Synthesis, optical properties, and blue electroluminescence of fluorene derivatives containing multiple imidazoles bearing polyaromatic hydrocarbons

Article abstract of DOI:10.1016/j.tet.2013.01.046

Fluorene decorated with multiple imidazole units in a non-conjugated fashion and featuring polyaromatic hydrocarbons such as anthracene or pyrene were synthesized and characterized as blue emitters suitable for application in organic light-emitting diodes. The optical absorption and emission peak profiles remained unaltered on increasing the imidazole loading on fluorene. But the molar extinction coefficient progressively increased on introduction of additional imidazole unit attributable to the increment in the chromophore density. Replacement of phenyl group at the C-2 of imidazole nucleus with polyaromatic hydrocarbons such as anthracene and pyrene led to a red-shift in the absorption and emission spectra due to their characteristic absorption features. All the compounds exhibited high thermal decomposition temperature in the range 395-509 °C showing significant thermal robustness. The anthracene and pyrene derivatives were demonstrated as efficient blue emitters in 4,4′-di(9H-carbazol-9-yl)-1,1′-biphenyl host for multilayered organic light-emitting diodes.

Full text of DOI:10.1016/j.tet.2013.01.046

Tetrahedron 69 (2013) 2594e2602
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis, optical properties, and blue electroluminescence of
fluorene derivatives containing multiple imidazoles bearing
polyaromatic hydrocarbons
,
b
b
Dhirendra Kumar a, K.R. Justin Thomas a , Yu-Lin Chen , Yung-Cheng Jou ,
*
,
Jwo-Huei Jou b
*
a Organic Materials Laboratory, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkeed247 667, India
b Department of Material Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Fluorene decorated with multiple imidazole units in a non-conjugated fashion and featuring poly-
aromatic hydrocarbons such as anthracene or pyrene were synthesized and characterized as blue
emitters suitable for application in organic light-emitting diodes. The optical absorption and emission
peak profiles remained unaltered on increasing the imidazole loading on fluorene. But the molar ex-
tinction coefficient progressively increased on introduction of additional imidazole unit attributable to
the increment in the chromophore density. Replacement of phenyl group at the C-2 of imidazole nucleus
with polyaromatic hydrocarbons such as anthracene and pyrene led to a red-shift in the absorption and
emission spectra due to their characteristic absorption features. All the compounds exhibited high
thermal decomposition temperature in the range 395e509 ꢀC showing significant thermal robustness.
The anthracene and pyrene derivatives were demonstrated as efficient blue emitters in 4,40-di(9H-car-
bazol-9-yl)-1,10-biphenyl host for multilayered organic light-emitting diodes.
Received 28 November 2012
Received in revised form 27 December 2012
Accepted 15 January 2013
Available online 1 February 2013
Keywords:
Polysubstituted fluorene
Imidazole
Pyrene
Anthracene
Blue emission
Organic light-emitting diodes
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Variety of organic compounds containing functional chromo-
phores such as styrylarylene,4 fluorene,5 fluoranthene,6 quinoline,7
Organic materials suitable for application in organic light-
emitting diodes (OLEDs) have been intensively studied due to
their potential application in electroluminescent devices used in
high-resolution, full-color, and flat-panel displays.1 To achieve
practical utility for such devices, organic or organometallic mate-
rials exhibiting emission in one of the three primary colors (blue,
green, or red), high electroluminescence (EL) efficiencies, good
thermal stabilities, and excellent charge-carrier injection/transport
abilities are required. Though the green- and red-emitting diodes2,3
have been demonstrated with satisfactory functionalities, the per-
formance of blue-emitting diodes remains below required excel-
lence limits. It is partially due to their intrinsic electronic features
such as wide band gap and low lying HOMO energy level, which
retard the injection of charge carriers in the molecular layer of blue-
emitting materials. So the researchers are actively involved in the
design and the synthesis of blue-emitting materials with color
purity in emission and good charge injection/transport capabilities
remains active.
quinoxaline,8 anthracene,9 carbazole,10 triarylamines,11 and pyr-
ene12 have been designed and synthesized toward realizing effi-
cient blue OLEDs. Among the various blue-emitting materials
reported, anthracene9 and pyrene containing12 molecular materials
have emerged as efficient blue emitters in OLEDs because of their
unique optical and electrochemical properties. But the drawback
with the anthracene and pyrene derivatives are their unwanted
longer-wavelength emission and emission-quenching in the solid
state or concentrated solutions attributed to excimer formation by
pep
stacking interactions.13 This drawback can be eliminated ei-
ther by structural alternations or by the use of appropriate device
architectures. Structurally, introduction of sterically demanding
alkyl or aryl units or non-planar structural elements at the lateral
position of the conjugation backbone has been found to impede the
aggregation propensity of such derivatives and enhances the OLED
performance.12c,14 Further employing device architectures in which
the emitting dye is doped in a host matrix composed of hole and/or
electron-transporting or hole/electron blocking materials were also
found to be beneficial for deep-blue emission. This kind of con-
centration dilution in solid solutions effectively suppresses the
aggregation and the correct choice of host will ensure efficient
* Corresponding authors. Tel.: þ91 1332 285376; e-mail addresses: krjt8fcy@
iitr.ernet.in, krjtiitk@gmail.com (K.R.J. Thomas), jjou@mx.nthu.edu.tw (J.-H. Jou).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.01.046

D. Kumar et al. / Tetrahedron 69 (2013) 2594e2602
2595
energy transfer to the emitting material to give efficient OLED
2.2. Photophysical properties
performance.12d,15
2,4,5-Trisubstituted imidazoles possess non-coplanar structure,
which is beneficial to retard aggregation.16 So the use of arylated
imidazoles as an emitting chromophore in a molecular design will
be beneficial for favorable properties such as emission color purity,
photoluminescence quantum yield, and thermal stability.17 Imid-
azole and arene-fused imidazole units such as benzimidazole,18
pyridoimidazole,19 pyrrolloimidazole,20 fluorenoimidazole,21 phe-
nanthroimidazole,22 pyrenoimidazole,23 etc. have been used in the
construction of multifunctional organic materials and appropri-
ately exploited for application in organic light-emitting diodes or
dye-sensitized solar cells. In this work, we report the synthesis and
optical, electrochemical, and thermal characterization of six
fluorene-bridged non-conjugated bis- and tris-imidazoles. The
imidazole chromophores were connected to the fluorene core via
a methylene linkage at C2 and C7 or C2, C5, and C7 positions of the
fluorene nucleus. Tri-substituted fluorene derivatives are relatively
scarce and their use in OLEDs remains unexplored.24 To impart
desirable emission properties, polyaromatic hydrocarbons such as
anthracene or pyrene are introduced at the C2 position of imidazole
nucleus. Due to the fused and rigid structure they are expected to
play a beneficial role to improve the thermal stability of the final
compounds. The molecules were designed to inherit the absorption
and the emission properties of polyaromatic hydrocarbons and
exhibit additional charge transporting capabilities attributable to
the imidazole unit.
The photophysical properties of the compounds were in-
vestigated by measuring absorption and emission spectra in a se-
ries of solvents of varying polarity viz. toluene (Tol),
dichloromethane (DCM), chloroform (CHCl3), N,N-dimethylforma-
mide (DMF), acetonitrile (ACN), and methanol (MeOH) at the
concentration of 2ꢁ10ꢂ5 M and 2ꢁ10ꢂ6 M, respectively. The ab-
sorption spectra of the imidazoles (4ae4c and 6ae6c) recorded in
toluene are displayed in Fig. 1 and the absorption parameters are
listed in Table 1.
80000
4a
4b
4c
60000
40000
20000
0
6a
6b
6c
300
350
400
450
2. Results and discussion
Wavelength (nm)
2.1. Synthesis and characterization
Fig. 1. Absorption spectra of 4ae4c and 6ae6c recorded in toluene.
The synthetic pathway employed to synthesize the fluorene-
bridged non-conjugated bis- and tris-imidazoles is shown in
Scheme 1. The non-conjugated imidazoles 4 and 6 were obtained
by N-alkylation of 3 with bis- and tris-bromomethylated fluorenes
2 and 5,24 under phase transfer catalytic conditions. The N-
unsubstituted imidazoles (3) required were constructed from the
corresponding diketones and aryl aldehydes as reported in our
earlier publication.25
Compounds containing 2,4,5-triphenylimidazole units (4a and
6a) exhibited poorly resolved absorption peaks originating from
fluorene and phenyl units below 320 nm. A hump observed at
w310 nm for all the compounds is attributed to the fluorene lo-
calized
pep
*
transition.26 Other derivatives (4b, 4c, 6b, and 6c)
displayed signature peaks attributable to the anthracene27 and
pyrene27b chromophores. Consequently, the anthracene containing
Scheme 1. Synthesis of non-conjugated fluorene-bridged imidazoles (4ae4c and 6ae6c).

2596
D. Kumar et al. / Tetrahedron 69 (2013) 2594e2602
Table 1
Absorption properties of 4ae4c and 6ae6c in different solvents
Dye
labs, nm (
3
ꢁ103, Mꢂ1 cmꢂ1
)
Tol
DCM
CHCl3
DMF
ACN
MeOH
4a
6a
4b
312 (42.5)
300 (47.1)
307 (61.4)
311 (40.9)
284 (62.9)
307 (58.4)
285 (83.9)
389 (17.0)
370 (17.3)
354 (11.8)
311 (27.8)
390 (23.3)
372 (24.3)
356 (16.6)
345 (55.5)
312 (45.5)
278 (96.2)
346 (77.0)
309 (53.8)
278 (118.6)
311 (39.3)
284 (64.1)
307 (56.4)
285 (85.2)
395 (15.1)
376 (18.6)
362 (14.6)
311 (26.8)
392 (22.7)
375 (25.6)
311 (39.8)
283 (59.3)
307 (57.3)
285 (78.8)
389 (16.6)
371 (17.2)
355 (11.6)
311 (25.9)
390 (22.9)
372 (23.9)
356 (16.0)
347 (53.8)
311 (44.5)
278 (93.4)
347 (75.9)
308 (55.7)
278 (123.2)
310 (39.0)
281 (65.9)
306 (56.3)
284 (85.4)
387 (17.3)
369 (17.6)
353 (11.8)
311 (25.9)
388 (23.3)
370 (24.6)
354 (16.8)
344 (55.9)
310 (44.3)
277 (97.7)
344 (77.4)
277 (127.1)
268 (107.5)
310 (33.7)
279 (67.6)
307 (49.9)
284 (89.5)
387 (16.8)
369 (18.0)
352 (12.5)
311 (24.4)
389 (23.2)
370 (25.8)
353 (17.9)
343 (59.5)
310 (40.7)
276 (93.7)
344 (80.6)
311 (46.5)
277 (125.3)
267 (104.7)
389 (16.5)
371 (16.0)
312 (28.0)
6b
4c
6c
390 (22.9)
373 (22.8)
347 (50.1)
312 (45.0)
350 (57.6)
311 (37.3)
280 (91.6)
349 (78.9)
280 (118.5)
347 (71.6)
310 (53.5)
derivatives (4b and 6b) showed most red-shifted absorption. On
comparing the bis-imidazoles with the corresponding tris-
imidazoles the following salient features emerge: (a) absorption
peaks mainly arise from the localized electronic transitions and the
inter-chromophoric electronic interactions are negligible (b) molar
extinction coefficients of the absorption peaks attributable to aryl
units (phenyl, anthracene, or pyrene) progressively increased with
the number of imidazole units on fluorene. The increment in the
chromophore density on incorporation of additional imidazole
moiety on fluorene nucleus is responsible for this hike in optical
intensity. The absorption profile of the compounds was insensitive
to the solvent polarity, which indicates that the effect due to the
interaction of the dyes with the solvent molecules in the ground
state is less important.
The emission spectra of the compounds recorded for toluene
solutions are shown in Fig. 2 and the relevant data compiled in
Table 2 for other solvents also. The emission peak values followed
a trend as per the nature of the aryl unit on imidazole: benze-
ne<pyrene<anthracene. This is reminiscent of an order expected
for the unsubstituted aromatic segments. It appears that the elec-
tronic delocalization from the aromatic segment into the imidazole
unit is very limited. The bis- and tris-imidazoles showed similar
emission profiles. Lophine derivatives showed emission in the
UV-region while the anthracene and pyrene derivatives emitted in
blue-region.
Though the lophine derivatives (4a and 6a) did not display al-
ternations in the emission profile on changing the solvent polarity,
the anthracene (4b and 6b) and pyrene (4c and 6c) derivatives
exhibited prominent positive solvatochromism (Table 2). The later
observation suggests the stabilization of excited state in polar sol-
vents for the anthracene and pyrene derivatives. Though there is no
inherent bipolar character in these molecules, it is probable that
electronic excitation induces significant change in the dipole mo-
ment for the molecules. Consequently the interaction of induced
molecular dipole with the solvent dipole is more favorable in polar
solvents with appreciable polarizability. Interestingly, a blue shift in
emission profile was observed for the anthracene and pyrene de-
rivatives when recorded in methanol. As this blue shift is not seen
for the lophine derivatives, we believe that hydrogen bonding in-
teraction with the solvent may not be a plausible reason. Alterna-
tively, pronounced hydrophobicity of the pyrene and anthracene
derivatives may be responsible for this unusual shift. Due to this
probably an ‘oil in water’ effect is operating and bringing the flu-
orophores together and leading to aggregates, which cause blue
shift in emission. The Stokes shifts of the lophine (4a and 6a) and
pyrene (4c and 6c) derivatives are significantly larger than those
observed for the anthracene derivatives (4b and 6b). This probably
suggests a reasonable structural reorganization in the excited state
for the former compounds.28
1.0
The emission spectra recorded for the solid films (Fig. 3) of the
compounds are slightly red-shifted than those observed in toluene
and dichloromethane solutions but exhibit close resemblance to
those observed in acetonitrile solutions. This probably indicates
that the dielectric constants for the solid films are approximately
close to that in corresponding acetonitrile solutions. It is very in-
teresting to note here that the red-shifted emission peaks observed
for anthracene29 and pyrene29a,30 excimers are not observed for
these derivatives. This clearly highlights the importance of the
present design in inhibiting the aggregation.
Photoluminescence quantum yields determined for the com-
pounds by using 2-aminopyridine (FF¼60% in 0.1 N H2SO4)31 and
coumarin-1 (FF¼99% in ethyl acetate)32 as standard are slightly
high (see Table 2) for the anthracene (4b and 6b) and pyrene (4c
and 6c) derivatives when compared to the lophine analogs (4a and
6a), which indicates the importance of anthracene and pyrene
acting as main fluorophore in these compounds. The exceptional
fluorescence quantum efficiency observed for the pyrene
4a
4b
4c
0.8
6a
6b
6c
0.6
0.4
0.2
0.0
350
400
450
500
550
600
Wavelength (nm)
Fig. 2. Emission spectra of 4ae4c and 6ae6c recorded in toluene.

D. Kumar et al. / Tetrahedron 69 (2013) 2594e2602
2597
Table 2
Emission properties and Stokes shift observed for the compounds in the different solvents
Dye
lem, nm (FF, %)a
Stokes shift, cmꢂ1
Tol
DCM
CHCl3
DMF
ACN
MeOH
Film
Tol
DCM
CHCl3
DMF
ACN
MeOH
4a
6a
4b
6b
4c
6c
383 (na)
383 (na)
456 (61)
456 (59)
440 (90)
440 (88)
382 (6)b
381 (4)b
464 (65)
462 (63)
449 (79)
449 (75)
385 (5)b
383 (4)b
456 (60)
457 (58)
447 (76)
448 (73)
384 (4)b
384 (3)b
479 (52)
475 (49)
462 (75)
459 (72)
382 (2)b
382 (1)b
476 (47)
473 (42)
460 (69)
458 (67)
381 (3)b
378 (2)b
453 (56)
455 (52)
444 (72)
443 (71)
383
388
469
475
460
459
5942
6464
3777
3711
6091
6091
5976
6327
4155
3996
6714
6630
6180
6464
3387
3628
6200
6332
6113
6532
4830
4588
7173
7032
6080
6502
4831
4632
7331
7236
6011
6118
3765
3729
6632
6496
na¼not analyzed.
a
Coumarin-1 (VF¼99% in ethyl acetate) as reference.
2-Aminopyridine (VF¼60% in 0.1 N H2SO4) as reference.
b
aggregation, or photochemical reactions. Near linear trend ob-
served for most of the solvents with the exception of methanol for
the reason described before (vide supra). The plot of Stokes shifts
versus the ET(30) parameter displayed for compounds 6b and 6c in
Fig. 4(b) further supports the positive solvatochromic behavior of
the compounds.
1.0
0.8
0.6
0.4
0.2
0.0
4a
6a
4b
6b
4c
6c
2.3. Electrochemical properties
The electrochemical characteristics of the compounds were
examined by using cyclic voltammetric (CV) and differential pulse
voltammetric (DPV) techniques. The redox potentials were cali-
brated using ferrocene as internal standard in each measurement
and the pertinent data compiled in Table 3. The pyrene containing
imidazoles showed three irreversible oxidation peaks while the
lophine and anthracene derivatives displayed single irreversible
oxidation wave (Fig. 5). The oxidation potential of the compounds
followed a trend reminiscent of the nature of C2 substituent in the
imidazole ring: phenyl>anthracene>pyrene. It is fairly in agree-
ment with the electron-richness of the aryl unit. The HOMO energy
of these derivatives was calculated by using ferrocene as a reference
(4.8 eV)35 and ranges from 5.52 to 5.71 eV. The LUMO energy was
calculated from HOMO energy and optical band gap estimated from
the intersection of absorption and emission profiles, which ranges
from 1.97 to 2.71 eV.
350
400
450
500
550
600
Wavelength (nm)
Fig. 3. Emission spectra of the spin cast films of 4ae4c and 6ae6c.
derivatives indicates that these compounds may be suitable for
application as emitting dopants in organic light-emitting diodes.
Since the compounds containing anthracene and pyrene units
displayed solvent dependent excited state properties, we have
attempted to correlate the Stokes shift with LipperteMataga
equation,33 and ET(30) parameters of the solvents.34 The Lip-
perteMataga plots of Stokes shift against the orientation polariz-
ability map the general interaction of dye molecules with the polar
and nonpolar solvents (Fig. 4(a)). A linear correlation in the Lip-
perteMataga plot is indicative of the presence of general sol-
ventesolute interactions while a deviation from linearity suggests
2.4. Thermal properties
The thermal properties of the compounds were investigated by
thermogravimetric analysis and the data are presented in Fig. 6 and
Table 3. The thermal decomposition temperature of the compounds
falls in the range 395e509 ꢀC. Pyrene containing derivatives (4c
a
solvent specific interactions such as hydrogen bonding,
7500
7500
6b
(a)
(b)
6c
R = 0.97
6000
6000
R = 0.95
6b
6c
4500
4500
R = 0.86
R = 0.84
3000
3000
35
40
45
E (30) (kcal mol )
T
50
55
0.0
0.1
0.2
0.3
-1
Orientation Polarizability
Fig. 4. Correlation of solvent-induced Stokes shift with (a) orientation polarizability and (b) ET(30) parameter for compounds 6b and 6c (data for methanol are omitted from linear
correlation).

2598
D. Kumar et al. / Tetrahedron 69 (2013) 2594e2602
Table 3
emitting materials have displayed pronounced thermal stability
attributable to the rigidity of pyrene segment.12b,36 All the com-
pounds were found to be crystalline in nature and did not display
inflection in the differential scanning calorimetric traces attribut-
able to the glass transition temperature.
Thermal and electrochemical properties of the compounds
Td, ꢀCa
Tonset,
ꢀCb
Eox, Vc
HOMO, eVd
LUMO, eVe
E0ꢂ0
f
Dye
4a
6a
4b
6b
4c
395
401
461
459
509
336
345
382
407
406
0.85
0.84
0.77
0.91
0.72, 0.98,
1.22
5.65
5.64
5.57
5.71
5.52
1.97
1.97
2.57
2.71
2.39
3.68
3.67
3.00
3.00
3.13
2.5. Electroluminescent characteristics
The electroluminescent characteristics of the blue-emitting
compounds 4b, 6b, 4c, and 6c were investigated by employing
them as dopant in a multilayered OLED device with the configu-
ration: ITO/PEDOT:PSS/CBPþ4b or 4c or 6b or 6c (10%)/TPBI/LiF/Al
where 125 nm ITO (indium tin oxide) was the anode, 35 nm
poly(3,4-ethylene-dioxythiophene)epoly-(styrenesulfonate)
(PEDOT:PSS) served as a hole-injection layer, 36 nm 4b/6b/4c/6c
(10 wt %) doped in 4,40-bis(9H-carbazol-9-yl)biphenyl (CBP) as
a solution processed emitting layer, 32 nm 1,3,5-tris(N-phenyl-
benzimidazol-2-yl)benzene (TPBI) as an electron-transporting
layer, 0.7 nm LiF as an electron injection layer and 150 nm Al as
a cathode.
6c
494
418
0.72, 0.80,
1.21
5.52
2.39
3.13
a
Heating rate 10 ꢀC/min in nitrogen.
Temperature corresponding to 5% weight loss.
Oxidation potentials with reference to the ferrocene, which was used as an in-
b
c
ternal standard.
d
Deduced from the equation HOMO¼Eoxþ4.8.
e
f
Deduced from the equations LUMO¼HOMOeE0ꢂ0
.
Derived from the optical edge.
0.0
Alignment of the energy levels in the materials used for the
fabrication of the devices, the current densityevoltage (IeV) plots
and luminanceecurrent density (LeI) characteristics of the OLEDs
fabricated using these dyes are shown in Fig. 7. The device perfor-
mance and EL emission characteristics are summarized in Table 4.
The bright-blue EL spectra (Fig. 8) observed for the devices using
anthracene-based dyes (4b and 6b) are broad and structureless
with the maximum peak at 448 nm and CIE coordinates of x¼0.16
and y¼0.14 while the devices containing pyrene-based dyes
showed EL at 432 nm with CIE coordinates very close to the Na-
tional Television Standards Committee (NTSC) standard blue
values. The EL spectra observed for the dyes are slightly blue shifted
(w8 nm) than those recorded in toluene, which clearly results due
to the dilution effect arising from the uniform dispersion of the
dyes in CBP.
-0.5
-1.0
4a
4b
6a
6b
Ferrocene
-1.5
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
The driving voltages (at 10 cd/m2) observed for the anthracene
derivatives (4b and 6b) were slightly lower than those realized for
devices based on pyrene derivatives (4c and 6c). This probably
points an effective charge transport in the molecular layers of the
former compounds arising due to the lower LUMO (Fig. 7(a)) ob-
served for them, due to which electron injection from the electron-
transporting layer is favored. Compound 6b showed best device
performance exhibiting a maximum luminance of 2060 cd/m2 as
well as maximum power and current efficiencies of 1.0 lm/W and
2.3 cd/A, respectively. This may be attributed to the efficient cap-
turing of energy by this molecule from the CBP excitons generated
due to its reasonable alignment of energy levels (Fig. 7(a)). Com-
pound 4c exhibited deep-blue electroluminescence with moderate
currentevoltageeluminance characteristics (maximum luminance
of 1280 cd/m2 and power and current efficiencies of 0.7 lm/W and
1.7 cd/A). The efficiency of the devices showed slight roll-off from
a low current density to a higher current density. The efficiency
increased from bis- to tris-derivative for anthracene containing
compounds while reverse result was observed for the pyrene an-
alogs (Table 4).
Potential vs Ferrocene (V)
Fig. 5. Differential pulse voltammograms recorded for selected compounds 4a, 6a, 4b,
and 6b in dichloromethane.
100
4a
4b
80
4c
6a
6b
6c
60
40
20
0
100 200 300 400 500 600 700 800 900
Temperature (°C)
3. Conclusions
In summary, a series of non-conjugated fluorene-bridged bis-
and tris-imidazole derivatives have been synthesized and charac-
terized by their spectral, thermal, and electrochemical properties.
Polyaromatic hydrocarbons present on the imidazole nucleus play
vital role to determine the absorption and emission properties of
these blue-emitting compounds. Anthracene and pyrene contain-
ing derivatives exhibited blue photoluminescence with high
quantum yield and thermal stability. The solution processed OLED
Fig. 6. Thermogravimetric traces observed for the compounds.
and 6c) showed higher Td than the corresponding anthracene (4b
and 6b) and lophine (4a and 6a) derivatives. This observation re-
veals that the rigid polyaromatic substituents are responsible for
the thermal stability of these compounds. Pyrene containing mo-
lecular materials demonstrated as potential hole transporting/

D. Kumar et al. / Tetrahedron 69 (2013) 2594e2602
2599
(a)
3000
280
210
140
70
(c)
(b)
CBP:10wt% 4b
CBP:10wt% 4b
CBP:10wt% 6b
CBP:10wt% 4c
CBP:10wt% 6c
CBP:10wt% 6b
CBP:10wt% 4c
CBP:10wt% 6c
2000
1000
0
0
0
50
100
150
200
250
0
3
6
9
12
2
Current density (A/m )
Voltage (V)
Fig. 7. (a) Energy level diagram of the materials used in the devices (b) the current densityevoltage (IeV) and (c) luminanceecurrent density (LeI) curves of the devices using 4b,
4c, 6b, and 6c as dopants.
Table 4
Device performance characteristics of the OLEDs fabricated using 4b, 6b, 4c, and 6c
Dye
Drive voltage, Va
Power efficiency,
Current efficiency,
Max. luminance,
cd/m2
EL peak, nm
CIE (x,y)
lm/Wb
cd/Ab
4bþCBP
6bþCBP
4cþCBP
6cþCBP
6.4
6.1
7.1
7.0
0.6/0.4
1.0/0.7
0.7/0.4
0.5/e
1.5/1.3
2.3/2.1
1.7/1.2
1.2/e
1550
2060
1280
827
448
448
432
432
0.16, 0.14
0.16, 0.14
0.16, 0.09
0.16, 0.08
a
At 10 cd/m2.
At 100 cd/m2 and 1000 cd/m2.
b
devices based on these derivatives, 4b, 6b, 4c, and 6c, as dopant
were successfully fabricated and found to exhibit decent device
parameters. They showed blue electroluminescence with CIE co-
ordinates of x¼0.16 and y¼0.14 for 4b and 6b, x¼0.16 and y¼0.09
for 4c and x¼0.16 and y¼0.08 for 6c. The results suggest that these
compounds are promising emitting materials for the blue OLEDs.
Further studies to use these materials as hosts for suitable triplet
emitters, in solution processed electrophosphorescent devices are
currently being pursued.
0.036
0.030
0.024
0.018
0.012
0.006
0.000
CBP:10wt% 4b
CBP:10wt% 6b
CBP:10wt% 4c
CBP:10wt% 6c
4. Experimental section
4.1. Materials and methods
350
400
450
500
550
600
Wavelength (nm)
Chemicals were purchased from commercial sources and used
as received. Solvents were dried and distilled prior to use by
standard procedure. Column chromatography purification was
Fig. 8. The electroluminescence spectra observed for the devices containing 4b, 6b, 4c,
and 6c as dopants.

2600
D. Kumar et al. / Tetrahedron 69 (2013) 2594e2602
performed with the use of silica gel (230e400 mesh, Rankem) as
the stationary phase in a column with 40 cm long and 3.0 cm di-
ameter. 1H NMR and 13C NMR were recorded in CDCl3 on a Bruker
column chromatography (10% EtOAc/CH2Cl2) to give pale-yellow
solid (0.71 g, 68%); Rf (5% EtOAc/CH2Cl2) 0.32; mp 223e225 ꢀC; IR
(KBr) 3058, 2951, 2927, 2860,1598,1490,1416,1377,1255,1130, 827,
AV 500
O
FT-NMR spectrometer operating at 500.13 and
752, 697 cmꢂ1
; dH (500.13 MHz, CDCl3) 8.56 (2H, s), 8.02e8.04 (4H,
125.77 MHz, respectively. High-resolution mass spectrometric
measurements were carried out using ESI mass spectrometer. The
IR spectra were recorded on THERMO NICOLET NEXUS FT-IR spec-
trometer. UVevis spectra were recorded at room temperature in
quartz cuvettes using UV-1800 Shimadzu spectrophotometer.
Fluorescence measurements were carried out on RF-5301-PC Shi-
madzu Spectrofluorimeter. Quantum yield of the dyes was calcu-
lated by following standard procedure and using 2-aminopyridine
(VF¼0.60 in 0.1 N H2SO4)31 or coumarin-1 (VF¼0.99 in ethyl ace-
tate)32 as reference. Corrections due to dye absorption and re-
fractive indices of the solvents used for measurement were
incorporated in the calculation. The cyclic voltammetric (CV) and
differential pulse voltammetric (DPV) measurements were carried
out on a CHI 620C electrochemical analyzer in dichloromethane by
using 0.1 M tetrabutylammonium perchlorate as supporting elec-
trolyte. The experiments were performed at room temperature in
nitrogen atmosphere with a convenient three-electrode cell con-
sisting of a platinum wire as auxiliary electrode, a non-aqueous Ag/
AgNO3 reference electrode, and a glassy carbon working electrode.
The highest occupied molecular orbital (HOMO) energy levels of
organic materials were measured from the oxidation potential
obtained from cyclic voltammetry and lowest unoccupied molec-
ular orbital (LUMO) energy levels were determined from the HOMO
energy levels and optical band gap estimated from the intersection
of absorption and emission. Thermogravimetric analyses were
performed on PerkineElmer (Pyris Diamond) at a heating rate of
10 ꢀC/min under a flow of nitrogen.
m), 7.88e7.90 (4H, m), 7.66e7.68 (4H, m), 7.44e7.48 (8H, m), 7.36
(10H, s), 7.21e7.24 (4H, m), 7.14e7.17 (2H, m), 6.93 (2H, d, J 8.0 Hz),
6.27 (2H, s), 6.20 (2H, d, J 7.5 Hz), 4.74 (4H, s), 1.39 (4H, q, J 7.5 Hz),
ꢂ0.26 (6H, t, J 7.5 Hz); dC (125.77 MHz, CDCl3) 149.9, 144.9, 139.9,
138.3, 135.3, 134.7, 132.2, 131.5, 131.4, 131.3, 129.5, 129.2, 129.0,
128.7, 128.2, 126.9, 126.8, 126.4, 125.9, 125.8, 125.4, 124.6, 121.2,
119.0, 55.5, 48.8, 32.1, 8.3; dC
(125.77 MHz, CDCl3) 131.3,
DEPT 45
129.2, 129.0, 128.7, 128.2, 126.9, 126.8, 126.4, 125.9, 125.8, 125.4,
121.2, 119.0, 48.8, 32.1, 8.3; HRMS (ESI): MNaþ, found 1061.4554.
C77H58N4Na requires 1061.4559.
4.2.3. 1,10-(9,9-Diethyl-9H-fluorene-2,7-diyl)bis(methylene)bis(4,5-
diphenyl-2-(pyren-1-yl)-1H-imidazole) (4c). The title compound
was synthesized by following similar procedure described for 4a by
using 2,7-bis(bromomethyl)-9,9-diethyl-9H-fluorene (0.41 g,
1 mmol) and 4,5-diphenyl-2-(pyren-1-yl)-1H-imidazole (0.92 g,
2.2 mmol). The crude product was purified by flash column chro-
matography (10% EtOAc/CH2Cl2) to give light-yellow solid (0.72 g,
66%); Rf (5% EtOAc/CH2Cl2) 0.28; mp 220e222 ꢀC; IR (KBr) 3056,
2956, 2927, 2857,1599,1466,1418,1342,1322,1248,1132,1069, 974,
819, 769, 748, 698 cmꢂ1
; dH (500.13 MHz, CDCl3) 8.30 (2H, d, J 9.5 Hz),
8.24 (4H, d, J 7.5 Hz), 8.13e8.17 (8H, m), 8.03e8.08 (4H, m), 7.73 (4H,
d, J 8.0 Hz), 7.37e7.41 (10H, m), 7.29 (4H, t, J 7.5 Hz), 7.18e7.22 (4H,
m), 6.49 (2H, d, J 8.0 Hz), 6.33 (2H, s), 5.02 (4H, s),1.48 (4H, q, J 7.5 Hz),
ꢂ0.19 (6H, t, J 7.5 Hz); dC (125.77 MHz, CDCl3) 149.9, 147.2, 140.1,
138.3,136.0,134.6,132.0,131.3,131.2,131.2,130.9,130.7,129.8,129.0,
128.8, 128.7, 128.4, 128.4, 128.2, 127.3, 126.9, 126.5, 126.3, 125.7,
125.6, 125.4, 125.1, 124.9, 124.8, 124.5, 120.7, 119.4, 55.7, 48.6, 32.2,
8.1; dC DEPT 45 (125.77 MHz, CDCl3) 131.2,129.0,128.8,128.69,128.40,
128.36, 128.2, 127.3, 126.9, 126.5, 126.3, 125.7, 125.6, 125.1, 124.8,
124.5, 120.7, 119.4, 48.6, 32.2, 8.1; HRMS (ESI): MNaþ, found
1109.4565. C81H58N4Na requires 1109.4559.
4.2. Synthesis and characterization
Compounds 2,7-bis(bromomethyl)-9,9-diethyl-9H-fluorene (2)
and 2,4,7-tris(bromomethyl)-9,9-diethyl-9H-fluorene (5) were
synthesized by following reported procedures.24
4.2.4. 1,10,100-(9,9-Diethyl-9H-fluorene-2,4,7-triyl)tris(methylene)
tris(2,4,5-triphenyl-1H-imidazole) (6a). The title compound was
synthesized by following similar procedure described for 4a by
using a mixture of 2,5,7-tris(bromomethyl)-9,9-diethyl-9H-fluo-
rene (0.50 g, 1 mmol), 2,4,5-triphenyl-1H-imidazole (0.98 g,
3.3 mmol), BTEAC (0.30 g), 10 mL of 50% sodium hydroxide, and
5 mL of benzene. The crude product was purified by flash column
chromatography (30% EtOAc/CH2Cl2) to give cream colored solid
(0.71 g, 62%); Rf (10% EtOAc/CH2Cl2) 0.19; mp 180e182 ꢀC; IR (KBr)
3056, 2928, 2864, 1599, 1498, 1456, 1379, 1259, 1135, 832, 741,
4.2.1. 1,10-(9,9-Diethyl-9H-fluorene-2,7-diyl)bis(methylene)bis(2,4,5-
triphenyl-1H-imidazole) (4a). A mixture of 2,7-bis(bromomethyl)-
9,9-diethyl-9H-fluorene (0.41 g, 1 mmol), 2,4,5-triphenyl-1H-imid-
azole (0.65 g, 2.2 mmol), BTEAC (0.20 g), 10 mL of 50% sodium hy-
droxide, and 5 mL of benzene was refluxed for 24 h. Aftercompletion
of the reaction, the reaction mixture was poured into beaker con-
taining hot water. It was allowed to stand overnight in a hood. The
solid formed was extracted with CH2Cl2, dried over Na2SO4, and
purified by flash column chromatography on silica gel (10% EtOAc/
CH2Cl2) to give the title compound as white solid (0.45 g, 54%); Rf (5%
EtOAc/CH2Cl2) 0.48; mp 194e196 ꢀC; IR (KBr) 3058, 2926, 2860,
692 cmꢂ1
; dH (500.13 MHz, CDCl3) 7.62e7.64 (2H, m), 7.55e7.57
(2H, m), 7.48e7.51 (4H, m), 7.45e7.47 (2H, m), 7.36e7.41 (5H, m),
7.33e7.35 (1H, m), 7.27e7.31 (2H, m), 7.18e7.23 (12H, m), 7.10e7.17
(12H, m), 7.06e7.09 (2H, m), 7.00e7.02 (2H, m), 6.68 (1H, dd. J
8.0 Hz, 1.5 Hz), 6.62 (1H, s), 6.5e6.51 (2H, m), 5.33 (2H, s), 5.13 (2H,
s), 5.09 (2H, s), 1.67e1.72 (4H, m), ꢂ0.07 (6H, t, J 7.5 Hz); dC
(125.77 MHz, CDCl3) 151.0, 150.7, 148.1, 147.7, 139.9, 138.5, 138.4,
138.3, 137.1, 136.5, 136.3, 134.4, 134.3, 133.4, 131.1, 131.01, 130.97,
130.9, 130.9, 130.7, 130.02, 129.99, 129.04, 129.01, 128.93, 128.85,
128.8, 128.7, 128.61, 128.58, 128.5, 128.4, 128.1, 128.0, 127.0, 126.93,
126.85, 126.44, 126.40, 126.3, 124.9, 122.7, 122.1, 120.6, 119.7, 55.6,
1596, 1491, 1451, 1376, 1255, 1129, 837, 746, 698 cmꢂ1
; dH
(500.13 MHz, CDCl3) 7.70e7.71 (4H, m), 7.62 (4H, d, J 8.5 Hz), 7.49
(2H, d, J 8.0 Hz), 7.29e7.40 (16H, m), 7.24 (4H, t, J 7.5 Hz), 7.16e7.18
(2H, m), 6.82 (2H, d, J 8.0 Hz), 6.71 (2H, s), 5.19 (4H, s), 1.80 (4H, q, J
7.0 Hz), 0.13 (6H, t, J 7.0 Hz); dC (125.77 MHz, CDCl3) 150.3, 148.1,
140.4, 138.1, 136.7, 134.5, 131.1, 131.0, 130.1, 129.1, 129.0, 128.9, 128.7,
128.6, 128.1, 126.8, 126.4, 124.8, 120.5, 119.9, 56.0, 48.5, 32.6, 8.3; dC
(125.77 MHz, CDCl3) 131.1, 129.1, 129.0, 128.9, 128.7, 128.6,
DEPT 45
128.1, 126.8, 126.4, 124.8, 120.5, 119.9, 48.5, 32.6, 8.3; HRMS (ESI):
MNaþ, found 861.3926. C61H50N4Na (MþNa) requires 861.3933.
48.3, 46.3, 32.7, 8.1; dC
(125.77 MHz, CDCl3) 131.1, 131.0,
DEPT 45
130.9, 129.04, 129.01, 128.94, 128.86, 128.8, 128.7, 128.61, 128.58,
128.5,128.4,128.1,128.0,127.96,126.93,126.9, 126.44,126.40,126.3,
124.9, 122.7, 122.1, 120.6, 119.7, 48.3, 46.3, 32.7, 8.1; HRMS (ESI):
MNaþ, found 1169.5262. C83H66N6Na requires 1169.5247.
4.2.2. 1,10-(9,9-Diethyl-9H-fluorene-2,7-diyl)bis(methylene)bis(2-
(anthracen-9-yl)-4,5-diphenyl-1H-imidazole) (4b). The title com-
pound was synthesized by following similar procedure described
for 4a by using 2,7-bis(bromomethyl)-9,9-diethyl-9H-fluorene
(0.41 g, 1 mmol) and 2-(anthracen-9-yl)-4,5-diphenyl-1H-imidaz-
ole (0.87 g, 2.2 mmol). The crude product was purified by flash
4.2.5. 1,10,100-(9,9-Diethyl-9H-fluorene-2,4,7-triyl)tris(methylene)
tris(2-(anthracen-9-yl)-4,5-diphenyl-1H-imidazole) (6b). The title

D. Kumar et al. / Tetrahedron 69 (2013) 2594e2602
2601
compound was synthesized by following similar procedure de-
scribed for 6a using a mixture of 2,5,7-tris(bromomethyl)-9,9-
diethyl-9H-fluorene (0.50 g, 1 mmol) and 2-(anthracen-9-yl)-4,5-
diphenyl-1H-imidazole (1.31 g, 3.3 mmol). The crude product was
purified by flash column chromatography (25% EtOAc/CH2Cl2) to
give yellow solid (1.13 g, 78%); Rf (10% EtOAc/CH2Cl2) 0.28; mp
233e235 ꢀC; IR (KBr) 3065, 2959, 2932, 2869,1603,1491,1417,1375,
9-yl)biphenyl (CBP), were presented and deposited by spin-coating
at 2500 rpm for 20 s, TPBI layer was deposited on it. Subsequently,
lithium fluoride and aluminum cathode were thermally evaporated
at 1.0ꢁ10ꢂ5 Torr.
Acknowledgements
1259, 1122, 820, 757, 692 cmꢂ1
; dH (500.13 MHz, CDCl3) 8.55 (1H, s),
This work was financially supported by a grant from Council of
Scientific Industrial Research (01(2480)/11/EMR-II).
8.36 (1H, s), 8.01e8.03 (2H, m), 7.84e7.88 (4H, m), 7.76 (1H, s),
7.68e7.72 (6H, m), 7.61 (2H, d, J 8.5 Hz), 7.28e7.47 (22H, m),
7.11e7.24 (14H, m), 6.88 (2H, t, J 7.5 Hz), 6.79 (2H, t, J 7.5 Hz), 6.19
(1H, s), 5.98 (1H, s), 5.87 (1H, d, J 8.0 Hz), 5.74 (1H, d, J 8.0 Hz), 5.51
(1H, s), 4.76 (2H, s), 4.58 (2H, s), 4.51 (2H, s), 0.89e0.95 (4H, m),
ꢂ0.86 (6H, t, J 7.5 Hz); dC (125.77 MHz, CDCl3) 150.0, 149.8, 145.2,
144.9, 144.8, 138.6, 138.5, 138.2, 138.0, 135.5, 135.3, 134.6, 134.49,
134.46, 134.4, 132.1, 131.6, 131.43, 131.38, 131.36, 131.3, 131.24,
131.22, 131.19, 131.1, 131.0, 130.9, 130.8, 130.4, 130.1, 129.5, 129.41,
129.36, 129.14, 129.07, 128.9, 128.8, 128.7, 128.64, 128.55, 128.31,
128.28, 127.1, 126.90, 126.89, 126.8, 126.6, 126.5, 126.0, 125.9, 125.7,
125.5, 125.4, 125.2, 125.0, 124.8, 124.41, 124.35, 124.0, 120.9, 120.4,
118.6, 54.2, 48.6, 48.2, 44.6, 32.0, 7.9; dC DEPT 45 (125.77 MHz, CDCl3)
131.4, 131.24, 131.22, 129.41, 129.36, 129.14, 129.07, 128.9, 128.8,
128.7, 128.64, 128.56, 128.31, 128.28, 127.1, 126.90, 126.89, 126.8,
126.60, 126.56, 126.5, 126.0, 125.9, 125.7, 125.5, 125.4, 125.2, 125.0,
124.8, 122.6, 120.9, 120.4, 118.62, 48.6, 48.2, 44.6, 32.0, 7.9; HRMS
(ESI): MNaþ, found 1470.6237. C107H78N6Na requires 1470.6219.
Supplementary data
Absorption and emission spectra of the dyes recorded in dif-
ferent solvents, NMR (1H and 13C) spectra are provided in Supple-
mentary data. Supplementary data associated with this article can
be found in the online version, at http://dx.doi.org/10.1016/
j.tet.2013.01.046. These data include MOL files and InChiKeys of the
most important compounds described in this article.
References and notes
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4.2.6. 1,10,100-(9,9-Diethyl-9H-fluorene-2,4,7-triyl)tris(methylene)
tris(4,5-diphenyl-2-(pyren-1-yl)-1H-imidazole) (6c). The title com-
pound was synthesized by following similar procedure described for
6a using a mixture of 2,5,7-tris(bromomethyl)-9,9-diethyl-9H-flu-
orene (0.50 g, 1 mmol) and 4,5-diphenyl-2-(pyren-1-yl)-1H-imid-
azole (1.39 g, 3.3 mmol). The crude product was purified by flash
column chromatography (30% EtOAc/CH2Cl2) to give cream colored
solid (1.05 g, 69%); Rf (10% EtOAc/CH2Cl2) 0.25; mp 210e212 ꢀC; IR
(KBr) 3055, 2954, 2925, 2853, 1598, 1461, 1419, 1340, 1329, 1241,
1139, 1061, 979, 829, 772, 751, 691 cmꢂ1
; dH (500.13 MHz, CDCl3)
8.09e8.25 (8H, m), 7.99e8.06 (7H, m), 7.84e7.95 (7H, m), 7.79 (1H, t,
J 7.5 Hz), 7.54e7.74 (12H, m), 7.28e7.31 (3H, m), 7.07e7.25 (19H, m),
6.59 (1H, d, J 8.0 Hz), 6.42 (1H, s), 5.97e6.00 (2H, m), 5.81 (1H, s), 5.08
(2H, s), 4.86 (2H, s), 4.74 (2H, s), 1.01e1.08 (4H, m), ꢂ0.84 (6H, t, J
8.0 Hz); dC (125.77 MHz, CDCl3) 150.0, 147.6, 147.3, 147.23, 139.17,
138.6, 138.4, 136.3, 136.1, 135.4, 134.79, 134.77, 132.7, 132.08, 132.05,
131.8, 131.33, 131.30, 131.17, 131.15, 131.1, 131.0, 130.93, 130.86, 130.8,
130.7, 130.41, 130.36, 130.2, 129.9, 129.4, 129.2, 129.1, 128.9, 128.8,
128.6,128.5,128.4,128.1,127.6, 127.4, 127.2, 127.1,126.7,126.5, 126.3,
126.1, 126.0, 125.83, 125.76, 125.64, 125.57, 125.5, 125.4, 125.3, 125.1,
125.0, 124.8, 124.7, 124.6, 124.3, 122.1, 122.0, 120.3, 119.6, 54.8, 48.7,
48.3, 45.6, 32.2, 7.6; dC DEPT 45 (125.77 MHz, CDCl3) 131.2, 131.1, 131.01,
129.95, 129.4, 129.2, 129.1, 129.0, 128.93, 128.88, 128.8, 128.6, 128.5,
128.4,128.3,128.1, 127.6, 127.4, 127.2, 127.1, 126.9, 126.8, 126.7,126.5,
126.3, 126.1, 126.0, 125.83, 125.76,125.64, 125.57, 125.5, 125.3, 124.8,
124.73, 124.65, 122.1, 122.0, 120.3, 119.6, 48.7, 48.3, 45.6, 32.2, 7.8;
HRMS (ESI) MNaþ, found 1542.6229. C113H78N6Na requires
1542.6219.
4.3. OLED device fabrication and characterization
The OLED device was fabricated on the pre-cleaned glass sub-
strate and composed of a 125 nm layer indium tin oxide as anode,
35 nm poly(3,4-ethylene-dioxythiophene)epoly-(styrenesulfo-
nate) (PEDOT:PSS) as hole-injection layer (HIL), emissive layer
(EML), TPBI as electron-transporting layer, a 0.7 nm LiF electron
injection layer (EIL), a 150 nm Al layer as cathode. The aqueous
solution of PEDOT:PSS was spin coated at 4000 rpm for 20 s to form
a 40 nm HIL layer. A series of EMLs, doped in 4,40-bis(9H-carbazol-
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