mmol) and Koser reagent (10) (20.4 mmol) was added acetonitrile
(10 ml) followed by glacial acetic acid (2 ml) which was stirred at 40 °C for 3
hours. The solution was cooled and water (60 ml) was added. After freezing this
mixture and agitation, the product crystallized, filtered and washed thoroughly with
water and hexanes. The product was dried over night after it was recrystallized
from toluene / hexanes (1:1) to afford crystals of 11 in 83 % yield. Mp (Lit): 119 –
121 °C, Mp (compound): 113 – 115 °C. MS (expected) m/z: 608.57, MS (found)
m/z: 608, 360, 204, 91.
(4-n-decyloxyphenyl)phenyliodonium hexafluoroantimonate (12). (4-n-
decyloxyphenyl)phenyliodonium tosylate (11) (8.2 mmol) and NaSbF
6
(8.2 mmol)
was stirred in acetone at room temperature for 1 hour. Sodium tosylate was filtered
off and the volume of acetone was reduced to 1/3 the original volume and poured
into distilled water. The aqueous layer was decanted and on cooling and stirring
the product crystallized. Purification was accomplished by dissolving the product
in minimal amounts of methanol and triturating with a large quantity of distilled
25
water. The product was isolated by filtration, washed with water and dried to afford
12 in 81 % yield. Mp (Lit): 74 – 76 °C, Mp (compound): 70 – 72 °C. λ
(max: MeOH)
:
202 nm, 247 nm.
1
H-NMR (250 MHz, d
6
-DMSO): δ = 8.30-8.27 (m, 2H), 7.85-
7.80 (m, 1H), 7.60-7.68 (m, 2H), 7.13-7.19 (m, 2H), 6.83-6.80 (m, 2H), 4.06-4.00
(m, 2H), 1.74 (q, 2H, J = 6.8 Hz), 1.26 (s, 14H), 0.86 (t, 3H).
Synthesis of N,N'-di-1-naphthyl-N,N'-(4,4'divinyl)diphenyl-1,1'-
biphenyl-4,4'diamine (DV-NPD) (15)
Traditionally organic light emitting diodes have been fabricated using N,N'-
di-1-naphthyl-N,N'-diphenyl-1,1'-biphenyl-4,4'diamine (NPD) due to their
improved operational stability in comparison to other hole transporting
materials.(Van Slyke, Chen and Tang 1996; Halls, Tripp and Schlegel 2001) DV-
NPD was synthesized as depicted in Scheme 5 and Scheme 6. The first step in the
synthetic pathway is the Vilsmeier-Haack formylation of the electron rich arene
system, followed by the Wittig reaction to yield the final divinylated product 15.
26
N
N
Vilsmeier-Haack Reagent
N
N
H
O
H
O
13
14
Scheme 5 Vilsmeier-Haack Reaction on NPD.
Wittig Reagent
N
N
14
15
N
N
H
O
H
O
Scheme 6 Wittig reaction yielding DV-NPD.
N,N'-di-1-naphthyl-N,N'-(4,4'-diformyl)diphenyl-1,1'-biphenyl-
4,4'diamine (DF-NPD) (14). Phosphoryl chloride (221 mmol) was dropped into
27
DMF (221 mmol) at 0 °C, stirred at this temperature for 15 minutes and than heated
to 40 °C for 5 minutes to prepare the Vilsmeier-Haack reagent. This reagent is than
dropped into a heterogeneous solution of DMF (10 ml) and N,N'-di-1-naphthyl-
N,N'-diphenyl-1,1'-biphenyl-4,4'diamine (NPD) (13) (1.70 mmol) and heated for 24
hours at 60 °C. Next the DMF is removed and a 1M NaOH was added and stirred
at 60 °C for 2 hours to yield 14 in 7 % yield.
1
H-NMR (250 MHz, CDCl
3
): δ =
9.79-9.77 (s, 2H), 7.94-7.83 (m, 6H), 7.67-7.61 (m, 4H), 7.51-7.39 (m, 12H), 7.30-
7.29 (m, 4H), 6.98-6.89 (m, 4H).
N,N'-di-1-naphthyl-N,N'-(4,4'-divinyl)diphenyl-1,1'-biphenyl-
4,4'diamine (DV-NPD) (15). n-BuLi (2.5 M in hexanes) (2.24 mmol) was
dropped into a THF (10 ml) solution of methyltriphenylphosphonium bromide
(0.56 mmol) at room temperature to form the Wittig reagent. A solution N,N'-di-1-
naphthyl-N,N'-(4,4'formyl)diphenyl-1,1'-biphenyl-4,4'diamine (DF-NPD) (14)
(0.14 mmol) in THF (5 ml) was slowly added to the Wittig reagent. This reaction
mixture was stirred over night at room temperature. The solvent was removed
under reduced pressure and the product purified by column chromatography on
silica gel yielding 15 in 50 % yield.
1
H-NMR (250 MHz, d
6
-DMSO): δ = 7.94-7.83
(m, 6H), 7.67-7.61 (m, 4H), 7.51-7.39 (m, 12H), 7.30-7.29 (m, 4H), 6.98-6.89 (m,
4H), 6.68 (dd, 1H, J = 17.6, 10.9 Hz), 5.60 (d, 1H, J = 17.6 Hz), 5.17 (d, 1H, J =
10.9 Hz).
28
Thin Film Fabrication
Thin films of cis/trans 9-(1-propenyl) carbazole (6 and 7) and N,N'-di-1-
naphthyl-N,N'-(4,4'-divinyl)diphenyl-1,1'-biphenyl-4,4'diamine (DV-NPD) (15)
with 1 mol % of IOC 10 (12) where spin cast from dichloromethane,
tetrahydrofuran, cyclohexane with a concentration of 0.1 mol/l with constant time
of 40 seconds while varying the rpm from 1000 rpm to 3000 rpm. The spin coater
used was purchased from Specialty Coating System Inc., Indianapolis, IN, USA.
The film thickness was measured on a Si-wafer from International Wafer Service
Inc., with a polished/etched grade surface and thickness of 450 – 550 mm.
Rotating Analyzer Photometric Ellipsometer 2000FT from Rudolph Technologies
Inc., Santa Clara, CA, USA was used to measure the thickness on 4 representative
points on the wafer to determine the average thickness of the film. Near-UV
irradiation (360 – 440 nm / 500 watt) was accomplished with an OAI 150 from
Optical Associates Inc. Infrared spectra were recorded using a FTIR Spectrometer
2000 from Perkin Elmer on a salt plate from a macroscopic film which was
fabricated from a solution of know concentration of the photoacid, photosensitizer
and DV-NPD while the solvent was evaporated using a nitrogen stream.
29
Results and Discussion
Thin films of cis/trans 9-(1-propenyl) carbazole (6 and 7) and IOC 10 (12)
where spin cast on Si – wafers from a dichloromethane, THF, cyclohexane
solutions respectively, while varying the rpm but keeping the time rotation constant
at 40 seconds. In Table 1 contains the summary of the film thickness as a function
of revolutions per minute (rpm). Film thicknesses were determined by way of
ellipsometry on 4 independent points on the same substrate.
30
Solvent:
CH
2
Cl
2
Measurement/Position
RPM 1 2 3 4
Average
High (Å)
High
Deviation
(Å)
3000 248 350 315 329 311 38
2000 507 580 575 635 574 44
1000 2673 2743 2744 2642 2701 44
Solvent:
THF
Measurement/Position
RPM 1 2 3 4
Average
High (Å)
High
Deviation
(Å)
1500 1404 1354 1358 1359 1352 20
1000 1519 1587 1561 1564 1557 24
Solvent:
Cyclohexane
Measurement/Position
RPM 1 2 3 4
Average
High (Å)
High
Deviation
(Å)
1500 2314 1759 2214 1815 1990 241
1000 416 2797 766 183 279 1035
Table 1 Film thickness of ally-carbazole / IOC 10 as a function of solvent and
rpm.
31
As can be seen in Table 1, optimal film thickness of 500 Å can be achieved
if the film is spun for 40 seconds at 2000 rpm (red highlight in table 1). Do to the
high vapor pressure of dichloromethane other solvent have been investigated at the
same rotation speed. The films spun from THF and cyclohexeane where 2 fold
larger in thickness and cyclohexane produced an uneven film with pin holes visible
to the eye without magnification, which is also reflected in the large highth
deviation between the different measured points on the Si-wafer. FTIR was used to
investigate both photosensitization as well as polymerization of 6 and 7.
Macroscopic films where fabricated by simultaneous evaporation of the solvent
with a nitrogen stream, while the monomeric / photoacid solution was deposited on
a salt plate. The diagnostic IR-band frequency for a 1-propenyl group is 1670 cm
-
1
.(Sangermano, Malucelli, Bongiovanni, Priola, Annby and Rehnberg 2002)
In Figure 9b, it can be clearly seen that the 1-propenyl group of the
carbazole moiety did not undergo photoinitiated polymerization even after an
exposure time of 9 minutes towards near-UV electromagnetic radiation.
32
Figure 9 a) FTIR spectrum of 9-(1-propenyl)carbazole and IOC 10. b)
Enlarged region from 1800 - 800 cm
-1
shows diagnostic IR-band frequency for
a 1-propenyl group at 1670 cm
-1
(black circle).
a)
b)
33
Hence the macroscopic film easily was dissolved in common organic solvents.
Therefore, another commonly used hole transporting material, N,N'-di-1-naphthyl-
N,N'-diphenyl-1,1'-biphenyl-4,4'diamine (NPD) was derivatized with vinyl groups
(compound 15) and introduced into the carbazole/IOC 10 mixture. The film
thickness of this mixture was determined to be 430 Å from a toluene solution with
the spin caster parameters set to 40 seconds and 4000 rpm. The concentration
ratios of DV-NPD: carbazole : IOC 10 where 10 mg : 3 mg : 1 mg respectively in 2
ml of toluene. In addition to the IR-band of the 1-propenyl group, the vinyl out of
plane C – H deformation frequency at 900 cm
-1
may be used to monitor the
polymerization.(Kotorlenko and Gardenina 1971).
34
Figure 10a) FTIR spectrum of 9-(1-propenyl)carbazole, DV-NPD and IOC 10.
b) Enlarged region from 1800 - 800 cm
-1
shows diagnostic IR-band frequency
for a 1-propenyl group at 1670 cm
-1
and out of plane deformation frequency at
900 cm
-1
(blue and black circles respectively).
a)
b)
35
In the enlarged few of the FITR spectrum (Figure 10b) the IR-band of 1-
propenyl group clearly has disappeared indicating a complete polymerization of the
carbazole moiety. On the other hand the diagnostic out of plane deformation
frequency of the vinyl C – H bond of the DV-NPD compound only appears to have
decrease, indicating an incomplete conversion. This observation is reflected in the
ease of solution of this film in common organic solvents.
As plausible reason for the solubility of the thin film, the following
hypothesis was established and depicted in Figure 11 and Figure 12.
36
N
N N
O
C
10
H
21
I
SbF
6
Figure 11 Cartoon of heterogeneous film of DV-NPD (green film), 9-(1-
propenyl)carbazole (orange bars) and IOC 10 (blue circles) on a substrate
(gray) before UV irradiation.
As can be seen in Figure 11, all reactants are statistically distributed within
the thin film after evaporation of the solvent. Upon near UV irradiation, only those
areas of the thin film will undergo a photoinitiated cationic radical polymerization
37
where photosensitizer, photoacid and DV-NPD are in immediate contact yielding a
random copolymer / oligomeric species islands (black squares in Figure 12).
N
N N
O
C
10
H
21
I
SbF
6
Figure 12 Cartoon of heterogeneous film of DV-NPD (green film), 9-(1-
propenyl)carbazole (orange bars) and IOC 10 (blue circles) on a substrate
(gray) after UV irradiation. The black squares represent a random copolymer
/ oligomeric species of DV-NPD and 9-(1-propenyl)carbazole.
Random copolymer /
Oligomeric species
38
This incomplete copolymerization is partially due to the lack of solvent
within the thin film. The Brownian motion and therefore the propagation of the
reactive species is greatly impeded, increasing the probability of polymerizations in
small domains, leading to termination of the reactive oligomeric species. Exposing
this near UV irradiated thin film to organic solvents will easily penetrate this
partially polymerized film rendering it soluble. Thus our research efforts have
shifted away from photoinitiated cross-linking polymerizations of monomeric
species towards polymeric system on which thermally cross-linkable groups are
attached. The elevated temperature and the inherent proximity of the cross-linkable
pendent groups on the polymer back bone, dramatically enhance the statistical
probability of the thermally initiated cross-linking reaction rendering an insoluble
thin film. Therefore a lower cross-linking density, that is the number of cross-
linkable units per main chain, is needed to afford an insoluble film.(Stevens 1999)
Introduction Thermally Cross-linkable Polymers
The benzocyclobutene (BCB) cross-linking unit can be thermally cross-
linked as shown in Scheme 7. The cross-linking occurs between 180 °C and 250 °C
through ring opening of the four-member butene ring and irreversible cycloaddition
to a cyclooctadiene ring.(Cava and Deana 1959)
39
!
200
o
C
Scheme 7 Thermal cross-linking of the reactive benzocyclobutene to the
dibenzocyclooctadiene.
Advantages of this robust cyclooctadiene cross-linking unit include stability
to air, moisture, light, and essentially no reactivity post cross-linking. Furthermore,
the low reactivity of the benzocyclobutene precursor below 150 °C allows the
performance of a wide range of chemical reactions in its presence.(Eric
Drockenmuller 2005; Kim, Pyun, Frechet, Hawker and Frank 2005; Leiston-
Belanger, Russell, Drockenmuller and Hawker 2005; Pyun, Tang, Kowalewski,
Frechet and Hawker 2005)
40
Experimental
Synthesis and Characterization
Synthesis of monomers, polymers, their characterization and polymer thin
film studies where carried out by Dr. Frank Lauterwasser and Dr. Marco Rolandi in
the lab of Prof. Dr. Jean M. Fréchet at the Department of Chemistry, University of
California, Berkeley, California 94720-1460. All reactions were carrier out under
Argon unless otherwise noted. Solvents were dried as follows: Methylene chloride,
THF, toluene, pyridine, DMF, and triethyamine were purchased from Fisher and
vigorously purged with nitrogen for 1h. The solvents were further purified by
passing them under nitrogen pressure through two packed columns (Glass Contour)
of neutral alumina (for THF and methylene chloride), neutral alumina and
copper(II) oxide (for toluene), or activated molecular sieves (for DMF).
Chromatography was carried out with Merck silica gel for flash columns (230-400
mesh). Extracts were dried over MgSO
4
and solvents were removed with a rotary
evaporator at aspirator pressure. NMR spectra were recorded on Bruker AM 400
instruments with TMS or solvent carbon signal as standards. Analytical size
exclusion chromatography (SEC) in tetrahydrofuran (THF) was performed at 35 °C
at a nominal flow rate of 1.0 mL/min calibrated with linear poly(styrene) standards
(162-2,100,000 Da) and fitted with three columns having pore sizes of 10
5
, 10
3
, and
500 Å, respectively. The SEC system consists of a Waters 510 pump, a Waters 717
auto sampler, a Waters 486 UV-Vis detector, a Wyatt DAWN-EOS multi-angle
41
laser light scattering detector, and a Wyatt Optilab differential refractive index
detector. Light scattering data were analyzed using Astra software from Wyatt, and
SEC data using the UV-Vis and differential refractive index detectors were
analyzed using Millennium software from Waters. In addition, the cross-linker unit
allows monitoring of the polymerization ratio and conversion by
1
H-NMR
spectroscopy. As hole-transporter were used 3-vinyl-triphenylamine a derivative of
TPA and 4-[N-(4-vinylphenyl)-N-(4-methylphenyl)amino]-4´-[N-phenyl-N-(4-
methylphenyl)-amino]-biphenyl a derivate of TPD. TPA and TPD have been
widely used and studied.(Van der Auweraer, De Schryver, Borsenberger and
Fitzgerald 1993)
The TPA derivate, 3-vinyl-triphenylamine (16) was prepared on a
multi-gram scale in a one step reaction from diphenylamine and 3-bromostyrene
using previously reported procedures.(Furuta, Deng, Garon, Thompson and Frechet
2004; Deng, Furuta, Garon, Li, Kavulak, Thompson and Frechet 2006)
42
N
+
O N
Ar, 123
o
C
20h
N
N O
m
n
N
+
O N
Ar, 123
o
C
20h
N O
m
n
16 17 18
N
N
N
23
17
19
Scheme 8 Polymerization of the cross-linkable monomer 4-vinyl-
benzocyclobutene 17 and the hole transporters 16 and 23 with nitroxide
initiator.
The TPD derivative was easily prepared on a multi-gram scale in a three
step synthesis (Scheme 8) starting from N,N´-diphenylbenzidine (20). In the first
step, the Hartwig-Buchwald amination conditions(Hartwig, Kawatsura, Hauck,
43
Shaughnessy and Alcazar-Roman 1999) were used to obtain the 4,4'-bis[N-phenyl-
N-(4-methylphenyl)amino]-biphenyl (21) intermediate with 70% yield, after 3 h at
rt Scheme 9.
HN
HN
20
Pd(dpa)
2
,
P(t-Bu)
3
,
NaOtBu,
p-Iodtoluene
rt, 3 h, 70%
DMF, POCl
3
1,2-Dichloroehane
N
N
60
o
C, 5h, 54%
N
N
H
O
N
N
n-BuLi, Methyl-
triphenylphosphonium
bromide, THF
rt, over night, 75%
21 22 23
Scheme 9 Monomer synthesis of MV-TPD derivative 23.
The Vilsmeier reaction gave the aldehyde 22 in 54% yield, while it was
possible to isolate 34% of the unreacted substrate and 7% of the substituted di-
aldehyde.(Zhang, Ya-Dong, Hreha, Richard D., Jabbour, Ghassan E., Kippelen,
Bernard, Peyghambarian, N. and Marder, Seth R. 2002) Transformation from the
aldehyde to the olefin was carried out under standard Wittig reaction conditions to
yield 78% of the desired vinyl monomer 23. The 4-vinyl-benzocyclobutene (17)
monomer was synthesized by previously reported methods.(Lloyd and Ongley
44
1965; Wieland and McCarty 1972; Sanders and Giering 1973; Harth, Horn, Lee,
Germack, Gonzales, Miller and Hawker 2002)
4-[N-(4-Vinylphenyl)-N-(4-methylphenyl)amino]-4´-[N-phenyl-N-(4-
methylphenyl)amino]biphenyl (23). (9.36 mmol)
Methyltriphenylphosphoniumbromide was dissolved in THF under an inert
atmosphere. The solution was cooled to – 78 °C. 3.44 ml of a 2.5 M solution of n-
BuLi in hexanes was added slowly. The mixture was allowed to warm to rt and
was stirred for 1 h before cooling to – 78 °C. (7.77 mmol) of 4-[N-(4-
Formylphenyl)-N-(4-methylphenyl)amino]-4´-[N-phenyl-N-(4-
methylphenyl)amino]biphenyl (22) was dissolved in THF and added slowly to the
mixture. The reaction mixture was allowed to warm to rt and stirred for 11 h at
ambient temperature. Water was added to the reaction mixture and the phases were
separated. The organic phase was washed with water and brine, dried over MgSO
4
and the solvent was evaporated in vacuo. Column chromatography with a
hexanes/ethyl acetate (9:1) solvent mixture yielded 78% of the product as a solid.
1
H-NMR (400 MHz, CDCl
3
): δ = 7.45 (dd, 4H, J = 8.5, 1.9 Hz), 7.34 – 7.22 (m,
5H), 7.15 – 7.04 (m, 16H), 7.01 (t, 1H, J = 7.4 Hz), 6.68 (dd, 1H, J = 17.6, 10.9
Hz), 5.60 (d, 1H, J = 17.6 Hz), 5.17 (d, 1H, J = 10.9 Hz), 2.35, (s, 6H).
13
C-NMR
(125 MHz, CDCl
3
): δ = 147.86, 147.50, 146.88, 146.53, 145.09, 144.85, 136.23,
134.66, 134.27, 133.09, 132.87, 131.54, 129.99, 129.95, 129.15, 127.19, 127.00,
125.12, 125.02, 123.83, 123.75, 123.50, 123.17, 122.36, 111.94, 20.85. MS (EI)
45
m/z: 543 (M
+
, 6), 542 (13), 495 (10), 494 (21), 216 (17), 156 (6), 137 (39), 121
(100), 109 (8), 91 (10). HRMS (C
40
H
34
N
2
): calc. 542.2722; found 542.2726.
General Polymerization
Monomer 16, 17, 23, initiator, and co-solvent were added to a test-tube with
magnetic stir bar. The mixture was subjected to 3 cycles of freeze-pump-thaw
under Argon and sealed. The reaction mixture was heated to 123 °C for 15 h. The
polymer was dissolved in CH
2
Cl
2
and precipitated into hexanes. To test the
reactivity of the monomer 23 under living radical polymerization conditions, the
polymerization was investigated using a nitroxide initiator developed by Hawker et
al.(Hawker, Bosman and Harth 2001) with tert-butylbenzene as a co-solvent at
123 °C under Argon for 15 h. The homopolymerization of monomer 23 yielded a
polymer with predictable molecular weight and low polydispersity of 1.12.
Following this, statistical random copolymers with the hole-transporting monomers
16 and 23 and the cross-linkable monomer 17 were prepared as shown in Scheme
8. Using this type of living radical polymerization it is possible to obtain polymers
with predictable molecular weight by simply varying the ratio between the
monomers and the initiator. Therefore it was possible to synthesize a small library
of copolymers with different molecular weight and incorporation ratio between
hole-transporter and cross-linker (Table 2).
46
Entry
monomer
m/n
Ratio m/n
of monomers
Yield
Mw
Styrene
standard
PDI
Ratio m/n
polymer
1 16/17 10/1 90% 8 kD 1.13 10/1
2 16/17 10/2 87% 30 kD 1.35 10/2
3 23/17 10/2 83% 27 kD 1.37 10/2
Table 2 Results of statistical copolymerization between hole transporter and
cross-linker.
The polymerization yield varied between 83-90%, therefore it was necessary to
monitor the ratio between cross-linker and hole-transporter in the final polymer by
1
H-NMR.
Cross-linking of the TPD and TPD Copolymers
To demonstrate that these cross-linkable polymers are insoluble after cross-
linking, initial experiments were carried out on a glass surface (2 cm x 2 cm) with
the polymer entry 1 in Table 2. On each glass carrier, the polymers were spin-
coated using exactly the same parameters thus rendering precisely the same amount
of polymer on each. These films were then thermally cross-linked inside a glove-
box on a hot plate (200 °C). After specific time increments the films were removed
from the hot plate and the inert atmosphere. Each film was treated with a precise
47
amount of THF (2 ml, 30 min.) and the sol fraction was quantified by size
exclusion chromatography (SEC). The results are shown in Figure 13.
Figure 13 SEC traces of the sol fraction of thin films extracted from THF.
The sol fraction for the film cross-linked for one hour already shows a
significant drop in intensity compared to the untreated one (black solid trace in
Figure 13). After 5 h the cross-linking is sufficient, and no significant amount of
polymer can be detected by SEC. Therefore, the cross-linkable polymer becomes
insoluble after thermal treatment lasting 5 h at 200 °C.
To investigate the surface properties of the films before and after the cross-
linking, AFM measurements were carried out with polymer samples on silicon
substrate, entry 1 in Table 2. The polymer was dissolved in CHCl
3
(5mg/ml) and
spin-coated on the surface. Thickness was measured by ellipsometry and ranged
from 35 to 36 nm for all the films. To insure a homogeneous, smooth surface, the
48
measurement was repeated on various locations of the polymer film. The RMS
roughness for the film prior to cross-linking was 1.27 nm. The cross-linking was
thermalyzed on a hot plate at 200 °C, under nitrogen atmosphere, with heating time
varying from 1 to 24 h. After cross-linking, the surface of the films became
increasingly uneven with features as tall as 100 nm with an RMS roughness of 0.9
nm for the sample annealed for 5 hours. The enhanced surface roughness on
thermal treatment could be due to several factors, including the relatively low
molecular weight of the polymer, the thickness of the film, and the ratio of cross-
link units and hole-transporter. The lack of film stability and smoothness led to the
pursuit of the higher molecular weight polymer entry 2 in Table 2 with a molecular
weight of 30 kD and a 10/2 ratio of hole-transporter to cross-linker. In addition, the
cross-linking procedure was varried; first the film was annealed for 2 h at 170 °C
(this is below the efficient temperature for the cross-linking of the BCB unit), and
then cross-linked at 210 °C for the desired time. The spin coating conditions and
concentration of the solution were also modified to obtain thicker films. The film
thicknesses were measured by ellipsometry to be between 95 and 101 nm for six
samples. Here, each measurement showed again a very smooth film before
annealing, with each film having a RMS surface roughness of 0.5nm as measured
by AFM. Next, the sol extraction method was repeated to determine cross-linking
times under the new conditions. Here it was observed that the higher molecular
weight and higher rate of cross-linking allows the reduction of cross-linking time to
one hour and still obtain insoluble films. The ellipsometry data after cross-linking
49
looked very promising, since the film thickness did not change significantly. The
film thickness measurements differ by <2 nm over the entire film after annealing
and cross-linking. To monitor the quality of the surface directly AFM was
performed on a non-cross-linked film and a film annealed for 2 h at 170 °C and
cross-linked for 5 h at 210 °C, giving RMS roughnesses of 0.5nm and 0.3 nm,
respectively. The improved composition and the thermal cross-linking conditions
make it possible to cross-link the new polymers with excellent film properties.
This is confirmed by a measurement of a smaller RMS roughness of 0.3nm. This is
probably an effect of heating the polymer blend above its glass transition
temperature. Investigation of several areas on the substrate as large as 50µm did
not reveal any pinholes or cracks. Long-range continuity is important to avoid
current leakage or short circuit behavior often observed in polymer OLED devices.
A cross sectional analysis was performed on a trench created in the film by
dynamic plowing with the tip of the AFM. A depth of 95nm was measured; this
value is in excellent agreement with the thickness of the same film recorded with
ellipsometry.
50
OLED Fabrication and Testing
OLED Fabrication
Prior to device fabrication, ITO on glass substrates was patterned as 2 mm
wide stripes with resistivity of 20 Ω/cm. The substrates were cleaned by sonication
in soap solution; rinsed with deionized water; boiled in trichloroethylene, acetone,
and ethanol for 5 min each; and dried with nitrogen. Finally, the substrates were
treated with UV-ozone for 10 min. The polymers were dissolved in chloroform at a
concentration of 20 mg/mL. The resulting solutions were filtered (2 µm
poly(vinylidene difluoride) filter) prior to use. The solutions were spin-cast at 3000
rpm for 40 s. The film thickness was determined by ellipsometry. A 500 Å layer
of Alq
3
was deposited by thermal evaporation from resistively heated tantalum
boats onto the polymer-coated substrates at a rate of 2.0 Å/s. The base pressure at
rt was (3-4) x 10
-6
Torr. After organic film deposition, the chamber was vented and
a shadow mask with a 2 mm wide stripe was placed onto the substrates
perpendicular to the ITO stripes. A cathode consisting of 10 Å LiF followed by
1100 Å of aluminum was deposited at a rate of 0.2 Å/s for LiF and 4-5 Å/s for
aluminum. OLEDs were formed at the 2 x 2 mm squares where the ITO (anode)
and Al (cathode) stripes intersect.
51
OLED Testing
The devices were tested in air within 2 h of fabrication. The electrical and
optical intensity characteristics of the devices were measured with a Keithly 2400
source meter, 2000 multimeter coupled to a Newport 1835-C optical meter,
equipped with a UV-818 Si photodetector. Only light emitting from the front face
of the device was collected and used in subsequent efficiency calculations. The
electroluminescence (EL) spectra were measured on a PTI QuantaMaster model C-
60SE spectrofluorimeter, equipped with a 928 PMT detector and corrected for
detector response.(J. Cui 2002; Kim, Chung, Hong, Chung, Kim, Lee and Jang
2002; Wang, Liu, Huang, Xu, Gong, Chen, Yi, Xu, Yu, Wan, Bai and Zhu 2004;
Li, Jones, Allen, Heikenfeld and Steckl 2006) The emission was found to be
uniform throughout the area of each device.
Results and Discussion
Device testing was based on a comparison of non-cross-linked, thermally
cross-linked, and the corresponding homopolymers. From these specific
experiments it can be deduced if the cyclobutene unit or the thermal cross-linking
step influenced device performance. The following device construction, was used;
the benzocyclobutene cross-linkable polymers (50 nm) were spin-coated on
ITO/glass, followed by thermal annealing, next tris(8-hydroxyquinolate)aluminum
(Alq
3
) layer (50 nm) was deposited on the top of the polymer layer under high
52
vacuum conditions. The cathode, consisting of lithium fluoride (1 nm) and
aluminum (110 nm) was deposited on top of the Alq
3
functioning both as an
electron-transporting and emitting material.(Tang, VanSlyke and Chen 1989; Shi
and Tang 1996; Schmitz, Poesch, Thelakkat and Schmidt 1999) This device
structure was chosen because it is straight forward to fabricate and compare with
closely related devices from literature (Figure 14). The key question to be
addressed is how well the cross-linked polymer hole transporting layer performs
relative to small molecule vacuum deposited analogs.(Shirota, Kuwabara, Inada,
Wakimoto, Nakada, Yonemoto, Kawami and Imai 1994; Baldo, O'Brien, You,
Shoustikov, Sibley, Thompson and Forrest 1998; Feast, Peace, Sage and Wood
1999)
In the initial tests, the TPA homopolymer, TPA-based non-cross-linked
polymer, and TPA based cross-linked polymer where compaired. As shown in
Figure 14a, the best result is observed for the TPA homopolymer. The quantum
efficiency reaches 0.74%, while the peak efficiency of the cross-linked polymer
device is only 0.24%.
53
Figure 14 a) Quantum efficiency versus current density for TPA
homopolymer, non-cross-linked, cross-linked polymer. b) Quantum efficiency
versus current density for TPD homopolymer, non-cross-linked, cross-linked
polymer and small molecule vacuum deposited TPD.
Due to the poor performance of this TPA copolymer, interest was turned
towards the copolymer based on the TPD structure. As shown in Figure 14b, there
are only small differences in the quantum efficiency between the homopolymer,
a)
b)
54
non-cross-linked and cross-linked polymer. The homopolymer and the non-cross-
linked polymer devices give peak efficiencies of 0.80% and the cross-linked
polymer reaches an E.Q.E. of 0.65%, encouraging further device testing with a
multilayer structure in the future using this insoluble cross-linked hole-injection
layer. The E.Q.E. values for cross-linked polymers discussed here, are in close
agreement to those found in literature for vacuum deposited small molecule based
OLEDs with Alq
3
emitting layers (up to 1%).(J. Cui 2002; Kim, Chung, Hong,
Chung, Kim, Lee and Jang 2002; Wang, Liu, Huang, Xu, Gong, Chen, Yi, Xu, Yu,
Wan, Bai and Zhu 2004; Li, Jones, Allen, Heikenfeld and Steckl 2006) The
comparison of the polymeric based devices with the analogous vacuum deposited
structure (i.e. ITO/TPD (500Å/Alq
3
(500Å)/LiF/Al) reveal only minor differences
in device performance as is depicted in Figure 14b and Figure 15b.
55
Figure 15 a)Brightness versus voltage for homopolymer, non-cross-linked and
cross-linked polymer based on TPA. b) Brightness versus voltage for TPD
homopolymer, non-cross-linked and cross-linked polymer and small molecule
vacuum deposited TPD.
While then unannealed copolymer has a similar brightness at 6V to that of
the TPD homopolymer (400 Cd/m
2
), cross-linking drops the brightness to 0.3
a)
b)
56
Cd/m
2
at 6V. Also, the turn on voltage at 0.1 Cd/m
2
is 5.5 V for the cross-linked
polymer, which is approximately 3 V higher than that for the homopolymer, non-
cross-linked polymer and vacuum deposited TPD device as depicted in Figure 15b.
This shift in turn on voltage may be linked to domain formation within the polymer
during the annealing process, leading to isolated regions of TPD within the film,
which are expected to trap holes, increasing the voltage required to achieve a given
current density. Further investigation has to be undertaken with high molecular
weight polymers in which the ratio of cross-linker and TPD is varied, to determine
if the voltage increase can be eliminated.
Analysis of the electroluminescence (EL) spectra for both the TPA and TPD
polymer devices give a voltage independent EL spectrum (4 – 10V), with a λ
max
of
515 nm, consistent with solely Alq
3
emission. This indicates that recombination
and emission occur exclusively within the Alq
3
layer and no emission is observed
from the polymeric hole-transporting material.
Conclusion
In the first part of this chapter, the photoinitiated cationic radical
polymerization of cross-linkable monomeric compounds was investigate in thin
films. Since these monomeric photoinitiated films did not render an insoluble hole-
injection layer after UV-irradiation, thermal cross-linkable hole-injector polymers
with predictable molecular weight and narrow polydispersity where synthesized.
57
The reactive BCB units can be cross-linked to form smooth insoluble films by
heating to 210 °C for 5 h (after and initial 2 h annealing process at 170 °C). Device
performance with cross-linked TPD polymer 23 had similar line shape to the homo-
and non-cross-linked-polymer; only a small drop of E.Q.E and brightness was
observed upon cross-linking. Comparison of the polymers to small molecule
vacuum deposited TPD showed minor deviation of the device performance,
suggesting that the polymeric system reported here will make a suitable
replacement for vacuum deposited hole-transporting layer, such as TPD. These
results are very promising in that a cross-linkable polymer will allow successful
preparation of spin-cast multilayer devices.
58
Chapter 3 Near-Infrared Electrophosphorescence from Pt-
Metalloporphyrin Complexes
Abstract
Highly efficient organic light emitting diodes (OLEDs) require
phosphorescent emitters with high luminescent quantum efficiency. Two classes of
phosphorescent complexes have been employed as dopants in near infrared (nIR)
emitting OLEDs. The first utilizes trivalent lanthanide cations (Ln
3+
) as the
emitting centers, (e.g. Er
3+
or Nd
3+
) chelated with luminescent ligands to sensitize
excitation energy transfer to the lanthanide ion. Secondly, phosphorescent Ir(III)
complexes with extended p-systems have been prepared that give peak wavelengths
of 910–920 nm, but the limitation of these types of systems is that the
photoluminescent (PL) efficiency is very low, in the order of a few percent to a
fraction of a percent. A family of metal complexes that have shown intense
absorption and emission in the red-to-nIR region of the spectrum are
metalloporphyrins. In this thesis we study the photophysical and
electroluminescence properties of two Pt-metalloporphyrins Pt(II)-
tetraphenyltetrabenzoporphyrin (Pt(TPBP) and Pt(II)-
tetraphenyltetranaphthoporphyrin (Pt(TPNP).
59
Introduction
Organic light emitting diodes (OLEDs) have been subject to an enormous
research effort for the past two decades with a focus on devices that emit almost
exclusively in the visible part of the electromagnetic spectrum.(Baldo, O'Brien,
You, Shoustikov, Sibley, Thompson and Forrest 1998; Heeger 2001) Recently,
there has been a growing interest in fabricating OLEDs that emit in the nIR region
(>700 nm).(Harrison, Foley, Bouguettaya, Boncella, Reynolds, Schanze, Shim,
Holloway, Padmanaban and Ramakrishnan 2001; Slooff, Polman, Cacialli, Friend,
Hebbink, van Veggel and Reinhoudt 2001; O'Riordan, O'Connor, Moynihan,
Nockemann, Fias, Van Deun, Cupertino, Mackie and Redmond 2006)
Applications for these nIR devices include areas such as laser technology(Xu,
Zhao, Majumdar, Jayasinghe and Shi 2003), sensors(Williams, Evan L., Li, Jian
and Jabbour, Ghassan E. 2006) and night-vision-readable displays(Schanze,
Reynolds, Boncella, Harrison, Foley, Bouguettaya and Kang 2003). However,
there have been very few reports of efficient near-IR (nIR) emitting OLEDs. The
efficiency of OLEDs are markedly improved when fluorescent emissive dopants
are replaced with phosphorescent heavy metal complexes that can effectively
harvest both the singlet and triplet excitons formed in the electroluminescent
process.(E. Holder 2005) The use of heavy metal phosphors has led to high EL
efficiencies for devices with energies ranging from the near-UV to red (λ
max
380–
650 nm).(Holmes, Forrest, Sajoto, Tamayo, Djurovich, Thompson, Brooks, Tung,
D'Andrade, Weaver, Kwong and Brown 2005)
60
Highly efficient OLEDs require phosphorescent emitters with high
luminescent quantum efficiency. Two classes of phosphorescent complexes have
been employed as dopants in nIR emitting OLEDs. The first utilizes trivalent
lanthanide cations (Ln
3+
) as the emitting centers, e.g. Er
3+
or Nd
3+
(Wolbers, van
Veggel, Snellink-Ruël, Hofstraat, Geurts and Reinhoud 1998; Koppe, Brabec,
Sariciftci, Eichen, Nakhmanovich, Ehrenfreund, Epstein and Heiss 2001;
O'Riordan, O'Connor, Moynihan, Nockemann, Fias, Van Deun, Cupertino, Mackie
and Redmond 2006; Yang, Gong, Nie, Lou, Bian, Guan, Huang, Lee and Baik
2006) chelated with luminescent ligands to sensitize excitation energy transfer to
the lanthanide ion.(Oh, Kim, Nah and Kim 2005) The organic ligands make the
lanthanide complexes stable in organic media, sensitize energy transfer to the metal
center and prevent aggregation effects that can lead to luminescent self-quenching
processes.(Oh, Paik, Ka, Roh, Nah and Kim 2004) The emission spectrum from
the 4-f transitions of the lanthanide ion is relatively unperturbed by the organic
ligands and has a narrow line width.(O'Riordan, O'Connor, Moynihan,
Nockenmann, Fias, Deun, Cupertino, Mackie and Redmond 2006) However,
lanthanide complexes have intrinsically long emission decay lifetimes caused by
the inherently small radiative rates of the 4-f transitions. These long lifetimes lead
to deleterious effects on the OLED efficiency, especially at high current densities,
caused by saturation phenomena.(Baldo, Adachi and Forrest 2000) Schanze et. al.
has used Ln
3+
in conjunction with a porphyrin/polystyrene matrix in a nIR light
emitting diode (LED). The external quantum efficiency (EQE) was reported in a
61
range of 8.0x10
-4
to 2.0x10
-4
% at about 1 mA/cm
2
depending on dopant
concentration.(Schanze, Reynolds, Boncella, Harrison, Foley, Bouguettaya and
Kang 2003) A related device with a Nd(phenalenone)
3
emitter also gave a low
EQE (λ
max
= 1065 nm, EQE = 0.007%).(O'Riordan, O'Connor, Moynihan,
Nockemann, Fias, Van Deun, Cupertino, Mackie and Redmond 2006) Ir
complexes have also been reported which give emission in the nIR region,
however, even though Ir complexes have performed outstandingly well as dopants
in OLEDs emitting in the visible part of the spectrum, they have not performed
well in nIR OLEDs. Shifting emission energies into the nIR can lead to strong
excited state coupling into non-radiative decay modes such as C-H vibrations,
dramatically reducing luminescent efficiencies of these types materials.(Caspar and
Meyer 1983) For example, phosphorescent Ir(III) complexes with extended p-
systems have been prepared that give peak wavelengths of 910–920 nm, but the
photoluminescent (Φ
PL
) efficiency is very low (Φ
PL
= 0.02).(Chen, Yang, Chi,
Chemg, Yeh, Chou, Hsieh, Liu, Peng and Lee 2006) Currently, the only nIR
electrophosphorescent device that used a transition metal dopant, are a
cyclometallated (pyrenyl-quinolyl)
2
Ir(acetonylacetonate) complex (λ
max
= 720 nm),
which has an EQE of 0.1% (Williams, E. L., Li, J. and Jabbour, G. E. 2006) and a
terdentate cyclometallated phosphorescent Pt(II) complexes (PtLnCl) which exhibit
exclusive NIR excimer emission peaking at wavelengths between 705 and 720 nm
for different ligands with an EQE of around 10%.(Cocchi, Virgili, Fattori, Williams
and Kalinowski 2007)
62
Introduction to Metalloporphyrins
It was recognized early that the intensity and color of porphyrins are
derived from the highly conjugated π-electron system. The absorption spectrum of
porphyrins has long been understood in terms of the highly successful “four-
orbital” (two highest occupied π orbitals and two lowest unoccupied π* orbitals)
model first applied by Gouterman in the late 50’s.(Gouterman 1959) The
electronic spectra of these porphyrin compounds are characterized by three basic
regions. The so-called Q-band is relatively weak (in the case of a non-metallated
porphyrin) and occurs in the visible region of the electromagnetic spectrum. The
intense Soret- or B-band occurs in the near-UV and is often accompanied by a
closely related N-band of lower intensity. The higher UV bands in the third region
are broad and are often near uniform intensity.(Hashimoto, Choe, Nakano and
Hirao 1999)
63
Figure 16 Typical absorption spectrum of a metallated porphyrin. Right:
Calculated energy levels of LUMO+1, LUMO, HOMO, HOMO-1 for
Pt(TPBP).
Figure 16 shows absorption spectrum of Pt(TPBP) (left) and calculated energies of
the highest occupied molecular orbital -1 (HOMO-1), the highest occupied
molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) and
the lowest unoccupied molecular orbital +1 (LUMO+1). The Intense Soret-Band is
attributed to the electronic transition from the HOMO-1 to the degenerate LUMO
and LUMO+1 orbitals. The less intense Q-Band is attributed to the electronic
transition from HOMO to the degenerate LUMO and LUMO+1
orbitals.(Hashimoto, Choe, Nakano and Hirao 1999) (Calculation where done
using the B3LYP / LACVP** level of theory).
A family of metal complexes that have shown intense absorption and
emission in the red-to-nIR region of the spectrum are metalloporphyrins. The large
64
number of known porphyrins and metalloporphyrins make these an ideal class of
materials to explore for a wide range of potential applications, such as non-linear
optics, gas sensors, photodynamic therapy and near infrared dyes.(Ongayi,
Gottumukkala, Fronczek and Vicente 2005) Porphyrin chromophores with fused
aromatic moieties at the β-pyrrole positions, e.g. tetrabenzoporphyrin (TBP),
exhibit a bathochromic shift of the absorption and emission energy due to the
expansion of the π-electronic system of the porphyrin core as compared to simple
pyrrolic analogs. Addition of bulky groups to the meso-positions of the porphyrin
macrocycles with b-substituted pyrroles leads to the formation of nonplanar
porphyrins and further red-shifts the absorption maxima of both the low and high
energy transitions.(Finikova, Aleshchenkov, Brinas, Cheprakov, Carroll and
Vinogradov 2005) The combined effects of these modifications can lower the
energy of the porphyrin Q-band almost 90 nm for highly distorted derivatives.(Xu,
Shen, Okujima, Ono and You 2006) Also this distortion prevents π-stacking in
solids, dramatically increasing the solubility in common organic solvents, making
these substituted porphyrins attractive for solution based device fabrication
methods. Addition of a heavy metal atom at the center will increase the rate of the
intersystem crossing between singlet and triplet states via spin-orbit coupling,
thereby enhancing the rate of radiative decay from the triplet state of the
metalloporphyrins. Finally, the rigidity of the porphyrin pyrrolic ring system
decreases energy transfer into non-radiative vibrational decay modes and
consequently improves the luminescent efficiency. This behavior is also expected
65
to be true for porphyrin derivatives having extended aromatic systems that shift the
emission energy deeper into the infrared. For the reasons mentioned above, the
preliminary focus was thus aimed towards the use of Pt(II)-
tetraphenyltetrabenzoporphyrin, Pt(TPBP) (Figure 17) as a phosphorescent dopant.
N
N
N
N Pt
Figure 17 Pt(II)-tetraphenyltetrabenzoporphyrin.
In order to extend the emission energy of these types of metalloporphyrins
deeper into the near infrared region of the electromagnetic spectrum, two different
approaches where taken. Firstly, changing the central metal atom of the
tetraphenyltetrabenzoporphyrin system (Pt, Pd, Ni, Zn) and secondly through the
extension of the annulated aromatic system. For these reasons, Pt(II)-
tetraphenyltetranaphthoporphyrin Pt(TPNP) (Figure 18) was also investigated.
66
N
N
N
N Pt
Figure 18 Pt(II) tetraphenyltetranaphthoporphyrin.
Experimental
Unless otherwise noted, all reagents and solvents where obtained from
Sigma-Aldrich and used without any further purification. Extracts where dried
over Na
2
SO
4
and solvents were removed with a rotary evaporator at aspirator
pressure. NMR spectra where recorded on Bruker AM 250 instrument with TMS as
standard signal in either deuterated dichloromethane or deuterated DMSO.
67
Synthesis and Characterization
The photophysical properties of Pt(benzoporphyrin) Pt(BP), were described
by Vogler(Vogler, Kunkely and Rethwisch 1980)
and Aartsma.(Aartsma,
Gouterman, Jochum, Kwiram, Pepich and Williams 1982) The Pt(BP) complex
was prepared by insertion of Pt(II) into the pre-formed BP-H
2
ring that was
obtained from demetallation of Zn(BP). An analogous synthetic procedure was
used to prepare Pt(TPBP),(Chen, Tomov, Dvornikov, Nakashima, Roach, Alabran
and Rentzepis 1996) however, this method is not amenable to large scale synthesis
of the complex needed for this OLED studies. Thus a different route was chosen to
prepare Pt(TPBP) utilizing a synthetic methodology developed by Baumann and
Waldner.(Baumann and Waldner 2001)
Synthesis of Pt(II) tetraphenyltetrabenzoporphyrin
Starting with 4,5,6,7-tetrahydroisoindole (3), which was prepared from a
Barton-Zard type condensation of nitrocyclohexene (1) and ethyl isocyano acetate
(2) followed by a decarboxylation step,(Donohoe, Raoof, Linney and Helliwell
2001) was coupled with benzaldehyde under Lindsey conditions to form the
corresponding porphyrinogen. Oxidation with DDQ (2,3-Dichloro-5,6-dicyano-p-
benzoquinone) followed by metallation with PtCl
2
in refluxing benzonitrile, gave
the cyclohexyl porphyrin derivative (6), which upon a second oxidation with DDQ
68
formed the final product (7).(Baumann and Waldner 2001) The complex is
thermally robust, giving analytically pure samples upon sublimation at 470°C.
The synthetic scheme used to prepare Pt(TPBP) (7) is depicted in Scheme 10.
NO
2
+
CN
OEt
O
1 2
NH
O
OEt
3
Base
Base
NH
4
i) BF
3
OEt
ii) DDQ
N
HN
N
NH
Ph Ph
Ph Ph
5
N
N
N
N
Ph Ph
Ph Ph
6
PtCl
2
Pt
N
N
N
N
Ph Ph
Ph Ph
Pt
DDQ
7
PhCHO
Scheme 10 Synthesis of Pt(II)-tetraphenyltetrabenzoporphyrin.
69
The synthesis of the free base porphyrin and platination were carried out by
procedures similar to those reported previously.(Sato, Matsumoto, Takishima and
Mochizuki 1997; Baumann and Waldner 2001; Finikova, Cheprakov, Beletskaya
and Vinogradov 2001; Ito, Uno, Murashima and Ono 2001; Finikova, Cheprakov,
Beletskaya, Carroll and Vinogradov 2004)
2H-isoindole-4,5,6,7-tetrahydro-1-carboxylic acid ethyl ester (3). 1-
Nitrocyclohexene (1) (39.2 mmol) and ethyl isocyanoacetate (2) (39.2 mmol)
where dissolved in dry THF (100 ml) in a round bottom flask equipped with
condenser and purged with N
2
. DBU (39.2 mmol) was slowly added to this solution
and the reaction was refluxed over night. The solvent was removed by rotary
evaporation and the crude product purified on a silica gel column (eluent: ether) to
afford pale yellow crystals in a 90% yield.
1
H-NMR (250 MHz, CDCl
3
): δ = 4.34-
4.25 (q, 2H), 2.81 (t, 2H, J = 5.8 Hz), 2.54 (t, 2H, J = 5.8 Hz), 1.81-1.67 (m, 4H),
1.34 (t, 3H, J = 5.6 Hz).
4,5,6,7-Tetrahydroisoindole (4). Isoindole (3) (15.5 mmol), KOH (223
mmol) and ethylene glycol (120 ml) where combined and the resulting
heterogeneous mixtures heated until the homogenous solution turned black and
cooled rapidly using an ice bath. CH
2
Cl
2
was added, the organic phase separated
and washed several times with water. The aqueous phases where collected and
back washed with CH
2
Cl
2
. The combined organic phase where washed with brine
and dried over Na
2
SO
4
. The solvent was removed by rotary evaporation to afford a
70
black solid in a 95% yield.
1
H-NMR (250 MHz, CDCl
3
): δ = 6.08 (s, 2H), 2.81 (t,
2H, J = 5.8 Hz), 2.54 (t, 2H, J = 5.8 Hz), 1.81-1.67 (m, 4H).
Porphyrin (5) via the Lindsey Method. To a round bottom reaction flask
with condenser was added 2.3 L of CH
2
Cl
2
and degassed for 1 h with N
2
. Next, the
entire apparatus was shielded from light before the addition of the isoindole (4)
(28.8 mmol) and benzaldehyde (16.6 mmol) in one portion. The mixture was
stirred in the dark for 20 min at room temperature. BF
3
·Et
2
O (3.3 mmol) was added
in one portion, and the mixture was stirred at room temperature for an additional 4
h. DDQ (2.8 mmol) was added, and the mixture was left overnight under
continuous stirring. The resulting dark green solution was washed with 10% aq.
Na
2
SO
3
, 5% aq. HCl, 10% aq. Na
2
CO
3
and dried over Na
2
SO
4
. The solvent was
removed by rotary evaporation to a volume of about 25 ml. Next ether was
carefully layered over the CH
2
Cl
2
and let stand over night. The dark green
precipitate was filtered, mother solution concentrated and once again redissolved in
CH
2
Cl
2
and layer with ether as described before yielding the desired compound in
58% yield. UV-Vis, CH
2
Cl
2
, λ
max
nm: Soret-band (430 nm, ε
430 nm
= 2.03 x 10
5
M
-1
cm
-1
) and Q-band (611 nm, ε
611 nm
= 1.35 x 10
5
M
-1
cm
-1
).
1
H-NMR (250 MHz,
CDCl
3
): δ = 8.43-8.34 (m, 8H), 7.91-7.78 (m, 12H), 2.56-2.42 (m, 8H), 2.09-1.92
(m, 8H), 1.72-1.58 (m, 8H), 1.18-1.06 (m, 8H).
Pt-tetraphenyltetracyclohexenoporphyrin (6). In a round bottom flask
with freshly distilled benzonitrile (100 ml) was added PtCl
2
(1.32 mmol) and
porphyrin (5) (0.601 mmol) in one portion and the reaction mixture refluxed over
71
night under N
2
. The solvent was evaporated under vacuum and the crude product
purified on a silica gel column (eluent: CH
2
Cl
2
). The red-brown fraction was
colleted, the solvent evaporated and the product used in subsequent oxidation step
without further purification. Yield of metallation was 85%. UV-Vis, CH
2
Cl
2
, λ
max
nm: Soret Band 411, Q-Bands 524, 560.
1
H-NMR (250 MHz, CDCl
3
): δ = 8.43-
8.34 (m, 8H), 7.91-7.78 (m, 12H), 2.56-2.42 (m, 8H), 2.09-1.92 (m, 8H), 1.72-1.58
(m, 8H), 1.18-1.06 (m, 8H).
Pt-tetraphenyltetrabenzoporphyrin (7). In a round bottom flask was
dissolved Pt-porphyrin (6) (0.214 mmol) and DDQ (2.14 mmol) in toluene (60 ml)
and refluxed for 45 min. After cooling to room temperature, the green organic
phase was washed with 10% aq. Na
2
SO
3
, brine and the solvent evaporated under
vacuum. The crude product was purified on a silica gel column (eluent: CH
2
Cl
2
).
The green fraction was colleted and the solvent evaporated. The residue was taken
up in toluene and precipitated with methanol rendering purple crystals in a yield of
30%. UV-vis, CH
2
Cl
2
, λ
max
nm: Soret Band 430, Q-Bands 563, 612.
1
H-NMR
(250 MHz, CDCl
3
): δ = 8.36-8.18 (m, 8H), 8.05-7.77 (m, 12H), 7.41-6,89 (m,
16H).
Sublimation of Pt-tetraphenyltetrabenzoporphyrin (7). Under high
vacuum in a 1.4 cm sublimation tube the Pt-Porphyrin (7) (0.20 mmol) was heated
to 470°C. Sublimation was continued until no considerable deposition of purple
crystals could be detected near the end of the sublimator (approx. 4h). Yield: 120
mg 60%.
72
Synthesis of M(II) tetraphenyltetrabenzoporphyrin: M=Pd, Ni, Zn
The free-base porphyrin (non-metallated) synthesis, metallalation and
oxidation of the cyclohexyl units was carried out as described for the synthesis of
Pt(TPBP). In the case of Pd, PdCl
2
was used as the metal source. Ni(OAc)
2
·4H
2
O
and Zn(OAc)
2
·2H
2
O where used to introduce Ni and Zn into the porphyrin core
respectively.
Synthesis of Pt(II) tetraphenyltetranaphthoporphyrin
The synthesis of Pt(II)-tetraphenyltetranaphthoporphyrin (Scheme 11) was
prepared as described by Finikova et. al.(Finikova, Aleshchenkov, Brinas,
Cheprakov, Carroll and Vinogradov 2005) A Birch type reduction of naphthalene
with elemental sodium yielding 1,4-dihydronaphthalene (8)(Menzek, Altundas,
Uuml and ltekin 2003) is the first step of the synthesis, which is subsequently
converted into the corresponding α-chlorosulfone (9).(Hopkins and Fuchs 1978)
Next the allylsulfone (10) was prepared under nonucleophilic basic conditions.
The Barton-Zard type condensation of the allylsulfone (10) and ethyl
isocyanoacetate (2) yielded the corresponding pyrrole ester (11) which was
decarboxylated to the pyrrole (12) under strong basic conditions. Porphyrin
formation (carried out under Lindesy conditions) and subsequent oxidation of the
cyclohexeno species yielded the tetraphenyltetranaphthoporphyrin (13).
Metallation with PtCl
2
in refluxing benzonitrile is the last step in the synthesis
73
yielding the final product (14). The compound is thermally robust, giving
analytically pure sample upon sublimation at 525˚C.
CN
OEt
O
2
Base
NH
12
i) BF
3
OEt
ii) DDQ
13
PtCl
2
PhCHO
Na
8
SH
N
O
O
Cl
+
S
Cl
SO
2
Ph
Cl
9
Base
SO
2
Ph
10
NH
O
OEt
11
Base
N
HN
N
NH
Ph Ph
Ph Ph
N
N
N
N
Ph Ph
Ph Ph
14
Pt
Scheme 11 Synthesis of Pt(II)-tetraphenyltetranaphthoporphyrin.
1,4-Dihydronaphthalene (8). To a solution of naphthalene (7.8 mmol) in
dry THF (16 ml) was added metallic sodium (19.6 mmol) in small pieces over a
period of 5 minutes. Next tert-BuOH (19.6 mmol) was added slowly to quench the
74
deep green naphthalene/sodium complex. After all of the complex has been
quenched, the solution was stirred for 3 hours at room temperature. Unreacted
sodium was removed by filtration and washed with THF. The combined solutions
were evaporated, diethyl ether (50 ml) added and washed with water, brine and
dried over Na
2
SO
4
. The solvent was removed under vacuum yielding a yellow oil,
which was distilled to afford a colorless liquid which crystallized into a white solid
(8) in 86 % yield.
1
H-NMR (250 MHz, CDCl
3
): δ = 7.20 (tt, 4H J = 5.0, 2.4 Hz),
6.02-5.98 (m, 2H), 3,48-3.43 (m, 4H).
2-chloro-1,2,3,4-tetrahydro-3-(phenylsulfonyl)-naphthalene (9). N-
chlorosuccinimide (6.12 mmol) was suspended in dry CH
2
Cl
2
(10 ml) under
nitrogen and thiophenyl (6 mmol) was added very slowly (exothermic reaction).
The color changed to a dark orange solution while maintaining reflux conditions
through the addition of the thiophenol. This solution was stirred at room
temperature for 30 minutes. Next dihydronaphthalene (8) (6 mmol) was diluted
with dry CH
2
Cl
2
(3 ml) and slowly added to the above reaction mixture at 0˚C,
which was allowed to warm to room temperature and stirred for an additional 2
hours after the addition. This mixture was placed into a freezer at -48˚C over night
and the precipitated succinimide removed by filtration. The remaining solution
was diluted with cold CH
2
Cl
2
to about 50 ml total volume and cooled to 0˚C. Solid
m-CPBA (3-Chloroperbenzoic acid) (15 mmol) was added gradually and the
mixture stirred for 1 hour at room temperature. An ice cold 10% aq. Na
2
SO
3
solution was added and the mixture transferred into a separator funnel. The organic
75
phase was separated, washed with a 10% aq. Na
2
SO
3
solution and a 10% aq.
Na
2
CO
3
solution, dried over Na
2
SO
4
and the solvent evaporated to dryness. The
resulting white needles where recrystallized from EtOH to yield 20% of the product
(9).
1
H-NMR (250 MHz, CDCl
3
): δ = 8.01-7.92 (m, 2H), 7.74-7.56 (m, 2H), 7.23-
7.08 (m, 5H), 4,84 (q, 1H, J = 4.5), 3.52-3.05 (m, 5H).
3,4-dihydro-3-(phenylsulfonyl)-naphthalene (10). The corresponding α-
chlorosulfone (9) (3.3 mmol) was dissolved in CH
2
Cl
2
and cooled to ice bath
temperature. Diluted DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene) (3.4 mmol) was
slowly added and upon completion stirred for 2 hours. Diethyl ether was added and
the organics washed with 5% aq. HCl solution, dried over Na
2
SO
4
and evaporated
under reduced pressure. The light yellow product was recrystallized from ethanol
to afford colorless crystals in yields up to 61%.
1
H-NMR (250 MHz, CDCl
3
): δ =
7.76-7.67 (m, 2H), 7.50-7.38 (m, 1H), 7.33-724 (m, 3H), 7.09-6.97 (m, 3H), 6.76
(dd, 1H, J = 7.8, 5.0), 6.61-6.56 (m, 1H), 6.06-6.00 (m, 1H), 4.07-3.98 (m, 1H),
3.53-3.42 (m, 1H), 3.31-3.19 (m, 1H).
4,9-dihydro-2H-Benz[f]isoindole-1-carboxylic acid ethyl ester (11). A
solution of allyl sulfone (10) (11.1 mmol) in dry THF (20 ml) was added in one
shot to a stirred suspension of of tert-BuOH (13.3 mmol) and ethyl isocyanoacetate
(2) (14.4 mmol) in dry THF (50 ml) and stirred for 15 minutes at room temperature.
The resulting mixture was refluxed for 1 hour, cooled and the volume reduced to
about 20 ml. The product was precipitated by the addition of EtOH and the lightly
yellow colored product collected by filtration. The filtrate was washed with
76
additional EtOH yielding 65 % of the final isoindole (11).
1
H-NMR (250 MHz,
CDCl
3
): δ = 7.93-7.79 (m, 2H), 7.56-7.46 (m, 2H), 6.88-6.82 (m, 1H), 4.48-4.33 (q,
2H), 4.21-4.14 (m, 2H), 3.93-3.86 (m, 2H), 1.48-1.36 (t, 3H).
4,9-dihydro-2H-Benz[f]isoindole (12). In a 250 ml 1-neck-round
bottom flask were combined isoindole (11) (15.5 mmol), KOH (223mmol) and
ethylene glycol (120 ml). This heterogeneous mixture was heated until the
homogenous solution turned black and then was cooled rapidly using an ice bath.
CH
2
Cl
2
was added and the organic phase washed several times with water. The
aqueous phases where collected and back washed with CH
2
Cl
2
. The combined
organic phase where washed with brine and dried over Na
2
SO
4
. The solvent was
removed by rotary evaporation to afford a black solid in a 95% yield.
1
H-NMR
(250 MHz, CDCl
3
): δ = 7.33-7.25 (m, 2H), 7.22-7.15 (m, 2H), 6.68 (d, 2H, J =
2.5), 4.03-3.98 (m, 4H).
Porphyrin (13) via the Lindsey Method. To a 1L 2-neck-round bottom
reaction flask with condenser was added 0.6 L of CH
2
Cl
2
and degassed for 1 h with
N
2
. Next, the entire apparatus was shielded from light before the addition of the
isoindole (12) (5.9 mmol) and benzaldehyde (26.6 mmol) in one portion. The
mixture was stirred in the dark for 20 min at room temperature. BF
3
·Et
2
O (1.5
mmol) was added in one portion, and the mixture was stirred at room temperature
for an additional 4 h. DDQ (26.6 mmol) was added, and the mixture was left
stirring for 2 hours and refluxed for 1 hour. The resulting purple solution washed
with 10% aq. Na
2
SO
3
, 5% aq. HCl, 10% aq. Na
2
CO
3
and dried over Na
2
SO
4
. The
77
solvent was removed by rotary evaporation and the compound purified by column
chromatography (eluent: dichloromethane) yielding the desired compound in 40%
yield. UV-Vis, CH
2
Cl
2
, λ
max
nm: Soret Band 500, Q-Bands 677, 728, 750.
1
H-
NMR (250 MHz, CDCl
3
): δ = 8.53-841 (m, 12H), 8.13-8.02 (m, 8H), 8.01-7.89 (m,
12H), 7.73-7.38 (b, 12H).
Pt-tetraphenyltetranaphthoporphyrin (14). In a round bottom flask with
freshly distilled benzonitrile (100 ml) was added PtCl
2
(1.32 mmol) and porphyrin
(13) (0.601 mmol) in one portion and the reaction mixture refluxed over night
under N
2
. The solvent was evaporated under vacuum and the crude product
purified on a silica gel column (eluent: CH
2
Cl
2
). The green fraction was colleted,
the solvent evaporated yielding the metallated product up to 65%. UV-Vis,
CH
2
Cl
2
, λ
max
nm: Soret Band 414, 432, Q-Bands 622, 688.
1
H-NMR (250 MHz,
CDCl
3
): δ = 8.53-841 (m, 12H), 8.13-8.02 (m, 8H), 8.01-7.89 (m, 12H), 7.73-7.38
(b, 12H).
Sublimation of Pt-tetraphenyltetranaphthoporphyrin (14). Under high
vacuum in a 1.4 cm sublimation tube the Pt-Porphyrin (14) (0.20 mmol) was heated
to 525°C. Sublimation was continued until no considerable deposition of purple
crystals could be detected near the end of the sublimator (approx. 4h). Yield: 120
mg 40%.
78
X-ray Crystallographic Procedure
X-ray crystals of Pt(II)-tetraphenyltetrabenzoporphyrin Pt(TPBP) (7) where
grown by slow diffusion of diethyl ether into a concentrated solution of Pt(TPBP)
in CH
2
Cl
2
at -40˚C. X-ray diffraction data were collected on a Bruker SMART
APEX CCD diffractometer with graphite-monochromatic Mo K
α
radiation (λ =
0.71073 Å) at 140(2) K. The cell parameters for the Pt(TPBP) were obtained from
the least-squares refinement of the spots (from 60 collected frames) using the
SMART program. A hemisphere of the crystal data was collected up to a
resolution of 0.75 Å, and the intensity data were processed using the Saint Plus
program. All calculations for the structure determination were carried out using the
SHELXTL package (version 6.14).(Sheldrick 1997) Initial atomic position were
located by direct methods using XS, and the structure was refined by the least
square methods using SHELX with 8108 independent reflections within the range
of theta 1.70 – 27.53
o
(completeness 75.4%). Absorption corrections were applied
by SADABS.
[2]
Calculated hydrogen position were input and refined in a riding
manner along with the corresponding carbons. CCDC 627735 contains the
supplementary crystallographic data for this crystal structure. This data can be
obtained free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
79
ORTEP Representation of the X-ray Structure of Pt(TPBP)
Figure 19 ORTEP structure of Pt(II)-tetraphenyltetrabenzoporphyrin.
Summary of Structure Determination of Pt(TPBP)
Empirical formula (with solvent) C60 H36 N4 Pt (C61 H40 Cl2 N4 O Pt)
Formula weight 1008.03 (1110.94)
Temperature 140(2) K
Wavelength 0.71073 Å
Crystal system Monoclinic
Space group P2(1)/n
Unit cell dimensions a = 19.0112(12) Å α= 90°.
b = 10.2045(6) Å β= 99.4500(10)°.
c = 24.3457(15) Å γ = 90°.
Volume 4659.0(5) Å
3
80
Z 4
Density (calculated) 1.581 Mg/m
3
Absorption coefficient 3.176 mm
-1
F(000) 2208
Crystal size 0.27 x 0.13 x 0.05 mm
3
Theta range for data collection 1.70 to 27.53°.
Index ranges -24<=h<=24, -13<=k<=11, -28<=l<=30
Reflections collected 19416
Independent reflections 8108 [R(int) = 0.0434]
Completeness to theta = 27.53° 75.4 %
Transmission factors min/max ratio: 0.692
Refinement method Full-matrix least-squares on F
2
Data / restraints / parameters 8108 / 0 / 622
Goodness-of-fit on F
2
1.010
Final R indices [I>2sigma(I)] R1 = 0.0455, wR2 = 0.1289
R indices (all data) R1 = 0.0609, wR2 = 0.1329
Largest diff. peak and hole 2.526 and -1.257 e.Å
-3
81
Normal Coordinate Structural Decomposition (NSD) Analysis of
Pt(TPBP)
NSD analysis of the Pt(TPBP) was accomplished using the web-based NSD
program (http://jasheln.unm.edu/). The results of the complete NSD decomposition
are given below.
NSD result generated from file Xray Ph4-Pt-TBP.pdb at Thu Sep 07 14:47:00 PDT 2006 Summary of the NSD
(in A):
basis Doop doop B2u B1u A2u Eg(x) Eg(y) A1u
min. 2.8274 0.0224 -2.8265 -0.0060 0.0381 0.0313 0.0497 -0.0003
ext. 2.8305 0.0048 -2.8242 -0.0060 0.0384 0.0303 0.0506 -0.0003
0.1214 -0.0007 0.0206 -0.0259 0.0394 -0.0030
-0.0046 0.0052 -0.0122 -0.0034 -0.0149
-0.0067 -0.0045
0.0038 -0.0152
comp. 2.8306 0.0000 2.8291 0.0079 0.0451 0.0415 0.0677 0.0030
basis Dip dip B2g B1g Eu(x) Eu(y) A1g A2g
min. 0.3078 0.0579 0.0011 -0.0315 0.0103 -0.0255 -0.3044 -0.0180
ext. 0.3620 0.0417 0.0017 -0.0310 0.0110 -0.0257 -0.2958 -0.0176
-0.0227 -0.0140 0.0092 -0.0316 -0.1846 0.0033
0.0011 0.0188 -0.0071 0.0070 0.2001 -0.0013
0.0195 -0.0011 -0.0016 -0.0037 0.0736 0.0174
0.0008 -0.0056 0.0193 0.0033 -0.0270 -0.0049
-0.0113 0.0144 -0.0047 -0.0031 -0.0278
0.0066 0.0044
0.0010 0.0045
0.0013 -0.0027
-0.0138 -0.0167
0.0030 0.0002
comp. 0.4255 0.0000 0.0324 0.0424 0.0297 0.0452 0.4178 0.0258
82
Electrochemistry
Cyclic voltammetry and differential pulse voltammetry were performed
using an EG&G potentiostat/galvanostat model 283. Anhydrous DCE (Aldrich)
was used as the solvent under inert atmosphere, and 0.1 M tetra(n-butyl)ammonium
hexafluorophosphate was used as the supporting electrolyte. A glassy carbon rod
was the working electrode, a platinum wire was used as the counter electrode, and a
silver wire as a pseudo-reference electrode. The redox potentials are based on
values measured from differential pulse voltammetry and are reported relative to a
ferrocenium/ferrocene (Cp2Fe+/Cp2Fe) redox couple which was used as an internal
reference,(Gagne, Koval and Lisensky 1980; Sawyer, Sobkowiak and Roberts
1995) while electrochemical reversibility was determined by cyclic voltammetry.
Density Functional Calculations
DFT calculations were performed using a Titan software package
(Wavefunction, Inc.) at a B3LYP/LACVP** level of theory. The HOMO and
LUMO energies were determined using minimized singlet geometries to
approximate the ground state. The minimized singlet geometries were than used to
calculate the triplet molecular orbitals and approximate the triplet HSOMO
(HSOMO = highest singly occupied molecular orbital).
83
Spectroscopic Measurements
Absorption spectra were recorded with an Agilent 8453 UV-Visible
spectrometer and corrected for solvent absorption in the background. Emission
spectra were recorded on a PTI QuantaMaster Model C-60SE spectrofluorometer
with a 928 PMT detector corrected for detector sensitivity. Lifetimes for Pt(TPBP)
were recorded with an IBH photon timing instrument with an IBH model TBX-04
photon detection module.
Lifetimes for Pt(TPNP) where measured in Prof. Matthias Selke labs at Cal
State LA, Los Angeles, CA, with the help of Dr. Dong Zhang. A nanosecond
Nd:YAG laser (model MiniLaseII/10 Hz; NewWave Research Inc., USA) tripled
(355 nm) in frequency is used as an excitation source. The detector is a cryogenic
germanium photodiode detector (model 403HS; Applied Detector Corp., USA)
cooled by liquid nitrogen and specialized for detection of near-infrared radiation.
Three different filters are used to remove undesired radiation. A Schott color glass
filter (model RG850; cut-on 850 nm; Newport, USA) is taped to the sapphire
entrance of the detector to block any additional ultraviolet and visible light from
entering.
84
Quantum Yield Measurement
The absolute quantum yield efficiencies were measured using a calibrated
Hamamatsu integrating sphere equipped with a xenon lamp (with an excitation
wavelength of 430 nm) and a photonic multi-channel analyzer C10027. The
accuracy of the quantum efficiency measurements is ± 5-10% error of
measurements. The quantum efficiencies data have been processed with PLQY
measurement software U6039-05. Samples, where placed in sealable quartz
cuvettes and a) bubble degassed with nitrogen and b) subjected to freeze-pump-
thaw cycles until no visible gas evolution from the sample solution was observed.
4 different sample solutions (with an absorbance lower than 0.4) where used to
determine the quantum yield of Pt(TPBP) and Pt(TPNP).
Device Fabrication and Testing
All organic and metal films were deposited in a high vacuum (≤10
-6
Torr)
thermal evaporation system. Devices were prepared by depositing the organic
materials onto an indium tin oxide (ITO) coated glass substrate. The devices were
then removed from the vacuum system, a shadow mask was applied for cathode
patterning, and returned to the vacuum system and a 10-Å thick layer of LiF
followed by 1100-Å of Al was added to complete the architecture for the Pt(TPBP)
devices. Testing was done in air within 2 h of fabrication. The electrical and
optical intensity characteristics were measured with a Keithly 2400 source meter
85
2000, multimeter coupled to a Newport 1835-C optical meter, equipped with a UV-
818 Si photodetector. Only light emitting from the front face of the device was
collected and used in subsequent efficiency calculations. The electroluminescence
(EL) spectra were measured on a PTI QuantaMaster model C-60SE
spectrofluorimeter, equipped with a 928 PMT detector and corrected for detector
response. The emission was found to be uniform across the entire area of each
device. The device used for lifetime studies (device structure = ITO/400Å
NPD/400Å Alq
3
with 6% Pt(TPBP)/ 400Å BAlq/10Å LiF/1000Å Al) was
fabricated under high vacuum conditions (1x10
-7
Torr), using a different deposition
system at Universal Display Corporation, Ewing, NJ 08618. The organic materials
were outgassed prior to thin film growth and upon completion, the device is
transferred from the vacuum system into a nitrogen glove box. The device was
encapsulated with a glass lid sealed with a UV-curable epoxy resin in a nitrogen
glove box (<1 ppm of H
2
O and O
2
) immediately after fabrication, and a moisture
getter was incorporated inside the package. The device lifetime was collected by
monitoring the decrease in EL emission over time with a photodiode detector under
constant current (DC) conditions. A calibrated constant current source is used to
apply the aging current. Device intensity is normalized to the initial intensity
which is determined at Τ=0 at the applied current.
86
Results and Discussion
Analysis of the Pt(TPBP) by x-ray crystallography reveals a non-planar
molecular structure with a saddle type distortion, similar to that found in other
TPBP derivatives.(Cheng, Chen, Wang and Cheng 1993) An analysis of Pt(TPBP)
using normal-coordinate structural decomposition (NSD) software(Jentzen, Song
and Shelnutt
1997)
,
(http://jasheln.unm.edu/jasheln/content/nsd/nsd__welcome.htm)] quantifies
the various distortions that accompany the macrocyclic deformation (Figure 20).
Figure 20 NSD analysis with edge view of Pt(TPBP) (meso-phenyl substituents
where removed for clarity).
87
The total out-of-plane distortion (D
oop
= 2.83 Å) is almost exclusively
described by saddling (B2u
,
= 2.83 Å) while doming (A2u = 0.045 Å), wave(x) and
wave(y) (deformation similar to a chair conformation in cyclohexane, in either the
x or y direction) (Eg = 0.042 Å and Eg = 0.068 Å, respectively) contribute only
slightly to the non-planarity. This distortion is greater than that found in Zn(TPBP)
(D
oop
= 2.35 Å),(Cheng, Chen, Wang and Cheng 1993) yet is less than that of a
Ni(TPBP(CO
2
Me)
8
) derivative (D
oop
= 3.43 Å).(Rozhkov, Vladimir V.,
Khajehpour, Mazdak and Vinogradov, Sergei A. 2003)
Electrochemical analysis (special thanks to M. Dolores Perez) of Pt(TPBP)
(7), versus an internal ferrocene reference, shows a reversible oxidation at 0.24 V
and quasireversible reduction at -1.76 V. The highest occupied (HOMO) and the
lowest unoccupied (LUMO) molecular orbitals calculated from these data are 4.9
and 2.5 eV in energy relative to vacuum, respectively.(D'Andrade, Datta, Forrest,
Djurovich, Polikarpov and Thompson 2005)
B3LYP density functional theory (DFT) calculation were carried out on the
Pt(TPBP) compound using a LACVP** basis set. The HOMO and LUMO orbitals
are shown in Figure 21.
88
Figure 21 B3LYP / LACVP** calculation of Pt(II)-
tetraphenyltetrabenzoporphyrin.
The HOMO and LUMO levels displayed in the Figure 21 have a symmetry,
with opposite phases above and below the molecular plane. The
1
HOMO (-4.567
eV) consists of a mixture of predominantly pyrrolic and benzoyl orbitals, while the
1
LUMO (-2.001 eV) is mainly pyrrolic in character with some contribution of the 2
opposing benzoyl orbitals. The triplet
3
HSOMO (-2.982 eV) has a phase and
spatial relation opposite and offset by 90˚ to those of the singlet LUMO. The
energy of the triplet state was estimated as the difference between the singlet
ground state (
1
HOMO) and triplet (
3
HSOMO) energies.(Brooks, Babayan,
Lamansky, Djurovich, Tsyba, Bau and Thompson 2002; Hay 2002) The theoretical
triplet emission energy of 1.585 eV (782 nm) is comparable to the actual emission
energy of the compound, which will be discussed in the following text.
89
The absorption spectrum (Figure 22) displays strong transitions for the
Soret-band (430 nm, ε
430 nm
= 2.03 x 10
5
M
-1
cm
-1
) and Q-band (611 nm, ε
611 nm
=
1.35 x 10
5
M
-1
cm
-1
), which differ slightly from the previously reported values.
These features are red-shifted from the peaks in the analogous Pd(TPBP) complex
(Soret band: 444 nm, ε = 2.46 x 10
5
M
-1
cm
-1
; Q-band: 629 nm, ε = 1.05 x 10
5
M
-1
cm
-1
).(Rogers, Nguyen, Hufnagle, McLean, Su, Gossett, Burke, Vinogradov,
Pachter and Fleitz 2003)
Figure 22 Room temperature absorption spectra (black squares), and
normalized emission spectra at room temperature (λ=765nm (green triangles))
and 77K (λ=751nm (red circles)) of Pt(TPBP) in 2-methyl-THF.
90
The luminescence spectrum of a thoroughly degassed Pt(TPBP) sample
displays exclusive phosphorescence with a peak at 765 nm (τ = 53 µsec) at room
temperature and 751 nm (τ = 73 µsec) at 77K (Figure 22). The blue shift in the
emission spectra is due to a rigidochromic effect at low temperature. Assuming
that the radiative rate constant does not show a temperature dependence between
298 K and 77 K, and the nonradiative decay rate is negligible at 77 K, the ratio of
the lifetimes gives the photoluminescence (PL) efficiency. The photophysical
properties of Pt-tetrabenzoporphrin [Pt(BP)] support these assumptions.(Aartsma,
Gouterman, Jochum, Kwiram, Pepich and Williams 1982)
The Φ
PL
estimated by this method is 0.7.(Borek, Hanson, Djurovich,
Thompson, Aznavour, Bau, Sun, Forrest, Brooks, Michalski and Brown 2007)
Radiative (k
r
=1.3×10
4
s
-1
) and nonradiative (k
nr
=5.8×10
3
s
-1
) decay rates were
estimated from the lifetime (τ) data (k
r
=1/τ
77K
; k
nr
=1/τ
298K
-k
r
). Since this
publication by Borek et. al., the Φ
PL
has been re-measured using a Hamamatsu
integrating sphere with know absorbance after both a bubble degas procedure and
multiple freeze-pump-thaw cycles until no visible gas was evolved from the
Pt(TPBP) solution. Figure 23a) shows the absorbance of the toluene solutions and
Figure 23b) their respective lifetimes of a bubble degassed sample using N
2
.
91
Figure 23 a) Absorption measurement of Pt(TPBP). b) Lifetime of bubble
degassed solutions with know absorbance from Figure 23a.
a)
b)
92
From the lifetime data, it can be deduced that the samples where thoroughly
degassed, since the phosphorescence emission lifetime is strongly quenched by
oxygen dissolved in the solution.(Bansal, Holzer, Penzkofer and Tsuboi 2006) The
average lifetime for these solutions is τ=48 µs. The Φ
PL
of each of the solutions
was than measured with the Hamamatsu integrating sphere.
Figure 24 Quantum yield analysis of a bubble degassed sample of Pt(TPBP).
From Figure 24 it can be seen that Φ ranges from 0.31 to 0.34. Since again
the emission and thus the quantum yield of Pt(TPBP) is strongly quenched by
oxygen in the sample solution, a more rigorous degassing method (freeze-pump-
thaw cycles) was applied, to determine if bubble degassing the solution with
nitrogen is efficient enough to remove all the dissolved oxygen. Figure 25a) shows
the absorbance of the toluene solutions and Figure 25b) their respective lifetimes of
the freeze-pump-thaw treated samples.
93
Figure 25 Absorption measurement of Pt(TPBP). b) Lifetime of freeze-pump-
thaw degassed solutions with know absorbance from Figure 25a.
a)
Quantum Yield of Freeze-Pump-Thaw Pt(TPBP)
Excitation
Line of
Sample
b)
94
Once again the lifetime data confirm that the degassing process was
complete and the emission not quenched by oxygen Figure 25b. This effect can be
clearly be seen on sample (A), which was not properly degassed, thus decreasing
the phosphorescence emission lifetime. The average lifetime (of the remaining 3
samples) is τ=46 µs. The Φ
PL
of each of the solutions was than again measured
with the Hamamatsu integrating sphere.
Figure 26 Quantum yield analysis of a freeze-pump-thaw degassed sample of
Pt(TPBP).
Just like the decrease in luminescence lifetime, the quantum yield shows a
decreased value (blue square Figure 26) due to oxygen quenching. Φ
PL
for the
remaining 3 samples ranges from 0.28 to 0.33. From these newly determined
values of bubble and freeze-pump-thaw degassed samples, the radiative and
nonradiative decay rates can be calculated using the following equations
95
Φ= k
r
/ (k
r
+ k
nr
) (1)
k
r
= Φ / τ (2)
The rates determined from equations (1) and (2) k
r
and k
nr
are summarized in
Table 3.
The nonradiative rate shown in Table 3 is an order of magnitude larger to
that reported for [Pd(TPBP)] (k
nr
=4.2×10
3
s
-1
), but the stronger spin-orbit coupling
of Pt increases the radiative rate by an order of magnitude compared to
[Pd(TPBP)]: k
r
=8.8×10
2
s
-1
). (Rogers, Nguyen, Hufnagle, McLean, Su, Gossett,
Burke, Vinogradov, Pachter and Fleitz 2003)
To minimize concentration quenching in Pt(TPBP) based OLEDs, a dopant-
host system using tris(8-hydroxyquinoline) aluminum (Alq
3
) as host was
applied.(Baldo, O'Brien, You, Shoustikov, Sibley, Thompson and Forrest 1998) It
Table 3 Radiative and nonradiative rates for bubble and freeze-
pump-thaw degassed samples of Pt(TPBP).
K
r
[s
-1
] K
nr
[s
-1
]
Bubble degassed Sample 6.8 x 10
3
1.4 x 10
4
Freeze-Pump-Thaw degassed
Sample
6.7 x 10
3
1.3 x 10
4
96
is important to choose the correct host-dopant combination, with regard to proper
matching of the HOMO and LUMO energies of the respective molecules. The
HOMO of the dopant must be deeper in energy as the HOMO of the host. Further
the LUMO of the dopant must be lower in energy than the LUMO of the host. This
general energetic scheme will predominantly prevent endothermic repopulation of
the LUMO of the host, which would have as an effect diminished device
performance. This dopant-host system may increase the theoretical efficiency do to
both Förster and Dexter energy transfer; with the energy transfer rate being
proportional to the spectral overlap between the donor exciton and the host. Alq
3
has an oxidation (+0.7V) and reduction (-2.3V) potentials that straddle those of
Pt(TPBP),(Gross, Anderson, Slaterbeck, Thayumanavan, Barlow, Zhang, Marder,
Hall, Nabor, Wang, Mash, Armstrong and Wightman 2000) making charge
trapping particularly favorable in this system.
Preliminary OLEDs were fabricated using the following architecture:
ITO/(400Å) NPD/(400Å) Alq
3
with 6% Pt(TPBP)/(10)Å LiF/(1100Å) Al (ITO =
indium tin oxide, NPD = N,N´-bis(1-naphthyl)- N,N´-diphenyl- 1,1´-biphenyl-
4,4´-diamine). The electroluminescence spectrum (EL) for this device displays a
strong near infrared emission at 765 nm (Figure 27). A weak emission from Alq
3
centered at 520 nm that also increases as the driving voltage is increased is similar
to the behavior previously reported by Baldo, et. al. for devices doped with
Pt(OEP).(Baldo, O'Brien, You, Shoustikov, Sibley, Thompson and Forrest 1998)
97
Figure 27 Normalized EL spectra for device architecture ITO/400Å
NPD/400Å Alq
3
with 6% Pt(TPBP)/10Å LiF/1100Å Al showing nIR emission
at 765 nm of Pt(TPBP) and Alq
3
host emission at 520 nm.
Therefore a second device was fabricated with the following architecture:
ITO/(400Å) NPD/(400Å) Alq
3
with 6% Pt(TPBP)/(500Å) Alq
3
/(10Å)
LiF/(1100Å) Al. The additional layer between the emissive layer and the cathode
was introduced to prevent Pt(TPBP) exciton migration to the cathode and
subsequent quenching at the cathode interface. This neat exciton blocking layer
(EBL) of Alq
3
diminishes the unwanted emission of the host molecule in the EL
spectra, thus indicating a complete energy transfer from host to dopant (Figure 28).
98
Figure 28 Normalized EL spectra for device architecture ITO/(400Å)
NPD/(400Å) Alq
3
with 6% Pt(TPBP)/ (500Å) Alq
3
/(10Å) LiF/(1100Å) Al
showing nIR emission at 765 nm of Pt(TPBP) and no Alq
3
host emission at 520
nm.
This device architecture is going to be referred to as device 1 in the following
description of the OLED performance data.
Devices where also prepared by Universal Display Corporation, Ewing, NJ,
08618 (Dr. Jason Brook and Lech Michalski) to test the operational stability with
the following architecture (here on out named device 2): ITO/(400Å) NPD/(400Å)
Alq
3
with 6% Pt(TPBP)/(400Å) BAlq /(10Å) LiF/(1000Å) Al, where BAlq is
aluminum (II) bis(2-methyl-8-quinolinato)4-phenylphenolate and serves as the
EBL, which has previously been demonstrated to give high efficiencies and long
electrophosphorescent device lifetimes.(Kwong, Nugent, Michalski, Ngo, Rajan,
Tung, Weaver, Zhou, Hack, Thompson, Forrest and Brown 2002; Kwong, Weaver,
Michael Lu, Tung, Chwang, Zhou, Hack and Brown 2003)
99
Figure 29 External quantum efficiency versus current density. Device 1:
ITO/(400 Å) NPD/(400 Å) Alq
3
+ 6wt% [Pt(TPBP)]/(500 Å) Alq
3
/(10 Å)
LiF/(1100 Å) Al; device 2: ITO/(400 Å) NPD/(300 Å) Alq
3
+ 6wt%
[Pt(TPBP)]/(400 Å) BAlq/(10 Å) LiF/(1000 Å) Al. Inset: Intensity and current
density versus voltage for device 1 and 2.
Device 1 has an EQE of 6.3% at 0.1 mA cm
-2
, which gradually decreases as
the current density is increased (Figure 29). The light output is roughly 0.1 µW cm
-
2
at 3V and 750 µW cm
-2
at 12V.
Sun et. al. (Sun, Borek, Hanson, Djurovich, Thompson, Brooks, Brown and
Forrest 2007) has further demonstrated an increase of the EQE to 8.5% at a low
current density of 4x10
-3
mA cm
-2
(Figure 30) by changing the device architecture
to: ITO/(400Å) NPD/(250Å) Alq
3
with 4% Pt(TPBP)/(400Å) BCP /(80Å)
LiF/(500Å) Al, where BCP is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline. The
higher efficiency indicates that BCP effectively confines holes and excitons within
the emission layer (EML) and a lower doping level (4% versus 6% of Pt(TPBP))
100
assists in reducing triplet – triplet annihilation, which is responsible for an overall
luminescence efficiency loss.
Figure 30 External quantum efficiency versus current density. Inset: device
architecture.
Device 2, which was fabricated to test operational stability, shows at low
current density of 0.1 mA cm
-2
a maximum EQE of 3% falling off to 1.1% at 10
mA cm
-2
. The electroluminescence spectrum of this device is identical to that for
device 1 (Figure 28). The device 2 was aged at a high constant current of 40 mA
cm
-2
corresponding to an initial intensity of 740 µW cm
-2
. The data in Figure 31
(see inset) suggests that device 2 will maintain greater than 90% of its initial
intensity after 1000 hours of operation.
101
Figure 31 Intensity versus operation time, normalized to initial intensity
(closed squares) and voltage (open circles). The initial intensity was 740 µW
cm
-2
. The insert shows the time versus radiance data plotted on a
semilogarithmic scale with extrapolation to 1000 hours.
A 0.5 V rise in voltage occurring during the first 50 hours corresponds to
the initial fast decay in device intensity. After 100 hours, the voltage stabilizes at
13.3 V. The operational voltage is sensitive to variations in temperature, which may
account for some of the observed fluctuations. These initial results demonstrate that
Pt(TPBP) devices are stable at high currents, although further study is needed to
determine the device lifetime at radiance suitable for display use, assuming the
viewer is using night-vision goggles. The long device lifetimes estimated for
Pt(TPBP)-based OLEDs are consistent with the previously reported lifetime for
Pt(OEP)-based OLEDs (> 10
6
hours measured at low luminance conditions)
(Burrows, Forrest, Zhou and Michalski 2000), thus illustrating the high device
stability of Pt-porphyrin-based OLEDs.
102
To investigate how different metals influence the position of Sort- and Q-
bands and thus emission energy, porphyrins containing Pd, Ni and Zn were also
synthesized (Figure 32).
Figure 32 Absorption spectra of M(II) tetraphenyltetrabenzoporphyrin. M =
Pt, Pd, Ni, Zn.
The spectral shift of the Soret- and Q-bands in the absorption spectra of the
Pt and Zn porphyrins is 26 nm and 39 nm respectively. This slight shift in
absorption energies, but more importantly the diminished spin-orbit coupling of the
first row transition metals (Ni and Zn) contribute to a loss of phosphorescence
N
N
N
N
Ph Ph
Ph Ph
Pt
N
N
N
N
Ph Ph
Ph Ph
Pd
N
N
N
N
Ph Ph
Ph Ph
Ni
N
N
N
N
Ph Ph
Ph Ph
Zn
103
efficiencies. It is report in literature that the Zn(TPBP) has a Φ
PL
of 0.00002 ±
0.00001(Rogers, Nguyen, Hufnagle, McLean, Su, Gossett, Burke, Vinogradov,
Pachter and Fleitz 2003) Further, thermal decomposition upon sublimation of the
Ni and Zn porphyrins, as well as the reduced spin orbit coupling of the Pd
porphyrin (Φ
PL
for Pd(TPBP) = 0.167, λ
max
=797 (Rogers, Nguyen, Hufnagle,
McLean, Su, Gossett, Burke, Vinogradov, Pachter and Fleitz 2003)) directed the
investigation towards extension of the aromatic π system of the annulated isoindole
precursor in order to achieve a market red shift in the emission color of the
compound.
Extension of the porphyrin ring by benzo and naphtho annulation results in
a red shift in both the ground (Soret- and B-bands) and triple excited state (T
1
-T
n
)
absorption spectra. It is know that red shifts of the ground state spectra may be
attributed to some extent due to saddling/ruffling of the porphyrin as well as
electronic effects due to increased π-conjugation.(Rogers, Nguyen, Hufnagle,
McLean, Su, Gossett, Burke, Vinogradov, Pachter and Fleitz 2003)
In case of the Pt(TPNP) (14), the electrochemical analysis shows a
reversible oxidation potential is at -0.13 V and quasireversible reduction at -1.85V.
The highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular
orbitals calculated from these data are 4.4 and 2.7 eV in energy relative to vacuum,
respectively.
104
As previously demonstrated in the case of Pt(TPBP), the difference in the
Φ
PL
between the two degassing methods (bubble degassed versus “freeze-pump-
thaw”) lays within the experimental error of the measurement. Therefore the Φ was
determined from a with nitrogen bubble degassed toluene solution having the
following absorption values (Figure 33).
Figure 33 Absorption measurement of Pt(TPNP).
Do to instrument limitations in the emission region of this particular
porphyrin (900 nm), the lifetime could not be directly determined from these
solutions. Never the less, the quantum yield was measured at the absorbances
shown in Figure 33 and determined to be Φ
PL
=0.08(Figure 34).
Quantum Yield of Bubble Degassed Pt(TPNP)
Excitation
Line of
Sample
105
Figure 34 Quantum yield analysis of a bubble degassed sample of Pt(TPNP).
This corresponds well to the Pd(TPNP) analog, which has a reported
quantum efficiency of 0.065.(Rogers, Nguyen, Hufnagle, McLean, Su, Gossett,
Burke, Vinogradov, Pachter and Fleitz 2003) The increase in quantum yield
efficiency can once again be attributed to the larger spin-orbit coupling of the Pt
analog.
Due to equipment limitations, the phosphorescence lifetime was measured
in the lab of Prof. Matthias Selke at Cal State LA, Los Angeles, CA with the help
of Dr. Dong Zhang. Below are shown the phosphorescence decay curves of two
bubble degassed solution having different absorbance (0.39 and 0.19) values. From
these decay curves the lifetime was determined to be τ=30 µsec (Figure 35).
106
Figure 35 Lifetime measurement of Pt(TPNP) in a degassed toluene solution.
The nonradiative rate is a slightly larger to that reported for [Pd(TPNP)]
(k
nr
=1.5×10
4
s
-1
), but the stronger spin-orbit coupling of Pt increases the radiative
rate by 1.6×10
4
s
-1
compared to [Pd(TPNP)]: k
r
=1.4×10
3
s
-1
). (Rogers, Nguyen,
Hufnagle, McLean, Su, Gossett, Burke, Vinogradov, Pachter and Fleitz 2003)
The absorption spectrum (Figure 36) displays strong transitions for the
Soret band: 463 nm, ε = 1.44 x 10
5
M
-1
cm
-1
and an even stronger transition for the
K
r
[s
-1
] K
nr
[s
-1
]
Bubble degassed Sample 2.6 x 10
3
3.0 x 10
4
Table 4 Radiative and nonradiative rates for a bubble degassed
sample of Pt(TPNP).
107
Q-band: 685 nm, ε = 2.27 x 10
5
M
-1
cm
-1
. These features are slightly blue shifted
from the peaks in the analogous Pd(TPNP) complex (Soret band: 463 nm, ε = 1.44
x 10
5
M
-1
cm
-1
; Q-band: 705 nm, ε = 1.68 x 10
5
M
-1
cm
-1
).(Rogers, Nguyen,
Hufnagle, McLean, Su, Gossett, Burke, Vinogradov, Pachter and Fleitz 2003)
Figure 36 Room temperature absorption spectra (black open squares) and
normalized emission spectra at room temperature (λ=923nm (red open
circles)) and 77K (λ=887nm (green open triangles)) of Pt(TPNP) in 2-methyl-
THF.
N
N
N
N
Ph Ph
Ph Ph
Pt
108
The luminescence spectrum of a thoroughly degassed Pt(TPNP) sample
displays exclusive phosphorescence with a peak at 923 nm at room temperature and
887 nm at 77K (Figure 36). The blue shift in the emission spectra is due to a
rigidochromic effect at low temperature.
Due to limited instrumentation in the lab at USC, Pt(TPNP) was send to
Yiru Sun at the University of Michigan in Prof. Stephen R. Forrest lab for device
fabrication and testing. There the device architecture was optimized (host/dopant
system, co-dopants for better energy transfer etc) in order to achieve pure nIR
emission. The following relative simple device structure: ITO/(400Å) NPD/(800Å)
S-TAD/(250Å) CBP with 9% Pt(TPNP)/(400Å) BCP /(10Å) LiF/(1000Å) Al
(where S-TAD is 2,2',7,7'-tetrakis(diphenylamino)-9,9'-spirobifluorene, CBP is
4,4'-Bis(carbazol-9-yl)biphenyl and BCP is 2,9-Dimethyl-4,7-diphenyl-1,10-
phenanthroline) achieved 92 % phosphorescence emission at 900 nm (Figure 37).
109
Figure 37 Normalized electroluminescence spectrum with l
max
= 900nm at
current density of 20 mA/cm
2
(red) and 100 mA/cm
2
(blue).
As can bee seen from Figure 37, most of the phosphorescence emission
comes from the Pt(TPNP). The visible contribution in the electroluminescence
spectrum is not due to recombination of excitons on other molecules within the
device architecture. Thus further investigation has to be undertaken to determine
the origin of these emissions. The quantum efficiency for the above mentioned
device is 1.9 at 0.1 mA/cm
2
(Figure 38).
110
Figure 38 Quantum efficiency versus current density (black) and current
density versus lumens (red) of a Pt(TPNP) device.
Using the external quantum efficiency of this particular device, the solution
based quantum yield measured by the Hamamatsu integrating sphere may be
verified. Adachi et. al. (Bulovic, Khalfin, Gu, Burrows, Garbuzov and Forrest
1998; Adachi, Baldo, Thompson and Forrest 2001) demonstrated that using highly
phosphorescent materials which harvest both singlet and triplet excitons, the
internal quantum efficiency of the device can approach 100%, in which case a 20%
external quantum efficiency for OLEDs may be anticipated. In other words, to
estimate if the solution based efficiency is in the approximate range, the external
quantum efficiency of the OLED has to be multiplied by 5 in order to correlate to
the solution based phosphorescence emission quantum yield. Thus multiplication
of 1.9 (OLED efficiency) times 5 gives a quantum yield 9.5% which is close to the
experimentally measured Φ
PL
with the integrating sphere.
111
Conclusion
In this chapter, the photophysical and electroluminescence properties of two
Pt-metalloporphyrins, Pt(II)-tetraphenyltetrabenzoporphyrin (Pt(TPBP) and Pt(II)-
tetraphenyltetranaphthoporphyrin (Pt(TPNP), were investigated. Pt(TPBP) has a
strong near infrared emission from a room temperature degassed solution at 765
nm, and OLEDs fabricated from this compound show an external quantum
efficiency (EQE) of 6.3% at 0.1 mA cm
-2
, which gradually decreases as the current
density is increased. The light output is roughly 0.1 µW cm
-2
at 3V and 750 µW
cm
-2
at 12V at a device emission of 765 nm. Sun et. al. (Sun, Borek, Hanson,
Djurovich, Thompson, Brooks, Brown and Forrest 2007) has further demonstrated
an increase of the EQE to 8.5% at a low current density of 4x10
-3
mA cm
-2
by
slightly changing the device architecture. The luminescence spectrum of a
thoroughly degassed Pt(TPNP) sample displays exclusive phosphorescence with an
emission maximum at 923 nm at room temperature and 887 nm at 77K.
Preliminary devices where also fabricated which achieved 92 % phosphorescence
emission at 900 nm with an EQE of 1.9% at 0.1 mA cm
-2
. Further device
optimization has to be undertaken to obtain exclusive near infrared emission at 900
nm.
112
Chapter 4 Near-Infrared Phosphorescence Emission from
Asymmetrical Benzo-/Naphtho Pt-Metalloporphyrin
Complexes
Abstract
In this chapter, a rational large scale synthesis of asymmetric porphyrins
was attempted in order to provide a phosphorescence dopant for the application in
organic light emitting diodes (OLED) devices. These asymmetric porphyrins are
expected to have a phosphorescence emission maximum between those reported for
Pt(II)(tetraphenylbenzoporphyrin) (Pt(TPBP)) and
Pt(II)(tetraphenylnaphthoporphyrin) (Pt(TPNP)) which were evaluated on their
photophysical properties in chapter 3 of this work. Do to scrambling during the
porphyrin formation reaction, systematic synthesis of different asymmetric
porphyrins was not possible on large scale without extensive work on improving
the reaction conditions in order to minimize scrambling and product purification.
Thus a statistical synthetic approach was chosen along with HPLC purification to
identify specific asymmetric compounds and to investigate their corresponding
phosphorescence emission energies.
113
Introduction
As described in the previous chapter 3, symmetrical Pt containing porphyrin
molecules with either benzo [Pt(II)(tetraphenylbenzoporphyrin) Pt(TPBP)] or
naphtho [ Pt(II)(tetraphenylnaphthoporphyrin) Pt(TPNP)] annulated aromatic
system have been synthesized and their photophysical properties analyzed. The
phosphorescence emission of Pt(TPBP) and Pt(TPNP) are 765 nm and 923 nm at
room temperature, respectively. Even though the synthesis of these symmetrical
porphyrins can be accomplished in relative high yields (> 50%)(Baumann and
Waldner 2001; Borek, Hanson, Djurovich, Thompson, Aznavour, Bau, Sun,
Forrest, Brooks, Michalski and Brown 2007) rational synthesis of asymmetric
porphyrins, on the other hand, is still problematic to this day. Traditionally,
asymmetric porphyrins are stitched together out of different dipyrromethane
fragments. MacDonald et. al. improved Fischer’s dipyrromethane condensation
procedure in the 1960.(Arsenault, Bullock and MacDonald 1960) The so called
“2+2” MacDonald’s condensation utilizes milder conditions thus allowing the use
of dipyrromethanes, which are compounds that are generally much more available
than dipyrromethenes.(Ongayi 2005) Fischer had previously reported that
dipyrromethanes were to unstable under the cyclization conditions,(Fischer and
Klarer 1929) but MacDonald’s route basically entils the condensation of 1,9-
diformyl-dipyrromethane with 1,9-unsubstituded-dipyrromethane affording the
desired porphyrin in yields up to 60% (Scheme 12).
114
N
HN
N
NH
R
2
R
1
R
4
R
3
R
6
R
5
R
8
R
7
+
H
N HN
N
H
NH
R
2
R
1
R
4
R
3
R
6
R
5
R
8
R
7
O
O
Scheme 12 Retrosynthetic analysis of asymmetric porphyrins.
However, the major drawback of this pathway is that dipyrromethanes are
inherently unstable toward most acidic reagents resulting in scrambling or
redistribution during the cyclization reaction.(Fischer and Orth 1937) Since then, a
lot of research efforts have been undertaken, for example by Timothy D. Lash
et.al., Noboru Ono et.al., Jonathan S. Lindsey et.al. and Sergei A. Vinogradov
et.al., just to mention a few, to understand the chemistry behind the asymmetric
porphyrin synthesis. Thus in this chapter 4, different routes of asymmetric benzo-
/naphthoporphyrin synthesis were evaluated on the basis of larger scale compound
synthesis for OLED applications based on the previous research already established
in this filed.
115
Experimental
Unless otherwise noted, all reagents and solvents where obtained from
Sigma-Aldrich and used without any further purification. Extracts where dried
over Na
2
SO
4
and solvents removed with a rotary evaporator at aspirator pressure.
NMR spectra where recorded on Bruker AM 250 instrument with TMS as standard
signal in either deuterated dichloromethane or deuterated DMSO.
Synthesis and Characterization
The asymmetric benzo-/naphthoporphyrins of interest in this chapter were
synthesized from known literature procedures, which describe the synthesis of
other asymmetric porphyrin moieties.(Degani and Fochi 1976; Barbero, Cadamuro,
Degani, Fochi, Gatti and Regondi 1988; Lash, Chandrasekar, Osuma, Chaney and
Spence 1998; Rao, Dhanalekshmi, Littler and Lindsey 2000; Cammidge and
Oeztuerk 2001; Rietveld, Kim and Vinogradov 2003; Rozhkov, V. V., Khajehpour,
M. and Vinogradov, S. A. 2003; Setsune, Tanabe, Watanabe and Maeda 2006)
One Pot Synthesis of
Pt(II)tetraphenyl[(benzo)
n
(naphtho)
m
]porphyrin
A one pot method was investigated where Zn acetate acts as a templating
agent around which both naphtho- and benzo dicarboxyimide moieties will arrange
in a statistical fashion to form the meso phenyl substituted porphyrin (Scheme 13),
116
1
+
H
N
O O
+
H
N
O O
OH
O
2
3
Zn(OAc)
2
2H
2
O
KHCO
3
N
N
Zn
n
m
Scheme 13 One pot asymmetric benzo-/naphthoporphyrin synthesis.
where n = 4 - m or m = 4 - n, respectively.
Zn(II)-tetraphenyl-[(benzo)
n
(naphtho)
m
]porphyrin. 2,3-
Naphthalenedicarboximide (1) (10.1 mmol), phthalimide (2) (10.1 mmol),
phenylacetic acid (3) (24 mmol), Zn(OAc)
2
•2H
2
O (14.2 mmol) and KHCO
3
(20.2
mmol) where combined in a U-tube and heated in a sand bath for 40 - 50 minutes at
>300˚C. The resulting black sludge was washed with hot water and dried.(Wilson
and Vinogradov 2002) Unfortunately, no isolatable product was obtained via this
method. Thus the attention turned towards a more rational asymmetric design as
described in the literature for similar compounds.
Synthesis of Pt(II) tetraphenyl-trans-benzonaphthoporphyrin
This rational synthetic approach is based on the design of an isoindole
precursor, functionalized in the 2,5-position with a phenylhydroxymethane moiety,
117
which introduces both the meso carbon and phenyl substituent into the porphyrin
ring (9) (Scheme 14). Using compound (9) and naphthopyrrole precursor under
acetic conditions, it has been shown to form porhyrins on small scale.
5
H
N
POCl
3
/ DMF
6
H
N
H
O
NC
OEt
O
7
H
N
NC
OEt
O
i) POCl
3
/DMF
H
N
H
O
ii) Na(OAc) 3H
2
O
H
O
8
Li
H
N
HO OH
9
Scheme 14 Synthetic scheme of the precursor isoindole (9) for the rational
design of trans-benzonaphthoporphyrin.
Cyclohexenopyrrole-1-carboxaldehyde (6). DMF (4.1 ml) was cooled in
an ice bath and POCl
3
(2.4 ml) was added drop wise to form the Vilsmeier
complex. The ice bath was removed and the solution stirred for 15-20 minutes.
Dry CH
2
Cl
2
(10 ml) was added and the temperature lowered to 0 to -2˚C. A
solution of pyrrole (5) in CH
2
Cl
2
(14 ml) was added drop wise over a period of 15
minutes, the ice bath removed and refluxed for 15 to 20 minutes. At room
temperature, an aqueous solution of Na(OAc)•3H
2
O (17.7g in 25 ml water) was
118
added drop wise and the reaction mixture refluxed for another 15 to 20 minutes.
The layers where separated and the aqueous layer extracted with CH
2
Cl
2
. The
organic layers where combined and washed with a saturated Na
2
CO
3
solution and
dried over Na
2
SO
4
affording compound (6) in 65% yield and used in the next step
without further purification.
1
H-NMR (250 MHz, CDCl
3
): δ = 9.50-9.41 (s, 1H),
6.76 (s, 1H), 2.85-2.71 (m, 2H), 2.58-2.39 (m, 2H), 1.77-1.64 (m, 4H).
1(2-cyano-2-ethoxycarbonylvinyl)cyclohexenopyrrole (7). To a crude
cyclohexenopyrrolecarboxaldehyde (6) solution in absolute ethanol (60 ml) was
added ethylcyano acetate (1.51 ml) and piperidine (~ 10 drops). The resulting dark
solution was refluxed for 3 - 3.5 hours and filtered while still hot. The filtered
solution was placed in a refrigerator over night and the crystallized product filtered
and washed with ice cold ethanol yielding 450 mg of product.
1
H-NMR (250 MHz,
CDCl
3
): δ = 6.96 (t, 1H, J = 3.4), 4.35-4.24 (q, 2H), 2.74-2.63 (m, 2H), 2.60-2.48
(m, 2H), 1.82-1,71 (m, 4H), 1.38-1.32 (t, 3H).
1,3-Dicarboxaldehyde-cyclohexenopyrrole (8). DMF (5.1 ml) was cooled
in an ice bath and POCl
3
(2.4 ml) was added drop wise to form the Vilsmeier
complex. The ice bath was removed and the solution stirred for 15-20 minutes.
Dry CH
2
Cl
2
(80 ml) was added and the temperature lowered to 0 to -2˚C. A
solution of the activated pyrrole (7) in CH
2
Cl
2
(10 ml) was added drop wise over a
period of 15 minutes, the ice bath removed and refluxed for 15 to 20 minutes. At
room temperature, an aqueous solution of Na(OAc)•3H
2
O (17.7g in 25 ml water)
was added drop wise and the reaction mixture refluxed for another 15 to 20
119
minutes. The layers where separated and the aqueous layer extracted with CH
2
Cl
2
.
The organic layers where combined and washed with a saturated Na
2
CO
3
solution
and dried over Na
2
SO
4
. The crude product was refluxed in a 3M NaOH solution
(70 ml) for 30 minutes. The reaction mixture was cooled to room temperature and
neutralized with a dilute aqueous H
2
SO
4
solution and the precipitate collected
through suction filtration. The filtrate was washed several times with water and
dried by standing under ambient conditions.
1
H-NMR (250 MHz, CDCl
3
): δ = 9.80
(s, 2H), 2.91-2.83 (m, 4H), 1.90-1.81 (m, 4H).
2,5-bis(phenylhydroxymethyl)cyclohexenopyrrole (9). Phenyllithium
(8.8 mmol) was added drop wise to a dry THF solution (30 ml) of the dialdehyde
(8) at -78˚C and stirred at this temperature for 2 to 2.5 hours, after which it was
gradually warmed to room temperature over 1 hour and than stirred at this
temperature for an additional hour. The reaction mixture was quenched with water
and the aqueous layer repeatedly extracted with ether. The organics were washed
with brine and dried over Na
2
SO
4
.
The work up which was described in literature (Setsune, Tanabe, Watanabe
and Maeda 2006) did not yield the desired product. Thus column chromatography
was used to purify the compound. Due to the reactive nature of this
phenylhydroxymethylpyrrole, the product reacted upon contact with the slightly
acidic silica gel. Also due to the fairly difficult synthetic procedure, a fairly large
scale synthesis of this compound was not feasible.
120
Therefore the attention turned towards another method developed in the late
70’s (Degani and Fochi 1976) which utilizes a benzoxathiolylium salt (12) in order
to introduce the meso phenyls on the porphyrin scaffold (Scheme 15).
5
H
N
H
N
O O
9
HBF
4
OEt
2
+
SH
OH
11
O Cl
10
O
S
BF
4
12
H
N
13
O
S S
O
Ph Ph
Hg(II)O
HBF
4
NaBH
4
H
N
HO OH
14
NH
N
HN
N
NH
Ph Ph
Ph Ph
15
16
Scheme 15 Synthetic scheme for the precursor (9) for the synthesis of Pt(II)-
tetraphenyl-trans-benzonaphthoporphyrin.
121
2-Phenyl-1,3-benzoxathiolylium tetrafluoroborate (12). An ether (5 ml)
solution of mercaptophenol (11) (0.01 mmol) and HBF
4
•OEt
2
(0,02 mmol) was
added to an ice cold solution of benzoylchloride (10) (0.01 mmol) in ether (10 ml).
The reaction was than warmed to 30˚C for 2 hours and the resulting precipitate
filtered and washed with ether yielding the product in 63% yield. Mp (Lit): 204 –
205 °C, Mp (compound): 205 – 206 °C.
The compound was used in the next reaction without further purification.
2,5-Bis(2-phenyl-1,3-benzoxathiolyl)cyclohexenopyrrole (13).
Benzoxathiolyl salt (12) (6.35 mmol), cyclohexenopyrrole (5) (2.11 mmol) (for
synthetic procedure see chapter 3 of this work), dry pyridine (6.35 mmol) and dry
acetonitrile (8 ml) where stirred at room temperature for 30 min, after which the
pyrrole and pyridinium BF
4
salt was filtered, washed with 2-3 ml of acetonitrile
and then redissolved in chloroform:water (500:100 ml). Organics where washed
with a 5% aqueous NaOH solution, water and with hot methanol affording the
product (13) in 45% yield.
1
H-NMR (250 MHz, CDCl
3
): δ = 7.60-7.51 (m, 4H),
7.36-7.26 (m, 6H), 7.08-6.98 (m, 4H), 6.92-6.86 (m, 4H), 2.26-2.10 (m, 2H), 2.08-
1.94 (m, 2H), 1.52-1.40 (m, 4H).
Bis(2,5-diacylated)cyclohexenopyrrole (14). 2,5-Bis(2-phenyl-1,3-
benzoxathiolyl)cyclohexenopyrrole (13) (1.83 mmol) was added to the hydrolysis
reagent constituted of mercury(II) oxide (3.66 mmol) in THF (10 ml) and 48%
aqueous HBF
4
solution (18 ml). The reaction mixture was heated to 50˚C and
stirred for 3 hours, after which it was extracted using hot toluene and washed with a
122
10% aqueous KI solution (2x80 ml), a 5% aqueous NaOH solution (80 ml) and
water (80 ml). The product was purified by column chromatography yielding the
compound (14) in a 22 % yield. Compound was characterized by NMR and mass
spectroscopy.
1
H-NMR (250 MHz, CDCl
3
): δ = 7.75-7.68 (m, 4H), 7.63-7.64 (m,
6H), 2.57-2.46 (m, 4H), 1,79-1.59 (m, 4H).
Tetraphenyl-trans-cyclohexeno-1,4-dihydronaphthoporphyrin (16).
Diacetylated pyrrole (14) (0.39 mmol) was dissolved in dry THF/Methanol (12
ml/1.2 ml) and NaBH
4
(7.8 mmol) was slowly added under nitrogen. The resulting
reaction mixture was stirred for 1 hour after completion of addition. A saturated
aqueous solution of NaBH
4
(20 ml) was added, stirred for 5 – 10 minutes and
extracted with CH
2
Cl
2
, dried with Na
2
SO
4
and the solvent evaporated under
reduced pressure. Immediately after solvent evaporation, the product (14) was
dissolved in 100 ml of degassed acetonitrile, shielded from light and TFA (4.68
mmol) added. Pyrrole (15) (for synthetic procedure see chapter 3 of this work) was
dissolved in degassed acetonitrile (20 ml) and added very slowly to the above
solution. After completion of addition, the reaction mixture was stirred for 10
minutes and DDQ (1.17 mmol) was added and the resulting mixture stirred for an
additional hour. Next triethylamine (4.68 mmol) was added and the porphyrin was
extracted with CH
2
Cl
2
, was washed with a 10% aqueous Na
2
SO
3
solution and
water. The product was purified with column chromatography (eluent:
CH
2
Cl
2
/Methanol 10:1). Since the product purification with a column did not
exclusively render the desired product, the fractions where recombined and used in
123
the next metallation step, since a priori only the porphyrin moieties in the reaction
mixture would metallate (Scheme 16).
N
HN
N
NH
Ph Ph
Ph Ph
16
PtCl
2
N
N
N
N
Ph Ph
Ph Ph
17
Pt
DDQ
N
N
N
N
Ph Ph
Ph Ph
18
Pt
Scheme 16 Synthetic scheme for the synthesis of Pt(II)-tetraphenyl-trans-
benzonaphthoporphyrin (18).
Pt(II)-tetraphenyl-trans-cyclohexeno-1,4-dihydronaphthoporphyrin
(17). trans-porphyrin (16) (0.18 mmol) was heated in benzonitrile (50 ml) and
exposed to atmospheric conditions during the addition of PtCl
2
(0.40 mmol) and
124
refluxed over night, after which the solvent was evaporated. The product was
purified by column chromatography (eluent: 100% CH
2
Cl
2
).
Pt(II)-tetraphenyl-trans-benzonaphthoporphyrin (18) Pt(TP-t-BNP).
The precursor porphyrin (17) (0.109 mmol) was dissolved in toluene (100 ml),
DDQ (1.09 mmol) added and refluxed for 30 minutes. The reaction mixture a
cooled to room temperature and washed with a 10% aqueous Na
2
SO
3
solution,
brine and dried over Na
2
SO
4
. Compound was characterized by NMR and mass
spectroscopy.
MALDI Mass Spectroscopy
As matrix for these experiments was used α-Cyano-4-hydroxycinnamic
acid, which was dissolved in methanol. The compound was added in a CH
2
Cl
2
solution. The mixture was than spotted on a MALDI plate and analyzed on a
Voyager DE STR Bio Spectrometry Workstation by Applied Biosystems. The
ionization was done by a nitrogen laser at 337 nm with a pulse length of 3 ns at a
max firing rate of 20Hz. Detection was accomplished in linear and or reflector
mode.
125
HPLC Purification Parameters
The reaction mixture was analyzed on an HPLC from Shimatzu with a
SCL-10A
vp
system controller, SIL-10AF
vp
auto injector, two LC 10 ATV
p
high
pressure pumps, SPD-10A
vp
UV-VIS detector (setting at 400nm). The column
used, LiChrospher© RP-18 with pore size 5µm and 250 mm x 4.6 mm in
dimensions, was purchased from Supelco Inc. Injection volume was 50µl and flow
rate was chosen to be either 1 µl/min or 0.750 µl/min.
Results and Discussion
The rational design described in Scheme 15 and Scheme 16 should lead to
only the trans-porphyrin (18). The absorption spectra (Figure 39) on the other
hand shows a mixture of compounds (broadening of both the Soret- and Q-bands)
which are bracketed by the two limiting molecules Pt(TPBP) and Pt(TPNP), black
and blue trace respectively.
126
Figure 39 Absorption spectrum of Pt(TP-t-BNP) red circles, Pt(TPBP) black
squares and Pt(TPNP) blue triangles.
Since the absorption spectra did not reveal a single compound, MALDI
analysis was undertaken in order to understand the compounds in this “reaction
mixture” (Figure 40).
127
Figure 40 MALDI mass spectrum of Pt(TP-t-BNP). Red numbers identify the
porphyrin.
The MALDI clearly shows four main porphyrin derivatives which comprise
the absorption spectrum in Figure 39. Below is the list of compounds that have
been identified from the rational synthetic approach (Figure 41). A priori it is not
19
20
18
21
128
possible to tell using only mass spectroscopic methods if compound 18 really is
trans or cis.
N
N
N
N
Ph Ph
Ph Ph
19
Pt
Chemical Formula: C
60
H
36
N
4
Pt
Exact Mass: 1007.26
N
N
N
N
Ph Ph
Ph Ph
20
Pt
Chemical Formula: C
64
H
38
N
4
Pt
Exact Mass: 1057.27
N
N
N
N
Ph Ph
Ph Ph
18
Pt
Chemical Formula: C
68
H
40
N
4
Pt
Exact Mass: 1107.29
N
N
N
N
Ph Ph
Ph Ph
21
Pt
Chemical Formula: C
72
H
42
N
4
Pt
Exact Mass: 1157.31
Figure 41 Porphyrin molecules identified by MALDI MS.
A more careful separation of this compound by column chromatography
(Eluent=70% Hexanes / 30% CH
2
Cl
2
) confirmed the presence of multiple
compounds found by MALDI-MS (Figure 42).
129
Figure 42 a) Chromatography result of the separation of Pt(TP-t-BNP). b)
UV-VIS absorption spectrum of the above fractions.
In order to understand which asymmetric porphyrin compounds compose
the above separated solution, MALDI analysis was once again used ( Figure 43).
Green
Fraction
Purple
Fraction
Yellow
Fraction
ion
a)
b)
130
Figure 43 MALDI Spectra of the a) green fraction, b) purple fraction and c)
the yellow fraction. Red numbers identify the porphyrin moiety.
Green
Fraction
20
18
19
a)
131
Figure 43 continued
20'
18
b)
N
HN
N
NH
Ph Ph
Ph Ph
20'
Chemical Formula: C
64
H
54
N
4
Exact Mass: 878.43
Purple
Fraction
132
Figure 43 continued
18
20
c)
Yellow
Fraction
133
The apparent rational design and synthesis of asymmetric porphyrins did not
exclusively render the desired product due to scrambling reaction during the
porphyrin formation. This phenomena has been well documented in literature for
all kinds of different porphyrin formation conditions. (Littler, Ciringh and Lindsey
1999; Rao, Dhanalekshmi, Littler and Lindsey 2000; Rao, Littler, Geier and
Lindsey 2000; Cammidge and Oeztuerk 2001; Gryko and Jadach 2001; Sharada,
Muresan, Muthukumaran and Lindsey 2005)
Due to the synthetically demanding route to asymmetric porphyrins via the
2,5-Bis(2-phenyl-1,3-benzoxathiolyl)cyclohexenopyrrole (13) (Scheme 15) and
more importantly that this synthetic route does not render a easily isolatable trans-
product, a more statistical synthesis was reverted, where n = 4 – m or m = 4 – n
respectively. This synthetic approach will be from here on referred to as “statistical
synthetic procedure”.
23 24
+ N
N
H
n
m
H
N
H
N
+
O H
25
Lindsey Cond.
Scheme 17 Random synthetic approach to asymmetric porphyrins.
134
The synthesis of compounds 23, 24, the porphyrin formation reaction,
metallation and oxidation where performed as described in the experimental section
of chapter 3 and 4. The product of the above reaction was purified by the standard
methods, i.e. as in the case of the symmetric porphyrins described in Chapter 3.
This “crude” product was than analyzed by MALDI ( Figure 44a).
135
Figure 44 a) MALDI spectrum of crudely purified statistical synthetic
procedure. Red numbers identify the porphyrin. b) structures of compounds
analyzed by MALDI MS from the statistical synthetic procedure.
a)
21
27
18
18'
21'
21''
27'
18''
136
Figure 44 continued
N
N
N
N
Ph Ph
Ph Ph
18'
N
N
N
N
Ph Ph
Ph Ph
21
Pt
Chemical Formula: C
68
H
48
N
4
Pt
Exact Mass: 1115.35
Pt
N
N
N
N
Ph Ph
Ph Ph
21'
Pt
Chemical Formula: C
72
H
46
N
4
Pt
Exact Mass: 1161.34
Chemical Formula: C
72
H
42
N
4
Pt
Exact Mass: 1157.31
N
N
N
N
Ph Ph
Ph Ph
18''
Pt
Chemical Formula: C
68
H
52
N
4
Pt
Exact Mass: 1119.38
b)
137
Figure 44 Continued
N
N
N
N
Ph Ph
Ph Ph
27'
Pt
Chemical Formula: C
76
H
52
N
4
Pt
Exact Mass: 1215.38
N
N
N
N
Ph Ph
Ph Ph
27
Pt
Chemical Formula: C
76
H
44
N
4
Pt
Exact Mass: 1207.32
N
N
N
N
Ph Ph
Ph Ph
21''
Pt
Chemical Formula: C
72
H
52
N
4
Pt
Exact Mass: 1167.38
Surprisingly, the trans/cis precursor (18', 18'') are formed statistically less
frequent. Further it can be noticed that the oxidation with DDQ is impeded for the
asymmetric porphyrins compared to the symmetric homologue such as the naphtho
cont.b)
138
porphyrin. Pt(TPBP) is not formed at all in this representative sample but the other
symmetric porphyrin Pt(TPNP) 27 is formed as the major product. Further study of
this reaction must be undertaken to understand this phenomena.
Since most of the compounds in this porphyrin mixture are not fully
oxidized, another oxidation step with DDQ was performed on the same sample
under the same conditions as before. Already, the UV-VIS show major change in
terms of Q-band structure (Figure 45) compared to the pre-second oxidation step.
Figure 45 Absorption spectrum of second oxidation step (black squares),
before the second oxidation (red circles) of the statistically synthesized
asymmetric porphyrin; For comparison symmetric porphyrins Pt(TPBP)
(blue triangles) and Pt(TPNP) (green stars).
139
Analysis by MALDI confirms that the majority of the non-oxidized species in fact
were oxidized by the second DDQ oxidation (Figure 46).
Figure 46 MALDI MS spectrum of reaction mixture after the second
oxidation step using DDQ. Red numbers identify the porphyrin.
27
18''
21
140
The second oxidation oxidized the compounds 18', 21', 21'' and 27' to their
corresponding fully oxidized compounds, but the trans/cis derivative did not
undergo full oxidation of the cyclohexenopyrrole units. Due to the similarity of all
the asymmetric porphyrins in the reaction mixture, silica gel is not sufficient for
separation. High Performance Liquid Chromatography (HPLC) was used to
receive a pure sample which, than may be studied for its photophysical properties.
This same reaction mixture (after the second oxidation) was used for this separation
experiment. The mobile phase was chosen to be Methanol/Toluene mixture, which
was run with two different gradients as depicted in Figure 47a. The flow rate was
constant at 1 µl/min on a RP-18 column.
Figure 47 a) Methanol:Toluene gradients used for the HPLC of second
oxidation reaction mixture (First run (blue trace) and second run (red trace)).
b) HPLC chromatogram corresponding to the blue trace. c) HPLC
chromatorgam corresponding to the red trace.
a)
141
Figure 47 continued
b)
c)
142
The second gradient was chosen since peak C through G are more equally
distributed throughout the elution time. The eluted compound was collected by
hand and analyzed by MALDI MS (Figure 48a).
143
Figure 48 MALDI analysis of Peaks A - Peak G. Red numbers identify the
porphyrin. b) Relative intensity plot versus type of porphyrin versus elution
peak.
Peak A
144
Figure 48 continued
Peak A'
21
N
N
N
N
Ph Ph
Ph Ph
21'''
Pt
Chemical Formula: C
72
H
70
N
4
Pt
Exact Mass: 1185.52
21'''
145
Figure 48 continued
Peak B
N
N
N
N
Ph Ph
Ph Ph
20''
Pt
Chemical Formula: C
64
H
52
N
4
Pt
Exact Mass: 1071.38
20''
18
21
21''
27
21'''
146
Figure 48 continued
Peak C
18
21
147
Figure 48 continued
Peak D
18
21
20''
21'' 21'
21'''
27
148
Figure 48 continued
Peak D'
27
21
20''
149
Figure 48 continued
Peak E
20''
21
27
150
Figure 48 continued
Peak F
18''
21
27
151
Figure 48 continued
Peak F'
18''
21
27
152
Figure 48 continued
Peak G
27
21
18''
153
Figure 48 continued
As can be seen from Figure 48b), asymmetric porphyrin 21 is eluted
throughout the entire chromatogram. The desired trans/cis product becomes eluted
in the fractions ranging from B to D. Thus the photophysical properties of peak C
(containing compounds 18 and 21,
Figure 49a) and Peak E (containing compounds 21 and 27,
Figure 49b) where investigated. Peak E shall function as a standard for the
emission of 18, since this eluted sample does not contain compound 18 but includes
b)
154
27. The phosphorescence characteristics of 27 have been investigated in Chapter 3
of this work.
Figure 49 a) MALDI spectrum of elution peak C. Inset: structure of
porphyrins 18 and 21. b) MALDI spectrum of elution peak E. Inset: structure
of porphyrins 20'', 21 and 27.
Peak C
18
21
N
N
N
N
Ph Ph
Ph Ph
18
Pt
Chemical Formula: C
68
H
40
N
4
Pt
Exact Mass: 1107.29
N
N
N
N
Ph Ph
Ph Ph
21
Pt
Chemical Formula: C
72
H
42
N
4
Pt
Exact Mass: 1157.31
a)
155
Figure 49 continued
Peak E
20''
21
27
N
N
N
N
Ph Ph
Ph Ph
21
Pt
Chemical Formula: C
72
H
42
N
4
Pt
Exact Mass: 1157.31
N
N
N
N
Ph Ph
Ph Ph
27
Pt
Chemical Formula: C
76
H
44
N
4
Pt
Exact Mass: 1207.32
N
N
N
N
Ph Ph
Ph Ph
20''
Pt
Chemical Formula: C
64
H
52
N
4
Pt
Exact Mass: 1071.38
b)
156
The normalized excitation and emission spectra of peaks C&E, as well as
Pt(TPNP), are shown in Figure 50. Compound 20'' is not expected to show
phosphorescence in the near infrared region of the electromagnetic spectrum due to
the fact that the annulated benzo and naphtho derivatives are not aromatized and
hence are not participating in the electronic transition. Therefore the molecule
should behave similar to Pt(OEP) with an emission at 630 – 650 nm.(Baldo,
O'Brien, You, Shoustikov, Sibley, Thompson and Forrest 1998)
Figure 50 Excitation and emission spectra of peak C (black circles), peak E
(red triangles) and Pt(TPNP) (blue squares) at 77K in 2Me-THF.
157
The compound mixture from peak C has a comparatively broad
phosphorescence emission with λ
max
= 849 nm and the compound mixture from
peak E has a narrow emission line with λ
max
= 856 nm. Since the difference of
emission maximum is only 7 nm, a pure sample of either the trans/cis porphyrin 18
or monobenzo-tris-naphtho porphyrin 21 needs to be obtained. Thus the same
sample was subject to another HPLC separation with longer elution times, a
decrease in the flow rate from 1 µl/min to 0.750µl/min and more samples taken at
shorter intervals than during the previous experiment (Figure 51a).
Figure 51 a) Methanol:Toluene gradients used for the HPLC of the second
oxidation reaction mixture (previous conditions (red trace) and current run
(green trace)). b) HPLC chromatogram corresponding to the green trace.
a)
158
Figure 51 continued
b)
159
Figure 52 MALDI analysis of peaks A - K1. Red numbers identify the
porphyrin. b) Relative intensity plot versus type of porphyrin versus elution
peak.
Peak A
Peak A1
Peak B Peak C
21
21'
27
18''
21
21'''
27
21
18
21'''
27
160
Figure 52 continued
Peak C1
Peak D
Peak E
Peak F
18
18''
21
21'''
27
18
21
27
18
21 21
18
21'''
27
161
Figure 52 continued
Peak F1
Peak F2
Peak F3 Peak F4
20''
18
21'''
27
21
20''
27
21 21
21
20''
27
162
Figure 52 continued
Peak F5
Peak G
Peak G1
Peak G2
20''
21
27
21
27
21
27
21
20''
27
163
Figure 52 continued
Peak G3
Peak H
Peak H1 Peak I
21
20''
27
18''
21
27
27
21
18''
18''
21
27
20''
164
Figure 52 continued
Peak I1
Peak J
Peak J1
Peak K
18''
21
27
18'
21
27
18''
21
27
21
21'''
27
165
Figure 52 continued
Peak K1
21'''
21
b)
166
From the analysis of all the MALDI spectra (Figure 52b) only one fraction
collected contained a single compound (Peak F3). Unfortunately, the trans/cis
porphyrin (18) was never obtained as a single compound, due to permanent
coelution of either compounds 21 or 27. This problem can be corrected via the
identification of a more suitable column, but this was not goal of this research
project.
The photophysical properties (measured at 77K in 2Me-THF) of peak F3
are shown in
Figure 53b.
167
Figure 53 a) MALDI spectrum of elution peak F3. Inset: structure of
porphyrin 21. b) Excitation and emission spectra of peak F3 (red squares) and
Pt(TPNP) (black circles) at 77K in 2Me-THF.
N
N
N
N
Ph
Ph
Ph
Ph
21
Pt
Chemical Formula: C
72
H
42
N
4
Pt
Exact Mass: 1157.31
a)
168
Figure 53 continued
The phosphorescence emission energy of compound 21 is λ
max
= 855 nm
with a relatively narrow line shape. Since peak C (
Figure 49a) contains only compounds 18 and 21, comparing the two
emission spectra will give some indication of what part of the emission spectra is
due to the emission of Pt(TP-t-BNP) (Figure 54 a and b).
b)
169
Figure 54 a) Excitation and emission spectra of peak F3 (black squares), peak
C (blue circles) and peak E (red triangles) at 77K in 2Me-THF. b) Emission
spectra of peak F3 (black squares), peak C (blue circles), peak E (red
triangles) and Pt(TPNP) (green stars) at 77K in 2Me-THF.
b)
a)
170
Figure 54b shows that the phosphorescence emission from peak F3 and
peak E have the same maximum energy (855 nm). Comparatively, the emission
bands of peak C is slightly blue shifted, due to the contribution of compound 18 in
the sample. Therefore it may be expected that Pt(TP-t-BNP) will have a
phosphorescence emission maximum of approximately 849 nm. Therefore it is
save to assume (without further computational or experimental studies) that the
emission energy of asymmetric porphyrins does not scale proportional to the nature
and or number of electron rich substituents on the porphyrin core.
Conclusion
The rational large scale synthesis of asymmetric porphyrins was attempted in order
to provide a phosphorescence dopant for the application in OLED devices. Do to
scrambling during the porphyrin formation reaction, systematic synthesis of
different asymmetric porphyrins was not possible on large scale without extensive
work on improving the reaction conditions (i.e. scrambling minimization) and
product purification. Thus a statistical synthesis approach was chosen (Scheme 17)
along with HPLC purification (Figure 48 and Figure 52), in order to identify
specific asymmetric compounds and their corresponding phosphorescence emission
energies. From the 77k phosphorescence spectra, it can be shown that the emission
energy of a sample of trans/cis porphyrin (λ
max
= 849 nm) differs only by a few
nanometers to a mono-benzo-tris-naphthoporphyrin (λ
max
= 855 nm). Therefore it
171
is save to assume (without further computational or experimental studies) that the
emission energy of asymmetric porphyrins does not scale proportional to the nature
and or number of electron rich substituents on the porphyrin core.
172
Chapter 5 Octasubstituted Cyclooctatetraene as Host for
Blue, Green, and Red Electrophosphorescence OLED.
Abstract
Phosphorescent electroluminescent materials and devices are a prime focus
of OLED research due to their ability to efficiently utilize both singlet and triplet
excitons. The most common design for phosphorescence-based OLEDs involves a
doped emissive region, where the emissive dopant is either an Ir or a Pt complex.
While high-efficiency green and red emitting colors could be
obtained readily by
doping in the commonly used host materials,
such as tris(8-
hydroxyquinolinato)aluminum (Alq
3
), a wider band gap host is
essential for the
efficient generation of blue dopant emission. Cyclooctatetraene (COT) is a highly
interesting class of organic molecules and despite having the same (CH)
n
formulation, benzene and COT have strikingly different properties for example,
COT is a non-planar tub-shaped molecule. But, the reduction of COT is
accompanied by a structural change of the tub shaped neutral molecule to a planar
ring, which has been both studied computationally and spectroscopically. For this
reason, the geometry of the COT has to be locked into place in order to keep the
high triplet energy need for efficient energy transfer to the blue phosphorescent
dopant. This may be accomplished through the addition of bulky substituents to
the COT scaffold.
173
Introduction
Organic light-emitting diode (OLED) has
received much attention due to its
potential application in flat
panel display and large area illumination light source.
Much effort
has been devoted to developing materials and device structures to
obtain higher efficiency which is part of the need towards
commercialization.(Gao,
Mi, Chen, Cheah, Cheng and Wen 2007) Among the three principal colors
necessary for display applications,
blue-emitting materials and devices are
particularly in need of improvement
in terms of efficiency, color purity and most
importantly durability, than those of
the green and red emitters. In recent years,
developing deep
blue electroluminescence (EL) color with a Commission
Internationale de l'Eclairage
(CIE) coordinate value of <0.15, has been considered
essential as
such emitters can effectively reduce the power consumption of a
full-
color OLED panel and can also be utilized to generate
emission of other colors by
energy transfer to a matching
emissive dopant.(Lee, Chen, Liao, Tsai and Chen
2004; M.-T. Lee 2005)
Phosphorescent electroluminescent materials and devices are a prime focus
of (OLED) research due to their ability to efficiently utilize both singlet and triplet
excitons.(Baldo, O'Brien, You, Shoustikov, Sibley, Thompson and Forrest 1998)
The most common design for phosphorescence-based OLEDs involves a doped
emissive region, where the emissive dopant is either an Ir or a Pt complex. The
large spin-orbit coupling for these heavy metals leads to efficient intersystem
crossing and, hence, short emissive lifetime phosphorescence. To achieve the
174
maximum efficiency in these host-dopant OLEDs, the triplet level of the host must
not quench dopant phosphorescence.
Although many blue host materials have been reported, such as
anthracene,(Kim, Shin, Kim, Ko, Yu, Chae and Kwon 2001)
di(styryl)arylene,(Hosokawa, Higashi, Nakamura and Kusumoto 1995)
tetra(phenyl)pyrene,(C. C. Yeh, M. T. Lee, H. H. Chen and Chen 2004)
terfluorenes,(C.-C. Wu, Lin, Wong, Chen and Chien 2004) and tetra(phenyl)silyl
derivatives,(L.-H. Chan 2001; Ren, Li, Holmes, Djurovich, Forrest and Thompson
2004) blue-doped emitter systems having all the attributes of high EL efficiency,
long operational lifetime, and deep-blue color, are rare. This is because designing a
fluorescent, deep-blue dopant capable of forming an amorphous glassy state upon
thermal evaporation with a much shortened p-conjugation length is a rather
daunting task. In addition, finding a deep-blue dopant with a small Stokes shift is
essential for efficient Förster energy transfer from the host to dopant molecules,
since the energy transfer efficiency is highly dependent on the spectral overlap
between the emission of the host and the absorption of the dopant.(M.-T. Lee 2005)
While high-efficiency green and red emitting colors could be
obtained readily by
doping in the commonly used host materials,
such as tris(8-
hydroxyquinolinato)aluminum (Alq
3
), a wider band gap host is
essential for the
efficient generation of blue dopant emission.
Cyclooctatetraene (COT) is a highly interesting class of organic molecules
and despite having the same (CH)
n
formulation, benzene and COT have strikingly
175
different properties which appear to forcefully substantiate the Hückel theory,
making COT obtain its historical role in aromaticity concepts.(Wang and Xi 2007)
Unlike benzene, COT is a non-planar tub-shaped molecule having D
2d
conformation, which is more stable than the planar D
4h
and the delocalized D
8h
moieties.(Frank-Gerrit 2001; Wiberg 2001) But, the reduction of COT is
accompanied by a structural change of the tub shaped neutral molecule to a planar
ring of D
8h
symmetry which has been both studied computationally and
spectroscopically.(Wenthold, Hrovat, Weston Thatcher and Lineberger 1996; Baik,
Schauer and Ziegler 2002) This planarization phenomena may also be observed if
the molecule is excited into the triplet state. DFT calculations on the
B3LYP/LACVP** level of theory also show the planarization (Figure 55).
Figure 55 DFT calculation of the triplet excited state showing the
planarization.
176
As can be seen from the calculation, the COT molecule relaxes from this
triplet state via planarization. If on the other hand, the singlet geometry is frozen
and the triplet calculation performed one more time in this locked geometry, the
energy level of this frozen geometry is considerably higher (Figure 56).
Figure 56 DFT calculation of the triplet excited state with frozen geometry.
For this reason, the geometry of the COT has to be locked into place in
order to keep the high triplet energy need for efficient energy transfer to the blue
phosphorescent dopant. This may be accomplished through the addition of bulky
substituents to the COT scaffold. Also, to maximize the HOMO-LUMO gaps in
this material, the strategy is to electronically isolate each phenyl ring in the
structure, avoiding any conjugating substituents, which would lower the HOMO-
LUMO gap. For this reason, but more importantly solubility and thus direct
177
determination of the triplet energy in solution, the focus has been on tetra-bipheny-
tetra-phenyl-cyclooctatetraene (BPP-COT) and tetra-m-tolyl-teraphenyl-
cyclooctatetraene (mTP-COT). Further a symmetrical COT, octa-p-tolyl-
cyclooctatetraene (OT-COT), was also attempted to be synthesized. (Lu, Hong,
Cai, Djurovich, Weber and Thompson 2000)
Experimental
Unless otherwise noted, all reagents and solvents where obtained from
Sigma-Aldrich and used without any further purification. Extracts where dried
over Na
2
SO
4
and solvents were removed with a rotary evaporator at aspirator
pressure. NMR spectra where recorded on Bruker AM 250 instrument with TMS as
standard signal in either deuterated dichloromethane or deuterated DMSO.
Synthesis and Characterization
4,4′-Dimethylbenzil (1) was purchased from Sigma-Aldrich and used
without further purification and reacted with hydrazine hydrate to form the
corresponding dihydrazone. (Tsuji, Takahashi and Kajimoto 1973)
178
Synthesis of Octa-p-tolyl-cycooctatetraene (OT-COT)
COT derivatives have been easily synthesized from substituted acetylenes
via low valent metal cyclization reactions.(Wang and Xi 2007) The approach taken
here was earlier reported by Lu et.al. (Lu, Hong, Cai, Djurovich, Weber and
Thompson 2000) which has been modified in order to synthesize OT-COT(Scheme
18). OT-COT is believed to be more soluble due to the introduction of 8 methyl
groups.
1
2
O O
NH
2
-NH
2
H
2
O
N N
Hg
2
O
3
Tol
Tol
Tol
Tol Tol
Tol
Tol
Tol
4
Li / I
2
H
2
N NH
2
Scheme 18 Synthesis of octa-p-tolyl-cyclooctatetraene (OT-COT) (4).
4,4′-Ditolyl-dihydrazone (2). To a solution of 4,4′-Dimethylbenzil (1)
(0.25 mol) in n-propanol (150 ml) was added hydrazine hydrate (ca. 1mol) under
stirring, after which reaction was refluxed for 60 hours. After cooling the reaction
mixture for 2 hours in an ice bath, the product was filtered and washed with ethanol
179
affording the product in 66% yield and high purity.
1
H-NMR (250 MHz, CDCl
3
): δ
= 7.48-7.44 (m, 4H), 7.14-7.10 (m, 4H), 5.64 (s, 4H), 2.35 (s, 6H).
4,4′-Ditolyl-acetylene (3). Dihydrazone (2) (18.77 mmol) was suspended
in toluene and 1/10
th
of 41.29 mmol of mercury (II) oxide added and reluxed for 1
hour. After cooling to room temperature, the rest of the mercury oxide was added
and stirred for an additional 3 hours. The solids were filtered of and washed with
toluene, the solvents evaporated and recrystallized from ethanol affording the
product in 60% yield and high purity.
1
H-NMR (250 MHz, CDCl
3
): δ = 7.46-7.40
(m, 4H), 7.20-7.13 (m, 4H), 2.36 (s, 6H).
Octa-p-tolyl-cyclooctatetraene (4). Li metal (12.8 mmol) was sonicated in
hexanes for 10 minutes and activated by dipping it quickly into a methanol
solution, rinsed with hexanes once more and added to a solution of alkyne (3) (9.7
mmol) in freshly destilled THF (20 ml). The reaction was stirred for 3 days at
room temperature, after which the color changed to dark brown. This complex was
quenched through the addition of this solution to an iodine solution (3.5 mmol) in
freshy destilled THF (20 ml) and stirred over night. The solids where filtered off,
refluxed in THF for 2 hours, filtered, cooled and solids filtered. Unfortunately, this
reaction procedure (even after several attempts) yielded an unidentifiable reaction
mixture, most likely, consisting of oligomeric alkenes.
180
Synthesis of Tetra-biphenyl-tetraphenyl-cyclooctatetraene (BPP-
COT)
Due to the fact that the para phenyl groups unexpectedly impede the COT
formation in some fashion, an asymmetric COT moiety was thus synthesized
(Scheme 19).
5
Br
+
Pd(Cl)
2
(PPh)
2
6 i) Li ii) I
2
Ph Ph-Ph
4
4
8 (I-IV)
7
CuI / Base
Scheme 19 Synthesis of Tetra-biphenyl-tetra-phenyl-cyclooctatetraene (BPP-
COT) (8).
The first step in this reaction is the Sonogashira Coupling of commercially
available phenylacetylene (5) and 4-bromo-biphenyl (6). Next the same COT
formation reaction was followed as described in the case of Octa-p-tolyl-
cyclooctatetraene (4).
181
Biphenyl-phenyl-acetylene (7). A mixture of diisopropylamine (50 ml)
and toluene (50 ml) were degassed for 10 minutes using a stream of nitrogen. Next
phenylacetylene (5) (49 mmol), 4-bromo-biphenyl (6) (54 mmol),
Bis(triphenylphosphine)palladium(II) dichloride (1.5 mmol) and CuI (3 mmol)
were added to the solvent and refluxed over night. The reaction mixture was
cooled to room temperature, and the insoluble byproducts removed via filtration
through a with celite filled funnel. The mother liquor was reduced in volume and
the product precipitated through the addition of ethanol, yielding the product (7) in
85% yield.
1
H-NMR (250 MHz, CDCl
3
): δ = 7.63-7.52 (m, 5H), 7.49-7.40 (m,
4H), 7.39-7.31 (m, 5H).
Tetra-biphenyl-tetra-phenyl-cyclooctatetraene (8). Li metal (12 mmol)
was sonicated in hexanes for 10 minutes and activated by dipping it quickly into a
methanol solution, rinsed with hexanes once more and added to a solution of
alkyne (7) (7.9 mmol) in dry diethylether (50 ml). The reaction was stirred over
night at room temperature, after which the color changed to dark brown. This
complex was quenched through the addition of this solution to an iodine solution
(4.3 mmol) in dry diethylether (50 ml) and stirred over night. The solvent was
evaporated and the product purified via column chromatography (hexanes /
dichloromethane (1:1)) and sublimation.
1
H-NMR (250 MHz, CDCl
3
): δ = 7.90-
7.75 (m, 6H), 7.50-7.39 (m, 20H), 7.27-7.16 (m, 20H), 6.98-6.81 (m, 10H).
MADLI-TOF (M caluclated): 1017.30, (MH
+
experimental): 1018.30. DSC: 430°C
(decomposition). UV-Vis, CH
2
Cl
2
, λ
max
nm: 275.
182
Synthesis of Tetra-m-tolyl-tetraphenyl-cycooctatetraene (mTP-
COT)
Again, the first step in this reaction is the Sonogashira Coupling of
commercially available phenylacetylene (5) and m-bromotoluene (9). Next the
same COT formation reaction was followed as described in the case of Octa-p-
tolyl-cyclooctatetraene (4) (Scheme 20).
5
10
Br
+
9
i) Li ii) I
2
Ph m-Tol
4
4
11 (I-IV)
Pd(Cl)
2
(PPh)
2
CuI / Base
Scheme 20 Synthesis of Tetra-m-tolyl-tetraphenyl-cycooctatetraene (mTP-
COT) (11).
1-Methyl-3-phenylethylynyl-benzene (10). A mixture of
diisopropylamine (25 ml) and toluene (25 ml) were degassed for 10 minutes using a
stream of nitrogen. Next phenylacetylene (5) (25 mmol), m-bromotoluene (9) (27
mmol), Bis(triphenylphosphine)palladium(II) dichloride (0.7 mmol) and CuI (1.5
mmol) were added to the solvent and refluxed over night. The reaction mixture
183
was cooled to room temperature, and the insoluble byproducts removed via
filtration through a with celite filled funnel and the solvent evaporated under
reduced pressure. Water was added and the product extracted with ethyl acetate,
dried and purified via column chromatography (hexanes) yielding the desired
product (10) in 70% yield as a colorless oil.
1
H-NMR (250 MHz, CDCl
3
): δ =
7.70-7.67 (m, 2H), 7.52-7.49 (m, 2H), 7.36 (t, 1H, J = 7.5 Hz), 7.26 (d, 1H, J = 7.6
Hz), 2.47 (s, 3H).
Tetra-m-tolyl-tetraphenyl-cycooctatetraene (11). Li metal (37 mmol) was
sonicated in hexanes for 10 minutes and activated by dipping it quickly into a
methanol solution, rinsed with hexanes once more and added to a solution of
alkyne (10) (28 mmol) in dry diethylether (15 ml). The reaction was stirred over
night at room temperature, after which the color changed to dark brown. This
complex was quenched through the addition of this solution to an iodine solution
(10 mmol) in dry diethylether (15 ml) and stirred over night. The product never
precipitated due to the formation of oligo-type compounds, which are indicated by
a very broad aromatic signal in the
1
H-NMR.
184
Synthesis of Tetra-1-naphthyl-tetraphenyl-cycooctatetraene (mTP-
COT)
Tthe Sonogashira Coupling of commercially available phenylacetylene (5)
and 1-naphthalene (12) was used to synthesize the asymmetric acetylene. Next the
same COT formation reaction was followed as described in the case of Octa-p-
tolyl-cyclooctatetraene (4) (Scheme 21).
5
Br
+
Pd(Cl)
2
(PPh)
2
12
i) Li ii) I
2
Ph Naphthyl
4
4
14 (I-IV)
13
CuI / Base
Scheme 21 Synthesis of Tetra-1-naphthyl-tetraphenyl-cycooctatetraene (NP-
COT) (14).
1-naphthal-phenyl-acetylene (13). A mixture of diisopropylamine (25 ml)
and toluene (25 ml) were degassed for 10 minutes using a stream of nitrogen. Next
phenylacetylene (5) (25 mmol), 1-bromnaphthalene (12) (27 mmol),
Bis(triphenylphosphine)palladium(II) dichloride (0.7 mmol) and CuI (1.5 mmol)
were added to the solvent and refluxed over night. The reaction mixture was
cooled to room temperature, and the insoluble byproducts removed via filtration
185
through a with celite filled funnel and the solvent evaporated under reduced
pressure. Water was added and the product extracted with ethyl acetate, dried and
purified via column chromatography (hexanes) yielding the desired product (13) in
70% yield as a colorless oil.
1
H-NMR (250 MHz, CDCl
3
): δ = 8.43 (t, 1H, J = 7.34
Hz), 7.84 (q, 2H, J = 7.31 Hz), 7.76 (d, 1H, J = 7.22 Hz), 7.68-7.61 (m, 4H), 7.58-
7.41 (m, 4H).
Tetra-1-naphthyl-tetraphenyl-cycooctatetraene (14). Li metal (37 mmol)
was sonicated in hexanes for 10 minutes and activated by dipping it quickly into a
methanol solution, rinsed with hexanes once more and added to a solution of
alkyne (13) (28 mmol) in dry diethylether (15 ml). The reaction was stirred over
night at room temperature, after which the color changed to dark brown. This
complex was quenched through the addition of this solution to an iodine solution
(10 mmol) in dry diethylether (15 ml) and stirred over night. The product never
precipitated due to the formation of oligo-type compounds, which are indicated by
a very broad aromatic signal in the
1
H-NMR.
Density Functional Calculations
DFT calculations were performed using a Titan software package
(Wavefunction, Inc.) at a B3LYP/6-31G* and Semi-Emperical/PM3 level of
theory. The HOMO and LUMO energies were determined using minimized singlet
geometries to approximate the ground state. The minimized singlet geometries
186
were than used to calculate the triplet molecular orbitals and approximate the triplet
HSOMO (HSOMO = highest singly occupied molecular orbital) for the cases of the
COT core. The triplet equilibrium geometries of the asymmetric substituted COTs
were calculated on a Semi-Empirical/PM3 level of theory, by taking the triplet
COT optimized structure and adding the substituents before the calculation.
Normal coordinate structural decomposition (NSD) analysis
NSD analysis of the asymmetrical COTs was accomplished using the web-based
NSD program (http://jasheln.unm.edu/). The results of the complete NSD
decomposition are given below.
NSD result generated from file BPP-COT-sp2-trip-pm3 wthout BPP.pdb at Wed Jun 04 16:54:10
PDT 2008
Summary of the NSD (in A):
basis Dip dip B2g B1g Eu(x) Eu(y) A1g A2g
min. 14.6449 1.4192 -0.5836 -0.5525 0.5277 0.1558 -14.5734 1.0679
ext. 14.8071 1.3999 -0.5807 -0.5685 0.5188 0.1747 -14.5719 1.0672
0.4176 -0.8484 -0.8144 1.7232 0.4954 -0.0722
comp. 16.6083 0.0002 2.1075 1.2047 2.1629 4.8052 15.3884 2.3179
basis Doop doop B2u B1u A2u Eg(x) Eg(y) A1u
min. 1.6302 0.7898 -1.0307 -1.0853 -0.0147 -0.0353 -0.0494 -0.6431
ext. 2.3347 0.7127 -1.0499 -1.0853 0.0041 -0.0467 -0.0871 -0.6431
-0.9977 -0.2212 0.4364 -0.3054 -1.0086 -0.6711
comp. 5.0943 0.0006 2.8522 1.2371 0.4433 0.9126 3.7938 0.9294
187
NSD result generated from file COT-sp2-trip-dft.pdb at Wed Jun 04 17:04:41 PDT 2008
Summary of the NSD (in A):
basis Dip dip B2g B1g Eu(x) Eu(y) A1g A2g
min. 12.9652 1.5806 0.3878 -0.8372 0.6746 -0.5744 -12.8912 -0.5279
ext. 13.1778 1.5145 0.3861 -0.8113 0.6848 -0.5911 -12.8905 -0.5221
-0.2586 1.3720 0.9258 -1.5195 0.2499 0.6153
comp. 15.2924 0.0004 1.7274 2.5072 4.1024 2.7251 14.0563 1.6593
basis Doop doop B2u B1u A2u Eg(x) Eg(y) A1u
min. 1.9715 0.6308 -0.3271 -1.7415 0.5054 0.6894 -0.0985 -0.0828
ext. 2.4826 0.6338 -0.3397 -1.7415 0.5511 0.7031 -0.1013 -0.0828
-0.6537 0.7612 1.0603 0.3668 -0.0753 -0.0864
comp. 4.6799 0.0003 0.7853 3.4495 1.1763 2.2943 1.6504 0.1197
NSD result generated from file NP-COT-triplet-pm3-without.pdb at Thu Jun 12 13:53:38 PDT
2008
Summary of the NSD (in A):
basis Dip dip B2g B1g Eu(x) Eu(y) A1g A2g
min. 13.1589 1.4692 1.0494 0.8298 -1.3308 -0.1943 -13.0045 0.6647
ext. 13.2255 1.4635 1.0491 0.8388 -1.3183 -0.1905 -13.0037 0.6667
-0.0453 0.4765 1.1354 0.3512 0.2609 0.2179
comp. 15.2575 0.0001 1.5100 1.7527 3.9646 1.8545 14.2556 2.2499
basis Doop doop B2u B1u A2u Eg(x) Eg(y) A1u
min. 2.6597 0.6539 0.0734 -2.2017 -0.7433 -0.4023 -0.2624 -1.1990
ext. 3.1090 0.6361 0.0816 -2.2017 -0.7788 -0.4021 -0.2761 -1.1990
0.4281 0.1809 -0.8229 0.0064 -0.3668 -1.2513
comp. 4.9763 0.0005 0.5570 3.1483 1.1089 1.6378 2.7615 1.7330
Spectroscopic Measurements
Absorption spectra were recorded with an Agilent 8453 UV-Visible
spectrometer and corrected for solvent absorption in the background. Emission
spectra were recorded on a PTI QuantaMaster Model C-60SE spectrofluorometer
with a 928 PMT detector corrected for detector sensitivity. Phosphorescence
lifetimes were measured with the same detector as mentioned above, by
observation of the phosphorescence emission at 77K in 2-Me-THF.
188
Results and Discussion
Assuming that the original connectivity of the acetylene starting material is
maintained, four isomeric structures are possible as depicted in Figure 57.
I
A B
A
B
A B
A
B
II
A A
B
B
A A
B
B
III
A A
B
A
B A
B
B
IV
A A
B
A
B B
A
B
Figure 57 Possible isomeric structures of asymmetric COT.
Since COTs are not planar but instead have a tub-shaped geometry,
compounds I and II have D
2
symmetry, III has C
1
and isomer IV C
2
symmetry and
is the preferred product and main product as described by Lu et.al. (Lu, Hong, Cai,
Djurovich, Weber and Thompson 2000). Unfortunately, the proton NMR of the
asymmetric COTs (8 and 11) is very ambiguous, thus assumption was made that
these asymmetrical COTs will have the same conformation as described by
compound IV in Figure 57.
189
To understand how the substituents effect the geometry of the COT core,
normal-coordinate structural decomposition (NSD) software(Jentzen, Song and
Shelnutt 1997)
,
(http://jasheln.unm.edu/jasheln/content/nsd/nsd__welcome.htm) was
used to quantify the boat depth and modes of deformation from planarity on
unsubstituted COT, BPP-COT and NP-CO. The results depicted in Figure 58.
190
Figure 58 NSD analysis of a) COT core, b) NP-COT and c) BPP-COT. The
substituents have been removed for clarity. B
2u
= saddling, B
1u
= ruffling, A
2u
= doming, E
g(x)
= wave in x axis, E
g(y)
= wave in y axis, A
1u
= propellering.
a)
b)
c)
191
The total out-of-plane distortion (D
oop
) and other major deformation modes
of these selected COTs is tabulated in Table 5.
COT [Å] NP-COT [Å] BPP-COT [Å]
D
oop
2.48 3.11 2.33
B
2u
-0.34 0.082 -1.05
B
1u
-1.74 -2.201 -1.085
A
2u
0.55 -0.778 0.0041
A
1u
-0.083 -1.199 -0.64
Table 5 NSD results of COT, OP-COT and BPP-COT. D
oop
= total out of
plane distortion, B
2u
= saddling, B
1u
= ruffling, A
2u
= doming, A
1u
=
propellering.
Not surprisingly the NP-COT has the deepest boat/saddle conformation and
thus is expected to have a triplet energy higher than its corresponding homologues.
BPP-COT, most likely due to the asymmetry, shows a large contribution of A
1u
breathing mode (propellering), which can also be seen in the calculated structure
(Figure 58).
192
Photophysical analysis of BPP-COT at 77K in 2-Me-THF shows a
phosphorescence emission at 525 nm with a lifetime τ = 1.3 sec (Figure 59).
Figure 59 Absorption (black open squares) and emission (blue open triangle)
spectrum of BPP-COT at 77K in 2-Me-THF with a phosphorescence emission
(red open circles) at 525 nm.
At closer investigation of the absorption spectrum, a minute absorption can
be seen at 380 nm, which is attributed to a fluorescence impurity not detectable by
means of NMR. Thus the result of an excitation at 380 nm, is an emission at 430
nm which has no contribution to the phosphorescence emission at 525 nm (Figure
60).
193
Figure 60 Fluorescence emission of impurity (brown open diamond) and
enlarged view (green open stars).
This result indicates that the bulky substituents around the COT core, only
slightly impedes the energy loss of the triplet through geometrical changes. Due to
the fact that the triplet energy of BPP-COT is in the green part of the
electromagnetic spectrum, only red and yellow phosphorescence dopants may be
incorporated into this host with efficient energy transfer to the dopant.
194
Conclusion
Photophysical investigation of substituted cyclooctatetraene (COT)
derivatives was undertaken to determine their triplet energy, in order to evaluate the
possible application as host materials for blue, green or red phosphorescent
dopants. The substituents of the COT core where chosen such as to a) increase the
solubility of the inherently non-soulable core and b) to minimize planarization
effects, which is one of the mechanism by which unsubstituted COT moieties relax
into the ground state. The geometry of tetra-biphenyl-tetraphenyl-
cyclooctatetraene (BPP-COT) was calculated on a PM3 level of theory and the boat
depth determined by NSD analysis and found to be 2.33 Å which is 0.78 Å less
than the boat depth calculated for tetra-1-naphthyl-tetraphenyl-cycooctatetraene
(NP-COT) on the same level of theory. The 77K emission spectrum of BPP-COT
show a phosphorescence emission centered 525 nm with a lifetime of τ = 1.3 sec.
Therefore this host would not be used for blue phosphorescent dopants, but rather
would find application for dopants emitting in the red and yellow part of the
electromagnetic spectrum.
195
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Abstract (if available)
Abstract
This dissertation describes the development of interesting materials for different organic layers, which compose the standard organic light emitting diode (OLED) architecture.
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Asset Metadata
Creator
Borek, Carsten (author)
Core Title
New materials for organic light emitting diodes
School
College of Letters, Arts and Sciences
Degree
Doctor of Philosophy
Degree Program
Chemistry
Publication Date
08/05/2008
Defense Date
06/20/2008
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
OAI-PMH Harvest,organic light emitting diodes,organometallic chemistry
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Thompson, Mark E. (
committee chair
), Chongwu, Zhou (
committee member
), Haw, James (
committee member
)
Creator Email
borek@usc.edu,carsten@redhotchiliputters.com
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-m1546
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UC1125383
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etd-Borek-2138 (filename),usctheses-m40 (legacy collection record id),usctheses-c127-111873 (legacy record id),usctheses-m1546 (legacy record id)
Legacy Identifier
etd-Borek-2138.pdf
Dmrecord
111873
Document Type
Dissertation
Rights
Borek, Carsten
Type
texts
Source
University of Southern California
(contributing entity),
University of Southern California Dissertations and Theses
(collection)
Repository Name
Libraries, University of Southern California
Repository Location
Los Angeles, California
Repository Email
cisadmin@lib.usc.edu
Tags
organic light emitting diodes
organometallic chemistry