De novo design for functional amorphous materials: Synthesis and thermal and light-emitting properties of Twisted anthracene-functionalized bimesitylenes

Article abstract of DOI:10.1021/ja8042905

The unique structural attributes inherent to D_2dsymmetric rigid tetraarylbimesityls render their close packing in the solid state difficult. We have exploited the indisposed tendency of such modules based on the bimesityl scaffold toward crystallization to design a novel class of amorphous functional materials with high glass transition temperatures and thermal stability (T _d > 400 °C). It is shown that a variety of 2- and 4-fold anthracene-functionalized bimesityls, 1-7, that exhibit excellent amorphous properties (T_g = ca. 190-330 °C) can be readily prepared via facile Pd(0)-mediated cross-coupling strategies. As the communication between the bimesityl core and the anchored anthracenes is negligible or only marginal, the trends observed for luminescence of model constituent anthracenes are reproduced in the condensed- phase photoluminescence and electroluminescence of 1-7. In other words, the emission characteristics, i.e., λ_max and quantum yields, are readily modulated via appropriate modification of the fluorophores. The functional behavior of this unique class of amorphous materials based on the bimesityl scaffold is demonstrated by fabrication of OLED devices. The 2-fold functionalized derivatives 1 and 2 lend themselves to sublimation techniques, so that the electroluminescence is captured with high efficiencies at low turn-on voltages (3.5-6.5 V). The device ITO/NPB (400 A)/1% 2:MADN (400 A)/TPBI (400 A)/LiF (10 A)/AI (1500 A) for 2 yields the highest luminance of ～13 900 cd/m² at 17.5 V, a maximum luminance efficiency of ～7.4 cd/A at 4.5 V, and a power efficiency of ～5.3 Im/W at 4.0 V. Further, at a brightness of 800 cd/m ² and a current density of 13.8 mA/cm², the device is found to exhibit excellent luminance efficiency of 5.8 cd/A, external quantum efficiency of 4.3% with a power efficiency of 2.2 Im/W, and pure blue light with a CIE_xy (x= 0.13, y= 0.18). The performance characteristics of the devices fabricated for 1 and 2 are remarkable. Although the 4-fold functionalized systems did not permit sublimation leading to spin-coating as a means for device fabrication, the observed electroluminescence for 4 and 5 attests to a broader scope and applicability of this new category of amorphous molecules for application in OLEDs.

Full text of DOI:10.1021/ja8042905

Published on Web 11/26/2008
De Novo Design for Functional Amorphous Materials:
Synthesis and Thermal and Light-Emitting Properties of
Twisted Anthracene-Functionalized Bimesitylenes
Jarugu Narasimha Moorthy,*,† Parthasarathy Venkatakrishnan,† Palani Natarajan,†
Duo-Fong Huang,‡ and Tahsin J. Chow*,‡
Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India, and Institute
of Chemistry, Academia Sinica, Taipei 115, Taiwan, Republic of China
Received October 4, 2007; E-mail: moorthy@iitk.ac.in (J.N.M.)
Abstract: The unique structural attributes inherent to D2d-symmetric rigid tetraarylbimesityls render their
close packing in the solid state difficult. We have exploited the indisposed tendency of such modules based
on the bimesityl scaffold toward crystallization to design a novel class of amorphous functional materials
with high glass transition temperatures and thermal stability (Td > 400 °C). It is shown that a variety of 2-
and 4-fold anthracene-functionalized bimesityls, 1-7, that exhibit excellent amorphous properties (Tg )
ca. 190-330 °C) can be readily prepared via facile Pd(0)-mediated cross-coupling strategies. As the
communication between the bimesityl core and the anchored anthracenes is negligible or only marginal,
the trends observed for luminescence of model constituent anthracenes are reproduced in the condensed-
phase photoluminescence and electroluminescence of 1-7. In other words, the emission characteristics,
i.e., λmax and quantum yields, are readily modulated via appropriate modification of the fluorophores. The
functional behavior of this unique class of amorphous materials based on the bimesityl scaffold is
demonstrated by fabrication of OLED devices. The 2-fold functionalized derivatives 1 and 2 lend themselves
to sublimation techniques, so that the electroluminescence is captured with high efficiencies at low turn-on
voltages (3.5-6.5 V). The device ITO/NPB (400 Å)/1% 2:MADN (400 Å)/TPBI (400 Å)/LiF (10 Å)/Al (1500
Å) for 2 yields the highest luminance of ∼13 900 cd/m2 at 17.5 V, a maximum luminance efficiency of ∼7.4
cd/A at 4.5 V, and a power efficiency of ∼5.3 lm/W at 4.0 V. Further, at a brightness of 800 cd/m2 and a
current density of 13.8 mA/cm2, the device is found to exhibit excellent luminance efficiency of 5.8 cd/A,
external quantum efficiency of 4.3% with a power efficiency of 2.2 lm/W, and pure blue light with a CIEx,y
(x ) 0.13, y ) 0.18). The performance characteristics of the devices fabricated for 1 and 2 are remarkable.
Although the 4-fold functionalized systems did not permit sublimation leading to spin-coating as a means
for device fabrication, the observed electroluminescence for 4 and 5 attests to a broader scope and
applicability of this new category of amorphous molecules for application in OLEDs.
and chemical properties” has been termed crystal engineering;3
Introduction
indeed, the crystal structures can now be treated as retrosynthetic
targets.4 The flip side of crystal engineering is that the
identification of features that thwart crystallization may be
elegantly exploited for the development of amorphous molecular
materials,5 which are of immense utility in optoelectronic
devices. Extensive investigations on structure-property (Tg,
glass transition temperature) relationships have led to the design
of a variety of molecular amorphous materials. Some examples
that are of intermediate dimension and exhibit an amorphous
property with high Tg values include tetrahedral, spiro, starburst,
dendritic, etc. shaped molecules; cf. Chart 1.6 Molecules with
rigid structural features exhibit packing difficulties, leading to
stable amorphous glasses with high morphological stability.6
The molecular packing/organization in crystals holds sway
on the solid-state properties such as nonlinear optical activity,1a
ferromagnetic behavior,1b conductivity,1b thermal and photo-
chemical reactivity,1c,d etc.1 The focus in solid-state organic
chemistry over the past 3-4 decades has been to understand
the correlation between molecular and crystal structures and
progressively achieve synthesis of crystals with predetermined
molecular ordering based on the attributes of the molecular
structures.2 The “understanding of intermolecular interactions
in the context of crystal packing and utilization of such an
understanding in the design of new solids with desirable physical
† Indian Institute of Technology.
‡ Academia Sinica.
(3) (a) Desiraju, G. R. Crystal Engineering: The Design of Organic Solids;
Elsevier: Amsterdam, 1989. (b) Lehn, J.-M. Supramolecular Chem-
istry: Concepts and PerspectiVes; VCH Publishers: New York, 1995.
(4) Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311.
(5) Lebel, O.; Maris, T.; Perron, M.-E.; Demers, E.; Wuest, J. D. J. Am.
Chem. Soc. 2006, 128, 10372.
(1) (a) Evans, O. R.; Lin, W. Acc. Chem. Res. 2002, 35, 511. (b) Jones,
W. Organic Molecular Solids: Properties and Applications; CRC
Press: Boca Raton, FL, 1998. (c) Toda, F. Top. Curr. Chem. 2005,
254, 1. (d) Schmidt, G. M. J. Pure Appl. Chem. 1971, 27, 647–678.
(2) Braga, D. Chem. Commun. 2003, 2751.
9
17320 J. AM. CHEM. SOC. 2008, 130, 17320–17333
10.1021/ja8042905 CCC: $40.75
2008 American Chemical Society

De Novo Design for Functional Amorphous Materials
A R T I C L E S
Chart 1. Structures of Typical Amorphous Organic Functional Molecules
We recently reported a de novo design, synthesis, and
hydrogen-bonded as well as metal-mediated self-assembly of
D2d-symmetric 3-dimensional modules, viz., tetraarylbimesityls
of general structure Tet-Bm shown in Chart 2, into 3-dimen-
sional architectures.7 The methyl groups at the ortho positions
of each of the mesitylene rings in bimesitylene render the two
rings orthogonal and thus make the core as a whole rigid. The
high symmetry and rigidity associated with the arylated modules
render the close molecular packing extremely difficult, leading
to an amorphous nature. The crystallization was found to occur
only when simple tetraarylbimesityls contain functional groups
such as Br, CN, OH, COOH, etc.; in fact, the hydroxyl- and
carboxy-functionalized tetraphenylbimesityls crystallized only
in the presence of solvents that may be included in the lattice.7c
We have failed in our deliberate attempts to crystallize extended
tetraarylbimesityls. To exploit the indisposed tendency of
3-dimensional extended tetraarylbimesityls to undergo crystal-
lization, we designed anthracene-functionalized bimesityls 1-7
(2-fold as well as 4-fold) as functional amorphous materials
for application as emitting materials (EMs) in OLEDs.8 The
latter have been widely studied over the past decade for their
application in flat-panel displays and also due to their inherent
advantages such as low cost, ease of casting, low turn-on
voltages, potential for wide-angle viewing, etc.9 For application
in OLEDs, the materials should ideally permit formation of
uniform films (without any pinholes) and exhibit both morpho-
logical (Tg) and thermal (Td) stability. Thus, in addition to
the structural attributes noted above, which impart amorphous
nature and high Tg (due to rigidity and high molecular weight),
the following considerations were deemed compelling to explore
the potential of anthracene-functionalized bimesitylenes 1-7
as amorphous functional materials.
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A R T I C L E S
Moorthy et al.
1. Diiodination of bimesityl7c with N-iodosuccinimide in the
presence of a catalytic amount of trifluoroacetic acid afforded
3,3′-diiodobimesityl in 83% yield (Scheme 1). 2-fold function-
alization of the bimesitylene was achieved via Pd(0)-mediated
Sonogashira coupling between 3,3′-diiodobimesityl and (trim-
ethylsilyl)acetylene. The subsequent deprotection using tetrabu-
tylammonium fluoride (TBAF) afforded 3,3′-diethynylbimesityl
in near quantitative yield. The Suzuki and Sonogashira coupling
of 3,3′-diiodobimesityl and 3,3′-diethynylbimesityl with the
corresponding coupling partners yielded 1-3 in 41-62% yields
(Scheme 1).
Tetraiodination of the readily prepared bimesityl with I2 in
an AcOH/HNO3-H2SO4 mixture yielded tetraiodobimesityl
(11).7c 4-Fold functionalization of the bimesityl core by four
anthracene moieties was successfully achieved via Pd(0)-
mediated Suzuki/Sonogashira coupling reactions. Thus, the
anthracene derivative 5 was synthesized starting from the
3,3′,5,5′-tetraethynylbimesityl (12), which in turn was readily
prepared from Sonogashira coupling of tetraiodobimesityl and
(trimethylsilyl)acetylene followed by deprotection of the TMS
group with TBAF (Scheme 1). The 4-fold Sonogashira coupling
between tetraethynylbimesityl and 9-bromo-10-phenylanthracene
(15) using Pd(0) as a catalyst, CuI as a cocatalyst, and Et3N as
a solvent led to isolation of 5 in 59% yield. In a similar manner,
the coupling reaction of 9-bromo-10-(4-methoxyphenyl)an-
thracene (16) and 9-bromo-10-(phenylethynyl)anthracene (17)
with tetraethynylbimesityl afforded the elaborated anthracene
derivatives 6 and 7 in 52% and 54% yields, respectively
(Scheme 1). The 4-fold diphenylanthracene-derivatized bimesityl
4 was obtained in good yields from the Suzuki coupling reaction
between 15 and the tetraboronate ester 13. Whereas the model
compounds 9 and 10 were commercially available, compound
8 was synthesized using 9-bromo-10-phenylanthracene and
phenylacetylene via Sonogashira coupling. All the compounds
Figure 1. Normalized absorption (left) and emission spectra (right) of the
anthracene-anchored bimesitylenes 1-7.
(1) A serious problem with π-conjugated systems is that the
aggregation of planar aromatic systems in thin films depresses
the luminescence quantum yield.10 The most efficient config-
uration that prevents luminescence quenching in the condensed
phase is supposedly the perpendicular orientation of long
molecular axes.11 In all of the anthracene-functionalized bi-
mesitylenes 1-7, the unique molecular topology was antici-
pated, a priori, to discourage luminescence quenching. Further,
the linkages at the meta positions were thought to enhance the
charge separation and charge recombination processes in
multilayered devices for electroluminescence (EL), as has been
reported recently.12
(2) Facile functionalization of the bimesityl core (both 2-fold
and 4-fold) via Pd(0)-mediated cross-coupling strategies13 offers
the opportunity to access readily a host of compounds.
(3) The potential of anthracenes in EMs has been vastly
explored, and the emission maximum of anthracenes can be
conveniently tuned by subtle structural modifications.14 As the
communication between the bimesityl core and the anchored
anthracenes is negligible or virtually nonexistent, the trends
observed for luminescence of various derivatives were expected
to be reproduced in the condensed-phase photoluminescence
and electroluminescence.
(6) For 1,3,5-triarylbenzene- and triarylamine-based systems, see: (a)
Shirota, Y. J. Mater. Chem. 2000, 10, 1. (b) Thelakkat, M. Macromol.
Mater. Eng. 2002, 287, 442. (c) Shirota, Y. J. Mater. Chem. 2005,
15, 75. (d) Forrest, S. R.; Thompson, M. E. Chem. ReV. 2007, 107,
923–1386. For spirobifluorene-based systems, see: (e) Salbeck, J.; Yu,
N.; Bauer, J.; Weisso¨rtel, F.; Bestgen, H. Synth. Met. 1997, 91, 209.
(f) Johansson, N.; Salbeck, J.; Bauer, J.; Weisso¨rtel, F.; Broms, P.;
erson, A.; Salaneck, W. R. AdV. Mater. 1998, 10, 1136. (g) Kim, Y.-
H.; Shin, D.-C.; Kim, S.-H.; Ko, C.-H.; Yu, H.-S.; Chae, Y.-S.; Kwon,
S.-K. AdV. Mater. 2001, 13, 1690. (h) Shen, W.-J.; Dodda, R.; Wu,
C.-C.; Wu, F.-I.; Liu, T.-H.; Chen, H.-H.; Chen, C. H.; Shu, C.-F.
Chem. Mater. 2004, 16, 930. For tetraarylmethane-based systems, see:
(i) Robinson, M. R.; Wang, S.; Bazan, G. C.; Cao, Y. AdV. Mater.
2000, 12, 1701. (j) Wang, S.; Oldham, W. J., Jr.; Hudack, R. A., Jr.;
Bazan, G. C. J. Am. Chem. Soc. 2000, 122, 5695. (k) Robinson, M. R.;
Wang, S.; Heeger, A. J.; Bazan, G. C. AdV. Funct. Mater. 2001, 11,
413. For hexaarylbenzene-based systems, see: (l) Wu, I.-Y.; Lin, J. T.;
Tao, Y.-T.; Balasubramaniam, E. AdV. Mater. 2000, 12, 668. (m)
Thomas, K. R. J.; Velusamy, M.; Lin, J. T.; Chuen, C. H.; Tao, Y.-T.
J. Mater. Chem. 2005, 15, 4453. For benzocoronene-based hole-
transporting systems, see: (n) Wu, J.; Baumgarten, M.; Debije, M. G.;
Warman, J. M.; Mullen, K. Angew. Chem., Int. Ed. 2004, 43, 5331.
For binaphthyl-based systems, see: (o) Ostrowski, J. C.; Hudack, R. A.,
Jr.; Robinson, M. R.; Wang, S.; Bazan, G. C. Chem.sEur. J. 2001,
7, 4500. (p) Benmansour, H.; Shioya, T.; Sato, Y.; Bazan, G. C. AdV.
Funct. Mater. 2003, 13, 883. (q) He, Q.; Lin, H.; Weng, Y.; Zhang,
B.; Wang, Z.; Lei, G.; Wang, L.; Qiu, Y.; Bai, F. AdV. Funct. Mater.
2006, 16, 1343. For biphenyl-based systems, see: (r) He, F.; Cheng,
G.; Zhang, H.; Zheng, Y.; Xie, Z.; Yang, B.; Ma, Y.; Liu, S.; Shen,
J. Chem. Commun. 2003, 2206. (s) He, F.; Xia, H.; Tang, S.; Duan,
Y.; Zheng, M.; Liu, L.; Li, M.; Zhang, H.; Yang, B.; Ma, Y.; Liu, S.;
Shen, J. J. Mater. Chem. 2004, 14, 2735. (t) 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. (u) 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.
Results
Synthesis of the 2- and 4-Fold Anthracene-Anchored
Bimesityls 1-7. The preparation of 1-7 (Chart 1) was ac-
complished according to the synthetic routes given in Scheme
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17322 J. AM. CHEM. SOC. VOL. 130, NO. 51, 2008

De Novo Design for Functional Amorphous Materials
A R T I C L E S
Chart 2. Structures of Amorphous Functional Materials 1-7 Based on a Bimesityl Core
were purified by silica gel column chromatographic techniques.
The starting materials, i.e., 15 and 16, were prepared by
following the reported procedures.15a,b The compound 17 was
prepared by Sonogashira coupling of 9,10-dibromoanthracene
with phenylacetylene.15c The boronic acid of diphenylanthracene
(14) was prepared from 9-(4-bromophenyl)-10-phenylan-
thracene, which in turn was prepared from 9-phenylanthracene
boronate ester; cf. the Supporting Information (SI).
All the intermediate compounds were characterized routinely
by 1H and 13C NMR spectroscopies, and the 2-fold/4-fold
functionalized anthracene derivatives 1-7 were characterized
1
by IR, H and 13C NMR, MS, and CHN analyses.
Photophysical Studies. The normalized UV-vis absorption
spectra for 1-7 are shown in Figure 1, a perusal of which shows
(8) In our recent preliminary investigations, we have shown that diary-
laminobiphenyl-functionalized bimesityls serve as blue light emitting
materials; see: (a) Moorthy, J. N.; Venkatakrishanan, P.; Huang, D.-
F.; Chow, T. J. Chem. Commun. 2008, 2146.
(7) (a) Natarajan, R.; Savitha, G.; Moorthy, J. N. Cryst. Growth Des. 2005,
5, 69. (b) Natarajan, R.; Savitha, G.; Dominiak, P.; Wozniak, K.;
Moorthy, J. N. Angew. Chem., Int. Ed. 2005, 44, 2115. (c) Moorthy,
J. N.; Natarajan, R.; Venugopalan, P. J. Org. Chem. 2005, 70, 8568.
(d) Moorthy, J. N.; Natarajan, R.; Savitha, G.; Venugopalan, P. Cryst.
Growth Des. 2006, 6, 919.
(9) (a) Organic Electroluminescence; Kafafi, Z. H., Ed.; Optical Engineer-
ing Vol. 94; Taylor and Francis: Boca Raton, FL, 2005. (b) Shirota,
Y. In Organic Electroluminescence; Kafafi, Z. H., Ed.; Optical
Engineering, Vol. 94; Taylor and Francis: Boca Raton, FL, 2005;
Chapter 4, p 147.
9
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A R T I C L E S
Moorthy et al.
Scheme 1. Synthesis of Anthracene-Functionalized Bimesitylenes 1-7
that the absorptions differ significantly depending on the
substitution of the anthracene moieties. The anthracene-anchored
bimesityls 1-7 exhibit a common short-wavelength absorption
band at ca. 280 nm and a long-wavelength band, whose
absorption maximum varies depending on the nature of the
anthracene moiety. The absorption features are similar for pairs
1 and 4 and 3 and 7. Likewise, 2, 5, and 6 exhibit similar
features. The molar extinction coefficients (ε) of the 4-fold
derivatives at the absorption maxima were found to be almost
twice that for 2-fold functionalized derivatives. For example,
the ε values for 1 and 4 are 57 200 (390 nm) and 123 820 (390
nm) M-1 cm-1, respectively. A similar trend was observed for
the pairs 2 and 5 and 3 and 7. The optical band gap values
(Egopt, eV) for 1-7 were determined from their absorption onset
potential edge of the absorption spectra (Table 1). These values
vary from 2.47 to 2.98 eV; cf. Table 1.
The excitation spectra recorded for 1-7 at two different
wavelengths were found to be similar to those of the absorption
spectra. To examine the fluorescence properties of anthracene-
anchored bimesityls 1-7, the photoluminescence (PL) studies
Figure 2. Typical cyclic voltammograms (oxidation waves) observed for
the anthracene derivatives 2, 5, and 6 in dichloromethane/0.1 M TBAP.
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De Novo Design for Functional Amorphous Materials
A R T I C L E S
Table 1. Optical Absorption, PL, PL Quantum Yields, and Thermal Properties of Compounds 1-10
a
e
λmax (nm) (ε × 105)
band gap,b λ (eV)
λ
maxfl(solution)c (nm)
λ
maxfl(film)d (nm)
Φfl(solution)
Tdf (°C)
Tgg (°C)
abs
compd
1
2
3
4
5
6
7
8
9
10
396 (0.57)
436 (1.63)
471 (0.54)
397 (1.23)
438 (2.36)
440 (2.19)
472 (1.10)
425
420, 2.95
458, 2.71
501, 2.47
416, 2.98
460, 2.69
464, 2.67
504, 2.46
446, 2.78
432 (460 s)
453 (476, 504 s)
488 (512, 552 s)
430 (463 s)
450 (477, 508 s)
456 (481, 510 s)
485 (514, 550 s)
441
455
502
572
447
493
486
547
499
0.84
0.59
0.55
0.76
0.86
0.75
0.57
0.92
0.96i
0.88j
518
430
422
471
485
455
486
283
260
466
191
297
247
234
244
202
330
153, 207h
190, 248h
-,k 250h
375i
430i
440j
507j
a The molar extinction coefficients were determined for dichloromethane solutions (ca. 1 × 10-5 M) at 390 nm. b Band gap values as calculated from
c
the onset potential (λ) of the absorption spectrum; see the text. All the emission spectra were recorded for dilute solutions (1 × 10-6 M) in
cyclohexane; the values in parentheses refer to other absorption bands and shoulders (s). d All the films were prepared by spin-coating of the solutions in
1,2-dichloroethane. e Quantum yields were calculated using anthracene as a standard and N2-bubbled cyclohexane as a solvent for λex ) 367 nm. The
reported values are an average of three independent determinations. f Obtained from thermogravimetric analysis under argon/nitrogen gas at a heating
rate of 10 or 5 °C/min. g Obtained from differential scanning calorimetry under argon/nitrogen gas at a heating rate of 10 or 5 °C/min. h Melting points
from the literature are given for comparison. i Reference 16. j Reference 17. k Not available.
were carried out on dilute solutions (1 × 10-6 M, Figure 1)
and thin films (solid state); cf. the SI. The results are collected
in Table 1. The emission maximum for 1 and 4 in the solution
state was observed at ca. 430 nm, which is blue. For 2 and 5
blue-green emissions were observed with maxima at 453 and
450 nm. The anthracene derivatives 3 and 7 were found to emit
in the green region with λmax at ca. 485-488 nm. The model
compound 8 showed an emission maximum at 441 nm. From
the spectra in Figure 1, one clearly observes a red shift in the
emission maximum for 7 relative to that for 5 and 6 by ca.
30-35 nm.
The PL studies on films were also performed to examine the
emission in the condensed phase. The films were made by spin-
casting of their solutions in 1,2-dichloroethane onto the quartz
plates. The films of bimesitylenes 1, 2, and 4 exhibited red-
shifted emission by ca. 20-50 nm as compared to the emission
in the solution state. The emission maxima for the films of 5
and 6 were found to be ca. 40 nm red-shifted from those
observed in their solution states. A red shift by ca. 60-80 nm
with a slight broadening toward the longer wavelength region
was observed for 3 and 7 (cf. the SI). The red shifts in the
emission maxima of 1, 2, and 4-6 by ca. 20-50 nm in the
spin-coated films is presumably due to varying degrees of
planarization of the phenyl/aryl rings with respect to central
anthracene rings within the amorphous phase; this may result
in a continuum of states that disturb the vibronic pattern, leading
to a broad red-shifted emission. The sterically hindered phenyl/
aryl rings cannot be expected to promote excimer formation.18
The fact that one observes a structured photoluminescence as
well as electroluminescence for vacuum-deposited films, e.g.,
for 2 (cf. the SI), lends credence to this.19 Furthermore, the
excitation spectra typically recorded for the film of 2 at various
wavelengths of emission maximum were identical. For bimesi-
tylenes 3 and 7, it is likely that the extended phenylethynyl arms
of the neighboring molecules within the amorphous phase
penetrate into each other, leading to partial charge transfer in
the excited state, which manifests in observed red-shifted
emission with associated broadening. It should be noted that,
even for these cases, excimer formation via cofacial arrangement
of the (phenylethynyl)anthracene chromophores cannot occur
due to the bimesityl scaffold. For all bimesitylenes 1-7, the
difference in the dielectric constants of the two media, i.e.,
solution and thin film, may additionally contribute to the
observed red-shifted emissions upon going from solutions to
films.20 The photoluminescence quantum yields (Φfl) for 1-7
were determined in cyclohexane using anthracene as a stan-
dard,21 and the results are recorded in Table 1. The efficiencies
for 1-7 fall in the range 0.55-0.86 and are comparable to those
of their respective model compounds 8-10.22
Thermal Properties. The thermal behavior of the anthracene-
anchored bimesityls 1-7 was examined by TGA and DSC
studies. All the compounds were found to exhibit a high
decomposition temperatures (Td) in the range between 420 and
520 °C; cf. the SI. In contrast, the simple model compounds
8-10 are characterized by relatively low melting points. One
observes a regular improvement in the thermal stability as we
move from disubstituted bimesitylene 2 to simple phenylan-
thracene-substituted bimesitylene 5 to the bulky (phenylethy-
nyl)anthracene-substituted bimesityl derivative 7 (Table 1). The
(14) For example, see: (a) Kim, Y. H.; Shin, D. C.; Kim, S, H.; Ko, C. H.;
Yu, H. S.; Chae, Y. S.; Kwon, S. K. AdV. Mater. 2001, 13, 1690. (b)
Kim, Y. H.; Jeong, H. C.; Kim, S. H.; Yang, K.; Kwon, S. K. AdV.
Funct. Mater. 2005, 15, 1799. (c) Zheng, S.; Shi, J. Chem. Mater.
2001, 13, 4405. (d) Kla¨rner, G.; Davey, M. H.; Chen, W. D.; Scott,
J. C.; Miller, R. D. AdV. Mater. 1998, 10, 993.
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(11) (a) Cornil, J.; Beljonne, D.; Calbert, J. P.; Bre´das, J. L. AdV. Mater.
2001, 13, 1053. (b) Hallenx, V. de.; Calbert, J. P.; Brocorens, P.;
Cornil, J.; Delercq, J. P.; Bre´das, J. L. AdV. Funct. Mater. 2004, 14,
649. (c) Cornil, J.; dos Santos, D. A.; Crispin, X.; Silbey, R.; Bre´das,
J. L. J. Am. Chem. Soc. 1998, 120, 1289. (d) 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.
(15) (a) Mosnaim, D.; Nonhebel, D. C.; Russell, J. A. Tetrahedron 1969,
25, 3485. (b) Stewart, F. H. C. Aust. J. Chem. 1960, 13, 478. (c)
Gimenenz, R.; Pinol, M.; Serrano, L. Chem. Mater. 2004, 16, 1377.
(16) Meech, S. R.; Phillips, D. J. Photochem. 1983, 23, 193.
(17) Hanhela, P. J.; Paul, D. B. Aus. J. Chem. 1984, 37, 553.
(18) Only strong electrostatic interaction between electrochemically gener-
ated di-ions of opposite charges leads to excimer emission with a
characteristic broad and red-shifted emission in sterically hindered
phenylanthracene-functionalized spirobifluorenes; see: Sartin, M.; Shu,
C.; Bard, A. J. J. Am. Chem. Soc. 2008, 130, 5354.
(12) Thompson, A. L.; Ahn, T. S.; Thomas, K. R. J.; Thayumanavan, S.;
Martinez, T. J.; Bardeen, C. J. J. Am. Chem. Soc. 2005, 127, 16348.
(13) (a) Negishi, E.-i., Ed. Handbook of Organopalladium Chemistry for
Organic Synthesis; Wiley Interscience: New York, 2002. (b) Miyaura,
N., Ed. Cross-Coupling Reactions: A Practical Guide; Topics in
Current Chemistry Series 219; Springer Verlag: New York, 2002.
(19) The differences in the emission spectra for spin-coated and vacuum-
deposited films point to differences in the molecular arrangements in
the two films.
9
J. AM. CHEM. SOC. VOL. 130, NO. 51, 2008 17325

A R T I C L E S
Moorthy et al.
Table 2. EL Data for 1, 2, 4, and 5 and the Model Compounds 8-10
b
ηext
substratea
Von
ηpb
ηlb
λmax
Lb (Vd)
Lmax
fwhm
CIE (x,y)
EL
1A
4.5
1.17
0.48
1.24
452
462
498
460
460
248 (8.1)
4460
76
0.15, 0.13
1.05c
2.89
0.35c
1.13
1.12c
3.82
1117c (10.1)
762 (10.6)
3195c (13.0)
129 (6.6)
1B
2A
2B
2C
4.0
3.5
3.5
6.5
15303
3199
78
82
54
48
0.15, 0.17
0.19, 0.38
0.13, 0.18
0.12, 0.13
1.42c
0.28
0.77c
0.31
3.20c
0.64
0.30c
4.24
0.24c
2.03
0.70c
5.70
697c
1139 (8.8)
4853c (11.8)
767 (11.5)
2897c (14.1)
168c (15.1)
155c (17.2)
59(9.4)
13874
8200
3.62c
3.78
1.29c
1.05
4.86c
3.84
2.86c
0.17c
0.07c
0.17
0.65c
0.04c
0.03c
0.09
2.91c
0.17c
0.16c
0.29
4
5
8A
7.5
7.5
5.5
432
492
454
332
227
831
0.18, 0.17
0.20, 0.37
0.20, 0.26
84
108
68
0.17c
1.79
0.08c
1.13
0.28c
2.79
279c (11.4)
557(7.8)
9A
4.0
3.5
4.5
450
452
526
10992
8147
517
0.17, 0.21
0.15, 0.10
0.23, 0.44
1.57c
2.82
0.84c
1.01
2.45c
2.54
2429c (9.2)
508(7.9)
9B
2.20c
0.17
0.06c
0.13
1.94c
0.47
1944c(9.8)
93(11.7)
10A
90
0.12c
0.07c
0.32c
316c (14.4)
a A, B, and C refer to device configurations, which are (device A) ITO/NPB (400 Å)/1 or 2 (100 Å)/TPBI (400 Å)/LiF (10 Å)/Al (1500 Å), (device
B) ITO/NPB (400 Å)/1% 1 or 2:MADN (400 Å)/TPBI (400 Å)/LiF (10 Å)/Al (1500 Å), and (device C) ITO/NPB (400 Å)/1% 2:CBP (400 Å)/TPBI
(400 Å)/LiF (10 Å)/Al (1500 Å). Those for 4 and 5 are ITO/PEDOT:PSS/5% 4:PVK/TPBI (400 Å)/LiF (10 Å)/Al (1500 Å) and ITO/PEDOT:PSS/10%
5:PVK/BCP (100 Å)/AlQ (300 Å)/LiF (10 Å)/Al (1500 Å). The films of 4 and 5 were prepared by spin-coating of solutions of 4 or 5 and PVK in
chlorobenzene. Von is the turn-on voltage, ηext is the external quantum efficiency, ηp is the power efficiency (lm/W), ηl is the luminance efficiency (cd/
A), L is the luminance (cd/m2), Vd is the driving voltage (V), Lmax is the maximum luminance achieved (cd/m2), fwhm is the full width at half-maximum
b
c
(nm), and CIE stands for 1931 Commission Internationale de l’Eclairage coordinates. Measured at 20 mA/cm2. Measured at 100 mA/cm2.
DSC experiments reveal that all compounds 1-7 possess stable
thermal properties upon heating and cooling cycles; cf. the SI.
Further, they exhibit high Tg values (Table 1). The X-ray powder
diffraction profiles of anthracene-anchored bimesitylenes 4-7
show a lack of crystallinity when compared with their model
analogue 8; cf. the SI.
Electrochemical Properties. Cyclic voltammetric (CV) studies
were performed to calculate the HOMO and LUMO values for
1-6; the poor solubility of 7 precluded estimation of its HOMO
by cyclic voltammetry. The CV experiments were carried out
in solutions of 0.1 M supporting electrolyte (tetra-n-butylam-
monium hexafluorophosphate, TBAP) and 1 mM substrate in
dry dichloromethane under an argon atmosphere using ferrocene
Figure 3. Electroluminescence spectra of 1 for two different devices, A
as an internal standard. The representative voltammograms for
2, 5, and 6 are shown in Figure 2. The oxidation potential
(E1/2) in each case was calculated by taking the average of the
anodic and cathodic peak potentials. The values of the HOMOs
for 1-6 thus calculated are 5.71 (1), 5.54 (2), 5.52 (3), 5.54
(4), 5.73 (5), and 5.69 (6) eV. From the HOMO values and the
optical band gap energy available from the UV-vis spectra
(Table 1), the energies of the LUMOs for 1-6 were calculated
as follows: 2.76 (1), 2.83 (2), 3.05 (3), 2.56 (4), 3.04 (5), and
3.02 (6) eV.
Electroluminescence Properties. The functional behavior of
the amorphous materials was examined typically for 1, 2, 4,
and 5 by constructing devices to capture electroluminescence.
In our attempts, we discovered that the 2-fold functionalized
anthracene derivatives 1 and 2 could be examined for elec-
troluminescence by vacuum sublimation. Three types of mul-
tilayer devices, i.e., A, B, and C, were fabricated as follows:
(device A) ITO/NPB (400 Å)/1 or 2 (100 Å)/TPBI (400 Å)/
LiF (10 Å)/Al (1500 Å); (device B) ITO/NPB (400 Å)/1% 1 or
2:MADN (400 Å)/TPBI (400 Å)/LiF (10 Å)/Al (1500 Å);
(device C) ITO/NPB (400 Å)/1% 2:CBP (400 Å)/TPBI (400
Å)/LiF (10 Å)/Al (1500 Å). Indium-tin-oxide (ITO)-coated
glass served as the anode, N,N′-bis(naphthalen-1-yl)-N,N′-
bis(phenyl)benzidine (NPB) as a hole transporting layer, 1 or 2
and B.
as an emitter, 2-methyl-9,10-bis(naphthalen-2-yl)anthracene
(MADN) and 4,4′-bis(carbazol-9-yl)biphenyl (CBP) as host
emitting materials,23 2,2′,2′′-(1,3,5-benzenetriyl)tris(1-phenyl-
1H-benzimidazole) (TPBI) as an electron transporting layer, and
LiF:Al as the composite cathode. The performance character-
istics (turn-on voltage, power efficiency, luminance efficiency,
quantum efficiency, and CIE coordinates) of these devices are
summarized in Table 2. As can be seen, both devices A and B
for compound 1 exhibited a low turn-on voltage, bright
luminance, and respectable external quantum efficiency. The
doped device B showed better performance in comparison to
that of the nondoped device A (Table 2). The EL spectra
captured from the above devices of 1 are shown in Figure 3. In
Figure 4 are shown the external efficiency vs current density
plot and current-voltage-luminance profiles for both 1A and
1B.
The EL spectra recorded likewise for compound 2 under three
different device configurations are shown in Figure 5. The spec-
tra reveal blue and blue-green emissions according to the CIE
(20) Salbeck, J.; Yu, N.; Bauer, J.; Weissortel, F.; Bestgen, H. Synth. Met.
1997, 91, 209.
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De Novo Design for Functional Amorphous Materials
A R T I C L E S
Figure 4. Device external quantum efficiency (left) and current-voltage-luminance (right) characteristics for 1A and 1B.
coated devices are collected in Table 2. The device for 5 emitted
a blue-green light with λmax ) 492 nm, and that for 4 emitted
blue light with λmax ) 432 nm. The EL spectra of 1, 2, 4, and
5 from all the devices exhibit maxima closer to those observed
in their PL (film) spectra.
For comparison, the electroluminescence of model com-
pounds 8-10 was examined under doped (configuration B) as
well as nondoped (configuration A) conditions. The performance
results of the devices for 8-10 are collected in Table 2. In view
of the best results noted above for compound 2 under doping
conditions, the device performance characteristics of its model
compound 9 were also examined under doping conditions. In
Figure 8 are shown the EL spectra of 9 for nondoped (A) and
doped (B) devices. The EL spectrum of device 9A is character-
ized by a major band at ca. 450 nm followed by a low-energy
band at ca. 510 nm, while the latter is conspicuously absent in
that of 9B. The origin of the long-wavelength band in device
9A was probed by recording excitation and emission spectra
for 9 in films of varying thickness; see the SI. The photolumi-
nescence spectra showed that the intensity of the long-
wavelength band increases relative to that of the major band at
450 nm with an increase (100-400 Å) in the thickness of the
film. We attribute this feature to some kind of aggregation that
manifests predominantly with increasing film thickness. Oth-
erwise, it is noteworthy that the EL is captured with better
efficiencies under doped conditions for this model compound
as well; for example, the external efficiency is ca. 2.8% as
compared to 1.8% under nondoped conditions; cf. Table 2.
Figure 5. Electroluminescence spectra for 2 from three different devices,
A, B, and C. The inset photographs show the blue-green to blue emitter
from the devices of 2.
chromaticity diagram. The current-voltage-luminance char-
acteristics for nondoped and doped (1% 2) devices are shown
in Figure 6. For device 2C with a CBP host, the current turns
on at 3.5 V and the light later turns on at 6.5 V. The external
quantum efficiency and brightness based on different types of
fabrications are also shown in Figure 6. The doped devices 2B
and 2C were found to be more efficient than the pure 2 device,
that is, 2A. As compared to the latter, a substantial increase in
the efficiency from 0.28% to 4.24% was observed for device
2B with a corresponding luminescence enhancement from 129
to 1139 cd/m2 at 20 mA/cm2. Figure 7 shows significantly
improved peak EL efficiency, which increases from 3.84 cd/A
or 1.05 lm/W (for 2C) to 5.70 cd/A or 2.03 lm/W (for 2B).
The 4-fold anthracene-functionalized bimesitylenes 4 and 5
could not be sublimed for construction of the devices via the
vacuum-sublimation technique. Therefore, the fabrication was
done by the spin-coating method. The doped device of config-
uration ITO/PEDOT:PSS/x% 4 or 5:PVK/TPBI or BCP/Alq3/
LiF/Al involving poly(vinylcarbazole) (PVK)24 as the host
improved the luminance and quantum efficiencies compared to
the nondoped device. In the above configuration, PEDOT:PSS
was the hole injection layer and a blend of PVK and compound
4 or 5 (x%, x ) 5-25) that had been spun onto the hole injection
layer from its solution in chlorobenzene served as the emitting
layer. BCP (bathocuprine) was a hole-blocking layer, and TPBI
or Alq3 (tris(8-hydroxyquinoline)aluminum) served as an
electron transporting layer. Finally, a composite cathode of LiF
and Al was thermally evaporated onto the emitting layer
sequentially. The performance characteristics of the above spin-
Discussion
As mentioned at the outset, there is a surge of interest in
electroluminescence from organic molecules at the current time.
Small-molecule-based OLEDs are considered more valuable
than the polymer counterparts because of the difficulty in
controlling the film thickness with polymers and the possibility
of dissolution of the first layer during the coating of the second
layer by spin-coating techniques. Further, the diversity of organic
molecules with varying molecular weights and wide-ranging
properties is simply unmatched by inorganic materials. Our
design of EL materials constitutes a new category, which entails
2-fold/4-fold attachment of fluorophores to the basic extended-
tetrahedral bimesitylene core by Suzuki/Sonogashira coupling
protocols; the fluorophores, i.e., diphenylanthracene16 and 9,10-
bis(phenylethynyl)anthracene17 in the present study, are so
chosen as to exhibit, as a primary criterion, high fluorescence
quantum yields in the solution state (Φfl ) ca. 0.9). The
motivations for consideration of anthracene derivatives were
(i) they are well-known to undergo facile electrochemical
(21) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229.
9
J. AM. CHEM. SOC. VOL. 130, NO. 51, 2008 17327

A R T I C L E S
Moorthy et al.
Figure 6. Current-voltage-luminance (left) and efficiency-luminance-current (right) characteristics for the different 2 devices, i.e., 2A, 2B, and 2c.
cence in anthracene crystals26 due to their blue emission, which
is crucial for development of OLEDs with white emission, vide
infra.
As shown in Scheme 1, the synthesis of a number of
anthracene derivatives is readily accomplished via Pd(0)-
mediated Suzuki and Sonogashira coupling reactions. The
unique advantage is 2-fold or 4-fold functionalization of the
bimesitylene selectively, which is rather tedious for spirobif-
luorene and heretofore unknown for tetraphenylmethane.27 That
the 3-dimensional molecular topology of 1-7 does indeed
manifest in the amorphous nature is clearly revealed from their
powder X-ray diffraction patterns (cf. the SI). The thermal
stability is an important property for a molecule to be used in
a device.28 Several strategies and methods have been explored
Figure 7. Luminance and power efficiencies for 2 under doping conditions
with device configurations B and C.
to achieve high thermal stabilities. The anthracene-anchored
bimesityls were isolated as highly amorphous (Tg ≈ 190-330
°C) compounds with a high thermal stability (decomposition
starts above 420 °C); for example, the Td values for 2-fold and
4-fold functionalized bimesitylenes 1 and 4 are 470-520 °C as
compared to 260 °C for its model monomeric compound 9. The
DSC analyses for 1-7 show that the Tg values increase with
an increase in the rigidity and bulkiness (Table 1). Thus, the
extended-tetrahedral geometry, the rigidity extant to the bimesi-
tylene core, and 2-fold/4-fold substitution with large and bulky
substituents such as anthracenes manifest not only in high glass
transition (Tg) temperatures, but also in thermal decomposition
(Td) temperatures.
As shown in Figure 1, 1-7 exhibit UV-vis absorption
profiles that are characteristic of anthracene. It is noteworthy
that the absorption maximum shifts toward the longer wave-
Figure 8. Electroluminescence spectra of 9,10-diphenylanthracene (9) under
length region by ca. 30-40 nm upon introduction of an
additional triple bond as we go from 4 and 5 to 6 and 7.
Replacement of ethyne by a phenyl ring results in a blue shift
(1 and 3, Table 1), evidently due to the break in conjugation,
since the aryl rings attached to the core remain perpendicular.7c
The optical band gap energy values (Egopt) calculated from the
tail end of the absorption vary from 2.46 to 2.98 eV (see Table
1).
nondoped (A) and doped (B) conditions.
oxidation (hole transporting) and reduction (electron transport-
ing) reactions25 and hence have found use in electroluminescent
devices and (ii) there is a continued interest in exploiting them
for electroluminescence since the discovery of electrolumines-
(22) When the films were subjected to heat treatment (annealing) under
vacuum, the emission intensity of the films was found to decrease
considerably by 2-3-fold with a broadening in their spectral profiles.
Presumably, the annealing leads to some kind of reorganization of
the molecules within the amorphous film in a way that the emission
is suppressed. That the thermal heating does not lead to recrystallization
was established from powder XRD of the bimesitylenes 1 and 5 before
and after annealing; cf. the SI.
All compounds 1-7 exhibit very high emission intensities
at concentrations as low as 10-5-10-6 M. The effect of an
(25) (a) Kim, Y. H.; Kwon, S. K.; Yoo, D. S.; Rubner, M. F.; Wrighton,
M. S. Chem. Mater. 1997, 9, 2699. (b) Shi, J.; Tang, C. W. Appl.
Phys. Lett. 2002, 80, 3201. (c) Benzman, R.; Faulkner, L. R. J. Am.
Chem. Soc. 1972, 94, 6317. (d) Garay, R. O.; Naarmann, H.; Mu¨llen,
K. Macromolecules 1994, 27, 1922. (e) Shih, H. T.; Lin, C. H.; Shih,
H. H.; Cheng, C. H. AdV. Mater. 2002, 14, 1409. (f) Danel, K.; Hwang,
T. H.; Lin, J. T.; Tao, Y. T.; Chen, C. H. Chem. Mater. 2002, 14,
3860. (g) Yu, M. X.; Duan, J. P.; Lin, C. H.; Chung, C. H.; Tao,
Y. T. Chem. Mater. 2002, 14, 3948.
(23) For use of MADN as a dopant, see: (a) Lee, M. T.; Chen, H. H.;
Liao, C. H.; Tsai, C. H.; Chen, C. H. Appl. Phys. Lett. 2004, 85, 3301.
(b) Lee, M. T.; Liao, C. H.; Tsai, C. H.; Chen, C. H. AdV. Mater.
2005, 17, 2493.
(24) For use of PVK as a host, see: Robinson, M. R.; Wang, S.; Bazan,
G. C.; Cao, Y. AdV. Mater. 2000, 12, 1701.
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17328 J. AM. CHEM. SOC. VOL. 130, NO. 51, 2008

De Novo Design for Functional Amorphous Materials
A R T I C L E S
increase in conjugation is clearly revealed in the emission
maxima, which vary from ca. 450 to 570 nm (blue to blue-
green to green). Thus, the emission properties can be modulated
by appropriate structural modifications. Further, the films do
not appear to reveal any aggregation behavior at least for 1, 2,
and 4-6. What is noteworthy is the conservation of photo-
physical properties of the constituent fluorophores, i.e., an-
thracenes, upon 4-fold functionalization, which further attests
to the fact that the conjugation is only weakly transmitted in
the meta linkage. The fluorescence quantum efficiencies of all
1-7 are quite high (Table 1) and vary from 0.55 to 0.86. It
should be noted that the attachment of the small molecular
fluorophores to the bimesityl core leading to branched systems
results only in marginal variations in the quantum yields as
compared to those of the free constituent fluorophores 8-10.
Figure 9. Energy diagram for the devices doped with anthracene-
functionalized bimesitylenes. Also shown are the molecular structures of
CBP and MADN, which are employed as host emitting materials.
The functional behavior of the branched amorphous systems
1-7 was demonstrated by capturing electroluminescence for
1, 2, 4, and 5 as representative cases. In general, the devices
based on anthracenes are renowned for their excellent
stability.23a For 2-fold anthracene-functionalized bimesitylenes
1 and 2, the OLEDs of two configurations, A (nondoped) and
B (doped), were fabricated by vacuum deposition. The host
material in the emitting layer of device C is CBP, while that in
device B is MADN. The results of devices summarized in Table
2 for bimesitylenes 1 and 2 as well as for the model compounds
8-10 suggest that the performance characteristics under non-
doped conditions (A) are unexceptional. Otherwise, it is pertinent
to note that the EL spectrum of nondoped 2-fold functionalized
anthracene 1, i.e., 1A (Figure 3), reveals the absence of the long-
wavelength emission at ca. 500 nm, which is prominently
observed in the EL of its model compound 9; see 9A, Figure 8.
Given that the long-wavelength emission presumably owes its
origin to aggregation as mentioned earlier, its absence in 1A
clearly attests to the advantage of the bimesitylene scaffold in
obviating such a manifestation of long-wavelength emission.
In contrast to the nondoped devices, the doped devices (B
and C) for 1 and 2 as well as for the model compound 9 can be
turned on between 3.5 and 6.5 V with intensities in the range
of 508-1140 cd/m2 at a current density of 20 mA/cm2. Further,
the external quantum efficiency increases by 2.5 times for 1B
as compared to that for 1A; the efficiency increases from 1.17%
to 2.89% and the luminescence efficiency from 248 to 762 cd/
m2. Device 1B shows a maximum luminance of ∼15 300 cd/
m2 with CIEx,y (x ) 0.15, y ) 0.17). Clearly, the energy transfer
from the MADN host to the 1 dopant is efficient.29 The EL
spectrum for device 2A shows a major peak at 498 nm and a
shoulder at 470 nm with an fwhm of 82 nm, Figure 5. While
both of the doped devices 2B and 2C exhibit a major peak at
460 nm and a shoulder at 484 nm, the fwhm decreases to 54
nm for the MADN host device (2B) and 48 nm for the CBP
host device (2C). The delay in the light emission turn-on for
2C suggests that the hole and electron carriers are not properly
balanced. The energy diagram for the two device configurations
B and C of compound 2 is shown in Figure 9. Due to the
mismatch in the LUMO levels of CBP and TPBI by 0.6 eV,
the electron injection is presumably more difficult than the hole
Figure 10. Absorption spectrum of 2 and photoluminescence spectrum of
MADN.
injection in device 2C. However, for MADN as the host, both
the current and light turn-on voltages occurred at 3.5 V. This
suggests that hole and electron carriers are perfectly balanced
to facilitate easy electron injection compared to the case when
the host used was CBP, which is evidently due to the energy
gap between the LUMO levels of MADN and TPBI being very
negligible. Further, the peak EL efficiency increases from 3.84
cd/A for 2C to 5.70 cd/A for 2B. Relatively poor efficiency in
the case of 2C is also presumably due to the possibility that
the high concentration of planar CBP host leads to crystalliza-
tion in the emitting layer film. For the devices with MADN
as the host and anthracene derivatives as blue-fluorescent guests,
the reduction in driving voltages, compared to the case when
CBP was used as the host, should be traceable to a narrower
HOMO-LUMO energy gap for MADN than for CBP. In other
words, the HOMO and LUMO energy levels of 2 lie well within
those of the host MADN (HOMO ) 5.75 eV, LUMO ) 2.70
eV) for Fo¨rster-type energy transfer to occur efficiently. Indeed,
the absorption spectrum of 2 exhibits a good overlap with the
emission spectrum of MADN (Figure 10). Among all the
devices, device 2B shows the highest luminance of ∼13 900
cd/m2 at 17.5 V, maximum luminance efficiency of ∼7.4 cd/A
at 4.5 V, and power efficiency of ∼5.3 lm/W at 4.0 V. Further,
at a brightness of 800 cd/m2 and a current density of 13.8 mA/
cm2, device 2B exhibits an excellent luminance efficiency of
5.8 cd/A, an external quantum efficiency of 4.3% with a power
efficiency of 2.2 lm/W, and pure blue light with a CIEx,y (x )
0.13, y ) 0.18). The morphological stability of all the devices
based on the bimesityl scaffold was found to be excellent.
Besides, the results with doping are comparable to or better
than those reported thus far for anthracene-based organic light
emitting diodes.30
(26) Tang, C. W.; Van Slyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(27) (a) Chiang, C. L.; Shu, C. F.; Chen, C. T. Org. Lett. 2005, 7, 3717.
For a very recent synthesis and electroluminescence study on 2-fold
anthracene-functionalized tetraarylsilicon, see: (b) Lyu, Y-Y.; Kwak,
J.; Lee, S.-H.; Kim, D.; Lee, C.; Char, K. AdV. Mater. 2008, 20, 2720.
(28) Tokito, S.; Tanaka, H.; Noda, K.; Okada, A.; Taga, T. Appl. Phys.
Lett. 1997, 70, 1929.
(29) Pope, M.; Swenberg. C. E. Electronic Processes on Organic Crystals;
Clarendon: Oxford, 1982.
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A R T I C L E S
Moorthy et al.
In contrast, the spin-coated devices made from the blend of
PVK and 4/5 were rather unsatisfactory. Although the EL
spectra of 4 and 5 are similar to their PL spectra, the significantly
low emission from PVK appears unavoidable due to the device
structures even with high doping concentrations (ca. 25%).
Further, the emission from TPBIsthe electron transporting/hole
blocking layersin the TPBI-based devices suggests that some
proportion of holes are transported very fast through the emitting
layer and recombine in the TPBI layer, leading to TPBI
emission. The change of solvent from chlorobenzene to toluene
for spin-coating does improve the efficiency by 2-fold, while
solvents such as THF, 1,2-dichloroethane, etc. do not produce
good-quality films. The comparative EL spectra of the Alq3-
and TPBI-based devices with varying doping concentrations are
collected in the Supporting Information. The poor device
efficiency in these fabricated devices may be attributed to poor
film morphology and rapid charge neutralization at the electrodes
via a self-quenching mechanism leading to poor recombination.
Clearly, more experimentation with the device fabrication via
spin-coating techniques is necessary to further delineate the
potential of this new class of 4-fold functionalized amorphous
materials. Otherwise, the efficient functional behavior as OLEDs
for the molecules that can be sublimed is amply demonstrated
from the results observed for 1 and 2.
Given that electroluminescence closely follows the trend
observed for photoluminescence from films or in the solution
state (under conditions that the emission is not observed from
dopants), the anthracene-functionalized bimesityls 1-7 consti-
tute an excellent series with blue and blue-green emission
properties with associated wide optical band gaps. Indeed, the
host emitters with wide band gap energies are of great
importance, as they can be used to dope the red emitting
materials with narrow band gap energies to elicit white light
emission.31 This, indeed, is the cause for a surge of interest in
blue light emitting materials. Several groups have achieved white
light emission from such wide band gap materials via downhill
energy transfer.32 White light emission is considered vital, as
all the colors can be derived from it to produce a full color
display.
alized bimesityls 1-7 correspond to a new and unique category.
It is shown that 1-7 can be readily synthesized via Pd(0)-
mediated Suzuki and Sonogashira protocols. While the rigidity
and 3-dimensional topology seemingly render the molecular
packing difficult, leading to an amorphous nature, the high
molecular weight in conjunction with anthracene fluorophores
that are linked to bimesityl appears to contribute to Tg values
as high as ca. 300 °C. Further advantages of the design are (i)
the emission wavelength can be modulated with a suitable choice
of the fluorophore and (ii) the fluorescence quantum yields of
the constituent fluorophores are reliably reproduced upon 2- or
4-fold functionalization. That the molecular attributes, which
are pivotal to predicting efficient molecular packing and failure
thereof, manifest in amorphous properties for application in
OLEDs with high Tg and Td values is demonstrated by
fabrication of OLED devices; the 2-fold functionalized deriva-
tives 1 and 2 lend themselves to sublimation techniques, so that
the electroluminescence is captured with high efficiencies at low
turn-on voltages. The performance characteristics of the devices
fabricated for 1 and 2 are far more superior to those reported
so far for the anthracene-based OLEDs. Although the 4-fold
functionalized systems did not permit sublimation, leading to
spin-coating as a means for device fabrication, the observed
electroluminescence for 4 and 5 attests to a broader scope and
applicability of this new category of amorphous molecules for
application in OLEDs.
Experimental Section
General Aspects. All the palladium-mediated cross-coupling
reactions were performed under a nitrogen gas atmosphere in oven-
dried pressure tubes. 1H and 13C NMR spectra were recorded on a
JEOL (400 MHz) or Bruker (300/400/500 MHz) spectrometer in
CDCl3 as a solvent. Chemical shifts are reported in δ scale
downfield from the peak for tetramethylsilane. The FAB mass
spectra were recorded on a JEOL SX-102/mass system using argon/
xenon as the FAB gas in the linear mode with m-nitrobenzyl alcohol
as the matrix. The PXRD results were recorded using a Seifert
powder X-ray diffractometer with Cu KR (λ ) 1.54056 Å) radiation.
The TGA and DSC measurements were carried out with a heating
rate of 10 °C/min under a nitrogen or argon gas atmosphere.
UV-vis absorption spectra were recorded using a Jasco-550
spectrophotometer. Photoluminescence spectra and PL quantum
yields were obtained from a Spex Fluorolog-2/Hitachi F-4500
spectrofluorimeter. The photoluminescence spectra of solutions were
measured in spectral grade solvents using a 90° angle of detection,
and those of the thin solid films were measured by front-face
detection. The current-voltage (I-V) characteristics were recorded
by a Keithley model 2400 electrometer. The Commission Interna-
tionale de l’Eclairage (CIE) coordinates of the devices were
measured by a PR650 spectroscan spectrometer. All measurements
were carried out at ambient conditions.
Conclusions
We have shown that the molecular scaffolds based on a D2d-
symmetric 3-dimensional bimesityl core can be exploited to
design rigid and high molecular weight 2- and 4-fold fluoro-
phore-functionalized derivatives. While a variety of design
strategies have been reported so far, the anthracene-function-
(30) (a) Shen, W.-J.; Dodda, R.; Wu, C.-C.; Wu, F.-I.; Liu, T.-H.; Chen,
H.-H.; Chen, C. H.; Shu, C.-F. Chem. Mater. 2004, 16, 930. (b) Kim,
Y.-H.; Shin, D.-C.; Kim, S.-H.; Ko, C.-H.; Yu, H.-S.; Chae, Y.-S.;
Kwon, S.-K. AdV. Mater. 2001, 13, 1690. (c) Benmansour, H.; Shioya,
T.; Sato, Y.; Bazan, G. C. AdV. Funct. Mater. 2003, 13, 883. (d) Shih,
P.-I.; Chuang, C.-Y.; Chien, C.-H.; Diau, E. W.-G.; Shu, C.-F. AdV.
Funct. Mater. 2007, 17, 3141. (e) Kim, Y.-H.; Jeong, H.-C.; Kim,
S.-H.; Yang, K.; Kwon, S.-K. AdV. Funct. Mater. 2005, 15, 1799. (f)
Lee, M.-T.; Liao, C.-H.; Tsai, C.-H.; Chen, C. H. AdV. Mater. 2005,
17, 2493. (g) 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. (h) Tao, S. L.; Hong, Z. R.; Peng, Z. K.; Ju, W. G.; Zhang, X. H.;
Wang, P. F.; Wu, S. K.; Lee, S. T. Chem. Phys. Lett. 2004, 397, 1. (i)
Shi, J.; Tang, C. W. Appl. Phys. Lett. 2002, 80, 3201. (j) Kan, Y.;
Wang, L.; Duan, L.; Hu, Y.; Wu, G.; Qiu, Y. Appl. Phys. Lett. 2004,
84, 1513. (k) Li, H.-C.; Lin, Y.-P.; Chou, P.-T.; Cheng, Y.-M.; Liu,
R.-S. AdV. Funct. Mater. 2007, 17, 520.
Dry tetrahydrofuran (THF) and toluene were freshly distilled over
sodium prior to use. Triethylamine and diisopropylamine were
distilled from KOH. All the reactions were monitored by analytical
thin layer chromatography (TLC) using commercial aluminum
sheets precoated with silica gel. Chromatography was conducted
on silica gel (Acme, Mumbai, 60-120 mesh). All commercial
chemicals were used as received.
The starting materials, viz., bimesityl6c and 11,6c were synthe-
sized according to the earlier reported procedures established from
our laboratories.6 15,13a,b 16,13a,b and 1713 were prepared according
to the literature procedures.13
Preparation of 3,3′-Diiodobimesityl. In a two-necked round-
bottom flask, bimesityl (0.40 g, 1.68 mmol) was dissolved in 8
mL of CH3CN. To this was added 0.83 g (3.69 mmol) of NIS
followed by 0.08 mL of TFA (catalytic amount) under vigorous
stirring and cold conditions. The reaction mixture was allowed to
(31) (a) 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. (b) Cheon,
K. O.; Shinar, J. Appl. Phys. Lett. 2002, 81, 1738.
9
17330 J. AM. CHEM. SOC. VOL. 130, NO. 51, 2008

De Novo Design for Functional Amorphous Materials
A R T I C L E S
stir for 8 h. Subsequently, the reaction mixture was concentrated,
and the organic matter was extracted with CHCl3. The combined
extract was washed with NaHCO3 solution followed by brine, dried
over anhydrous Na2SO4, filtered, and evaporated to dryness. Careful
silica gel column chromatography using CHCl3 and petroleum ether
afforded the colorless diiodobimesityl (0.68 g) in 83% yield: 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.
(M+); HRMS m/z calcd for C70H54 894.4226, found 894.4209 (M+).
Anal. Calcd for C70H54: C, 93.92; H, 6.08. Found: C, 93.93; H,
5.71.
General Procedure for the Synthesis of Tetraphenylbimesi-
tyl via 4-Fold Suzuki Coupling. The following is a representative
procedure for 4-fold Suzuki coupling.
Preparation of 3,3′,5,5′-Tetraphenylbimesityl. 11 (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
pressure tube under N2. To this mixture were added dry toluene
(15 mL) and saturated NaHCO3 solution (15 mL). The reaction
mixture was heated slowly from 80 to 120 °C and maintained at
the latter for 12 h with constant stirring. After this period, the
reaction mixture was cooled, and the contents were extracted with
CHCl3. Silica gel column chromatography using 5% chloroform
in hexane yielded 1.25 g (77%) of tetraphenylbimesityl as a
colorless solid material: IR (KBr, cm-1) 3020, 2921, 1598, 1492,
Preparation of 3,3′,5,5′-Tetrakis[(trimethylsilyl)ethynyl]-
bimesityl. An oven-dried pressure tube was cooled under nitrogen
gas and charged with Pd(PPh3)4 (0.078 g, 0.06 mmol), CuI (0.0065
g, 0.03 mmol), 11 (0.50 g, 0.67 mmol), (trimethylsilyl)acetylene
(0.33 g, 3.36 mmol), and Et3N (12.5 mL). Subsequently, the tube
was sealed, and the mixture was heated at 90 °C for 36 h. After
the usual workup, a short pad filtration was performed to obtain
1
1438; H NMR (400 MHz, CDCl3) δ 1.71 (s, 12H), 1.72 (s, 6H),
1
the compound as a pure material (0.41 g, 97%): H NMR (400
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.
MHz, CDCl3) δ 0.26 (s, 36H), 1.99 (s, 12H), 2.63 (s, 6H); 13C
NMR (100 MHz, CDCl3) δ 0.10, 18.6, 20.0, 102.5, 102.9, 121.4,
137.0, 138.5, 142.3.
Preparation of 3,3′5,5′-Tetrakis(4-bromophenyl)bimesityl. A
solution of tetraphenylbimesityl (1.25 g, 2.3 mmol) in CHCl3 was
treated, under constant stirring and in the presence of Fe powder
(0.2 mmol) as a catalyst, with liquid bromine (0.5 mL, 9.66 mmol)
and stirred at room temperature for 4 h. After this period, it was
diluted with CHCl3, washed well with 2 N NaOH solution, and
dried over anhydrous Na2SO4. After removal of the solvent under
vacuum, the crude product was recrystallized from EtOAc, yield
1.68 g (86%), colorless solid: IR (KBr, cm-1) 2921, 2855, 1487,
1445, 1386; 1H NMR (400 MHz, CDCl3) δ 1.63 (s, 12H), 1.67 (s,
6H), 7.06 (d, J ) 8.3 Hz, 8H), 7.54 (d, J ) 8.3 Hz, 8H); 13C NMR
(100 MHz, CDCl3) δ 18.2, 19.3, 120.5, 131.2, 131.6, 132.0, 132.6,
138.5, 139.0, 140.9.
Preparation of 3,3′-Bis[(trimethylsilyl)ethynyl]bimesityl. This
compound was prepared by following the procedure described
above. Diiodobimesitylene and 4 equiv of (trimethylsilyl)acetylene
were used as coupling agents, and Pd(PPh3)4 (10 mol %) was used
as the catalyst. The desired compound was obtained in a yield of
1
99%: H NMR (300 MHz, CDCl3) δ 0.24 (s, 18H), 1.79 (s, 6H),
1.97 (s, 6H), 2.43 (s, 6H), 6.95 (s, 6H); 13C NMR (100 MHz,
CDCl3) δ 0.19, 18.1, 20.1, 21.0, 102.0, 103.5, 120.9, 128.6, 136.1,
137.0, 138.2, 139.4.
Preparation of 3,3′,5′,5′-Tetraethynylbimesityl, 12. Desilyla-
tion was carried out by adding n-Bu4NF (3.15 mL of a 1 M solution
in THF) dropwise at 0 °C to a solution of tetrakis[(trimethylsilyl-
)ethynyl]bimesityl (0.41 g, 0.65 mmol) in 10 mL of dry THF under
a nitrogen atmosphere. The reaction mixture was slowly allowed
to attain room temperature, and the mixture was stirred for a further
6 h. The solvent was removed in vacuo, and the residue was poured
into water and extracted with dichloromethane. The organic matter
thus obtained was subjected to rapid column chromatography on
Preparation of Tetraboronate Ester 13. A solution of tet-
rakis(4-bromophenyl)bimesityl (0.60 g, 0.69 mmol) in dry THF (40
mL) was cooled to -78 °C. A 1.6 M solution of n-BuLi in hexanes
(2.6 mL, 4.19 mmol) was introduced slowly dropwise into the above
well-stirred solution. The reaction mixture turned red-brown in
color. It was allowed to stir for a further 15-20 min, and
triisopropyl borate (1.9 mL, 8.38 mmol) was slowly introduced.
Subsequently, the reaction mixture was stirred for 30 min at the
same temperature and allowed to warm to room temperature over
a period of 5 h. The solvent was removed under reduced pressure
and dried thoroughly to obtain a pale yellow solid. The isopropyl
boronate ester was dissolved in dry toluene (150 mL), and pinacol
(1.35 g, 11.4 mmol) was introduced. The contents were heated at
reflux overnight with a Dean-Stark apparatus. At the end of the
reaction, the reaction mixture was concentrated, and the organic
matter was extracted with CHCl3. The organic layer was washed
with brine, dried over anhydrous Na2SO4, filtered, and evaporated
to yield a crude product. Rapid column chromatography using 25%
EtOAc in petroleum ether afforded pure boronate ester as a colorless
solid, yield 0.84 g (86%): IR (KBr, cm-1) 2978, 2927, 1609, 1447,
1
silica gel to obtain the pure product (0.21 g, 97%): H NMR (400
MHz, CDCl3) δ 1.98 (s, 12H), 2.65 (s, 6H), 3.50 (s, 4H); 13C NMR
(100 MHz, CDCl3) δ 18.6, 19.9, 76.6, 77.0, 77.3, 81.2, 85.3, 120.6,
137.0, 139.0, 142.9.
Preparation of 3,3′-Diethynylbimesityl. This compound was
prepared starting from 3,3′-bis[(trimethylsilyl)ethynyl]bimesityl by
1
following the procedure described above (yield 96%): H NMR
(400 MHz, CDCl3) δ 1.82 (s, 6H), 1.99 (s, 6H), 2.45 (s, 6H), 3.47
(s, 2H), 6.98 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 18.1, 20.1,
20.9, 81.9, 84.8, 119.9, 128.7, 136.3, 137.0, 138.5, 139.6.
Synthesis of 1. Diiodobimesityl (0.627 g, 1.28 mmol), 9-(4-
bromophenyl)-10-phenylanthraceneboronic acid (14) (1.40 g, 3.74
mmol), and Pd(PPh3)4 (0.118 g, 8 mol %) were introduced into an
initially oven-dried pressure tube under N2 gas. To this mixture
were added dry toluene (10 mL), dry THF (5 mL), and 2 M K2CO3
solution (5 mL). The reaction mixture was heated slowly from 70
to 110 °C and maintained at this temperature for 36 h with constant
stirring. After this period, the reaction mixture was cooled and the
contents were extracted with CHCl3. Careful column chromatog-
raphy over silica gel with 10-25% dichloromethane in petroleum
ether followed by purification via precipitation of the compound
from its solution in chloroform by adding methanol led to 0.45 g
1
1396; H NMR (400 MHz, CDCl3) δ 1.36 (s, 48H), 1.64 (s, 6H),
1.66 (s, 12H), 7.23 (d, J ) 8.0 Hz, 8H), 7.87 (d, J ) 8.0 Hz, 8H);
13C NMR (100 MHz, CDCl3) δ 18.3, 19.3, 24.9, 83.7, 129.0, 131.6,
132.2, 134.8, 138.6, 140.0, 145.6.
Synthesis of 4. To an oven-dried pressure tube cooled under N2
were added tetraboronate ester 13 (0.104 g, 0.10 mmol), 15 (0.26
g, 0.79 mmol), Pd(PPh3)4 (0.009 g, 10 mol %), dry toluene (4.0
mL), and 2 M Na2CO3 (2.0 mL). The solution was purged
thoroughly with nitrogen gas, the tube sealed tightly, and the
solution heated at 110 °C. A pale brown solid began to form after
10 h. After 24 h, the reaction mixture was worked up in the usual
manner. The residue was purified by silica gel column chromatog-
raphy using chloroform-petroleum ether as the eluent. The product
obtained was further purified by precipitation from CHCl3 solution
using petroleum ether/methanol, which led to isolation of the pure
1
(41%) of 1 as a colorless solid material: H NMR (500 MHz,
CDCl3) δ 1.89 (s, 6H), 2.07 (s, 6H), 2.29 (s, 6H), 7.19 (s, 2H),
7.30-7.45 (m, 10H), 7.46-7.67 (m, 16H), 7.69-7.86 (m, 8H);
13C NMR (125 MHz, CDCl3) δ 18.2, 20.1, 21.2, 125.0, 127.0, 127.4,
128.4, 128.9, 129.5, 129.9, 130.0, 131.2, 131.3, 133.7, 134.3, 134.5,
136.9, 137.0, 137.2, 138.3, 139.1, 139.8, 141.1; FAB-MS m/z 894
9
J. AM. CHEM. SOC. VOL. 130, NO. 51, 2008 17331

A R T I C L E S
Moorthy et al.
product (0.15 g, 52%) as a colorless powder: IR (KBr, cm-1) 2920,
1603, 1442, 1388, 1022; H NMR (500 MHz, CDCl3) δ 2.09 (s,
FAB-MS m/z 1344 (M + 1). Anal. Calcd for C106H70: C, 94.75; H,
5.25. Found: C, 94.32; H, 5.53.
1
12H), 2.3 (s, 6H), 7.32-7.40 (m, 16H), 7.46-7.48 (m, 8H),
7.55-7.62 (m, 28H), 7.65-7.73 (m, 8H), 7.75 (t, J ) 5.0 Hz, 8H);
13C NMR (125 MHz, CDCl3) δ 18.8, 20.0, 125.0, 127.0, 127.4,
128.4, 129.7, 129.9, 130.0, 131.3, 132.4, 132.8, 132.9, 137.0, 137.1,
137.2, 139.1, 140.2, 141.8; FAB-MS m/z 1550 (M+). Anal. Calcd
for C122H86: C, 94.41; H, 5.59. Found: C, 94.18; H, 5.75.
Synthesis of 6. This compound was prepared starting from
3,3′,5′,5′-tetraethynylbimesityl by following the general procedure
described above. 16 was used as the coupling partner with Pd(PPh3)4
(10 mol %) as the catalyst. Isolation and purification were carried
out as described for 5 above, yield 52%, deep yellow fluorescent
solid: IR (KBr, cm-1) 2920, 2179, 2050, 1607, 1511, 1435, 1245,
1
1032, 765; H NMR (500 MHz, CDCl3) δ 2.66 (s, 12H), 3.38 (s,
General Procedure for the 2-Fold Coupling of 3,3′-Diethy-
nylbimesityl with Aryl Bromide via Sonogashira Coupling. A
typical procedure for the synthesis of 2 is as follows. Sonogashira
coupling is described below for the synthesis of 2 as a representative
case.
6H), 3.96 (s, 12H), 7.14 (d, J ) 8.5 Hz, 8H), 7.37 (d, J ) 8.5 Hz,
8H), 7.43 (t, J ) 8.0 Hz, 8H), 7.66 (t, J ) 8.0 Hz, 8H), 7.77 (d, J
) 9.0 Hz, 8H), 8.90 (d, J ) 9.0 Hz, 8H); 13C NMR (125 MHz,
CDCl3) δ 19.9, 21.5, 55.4, 95.0, 99.0, 113.9, 117.8, 122.7, 125.5,
126.4, 126.9, 127.6, 130.3, 130.5, 132.3, 132.4, 137.9, 138.3, 138.6,
141.5, 159.2; FAB-MS m/z 1463 (M+). Anal. Calcd for C110H78O4:
C, 90.26; H, 5.37; O, 4.37. Found: C, 89.48; H, 5.60.
An oven-dried pressure tube was charged with 3,3′-diethynyl-
bimesityl (0.45 g, 1.57 mmol), 15 (1.88 g, 5.66 mmol), Pd(PPh3)4
(0.145 g, 8 mol%), CuI (0.006 g, 4 mol %), and Et3N:THF (20:10
mL) under a N2 atmosphere and sealed. The contents were heated
in an oil bath at 90 °C for 48 h. The reaction mixture turned yellow
in color within 10 min and then to yellow-brown with time. After
the usual workup, a short pad filtration over silica gel was performed
with petroleum ether to isolate the unreacted 9-bromo-10-pheny-
lanthracene and 9-phenylanthracene. Further elution with chloro-
form yielded a mixture of mono- and dicoupled products. Careful
silica gel column chromatography of the mixture with 10%
dichloromethane in petroleum ether led to a small amount of the
monocoupled product first (10-15%) followed by the required
difunctionalized bimesitylene 2 as a bright yellow solid. It was
further purified by precipitation of its solution in dichloromethane/
chloroform using ethanol, yield 0.63 g (51%): 1H NMR (500 MHz,
CDCl3) δ 2.04 (s, 6H), 2.47 (s, 6H), 2.83 (s, 6H), 7.20 (s, 2H),
7.37-7.45 (m, 10H), 7.54-7.61 (m, 8H), 7.68 (d, J ) 8.5 Hz,
4H), 8.86 (d, J ) 9.0 Hz, 4H); 13C NMR (125 MHz, CDCl3) δ
19.1, 20.4, 21.9, 94.4, 99.5, 118.4, 121.6, 125.5, 126.2, 127.0, 127.4,
127.6, 128.4, 129.0, 130.0, 131.2, 132.1, 136.4, 137.4, 138.0, 138.1,
138.6, 139.1; FAB-MS m/z 790 (M+); HRMS m/z calcd for C62H46
790.3600, found 790.3616 (M+). Anal. Calcd for C62H46: C, 94.14;
H, 5.86. Found: C, 94.19; H, 5.67.
Synthesis of 7. This compound was prepared starting from
3,3′,5′,5′-tetraethynylbimesityl by following the general procedure
described above. 17 was used as the coupling partner with Pd(PPh3)4
(10 mol %) as the catalyst. Isolation and purification were carried
out as described for 5 above, yield 54%, bright orange red powder:
IR (KBr, cm-1) 2922, 2367, 2182, 1597, 1435, 1023, 758; 1H NMR
(500 MHz, CDCl3) δ 2.64 (s, 12H), 3.36 (s, 6H), 7.43-7.48 (m,
12H), 7.67-7.70 (m, 16H), 7.80-7.81 (m, 8H), 8.73-8.75 (m, 8H),
8.85-8.87 (m, 8H); 13C NMR (125 MHz, CDCl3) δ 19.9, 21.6,
86.5, 95.1, 100.4, 102.5, 118.5, 118.9, 122.6, 123.4, 126.8, 127.0,
127.3, 127.4, 128.6, 128.7, 131.7, 132.2, 138.9, 141.8; FAB-MS
m/z 1439 (M+). Anal. Calcd for C114H70: C, 95.10; H, 4.90. Found:
C, 95.72; H, 4.83.
PL Quantum Yield Measurements in Solutions. For determi-
nation of fluorescence quantum yields, the solutions of bimesitylene-
based EMs 1-7 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 the right angle mode (375-650 nm). The quantum yield was
calculated from the following relation:
Synthesis of 3. This compound was prepared by following the
general procedure described above. 17 was used as the coupling
partner with Pd(PPh3)4 (8 mol %) as the catalyst. Isolation via silica
gel column chromatography and purification as described above
φu ) φs(As/Au)(Iu/Is)(ηu/ηs)2
(1)
where the subscripts “s” and “u” refer to standard and unknown
samples, Au and As to absorbances of the sample and the standard
at the excitation wavelength, Iu and Is to the integrated emission
intensities (i.e., areas under the emission curves) of the sample and
the standard, and ηu and ηs to the refractive indexes of the
corresponding solutions (pure solvents are assumed). Anthracene
was chosen as a reference for quantum yield determination, for
which the reported quantum yield in ethanol is 0.27.16
Electrochemical Measurements. The cyclic voltammetry ex-
periments were performed on a BAS-100 B electrochemical
analyzer. The data were collected and analyzed using electrochemi-
cal analysis software. All experiments were carried out in a three-
electrode compartment cell with a Pt wire counter electrode, a glassy
carbon working electrode, and a Ag/AgNO3 (0.1 M) reference
electrode at varying scan rates. The supporting electrolyte used was
0.1 M tetrabutylammonium hexafluorophosphate solution in dry
dichloromethane. 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 potentials of 1-7 were
subsequently determined from the average of the anodic and
cathodic 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 oxidation
potential by using the following equations: EHOMO ) Eox + 4.8
eV; ELUMO ) EHOMO - Egopt. The HOMO of ferrocene lies at 4.8
eV.
1
for 2 yielded 3 in 62% yield, bright orange fluorescent solid: H
NMR (500 MHz, CDCl3) δ 2.03 (s, 6H), 2.45 (s, 6H), 2.82 (s,
6H), 7.20 (s, 2H), 7.42-7.48 (m, 6H), 7.60-7.66 (m, 8H),
7.77-7.79 (m, 4H), 8.69-8.71 (m, 4H), 8.79-8.81 (m, 4H); 13C
NMR (125 MHz, CDCl3) δ 19.1, 20.4, 21.9, 86.6, 94.4, 101.1,
102.2, 118.0, 119.4, 121.5, 123.5, 126.7, 126.8, 127.3, 127.4, 128.6,
128.7, 129.2, 131.7, 132.0, 132.1, 136.7, 137.4, 138.2, 139.2; FAB-
MS m/z 838 (M+); HRMS m/z calcd for C66H46 838.3600, found
836.3610 (M+). Anal. Calcd for C66H46: C, 94.47; H, 5.53. Found:
C, 94.26; H, 5.70.
Synthesis of 5. This compound was prepared starting from
3,3′,5′,5′-tetraethynylbimesityl by following the general procedure
described above. 15 was used as the coupling partner with Pd(PPh3)4
(10 mol %) as the catalyst. With a short pad filtration of the reaction
mixture over silica gel using petroleum ether, the unreacted
9-bromo-10-phenylanthracene and dehalogenated 9-phenylan-
thracene were isolated. Further filtration with CHCl3 yielded a
mixture of products, which was subjected to SiO2 column chro-
matography. Gradual elution with 2-10% chloroform/petroleum
ether led to a more polar component, which was identified as the
required bimesitylene 5. Further purification was accomplished via
precipitation of its solution in dichloromethane/chloroform, yield
59%, bright yellow fluorescent solid: IR (KBr, cm-1) 2921, 2362,
1
2183, 1435, 1024; H NMR (400 MHz, CDCl3) δ 2.68(s, 12H),
3.39 (s, 6H), 7.44 (m, 16H), 7.61 (m, 20H), 7.72 (d, J ) 8.8 Hz,
8H), 8.93 (d, J ) 8.8 Hz, 8H); 13C NMR (100 MHz, CDCl3) δ
19.9, 21.5, 94.9, 99.0, 117.0, 118.0, 122.7, 125.6, 126.4, 126.9,
127.5, 127.6, 128.4, 130.0, 131.1, 132.3, 138.4, 138.5, 138.6, 141.5;
9
17332 J. AM. CHEM. SOC. VOL. 130, NO. 51, 2008

De Novo Design for Functional Amorphous Materials
A R T I C L E S
Fabrication and Characterization of OLEDs. The multilayer
OLEDs were fabricated by employing the anthracene-anchored
bimesitylenes as emitting materials. The ITO-coated glass substrates
(Merck Display Technology, Taiwan) with a sheet resistance of
<50 ΩU/sq were cleaned sequentially in an ultrasonicator using
acetone, detergent solution, deionized water, ethanol, and 2-propanol
and then subjected to oxygen plasma.
Spin-Coating. The commercially available solution of PEDOT
(Bayer) was filtered using syringe filters of 0.45 µM. A PEDOT
layer of 400 Å thickness was spin-coated onto the ITO glass
substrate and dried at 100 °C for 1 h under a vacuum. Further, a
blend of PVK and anthracene-functionalized bimesitylenes (5-25
wt % 4 and 5) in chlorobenzene (5 mL) was spin-coated above the
PEDOT layer (2000 rpm for 30 s) and dried at 50 °C overnight in
vacuo. The ITO glass was transferred to the vacuum deposition
chamber (ULVAC cryogenics) and sealed under a vacuum.
Subsequently, the layers of TPBI or BCP/AlQ, LiF, and Al were
deposited under a pressure of 10-6 Torr at a rate of 0.1-1.0 Å/s.
The devices were sealed under an inert atmosphere in a glovebox,
and the EL characteristics were measured.
Vacuum Deposition. Vacuum deposition of the organic materials
such as NPB, 1% 1/2/8/9/10 with MADN, TPBI, LiF, and Al was
carried out under a pressure of 10-6 Torr on top of an etched ITO
glass substrate sequentially. The rate of deposition of organic
materials was maintained in a range of 0.1-0.5 Å/s. The evapora-
tion rate and thickness of the organic layers were monitored by a
quartz oscillator. After the vacuum deposition, the devices were
sealed, under an inert atmosphere, in a glovebox. EL spectra of
the devices were obtained using a diode-array rapid analyzer system.
Electroluminescence Measurements. Current-voltage and light
intensity measurements were performed on a Keithley 2400 source
meter and a Newport 1835C optical meter equipped with a Newport
818-ST 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 “underfilling”, satisfying the
measurement protocol.33
Acknowledgment. J.N.M. is thankful to the Department of
Science and Technology (DST), India, for generous financial
support. P.V.K. gratefully acknowledges the research fellowship
from the CSIR, India. P.N. acknowledges the research fellowship
from the UGC, India. We are thankful to the anonymous reviewers
for their invaluable suggestions and criticism.
Supporting Information Available: Synthesis of starting
1
materials and their spectral data, H and 13C NMR spectra of
all the compounds, PL spectra (film), X-ray powder diffraction
profiles, and EL data (EL spectrum of PVK-doped devices,
luminescence, and external quantum efficiencies). This material
is available free of charge via the Internet at http://pubs.acs.org.
JA8042905
(32) For example, see: Kido, J.; Hongawa, K.; Nagai, K. Macromol. Symp.
1994, 84, 81.
(33) Forrest, S. R.; Bradley, D. D. C.; Thompson, M. E. AdV. Mater. 2003,
15, 1043.
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