Polymers with carbazole-oxadiazole side chains as ambipolar hosts for phosphorescent light-emitting diodes

Article abstract of DOI:10.1021/cm201562p

Polymethacrylates, polystyrenes, and polynorbornenes bearing 2-(3-(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole, 2-(4-(carbazol-9-yl) phenyl)-5-phenyl-1,3,4-oxadiazole, 2-(3,5-di(carbazol-9-yl)phenyl)-5-phenyl-1,3, 4-oxadiazole) groups linked to the po

Full text of DOI:10.1021/cm201562p

ARTICLE
pubs.acs.org/cm
Polymers with Carbazole-Oxadiazole Side Chains as Ambipolar Hosts
for Phosphorescent Light-Emitting Diodes
Yadong Zhang,†,‡ Carlos Zuniga,†,‡ Sung-Jin Kim,†,§ Dengke Cai,†,§ Stephen Barlow,†,‡ Seyhan Salman,†,‡
Veaceslav Coropceanu,†,‡ Jean-Luc Brꢀedas,†,‡ Bernard Kippelen,†,§ and Seth Marder*,†,‡
†Center for Organic Photonics and Electronics (COPE), ‡School of Chemistry and Biochemistry, and §School of Electrical and
Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States
S Supporting Information
b
ABSTRACT: Polymethacrylates, polystyrenes, and polynorbornenes bearing 2-(3-(carbazol-9-yl)-
phenyl)-5-phenyl-1,3,4-oxadiazole, 2-(4-(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole, 2-(3,5-di-
(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole) groups linked to the polymer backbone through
the 3-position of their terminal phenyl groups have been synthesized for use as solution-processable
ambipolar hosts for phosphorescent light-emitting diodes. The polymers exhibit good thermal
stabilities, with no weight loss below 350 °C, and have glass-transition temperatures in the range
118ꢀ209 °C. Spectroscopic, electrochemical, and quantum-chemical data for small-molecule model
compounds suggest that the side chain groups are suitable hosts for a range of phosphors, including
0
the green-emitter fac-tris(2-phenylpyridinato-N,C2 )iridium (Ir(ppy)3). Light-emitting diodes were fabricated, each with a solution-
processed photo-cross-linked hole-transport layer, a solution-processed emissive layer composed of a carbazole-oxadiazole-function-
alized polymethacrylate doped with Ir(ppy)3, and an evaporated electron-transport layer. The polymer with 2-(3,5-(dicarbazol-9-yl)-
phenyl)-5-phenyl-1,3,4-oxadiazole units in the side chain exhibited the lowest turn-on voltage (6.0 V), and the highest efficiency
(external quantum efficiency of 10.0%, current efficiency of 34.1 cd/A).
KEYWORDS: oxadiazole, carbazole, light-emitting diode, phosphorescence, side-chain polymer, triplet energy
’ INTRODUCTION
hole-transporting donor and electron-transporting acceptor
functionalities, either in the same molecule, in the same polymer,
or in blends of different materials. To achieve efficient energy
transfer from the host to the guest, and to prevent back energy
transfer from guest to host, the singlet and triplet excited-state
energies of the host should generally be higher than the
respective energies for the guest,23 placing significant constraints
on the design of ambipolar host materials; care must be taken
that charge-transfer-type states associated with donorꢀacceptor
interactions are sufficiently high in energy for compatibility with
the phosphor of interest. However, many hosts that function well
as hosts for red, green, and, in some cases, blue-green phosphors
have been developed based on carbazole donors and oxadiazole
acceptors: these include blends of poly(N-vinylcarbazole) with
small-molecule or polymeric oxadiazoles,19,20,22,24ꢀ26 copoly-
mers of carbazole, and oxadiazole-containing monomers (in
some cases also incorporating the phosphor as a monomer),27
main-chain carbazole polymers with side-chain oxadiazoles,28
and small molecules in which the carbazoles are linked to
oxadiazole through various conjugated or nonconjugated brid-
ging groups.29ꢀ33 In addition, some carbazole-oxadiazole mate-
rials have been used as fluorophores or hosts for other longer-
wavelength emitting fluorophores in fluorescent OLEDs.34ꢀ36
Phosphorescent organic light-emitting diodes (OLEDs)
are theoretically capable of luminescence with internal quan-
tum efficiencies of up to 100%, fully harvesting both the
singlet and the triplet excitons created by holeꢀelectron
recombination events; they have, therefore, attracted much
attention for their potential applications in color displays and
solid-state lighting,1ꢀ12 largely stimulated by Forrest and
Thompson’s demonstration of efficient phosphorescent
OLEDs using iridium(III)-based phosphors.13 To minimize
self-quenching associated with tripletꢀtriplet annihilation,2,14 the
phosphorescent emitter is typically used at a relatively low
concentration as a guest in a charge-transporting host material
(alternative approaches have been, however, used in which self-
quenching is minimized by dendronization of the phosphor-
escent center15).
Although host materials with either predominantly hole- or
electron-transporting properties have been widely used, ambi-
polar hosts have potential advantages arising from more balanced
injection and transport of holes and electrons. For example, more
balanced carrier injection and transport leads to a broader
recombination zone within the emissive layer, with a concomi-
tant reduction of the local density of phosphor excitons and,
therefore, of the rate of tripletꢀtriplet annihilation and other
triplet quenching processes.16,17 In many cases the use of
ambipolar host materials permits the fabrication of efficient
OLEDs with relatively simple device architectures.18ꢀ22 One
approach to the design of ambipolar hosts is to combine known
Received: June 2, 2011
Revised:
August 2, 2011
Published: August 19, 2011
r
2011 American Chemical Society
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Figure 1. Carbazole-oxadiazole motifs used in this work with (a) meta- and (b) para-phenylene linkages between single carbazole and oxadiazole
moieties, and (c) meta-linkages between two carbazoles and an oxadiazole.
We have been interested in the side-chain polymer approaches
to OLEDs that potentially combine facile solution processability
with morphological stability and with control of the optical and
electronic properties through the choice of side chain. In this
paper, we report the preparation of homopolymers based on
polymethacrylate, polystyrene, and polynorbornene backbones
and bearing pendant carbazole-oxadiazole transport functional-
ities. 2-(3-(Carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole, 2-
(4-(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole, and 2-(3,5-
di(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole groups (Figure 1a,
b, and c, respectively) were chosen asambipolar transportmoieties
sincesimilarstructuresstudiedintheliterature(IVꢀVIin Figure3,
see below) have been shown to have triplet energies suitable for
use with green and blue-green phosphors.31,33 Alkoxy groups were
used to link the polymerizable groups to the terminal phenyl
groups, a meta-linkage being chosen to avoid the π-donating
alkoxy substituent significantly influencing the spectroscopic or
electrochemical properties of the carbazole-oxadiazole moieties.
The optical and electrochemical properties of the polymers have
been assessed using the corresponding methoxy derivatives of the
carbazole-oxadiazole groups as model compounds, while phos-
glassy carbon electron was used as the working electrode a platinum wire
was used as an auxiliary electrode, and a silver wire anodized with AgCl
was used as a pseudoreference electrode. Potentials were referenced to
ferrocenium/ferrocene (FeCp2+/0) by addition of ferrocene to the cell.
All polymerizations were carried out under a nitrogen atmosphere
using Schlenk techniques. The solvents for radical polymerizations were
purified by reflux over and distillation from CaH2. AIBN was purified by
recrystallization from ethanol. 2-Bromoethyl methacrylate,37 5-(5-
bromopentyl)bicyclo[2,2,1]hept-2-ene (mixture of endo- and exo-
isomers),38 methyl 3-iodobenzoate (1),39 methyl 3,5-diiodobenzoate
(3),40 (5-norbornen-2-yl)methyl p-tolenesulfonate,41 and Poly-TPD-
F42,43 were synthesized according to the published procedures. THF
used for free-radical polymerization was purified by reflux over, and
distillation from, sodium wire. Dichloromethane for the ring-opening
metathesis polymerization was obtained from an MBRAUN MB-SPS
solvent purification system. Other reagents and solvents were used as
received without further purification unless otherwise stated.
3-Iodobenzohydrazide (4). To methyl 3-iodobenzoate, 1 (25.0 g,
95.4 mmol) in ethanol (120 mL), was added hydrazine hydrate (50 mL).
The reaction mixture was heated to reflux for 18 h. After cooling to room
temperature, water (300 mL) was added. The white solid that formed was
collected by filtration. The product was washed with water and dried
under vacuum to give a white solid, 23.0 g (92%). 1H NMR (400 MHz,
DMSO-d6) δ: 9.85 (s, br, 1H, NH), 8.14 (t, J = 1.6 Hz, 1H), 7.85 (dt, J =
8.0, 1.6 Hz, 1H), 7.82 (dt, J = 8.0, 1.6 Hz, 1H), 7.25 (t, J = 8.0 Hz, 1H),
4.50 (s, 2H, NH2). 13C{1H} NMR (100 MHz, DMSO-d6) δ: 164.73,
140.06, 135.94, 135.76, 130.98, 126.75, 95.10. MS(EI): m/z 262 (M+).
Anal. Calcd for C7H7IN2O: C, 32.08; H, 2.69; N, 10.69. Found: C, 32.23;
H, 2.61; N, 10.75.
phorescent OLEDs have been fabricated using the three metha-
0
crylate polymers as hosts and fac-tris(2-phenylpyridinato-N,C2 )
iridium (Ir(ppy)3) as a guest phosphor.
’ EXPERIMENTAL SECTION
1H NMR and 13C{1H} NMR spectra were recorded on a 400 MHz
Varian Mercury spectrometer and were referenced to Me4Si using the
residual solvent proton and carbon signals. Elemental analyses results
were performed by Atlantic Microlab Inc. Mass spectra (EI) and
(MALDI) were recorded on a Micromass Autospace mass spectrometer
and Applied Biosystems 4700 Proteomics Analyzer mass spectrometer,
respectively. The molecular weights of the polymers were estimated in
tetrahydrofuran (THF) by using gel permeation chromatography using
a Waters 1515 pump, a Waters 2489 UVꢀvis detector, a Styragel HR 5E
THF 4.6 ꢁ 300 mm column, and linear poly(styrene) standards.
Differential scanning calorimetry (DSC) data was obtained using a TA
DSCQ200 at a heating rate of 10 °C/min from 40 to 300 °C under a
nitrogen atmosphere. Thermogravimetric analysis (TGA) was per-
formed on a NETZSCH STA 449C instrument at a heating rate of
20 °C/min from 20 to 500 °C. UVꢀvis absorption spectra were
measured on a Hewlett-Packard 8453 spectrometer. Fluorescence
spectra were recorded in dichloromethane on a Fluorolog III ISA
spectrofluorimeter. The phosphorescence spectrum for 15 was recorded
in 2-methyltetrahydrofuran solution at 77 K using a HORIBA Jobin
Yvon Fluoromax-4P spectrofluorimeter at an excitation wavelength of
300 nm. Solution cyclic voltammetry (CV) was performed on a CH
Instruments electrochemical workstation at room temperature in nitro-
gen-purged anhydrous THF with 0.1 M tetrabutylammonium hexa-
fluorophosphate as a supporting electrolyte at scan rate of 50 mV/s. A
4-Iodobenzohydrazide (5). Compound 5 (26.0 g, 87%) was
synthesized from methyl 4-iodobenzoate, 2 (30.0 g, 114 mmol), using a
1
procedure analogous to that for compound 4. H NMR (400 MHz,
DMSO-d6) δ: 9.84 (s, 1H, NH), 7.82 (d, J = 8.4 Hz, 2H), 7.59 (d, J = 8.4
Hz, 2H), 4.50 (s, 2H, NH2). 13C{1H} NMR (100 MHz, DMSO-d6) δ:
165.62, 137.65, 133.15, 129.37, 99.05. MS(EI): m/z 262 (M+). Anal.
Calcd for C7H7IN2O: C, 32.08; H, 2.69; N, 10.69. Found: C, 32.30; H,
2.64; N, 10.69.
3,5-Diiodobenzohydrazide (6). Compound 6 (4.6 g, 92%) was
synthesized from methyl 3,5-diiodobenzoate, 3 (5.0 g, 13 mmol), using a
1
procedure analogous to that for compound 4. H NMR (400 MHz,
DMSO-d6) δ: 9.92 (s, 1H, NH), 8.22 (t, J = 1.6 Hz, 1H), 8.12 (d, J =
1.6 Hz, 2H), 4.53 (s, 2H, NH2). 13C{1H} NMR (100 MHz, DMSO-d6)
δ: 163.26, 146.96, 137.04, 135.37, 96.48. MS(EI): m/z 388 (M+). Anal.
Calcd for C7H6I2N2O: C, 21.67; H, 1.56; N, 7.22. Found: C, 21.88; H,
1.44; N, 7.23.
3-Iodo-N0-(3-methoxybenzoyl)benzohydrazide (7). To a
solution of 4 (10.0 g, 38.2 mmol) in a mixture of dry THF (100 mL)
and dimethylformamide (DMF, 10 mL) was slowly added 3-methoxy-
benzoyl chloride (7.0 g, 42 mmol) at 0 °C under nitrogen, during which
time a white solid appeared. After the addition was complete, the
reaction mixture was allowed to warm to room temperature and then
stirred for 19 h at room temperature. Pyridine (20 mL) was added, and
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the mixture was stirred for an additional 45 min. Water (300 mL) was
then added to the reaction mixture; the white solid that formed was
collected by filtration and washed with water, recrystallized from
methanol, and dried under vacuum to give compound 7 as a white
MS(EI): m/z 378 (M+). Anal. Calcd for C15H11IN2O2: C, 47.64; H,
2.93; N, 7.41. Found: C, 47.77; H, 2.81; N, 7.18
2-(3,5-Diiodophenyl)-5-(3-methoxyphenyl)-1,3,4-oxadia-
zole (12). Compound 12 was obtained as a white solid (3.4 g, 71%)
from 9 (5.0 g, 9.6 mmol) by a procedure analogous to that used for
compound 10 (using the same eluent as for 10). 1H NMR (400 MHz,
CDCl3) δ: 8.43 (d, J = 1.6 Hz, 2H), 8.23 (t, J = 1.6 Hz, 1H), 7.71 (d, J =
8.0 Hz, 1H), 7.66 (m, 1H), 7.46 (t, J = 8.0 Hz, 1H), 7.12 (dd, J = 8.0, 2.4
Hz, 1H), 3.92 (s, 3H, OCH3). 13C{1H} NMR (100 MHz, CDCl3) δ:
165.06, 161.67, 159.98, 148.05, 134.63, 130.31, 126.95, 124.44, 119.44,
118.56, 111.66, 94.98, 55.59. MS(EI): m/z 504 (M+). Anal. Calcd for
C15H10I2N2O2: C, 35.74; H, 2.00; N, 5.56. Found: C, 35.58; H, 1.92;
N, 5.60.
1
powder in 12.4 g (82%). H NMR (400 MHz, DMSO-d6) δ: 10.62
(s, 1H, NH), 10.56 (s, 1H, NH), 8.25 (t, J = 1.6 Hz, 1H), 7.96 (d, J =8.0 Hz,
1H), 7.91 (d, J = 8.0 Hz, 1H), 7.49 (d, J = 8.0 Hz, 1H), 7.45 (d, J = 2.4 Hz,
1H), 7.41 (d, J = 8.0 Hz, 1H), 7.34 (t, J = 8.0 Hz, 1H), 7.16 (dd, J = 8.0,
2.4 Hz, 1H), 3.81 (s, 3H, OCH3). 13C{1H} NMR (100 MHz, DMSO-
d6) δ: 166.00, 164.92, 159.67, 140.89, 136.37, 135.00, 134.24, 131.21,
130.17, 127.33, 120.17, 118.27, 113.00, 95.22, 55.78 ppm. MS(MALDI):
m/z 397 (MH+). Anal. Calcd for C15H13IN2O3: C, 45.47; H, 3.31; N,
7.07. Found: C, 45.24; H, 3.30; N, 6.96.
4-Iodo-N0-(3-methoxybenzoyl)benzohydrazide (8). Com-
pound 8 was obtained as a white powder (7.2 g, 95%) from 5 (5.0 g,
19 mmol) and 3-methoxybenzoyl chloride (3.5 g, 21 mmol) using a
procedure analogous to that used for compound 7. 1H NMR (400 MHz,
DMSO-d6) δ: 10.60 (s, 1 H, NH), 10.52 (s, 1 H, NH), 7.91 (d, J =
8.0 Hz, 2H), 7.68 (d, J = 8.0 Hz, 2H), 7.50 (d, J = 8.0 Hz, 1H), 7.45 (d, J =
2.4 Hz, 1H), 7.41 (d, J = 8.0 Hz, 1H), 7.15 (dd, J = 8.0, 2.4 Hz, 1H), 3.81
(s, 3H, OCH3). 13C{1H} NMR (100 MHz, DMSO-d6) δ: 166.00,
165.74, 159.67, 137.92, 134.28, 132.42, 130.17, 129.80, 120.14, 118.25,
112.97, 100.10, 55.77. MS(MALDI): m/z 397 (MH+). Anal. Calcd for
C15H13IN2O3: C, 45.47; H, 3.31; N, 7.07. Found: C, 45.35; H, 3.39;
N, 6.95.
2-(3-(Carbazol-9-yl)phenyl)-5-(3-methoxyphenyl)-1,3,4-ox-
adiazole (13). A mixture of 10 (3.0 g, 7.9 mmol), carbazole (1.5 g, 9.0
mmol), Cu (2.0 g, 32 mmol), and potassium carbonate (4.0 g, 29 mmol)
in DMF (20.0 mL) was stirred at 150 °C for 5 h. After allowing it to cool
to room temperature, the reaction mixture was diluted with THF
(100 mL) and filtered. The solid residues were carefully washed with
THF. The combined THF extracts were evaporated, and water
(150 mL) was added; the resulting brown solid was collected by
filtration. The crude product was purified by column chromatography
on silica gel using dichloromethane/ethyl acetate (19:1) as eluent. The
white solid obtained after evaporating the solvent was recrystallized from
acetone/methanol and finally dried under vacuum to give 13 as a white
solid in 3.15 g (95%) yield. 1H NMR (400 MHz, CDCl3) δ: 8.34 (t, J =
1.6 Hz, 1H), 8.27 (m, 1H), 8.18 (dt, J = 8.0, 0.8 Hz, 2H), 7.78 (m, 2H),
7.71ꢀ7.65 (m, 2H), 7.46ꢀ7.41 (m, 5H), 7.36ꢀ7.31 (m, 2H), 7.09 (dd,
J = 8.0, 1.2 Hz, 1H), 3.89 (s, 3H, OCH3). 13C{1H} NMR (100 MHz,
CDCl3) δ: 164.85, 163.84, 159.96, 140.60, 138.70, 130.85, 130.32,
130.25, 126.19, 125.84, 125.40, 124.72, 123.55, 120.45, 120.37, 119.39,
118.44, 111.52, 109.55, 55.55 (1 resonance not observed, presumably
because of overlap). MS(EI): m/z 417 (M+). Anal. Calcd for
C27H19N3O2: C, 77.68; H, 4.59; N, 10.07. Found: C, 77.47; H, 4.51;
N, 10.06.
3,5-Diiodo-N0-(3-methoxybenzoyl)benzohydrazide (9).
Compound 9 was obtained as white powder (5.05 g, 83%) from 6
(4.5 g, 12 mmol) and 3-methoxybenzoyl chloride (2.2 g, 13 mmol) using
1
a procedure analogous to that used for compound 7. H NMR (400
MHz, DMSO-d6) δ: 10.72 (s, 1H, NH), 10.62 (s, br, 1H, NH), 8.33
(t, J = 1.6 Hz, 1H), 8.23 (d, J = 1.6 Hz, 2H), 7.50ꢀ7.41 (m, 3H), 7.15 (m,
1H), 3.81 (s, 3H, OCH3). 13C{1H} NMR (100 MHz, DMSO-d6) δ:
165.91, 163.57, 159.68, 147.82, 136.23, 135.82, 134.11, 130.18, 120.17,
118.33, 112.99, 96.67, 55.79. MS(MALDI): m/z 523 (MH+). Anal.
Calcd for C15H12I2N2O3: C, 34.51; H, 2.32; N, 5.37. Found: C, 34.78;
H, 2.25; N, 5.42.
2-(4-(Carbazol-9-yl)phenyl)-5-(3-methoxyphenyl)-1,3,4-ox-
adiazole (14). Compound 14 was obtained as a white solid (3.2 g,
97%) from 11 (3.0 g, 7.9 mmol) and carbazole (1.5 g, 8.97 mmol) by a
2-(3-Iodophenyl)-5-(3-methoxyphenyl)-1,3,4-oxadiazole
(10). Compound 7 (11.0 g, 27.8 mmol) was suspended in POCl3
(60.0 mL), and the reaction mixture was stirred at 100 °C. During
heating the white solid starting materials dissolved to give a clear
solution. After 2 h the reaction mixture was brought to room tempera-
ture and was carefully added to iceꢀwater (1000 mL). The white solid
that precipitated was collected by filtration, washed with water, and
dried. The crude product was purified by column chromatography on
silica gel, eluting with dichloromethane/ethyl acetate (19:1). The solid
obtained after evaporation of the solvents was recrystallized from
acetone/water and then dried under vacuum to give 10 as white solid
(6.4 g, 61%). 1H NMR (400 MHz, CDCl3) δ: 8.47 (t, J = 1.6 Hz, 1H),
8.12 (dt, J = 8.0, 1.6 Hz, 1H), 7.88 (m, 1H), 7.70 (dt, J = 8.0, 1.6 Hz, 1H),
7.67 (m, 1H), 7.45 (t, J = 8.0 Hz, 1H), 7.28 (t, J = 8.0 Hz, 1H), 7.10 (m,
1H), 3.92 (s, 3H, OCH3). 13C{1H} NMR (100 MHz, CDCl3) δ: 164.79,
163.07, 159.94, 140.58, 135.46, 130.68, 130.26, 126.03, 125.68, 124.69,
119.36, 118.36, 111.59, 94.39, 55.56. MS(EI): m/z 378 (M+). Anal.
Calcd for C15H11IN2O2: C, 47.64; H, 2.93; N, 7.41. Found: C, 47.65; H,
2.85; N, 7.54.
2-(4-Iodophenyl)-5-(3-methoxyphenyl)-1,3,4-oxadiazole
(11). Compound 11 was obtained as a white solid (5.8 g, 87%) from 8
(7.0 g, 18 mmol) by a procedure analogous to that used for compound
10, differing only in the eluent used for column chromatography (25:1
dichloromethane/ethyl acetate). 1H NMR (400 MHz, CDCl3) δ: 7.86
(d, J = 8.4 Hz, 2H), 7.83 (d, J = 8.4 Hz, 2H), 7.67 (dt, J = 8.0, 1.2 Hz, 1H),
7.63 (m, 1H), 7.42 (t, J = 8 Hz, 1H), 7.07 (m, 1H), 3.88 (s, 3H, OCH3).
13C{1H} NMR (100 MHz, CDCl3) δ: 164.66, 164.00, 159.92, 138.32,
130.23, 128.21, 124.73, 123.26, 119.29, 118.26, 111.56, 98.60, 55.52.
1
procedure analogous to that used for compound 13. H NMR (400
MHz, CDCl3) δ: 8.38 (m, 2H), 8.16 (d, J = 8.0 Hz, 2 Hz), 7.81ꢀ7.72 (m,
4H), 7.52ꢀ7.43 (m, 5H), 7.33 (m, 2H), 7.12 (m, 1H). 13C{1H} NMR
(100 MHz, CDCl3) δ: 164.72, 164.02, 159.99, 140.90, 140.21, 130.28,
128.58, 127.21, 126.21, 124.88, 123.76, 122.40, 120.57, 120.48, 119.33,
118.29, 111.61, 109.68, 55.55 MS(EI): m/z 417 (M+). Anal. Calcd for
C27H19N3O2: C, 77.68; H, 4.59; N, 10.07. Found: C, 77.71; H, 4.47;
N, 10.04.
2-(3,5-Di(carbazol-9-yl)phenyl)-5-(3-methoxyphenyl)-1,3,
4-oxadiazole (15). Compound 15 was obtained as white solid (0.99 g,
86%) from 12 (1.0 g, 2.0 mmol) and carbazole (1.0 g, 6.0 mmol) by a
procedure analogous to that used for compound 13, but differing in the
use of toluene/ethyl acetate (97:3) as eluent and THF/methanol for
1
recrystallization. H NMR (400 MHz, CDCl3) δ: 8.47 (d, J = 2.4 Hz,
2H), 8.16 (d, J = 8.0 Hz, 4H), 8.01 (t, J = 1.6 Hz, 1H), 7.69ꢀ7.64 (m,
2H), 7.58 (d, J = 8.0 Hz, 4H), 7.45 (m, 4H), 7.41 (t, J = 8.0 Hz, 1H), 7.34
(m, 4H), 7.08 (m, 1H), 3.87 (s, 3H, OCH3). 13C{1H} NMR (100 MHz,
CDCl3) δ: 165.19, 163.30, 159.98, 140.46, 140.28, 130.31, 128.03,
127.38, 126.43, 124.49, 123.81, 123.72, 120.80, 120.60, 119.48, 118.66,
111.55, 109.52, 55.57. MS(EI): m/z 582 (M+). Anal. Calcd for
C39H26N4O2: C, 80.39; H, 4.50; N, 9.62. Found: C, 80.32; H, 4.41;
N, 9.60.
3-(5-(3-Carbazol-9-ylphenyl)-1,3,4-oxadiazol-2-yl)phenol
(16). A solution of BBr3 (30.0 mL of a 1 M solution in dichloromethane)
was added dropwise to a solution of 13 (3.0 g, 7.2 mmol) in dichloro-
methane (20.0 mL) at ꢀ78 °C (dry ice/acetone) under nitrogen. After
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addition of BBr3 solution, the reaction was allowed to warm to room
temperature and kept at room temperature for 6 h. The reaction mixture
was poured into iceꢀwater (100 mL). The dichloromethane was
evaporated under reduced pressure. The white solid that formed was
collected by filtration. The product was purified by recrystallization from
acetone/water. After drying under vacuum, compound 16 was obtained
as a white solid (2.9 g, 100%). 1H NMR (400 MHz, acetone-d6) δ: 8.89
(s, br, 1H, OH), 8.41 (m, 1H), 8.34 (m, 1H), 8.26 (m, 2H), 7.92 (m,
2H), 7.66 (m, 2H), 7.53ꢀ7.41 (m, 5H), 7.33 (m, 2H), 7.10 (m, 1H) .
13C{1H} NMR (100 MHz, acetone-d6) δ: 164.71, 163.74, 157.93,
140.65, 138.61, 131.28, 130.52, 130.23, 126.29, 126.14, 125.74,
125.07, 125.06, 123.54, 120.39, 120.38, 119.01, 118.08, 113.30,
109.60. MS(EI): m/z 403 (M+). Anal. Calcd for C26H17N3O2: C,
77.41; H, 4.25; N, 10.42. Found: C, 77.14; H, 4.15; N, 10.34.
3-(5-(4-Carbazol-9-ylphenyl)-1,3,4-oxadiazol-2-yl)phenol
(17). Compound 17 was obtained as a white solid (0.97 g, 100%) from
14 (1.0 g, 2.40 mmol) by a procedure analogous to that used for
compound 16. 1H NMR (400 MHz, acetone-d6) δ: 8.91 (s, 1H, OH),
8.48 (m, 2H), 8.24 (dt, J = 8.0, 1.2 Hz, 2H), 7.91 (m, 2H), 7.71 (t, J = 1.2
Hz, 1H), 7.69 (t, J = 1.6 Hz, 1H), 7.56 (d, J = 0.8 Hz, 1H), 7.54 (t, J = 0.8
Hz, 1H), 7.46 (m, 3H), 7.32 (m, 2H), 7.12 (m, 1H), 3.93 (s, 3H, OCH3).
13C{1H} NMR (100 MHz, acetone-d6) δ: 165.43, 164.72, 158.87,
141.48, 141.12, 131.44, 129.41, 128.23, 127.17, 126.04, 124.56,
123.65, 121.43, 121.26, 119.88, 118.89, 114.18, 110.62. MS(EI): m/z
403 (M+). Anal. Calcd for C26H17N3O2: C, 77.41; H, 4.25; N, 10.42.
Found: C, 77.40; H, 4.19; N, 10.50.
3-(5-(3,5-Dicarbazol-9-ylphenyl)-1,3,4-oxadiazol-2-yl)phenol
(18). Compound 18 was obtained as a white solid (0.92 g, 99%) from 15
(0.95 g, 1.6 mmol) by a procedure similar to that used for compound 16,
but recrystallizing from acetone/methanol. 1H NMR (400 MHz,
DMSO-d6) δ: 10.01 (s, br, 1H, OH), 8.46 (d, J = 1.6 Hz, 2H), 8.28
(d, J = 8.0 Hz, 4H), 8.17 (t, J = 1.6 Hz, 1H), 7.66 (d, J = 8.0 Hz, 4H), 7.59
(d, J = 8.0 Hz, 1H), 7.50 (m, 5H), 7.34 (m, 5H), 7.00 (dd, J = 8.4, 2.4 Hz,
1H). 13C{1H} NMR (100 MHz, acetone-d6) δ: 166.00, 164.37, 158.90,
141.54, 141.36, 131.55, 129.21, 128.77, 127.45, 125.90, 124.86, 124.72,
121.67, 121.44, 120.11, 119.18, 114.33, 110.79. MS(EI): m/z 568 (M+).
Anal. Calcd for C38H24N4O2: C, 80.27; H, 4.25; N, 9.85. Found: C,
80.38; H, 4.23; N, 9.79.
2-(3-(5-(3-Carbazol-9-ylphenyl)-1,3,4-oxadiazol-2-yl)phe-
noxy)ethyl Methacrylate (M1). To a stirred solution of 16 (0.80 g,
2.0 mmol) and 2-bromoethyl methacrylate (0.40 g, 2.1 mmol) in DMF
(15 mL) was added K2CO3 (4.0 g, 29 mmol). After stirring at room
temperature for 21 h, water (100 mL) was added and the resultant brown
solid was collected by filtration. The crude product was purified by silica
gel column chromatography using dichloromethane/ethyl acetate (19:1)
as eluent. After removal of the solvents under reduced pressure, a viscous
material was obtained; addition of methanol (120 mL) and removal under
reduced pressure caused the material to solidify. The white solid product
was suspended indeionizedwaterand filtered; it was then dried in vacuum
to give M1 (0.78 g, 76%). 1H NMR (400 MHz, CDCl3) δ: 8.34 (m, 1H),
8.26 (m, 1H), 8.15 (dd, J = 8.0, 0.8 Hz, 2H), 7.78ꢀ7.66 (m, 4H), 7.42
(m, 5H), 7.31 (m, 2H), 7.10 (m, 1H), 6.12 (t, J = 1.2 Hz, 1H), 5.56 (m,
1H), 4.51 (m, 2H, OCH2), 4.27 (m, 2H, OCH2), 1.93 (t, J = 1.2 Hz, 3H,
CH3). 13C{1H} NMR (100 MHz, CDCl3) δ: 167.25, 164.71, 163.88,
158.97, 140.61, 138.73, 135.86, 130.86, 130.37, 126.19, 125.85, 125.78,
125.43, 124.80, 123.56, 120.45, 120.37, 119.85, 118.88, 112.42, 109.54,
66.21, 62.84, 18.27. MS(EI): m/z 515.2 (M+). Anal. Calcd for
C32H25N3O4: C, 74.55; H, 4.89; N, 8.15. Found: C, 74.26; H, 4.83; N, 8.03.
2-(3-(5-(4-Carbazol-9-ylphenyl)-1,3,4-oxadiazol-2-yl)phe-
noxy)ethyl Methacrylate (M2). M2 was synthesized from 17 (1.0 g,
2.5 mmol) and 2-bromoethyl methacrylate (0.5 g, 2.6 mmol) using the
same procedure as used for M1. The crude product was purified by silica
gel column chromatography using dichloromethane/ethyl acetate
(97:3) as eluent. After removal of solvent under reduced pressure, the
residue was dissolved in acetone, and the solution was added dropwise
into a stirred mixture of methanol and water (3:1, 90 mL); the resultant
precipitate was collected by filtration and dried in vacuum to give a white
solid (0.98 g, 77%). 1H NMR (400 MHz, CDCl3) δ: 8.37 (d, J = 8.0 Hz,
2H), 8.14 (dd, J = 8.0, 0.8 Hz, 2H), 7.78ꢀ7.72 (m, 4H), 7.50ꢀ7.41 (m,
5H), 7.31 (m, 2H), 7.12 (dd, J = 8.0, 2.4 Hz, 1H), 6.16 (d, J = 0.4 Hz, 1H,
CdCꢀH), 5.59 (t, J = 0.8 Hz, 1H, CdCꢀH), 4.55 (t, J = 4.8 Hz, 2H,
OCH2), 4.33 (t, J = 4.8 Hz, 2H, OCH2), 1.96 (s, 3H, CH3). 13C{1H}
NMR (100 MHz, CDCl3) δ: 167.28, 164.57, 164.06, 140.94, 140.22,
135.89, 130.40, 128.59, 127.21, 126.21, 124.97, 123.78, 122.34, 120.58,
120.47, 119.79, 118.74, 112.47, 109.87, 66.22, 62.86, 18.30. MS(EI):
m/z 515 (M+). Anal. Calcd for C32H25N3O4: C, 74.55; H, 4.89; N, 8.15.
Found: C, 74.29; H, 4.79; N, 8.13.
2-(3-(5-(3,5-Dicarbazol-9-ylphenyl)-1,3,4-oxadiazol-2-yl)-
phenoxy)ethyl Methacrylate (M3). M3 was synthesized from 18
(0.70 g, 1.2 mmol) and 2-bromoethyl methacrylate (0.25 g, 1.3 mmol)
by the same procedure for M1. The crude product was purified by silica
gel column chromatography using dichloromethane/ethyl acetate
(19:1) as eluent. After removal of solvent, the residue was dissolved in
THF (10 mL); addition of methanol (120 mL) to this stirred solution
gave a precipitate which was collected by filtration and dried under
vacuum to give a white solid (0.70 g, 84%). 1H NMR (400 MHz, CDCl3)
δ: 8.47 (d, J = 1.6 Hz, 2H), 8.16 (d, J = 8.0 Hz, 4H), 8.01 (d, J = 1.6 Hz,
1H), 7.71 (dt, J = 8.0, 1.6 Hz, 1H), 7.67 (m, 1H), 7.57 (d, J = 8.4 Hz, 4H),
7.49ꢀ7.40 (m, 5H), 7.34 (m, 4H), 7.10 (m, 1H), 6.11 (t, J = 1.2 Hz, 1H),
5.55 (m, 1H), 4.49 (t, J = 4.4 Hz, 2H, OCH2), 4.29 (t, J = 4.4 Hz, 2H,
OCH2), 1.92 (t, J = 1.2 Hz, 3H, CH3). 13C{1H} NMR (100 MHz,
CDCl3) δ: 167.24, 165.05, 163.35, 159.01, 140.51, 140.32, 135.85,
130.43, 128.12, 127.36, 126.43, 126.16, 124.60, 123.84, 123.77, 120.82,
120.61, 119.95, 119.09, 112.50, 109.52, 66.24, 62.81, 18.26. MS(FAB):
m/z 680 (M+). Anal. Calcd for C44H32N4O4: C, 77.63; H, 4.74; N, 8.23.
Found: C, 77.49; H, 4.69; N, 8.21.
2-(3-Carbazol-9-ylphenyl)-5-(3-(4-vinylbenzyloxy)phenyl)-
1,3,4-oxadiazole (M4). To a stirred solution of 16 (3.0 g, 7.4 mmol)
and 1-(chloromethyl)-4-vinylbenzene (1.3 g, 8.5 mmol) in DMF
(40 mL) was added K2CO3 (12.0 g, 86.8 mmol) at room temperature.
After stirring for 48 h, water (200 mL) was added; the resulting
precipitate was collected by filtration, washed with water and methanol,
and dried under reduced pressure. The crude product was purified by
silica gel column chromatography using dichloromethane/ethyl acetate
(19:1) as eluent. After removal of solvent under reduced pressure, the
resulting solid was transferred to a filter with methanol and dried in
vacuum to give a white solid (3.72 g, 96%). 1H NMR (400 MHz, CDCl3)
δ: 8.32 (m, 1H), 8.26 (m, 1H), 8.17 (dd, J = 8.0, 1.2 Hz, 2H), 7.78ꢀ7.69
(m, 4H), 7.44ꢀ7.39 (m, 9H), 7.30 (m, 2H), 7.12 (m, 1H), 6.68 (dd, J =
17.6, 11.2 Hz, 1H, CdCꢀH), 5.72 (dd, J = 17.6, 0.8 Hz, 1H, CdCꢀH),
5.23 (dd, J = 11.2, 0.8 Hz, 1H, CdCꢀH), 5.11 (s, 2H, OCH2). 13C{1H}
NMR (100 MHz, CDCl3) δ: 164.77, 163.83, 159.06, 140.61, 138.71,
137.49, 136.30, 135.78, 130.84, 130.31, 127.73, 126.44, 126.19, 125.81,
125.40, 124.75, 123.56, 120.44, 120.38, 119.65, 119.07, 114.22, 112.70,
109.54, 104.96, 70.00. MS(EI): m/z 519 (M+). Anal. Calcd for
C35H25N3O2: C, 80.90; H, 4.85; N, 8.09. Found: C, 80.71; H, 4.85;
N, 8.03.
2-(3,5-Dicarbazol-9-ylphenyl)-5-(3-(4-vinylbenzyloxy)-
phenyl)-1,3,4-oxadiazole (M5). M5 was synthesized starting from
18 (3.0 g, 5.3 mmol) and 1-(chloromethyl)-4-vinylbenzene (1.0 g,
6.6 mmol) using the same procedure as used for M4. The crude product
was purifiedbysilica gel column chromatography usingdichloromethane/
ethyl acetate (39:1) as eluent. After removal of solvent under reduced
pressure, the residue was dissolved in dichloromethane; this solution was
added dropwise into stirred methanol (100 mL), and the resultant
precipitate was collected by filtration and dried under vacuum to give a
white solid (2.45 g, 68%). 1H NMR (400 MHz, CDCl3) δ: 8.46 (d, J =
2.0 Hz, 2H), 8.17 (dd, J = 8.0, 0.8 Hz, 4H), 8.01 (t, J = 2.0 Hz, 1H),
4005
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7.72ꢀ7.70 (m, 2H), 7.58 (d, J = 8.0 Hz, 4H), 7.50ꢀ7.33 (m, 13H), 7.14
(m, 1H), 6.66 (dd, J = 17.6, 11.2 Hz, 1H, CdCꢀH), 5.70 (dd, J = 17.6, 0.8
Hz, 1H, CdCꢀH), 5.30 (dd, J = 11.2, 0.8 Hz, 1H, CdCꢀH), 5.11 (s, 2H,
OCH2). 13C{1H} NMR (100 MHz, CDCl3) δ: 165.09, 163.28, 159.07,
140.47, 140.31, 137.47, 136.27, 135.73, 130.36, 128.04, 127.68, 127.37,
126.46, 126.42, 124.53, 123.82, 123.70, 120.81, 120.60, 119.73, 119.31,
114.21, 112.72, 109.51, 70.00. MS(FAB): m/z 684 (M+). Anal. Calcd for
C47H32N4O2: C, 82.44; H, 4.71; N, 8.18. Found: C, 82.18; H, 4.71;
N, 8.20.
2-(3,5-Dicarbazol-9-ylphenyl)-5-(3-(5-(bicyclo[2,2,1]hept-
5-en-2-yl)pentyloxy)phenyl)-1,3,4-oxadiazole (Mixture of
endo and exo Isomers) (M8). M8 was synthesized from 18 (0.90 g,
1.6 mmol) and 5-(5-bromopentyl)-bicyclo[2,2,1]hept-2-ene (mixture
of endo and exo isomers) (0.5 g, 2.1 mmol) using a procedure analogous
to that used for M7. After purification by silica-gel column chromato-
graphy using toluene/ethyl acetate (99:1) as eluent, a glassy solid was
obtained; this was dissolved in acetone (12.0 mL) and the acetone
solution was added dropwise into stirred methanol (120 mL) to give a
white precipitate, which was isolated by filtration and dried under
2-(3-Carbazol-9-ylphenyl)-5-(3-(5-(bicyclo[2,2,1]hept-5-en-
2-ylmethoxy)phenyl)-1,3,4-oxadiazole (Mixture of endo and
exo Isomers) (M6). To a stirred solution of 16 (1.0 g, 2.5 mmol) and
(5-norbornen-2-yl)methyl p-toluenesulfonate (mixture of endo and exo
isomers) (0.8 g, 3 mmol) in DMF (20 mL) was added Cs2CO3 (1.6 g,
4.9 mmol) at room temperature. The reaction was heated to 100 °C for
3 h; after the reaction mixture was cooled to room temperature, water
(120 mL) was added, and the resultant brown solid product was
collected by filtration, and purified by silica gel column chromatography
using toluene/ethyl acetate (19:1) as eluent. After removal of solvents,
the residue was dissolved in acetone (3.0 mL); the solution was added
dropwise into a stirred mixture of methanol and water (3:1, 100 mL) and
the resulting white precipitate was collected by filtration and dried under
vacuum to give a white solid (1.07 g, 85%). 1H NMR (400 MHz, CDCl3)
δ: 8.34 (t, J = 1.2 Hz, 1H), 8.27 (m, 1 Hz), 8.17 (d, J = 8.0 Hz, 2H),
7.81ꢀ7.76 (m, 2H), 7.70ꢀ7.61 (m, 2H), 7.46ꢀ7.31 (m, 7H), 7.07 (m,
1H), 6.19ꢀ5.95 (m, 2H, CdCꢀH), 4.10 (dd, J = 8.8, 6.0 Hz, 0.6H,
0.3 ꢁ OCH2), 3.92 (t, J = 8.8 Hz, 0.4H, 0.2 ꢁ OCH2), 3.78 (dd, J = 8.8,
6.0 Hz, 0.4H, 0.2 ꢁ OCH2), 3.62 (t, J = 8.8 Hz, 0.6H, 0.3 ꢁ OCH2), 3.05
(s, br, 0.4H), 2.86 (m, br, 1.6H), 2.57 (m, 1H), 1.92 (m, 1H), 1.50ꢀ1.24
(m, 3H), 0.65 (m, 1H). 13C{1H} NMR (100 MHz, CDCl3) δ: 164.91,
163.81, 163.83, 159.55, 140.62, 138.71, 137.65, 136.88, 136.36, 132.26,
130.83, 130.31, 130.28, 130.21, 130.16, 126.19, 125.85, 125.42, 124.68,
124.63, 123.56, 120.45, 120.37, 119.23, 119.13, 118.84, 118.81, 112.28,
112.23, 109.55, 72.55, 71.75, 49.41, 45.04, 43.86, 43.68, 42.21, 41.58,
38.52, 38.30, 29.60, 28.98. MS(EI): m/z 509 (M+). Anal. Calcd for
C34H27N3O2: C, 80.13; H, 5.35; N, 8.25. Found: C, 80.00; H, 5.33;
N, 8.19.
2-(4-Carbazol-9-ylphenyl)-5-(3-(5-(bicyclo[2,2,1]hept-5-en-
2-yl)pentyloxy)phenyl)-1,3,4-oxadiazole (Mixture of endo
and exo Isomers) (M7). To a solution of 17 (0.90 g, 2.2 mmol)
and 5-(5-bromopentyl)bicyclo[2,2,1]hept-2-ene (mixture of endo and
exo isomers) (0.74 g, 3.0 mmol) in DMF (10.0 mL) was added K2CO3
(5.0 g, 36 mmol) at room temperature with stirring. After the reaction
was carried out at room temperature for 24 h, water (150 mL) was added
and the resulting brown semisolid product was obtained by filtration.
The crude product was purified by silica-gel column chromatography
using toluene/ethyl acetate (19:1) as eluent. After removal of solvents,
acetone (3.0 mL) was added to the resulting glassy solid, which
disappeared and then reappeared; methanol was added into the stirred
acetone solution to complete the precipitation, and the precipitate was
collected by filtration and dried in vacuum to give a white solid (0.99 g,
79%). 1H NMR (400 MHz, CDCl3) δ: 8.38 (d, J = 8.4 Hz, 2H), 8.15 (d,
J = 8.0 Hz, 2H), 7.80ꢀ7.70 (m, 4H), 7.52ꢀ7.43 (m, 5H), 7.33 (m, 2H),
7.10 (dd, J = 8.0, 1.6 Hz, 1H), 6.11 (q, J = 2.8 Hz, 0.7H), 6.09 (q, J = 2.4
Hz, 0.3H), 6.01 (q, J = 2.4 Hz, 0.3H), 5.92 (q, J = 2.8 Hz, 0.7H), 4.06 (t,
J = 6.4 Hz, 2H, OCH2), 2.76 (m, br, 1.7H), 2.52 (s, br, 0.3H), 1.97 (m,
1H), 1.84 (m, 2.5H), 1.51ꢀ1.23 (m, 5H), 1.21ꢀ1.06 (m, 2.5H), 0.50
(m, 1H). 13C{1H} NMR (100 MHz, CDCl3) δ: 164.76, 163.97, 159.54,
140.86, 140.20, 136.94, 132.34, 130.22, 128.55, 127.18, 126.20, 124.79,
123.75, 122.40, 120.55, 120.45, 119.10, 118.66, 112.26, 109.67, 68.31,
49.53, 45.37, 42.48, 38.65, 34.67, 32.38, 29.18, 28.37, 26.24. MS(EI):
m/z 565 (M+). Anal. Calcd for C38H35N3O2: C, 80.68; H, 6.24; N, 7.43.
Found: C, 80.62; H, 6.15; N, 7.38.
1
vacuum to give a white solid (1.05 g, 91%). H NMR (400 MHz,
CDCl3) δ: 8.48 (d, J = 1.6 Hz, 2H), 8.17 (d, J = 8.0 Hz, 4H), 8.02 (t, J =
1.6 Hz, 1H), 7.68ꢀ7.58 (m, 6H), 7.50ꢀ7.33 (m, 9H), 7.07 (dd, J = 8.0,
2.0 Hz, 1H), 6.09 (q, J = 2.8 Hz, 0.7H), 6.08 (q, J = 2.8 Hz, 0.3H), 6.00
(q, J = 2.8 Hz, 0.3H), 5.90 (q, J = 2.8 Hz, 0.7H), 4.00 (t, J = 6.4 Hz, 2H,
OCH2), 2.74 (s, br, 1.7H), 2.49 (s, br, 0.3H), 1.96 (m, 1H), 1.81 (m,
2.5H), 1.46ꢀ1.03 (m, 7.5H), 0.47 (m, 1H). 13C{1H} NMR (100 MHz,
CDCl3) δ: 165.24, 163.26, 159.58, 140.47, 140.31, 136.91, 132.33,
130.23, 128.02, 127.42, 126.42, 124.44, 123.81, 123.72, 120.79, 120.59,
119.25, 118.96, 112.29, 109.52, 68.34, 49.52, 45.37, 42.48, 38.64, 34.63,
32.37, 29.13, 28.34, 26.20. MS(FAB): m/z 730 (M+). Anal. Calcd for
C50H42N4O2: C, 82.16; H, 5.79; N, 7.67. Found: C, 82.31; H, 5.77;
N, 7.68.
General Procedure for Free-Radical Polymerization of
Methacrylate Monomers. A Schlenk flask was charged with mono-
mer (ca. 0.15 M M1, M2, or M3), AIBN (0.015 equiv), and dry THF.
The polymerization mixture was purged with nitrogen to remove
oxygen, sealed under nitrogen, and heated to 60 °C with stirring for 3
d. After allowing to cool to room temperature, the polymer was
precipitated from the reaction mixture with ethanol and collected by
filtration, redissolved in dichloromethane, and reprecipitated with
ethanol again; this dissolution/precipitation procedure was repeated
an additional three times. The product from the final precipitation was
then dried under vacuum to give a white solid.
Polymer P1. Yield: 0.46 g (92%). 1H NMR (400 MHz, CDCl3) δ:
8.06 (s, br, 1H), 7.93 (m, br, 2H), 7.44 (s, br, 1H), 7.22 (m, br, 8H), 7.10
(s, br, 2H), 6.81 (m, br, 2H), 4.04 (m, br, 4H), 1.82 (s, br, 2H), 1.00 (m,
br, 3H). Anal. Calcd for (C32H25N3O4)n: C, 74.55; H, 4.89; N, 8.15.
Found: C, 73.95; H, 4.72; N, 8.02.
Polymer P2. Yield: 0.47 g (92%). 1H NMR (400 MHz, CDCl3) δ:
8.01 (s, br, 2H), 7.94 (s, br, 2H), 7.43 (m, br, 4H), 7.23 (m, br, 5H), 7.13
(m, br, 2H), 6.89 (m, br, 1H), 4.12 (m, br, 4H), 1.97 (s, br, 2H), 1.00 (m,
br, 3H). Anal. Calcd for (C32H25N3O4)n: C, 74.55; H, 4.89; N, 8.15.
Found: C, 73.64; H, 4.80; N, 7.94.
Polymer P3. Yield: 0.46 g (92%). 1H NMR (400 MHz, CDCl3) δ:
8.10 (s, br, 2H), 7.82 (s, br, 4H), 7.62 (s, br, 1H), 7.31 (m, br, 5H), 7.23
(m, br, 2H), 7.17 (s, br, 4H), 7.04 (s, br, 4H), 6.78 (m, br, 1H), 3.90 (m,
br, 4H), 1.73 (m, br, 2H), 0.81 (m, br, 3H). Anal. Calcd for
(C44H32N4O4)n: C, 77.63; H, 4.74; N, 8.23. Found: C, 77.12; H,
4.63; N, 8.16.
General Procedure for Free-Radical Polymerization of
Styrene Monomers. A Schlenk flask was charged with monomer
(ca. 0.1 M M4 or M5), AIBN (0.025 equiv), and dry THF. The
polymerization mixture was purged with nitrogen (removal of oxygen),
securely sealed under nitrogen, and heated to 60 °C with stirring for 7 d.
After allowing it to cool to room temperature, the polymer was
precipitated with acetone, collected by filtration, redissolved in dichloro-
methane, and reprecipitated with acetone again; this dissolution/pre-
cipitation (dicloromethane/acetone) procedure was repeated an addi-
tional three times. The solid from the final precipitation was dried under
vacuum to give a white solid.
Polymer P4. Yield: 0.90 g (90%). 1H NMR (400 MHz, CDCl3) δ:
8.10 (s, br, 1H), 7.97 (s, br, 2H), 7.49 (s, br, 2H), 7.26 (m, br, 7H), 7.15
(s, br, 3H), 7.05 (m, br, 2H), 6.86 (m, br, 1H), 6.45 (m, br,
4006
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Chart 1
2H), 4.77 (s, br, 2H), 2.00ꢀ1.00 (m, br, 3H). Anal. Calcd for
(C35H25N3O2)n: C, 80.90; H, 4.85; N, 8.09. Found: C, 80.62; H,
4.76; N, 8.09.
solution of the cross-linkable polymer Poly-TPD-F (Chart 1) in dry
deoxygenated toluene (10 mg/mL) onto air-plasma-treated indium tin
oxide (ITO) coated glass substrates with a sheet resistance of 20 Ω/0
(Colorado Concept Coatings, L.L.C.); the polymer was then photo-
cross-linked by irradiating for 1.0 min with a standard broad-band UV
light with a 0.7 mW/cm2 power density. Subsequently, a 40 nm-thick
Polymer P5. Yield: 0.82 g (82%). 1H NMR (400 MHz, CDCl3) δ:
8.17 (s, br, 2H), 7.87 (s, br, 4H), 7.68 (s, br, 1H), 7.38 (s, br, 9H), 7.09
(m, br, 4H), 6.95 (m, br, 5H), 6.29 (m, br, 2H), 4.67 (m, br, 2H),
2.00ꢀ0.60 (m, br, 3H). Anal. Calcd for (C47H32N4O2)n: C, 82.44; H,
4.71; N, 8.18. Found: C, 82.36; H, 4.69; N, 8.29.
emissive layer of host polymer (P1, P2, or P3) doped with fac-tris-
0
(2-phenylpyridinato-N,C2 ) iridium, Ir(ppy)3 (Chart 1) at a concentration
of 6 vol. %, was spin-coated (60 s at 1000 rpm, acceleration 10 000) from a
chlorobenzene solution (9.4 mg/mL polymer and 0.6 mg/mL in
Ir(ppy)3) on top of the insoluble photocross-linked hole-transport layer.
A 40 nm-thick layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline,
BCP, (Chart 1, purified by gradient zone sublimation) was thermally
evaporated at a rate of 0.4 Å/s and at a pressure below 1 ꢁ 10ꢀ7 Torr on
top of the emissive layer to serve as an electron-transport/hole-blocking
layer. Finally, 2.5 nm of lithium fluoride (LiF) as an electron-injection
layer and a 200 nm-thick aluminum cathode were deposited in a vacuum
chamber at a pressure below 1 ꢁ 10ꢀ6 Torr and at rates of 0.1 Å/s and
2 Å/s, respectively. A shadow mask was used for the evaporation of the
metal to form five devices with an area of 0.1 cm2 per substrate; the
devices were tested immediately after cathode deposition. At no point
during fabrication or testing were the devices exposed to air.
General Procedure for Ring-Opening Metathesis Poly-
merization of Norbornene Monomers. To a stirred dichloro-
methane solution of the norbornene monomer (M6, M7, or M8, ca. 0.1
M) was added a solution of the Grubbs first generation initiator in
dichloromethane (ca. 0.008 M, 0.01 equiv) at room temperature in an
inert-atmosphere glovebox. After stirring overnight, the polymerization
mixture was taken out of the glovebox and ethyl vinyl ether (2 mL) was
added with stirring to quench the polymerization. After stirring 30 min,
the polymer was precipitated with ethanol, collected by filtration,
redissolved in dichloromethane, and reprecipitated with ethanol; this
dissolution/precipitation procedure was repeated two more times.
Drying the product from the final precipitation under vacuum gave an
off-white solid.
Polymer P6. Yield: 0.38 g (76%). 1H NMR (400 MHz, CDCl3) δ:
8.26 (s, br, 1H), 8.08 (s, br, 2H), 7.62 (m, br, 3H), 7.36 (s, br, 4H), 7.26
(s, br, 4H), 6.96 (m, br, 2H), 5.34 (m, br, 2H), 3.74 (m, br, 2H), 2.65 (m,
br, 1H), 2.42 (m, br, 2H), 1.90 (m, br, 2H), 1.20 (m, br, 2H). Anal. Calcd
for (C34H27N3O2)n: C, 80.13; H, 5.34; N, 8.11. Found: C, 79.55; H,
5.22; N, 8.11.
Computational Methodology. The molecular geometries of the
ground singlet (S0) state and lowest triplet (T1) state of 13, 14, and 15
were optimized at the density functional theory (DFT) level using the
B3LYP functional and the 6-31G(d,p) basis set, as implemented in the
Gaussian03 program.44 Ground-state geometries were confirmed to be
minima of the corresponding adiabatic potential surfaces by performing
additional vibrational frequency calculations. The excited-state energies
and related transition dipole moments (oscillator strengths) were
derived by means of the time-dependent density functional theory
(TD-DFT) calculations that were carried out at the same B3LYP/6-
31G(d,p) level of theory. Electronic structure calculations for Ir(ppy)3
were also carried out for comparison.
Polymer P7. Yield: 0.41 g (82%). 1H NMR (400 MHz, CDCl3) δ:
8.28 (s, br, 2H), 8.08 (m, br, 2H), 7.68 (m, br, 4H), 7.38 (m, br, 3H),
7.28 (m, br, 2H), 7.26 (m, br, 2H), 7.02 (m, br, 1H), 5.26 (m, br, 2H),
3.96 (m, br, 2H), 2.87ꢀ2.72 (m, br, 1H), 2.51ꢀ2.37 (m, br, 2H),
1.85ꢀ1.75 (m, br, 4H), 1.75ꢀ1.12 (m, br, 8H). Anal. Calcd for
(C38H35N3O2)n: C, 80.68; H, 6.24; N, 7.43. Found: C, 79.87; H,
6.16; N, 7.30.
Polymer P8. Yield: 0.38 g (76%). 1H NMR(400 MHz, CDCl3) δ:
8.39 (s, br, 2H), 8.07 (s, br, 4H), 7.91 (s, br, 1H), 7.53 (d, br, 6H), 7.39 (s,
br, 4H), 7.25 (s, br, 5H), 6.94 (m, br, 1H), 5.21 (m, br, 2H), 3.87 (m, br,
2H), 2.85ꢀ2.69 (m, br, 1H), 2.47ꢀ2.32 (m, br, 2H), 1.80ꢀ1.66 (m, br,
6H), 1.25ꢀ1.06 (m, br, 6H). Anal. Calcd for (C50H42N4O2)n: C, 82.16;
H, 5.79; N, 7.67. Found: C, 81.17; H, 5.74; N, 7.58.
’ RESULTS AND DISCUSSION
Preparation of Carbazole-Oxadiazole Monomers. The
syntheses for the monomers are shown in Scheme 1. The
iodo-substituted benzoates, 1ꢀ3, reacted with hydrazine in
ethanol to give iodo-substituted benzohydrazides, 4ꢀ6, respec-
tively, in good yields, which can be used for the next step without
OLED Device Fabrication. A 35 nm-thick hole-transport layer
was fabricated by spin-coating (60 s at 1500 rpm, acceleration 10 000) a
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Scheme 1
purification. The benzohydrazides, 4ꢀ6, reacted with 3-methoxy-
benzoyl chloride to give the corresponding iodo-substituted-
N0-(3-methoxybenzoyl)benzohydrazides 7ꢀ9, which can also be
used without further purification. The various iodinated deriva-
tives of 2-(phenyl)-5-(3-methoxyphenyl)-1,3,4-oxadiazole,
10ꢀ12, were obtained by heating 7ꢀ9 in phosphorus
oxychloride. The carbazole units were then introduced under
Ullmann conditions to generate the methoxy-functionalized
carbazole/oxadiazole compounds 13ꢀ15 in excellent yields
(86ꢀ96%). The corresponding phenols 16ꢀ18 were obtained
from by the reaction of 13ꢀ15 with boron tribromide in
dichloromethane. The monomers M1ꢀM8 were obtained in
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Table 1. Molecular Weights and Thermal Properties of
P1ꢀP8
a
a
Mw
Mn
PDI
Tg (°C)b
Td (°C)c
P1
P2
P3
P4
P5
P6
P7
P8
32 000
46 000
45 000
29 000
25 000
66 000
50 000
60 000
13 000
17 000
16 000
19 000
16 000
46 000
30 000
35 000
2.5
2.7
2.8
1.5
1.6
1.4
1.6
1.7
132
141
190
150
209
138
118
160
375
376
400
385
414
383
390
412
a Molecular weights estimated by GPC using polystyrene standards.
b Measured by DSC at heating rate of 10 °C/min. c Measured by TGA at
heating rate of 20 °C/min.
good yields (67ꢀ96%) by nucleophilic substitution reactions
between the phenols, 16ꢀ18, and the appropriate methacrylate,
styrene, or norbornene-functionalized alkyl bromide or tosylate.
Preparation and Characterization of Carbazole-Oxadia-
zole Polymers. The syntheses of homopolymers P1ꢀP8 are
also shown in Scheme 1. The methacrylate polymers, P1ꢀP3,
and styrene polymers, P4ꢀP5, were obtained by thermal
(60 °C) free-radical polymerization of the corresponding mono-
mers in THF in inert atmosphere, using 1.5 and 2.5 mol % of 2,20-
azobisisobutyronitrile (AIBN) as an initiator for methacrylate
and styrene polymerizations, respectively. Methacrylate poly-
mers were obtained in high yields (>90%) after reaction for
3 days, while relatively low yields of polystyrenes were obtained
in the same time; good yields (>80%) of polystyrenes were,
however, obtained after reaction for 7 days. Poly(norbornene)s
P6ꢀP8 were synthesized in good yield (>80%) by ring-opening
metathesis polymerization (ROMP) of the corresponding
monomers in dichloromethane using 1.0 mol % of the Grubbs
first generation initiator in a glovebox at room temperature. All
the polymers were purified by repeated dissolution and precipi-
1
tation and were characterized by H NMR spectroscopy, ele-
mental analysis, and gel-permeation chromatography (GPC).
The number-average and weight-average molecular weights, Mn
and Mw respectively, determined against polystyrene standards,
are summarized in Table 1. The number-average molecular
weights are smaller for the polymers obtained from free-radical
polymerization, P1ꢀP5, than for the ROMP polymers, P6ꢀP8.
The polydispersity indices (PDI = Mw/Mn) of the polymers with
methacrylate backbones fall in the range 2.5ꢀ2.8, while the
styrene- and norbornene-based polymers have narrower molec-
ular-weight distributions with PDIs of 1.4ꢀ1.7. The number-
average degrees of polymerization suggested by the GPC data for
the poly(norbornene)s fall in the range 47ꢀ90, thus, in some
cases falling only a little short of the value of 100 expected from
the initiator/monomer ratio employed.
Thermal Properties. The thermal stabilities of the polymers
were investigated by the thermal gravimetric analysis (TGA)
under nitrogen (see Supporting Information, Figure S1). The
decomposition temperatures (Td), defined as the temperatures
at which 5% weight loss was observed at a heating rate of 20 °C/min,
are summarized in Table 1 and suggest all have reasonably good
thermal stability. The decomposition temperatures seem to be
influenced by the choice both of the carbazole-oxadiazole
pendant group and of the backbone: for a given side chain
the polymethacrylates show slightly lower decomposition
Figure 2. Normalized absorption and fluorescence spectra of 13 (top),
14 (middle), and 15 (bottom) in dichloromethane.
temperatures than the corresponding polystyrenes and polynor-
bornenes, while the polymers (P3, P5, P8) derived from the
bis(carbazole)-oxadiazole phenol 18 show somewhat higher
decomposition temperatures than comparable polymers with
one carbazole in the pendant group (derived from phenols 16
and 17). The polymers were also studied using differential
scanning calorimetry (DSC, see Supporting Information, Figure
S2); in each case the only events detected were glass transitions,
suggesting that these materials may form amorphous films. The
three polymers derived from bis(carbazole)-oxadiazole phenol
18 exhibit higher glass-transition temperatures (Tg) that those
from 16 and 17, with the highest value being observed for the
polystyrene P5.
Photophysical Properties. The optical properties of the three
carbazole-oxadiazole groups used (Figure 1) were compared using
the intermediates13, 14, and 15 asmodelcompounds. Absorption
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Table 2. Experimental and Quantum-Mechanical Optical Data for 13, 14, 15, and Ir(ppy)3
experimental
theoretical transition energies (eV)
(oscillator strengths in parentheses)
absorptiona
fluorescencea
phosphorescenceb
c
c
d
λmax (nm)
Emax (eV)
Eonset (eV) λmax (nm) Emax (eV) λ0,0 (nm)
E0,0 (eV)
vert. S0fS1
vert. S0fT1
ET
13
241, 292, 339 5.15, 4.25, 3.66
237, 284, 341 5.23, 4.37, 3.64
234, 292, 337 5.30, 4.25, 3.68
3.46
3.26
3.31
424
414
434
2.92
3.00
2.86
3.25 (0.01)
3.26 (0.4)
3.10 (0.02)
2.74
2.65
2.71
2.52
2.60
2.52
2.59
2.36
14
15
456
2.72
Ir(ppy)3
2.80 (0.002)
a In CH2Cl2 at room temperature. b Absorption maxima for the highest energy feature in a spectrum recorded in 2-methyltetrahydrofuran at 77 K and
taken as an estimate of the adiabatic triplet energy. c TD-DFT/B3LYP estimates of the vertical transition energies at ground-state (S0) geometry for
S0 fS1 and S0 f T1 transitions. d ΔSCF estimates of adiabatic energies of the first triplet state.
Figure 3. Some small-molecule carbazole-oxadiazole and carbazole-triazole compounds studied in the literature.
and fluorescence spectra obtained in dilute dichloromethane
solution are shown in Figure 2; the absorption and emission
maxima are listed in Table 2. In all three compounds the lowest
energy distinct absorption maxima are at about 340 nm (ca.
3.6 eV). Maxima at similar wavelengths have been previously
reported for other carbazole-oxadiazole molecules with similar
structural motifs including IꢀVI (Figure 3, values range from
338 to 342 nm, except for II, for which the maximum is reported
at 361 nm).31ꢀ33,35,36 2,5-Diphenyl-1,3,4-oxadiazole absorbs at
much shorter wavelength (284 nm, 4.37 eV in chloroform),31
while carbazole and N-phenylcarbzole have weak vibronically
structured absorptions in the same region as the maxima observed
for the present compounds (341 and 333 nm, respectively, in
chloroform).45,46 For compounds 13 and 15, in which the
carbazole(s) and oxadiazole are linked by meta-phenylene bridges
(motifs (a) and (c) in Figure 1) the low-energy absorption is weak
relative to the other shorter wavelength absorptions, whereas the
lowest energy band of para-phenylene-linked 14 (motif of
Figure 1b) is relatively strong, consistent with what is seen in
comparing other meta- and para-linked donorꢀacceptor systems,
including the closely related carbazole-oxadiazole derivatives IV
and V33 and the carbazole-triazole series VIIꢀIX (Figure 3).47 It
should be noted that, at least for 15, there appears to be a broad tail
extending well beyond the sharp feature giving rise to the lowest
resolvable maximum, suggesting the possibility of more than one
weak low-energy transition (see below).
TD-DFT calculated energies for the S0fS1 transition of
isolated gas-phase molecules are somewhat overestimated com-
pared to the experimental absorption maxima, but, consistent
with experiment, show only minor variations in transition energy
between the three compounds. The TD-DFT calculations in-
dicate that for 13ꢀ15 the S0fS1 transitions are dominated by
HOMO-to-LUMO excitation. As shown in Figure 4, the
HOMOs are strongly localized on the carbazole moieties, with
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Figure 5. Illustrations of the natural transition orbitals for the T1 state
of compounds 13, 14, and 15.
Figure 4. Illustration of the frontier molecular orbitals of 13, 14, and 15.
reported for the other meta-linked carbazoles-oxadiazoles IV
and VI (Figure 3),31,33,50,51 and somewhat lower than values
reported for either 2,5-diphenyl-1,3,4-oxadiazole (3.00 eV)31 or
N-phenylcarbazole (2.88 eV in 2-methyltetrahydrofuran).49
DFT calculations of the adiabatic triplet energies suggest values
of about 2.6 eV for 13 and 15, close to the experimental value for
15, and a value about 0.1 eV lower for 14. This is consistent with
results for IV and V, where the experimental triplet energy is also
about 0.1 eV lower for the para-linked derivative V than for its
meta-analogue IV.33 Natural transition orbital (NTO) analyses
were performed based on TD-DFT calculations carried out at the
optimized T1 geometries to obtain a better insight into the nature
of the T1 states of 13ꢀ15. The obtained NTOs (Figure 5)
indicate that the T1 state in all three systems is largely localized
on the diphenyl oxadiazole portion of the molecule, in contrast to
the carbazole-to-oxadiazole charge-transfer character of the S1
state. This is consistent with the observation that the experi-
mental adiabatic triplet energies of 15 and 2,5-diphenyl-1,3,4-
oxadiazole differ less from one another than do the fluorescence
energies (see above). In the case of 14 both NTOs show some
density on the carbazole unit, suggesting that, in this case, there is
some interaction between local carbazole- and diphenyloxadia-
zole-localized triplet states. Presumably, it is this interaction that
causes the T1 state of 14 to be slightly lower in energy than that
for the other two systems. In the context of the OLED properties
investigated here, it is important to note that the experimental
triplet energy of 15 is about 0.2 eV higher than that of the widely
used green phosphor, Ir(ppy)3 (2.4 eV in both solution and
various solid matrixes),52,53 which is used as the emitter in this
work; moreover, the calculated triplet energies of all three
structures are higher than that calculated for Ir(ppy)3 in the
same way (Table 2). 15 is also likely to be suitable for hosting
the blue-green phosphor bis[(4,6-difluorophenyl)pyridinato-N,
much smaller coefficients on the adjacent bridging phenylene
group, while the LUMOs extend over the diphenyloxadiazole
portion of the molecule, meaning that the first singlet excited
states (S1) in all these systems have considerable carbazole-to-
diaryloxadiazole charge-transfer character.48 The TD-DFT cal-
culations also reproduce the variations in the relative strength of
the experimental low-energy maxima; the variations in the
S0fS1 oscillator strength (Table 1) can be attributed to the
differences in HOMOꢀLUMO overlap on the bridging pheny-
lene group for para and meta cases. The TD-DFT calculations
indicate the presence of several weak low-energy transitions with
charge-transfer character for 13 and 15, consistent with the
appearance of the long-wavelength part of the spectrum of 15
discussed above.
All three molecules show blue fluorescence with maxima in
dichloromethane at 414ꢀ434 nm (2.85ꢀ3.00 eV), in a similar
range to that previously reported for other related carbazole-
oxadiazole molecules (416ꢀ439 nm for IIꢀVI).31ꢀ33,36 The
emission maxima of 13ꢀ15 are bathochromically shifted by
0.5ꢀ0.7 eV from those of either 2,5-diphenyl-1,3,4-oxadiazole
(346 nm, 3.58 eV in CHCl3)31 or N-phenylcarbazole (350 nm,
3.54 eV in CHCl3)45,49 and the spectra lack the resolved vibronic
structure seen for those compounds, consistent with the S1 state
of the present compounds exhibiting considerable carbazole-
oxadiazole charge-transfer character. As with the comparison of
IV and V,33 the meta-linked species in the present series exhibit
the more bathochromically shifted emissions. The emissions of
15 are also significantly bathochromically shifted relative to those
of the carbazole-triazoles VIIꢀIX, where any charge-transfer-
type state would be expected to be higher in energy owing to the
expected effect of replacing O with the less electronegative N on
the acceptor strength of the five-membered heterocycle.47
The phosphorescence spectrum of 15 was obtained in
2-methyltetrahydrofuran at 77 K (see Supporting Information,
Figure S3). An adiabatic triplet energy of 2.72 eV was estimated
from the highest energy maximum in the spectrum, identical
within experimental uncertainty with the value previously
2
0
C ](picolinato-N,O) iridium, FIrpic, for which a triplet energy of
about 2.6 eV is reported.54,55
Electrochemical Properties. As with the optical data,
the methoxy-substituted carbazole-oxadiazole small molecules
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Table 3. Reduction Potentials,a Estimated Solid-State Elec-
tron Affinities (EA)b and Ionization Potential (IP), and DFT/
B3LYP Gas-Phase IPs and EAs of 13, 14, and 15
relation EA(s) ≈ 4.8 eV + eE1/20/ꢀ, where E1/2
is quoted
0/ꢀ
relative to ferrocenium/ferrocene.60 Because of the irreversibility
of the oxidation processes, the solid-state IPs were derived from
the EA estimates and the absorption onset recognizing that the
optical gap is likely to be smaller than the electrochemical gap
(which, in turn, may also be smaller than the true transport gap)
because of the effect of the exciton-binding energy.
EA(g) (eV)b
IP(g) (eV)
0/ꢀ
E1/2
(V)a
EA(s)
(eV)b,c (eV)d
IP(s)
KTe
ΔSCFf
KTe
ΔSCFf
IPs and EAs for isolated gas-phase molecules were also
obtained from DFT calculations, both using Koopmans’ theorem
(KT) and ΔSCF calculations (Table 3). EAs obtained using
either approach reproduce the trends seen in the experimental
potentials and estimated EAs, with 15 being some 0.2 eV more
readily reduced than 13 and 14. It should be noted that the large
discrepancy between the ΔSCF IPs and EAs (which are in
general expected to be more accurate than those obtained at
the KT level) and the experimentally estimated values is because
the calculations are performed on isolated molecules and, there-
fore, do not account for the additional stabilizations of the ionic
species by electronic polarization of the surrounding medium.61
In the case of IPs, KT and ΔSCF calculations predict slightly
different trends; however, both approaches indicate that the
IPs of the present systems are very similar and differ among
molecules at most by 0.14 eV.
13
14
15
ꢀ2.53
ꢀ2.54
ꢀ2.33
2.3
2.3
2.5
5.7
5.5
5.8
1.78
1.77
1.92
0.57
0.61
5.54
5.49
5.53
6.80
6.65
6.64
0.84
n
a Reduction potential vs FeCp2 in 0.1 M Bu4NPF6 THF solution.
+/0
b Defined as ꢀΔG for M + e f Mꢀ. c Estimated from EA = 4.8 eV +
.
d Estimated from IP = EA + Eonset using Eonset values from
0/ꢀ
eE1/2
Table 2; of the effects of exciton binding energy mean that these values
are likely to be underestimates. e Estimated according to Koopmans’
theorem, i.e., IP = ꢀEHOMO and EA = ꢀELUMO. f Vertical.
Use as Host Materials. LEDs were fabricated to compare the
polymers P1ꢀP3, which have a common polymethacrylate
backbone and the three different carbazole-oxadiazole motifs as
side groups, as host matrixes for the green phosphor Ir(ppy)3
(Chart 1). As in some of our previous work,26,62,63 poly-TPD-
F42,43—a copolymer of monomers functionalized with a bis-
(diarylamino)biphenyl hole-transport moiety and a photo-cross-
linkable cinnamate group, shown in Chart 1—was used as a
solution-processable hole-transport layer; cross-linking after
1 min of UV irradiation renders this film insoluble permitting
the deposition of the polymeric emissive layer from solution. 2,9-
Dimethyl-4,7-diphenyl-1,10-phenanthroline, BCP (Chart 1),
was vacuum-deposited as an electron-transport layer, and finally
a LiF/Al cathode was deposited to give an overall device
structure ITO/poly-TPD-F(35 nm)/polymer:Ir(ppy)3(6.0%)-
(40 nm)/BCP(40 nm)/LiF(2.5 nm)/Al(200 nm). This device
architecture has not been optimized, but is based on that of
efficient green phosphorescent OLEDs with ambipolar poly-
meric hosts (differing from the current work in that these hosts
are blends of PVK and oxadiazole side-chain polymers).26
Devices IꢀIII refer to those using P1ꢀP3, respectively, as hosts.
All three devices show typical Ir(ppy)3-based green emission.
The behavior of devices IꢀIII is summarized in Figures 6 and 7
and in Table 4. Devices I and III are characterized by similar turn-
on voltages (defined as the voltage required to obtain a bright-
ness of 10 cd/m2) and external quantum efficiencies (EQEs) to
one another and by fairly high maximum luminance values.
These device characteristics are similar to those previously
reported for similar devices using an emissive layer composed
of PVK, oxadiazole side-chain polymers, and Ir(ppy)3;26,64 this is
perhaps not too surprising given the use of essentially the same
device architecture and given the similarity in the transport levels
estimated for the emissive layers. On the other hand, device II
shows a turn-on voltage about 2 V higher than those of devices I
and III, along with about 6ꢀ10 times lower luminance at the
same applied voltage and the lowest efficiency. These observa-
tions presumably arise from a combination of the effects of
differences in hole and electron mobility values for the polymers
with meta and para carbazole-oxadiazole linkages, and of
Figure 6. J-V characteristics of devices: I (squares), II (circles), and III
(triangles) (inset shows the same data on a semilogarithmic scale).
13, 14, and 15 were used as models for the electrochemical
behavior of the corresponding side-chain polymers. Cyclic
voltammograms were obtained in THF with 0.1 M tetrabuty-
lammonium hexafluorophosphosphate as the supporting elec-
trolyte. All three show reversible reductions (see Supporting
Information, Figure S4), while the molecular oxidations (not
shown) are highly irreversible, as is typical for carbazoles without
3,6 substitution because of coupling of the radical cations
through these positions,56,57 meaning values of the redox
potential for this process are not easily or reliably determined.
The potentials (see Table 3) are similar to those reported for
other oxadiazoles (2,5-diphenyl-1,3,4-oxadiazole is reduced at
+/0
ꢀ2.47 V vs FeCp2
in acetonitrile58,59 and the carbazole-
oxadiazoles, IIIꢀV, shown in Figure 3 are reduced at about
ꢀ2.3 V in THF33) consistent with the localization of the lowest
unoccupied molecular orbitals (LUMOs) on the diphenylox-
adiazole portion of the molecules. Interestingly, the meta- and
para-monocarbazole derivatives, 13 and 14, are reduced at
similar potential to one another, while the bis(carbazole)
derivative, 15, is about 0.2 V more easily reduced, thus
suggesting that the influence of the carbazole moieties on the
redox potential of the oxadiazole moiety is largely through
inductive electron withdrawal. The solid-state electron affinities
(EAs) of the molecules 13, 14, and 15 can be estimated as
2.3ꢀ2.5 eV from the reduction potentials via the widely used
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Table 4. Performance of Green Phosphorescent OLEDs
Using P1, P2, and P3 Hosts
current
efficiency
at 100 cd/m2
(cd/A)
turn-on
maximum
luminance 100 cd/m2
EQE at
voltage at
device host 10 cd/m2 (V) (cd/m2)
(%)
I
P1
P2
P3
6.2
8.0
6.0
6500
1000
9.6
8.8
32.9
30.0
34.1
II
III
10000
10.0
devices, including examples that we have previously reported
using an emissive layer composed of a copolymer of 2,7-bis-
(carbazol-9-yl)fluorene host moieties and Ir(ppy)3-based phos-
phor guests.65 It should be noted that considerably more efficient
devices (EQE up to 18.8%) have been reported that use solution-
processed emissive layers based on PVK/small-molecule-oxadia-
zole/Ir(ppy)3 blends processed onto cross-linked bis(diarylamino)-
biphenyl-oxetane hole-transport layers.24 However, compared to
other solution-processed ambipolar hosts comprising poly-
merꢀpolymer or small-molecule-polymer blends, the current
polymers may be advantageous in that phase separation be-
tween hole-transport and electron-transport components is not
possible; although phase separation of the phosphor is still
potentially an issue, this should be addressable by copolymer-
ization of phosphor-functionalized monomers, such as those
reported in ref 65 with host monomers such as 8. Moreover,
improvements might be anticipated after optimization of layer
thicknesses and perhaps variation of the other organic materi-
als. In particular, depending on the location of the recombina-
tion zone within the emissive layer, triplet energy transfer to
poly-TPD-F (the triplet energy of which is presumably close to
the 2.3 eV reported for 4,40-bis(phenyl-m-tolylamino)biphenyl,
TPD52) could have an adverse effect on the device performance,
while if the polymers are to be used as hosts for blue-green
phosphors such as FIrpic then BCP (ET = 2.5 eV52) may also have
to be replaced by a higher-triplet-energy species to effectively
confine triplet excitons to the emissive layer.
’ CONCLUSIONS
Polymethacrylates, polystyrenes, and polynorbornenes have
been prepared with three different phenylene-linked carbazole-
oxadiazole side chains; the polymers exhibit high decomposi-
tion temperatures and moderate to high glass-transition tem-
peratures that depend on the identity of both the backbone and
the side chain. Spectroscopic and electrochemical data for
model small compounds suggest that these polymers should
be suitable for use as hosts for green and, at least in some cases,
blue-green phosphors. Preliminary unoptimized OLEDs in
which these materials are used as hosts for the green phosphor
Ir(ppy)3 exhibit external quantum efficiencies of about 10%,
suggesting that the use of polymers with pendant ambipolar
side chains may be a promising approach to solution-processed
phosphorescent OLEDs.
Figure 7. Luminance (squares) and external quantum efficiency
(circles) as a function of the applied voltage for devices I (top), II
(middle), and III (bottom).
differences in charge-injection properties, but at this point any
detailed explanation of the differences in device behavior would
be highly speculative. In green electrophosphorescent devices
based on related small molecules IIIꢀV (Figure 3) the para-
linked species V also exhibits lower efficiency than its meta-linked
analogue IV, but shows a similar turn-on voltage.33 While much
higher efficiencies have been reported for devices based on
related ambipolar small molecules such as IIIꢀV,33 those devices
were fully vacuum-processed whereas the current devices are
partially solution-processed. As noted above, devices I and III
show similar performance to devices using polymer blends
as ambipolar hosts26 and give higher efficiencies than many
other solution-processed polymer green electrophosphorescent
’ ASSOCIATED CONTENT
S
Supporting Information. Figures showing TGA, DSC,
b
phosphorescence, and electrochemical data. This material is
available free of charge via the Internet at http://pubs.acs.org.
4013
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Chemistry of Materials
ARTICLE
’ AUTHOR INFORMATION
(28) Zhang, K.; Tao, Y.; Yang, C.; You, H.; Zou, Y.; Qin, J.; Ma, D.
Chem. Mater. 2008, 20, 7324.
Corresponding Author
*E-mail: seth.marder@chemistry.gatech.edu.
(29) Brunner, K.; van Dijken, A.; B€orner, H.; Bastiaansen, J. J. A. M.;
Kiggen, N. M. M.; Langeveld, B. M. W. J. Am. Chem. Soc. 2004, 126, 6035.
(30) Guan, M.; Chen, Z. Q.; Bian, Z. Q.; Liu, Z. W.; Gong, Z. L.;
Baik, W.; Lee, H. J.; Huang, C. H. Org. Electron. 2006, 7, 330.
(31) Leung, M.-k.; Yang, C.-C.; Lee, J.-H.; Tsai, H.-H.; Lin, C.-F.;
Huang, C.-Y.; Su, Y. O.; Chiu, C.-F. Org. Lett. 2007, 9, 235.
(32) Tao, Y.; Wang, Q.; Yang, C.; Wang, Q.; Zhang, Z.; Zou, T.; Qin,
J.; Ma, D. Angew. Chem., Int. Ed. 2008, 47, 8104.
(33) Tao, Y.; Wang, Q.; Yang, C.; Zhong, C.; Zhang, K.; Qin, J.; Ma,
D. Adv. Funct. Mater. 2010, 20, 304.
(34) Jiang, X.; Register, R. A.; Killeen, K. A.; Thompson, M. E.;
Pschenitzka, F.; Sturm, J. C. Chem. Mater. 2000, 12, 2542.
(35) Guan, M.; Bian, Z. Q.; Zhou, Y. F.; Li, F. Y.; Li, Z. J.; Huang,
C. H. Chem. Commun. 2003, 2003, 2708.
’ ACKNOWLEDGMENT
Support from Solvay SA and from the Science and Technology
Center Program of the National Science Foundation (DMR-
0120967) is gratefully acknowledged. We are grateful to Jean-
Pierre Catinat and Vꢀeronique Mathieu for measuring the adiabatic
triplet energy of 15.
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4014
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Chemistry of Materials
ARTICLE
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