Nondoped pure-blue OLEDs based on amorphous phenylenevinylene- functionalized twisted bimesitylenes

Article abstract of DOI:10.1021/jo1001565

The twisted bimesitylene scaffold hinders crystallization and imparts amorphous nature to the oligophenylenevinylenes (OPVs) generated by 2- and/or 4-fold functionalization. The resultant phenylenevinylenes 1?5 with unique molecular topology exhibit excellent thermal and solid-state luminescence properties. The amorphous nature permits their application as pure-blue emissive materials in OLEDs. Under nondoped conditions, the device performances observed surpass those for analogous and simple oligophenylenevinylenes known so far; for example, the device based on OPV 2 as an emitting material and structurally analogous Bim-DPAB as a hole-transporting material yields pure-blue electroluminescence with an external quantum efficiency of ca. 4.70% at 20 mA/cm², which is higher than those reported for nondoped pure-blue OPV emitters.

Full text of DOI:10.1021/jo1001565

pubs.acs.org/joc
Nondoped Pure-Blue OLEDs Based on Amorphous
Phenylenevinylene-Functionalized Twisted Bimesitylenes
Jarugu Narasimha Moorthy,*,† Parthasarathy Venkatakrishnan,† Palani Natarajan,†
Zhenghuan Lin,‡ and Tahsin J. Chow*,‡
†Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India, and
‡Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115, Republic of China
moorthy@iitk.ac.in; tjchow@chem.sinica.edu.tw
Received January 29, 2010
The twisted bimesitylene scaffold hinders crystallization and imparts amorphous nature to the
oligophenylenevinylenes (OPVs) generated by 2- and/or 4-fold functionalization. The resultant
phenylenevinylenes 1-5 with unique molecular topology exhibit excellent thermal and solid-state
luminescence properties. The amorphous nature permits their application as pure-blue emissive
materials in OLEDs. Under nondoped conditions, the device performances observed surpass those
for analogous and simple oligophenylenevinylenes known so far; for example, the device based on
OPV 2 as an emitting material and structurally analogous Bim-DPAB as a hole-transporting material
yields pure-blue electroluminescence with an external quantum efficiency of ca. 4.70% at 20 mA/cm2,
which is higher than those reported for nondoped pure-blue OPV emitters.
1. Introduction
the fluorescence in the solid state is efficiently quenched, while
emission occurs in the solution state with a very high efficiency.3
The quenching in the solid state has been attributed to inter-
molecular π-π stacking interactions and H-type aggregation,
which lead to the formation of excimers.4 Thus, various
strategies have been explored to obviate quenching in the solid
state and increase photoluminescence efficiency via suppression
of molecular aggregation. The tetrahedrally linked DSBs, viz.
The organic π-conjugated systems based on polyphenyle-
nevinylenes (PPV) and oligophenylenevinylenes (OPV) are
of immense interest at the present time due to their applica-
tions in light-emitting diodes (LEDs), semiconducting and
photoconducting devices, nonlinear optics, light-harvesting
antennas, etc.1 Insofar as organic LEDs are concerned,
OPVs constitute excellent materials for light emission.2
However, a serious drawback, as exemplified by the repre-
sentative distyrylbenzene (DSB) and its derivatives, is that
5a
tetrakis(4-tert-butylstyrylstilbenyl)methane C(tBuSSB)4 and
tetrakis(4-(40-(300,500-dihexyloxystyryl)styryl)stilbenyl)methane
(1) (a) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, Plenum
Press: New York, 1999. (b) Meier, H. Angew. Chem., Int. Ed. Engl. 1992, 31,
1399. (c) Tessler, N.; Denton, G. J.; Friend, R. H. Nature 1996, 382, 695. (d) Tour,
J. M. Chem. Rev. 1996, 96, 537. (e) Kraft, A.; Grimsdale, A. C.; Holmes, A. B.
Angew. Chem., Int. Ed. 1998, 37, 402. (f) Organic Electroluminescence; Kafafi,
Z. H., Ed.; Taylor and Francis: Boca Raton, FL, 2005; Optical Engineering Vol. 94.
(g) Organic Light Emitting Devices. Synthesis, Properties and Electronic
Applications; Mullen, K., Scherf, U., Eds.; Wiley: Weinheim, Germany, 2006.
(2) (a) Electronic Materials: The Oligomer Approach; M€ullen, K., Ed.;
Wiley-VCH: Weinheim, Germany, 1998. (b) Liu, B.; Bazan, G. C. Organic
Electroluminescence; Kafafi, Z. H., Ed.; Taylor and Francis: Boca Raton, FL,
2005; Optical Engineering Vol. 94.
(3) (a) Oelkrug, D.; Tompert, A.; Egelhaaf, H.-J.; Hanack, M.; Steinhuber,
E.; Hohloch, M.; Meier, H.; Stalmach, U. Synth. Met. 1996, 83, 231. (b) Oelkrug,
D.; Tompert, A.; Gierschner, J.; Egelhaaf, H.-J.; Hanack, M.; Hohloch, M.;
Steinhuber, E. J. Phys. Chem. B 1998, 102, 1902. (c) Ruiz, P. M.; Behnisch, B.;
Schweikart, K. H.; Hanack, M.; Ler, L.; Oelkrug, D. Chem.;Eur. J. 2000, 6,
1294.
€
(4) Wurthner, F.; Thalacker, C.; Dieke, S.; Tschierske, C. Chem.;Eur. J.
2001, 7, 2245.
(5) (a) Wang, S.; Oldham, W. J. Jr.; Hudack, R. A. Jr.; Bazan, G. C.
J. Am. Chem. Soc. 2000, 122, 5695. (b) Robinson, M. R.; Wang, S.; Bazan,
G. C.; Cao, Y. Adv. Mater. 2000, 12, 1701. (c) Gu, T.; Accorsi, G.; Armaroli,
N.; Guillon, D.; Nierengarten, J. F. Tetrahedron Lett. 2001, 42, 2309.
DOI: 10.1021/jo1001565
r
Published on Web 03/23/2010
J. Org. Chem. 2010, 75, 2599–2609 2599
2010 American Chemical Society

Moorthy et al.
JOCArticle
CHART 1. Structures of the OPVs 1-5
T-4R-C6H13,5b and 4-fold DSB-functionalized calix[4]arenes5c
are some examples of rational approaches aimed at enhancing
the emission in the solid state. Incidentally, the device perfor-
mances with such molecular systems are not entirely encour-
aging.5a,b A classical blue-emitting dopant such as 4,40-bis(2,2-
diphenylvinyl)biphenyl (DPVBi) has been found to exhibit
bluish-green electroluminescence based on color chromaticity
diagram in addition to the fact that its glass transition tem-
perature (Tg) is not appreciably high (below 100 °C).6 Indeed,
OPVs that exhibit pure-blue emission with a high EL efficiency,
brightness, and stability are still rare.
We have recently shown that the twisted anthracene-
functionalized bimesitylenes exhibit amorphous behavior
and that they can be exploited as brilliant emitting materials
for application in LEDs.7a We have also demonstrated
the utility of diarylamino end-capped bimesitylenes (e.g.,
Bim-DPAB in Chart 1) as emitting as well as hole-transport-
ing materials in organic LEDs.7b In continuation of these
studies, we surmised that the twisted bimesityl core can
be beneficially exploited to develop OPVs with better optical
properties and high Tg values. Thus, we designed 2- and
4-fold OPVs 1-5 in Chart 1 with the following rationale:
• the “twisted” geometry introduced by the bimesityl core
supposedly hinders close packing of molecules, leading
to amorphous property,8 which is essential for applica-
tion as materials in OLEDs.
• the difficulty associated with the close packing of three-
dimensional molecules with orthogonal planes typified
by bimesityl presumably suppresses the self-quenching,
such that the emission quantum yields are enhanced
in the solid state as compared to those in the solution
state.
•
the unique molecular topology with meta linkage
restricts the conjugation and hence conserves the
optical properties of the chromophoric units that are
attached to the core.
(6) Matsurra, M.; Kusumoto, T.; Tokailin, H. US Patent 5 516 577, 1996.
(7) (a) Moorthy, J. N.; Venkatakrishnan, P.; Natarajan, P.; Huang, D.-F.;
Chow, T. J. J. Am. Chem. Soc. 2008, 130, 17320. (b) Moorthy, J. N.;
Venkatakrishnan, P.; Huang, D.-F.; Chow, T. J. Chem. Commun. 2008, 2146.
(8) (a) Shirota, Y. Organic Electroluminescence; Kafafi, Z. H., Ed.; Taylor
and Francis: Boca Raton, FL, 2005; Optical Engineering Vol. 94, Chapter 4, p 147.
(b) Shirota, Y. J. Mater. Chem. 2005, 15, 75. (c) Shirota, Y.; Kageyama, H. Chem.
Rev. 2007, 107, 953.
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SCHEME 1. Synthesis of OPVs 1-5a
aReagents and conditions: (a) NIS, TFA (catalytic amount), CH3CN, 5 °C to rt, 8 h, 83%; (b) I2, HNO3/H2SO4 (1:1), CHCl3, AcOH, 0 °C to rt, 3 h, 86%;
(c) 4-formylphenylboronic acid, Pd(PPh3)4, 2 M K2CO3, toluene/THF (2:1), 90 °C, 18 h, 82-97%; (d) 40-formylbiphenyl-4-boronic acid, Pd(PPh3)4, 2 M
K2CO3, toluene/THF (2:1), 90 °C, 24 h, 80-95%; (e) PhB(OH)2, Pd(PPh3)4, sat NaHCO3, toluene, 90-100 °C, 12 h, 77%; (f) anhyd AlCl3, C6H5COCl,
CS2, reflux, 2.5 h, 81%; (g) excess diphenylmethyl lithium, -20 °C to rt, 3 h, then rt, overnight; (h) PTS, toluene, reflux, overnight, 56-65%.
• the cross arrangement of the dipoles due to the styryl
groups is expected to increase luminescence efficiency
in the solid state, as has been shown recently by Ma
et al.9
Systems Committee (NTSC) standards and exhibit Tg
values, EL efficiencies, and brightness that surpass those
reported heretofore for simple OPVs lacking amino func-
tionalities.
Herein, we report the synthesis, photophysical, thermal,
and electrochemical properties of OPVs 1-5. Their func-
tional behavior is demonstrated by constructing electrolu-
minescent (EL) devices for OPVs 1-3. It is shown that
amines with analogous topological features, viz., Bim-
DPAB, can be employed as a hole-transporting material
(HTM) to yield device electroluminescence efficiency that
is ca. 2.5-fold better than with a conventional HTM, viz., N,
N0-bis(naphthalen-1-yl)-N,N0-bis(phenyl)benzidine (NPB).
The three-dimensional bimesitylene-based OPVs 1-3 exhi-
bit pure-blue emission close to the National Television
2. Results
Synthesis of 2- and 4-Fold Phenylenevinylene-Functiona-
lized Bimesitylenes 1-5. The synthesis of OPVs 1-5 was
accomplished starting from 3,30-diiodobimesityl 67 and
3,30,5,50-tetraiodobimesityl 7,7a,10 as shown in Scheme 1.
The Suzuki coupling of the diiodobimesityl 6 with 4-formyl-
phenylboronic acid11 and 4-formylbiphenyl-40-boronic
acid10 gave the dialdehydes 8 and 9, respectively. Further
treatment of the dialdehydes 8 and 9 with excess of diphe-
nylmethyl lithium followed by acid-catalyzed dehydration
(9) (a) Xie, Z.; Yang, B.; Li, F.; Cheng, G.; Liu, L.; Yang, G.; Xu, H.; Ye,
L.; Hanif, M.; Liu, S.; Ma, D.; Ma, Y. J. Am. Chem. Soc. 2005, 127, 14152. (b)
He, F.; Xu, H.; Yang, B.; Duan, Y.; Tian, L.; Huang, K.; Ma, Y.; Liu, S.;
Feng, S.; Shen, J. Adv. Mater. 2005, 17, 2710.
(10) Moorthy, J. N.; Natarajan, R.; Venugopalan, P. J. Org. Chem. 2005,
70, 8568.
(11) Cousin, D.; Mann, J.; Nieuwenhuyzen, M.; van den Berg, H. Org.
Biomol. Chem. 2006, 4, 54.
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TABLE 1. Optical Absorption, Photoluminescence (PL), PL Quantum Yield, Electrochemical Data, and Thermal Properties of 1-5
band gap,b
nm (eV)
λ
max (PL),c soln
(solid), nm
Φfl,d soln
(solid)
HOMO, LUMO,e
eV
OPV
λ
max,a nm
Td,f °C
Tg,g °C
1
2
3
4
5
312
326
314
329
315
397 (3.12)
390 (3.18)
366 (3.38)
399 (3.11)
381 (3.25)
439 (447)
450 (448)
441 (440)
444 (473)
474 (474)
0.015 (0.11)
0.01 (0.29)
0.012 (0.06)
0.06 (0.22)
0.004 (0.26)
6.03 (6.08), 2.91
5.82 (6.05), 2.68
6.05 (6.06), 2.68
5.82 (6.08), 2.71
6.17 (6.08), 2.92
438
460
463
495
508
94
123
133, 195h
155, 254h
152, 251h
aIn cyclohexane solutions (10-5 M). bCalculated from the absorption onset values. cAll the emission spectra were recorded for dilute solutions (10-6
M) in cyclohexane, λex = 367 nm for the solution state and λex = 325 nm for the solid state. dQuantum yields were determined using anthracene as the
standard and N2 bubbled cyclohexane as the solvent with λex = 367 nm. The reported values refer to an average of three independent determinations.
The values in parentheses refer to quantum yields for the solid sample/film. eHOMO values were calculated from oxidation potentials, while LUMO
values were calculated by subtracting the band gap values from the HOMO values. The HOMO values as obtained from UVPES are given in parentheses
for comparison. fDetermined from TGA analyses performed under nitrogen at a heating rate of 10 °C/min. gDetermined from DSC analyses performed
under nitrogen at a heating rate of 10 °C/min. hMelting temperature.
afforded the desired disubstituted OPVs 1 and 2. A similar
reaction sequence as above starting from tetraiodobimesityl
7 led to tetrasubstituted diphenylvinyl analogues 3 and 4
in good yields (62-65%). The OPV 5 containing the
tetraphenylethylene moiety was prepared starting from 7 in
56% yield via Suzuki coupling, Friedel-Crafts benzoyla-
tion, followed by treatment with excess of diphenylmethyl
lithium and subsequent dehydration12 (cf. Experimental
Section). The preparation of Bim-DPAB has been reported
previously.7b
Photophysical Properties. In Figure 1, a comparative
picture of the UV-vis absorption spectra for 1-5 is shown,
which shows that all OPVs exhibit λmax within a narrow
region (i.e., 312-329 nm). Extension of the arms by changing
phenyl to biphenyl (1f2 or 3f4) results in a slight bath-
ochromic shift by ca. 14 nm in the absorption maximum.
Introduction of an additional phenyl group into the vinyl
double bond does not result in a significant red shift, as is
evident from a comparison of the spectra for 3 and 5. The
UV-vis absorption maxima and the band gap energies for
all 1-5 calculated from their absorption onset potentials are
given in Table 1. The solution-state photoluminescence
studies for the phenylenevinylenes 1-5 show that the emis-
sion maxima for 1-5 fall in the range of 439-474 nm, with 5
exhibiting a maximum red-shifted emission at 474 nm.
Remarkably, all compounds 1-5 exhibit blue emission.
While the emission for all 1-5 was found to be weak in
FIGURE 1. UV-vis absorption and PL spectra in cyclohexane
the solution state (Φfl = 0.004-0.06), strong blue emis-
sion was observed in the solid state (Φfl = 0.06-0.29). A
comparison of the emission spectra for 1-5 recorded in the
solution and solid states (Figure 1) reveals a red shift by
ca. 10-30 nm for1, 2, and 4, while virtually no difference
is found in the emission maxima for 3 and 5. The emis-
sion maxima for compounds 1-5 along with their photolumi-
nescence quantum yields in both solid and solution states are
given in Table 1.
(top) and emission spectra in the solid state (bottom) for OPVs 1-5.
phenylenevinylene derivatives of bimesitylene 1-5 were
found to exhibit stable thermal properties upon several
heating and cooling cycles, as revealed by their DSC scans
(cf. Supporting Information). The OPV 1 showed Tg at ca.
94 °C without any noticeable presence of crystallization and
melting temperatures. In contrast, it is noteworthy that the
commercial blue-emitter, viz., 4,40-diphenylvinylbiphenyl,
shows glass transition at 64 °C and crystallizes at 106 °C.5a,6
The 4-fold-functionalized analogue 3 exhibited glass transition
at 133 °C followed by an endothermic transition at 195 °C. The
phenyl extended disubstituted bimesityl derivative 2 displayed
a stable glass transition temperature at ca. 123 °C without
melting up to 400 °C. The analogous 4-fold derivative 4 showed
glass transition at ca. 155 °C. The tetraphenylethylene-functio-
nalized bimesityl 5 exhibited Tg at 152 °C but revealed
a broad endothermic melting transition at around 251 °C.
Clearly, the three-dimensional D2d-symmetric bimesitylene
Thermal Properties. The morphological stabilities of
OPVs 1-5 were gauged by thermogravimetric analysis
(TGA) and differential scanning calorimetric (DSC) studies.
The TGA analyses show that the thermal decomposition
(Td) values for these compounds are relatively higher (>400
°C) when compared with their simple DSB analogues
(Table 1). The Td is found to increase with increasing number
of phenyl rings attached to the bimesityl core. All of the
(12) Banerjee, M.; Emond, S. J.; Lindeman, S. V.; Rathore, R. J. Org.
Chem. 2007, 72, 8054.
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˚
FIGURE 2. EL spectrum (left) and the I-V-L curves (right) of the devices of 1-3. The device configurations are (1A) ITO/NPB (400 A)/1
(100 A)/BCP (100 A)/Alq3 (300 A)/LiF (10 A)/Al (1500 A); (1B) ITO/NPB (400 A)/1 (100 A)/TPBI (400 A)/LiF (10 A)/Al (1500 A); (1C) ITO/
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
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NPB (300 A)/CBP (200 A)/1 (100 A)/TPBI (400 A)/LiF (10 A)/Al (1500 A); (2A) ITO/NPB (400 A)/2 (100 A)/BCP (100 A)/ Alq3 (300 A)/LiF
˚
˚
˚
˚
˚
˚
˚
(10 A)/Al (1500 A); (2B) ITO/NPB (400 A)/2 (100 A)/TPBI (400 A)/LiF (10 A)/Al (1500 A); (2C) ITO/NPB (300 A)/CBP (200 A)/2 (100 A)/
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
TPBI (400 A)/LiF (10 A)/Al (1500 A); (3A) ITO/NPB (400 A)/3 (100 A)/BCP (100 A)/Alq3 (300 A)/LiF (10 A)/Al (1500 A); (3B) ITO/NPB (400
˚
˚
A)/3 (100 A)/TPBI (400 A)/LiF (10 A)/Al (1500 A).
˚
˚
˚
˚
core discourages close packing of molecules and hence crystal-
lization to impart amorphous glassy nature.
Electroluminescence Properties. The functional behavior
of OPVs 1-5 was investigated by fabricating devices for
1-3, as representative cases, to capture electroluminescence.
The 2-fold-functionalized (1 and 2) and 4-fold-functiona-
lized bimesitylene (3) were conveniently vacuum sublimed to
construct multilayer nondoped devices, which comprised
Electrochemical Properties. The cyclic voltammetric (CV)
measurements were carried out for 1-5 in order to deter-
mine the HOMO and LUMO energy levels (cf. Supporting
Information). The HOMO values were calculated from the
oxidation potentials (Eox) and were compared with the
ionization potentials determined from the UV photo-
electron spectroscopic measurements. The LUMOs were
calculated from their respective onset potentials (band
gap energy) obtained from the tail-end of the absorption
spectra and HOMO values. The di- and tetra-substituted
bimesitylene derivatives exhibited similar HOMO and LUMO
values (Table 1), which attests to the absence of communica-
tion between the chromophores linked at the meta sites.
The HOMO values of the OPVs were found to be as low as
ca. 6.0 eV with optical band gap energy higher than 3.0 eV.
˚
˚
four types: (A) ITO/NPB (400 A)/1-3 (100 A)/BCP (100
A)/ Alq3 (300 A)/LiF (10 A)/Al (1500 A), (B) ITO/NPB (400
˚
˚
˚
˚
˚
˚
˚
˚
˚
A)/1-3 (100 A)/TPBI (400 A)/LiF (10 A)/Al (1500 A), (C)
˚
˚
˚
˚
ITO/NPB (300 A)/CBP (200 A)/1-2 (100 A)/TPBI (400 A)/
LiF (10 A)/Al (1500 A), and (D) ITO/Bim-DPAB (400 A)/2
˚
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(100 A)/TPBI (400 A)/LiF (10 A)/Al (1500 A), where ITO
(indium tin oxide) was the anode, NPB or Bim-DPAB (3,30-
bis(4-diphenylaminobiphenyl-40-yl)bimesityl served as the
hole-transporting layer, CBP (4,40-bis(carbazol-9-yl)biphenyl)
served as the electron-blocking layer, 1/2/3 as the emitting
layer, BCP (bathocuprine) as a hole-blocking material, TPBI
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Moorthy et al.
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TABLE 2. Electroluminescence Data for 1-3
led to considerable increase in the device efficiency (up to
2.22% for 1) as well as in the luminance efficiency (up to 2.28
cd/A for 2) with a marginal change in color purity (cf.
Supporting Information). Remarkably, replacement of the
hole-transporting layer involving NPB with Bim-DPAB, an
HTM with structural characteristics similar to those of OPVs
1-5, in the case of the device 2B for OPV 2, that is, ITO/
devicea Von
ηex
ηp
ηle
λmax
Lmax
CIEg x, y
b
c
d
EL
f
1A
1B
1C
2A
2B
2C
2D
3A
3B
a
4.5
1.52 0.55 1.22
1.51 0.66 1.29
2.21 0.93 1.90
1.07 0.49 1.38
1.95 0.95 1.80
2.11 1.10 2.24
4.70 2.02 5.40
1.06 0.39 1.15
1.08 0.43 1.07
448
452
450
462
452
456
460
452
452
5415 0.15, 0.09
4575 0.14, 0.08
7599 0.15, 0.10
6129 0.10, 0.16
9031 0.14, 0.11
9500 0.15, 0.13
15361 0.15, 0.17
1466 0.16, 0.13
1727 0.15, 0.12
4.5
5.0
4.5
4.5
5.0
4.5
5.0
5.5
˚
˚
˚
˚
Bim-DPAB (400 A)/2 (100 A)/TPBI (400 A)/LiF (10 A)/Al
˚
(1500 A), resulted in a dramatic improvement in the device
efficiencies; the external quantum efficiency of the above device
reached as high as 4.70% (20 mA/cm2), which is higher than
those reported for nondoped pure-blue emitters based on OPVs
reported so far.9,14
˚
˚
The four types of devices are (A) ITO/NPB (400 A)/1-3 (100 A)/
˚
˚
˚
˚
BCP (100 A)/Alq3 (300 A)/LiF (10 A)/Al (1500 A), (B) ITO/NPB (400
˚
˚
˚
˚
˚
A)/1-3 (100 A)/TPBI (400 A)/LiF (10 A)/Al (1500 A), (C) ITO/NPB
(300 A)/CBP (200 A)/1-2 (100 A)/TPBI(400 A)/LiF (10 A)/Al (1500 A),
(D) ITO/Bim-DPAB (400 A)/2 (100 A)/TPBI (400 A)/LiF (10 A)/Al
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
b
c
˚
3. Discussion
(1500 A). Turn-on voltage. External quantum efficiency (%) at 20 mA/
cm2. dPower efficiency (lm/W) at 20 mA/cm2. eLuminance efficiency (cd/
A) at 20 mA/cm2. Maximum luminance achieved (cd/m2). g1931 CIE
chromaticity coordinates.
f
Enormous commercial success with π-conjugated poly-
mers/oligomers in OLEDs has spurred a great deal of interest
in the development of blue-light-emitting π-conjugated mo-
lecules that display desirable optoelectronic properties.
There has been an emphasis of interest in blue-light-emitting
materials in particular, as they can be used to generate white
light emission.14a,b,15 Besides, it has been shown that the
purity of blue emission is inversely related to the consump-
tion of power for full-color displays. Thus, the blue-light-
emitting molecular materials based on OPVs 1-5, which are
of intermediate dimension, offer tremendous advantage over
other materials from the point of view of ease of processa-
bility. The linkage of additional phenyl rings, as in 4 and 5,
leads to increase in decomposition (Td) as well as the glass
transition temperatures (Tg) considerably (Table 1). The
attachment of additional phenyl rings evidently contributes
to some steric congestion and bulkiness of the molecules,
leading to better and stable morphological behavior, as
reflected by the device characteristics; highly stable glassy
nature is what is most desired of a material to be employed in
OLEDs. The extension of π-conjugation by way of attach-
ment of an additional phenyl ring does result in a marginal
bathochromic shift of the λmax (UV-vis or PL) such that the
optical properties may be appropriately modulated. In the
case of 4-fold derivatives 3-5, the π-conjugation is limited to
the attached unit, and there exists only a weak meta-con-
jugation or communication between the units linked to the
central mesitylene ring. The presence of the additional
phenyl groups on the periphery of OPVs 3-5 may force a
nonplanar geometry of the π-system (CdC) due to sterically
congested environment around the double bond.16 The effect
of meta-conjugation and the effect of attachment of addi-
tional phenyl ring on fluorescence can be readily discerned
when moving from 1 to 3 to 5.16b,17 It is well-known that the
emission of multisubstituted olefins depends not only on the
nature of the aryl groups but also on the position of their
attachment on to the olefins.16 A marginal red shift observed
in the emission of solid films, in general, may be attributed to
the difference in the media. The solid-state quantum effi-
ciencies of all the OPVs are greater than those observed in the
solution state. The poor solution-state quantum efficiencies
(2,20,200-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole)/
Alq3 (tris(8-hydroxyquinolinato)aluminum) as an electron-
transporting layer, and LiF/Al as the composite cathode. The
EL spectra and I-V-Lcharacteristics of the devices 1A-C, 2A-
C, and 3A-B are shown in Figure 2. The performance char-
acteristics for these devices are presented in Table 2. Clearly, the
nondoped devices constructed for OPVs 1-3 revealed excellent
pure-blue emission according to the NTSC standards with CIE
coordinates (Commission Internationale de l’Eclairge) equal to
or nearest to x = 0.14, y = 0.08; the chromaticity diagram for
the blue light emission from the EL devices of 1-3 (types A and
B) is displayed in Figure 3. The device turn-on voltages were as
low as 4.5-5.5 V. The external quantum efficiencies of these
nondoped devices of standard configurations were found to
vary from 1.1 to 1.5% for BCP/Alq3-based devices and
1.1-2.0% for TPBI-based devices at a current density of
20 mA/cm2. The color purity as well as the performance of
TPBI-based devices is, in general, better as compared to that of
BCP/Alq3-based devices.13 Hence, further modifications were
attempted on the device structure of the TPBI-based devices
(type B) in order to improve the device efficiencies for 1 and 2.
Insertion of additional electron-blocking CBP (4,40-bis-
(carbazol-9-yl)biphenyl) layer between the emitting layer and
the hole-transporting layer in device type B for OPVs 1 and 2
(13) For example, see: (a) Tao, Y. T.; Balasubramaniam, E.; Daniel, A.;
Wilsa, A.; Tomasik, P. J. Mater. Chem. 2001, 11, 768. (b) Thomas, K. R. J.;
Velusamy, M.; Lin, J. T.; Chuen, C. H.; Tao, Y. T. J. Mater. Chem. 2005, 15,
4453.
(14) (a) van Hutten, P. F.; Wildeman, J.; Meetsma, A.; Hadziioannoou,
G. J. Am. Chem. Soc. 1999, 121, 5910. (b) Cheon, K. O.; Shinar, J. Appl. Phys.
Lett. 2002, 81, 1738. (c) Xie, W.; Hou, J.; Liu, S. Y. Semicond. Sci. Technol.
2003, 18, 42.
(15) (a) Kido, J.; Kimura, M.; Nagai, K. Science 1995, 267, 1332. (b)
Strukelj, M.; Jordan, R. H.; Dodabalapur, A. J. Am. Chem. Soc. 1996, 118,
1213. (c) D’Andrade, B. W.; Thompson, M. E.; Forrest, S. R. Adv. Mater.
2000, 12, 130. (d) D’Andrade, B. W.; Brooks, J.; Adamovich, V.; Thompson,
M. E.; Forrest, S. R. Adv. Mater. 2002, 14, 1032. (e) Zhang, X. H.; Liu,
M. W.; Wong, O. Y.; Lee, C. S.; Kwong, H. L.; Lee, S. T.; Wu, S. K. Chem.
Phys. Lett. 2003, 369, 478. (f) Chuen, C. H.; Tao, Y. T.; Wu, F. I.; Shu, C. F.
Appl. Phys. Lett. 2004, 85, 4609. (g) Mita, Y.; Kobayashi, T.; Miyamoto, Y.;
Ishii, O.; Sawanobori, N. Phys. Status Solidi B 2004, 241, 2672. (h)
D’Andrade, B. W.; Forrest, S. R. Adv. Mater. 2004, 16, 1585. (i) Mazzeo,
M.; Vitale, V.; Sala, F. D.; Anni, M.; Barbarella, G.; Favaretto, L.; Sotgiu,
G.; Cingolani, R.; Gigli, G. Adv. Mater. 2005, 17, 34. (j) Cheng, G.; Xie, Z.;
Zhao, Y.; Zhang, Y.; Xia, H.; Ma, Y.; Liu, S. Thin Solid Films 2005, 484, 54.
(k) Yang, Y.; Lowry, M.; Schowalter, C. M.; Fakayode, S. O.; Escobedo, J. O.; Xu,
X.; Zhang, H.; Jensen, T. J.; Fronczek, F. R.; Warner, I. M.; Strongin, R. M. J. Am.
Chem. Soc. 2006, 128, 14081 and references cited therein.
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Chem. 2007, 72, 8054.
2604 J. Org. Chem. Vol. 75, No. 8, 2010

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FIGURE 3. CIE chromaticity diagram (left) for the nondoped devices (A and B) constructed from OPVs 1-3. The circle indicates the pure-
blue emission region according to the NTSC standards. The enlarged view of this region is shown in the right. The inset photograph shows the
blue electroluminescence captured from device 1B.
in all of these cases are undoubtedly due to efficient isomeri-
zation, which is characteristic of stilbenes.18 The respectable
solid-state quantum efficiencies of OPVs 1-5 and the ab-
sence of long wavelength emission band in their PL of solid/
films suggests that the molecular topology prevents efficient
molecular packing and hence aggregation, thereby reducing
the intermolecular interactions and hence fluorescence
quenching, leading to significantly improved solid-state
luminescence properties (ca. 10-60-fold). Very recently,
several examples such as styrylbenzene derivatives,19 ary-
lethynylenes,20 siloles,21 polymers,22 and others23 have been
shown to display better emission in the solid state than in the
solution state. Though the origin of such enhanced emission
in the solid state is still not unambiguously clear, it is
presumed that the sole phenomenon of change in emission
is mostly linked to the inter- and intramolecular effects; while
the twisted (solution state) and the planar (solid state)
conformations affect (suppress and promote the radiation
process, respectively) the fluorescence change intramolecu-
larly,19a,g the type of aggregation (H- or J-aggregation)
influences the emission efficiency. H-aggregates induce
nonradiative deactivation process, while J-aggregates trigger
the radiative process intermolecularly.24 The nonplanar
sterically congested environment of the vinylene/ethylene
double bond in addition to the crossed arrangement of the
chromophores in the D2d-symmetric 4-fold-functionalized
OPVs 3-5 presumably contributes to the restricted molecu-
lar motions that promote radiative fluorescence enhance-
ment in the solid state.16b,c As mentioned earlier, it has been
predicted by Bredas et al.25 from theoretical analysis that the
nonplanar perpendicular arrangement of transition dipoles
of the chromophores avoids fluorescence quenching and
promotes the solid-state luminance efficiency. Also, Ma et
al. have recently demonstrated this feature in dimeric and
trimeric DSBs.9b,26 Indeed, such materials appear to be
relevant for application in solid-state lasers.9a,27
Remarkably, all OPVs exhibit a uniform oxidation beha-
vior in the CV in that the di- and the tetra-substituted
derivatives show similar oxidation potentials. These blue-
emitting OPVs with reasonable band gap energies (ca.
3.11-3.38 eV) may in principle be used in the generation of
full-color displays. The ready sublimation of OPVs 1-3
permitted facile fabrication of multilayer devices to capture
the pure-blue electroluminescence (λmax = 448-462 nm).
Although the OPVs 1-5 possess large band gap energies,
their low turn-on voltages from 4.5 to 5.5 V reveal that the
charge injection in these materials is really not difficult. It is
noteworthy that the blue-emitting materials suffer from charge
injection in general. Of the two types of nondoped devices
(A and B) fabricated, the device B was found to perform better
(18) Waldeck, D. H. Chem. Rev. 1991, 91, 415.
(19) (a) An, B. K.; Kwon, S. K.; Jung, S. D.; Park, S. Y. J. Am. Chem. Soc.
2002, 124, 14410. (b) Lim, S. J.; An, B. K.; Jung, S. D.; Chung, M. A.; Park,
S. Y. Angew. Chem., Int. Ed. 2004, 43, 6346. (c) Yeh, H. C.; Yeh, S. J.; Chen,
C. T. Chem. Commun. 2003, 2632. (d) Chen, J.; Xu, B.; Ouyang, X.; Tang,
B. Z.; Cao, Y. J. Phys. Chem. A 2004, 108, 7522. (e) Bhongale, C. J.; Chang,
C. W.; Lee, C. S.; Diau, W. G.; Hsu, C. S. J. Phys. Chem. B 2005, 109, 7522. (f)
Xie, Z.; Yang, B.; Cheng, G.; Liu, L.; He, F.; Shen, F.; Ma, Y.; Liu, S. Chem.
Mater. 2005, 17, 1287. (g) Xie, Z.; Yang, B.; Xie, W.; Liu, L.; Shen, F.; Wang,
H.; Yang, X.; Wang, Z.; Li, Y.; Hanif, M.; Yang, G.; Ye, L.; Ma, Y. J. Phys.
Chem. B 2006, 110, 20993.
(20) Levitus, M.; Schmieder, K.; Ricks, H.; Shimizu, K. D.; Bunz,
U. H. F.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2001, 123, 4259.
(21) Fan, X.; Sun, J.; Wang, F.; Chu, Z.; Wang, P.; Dong, Y.; Hu, R.;
Tang, B. Z.; Zou, D. Chem. Commun. 2008, 2989 and references cited therein.
(22) (a) Deans, R.; Kim, J.; Machacek, M. R.; Swager, T. M. J. Am.
Chem. Soc. 2000, 122, 8565. (b) Belton, C.; O’Brien, D. F.; Blau, W. J.;
Cadby, A. J.; Lane, P. A.; Bradley, D. D. C.; Byrne, H. J.; Stockman, R.;
˚
than A. The device from OPV 2 [2B: ITO/NPB (400 A)/2
˚
˚
˚
˚
(100 A)/TPBI (400 A)/LiF (10 A)/Al (1500 A)] shows best
performance with a bright blue luminance of 9031 cd/m2 and a
luminance efficiency of 1.8 cd/A at 20 mA/cm2. The external
quantum efficiency and the CIE coordinates of the above
device are 2.0% and x = 0.14 and y = 0.11, respectively
ꢀ
(25) Cornil, J.; Beljonne, D.; Calbert, J. P.; Bredas, J. L. Adv. Mater. 2001,
13, 1053.
(26) (a) He, F.; Cheng, G.; Zhang, H. Q.; Zheng, Y.; Xie, Z. Q.; Yang, B.;
Ma, Y.; Liu, S. Y.; Chen, J. C. Chem. Commun. 2003, 2206. For cruciform
DPVBi, see: (b) Liu, S.; He., F.; Wang, H.; Xu, H.; Wang, C.; Li, F.; Ma, Y.
J. Mater. Chem. 2008, 18, 4802.
(27) (a) Johansson, N.; Salbeck, J.; Bauer, J.; Weissortel, F.; Broms, P.;
Anderson, A.; Salaneck, W. R. Adv. Mater. 1998, 10, 1136. (b) Spehr, T.;
Pudzich, R.; Fuhrmann, T.; Salbeck, J. Org. Electron. 2003, 4, 61.
€
Horhold, H. H. Appl. Phys. Lett. 2001, 78, 1059.
(23) Wang, Z.; Shao, H.; Ye, J.; Tang, L.; Lu, P. J. Phys. Chem. B 2005,
109, 19627.
€
€
(24) Rosch, U.; Yao, S.; Wortmann, R.; Wurthner, F. Angew. Chem., Int.
Ed. 2006, 45, 7026 and references therein.
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Moorthy et al.
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FIGURE 4. External and luminance quantum efficiencies of the nondoped devices 1A-C and 2A-C.
FIGURE 5. EL spectrum and external and luminance quantum efficiencies of the device 2D. The device configuration of 2D was ITO/Bim-
˚
DPAB (400 A)/2 (100 A)/TPBI (400 A)/LiF (10 A)/Al (1500 A).
˚
˚
˚
˚
(Figure 4 and Table 2). The best device of 1 was achieved by
insertion of the additional CBP layer (HOMO = 5.7 eV and
LUMO = 2.0 eV)28 between NPB and the emitting material
other devices that were fabricated. The external quantum
efficiency for the device 2D with Bim-DPAB increased
dramatically from 1.97% (2B) to 4.70% (2.5-fold) with a
corresponding enhancement in the luminance efficiency
from 1.80 cd/A (2B) to 5.40 cd/A at current density = 20
mA/cm2; the maximum external and luminance efficiencies
were 4.79 and 5.52%, respectively, at a current density of
14.22 mA/cm2 (9.0 V) with the power efficiency being 2.44
lm/W at 6 V (0.84 mA/cm2). The EL spectrum and the
performance efficiency of the device 2D are given in Figure 5.
Given that the HOMO values of NPB29 and Bim-DPAB
(5.54 eV) are closer, the origin of the excellent performance
of device 2D should be attributed to the morphological
compatibility of the hole-transporting and emitting layers,
both of which are uniquely based on the bimesitylene scaf-
fold.
It is well-known that amorphous layers that are pinhole-
free facilitate charge mobilities and their recombination
efficiently. Thus, the fantastic amorphous property imparted
by the bimesitylene core with orthogonal planes in bringing
about morphological compatibility of the two layers is
compellingly evident from the device characteristics. There
are only scant reports of layer compatibility studies so far in
OLEDs with appropriately designed materials; for example,
the EL device constructed using spiro-based HTL (spiro-
TAD) and ETL (spiro-PBD) showed good performance.30
˚
˚
1 [1C: ITO/NPB (300 A)/CBP (200 A)/1 (100 A)/TPBI (300
˚
˚
˚
A)/LiF (10 A)/Al (1500 A)]. This promotes the efficient
hole-electron recombination to occur in the emitting layer
(1) by reducing the energy barrier for hole injection between
NPB (HOMO = 5.4 eV)29 and 1, and by blocking the
electrons from entering into NPB very readily. Thus, the
modified device showed a maximum luminance of 7599 cd/
m2 with an increased external quantum efficiency of 2.22%
and a luminance efficiency of 1.90 cd/A at 20 mA/cm2
(Figure 4). At a brightness of 800 cd/m2, devices 1C and
2C exhibited external quantum efficiencies of 2.14 and
2.04% at current density = 41.75 mA/cm2 (7.71 V) and a
luminance efficiencies of 1.84 and 2.18 cd/A at current
density = 35.06 mA/cm2 (8.49 V). Such high external
quantum and luminance efficiencies based on a simple device
structure for OPVs 1 and 2 are indeed remarkable. To our
excitement, the device 2D with a HTL based on structurally
analogous bimesityl-based amine, viz., Bim-DPAB, led to
excellent performance characteristics when compared to all
(28) (a) Chen, C.-H.; Wu, F.-I.; Shu, C.-F.; Chien, C.-H.; Tao, Y.-T. J.
Mater. Chem. 2004, 14, 1585. (b) Chow, T. J.; Lin, R.; Ko, C.-W.; Tao, Y.-T.
J. Mater. Chem. 2002, 1, 42.
(29) (a) Tong, Q.-X.; Lai, S.-L.; Chan, M.-Y.; Lai, K.-H.; Tang, J.-X.;
Kwong, H.-L.; Lee, C.-S.; Lee, S.-T. Chem. Mater. 2007, 19, 5851. (b) Wu, Z.;
Yang, H.; Duan, Y.; Xie, W.; Liu, S.; Zhao, Y. Semicond. Sci. Technol. 2003,
18, 49.
(30) Salbeck, J.; Yu, N.; Bauer, J.; Weissortel, F.; Bestgen, H. Synth. Met.
1997, 91, 209.
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The characteristics of the nondoped devices of OPVs 1-3
described herein are comparatively better than those for the
commonly used blue dopant such as DPVBi,14b,c and oligo-
meric PPVs do not contain amino substituents.5a,9,26,31 It
should be mentioned that even the 4-fold-functionalized
bimesitylenes such as 3 can be comfortably sublimed without
any residual traces.
using commercial aluminum sheets precoated with silica gel.
Chromatography was conducted on silica gel of mesh size
60-120. All of the commercial chemicals were used as received.
The starting materials, viz., bimesityl,7 diiodobimesityl,7 tetra-
iodobimesityl,7 3,30,5,50-tetrakis(4-formylphenyl)bimesityl,10
and 3,30,5,50-tetrakis[4(40-formyl)biphenyl]bimesityl,10 were
synthesized according to the earlier reported procedures
established in our laboratories. The detailed synthetic proce-
dures for preparing 1-5 are given below (cf. Scheme 1).
Preparation of 3,30-Diiodobimesityl (6):7. In a two-necked
round-bottom flask, bimesityl (0.40 g, 1.68 mmol) was dissolved
in CH3CN (8 mL). To this was added NIS (0.83 g, 3.69 mmol)
followed by TFA (0.08 mL) under vigorously stirring cold
conditions (5 °C). The reaction mixture was allowed to stir at
room temperature for 8 h. Subsequently, the reaction mixture
was concentrated, and the organic matter was extracted with
CHCl3. The combined extract was washed with NaHCO3 solu-
tion followed by brine, dried over anhyd Na2SO4, filtered, and
evaporated to dryness. A careful silica gel column chromatog-
raphy using CHCl3 and petroleum ether mixture (1:9) afforded
the colorless compound diiodobimesityl 6 in 83% yield (0.68 g):
IR (KBr) cm-1 2970, 2947, 2915, 1449, 1375; 1H NMR (CDCl3,
400 MHz) δ 1.78 (s, 6H), 2.07 (s, 6H), 2.48 (s, 6H), 7.03 (s, 2H);
13C NMR (CDCl3, 100 MHz) δ 19.6, 26.7, 29.9, 106.2, 129.0,
135.2, 138.3, 138.8, 140.7.
4. Conclusions
The 2-fold- and 4-fold-functionalized phenylenevinylenes
1-5 based on three-dimensional D2d-symmetric bimesitylene
scaffold constitute a new family OPVs, whose advantages
include unique pattern of substitution, ease of synthesis,
and molecular topology that prevents ready crystallization
leading to amorphous nature. The bimesityl core is simply a
bystander permitting the optical properties of the attached
chromophores to be reproduced, while assuring enhance-
ment of fluorescence emission efficiency in the solid state via
crossed arrangement of chromophores, amorphous nature,
and high Tg values. The increase in the conjugation of the
chromophores leads to corresponding variation in the ab-
sorption and emission properties, whereby the modulation
of photophysical properties is readily achieved. The func-
tional behavior of this new class of amorphous materials is
demonstrated from electroluminescence studies, which re-
veal emission properties that surpass the ones reported for
nondoped blue-light emitters based on the same category. It
is shown that both hole-transporting and emitting materials
(i.e., Bim-DPAB and OPV 2) based on analogous structural
characteristics show remarkable layer compatibility to per-
mit electroluminescence with extraordinary efficiencies.
Clearly, the strategy based on the bimesitylene scaffold
opens way for a myriad of amorphous pure-blue emitting
materials with desirable thermal stability.
Preparation of 3,30-Bis(4-formylphenyl)bimesityl (8): A 120
mL pressure tube, removed hot from oven, was cooled under N2
atmosphere and charged with diiodobimesityl 6 (1.80 g, 3.67
mmol), 4-formylphenylboronic acid (1.65 g, 11.02 mmol), Pd-
(PPh3)4 (0.254 g, 0.22 mmol), toluene (20 mL), THF (10 mL),
and 2 M K2CO3 solution (10 mL). The mixture was bubbled
with a N2 gas for 20 min, the pressure tube was capped tightly,
and the reaction mixture was heated at 90 °C. The yellow turbid
solution became clear in 2 h. The heating was continued for a
further 16 h, after which time the color of the reaction mixture
turned dark brown. At this stage, the reaction mixture was
cooled and extracted with ethyl acetate. The combined extracts
were washed with water and brine, dried over anhyd Na2SO4,
filtered, and concentrated. The pure product 8 was isolated as a
pale yellow solid (1.6 g, 97%) by column chromatography with
silica gel using ethyl acetate and petroleum ether mixture (5:95)
as an eluent: mp 242-244 °C; IR (KBr) cm-1 3003, 2918, 2820,
1706, 1603; 1H NMR (CDCl3, 500 MHz) δ 1.59 (s, 6H), 1.94 (s,
6H), 2.01 (s, 6H), 7.06 (s, 2H), 7.34 (dd, J1 = 16.0 Hz, J2 = 7.75
Hz, 4H), 7.93 (t, J = 8.2 Hz, 4H), 10.05 (s, 2H); 13C NMR
(CDCl3, 125 MHz) δ 17.8, 19.9, 20.7, 129.2, 129.90, 129.92,
130.3, 132.8, 133.7, 134.9, 135.1, 138.0, 138.7, 148.7, 192.0;
FAB-MS (M þ H)þ 447.
5. Experimental Section
General Aspects: All Pd(0)-mediated cross-coupling reactions
were performed under a nitrogen gas atmosphere in oven-dried
1
pressure tubes. H and 13C NMR spectra were recorded on a
400/500 MHz spectrometer in CDCl3 as a solvent. Chemical
shifts are reported in δ scale downfield from tetramethylsilane.
The FAB-mass spectra were recorded on a mass system using
argon/xenon as the FAB gas in a linear mode with m-nitrobenzyl
alcohol as the matrix. The TGA and DSC measurements were
carried out at 10 °C/min under a nitrogen gas atmosphere. The
photoluminescence spectra in the solution state were recorded
using spectral grade solvents, and those of the thin solid films
were measured by front-face excitation and detection. The
ionization potentials were calculated by photoelectron spectros-
copy with UV intensity of 5-30 nW. The Commission Inter-
nationale de l’Eclairage (CIE) coordinates of the devices were
measured by a spectroscan spectrometer.
Preparation of 3,30-Bis(4(40-formyl)biphenyl)bimesityl (9):
This compound was synthesized using 40-formylbiphenyl-4-
boronic acid following the procedure used for the preparation
of 3,30-bis(4-formylphenyl)bimesityl, 8: yield 95%; mp 196-
198 °C; IR (KBr) cm-1 3024, 2917, 2855, 1695, 1601; 1H NMR
(CDCl3, 500 MHz) δ 1.69 (s, 6H), 1.98 (s, 6H), 2.08 (s, 6H), 7.08
(s, 2H), 7.29 (dd, J1 =14.4 Hz, J2 = 8.0 Hz, 4H), 7.70 (t, J =
8.94 Hz, 4H), 7.82 (d, J = 8.2 Hz, 4H), 7.96 (d, J = 8.2 Hz, 4H),
10.05 (s, 2H); 13C NMR (CDCl3, 125 MHz) δ 17.9, 19.9, 20.9,
127.3, 127.5, 128.9, 130.2, 130.3, 133.3, 134.1, 134.6, 135.1,
137.6, 138.1, 139.2, 142.3, 146.9, 191.9; FAB-MS (M þ H)þ 599.
Preparation of 3,30,5,50-Tetrakis(4-formylphenyl)bimesityl:10.
This compound was synthesized using 4-formylphenylboronic
acid following the procedure used for the preparation of 3,30-
bis(4-formylphenyl)bimesityl, 8: yield 82%; mp 280-282 °C
Materials: Dry tetrahydrofuran (THF) and toluene were
freshly distilled over sodium prior to use. All of the reactions
were monitored by analytical thin layer chromatography (TLC)
(31) (a) Ostrowski, J. C.; Hudack, R. A. Jr.; Robinson, M. R.; Wang, S.;
Bazan, G. C. Chem.;Eur. J. 2001, 7, 4500. (b) Robinson, M. R.; Wang, S.;
Heeger, A. J.; Bazan, G. C. Adv. Funct. Mater. 2001, 11, 413. (c) Cheng, G.;
He, F.; Zhao, Y.; Duan, Y.; Zhang, H.; Yang, B.; Ma, Y.; Liu, S. Semicond.
Sci. Technol. 2004, 19, 78. (d) He, F.; Xia, H.; Tang, S.; Duan, Y.; Zeng, M.;
Liu, L.; Li, M.; Zhang, H.; Yang, B.; Ma, Y.; Liu, S.; Shen, J. J. Mater. Chem.
2004, 14, 2735. (e) Niazimbetova, Z. I.; Christian, H. Y.; Bhandari, Y. J.;
Beyer, F. L.; Galvin, M. E. J. Phys. Chem. B 2004, 108, 8673. (f) Rathnayake,
H. P.; Cirpan, A.; Lahti, P. M.; Karasz, F. E. Chem. Mater. 2006, 18, 560.
1
(dec); IR (KBr) cm-1 3045, 1699, 1603; H NMR (400 MHz,
CDCl3) δ 1.67 (s, 6H), 1.68 (s, 12 H), 7.39 (d, 8H, J = 8.04 Hz),
7.96 (d, 8H, J = 8.04 Hz), 10.07 (s, 4H); 13C NMR (100 MHz,
CDCl3) δ 18.3, 19.2, 130.0, 130.2, 131.4, 132.6, 135.0, 138.5,
139.3, 148.7, 192.0; FAB-MS (M þ 1) 655.
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Moorthy et al.
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Preparation of 3,30,5,50-Tetrakis(4(40-formyl)biphenyl)bi-
mesityl:10. This compound was synthesized using 40-formyl-
biphenyl-4-boronic acid following the procedure used for the
preparation of 3,30-bis(4-formylphenyl)bimesityl, 8: yield 80%;
(m, 36H); 13C NMR (100 MHz, CDCl3) δ 18.2, 19.1, 126.3,
126.4, 127.4, 127.6, 128.8, 131.2, 131.35, 131.4, 131.8, 132.2,
138.6, 139.7, 140.6, 141.0, 141.1, 141.8, 143.5, 143.6, 143.8;
FAB-MS 1558 (M þ 1). Anal. Calcd for C122H94: C, 93.93; H,
6.07. Found: C, 93.50; H, 6.25.
1
mp 160-162 °C, IR (KBr) cm-1 3028, 2923, 1697; H NMR
(CDCl3, 400 MHz) δ 1.78 (s, 12 H), 1.81 (s, 6H), 7.36 (d, 8H, J =
8.32 Hz), 7.74 (d, 8H, J = 8.32 Hz), 7.85 (d, 8H, J = 8.32 Hz),
7.98 (d, 8H, J = 8.32 Hz), 10.07 (s, 4H); 13C NMR (CDCl3, 100
MHz) δ 18.5, 19.5, 127.4, 127.5, 130.2, 130.3, 132.7, 135.1, 137.7,
138.8, 139.6, 142.7, 192.6; FAB-MS (M þ 1) 960.
3,30-Bis-p-(2,2-diphenylvinyl)phenylbimesityl (1): This com-
pound was synthesized starting from 3,30-bis(4-formylphenyl-
)bimesityl, 8, following the above procedure used for the
preparation of 5: yield 62%; colorless powder; IR (KBr) cm-1
3020, 2916, 1492, 1443; 1H NMR (400 MHz, CDCl3) δ 1.49 (s,
6H), 1.81 (s, 6H), 1.93 (s, 6H), 6.82-7.00 (m, 12H), 7.20-7.30
(m, 20H); 13C NMR (100 MHz, CDCl3) δ 17.8, 19.9, 20.8,
127.36, 127.40, 127.5, 128.1, 128.2, 128.6, 128.7, 129.1, 129.4,
130.3, 133.4, 134.0, 134.2, 135.4, 138.0, 139.5, 140.4, 140.5,
142.2, 143.5; FAB-MS 746 (M þ 1). Anal. Calcd for C58H50:
C, 93.25; H, 6.75. Found: C, 93.21; H, 6.72.
1,1-Diphenyl-2-biphenylethene-Substituted Bimesitylene (2):
This compound was synthesized starting from 3,30-bis(4(40-
formyl)biphenyl)bimesityl following the above procedure used
for the preparation of 5: yield 58%; colorless powder; IR (KBr)
cm-1 3053, 3021, 2919, 1493, 1443, 1376; 1H NMR (300 MHz,
CDCl3) δ 1.66 (s, 6H), 1.96 (s, 6H), 2.05 (s, 6H), 7.02 (s, 2H), 7.06
(s, 2H), 7.11 (d, J = 8.2 Hz, 4H), 7.14-7.23 (m, 4H), 7.24-7.39
(m, 16H), 7.46 (d, J = 8.2 Hz, 4H), 7.58-7.64 (m, 4H); 13C
NMR (100 MHz, CDCl3) δ 17.9, 19.9, 20.9, 126.4, 126.6, 127.4,
127.5, 127.6, 127.8, 128.2, 128.7, 128.8, 129.9, 130.0, 130.4,
133.5, 134.2, 134.4, 136.4, 138.1, 138.4, 139.0, 139.5, 140.4,
140.9, 142.6, 143.4; FAB-MS 898 (Mþ). Anal. Calcd for
C71H62: C, 93.17; H, 6.83. Found: C, 92.96; H, 6.68.
3,30,5,50-Tetrakis-p-(2,2-diphenylvinyl)phenylbimesityl (3):
This compound was synthesized starting from 3,30,5,50-
tetrakis(4-formylphenyl)bimesityl following the above proce-
dure used for the preparation of 5: yield 65%; colorless
powder; IR (KBr) cm-1 3053, 3021, 2919, 1597, 1493, 1443;
1H NMR (400 MHz, CDCl3) δ 1.50 (s, 18H), 6.80-6.90
(m, 6H), 6.92-7.00 (m, 10H), 7.14-7.35 (m, 46H); 13C NMR
(100 MHz, CDCl3) δ 18.2, 19.3, 127.3, 127.4, 127.5, 128.1,
128.2, 128.6, 129.2, 129.4, 130.3, 131.8, 132.2, 135.4, 138.6,
139.6, 140.5, 140.9, 142.2, 143.5; FAB-MS 1255 (M þ 1).
Anal. Calcd for C98H78: C, 93.74; H, 6.26. Found: C, 93.73;
H, 6.23.
Preparation of 3,30,5,50-Tetraphenylbimesityl: Tetraiodobi-
mesityl 7 (2.22 g, 3.0 mmol), phenylboronic acid (2.90 g, 24.0
mmol), and Pd(PPh3)4 (0.69 g, 20 mol %) were introduced into
an initially oven-dried and cooled (under N2) pressure tube. To
this mixture, dry toluene (15 mL) and saturated NaHCO3
solution (15 mL) were added, and the resulting solution was
bubbled with a nitrogen gas for 15 min. The pressure tube was
sealed under nitrogen, and the reaction mixture was heated
to 90-100 °C slowly and maintained at this temperature for
12 h with constant stirring. After this period, the reaction
mixture was cooled and the contents were extracted with
dichloromethane. A short pad filtration over silica gel using
dichloromethane and hexane (1:19) yielded 3,30,5,50-tetraphe-
nylbimesityl (1.25 g, 77%) as a colorless solid material: IR (KBr)
cm-1 3020, 2921, 1598, 1492, 1438; 1H NMR (400 MHz, CDCl3)
δ 1.71 (s, 12H), 1.72 (s, 6H), 7.23 (m, 8H), 7.32 (m, 4H), 7.42 (m,
8H); 13C NMR (100 MHz, CDCl3) δ 18.3, 19.3, 126.2, 128.2,
129.5, 131.9, 132.4, 138.7, 140.0. 142.5.
Preparation of 3,30,5,50-Tetrakis(4-benzoylphenyl)bimesityl: A
solution of AlCl3 (1.44 g, 10.8 mmol) in CS2 (20 mL) contained
in a 50 mL two-necked round-bottom flask was cooled to 0 °C.
Benzoylchloride (1.0 mL, 9.0 mmol) was slowly introduced into
the flask at 10 °C. The solution of the 3,30,5,50-tetraphenylbi-
mesityl (0.50 g, 0.9 mmol) in CS2 (8 mL) was added to the reaction
mixture dropwise over a period of 15 min at the same temperature.
The resulting dark brown mixture was heated at reflux for 2.5 h.
Subsequently, the contents were poured into an ice-cold 10% HCl
solution. The organic material was extracted with ethyl acetate,
washed with water, dried over anhyd Na2SO4, and concentrated.
The pure product was obtained (0.70 g, 81%) after silica gel
column chromatography using ethyl acetate/petroleum ether
1
(1:9 f 1:4) as an eluent: IR (KBr) cm-1 3053, 2923, 1722; H
1,1-Diphenyl-2-biphenylethene-Substituted Bimesitylene (4):
This compound was synthesized starting from 3,30,5,50-tetrakis-
(4(40-formyl)biphenyl)bimesityl following the above procedure
used for the preparation of 5: yield 61%; colorless powder; IR
(KBr) cm-1 3052, 2920, 2852, 1597, 1493, 1442, 1389; 1H NMR
(400 MHz, CDCl3) δ 1.65 (s, 12H), 1.68 (s, 6H), 6.94 (s, 4H), 7.03
(d, J = 8.0 Hz, 8H), 7.13-7.19 (m, 16H), 7.20-7.32 (m, 32H),
7.38 (d, J = 8.0 Hz, 8H), 7.45 (d, J = 8.0 Hz, 8H); 13C NMR
(100 MHz, CDCl3) δ 18.4, 19.5, 126.3, 126.6, 127.0, 127.4, 127.5,
127.6, 127.7, 128.2, 128.7, 130.0, 130.4, 132.1, 132.5, 136.3,
138.3, 138.7, 138.9, 139.7, 140.4, 141.5, 142.6, 143.4; FAB-MS
1558 (M þ 1). Anal. Calcd for C122H94: C, 93.93; H, 6.07.
Found: C, 94.00; H, 6.00.
PL Quantum Yield Measurements:. Solution State For deter-
mination of fluorescence quantum yields, the solutions of
bimesitylene-based OPVs 1-5 were prepared in cyclohexane
(spectral grade) such that their absorbance at λ = 367 nm was
ca. 0.025. These solutions were deaerated using nitrogen gas
before their fluorescence was recorded at 298 K. The excitation
wavelength was chosen as 367 nm for all the samples, and the
emission in each case was recorded in a right angle mode. The
quantum yield was calculated from the following relation:
NMR (CDCl3, 400 MHz) δ 1.67 (s, 12H), 1.69 (s, 6H), 7.29 (d,
J = 8.1 Hz, 8H), 7.44 (t, J = 7.4 Hz, 8H), 7.54 (t, J = 7.4 Hz,
4H), 7.80 (d, J = 7.1 Hz, 8H), 7.84 (d, J = 8.1 Hz, 8H); 13C
NMR (CDCl3, 100 MHz) δ 18.4, 19.4, 128.3, 129.5, 130.0, 130.5,
131.7, 132.4, 132.6, 135.8, 137.6, 138.7, 139.4, 146.8, 196.6.
Representative Procedure for the Synthesis of Tetrapheny-
lethene-Substituted Bimesitylene (5): A solution of 3,30,5,50-
tetrakis(4-benzoylphenyl)bimesityl (0.44 g, 0.46 mmol) in
THF (20 mL) was added slowly to the solution of diphenyl-
methyl lithium (generated from diphenylmethane (1.23 g, 7.34
mmol) and n-BuLi (4.0 mL of 1.6 M, 6.43 mmol) at -20 °C. The
solution was slowly allowed to warm up to room temperature
over a period of 3 h. The solution turned deep brown in color.
After stirring for overnight at this temperature, the reaction
mixture was quenched with water and the organic contents were
extracted with ethyl acetate. The crude tetrahydroxy compound
thus obtained was taken as it is to the next step without further
purification. The crude product was later subjected to acid-
catalyzed dehydration using dry toluene and a catalytic amount
of PTS in a Dean-Stark apparatus. After reflux overnight, the
solvent was evaporated to dryness. Silica gel column chroma-
tography using chloroform and hexane mixture (1:4 f 1:1)
afforded the required compound 5 as a colorless powder (yield
2
φu ¼ φsðAs=AuÞðIu=IsÞðηu=ηsÞ
where the subscripts s and u refer to standard and unknown
samples, Au and As to absorbances of the sample and the
1
56%): IR (KBr) cm-1 3053, 3023, 1598, 1493, 1442; H NMR
(400 MHz, CDCl3) δ 1.50 (s, 12H), 1.58 (s, 6H), 6.83 (d, J =
8.0 Hz, 8H), 6.96 (br s, 24H), 6.98 (d, J = 8.0 Hz, 8H), 7.01
2608 J. Org. Chem. Vol. 75, No. 8, 2010

Moorthy et al.
JOCArticle
standard at the excitation wavelength, Iu and Is to the integrated
emission intensities (i.e., areas under the emission curves) of the
sampleandthe standard, andηu and ηs tothe refractiveindexes of
the corresponding solutions (pure solvents are assumed). Anthra-
cene was chosen as a reference for quantum yield determination,
for which the reported quantum yield in ethanol is 0.27.32
Solid State. The films of compounds 1-5 were prepared on
quartz plates (1 mm ꢀ 2 cm) by spin-coating of the solutions in
1,2-dichloroethane (10 mg/mL) at a constant rate of 300 rpm or
by vacuum sublimation to a thickness of (1000 A). The spin-
coated quartz plates were dried in a vacuum oven, and the solid-
state quantum efficiencies of the films were measured using an
integrating sphere method.33 The excitation source was 325 nm
laser beam for all the samples.
Electrochemical Measurements. The cyclic voltammetry ex-
periments were performed on an electrochemical analyzer, and
the data were collected and analyzed using electrochemical
analysis software. All experiments were carried out in a three-
electrode compartment cell with Pt wire counter electrode,
glassy carbon working electrode, and Ag/AgNO3 (0.1 M)
reference electrode at varying scan rates. The supporting elec-
trolyte used was 0.1 M tetrabutylammoniumhexafluoropho-
sphate solution in dry dichloromethane, and the cell
containing the solution of the sample (1 mM) and the supporting
electrolyte was purged with a nitrogen gas thoroughly before
scanning for its oxidation and reduction properties. For each
determination, the CV was run independently for ferrocene as a
reference. The oxidation and reduction potentials of 1-5 were
subsequently calculated from the average of anodic and catho-
dic peak potentials and were corrected according to the values
observed for ferrocene oxidation. The HOMO and LUMO
values were thus calculated with reference to the ferrocene
Device Fabrication and Characterization of OLEDs. The
multilayer OLEDs were fabricated by employing the OPVs
1-3 as emitting materials. The ITO-coated glass substrates with
a sheet resistance of <50 Ω/0 were cleaned sequentially in an
ultrasonicator using acetone, detergent solution, deionized
water, ethanol, and 2-propanol and then subjected to oxygen
plasma and UV treatments.
Vacuum Deposition. Vacuum deposition of the organic ma-
terials such as NPB, Bim-DPAB, CBP, 1 or 2 or 3, BCP, Alq3,
TPBI, LiF, and Al was carried out under a pressure of 10-6 Torr
on top of etched ITO glass substrates sequentially. The rate of
deposition for organic materials was maintained in the range
ꢀ
˚
˚
0.1-0.5 A/s. The evaporation rate and thickness of the organic
layers were monitored by a quartz oscillator. After the vacuum
deposition, the devices were sealed in an inert atmosphere
glovebox. EL spectra of the devices were obtained using a
diode-array rapid analyzer system.
Electroluminescence Measurements. Current voltage and light
intensity measurements were done on a source meter and an
optical meter equipped with a silicon photodiode, respectively,
The device was placed close to the photodiode such that all the
forward light entered the photodiode. The effective size of the
emitting diode was 4.0 mm2, which is significantly smaller than
the active area of the photodiode detector, a condition known as
“under-filling”, satisfying the measurement protocol.34
Acknowledgment. J.N.M. is thankful to the Department
of Science and Technology (DST), India, for generous
financial support. P.N. gratefully acknowledges the research
fellowship from UGC-CSIR, India.
Supporting Information Available: 1H and 13C NMR char-
acterization of the OPVs 1-5, CV and DSC scans of the OPVs,
and CIE coordinates obtained for the OLEDs. This material is
available free of charge via the Internet at http://pubs.acs.org.
oxidation potential by using the following general equations:
E
opt
.
HOMO = Eox þ 4.8 eV; ELUMO = EHOMO - Eg
(32) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229.
(33) (a) de Mello, J. C.; Wittmann, H. F.; Friend, R. H. Adv. Mater. 1997,
9, 230. (b) Chiang, C.; Wu, M. F.; Dai, D. C.; Wen, Y. S.; Wang, J. K.; Chen,
C. T. Adv. Funct. Mater. 2005, 15, 231.
(34) Forrest, S. R.; Bradley, D. D. C.; Thompson, M. E. Adv. Mater.
2003, 15, 1043.
J. Org. Chem. Vol. 75, No. 8, 2010 2609