Pyrenoimidazole-based deep-blue-emitting materials: Optical, electrochemical, and electroluminescent characteristics

Article abstract of DOI:10.1002/asia.201300271

A series of pyrenoimidazoles that contained various functional chromophores, such as anthracene, pyrene, triphenylamine, carbazole, and fluorene, were synthesized and characterized by optical, electrochemical, and theoretical studies. The absorption spectra of the dyes are dominated by electronic transitions that arise from the pyrenoimidazole core and the additional chromophore. All of the dyes exhibited blue-light photoluminescence with moderate-to-high quantum efficiencies. They also displayed high thermal stability and their thermal-decomposition temperatures fell within the range 462-512 °C; the highest decomposition temperature was recorded for a carbazole-containing dye. The oxidation propensity of the dyes increased on the introduction of electron-rich chromophores, such as triphenylamine or carbazole. The application of selected dyes that featured additional chromophores such as pyrene, carbazole, and triphenylamine as blue-emissive dopants into multilayered organic light-emitting diodes with a 4,4′-bis(9H-carbazol-9-yl)biphenyl (CBP) host was investigated. Devices that were based on triphenylamine- and carbazole-containing dyes exhibited deep-blue emission (CIE 0.157, 0.054 and 0.163, 0.041), whereas a device that was based on a pyrene-containing dye showed a bright-blue emission (CIE 0.156, 0.135). Deep-blue something: New pyrenoimidazoles showed blue emission with a high quantum efficiency and thermal stability and low oxidation potential. The 10-pyrenyl pyrenoimidazole gave a driving voltage of 5.3 eV and a brightness of 2736 cd m^-2. Copyright

Full text of DOI:10.1002/asia.201300271

FULL PAPER
FULL PAPER
OLEDs
Dhirendra Kumar, K. R.
Justin Thomas,* Ching-Chiao Lin,
Jwo-Huei Jou*
&&&& — &&&&
Pyrenoimidazole-Based Deep-Blue-
Emitting Materials: Optical, Electro-
chemical, and Electroluminescent
Characteristics
Deep-blue something: New pyrenoimi-
dazoles showed blue emission with
a high quantum efficiency and thermal
stability and low oxidation potential.
The 10-pyrenyl pyrenoimidazole gave
a driving voltage of 5.3 eV and a bright-
ness of 2736 cdmꢀ2.
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These are not the final page numbers! ÞÞ

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K. R. Justin Thomas, Jwo-Huei Jou et al.
DOI: 10.1002/asia.201300271
Pyrenoimidazole-Based Deep-Blue-Emitting Materials: Optical,
Electrochemical, and Electroluminescent Characteristics
Dhirendra Kumar,[a] K. R. Justin Thomas,*[a] Ching-Chiao Lin,[b] and Jwo-Huei Jou*[b]
Abstract: A series of pyrenoimidazoles
that contained various functional chro-
mophores, such as anthracene, pyrene,
triphenylamine, carbazole, and fluo-
rene, were synthesized and character-
ized by optical, electrochemical, and
theoretical studies. The absorption
spectra of the dyes are dominated by
electronic transitions that arise from
the pyrenoimidazole core and the addi-
tional chromophore. All of the dyes ex-
hibited blue-light photoluminescence
with moderate-to-high quantum effi-
ciencies. They also displayed high ther-
mal stability and their thermal-decom-
position temperatures fell within the
range 462–5128C; the highest decom-
position temperature was recorded for
a carbazole-containing dye. The oxida-
tion propensity of the dyes increased
on the introduction of electron-rich
chromophores, such as triphenylamine
or carbazole. The application of select-
ed dyes that featured additional chro-
mophores such as pyrene, carbazole,
and triphenylamine as blue-emissive
dopants into multilayered organic
light-emitting diodes with
a
4,4’-
bis(9H-carbazol-9-yl)biphenyl (CBP)
host was investigated. Devices that
were based on triphenylamine- and car-
bazole-containing dyes exhibited deep-
blue emission (CIE 0.157, 0.054 and
0.163, 0.041), whereas a device that was
based on
a pyrene-containing dye
Keywords: density functional calcu-
showed a bright-blue emission (CIE
0.156, 0.135).
lations
cence
·
·
heterocycles
organic light-emitting
· lumines-
diodes · pyrenoimidazoles
Introduction
terials are of particular importance because of their applica-
tions as emitters[8] and hosts for green- or red-phosphores-
cent materials[9] if they possess suitable triplet energy levels.
However, of the three basic colors that are required for full-
color OLED devices, only a limited number of blue-light-
emitting small molecules have been demonstrated to exhibit
high efficiency, stability, and color purity.[10] In view of such
scarcity, there is much active research on blue-light-emitting
chromophores. So far, the most-promising blue-light-emit-
ting materials have been derived from polyaromatic hydro-
carbons, such as fluorene[11] and anthracene.[12] Recently,
pyrene-based[13] molecular materials that possess other func-
tional chromophores have also been developed and demon-
strated as deep-blue-light emitters.
Pyrene contains an extensive p-conjugated system and
displays high photoluminescence efficiency, enhanced ther-
mal stability, and also promising carrier mobility.[14] Because
of its unique optical and electronic properties, pyrene has at-
tracted much attention as an important scaffold in organic
materials for application in OLEDs[15] and organic field-
effect transistors (OFETs),[15b,16] as well as in photovoltaic
devices, such as bulk heterojunction (BHJ) solar cells[17] and
dye-sensitized solar cells (DSSCs).[18] The optical and elec-
trochemical properties of pyrene derivatives can be easily
tuned by incorporating electron-donors or acceptors and
also by simply altering the chromophore loading through
substitution at different positions.[15b,19] Despite these advan-
tages, pyrene suffers from its propensity to form p aggre-
gates in the solid state, which significantly decreases its utili-
The development of suitable organic dyes for use in organic
light-emitting diodes (OLEDs) has received immense atten-
tion owing to their applications in flexible, lightweight, and
power-saving displays and illumination devices.[1] Such dyes
are often used as primary-color emitters,[2] charge transport-
ers,[3] charge injectors,[4] charge blockers,[5] and even as
hosts[6] for singlet or triplet emitters in such devices. The de-
velopment of new organic materials with multiple functional
properties has been extensively explored for improving the
device performance and minimizing the number of layers
that are involved in the electroluminescent devices. For the
commercialization of OLEDs, the development of stable
and highly efficient three-primary-color (red, green, and
blue) emitters is important.[7] Deep-blue-light-emitting ma-
[a] D. Kumar, Dr. K. R. J. Thomas
Organic Materials Laboratory, Department of Chemistry
Indian Institute of Technology Roorkee
Roorkee 247 667 (India)
Fax : (+91)1332-286202
E-mail: krjt8fcy@iitr.ernet.in
[b] C.-C. Lin, Prof. J.-H. Jou
Department of Material Science and Engineering
National Tsing Hua University
Hsinchu 30013 (Taiwan)
E-mail: jjou@mx.nthu.edu.tw
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201300271.
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K. R. Justin Thomas, Jwo-Huei Jou et al.
ty in OLEDs.[20] However, by making subtle structural varia-
tions, in particular through the introduction of sterically de-
manding groups, such as tert-butyl or diarylamine units,
pyrene derivatives have been tuned to exhibit useful emis-
sion characteristics.[21] Pyrene derivatives those have been
exploited in efficient blue OLEDs include polypyrenes,[21d,22]
dipyrenylbenzene,[23] alkynylpyrene,[21c,24] pyrene-functional-
ized calix[4]arenes,[25] and aryl-functionalized pyrene-
[24b,27]
s,[13a,21a,26] as well as pyrene carbazole,
pyrene fluo-
ꢀ
ꢀ
rene,[28] pyrene triphenylamine, and pyrene anthracene
hybrids. In general, it has been observed that the hybridiza-
tion of two functional units often leads to improvement in
the device performance.[27–30] With this result in mind, we
have focused our attention on the less-explored pyrenoimi-
[29]
[30]
ꢀ
ꢀ
ꢀ
Scheme 1. Benzimidazole (I), phenanthroline imidazole (II), phenan-
throimidazole (III), fluorenoimidazole (IV), and pyrenoimidazole (V).
dazole derivatives. Fused imidazoles (Scheme 1), such as
[32]
benzimidazole (I),[31] phenanthroline imidazole (II), phe-
ꢀ
nanthroimidazole (III),[33] and fluorenoimidazole (IV),[34]
moieties has also been shown to be good electron-transport-
ing materials[32a,35] for OLEDs, owing to their strong electron
affinity. In contrast, phenanthroimidazole[33] and fluorenoi-
midazole[34] derivatives have displayed low oxidation poten-
tials and longer-wavelength photophysical characteristics
compared to their corresponding benzimidazole analogues,
owing to elongation of the conjugation in the former com-
pounds. On this basis, we expected that the pyrenoimidazole
moiety (V), with their outer p-electron cloud, would defi-
nitely enhance the electron density on the imidazole seg-
ment and the resulting materials might display unique elec-
tronic properties.[36]
have been used in the construction of blue-light-emitting
ꢀ
materials. Benzimidazole and phenanthroline imidazole
Abstract in Hindi:
We have previously used the pyrenoimidazole moiety in
ꢀ
the construction of organic dyes that possessed a donor ac-
ceptor molecular configuration and demonstrated them to
be promising sensitizers in dye-sensitized solar cells.[37]
Herein, we report eight new pyrenoimidazole-based lumi-
nescent materials and evaluate their photophysical, electro-
chemical, and electroluminescence characteristics. Time-de-
pendent density functional theory (TDDFT) computations
have also been performed to understand the electronic
structure of these molecules. Molecular hybrids between
pyrenoimidazole and fluorene, anthracene, pyrene, triphe-
nylamine, and carbazole have been developed by using
simple synthetic procedures. The optical properties of this
class of compounds were affected by the nature of the chro-
mophore that was attached onto the pyrenoimidazole
moiety, thus revealing strong electronic coupling between
the constituent units. Subsequently, emitters of deep-blue-
to-cyan light have been realized and the deep-blue-light
emitters have been found to effectively function as doped
emitters in multilayered organic light-emitting diodes.
Results and Discussion
Synthesis and Characterization
The synthetic pathways that we employed to obtain the
target compounds are shown in Scheme 2. The pyrene-4,5-
dione was prepared by the oxidation of pyrene in the pres-
ence of ruthenium trichloride and sodium periodate.[38] It
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K. R. Justin Thomas, Jwo-Huei Jou et al.
jugation in the pyrenoimidazole
unit. The molar extinction coef-
ficients of this transition fol-
lowed the order: 4a (para-
tolyl)<4b
(anthracene)<4e
(carbazole)<4c (pyrene)<4d
(triphenylamine) for the mono-
imidazoles and 5c (carbazole)<
5b (triphenylamine)<5a (fluo-
rene) for the bis-imidazoles.
The bis-derivatives showed
more
than
two-fold-higher
molar extinction coefficients
than the corresponding mono-
derivatives at lower energies,
thus indicating a doubled chro-
mophore density and confirm-
ing the origin of absorption
from the pyrenoimidazole unit.
The absorption spectra of the
pyrenoimidazole derivatives are
less sensitive towards the sol-
vent polarity, which indicates
Scheme 2. Synthesis of pyrenoimidazole derivatives 4a–4e and 5a–5c.
was then reacted with the corresponding aldehyde in the
presence of ammonium acetate to form the sparingly soluble
imidazoles. Alkylation was either performed under phase-
transfer catalysis conditions to afford mono-imidazoles (4a–
4e) or by using K2CO3/DMF to afford bis-imidazoles (5a–
5c). Reasonably soluble products were obtained by intro-
ducing n-butyl groups into the mono-imidazoles or 2-ethyl-
hexyl units into the bis-imidazoles. The compounds were
thoroughly characterized by IR and NMR (1H and 13C)
spectroscopy and by mass spectrometry. The analytical data
were found to be consistent with the proposed structures.
Photophysical Properties
The photophysical properties of the dyes were investigated
by absorption and fluorescence spectroscopy. The absorp-
tion and emission spectra of the compounds were recorded
in various solvents, such as toluene, CH2Cl2, CHCl3, DMF,
MeCN, and MeOH, to study the effect of solvent polarity
on their optical properties.
Figure 1. Absorption spectra of compounds 4a–4e and 5a–5c in toluene.
that the interaction of the dyes with the solvents in the
ground state is less significant.
The absorption spectra of pyrenoimidazole derivatives in
toluene are shown in Figure 1 and the pertinent data are
compiled in Table 1. All of the compounds showed complex
The emission spectra of the pyrenoimidazoles were mea-
sured in a series of solvents with different polarity. The
emission spectra of the compounds in toluene are shown in
Figure 2. In contrast to the absorption properties, the emis-
sion properties depend on the auxiliary chromophore that is
present on the pyrenoimidazole segment. If the emission
purely originates from the pyrenoimidazole-localized excited
state, then the emission profile should be similar, irrespec-
tive of the chromophore that is present. Interestingly, the
compounds that contained para-tolyl and carbazole moieties
(4a, 4e, and 5c) exhibited similar emission profiles within
the region 360–460 nm. This result suggests that the emis-
ꢀ
absorption profiles, owing to multiple localized p p* transi-
tions, which originated from the pyrenoimidazole segment
and the chromophore that was tethered at the C10 position.
The observed peak in the longer-wavelength region with vi-
brational fine structure is attributed to pyrenoimidazole-lo-
ꢀ
calized p p* electronic excitation. This peak is significantly
red-shifted compared to the analogous absorptions for the
phenanthroimidazole and fluorenoimidazole analogues.[33-
d,34a]
This result suggests the presence of more-extended con-
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K. R. Justin Thomas, Jwo-Huei Jou et al.
Table 1. Absorption properties of compounds 4a–4e and 5a–5c in different solvents.
tureless emission profiles (Figure 2) for these two
dyes indicated the presence of strong inter-chromo-
phoric electronic coupling in the excited state. In
contrast, the para-tolyl (4a) and carbazole deriva-
tives (4e and 5c) exhibited very small Stokes shifts
(204–336 cmꢀ1, Table 2).
With the exception of compounds 4a, 4e, and 5c,
all of these compounds showed positive solvato-
chromism, as shown for representative examples in
Figure 3. The relatively poor sensitivity of com-
pounds 4a, 4e, and 5c towards the solvent environ-
Dye
labs [nm] (eꢁ103 [mꢀ1 cmꢀ1])
Toluene
CH2Cl2
CHCl3
DMF
MeCN
MeOH
4a
382 (7.2)
356 (19.5)
296 (36.9)
381 (6.5)
353 (19.2)
292 (39.2)
376 (6.2)
357 (5.2)
339 (29.6)
324 (27.0)
278 (36.3)
394 (8.2)
375 (10.7)
355 (8.3)
338 (26.4)
323 (17.6)
309 (11.8)
278 (33.2)
380 (15.2)
341 (39.5)
280 (51.0)
381 (7.8)
353 (29.9)
292 (40.2)
380 (6.6)
349 (18.2)
290 (38.6)
377 (5.6)
345 (18.8)
332 (16.3)
314 (14.7)
286 (38.2)
386 (9.7)
376 (8.5)
366 (10.1)
345 (20.3)
331 (15.2)
285 (33.4)
4b
387 (9.8)
354 (20.5)
291 (33.9)
387 (10.5)
351 (20.8)
290 (34.7)
388 (10.6)
381 (9.7)
351 (21.2)
290 (35.2)
385 (11.0)
360 (14.4)
348 (20.6)
288 (34.2)
ꢀ
ment (Table 2) may indicate a less-polar p p* ex-
cited state that is localized within the pyrenoimida-
zole unit. Although the peak positions of com-
pounds 4a, 4e, and 5c are not affected by the sol-
vent polarity, the intensity ratio of the vibronic
peaks undergo changes (Figure 3b) with the solvent
polarity, analogous to those reported for polyaro-
matic hydrocarbons, such as anthracene and
pyrene.[40] The solvent-dependent fluorescence of
compounds 4b, 4c, 4d, 5a, and 5b can be explained
4c
382 (15.7)
355 (36.8)
343 (38.8)
291 (49.2)
381 (14.9)
344 (43.7)
289 (50.7)
279 (52.0)
382 (16.4)
344 (44.3)
289 (52.1)
279 (51.2)
380 (14.5)
342 (43.5)
288 (50.0)
277 (50.6)
377 (13.6)
342 (49.5)
329 (38.4)
285 (57.5)
277 (56.4)
379 (16.1)
335 (47.2)
287 (44.1)
4d
4e
384 (18.6)
364 (29.7)
341 (39.5)
295 (36.6)
383 (10.5)
351 (23.3)
299 (51.3)
383 (17.1)
339 (39.6)
292 (36.7)
383 (31.1)
345 (39.8)
279 (33.9)
269 (30.2)
379 (12.6)
359 (14.1)
340 (36.7)
328 (34.0)
280 (37.7)
383 (38.3)
340 (86.5)
325 (59.2)
310 (37.2)
279 (72.6)
385 (47.3)
364 (51.5)
350 (56.7)
280 (45.2)
382 (24.0)
343 (50.6)
324 (50.7)
293 (58.3)
282 (58.3)
383 (19.0)
363 (29.3)
339 (41.7)
292 (39.1)
382 (11.0)
349 (23.6)
298 (57.5)
382 (16.8)
336 (41.6)
290 (39.9)
381 (9.4)
349 (22.4)
298 (51.9)
381 (9.2)
348 (21.8)
297 (53.3)
266 (36.2)
378 (8.4)
346 (25.3) on the basis of general solvation effects with the
334 (24.8)
289 (58.6)
268 (39.6)
Lippert–Mataga equation[41] and the ET(30) param-
eter[42] (Figure 4). The emission peak of these dyes
shifted to longer wavelength on increasing the sol-
5a
387 (44.7)
364 (56.4)
353 (66.3)
291 (57.6)
385 (43.0)
350 (71.5)
291 (63.1)
386 (45.1)
364 (54.9)
351 (69.9)
291 (57.3)
384 (42.8)
362 (51.9)
348 (70.7)
289 (59.4)
381 (40.2)
vent polarity. This positive solvatochromism is indi-
cative of a more-efficient solvation of the molecules
in the excited state, that is, the stabilization of the
excited state in polar solvents as compared to the
ground state.[43] The blue-shifted emission in MeOH
360 (46.2)
346 (77.5)
285 (69.8)
5b
5c
385 (42.8)
365 (58.2)
352 (64.0)
293 (52.5)
383 (22.8)
363 (42.0)
349 (44.2)
321 (54.0)
297 (73.4)
384 (37.7)
363 (53.4)
351 (62.0)
292 (51.3)
382 (19.6)
361 (36.4)
348 (40.5)
295 (72.9)
384 (38.5)
364 (51.0)
351 (59.0)
292 (48.3)
382 (21.8)
362 (37.1)
348 (41.4)
296 (73.4)
383 (38.3)
362 (52.7)
349 (64.4)
289 (53.4)
381 (19.4)
360 (34.0)
346 (41.0)
293 (75.0)
381 (36.2)
360 (49.1)
347 (67.6) may be ascribed to hydrogen-bonding interactions,
286 (55.6)
379 (18.5)
359 (30.9)
344 (46.4)
312 (57.3)
288 (79.8)
which probably retard the reorientation of the fluo-
rophore in the excited state by hindering the rota-
tion of the aryl unit (Figure 5).[44]
An abnormal red-shifted emission was observed
in CHCl3 for all of these compounds, which may be
attributed to the involvement of carbene species
that are generated upon the photoexcitation of
sion from these compounds mainly arises from the pyrenoi-
midazole-localized electronic transition and the participa-
tion of the auxiliary chromophores in the transition is negli-
gible. However, the dyes that possess triphenylamine (4d
and 5b), fluorene (5a), pyrene (4c), and anthracene units
(4b) exhibited red-shifted emission in that order. This result
probably indicates the involvement of the auxiliary chromo-
phores in the excited state of these compounds. It is likely
that the excited state assumes a more-planar structure, thus
giving room for the extension of conjugation to the auxiliary
chromophore.[39] A slight red-shift was observed for the bis-
imidazole derivatives (5b and 5c) when compared to their
corresponding mono-imidazole analogues (4d and 4e). This
result further attests the partial involvement of auxiliary
chromophores in the electronically excited state.
Among these dyes, the anthracene- (4b) and pyrene-con-
taining dyes (4c) displayed pronounced Stokes shifts. This
result could either be due to a significant structural reorien-
tation in the excited state or to excimer formation. The fea-
Figure 2. Emission spectra of compounds 4a–4e and 5a–5c in toluene.
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K. R. Justin Thomas, Jwo-Huei Jou et al.
compounds showed featureless,
Table 2. Emission properties and Stokes shift for compounds 4a–4e and 5a–5c in various solvents.
broad, red-shifted emissions in
films compared to those in tolu-
ene, which may be indicative of
a close association of molecules
in the solid state. Interestingly,
the anthracene- (4b) and
pyrene-containing derivatives
(4c) showed a smaller shift,
which was suggestive of their
non-planar arrangement (see
above), thus inhibiting an or-
derly arrangement in the solid
state. The triphenylamine-con-
taining mono-imidazole (4d)
showed the most-red-shifted
emission in the film compared
to the corresponding bis-deriva-
tive (5b). The poor propensity
Dye
lem [nm] (FF [%])[a]
Stokes Shift [cmꢀ1
Film Toluene CH2Cl2 DMF MeCN MeOH
]
Toluene CH2Cl2
DMF
MeCN
MeOH
4a
385,
407,
384,
406,
384,
406,
383,
405,
380,
401,
464
204
205
205
206
209
430 (58) 428 (70) 428 (65) 427 (62) 424 (63)
4b
4c
4d
467 (62) 485 (34) 504 (26) 507 (22) 486 (28) 483 4427
451 (97) 472 (84) 483 (76) 487 (73) 469 (75) 484 4005
5221
5060
2834
5932 6250
5474 5782
3638 3906
5331
5203
3397
406,
429 (79) 445 (72) 449 (70) 435 (67) 494 1411
427 (88)
388,
4e
5a
388,
389,
410 (81) 412,
388,
410,
383,
404,
478
336
474
471
474
345
410,
428 (75)
425,
427 (78) 428 (74) 425 (73)
445 (98) 449 (94) 450 (92) 443 (94) 484 2310
3502
3635 3819
2622 2962
3673
447 (99)
5b
5c
417 (87) 421 (90) 427 (88) 432 (84) 426 (82) 451 1993
388, 388, 389, 387, 383, 469 336
410 (69) 410 (67) 411 (64) 409 (61) 405,
426 (60)
2289
405
2773
276
471
407
[a] Coumarin-1 (FF =99% in EtOAc) was used as a reference.
CHCl3. The effect of the acidic nature of halogenated sol-
vents, such as CCl4 and CHCl3, and their influence on the
emission behavior of polyaromatic hydrocarbons are well-
known in the literature.[45] Notably, the electron-rich nature
of the molecule dictated the outcome of such photochemical
reactions. The deviations in the emission profile in CHCl3
can be correlated with the p-accepting or p-donating ability
of the aromatic unit that is linked to the pyrenoimidazole
nucleus. The compound that contains pyrene and anthracene
units are not affected by CHCl3, whereas the other deriva-
tives display shifts in accordance with their electron density,
as identified from the oxidation potentials of the compounds
(see above) of the units: fluorene (5a)<para-tolyl (4a)<
carbazole (4e and 5c)<triphenylamine (4d and 5b; also see
the Electrochemical Properties Section). The Stokes shift
for derivatives 4a, 4e, and 5c were less affected by the sol-
vent polarity, which supported the rigidity of these mole-
cules.[46]
for aggregation of the more-twisted bis-imidazole with a pyr-
amidal triphenylamine segment is the reason for this obser-
vation. However, the carbazole-based mono- and bis-imida-
zoles exhibited similar shifts.
The photoluminescence quantum yields were determined
by using coumarin-1 as a standard (FF =99% in EtOAc).[47]
All of the compounds showed very high quantum yields, in
particular fluorene-, pyrene-, and triphenylamine-containing
derivatives, which showed excellent fluorescence quantum
yields (87–99%) in solution. The high quantum yields for
the triphenylamine derivatives overruled the presence of
pronounced intramolecular charge transfer in these com-
pounds, even in the excited state. However, a slight decrease
in the quantum yield on moving from non-polar solvents to
polar solvents suggest a weak dipole–dipole interaction be-
tween the molecules and the solvent.[48]
The emission spectra of the solid (drop-cast) films of pyr-
enoimidazole derivatives are shown in Figure 6. All of the
Figure 3. Emission spectra of compounds 4b (a) and 4e (b) in different solvents.
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K. R. Justin Thomas, Jwo-Huei Jou et al.
Figure 6. Emission spectra for the dyes as films.
pulse voltammetry (DPV). The redox potentials were cali-
brated by using the ferrocene/ferrocenium redox couple as
an internal standard in each measurement. Selected data are
collected in Table 3. All of the compounds exhibited irrever-
sible peaks, which originated from the oxidation of pyrenoi-
midazole and/or auxiliary chromophores. Interestingly, the
triphenylamine-based derivatives (4d and 5b) showed
a quasi-reversible redox couple in addition to the irreversi-
ble peak that was due to the removal of electron from the
pyrenoimidazole moiety. The quasi-reversible oxidation pro-
cess that occurred at high potential is attributed to the oxi-
dation of the triphenylamine unit, in comparison to the
value for the unsubstituted triphenylamine.[49]
The first oxidation potentials for these compounds follow
the order: 4b (0.70 V)>4c (0.67 V), 4a (0.65 V)>4e
(0.56 V)>4d (0.50 V) for the mono-imidazoles and 5a
(0.68 V)>5c (0.55 V)>5b (0.53 V) for the bis-imidazoles.
The more-electron-rich substituents facilitate the oxidation,
as evidenced from the lower half-wave potentials for com-
pounds 4d and 5b. Anthracene-containing derivative 4b
showed the highest oxidation potential because of its rela-
tively enhanced p-accepting nature, whereas the electron-
rich triphenylamine substituent caused the lowest oxidation
potential in these series. The interplanar angle between the
pyrenoimidazole and aromatic moieties also influenced the
oxidative ability of the compounds, besides the electron den-
sity on the substituents, because it could alternate the elec-
tronic coupling between the two units. The dihedral angles
(see above) in the imidazoles that contain aromatic substitu-
ents follow the trend: para-tolyl (4a)<pyrene (4c)<anthra-
cene (4b), in accordance with their redox potentials. Pre-
sumably, a more-coplanar structure ensures the stabilization
of the radical cation that is formed after oxidation of the
imidazole unit. The lower oxidation potentials for these de-
rivatives compared to the other reported fused imidazo-
les[31d,32,33b] suggest the electron richness of the pyrenoimida-
zole moiety.
Figure 4. Correlation of the solvent-induced Stokes shift with a) orienta-
tion polarizability and b) the ET(30) parameter for compounds 4b–4d,
5a, and 5b.
Figure 5. Hydrogen-bonding explanation for the observed blue-shifted
emission in MeOH.
Electrochemical Properties
The electrochemical properties of these compounds were
examined by using cyclic voltammetric (CV) and differential
The HOMO energies of these compounds were calculated
by using the ferrocene/ferrocenium redox couple as a refer-
7
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K. R. Justin Thomas, Jwo-Huei Jou et al.
Table 3. Thermal and electrochemical properties of compounds 4a–4e and 5a–5c.
M5a–M5c; see the Supporting Information, Fig-
ure S57) to save computational time. We supposed
that the replacement of the long alkyl chains by
methyl groups would not affect the parameters that
were relevant to this discussion. The unconstrained
geometries of these compounds under vacuum were
optimized by using DFT calculations with the
Becke’s three-parameter functional[54] that was hy-
bridized with the Lee–Yang–Parr correlation func-
[f]
Dye Td
[8C][a]
Tonset
Eox [V][c] (DEp
[mV])
HOMO
[eV][d]
LUMO
[eV][e]
E0ꢀ0
[8C][b]
4a
4b
4c
4d
462
493
500
505
356
269
447
403
0.65, 1.11
0.70, 1.08
0.67, 1.08
0.50, 0.72 (101),
1.05, 1.14
0.56, 1.04,
1.21
5.45
5.50
5.47
5.30
2.17
2.53
2.46
2.12
3.28
2.97
3.01
3.18
4e
512
415
5.36
2.10
3.26
tional[55] and the 6-31G
ACHTNUTRGNEUNG(d,p) basis set.
5a
5b
470
464
427
430
0.68, 1.08
0.53, 0.69 (91),
0.82, 1.02,
1.10
5.48
5.33
2.40
2.20
3.08
3.13
The computed interplanar angles between the
pyrenoimidazole and aromatic segments, the dipole
moments, absorptions wavelengths, and oscillator
strengths (f) for the most-appropriate transitions of
the absorption bands of these compounds are re-
ported in Table 4. The optimized geometries of
these compounds showed that there was significant
non-coplanarity between the pyrenoimidazole and
C10-substituted segments. The anthracene-contain-
5c
467
407
0.55, 1.07
5.35
2.09
3.26
[a] Heating rate: 108Cminꢀ1 under a nitrogen atmosphere. [b] Temperature that corre-
sponds to a 5% weight loss. [c] Oxidation potentials with reference to the ferrocene/
ferrocenium redox couple as an internal standard. [d] Determined from the equation
HOMO=Eox+4.8. [e] Determined from the equation LUMO=HOMOꢀE0ꢀ0. [f] De-
rived from the optical edge.
ence (4.8 eV)[50] and were within the range 5.30–5.50 eV.
The LUMO energies was calculated from their correspond-
ing HOMO energies and the optical band gap as estimated
from the intersection of the absorption and emission peaks,
which ranged from 2.09 to 2.53 eV. The anthracene and
pyrene derivatives showed low-lying LUMOs, in accordance
with their p-accepting nature.[27a,51]
ing derivative showed a higher interplanar angle (about 698)
than the pyrene-containing derivative (about 498). The other
derivatives were moderately twisted from the plane of the
imidazole unit (35–378). The HOMOs of these compounds
were generally localized over the entire molecule (Figure 7),
whilst the LUMOs were generally spread over the different
parts of the molecules. The p-aromatic segments, such as an-
thracene, pyrene, and fluorene, exclusively participated in
constructing the LUMOs in compounds 4b, 4c, and 5a.
However, for the derivatives that contained para-tolyl, car-
Thermal Properties
The thermal properties of the pyrenoimidazole derivatives
were investigated by using thermal gravimetric analysis; the
pertinent data are presented in Table 3. All of the com-
pounds showed very high thermal stability, as illustrated by
their high thermal decomposition temperatures (Td =462–
5128C). The rigid pyrenoimidazole moiety is responsible for
the high thermal stability. The substituents on the pyrenoi-
midazole nucleus also played an important role in determin-
ing the thermal stability of these compounds. Consequently,
the thermal stability increased in the order: para-tolyl
Table 4. Computed interplanar angle, dipole moment, absorptions, and
oscillator strengths (TDDFT/B3LYP/6-31GACTHNUTRGNE(NUG d,p)) of structures M4a–M4e
and M5a–M5c.
Dye Angle
mg
labs
f
Assignment (%)
[8][a]
[Debye] [nm]
M4a 36.64
M4b 68.51
3.47
3.33
356.2 0.2306
455.3, 0.0706,
HOMO!LUMO (90)
HOMO!LUMO (99)
387.4, 0.1224, HOMOꢀ1!LUMO (97)
352.3 0.1197
425.7, 0.3429,
362.2 0.2381
HOMO!LUMO+1 (84)
HOMO!LUMO (97)
HOMO!LUMO+1 (65)
HOMOꢀ1!LUMO (23)
HOMO!LUMO (89)
HOMO!LUMO+1 (85)
HOMOꢀ2!LUMO (5)
HOMO!LUMO (90)
HOMO!LUMO+1 (62)
HOMO!LUMO+2 (24)
HOMO!LUMO (96)
HOMO!LUMO+2 (78)
HOMO!LUMO (77)
HOMO!LUMO+2 (17)
HOMO!LUMO+3 (52)
HOMOꢀ1!LUMO (28)
HOMOꢀ1!LUMO+2 (8)
HOMO!LUMO (87)
HOMO!LUMO+3 (6)
HOMOꢀ1!LUMO (93)
M4c 48.77
M4d 34.95
M4e 36.21
3.08
3.56
4.16
(4a)<anthracene
(4b)<pyrene
(4c)<triphenylamine
(4d)<carbazole (4e) for the mono-imidazoles and fluorene
(5a)<triphenylamine (5b)<carbazole (5c) for the bis-imi-
dazoles. The bis-derivatives (5b and 5c) possessed lower Td
values than their corresponding mono-derivatives (4d and
4e), presumably owing to the presence of long alkyl chains
in the former compounds. The carbazole-containing deriva-
tive (4e) showed remarkable thermal stability (Td =5128C)
in the series. Thermal stability of the carbazole derivatives is
well-established in the literature.[52]
378.8, 0.4937,
362.3 0.4401
364.6, 0.1778,
350.3 0.1435
M5a 34.39
37.30
M5b 35.39
35.40
1.06
6.77
413.9, 1.2883,
359.2 0.1433
394.9, 1.2905,
351.7 0.2769
Theoretical Calculations
M5c 36.49
1.41
369.2, 0.2087,
354.4 0.1654
To gain more insight into the photophysical behavior of
these compounds, DFT calculations were performed for the
pyrenoimidazole derivatives by using the Gaussian 09 pro-
gram package.[53] The lengthy alkyl chains were replaced by
methyl groups in the model structures (M4a–M4e and
36.54
[a] Interplanar angle between the pyrenoimidazole moiety and the sub-
stituent at the C10 position.
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K. R. Justin Thomas, Jwo-Huei Jou et al.
Figure 7. Frontier molecular orbitals (HOMOs and LUMOs) of structures M4a–M4e (top) and M5a–M5c (bottom).
bazole, and triphenylamine units, the LUMOs were mainly
localized on the pyrenoimidazole unit. The trend in the ex-
perimentally observed longer-wavelength absorptions for
these compounds matched well with the theoretical comput-
ed low-energy excitations.
were also calculated. The EAs of these compounds followed
the trend: 4e<4a<4d<4c<4b for the mono-imidazoles
and 5c<5b<5a for the bis-imidazoles, in good agreement
with the experimentally determined LUMO energy values.
The adiabatic and vertical ionization potentials for the
compounds were also computed (Table 5) to gain insight
into their charge-transport properties. The computed ioniza-
tion potentials followed the order: 4a>4b>4c>4e>4d
for the mono-imidazoles and 5a>5c>5b for the bis-imida-
Electroluminescence Properties
The use of dyes 4c, 5b, and 5c in multilayered OLED devi-
ces was also evaluated. These dyes were either simply ap-
plied as emitting materials, ITO/PEDOT:PSS/4c or 5b or
5c/TPBI/LiF/Al (device I), or as emitting dopants in a CBP
host, ITO/PEDOT:PSS/CBP+4c or 5b or 5c (3, 6, or
9 wt.% each)/TPBI/LiF/Al (devices II–IV). The alignment
of the energy levels in the materials that were used for the
Table 5. Computed ionization potentials and electron affinities for struc-
tures M4a–M4e and M5a–M5c.
Dye Adiabatic Ip
Vertical Ip
Adiabatic Ea
Vertical Ea
[eV]
[eV]
[eV]
[eV]
M4a ꢀ6.256
M4b ꢀ6.155
M4c ꢀ6.071
M4d ꢀ5.810
M4e ꢀ5.976
M5a ꢀ5.803
M5b ꢀ5.631
M5c ꢀ5.796
ꢀ6.342
ꢀ6.258
ꢀ6.155
ꢀ5.912
ꢀ6.079
ꢀ5.856
ꢀ5.694
ꢀ5.841
0.086
0.683
0.660
0.250
0.064
0.783
0.558
0.454
ꢀ0.045
0.560
0.542
0.083
ꢀ0.015
0.675
0.486
0.396
zoles. This trend matched that observed for the experimen-
tally determined HOMO levels. In general, the presence of
electron-rich groups, such as triarylamines and carbazole
groups, or the elongation of conjugation in the molecule de-
creased the ionization energy. Triphenylamine-containing
derivatives 4d and 5b showed the lowest Ip values, owing to
the relatively high electron-donating nature of this group.
The molecules with lower ionization potentials are expected
to possess good hole-transporting capability. The adiabatic
and vertical electron affinities (EA) of these compounds
Figure 8. Energy-level diagram of the materials that were used for the
fabrication of devices.
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K. R. Justin Thomas, Jwo-Huei Jou et al.
fabrication of these devices is shown in Figure 8. Figure 9
shows the current-density–voltage (I–V) and luminance–cur-
rent-density (L–I) plots of the OLEDs that were fabricated
by using these dyes as emitting dopants. The device-perfor-
mance and electroluminescence (EL) emission characteris-
tics are listed in Table 6. The devices with these dyes as neat
emitters exhibit lower turn-on voltages when compared to
the devices with these dyes as emitting dopants. This result
is probably due to the larger hole-blocking barrier (1.1 eV)
for the doped devices when compared to the neat devices
(0.4–0.6 eV).
The EL spectra of the devices that incorporated these
dyes as either neat films or as dopants at different concen-
trations are shown in Figure 10. The neat-film-based devices
showed a red-shifted EL, similar to the photoluminescence
(PL) observed for the film, which further confirmed the in-
volvement of aggregation in the solid state. The devices with
3 wt.% dyes (device II) showed the most-blue-shifted EL,
which was almost unaffected on increasing the concentration
of the dye up to 9 wt.%. However the brightness varied in
the cases of dyes 4c and 5b: Maximum brightness was ob-
served for the devices that were doped with 6 wt.% of com-
pound 4c (2980 cdmꢀ12) and with 9 wt.% of compound 5b
(1154 cdmꢀ12). The device that was doped with compound
4c showed bright-blue EL, whereas deep-blue EL was ob-
served for the devices that were doped with compounds 5b
and 5c. The highest brightness, which was obtained for the
compound-4c-based devices, may be attributed to the bal-
anced confinement of holes and electrons within the emit-
ting layer, owing to the favorable hole- and electron-injec-
tion barriers at the HTL/EML and EML/ETL interfaces.
The efficiencies of the neat devices and the OLEDs that
contained higher concentrations (6–9 wt.%) of the dyes
were inferior to those of the devices that contained 3 wt.%
of the dyes in the CBP host. Among these devices, the best
performance was achieved for the device that employed
compound 4c (3 wt.%) as an emitting dopant (driving volt-
age: 5.3 eV; hext =2.24%; hc =2.4 cdAꢀ1; hp =1.2 LmWꢀ1
;
CIE (0.156, 0.135); brightness: 2736 cdmꢀ2). In general, the
performance of the doped devices (devices II–IV) were su-
perior to those of the neat-film-based devices (device I),
which may be attributed to aggregation of the dyes in the
pure film, owing to their planar structural features. In addi-
tion, the efficient capturing of energy by the dyes from the
CBP excitons is responsible for the better performance of
the doped devices. The EL did not fluctuate with the varia-
tion in the cell voltage, which is beneficial for applications
in commercial full-color displays.
Conclusions
A series of pyrenoimidazole-based luminescent materials
have been synthesized and characterized. Pyrenoimidazoles
that contained different chromophores exhibited blue lumi-
nescence with excellent photoluminescence quantum effi-
ciency. The para-tolyl- and carbazole-containing derivatives
showed featured emission with very low Stokes shifts in
most of the solvents tested, thus revealing their rigid struc-
ture. Moreover, all of the compounds studied herein showed
high thermal stability and lower oxidation potentials, which
strengthened their eligibility in electroluminescent and hole-
transporting materials for organic light-emitting devices. So-
lution-processed OLED devices based on compounds 4c,
5b, and 5c as a dopant were successfully fabricated and
showed decent device parameters. Devices based on com-
Figure 9. Current-density–voltage (I–V) and luminance–current-density
(L–I) curves (inset) for devices that were doped with compounds 4c (a),
5b (b), and 5c (c).
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K. R. Justin Thomas, Jwo-Huei Jou et al.
Table 6. Performance characteristics of devices based on compounds 4c, 5b, and 5c.
OLED Device Fabrication
Material Conc. Driving Power ef- Current
External
CIE coordinates
Max. lumi-
The OLED devices were fabricated on
a pre-cleaned glass substrate and were
[wt.%] voltage ficiency
efficiency quantum ef-
nance
[V]
[LmWꢀ1
]
[cdAꢀ1
]
ficiency [%]
[cdmꢀ2
]
composed of
a layer of indium tin
@ 100/1000 cdmꢀ2
oxide (thickness: 125 nm) as an anode,
poly(3,4-ethylene-dioxythiophene)-
poly(styrenesulfonate) (PEDOT:PSS,
thickness: 35 nm) as a hole-injection
layer (HIL), an emissive layer (EML),
TPBI as an electron-transport layer,
4c
4c+CBP
neat 3.7
0.9/0.8
1.2/0.8
0.9/0.8
1.0/0.8
0.7/0.5
0.2/–
0.3/–
0.4/0.3
0.5
1.2/1.5
2.4/2.1
1.9/2.3
2.0/2.1
0.9/0.9
0.4/–
0.7/–
0.8/0.7
0.7
0.55/0.69
2.24/2.12
1.60/2.06
1.58/1.75
0.71/0.76
1.53/–
1.86/–
1.87/1.56
0.47
(0.196, 0.345)/(0.188, 0.330) 2510
(0.156, 0.135)/(0.156, 0.126) 2736
(0.155, 0.145)/(0.154, 0.137) 2980
(0.156, 0.153)/(0.155, 0.145) 2931
(0.176, 0.176)/(0.170, 0.160) 1227
(0.158, 0.045)/–
(0.157, 0.049)/–
(0.157, 0.054)/(0.157, 0.057) 1154
(0.185, 0.232)
(0.163, 0.041)
(0.163, 0.045)
(0.163, 0.061)
3
6
9
5.3
5.7
5.6
5b
5b+CBP
neat 3.6
a
LiF electron-injection layer (EIL,
3
6
9
5.9
5.5
5.5
609
939
thickness: 0.7 nm), and an Al layer
(thickness: 150 nm) as a cathode. The
aqueous solution of PEDOT:PSS was
spin-coated at 4000 rpm for 20 s to
form the HIL layer (thickness: 35 nm).
The dyes (4c, 5b, and 5c), either neat
or doped into 4,4’-bis(9H-carbazol-9-
yl)biphenyl (CBP), were deposited by
spin-coating at 2500 rpm for 20 s; then,
the TPBI layer was deposited onto it.
5c
neat 3.7
736
436
435
432
5c+CBP
3
6
9
5.9
5.8
5.9
0.1
0.1
0.2
0.3
0.3
0.4
1.94
1.72
1.30
pounds 5b (CIE 0.157, 0.054) and 5c (CIE 0.163, 0.041) ex-
hibited deep-blue-light emission and bright-blue-light emis-
sion was observed for the device that was doped with com-
pound 4c (CIE 0.156, 0.135). The 3 wt.% compound-4c-
doped device showed the best performance, with a driving
voltage of 5.3 eV and a brightness of 2736 cdmꢀ2, with an
external efficiency, current efficiency, and power efficiency
of 2.24%, 2.4 cdAꢀ1, and 1.2 LmWꢀ1, respectively.
Subsequently, lithium fluoride and the aluminum cathode were thermally
evaporated at a pressure of 1.0ꢁ10ꢀ5 Torr.
Synthesis
Pyene-4,5-dione (2),[38] 4-(diphenylamino)benzaldehyde,[56] 4,4’-(phenyla-
zanediyl)dibenzaldehyde,[56] 9-butyl-9H-carbazole-3-carbaldehyde,[56] and
9-butyl-9H-carbazole-3,6-dicarbaldehyde[57] were synthesized according
to literature procedures.
General Procedure for the Synthesis of 10-Aryl-9H-pyreno
ACHTUNGTNER[NUNG 4,5-
d]imidazole (3a–3e)
Pyrene-4,5-dione (348 mg, 1.5 mmol), the corresponding aryl aldehyde
(1.5 mmol), and ammonium acetate (578 mg, 7.5 mmol) were dissolved in
acetic acid (10 mL) and the resultant mixture was heated at reflux for
24 h. Next, the reaction mixture was cooled and poured into water to
obtain a precipitate, which was dried and washed with hot CHCl3 to
obtain the title compounds with sufficient purity for their direct use in
further reactions.
Experimental Section
Material and Methods
Chemicals were purchased from Aldrich, SD Fine Chem, Spectrochem,
or Thomas Baker and used as received. Solvents were dried and distilled
immediately prior to use by using standard procedures. Purification by
column chromatography was performed on silica gel (230–400 mesh,
Rankem) as the stationary phase (column dimensions: 40 cmꢁ3.0 cm).
1H NMR and 13C NMR spectra were recorded in CDCl3 or [D6]DMSO
10-para-Tolyl-9H-pyrenoACTHNUGRTNEUNG[4,5-d]imidazole (3a)
Pale-yellow solid; 66% yield; M.p. 220–2228C; 1H NMR ([D6]DMSO,
500.13 MHz): d=13.66 (s, 1H), 8.81–8.86 (m, 2H), 8.29 (d, J=8.5 Hz,
2H), 8.24 (dd, J=8.0 Hz, 1.5 Hz, 2H), 8.12–8.19 (m, 4H), 4.43 (d, J=
8.0 Hz, 2H), 2.41 ppm (s, 3H); 13C NMR ([D6]DMSO, 125.77 MHz): d=
150.1, 139.5, 138.0, 132.1, 131.9, 130.0, 128.5, 128.3, 128.2, 127.9, 126.8,
126.7, 126.6, 124.6, 124.5, 122.3, 122.2, 122.1, 119.5, 119.4, 21.5 ppm;
HRMS (ESI): m/z calcd for C24H17N2: 333.1386 [M+H]; found: 333.1395.
on
a Bruker AV 500 O FT NMR spectrometer at 500.13 and
125.77 MHz, respectively. High-resolution mass spectrometry was per-
formed on an ESI mass spectrometer. UV/Vis spectra were recorded at
RT in quartz cuvettes on a UV-1800 Shimadzu spectrophotometer. Fluo-
rescence measurements were performed on a RF-5301-PC Shimadzu
spectrofluorophotometer. The quantum yield of the dyes were calculated
by following a standard procedure and using coumarin-1 (FF =0.99 in
EtOAc)[47] as a reference. Corrections owing to dye absorption and re-
fractive indices of the solvents were incorporated into the calculations.
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV)
measurements were performed on a CHI 620C electrochemical analyzer
in THF with 0.1m tetrabutylammonium perchlorate as a supporting elec-
trolyte. The experiments were performed at RT under a nitrogen atmos-
phere in a three-electrode cell that consisted of a Pt wire as an auxiliary
electrode, a non-aqueous Ag/AgNO3 reference electrode, and a glassy
carbon working electrode. The HOMO energy levels of the organic mate-
rials were measured from the oxidation potentials, as obtained by cyclic
voltammetry, and the LUMO energy levels were determined from the
HOMO energy levels and the optical band gaps, as estimated from the
intersection of the absorption and emission bands. Thermogravimetric
analysis/differential thermal analysis/differential thermogravimetry
(TGA-DTA-DTG) measurements were performed on a Perkin–Elmer
Pyris Diamond at a heating rate of 108Cminꢀ1 under a flow of nitrogen.
All of the calculations for the geometrical and electronic properties of
the dyes were performed in the gas phase by using the Gaussian 09 pro-
gram package.
10-(Anthracen-9-yl)-9H-pyrenoACHTUNTRGNEUNG[4,5-d]imidazole (3b)
Yellow solid; 65% yield; M.p. 278–2808C; 1H NMR ([D6]DMSO,
500.13 MHz): d=14.16 (s, 1H), 8.92 (s, 1H), 8.86 (d, J=7.5 Hz, 1H), 8.65
(d, J=7.5 Hz, 1H), 8.25–8.30 (m, 6H), 8.15 (quin, J=7.5 Hz, 2H), 7.87
(d, J=8.5 Hz, 2H), 7.62 (t, J=7.5 Hz, 2H), 7.55 ppm (t, J=7.5 Hz, 2H);
13C NMR ([D6]DMSO, 125.77 MHz): d=147.2, 137.4, 131.6, 131.5, 131.0,
130.8, 128.9, 128.6, 128.0, 127.97, 127.5, 127.0, 126.5, 126.3, 125.9, 125.7,
124.3, 124.1, 121.84, 121.80, 121.7, 119.2, 118.7 ppm; HRMS (ESI): m/z
calcd for C31H19N2: 419.1543 [M+H]; found: 419.1552.
10-(Pyren-1-yl)-9H-pyrenoACTHNUGTRNEUNG[4,5-d]imidazole (3c)
Yellow solid; 75% yield; M.p. 258–2608C; 1H NMR ([D6]DMSO,
500.13 MHz): d=14.09 (s, 1H), 9.68 (d, J=7.5 Hz, 1H), 8.99 (d, J=
7.5 Hz, 1H), 8.91 (d, J=7.0 Hz, 1H), 8.77 (d, J=8.0 Hz, 1H), 8.56 (d, J=
8.0 Hz, 1H), 8.40–8.43 (m, 3H), 8.30–8.33 (m, 4H), 8.17–8.25 ppm (m,
5H); 13C NMR ([D6]DMSO, 125.77 MHz): d=149.8, 149.6, 131.5, 131.4,
130.9, 130.4, 128.8, 128.6, 128.5, 128.3, 127.4, 127.3, 127.2, 126.6, 126.3,
125.8, 125.7, 125.5, 125.3, 125.0, 124.9, 124.8, 124.4, 124.3, 123.8, 121.9,
11
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10-(9-Butyl-9H-carbazol-3-yl)-9H-pyrenoACTHNUTRGNEUGN[4,5-d]imidazole (3e)
White solid; 56% yield; M.p.>3008C; 1H NMR ([D6]DMSO,
500.13 MHz) 9.27 (s, 1H), 8.95 (d, J=7.5 Hz, 2H), 8.59 (dd, J=8.5 Hz,
1.5 Hz, 1H), 8.32–8.35 (m, 1H), 8.20–8.23 (m, 4H), 8.14 (t, J=7.5 Hz,
2H), 7.81 (d, J=8.5 Hz, 1H), 7.67 (d, J=8.0 Hz, 1H), 7.52 (dt, J=
7.5 Hz, 1 Hz, 1H), 7.29 (t, J=7.5 Hz, 1H), 4.48 (t, J=7.5 Hz, 2H), 1.83
(quin, J=7.5 Hz, 2H), 1.37 (sext, J=7.5 Hz, 2H), 0.92 ppm (t, J=7.5 Hz,
3H); 13C NMR ([D6]DMSO, 125.77 MHz): d=151.2, 140.58, 140.56,
131.5, 127.6, 126.3, 124.6, 123.8, 122.3, 122.1, 121.6, 121.4, 120.5, 119.3,
119.0, 118.5, 109.71, 109.66, 42.2, 30.7, 19.8, 13.7 ppm; HRMS (ESI): m/z
calcd for C33H26N3: 464.2121 [M+H]; found: 464.2128.
General Procedure for the Synthesis of 10-Aryl-9-butyl-9H-pyrenoACHTUNGTRENNUNG[4,5-
d]imidazole (4a–4e)
A mixture of 10-aryl-9H-pyrenoACHTUNTRGNEU[GN 4,5-d]imidazole (1 mmol), 1-bromobu-
tane (0.21 g, 1.5 mmol), benzyltriethylammonium chloride (BTEAC
0.10 g), a 50% aqueous solution of NaOH (10 mL), and benzene (5 mL)
was heated at reflux for 6 h. After a clear organic layer had formed, the
reaction mixture was poured into hot water and allowed to stand over-
night in the hood. The solid that formed was extracted several times with
CH2Cl2 and the extract was dried over anhydrous Na2SO4. The crude
compound that was obtained on evaporation of the extract was purified
by column chromatography on silica gel (CHCl3/EtOAc).
9-Butyl-10-para-tolyl-9H-pyrenoACTHNUTRGNEUNG[4,5-d]imidazole (4a)
Light-yellow solid; 81% yield; M.p. 104–1068C; 1H NMR (CDCl3,
500.13 MHz): d=9.07 (dd, J=7.5 Hz, 1 Hz, 1H), 8.44 (d, J=8.0 Hz, 1H),
8.16 (d, J=7.5 Hz, 1H), 8.09–8.13 (m, 3H), 7.99–8.06 (m, 2H), 7.69 (d,
J=8.0 Hz, 2H), 7.39 (d, J=7.5 Hz, 2H), 4.69 (t, J=7.5 Hz, 2H), 2.49 (s,
3H), 1.97 (quin, J=7.5 Hz, 2H), 1.29 (sext, J=7.5 Hz, 2H), 0.84 ppm (t,
J=7.5 Hz, 3H); 13C NMR (CDCl3, 125.77 MHz): d=153.1, 139.5, 138.5,
132.4, 131.6, 130.0, 129.5, 128.2, 128.1, 127.6, 126.8, 126.6, 126.3, 125.6,
124.3, 124.1, 123.4, 123.0, 122.7, 119.7, 117.6, 46.8, 32.6, 21.5, 19.7,
13.6 ppm; HRMS (ESI): m/z calcd for C28H25N2: 389.2012 [M+H];
found: 389.2013.
10-(Anthracen-9-yl)-9-butyl-9H-pyrenoACTHNUTRGENUG[N 4,5-d]imidazole (4b)
Yellow solid; 78% yield; M.p. 164–1668C; 1H NMR (CDCl3,
500.13 MHz): d=9.13 (d, J=7.5 Hz, 1H), 8.70 (s, 1H), 8.56 (d, J=8.0 Hz,
1H), 8.22 (d, J=8.0 Hz, 2H), 8.07–8.18 (m, 6H), 7.73 (d, J=8.5 Hz, 2H),
7.53 (t, J=7.5 Hz, 2H), 7.44 (t, J=7.5 Hz, 2H), 4.35 (t, J=8.0 Hz, 2H),
1.83 (quin, J=7.5 Hz, 2H), 1.08 (sext, J=7.5 Hz, 2H), 0.54 ppm (t, J=
7.5 Hz, 3H); 13C NMR (CDCl3, 125.77 MHz): d=149.6, 139.0, 132.6,
132.4, 131.7, 131.4, 129.5, 128.7, 128.2, 127.6, 127.0, 126.8, 126.6, 126.5,
125.9, 125.7, 125.6, 124.53, 124.45, 124.4, 123.5, 123.0, 122.8, 120.0, 117.8,
46.8, 32.4, 19.7, 13.2 ppm; HRMS (ESI): m/z calcd for C35H27N2: 475.2169
[M+H]; found: 475.2169.
9-Butyl-10-(pyren-1-yl)-9H-pyrenoACTHNUTRGNEUNG[4,5-d]imidazole (4c)
Dark-yellow solid; 84% yield; M.p. 154–1568C; 1H NMR (CDCl3,
500.13 MHz): d=9.16 (dd, J=7.5 Hz, 1 Hz, 1H), 8.57 (d, J=8.0 Hz, 1H),
8.36 (q, J=8.0 Hz, 2H), 8.28 (dd, J=8.0 Hz, 1 Hz, 1H), 8.20–8.24 (m,
5H), 8.13–8.19 (m, 3H), 8.06–8.11 (m, 4H), 4.58 (s, 2H), 1.90 (quin, J=
7.5 Hz, 2H), 1.11 (sext, J=7.5 Hz, 2H), 0.62 ppm (t, J=7.5 Hz, 2H);
13C NMR (CDCl3, 125.77 MHz): d=151.8, 139.0, 132.5, 132.4, 131.7,
131.3, 131.2, 130.9, 128.9, 128.7, 128.6, 128.2, 127.7, 127.4, 126.73, 126.67,
126.5, 126.4, 125.9, 125.8, 125.7, 125.4, 124.8, 124.7, 124.6, 124.52, 124.48,
124.4, 123.6, 123.1, 122.9, 119.9, 117.8, 47.0, 32.4, 19.6, 13.5 ppm; HRMS
(ESI): m/z calcd for C37H27N2: 499.2169 [M+H]; found: 499.2176.
Figure 10. Electroluminescence spectra for the devices (I–IV) that were
fabricated from compounds 4c (a), 5b (b), and 5c (c).
121.7, 119.4, 119.2, 119.0 ppm; HRMS (ESI): m/z calcd for C33H19N2:
443.1543 [M+H]; found: 443.1548.
N,N-Diphenyl-4-(9H-pyrenoACTHNURTGNEU[GN 4,5-d]imidazol-10-yl)aniline (3d)
Light-yellow solid; 41% yield; M.p. 285–2878C; 1H NMR ([D6]DMSO,
500.13 MHz): d=13.59 (s, 1H), 8.80 (s, 2H), 8.23–8.27 (m, 4H), 8.20 (s,
2H), 8.14 (t, J=7.5 Hz, 2H), 7.36–7.40 (m, 4H), 7.16–7.18 (m, 2H), 7.12–
7.15 ppm (m, 6H); 13C NMR ([D6]DMSO, 125.77 MHz): d=149.5, 148.3,
146.7, 137.5, 131.5, 131.4, 129.7, 128.0, 127.9, 127.5, 126.3, 126.2, 126.0,
124.7, 124.0, 123.8, 122.1, 121.6, 119.1, 118.8 ppm; HRMS (ESI): m/z
calcd for C35H24N3: 486.1965 [M+H]; found: 486.1974.
4-(9-Butyl-9H-pyrenoACTHNUTRGNE[NUG 4,5-d]imidazol-10-yl)-N,N-diphenylaniline (4d)
Red solid; 72% yield; M.p. 148–1508C; 1H NMR (CDCl3, 500.13 MHz):
d=9.04 (dd, J=7.5 Hz, 1 Hz, 1H), 8.43 (d, J=8.0 Hz, 1H), 8.13 (d, J=
7.5 Hz, 1H), 8.06–8.10 (m, 3H), 7.97–8.03 (m, 2H), 7.64–7.66 (m, 2H),
7.30–7.33 (m, 4H), 7.19–7.24 (m, 6H), 7.09 (t, J=7.5 Hz, 2H), 4.72 (t, J=
7.5 Hz, 2H), 1.98 (quin, J=7.5 Hz, 2H), 1.32 (sext, J=7.5 Hz, 2H),
0.84 ppm (t, J=7.5 Hz, 3H); 13C NMR (CDCl3, 125.77 MHz): d=153.0,
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149.1, 147.4, 138.6, 132.4, 131.6, 131.0, 129.5, 128.1, 127.6, 126.8, 126.6,
126.3, 125.6, 125.1, 124.8, 124.3, 124.2, 124.1, 123.6, 123.4, 123.0, 122.7,
119.7, 117.6, 46.8, 32.6, 19.6, 13.6 ppm; HRMS (ESI): m/z calcd for
C39H31N3Na: 564.2410 [M+Na]; found: 564.2412.
2H), 8.58 (d, J=8.0 Hz, 2H), 8.05–8.20 (m, 12H), 7.98–8.01 (m, 2H),
7.68 (d, J=8.0 Hz, 2H), 4.84–4.91 (m, 4H), 4.50 (t, J=7.5 Hz, 2H), 2.19–
2.24 (m, 2H), 2.01 (quin, J=7.5 Hz, 2H), 1.49 (sext, J=7.5 Hz, 2H),
1.01–1.09 (m, 19H), 0.62–0.67 ppm (m, 12H); 13C NMR (CDCl3,
125.77 MHz): d=154.0, 140.3, 137.8, 131.3, 130.6, 127.3, 126.9, 126.5,
126.0, 125.6, 125.2, 124.3, 123.2, 123.0, 122.3, 122.2, 122.0, 121.9, 121.8,
121.2, 118.7, 117.1, 108.3, 50.2, 42.2, 38.0, 30.0, 28.7, 26.7, 22.0, 21.8, 19.5,
12.9, 12.8, 9.1 ppm; HRMS (ESI): m/z calcd for C66H66N5: 928.5313
[M+H]; found: 928.5309.
9-Butyl-10-(9-butyl-9H-carbazol-3-yl)-9H-pyrenoACHTNUTRGNEUNG[4,5-d]imidazole (4e)
Light-yellow solid; 77% yield; M.p. 100–1028C; 1H NMR (CDCl3,
500.13 MHz): d=9.14 (dd, J=7.5 Hz, 1.0 Hz, 1H), 8.56 (d, J=1.0 Hz,
1H), 8.51 (d, J=8.0 Hz, 1H), 8.19 (d, J=7.5 Hz, 2H), 8.12–8.15 (m, 3H),
8.02–8.09 (m, 2H), 7.89 (dd, J=8.0 Hz, J=1.5 Hz, 1H), 7.52–7.58 (m,
2H), 7.48 (d, J=8.0 Hz, 1H), 7.31 (t, J=7.5 Hz, 1H), 4.81 (t, J=7.5 Hz,
2H), 4.37 (t, J=7.5 Hz, 2H), 2.03 (quin, J=7.5 Hz, 2H), 1.92 (quin, J=
7.5 Hz, 2H), 1.44 (sext, J=7.5 Hz, 2H), 1.30 (sext, J=7.5 Hz, 2H), 0.99
(t, J=7.5 Hz, 3H), 0.81 ppm (t, J=7.5 Hz, 3H); 13C NMR (CDCl3,
125.77 MHz): d=154.3, 141.0, 138.5, 132.5, 131.7, 128.1, 127.60, 127.57,
126.8, 126.7, 126.3, 126.2, 125.6, 124.3, 124.0, 123.4, 123.14, 123.08, 122.82,
122.77, 122.4, 121.4, 120.7, 119.8, 119.4, 117.6, 109.1, 109.0, 47.0, 43.1,
32.7, 31.2, 20.6, 19.7, 13.9, 13.6 ppm; HRMS (ESI): m/z calcd for
C37H33N3Na: 542.2567 [M+Na]; found: 542.2569.
Acknowledgements
K.R.J.T. is thankful to the Council of Scientific and Industrial Research,
New Delhi (India) for financial support (01ACTHNUTRGNE(NUG 2480)/11/EMR-II). D.K. ac-
knowledges the CSIR for a Research Fellowship.
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General Procedure for the Synthesis of 10,10’-Aryl-bis(9-(2-ethylhexyl)-
9H-pyrenoACHTUNGTRENNUNG[4,5-d]imidazole) (5a–5c)
Pyrene-4,5-dione (0.46 g, 2.0 mmol), the corresponding aldehyde
(1.0 mmol), and ammonium acetate (0.62 g, 8 mmol) were dissolved in
acetic acid (20 mL) and the resultant mixture was heated at reflux for
24 h. Then, the reaction mixture was cooled and poured into water to
obtain a precipitate, which was thoroughly washed with water and dried.
The crude product could not be purified because it was not soluble in
most organic solvents. Thus, it was suspended in DMF along with K2CO3
(0.83 g, 6 mmol) and the resultant mixture was degassed with nitrogen.
The mixture was heated at 508C for 1 h, 2-ethylhexylbromide (0.87 g,
4.5 mmol) was added, and the reaction was continued for a further 48 h.
Then, the mixture was poured into water and extracted with CH2Cl2. The
organic layer was washed several times with brine, dried with Na2SO4,
and removed under vacuum. The residue was purified by column chro-
matography on silica gel (CH2Cl2/EtOAc).
10,10’-(9,9-Diethyl-9H-fluorene-2,7-diyl)bis(9-(2-ethylhexyl)-9H-
pyrenoACHTUNGTRENNUNG[4,5-d]imidazole) (5a)
Yellow solid; 45% yield; M.p. 126–1288C; 1H NMR (CDCl3,
500.13 MHz): d=9.11 (dd, J=7.5 Hz, 1.0 Hz, 2H), 8.57 (d, J=8.0 Hz,
2H), 8.20 (t, J=7.5 Hz, 4H), 8.06–8.16 (m, 8H), 8.01 (d, J=7.5 Hz, 2H),
7.89 (d, J=7.5 Hz, 2H), 7.85 (s, 2H), 4.83 (d, J=5.5 Hz, 4H), 2.21–2.23
(m, 6H), 1.12 (s, 16H), 0.65–0.76 (m, 12H), 0.47 ppm (t, J=5.0 Hz, 6H);
13C NMR (CDCl3, 125.77 MHz): d=154.4, 150.8, 142.1, 138.9, 132.4,
131.7, 130.4, 129.8, 128.1, 127.7, 127.4, 126.6, 126.5, 125.5, 125.0, 124.5,
124.3, 123.5, 123.2, 122.9, 120.6, 119.9, 118.2, 56.9, 51.4, 38.9, 33.0, 29.9,
28.0, 23.0, 14.0, 10.0, 8.7 ppm; HRMS (ESI): m/z calcd for C67H67N4:
927.5360 [M+H]; found: 927.5356.
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4-(9-(2-Ethylhexyl)-9H-pyreno
ACHTUNGTRENN[UNG 4,5-d]imidazol-10-yl)-N-(4-(9-(2-
ethylhexyl)-9H-pyreno[4,5-d]imidazol-10-yl)phenyl)-N-phenylaniline (5b)
ACHTUNGTRENNUNG
Light-brown solid; 50% yield; M.p. 106–1088C; 1H NMR (CDCl3,
500.13 MHz): d=9.07 (dd, J=7.5 Hz, 1.0 Hz, 2H), 8.55 (d, J=8.0 Hz,
2H), 8.18 (t, J=8.0 Hz, 4H), 8.08–8.14 (m, 6H), 8.06 (t, J=7.5 Hz, 2H),
7.76 (d, J=8.5 Hz, 4H), 7.36–7.41 (m, 6H), 7.28 (d, J=7.5 Hz, 2H), 7.18
(t, J=7.5 Hz, 1H), 4.79 (d, J=7.0 Hz, 4H), 2.20–2.24 (m, 2H), 1.17 (s,
16H), 0.81–0.84 (m, 6H), 0.70 ppm (s, 6H); 13C NMR (CDCl3,
125.77 MHz): d=153.9, 148.4, 146.8, 138.6, 132.4, 131.7, 131.4, 129.7,
128.1, 127.6, 127.2, 126.5, 126.4, 125.5, 125.4, 125.3, 124.5, 124.3, 124.2,
123.8, 123.4, 123.2, 122.9, 119.8, 118.1, 50.9, 39.0, 29.7, 27.8, 23.0, 14.0,
9.9 ppm; HRMS (ESI): m/z calcd for C68H63N5Na: 972.4976 [M+Na];
found: 972.4979.
10,10’-(9-Butyl-9H-carbazole-3,6-diyl)bis(9-(2-ethylhexyl)-9H-pyreno
d]imidazole) (5c)
ACHTUNGTNER[NUNG 4,5-
Light-brown solid; 52% yield; M.p. 122–1248C; 1H NMR (CDCl3,
500.13 MHz): d=9.11 (dd, J=7.5 Hz, 1.0 Hz, 2H), 8.65 (t, J=7.0 Hz,
13
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K. R. Justin Thomas, Jwo-Huei Jou et al.
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