Optical properties of pyrene and anthracene containing imidazoles: Experimental and theoretical investigations

Article abstract of DOI:10.1016/j.jphotochem.2010.12.018

A series of anthracene and pyrene containing imidazole derivatives have been synthesized and characterized by spectroscopy. Optical absorption and emission properties of the imidazole derivatives were examined in different solvents of varying polarity. Th

Full text of DOI:10.1016/j.jphotochem.2010.12.018

Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 162–173
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Optical properties of pyrene and anthracene containing imidazoles:
Experimental and theoretical investigations
Dhirendra Kumar, K.R. Justin Thomas∗
Organic Materials Lab, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of anthracene and pyrene containing imidazole derivatives have been synthesized and char-
acterized by spectroscopy. Optical absorption and emission properties of the imidazole derivatives
were examined in different solvents of varying polarity. The absorption profile remained unchanged on
increasing the solvent polarity, while the emission maxima red-shifted progressively. The solvatochromic
shifts of the emission spectra were correlated with ET(30), orientation polarizability and Kamlet–Taft. A
better correlation obtained for the Kamlet–Taft treatment suggested the involvement of non-specific
interactions in the excited state. The TDDFT calculation results suggested a negligible contribution of
charge transfer to the optical properties and more planar excited states. The pyrene containing imida-
zoles showed higher decomposition temperature when compared to the analogous compounds bearing
anthracene. Electrochemical data revealed that the pyrene based imidazoles are electron rich than the
corresponding anthracene analogs as indicated by their facile oxidation.
Received 27 September 2010
Received in revised form 2 December 2010
Accepted 27 December 2010
Available online 31 December 2010
Keywords:
Imidazole
Pyrene
Anthracene
Emission
Solvatochromism
TDDFT
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
the emission characteristics of otherwise non-emissive imidazole
core.
In recent years, organic heterocyclic compounds such as pyri-
dine, oxadiazole, pyrazine and imidazole have received immense
attention owing to their potential use in molecular designs tar-
geting biological and electronic applications. Imidazole is quite
attractive due to their biological activities [1–3] and sensor
applications [4–8]. Fused imidazole derivatives such as benzoim-
idazoles and phenanthroimidazoles [9–13] have also been used
in the fabrication of light-emitting devices, employing them as
electron-transporting layer and as sensitizers in dye-sensitized
solar cells [14–18]. Imidazoles when conjugated with the triary-
lamines served as effective ambipolar materials [11]. Functional
imidazoles have been mainly developed due to their wide optical
absorption, bright luminescence and bipolar transport character-
istics. There is a growing interest in the design and synthesis of
imidazoles enriched with functional chromophores to enhance
their optical and charge transport properties. It is also a fun-
damental academic curiosity to study the optical properties of
the functional hybrids to understand the interaction between the
different chromophores present in the molecule. Keeping this
in mind we have synthesized imidazoles containing anthracene
and pyrene segments. It is our hypothesis that incorporation of
fluorophoric groups such as anthracene or pyrene will enhance
Imidazoles containing polyaromatic hydrocarbon hybrids are
interesting due to the fact that excellent emission and ther-
mal properties of the polyaromatic hydrocarbon will add to the
electron-transporting ability of the imidazole segment and result
in multi-functional materials. Polyaromatic segments such as
anthracene and pyrene were widely used in the development
of functional materials displaying bright emission and unique
charge transporting properties [19–25]. Incorporation of pyrene
or anthracene in molecular materials has been demonstrated to
improve the optical and thermal properties. Pyrene and anthracene
segments due to their planar and rigid architecture were prone
to facilitate ␲–␲ stacking and enhance charge migration in solid
state. In this paper, we describe the steady state optical properties
of a series of imidazoles along with their electrochemical and ther-
mal properties. These derivatives are emitting in the blue region
and display marked thermal stability. Blue-emitting materials are
attractive for application in OLEDs as hosts for phosphorescent
materials.
2. Experimental
2.1. Reagents and instruments
All the chemicals were commercially available and they were
used without further purification. All the solvents were dried
using standard methods prior to use. 1H NMR and 13C NMR
∗
Corresponding author. Tel.: +91 1332 285376; fax: +91 1332 286202.
E-mail address: krjt8fcy@iitr.ernet.in (K.R.J. Thomas).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.12.018

D. Kumar, K.R.J. Thomas / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 162–173
163
were recorded on a Bruker AV 500 O FT-NMR spectrometer.
Deuterated chloroform (CDCl3), acetone-d6 or dimethylsulfox-
ide (DMSO-d6) was used as solvent and tetramethylsilane
(TMS) as internal standard. UV–Vis spectra were recorded at
room temperature in quartz cuvettes using Shimadzu UV-1800
spectrophotometer for cyclohexane, toluene, dichloromethane
and N,N-dimethylformamide (DMF), acetonitrile and methanol
solutions. Fluorescence spectra were obtained from Shimadzu
RF-5301-PC spectrofluorophotometer for cyclohexane, toluene,
dichloromethane, N,N-dimethylformamide (DMF), acetonitrile and
methanol solutions. Quantum yield of the dyes were calculated by
following standard procedure and using Coumarin-1 (˚F = 0.99 in
ethyl acetate) or Coumarin-6 (˚F = 0.78 in ethanol) as reference.
Corrections due to dye absorption and refractive indices of the sol-
vents used for measurement were incorporated in the calculation.
TGA-DTA-DTG measurements were performed on Perkin-Elmer
(Pyris Diamond) at a heating rate of 10 ◦C/min under a flow of nitro-
gen. The electrochemical properties were investigated by cyclic
voltammetry (CV) and differential pulse voltammetry (DPV) in
CH2Cl2 and DMF (2c) by using 0.1 M tetrabutylammonium hex-
afluorophosphate as supporting electrolyte. The experiments were
performed at room temperature with a three-electrode cell con-
sisting of a platinum wire as auxiliary electrode, a non-aqueous
Ag/AgNO3 reference electrode and a glassy carbon working elec-
trode.
was purified by recrystallization with dichloromethane and
hexane.
2.2.2.1. 2-(10-Bromoanthracen-9-yl)-1-butyl-4,5-diphenyl-1H-
imidazole (3b). Orange crystalline solid, yield 84%. Recently it has
been reported that the title compound can also be synthesized
by the reaction of 2-(anthracen-9-yl)-1-butyl-4,5-diphenyl-1H-
imidazole with Br2 in CS2 [26]. 1H NMR (CDCl3, 500 MHz): ı 8.63
(d, J = 8.5 Hz, 2 H), 7.78 (d, J = 8.5 Hz, 2 H), 7.62–7.65 (m, 4 H),
7.50–7.57 (m, 7 H), 7.21–7.24 (m, 2 H), 7.15–7.17 (m, 1 H), 3.45
(t, J = 7.5 Hz, 2 H), 1.00 (quin, J = 7.5 Hz, 2 H), 0.68 (sext, J = 7.5 Hz,
2 H), 0.26 (t, J = 7.5 Hz, 3 H). 13C NMR (CDCl3, 125 MHz): ı 142.64,
136.88, 133.43, 131.51, 130.35, 130.00, 129.22, 128.24, 127.99,
127.61, 126.97, 126.11, 125.78, 125.60, 125.25, 125.18, 124.90,
124.33, 43.27, 31.20, 18.08, 11.73.
2.2.2.2. 1-Butyl-4,5-diphenyl-2-(pyren-1-yl)-1H-imidazole
(3c).
Pale yellow crystalline solid, yield 65%. 1H NMR (CDCl3, 500 MHz):
ı 8.29 (d, J = 7.5 Hz, 1 H), 8.21–8.26 (m, 3 H), 8.13–8.18 (m, 4
H), 8.04–8.08 (m, 1 H), 7.64–7.66 (m, 2 H), 7.49–7.56 (m, 5 H),
7.22–7.26 (m, 3 H), 7.15–7.18 (m, 1 H), 3.75 (t, J = 7.5 Hz, 2 H), 1.13
(quin, J = 7.5 Hz, 2 H), 0.77 (sext, J = 7.5 Hz, 2 H), 0.37 (t, J = 7.5 Hz, 3
H). 13C NMR (CDCl3, 125 MHz): ı 146.39, 137.58, 134.43, 131.66,
131.39, 130.98, 130.78, 130.64, 130.48, 129.11, 128.83, 128.36,
128.24, 128.03, 128.00, 127.82, 127.08, 126.58, 125.98, 125.94,
125.72, 125.34, 125.26, 124.69, 124.59, 124.28, 44.30, 32.15, 18.95,
12.81. HR MS (ESI) m/z calcd. for C35H28N2: 476.2252; Found:
476.2231.
2.2. Synthesis
The molecular structure and synthetic routes employed to
obtain the imidazole derivatives are shown in Scheme 1. The parent
imidazoles 2-(anthracen-9-yl)-4,5-diphenyl-1H-imidazole (2a),
4,5-diphenyl-2-(pyren-1-yl)-1H-imidazole (2c) and 2-(anthracen-
9-yl)-1-butyl-4,5-diphenyl-1H-imidazole (3a) were prepared by
reported methods [26,27]. The NMR spectral data matched well
with the literature data. Analytical data of the hitherto unknown
compounds are compiled here.
2.2.3. General procedure for the synthesis of
4,5-diphenyl-1,2-disubstituted imidazoles
A
mixture of 1,2-bis(4-methoxyphenyl)ethane-1,2-dione
(2.70 g, 10 mmol), the corresponding aldehyde (11 mmol), suitable
amine (15 mmol), ammonium acetate (1.54 g, 20 mmol) and
acetic acid (20 mL) was refluxed for 48 h. After the completion
of the reaction, it was quenched by pouring into ice water. The
precipitated solid was filtered and thoroughly washed with water.
It was purified by column chromatography on silica gel using
dichloromethane-hexane mixture as eluant. Analytically pure
samples were obtained by crystallization from dichloromethane
and ethanol mixture.
2.2.1. General procedure for synthesis of
4,5-diphenyl-2-substituted-1H-imidazoles
A mixture of benzil (2.10 g, 10 mmol), corresponding aldehyde
(10 mmol), ammonium acetate (3.08 g, 40 mmol) and acetic acid
(20 mL) was heated to reflux for 48 h. After the completion of the
reaction, it was poured into ice-cold water to obtain a tan solid. It
was recrystallized from dichloromethane and ethanol mixture to
yield the analytically pure product.
2.2.3.1. 2-(Anthracen-9-yl)-4,5-bis(4-methoxyphenyl)-1-p-tolyl-
1H-imidazole (4a). Light yellow powder, yield 72%. 1H NMR
(CDCl3, 400 MHz): ı 8.41 (s, 1 H), 7.91–7.93 (m, 2 H), 7.82–7.84 (m,
2 H), 7.62–7.65 (m, 2 H), 7.36–7.43 (m, 4 H), 7.16–7.19 (m, 2 H),
6.79–6.82 (m, 4 H), 6.67 (d, J = 8 Hz, 2 H), 6.55 (d, J = 8 Hz, 2 H), 3.78
(s, 6 H), 1.95 (s, 3 H). 13C NMR (CDCl3, 125 MHz): ı 159.10, 158.38,
144.69, 137.86, 137.27, 133.82, 132.24, 132.29, 131.05, 128.88,
128.78, 128.69, 128.57, 128.34, 127.67, 126.84, 126.30, 126.26,
125.64, 125.15, 123.32, 113.97, 113.64, 55.24, 55.15, 20.82. HR MS
(ESI) m/z calcd. for C38H30N2O2: 546.2307; Found: 546.2286.
2.2.1.1. 2-(10-Bromoanthracen-9-yl)-4,5-diphenyl-1H-imidazole
(2b). Yellow powder, yield 83%. 1H NMR (acetone-d6, 500 MHz): ı
11.96 (s, 1 H), 8.61 (d, J = 9 Hz, 2 H), 8.06 (d, J = 9 Hz, 2 H), 7.68–7.79
(m, 6 H), 7.58–7.62 (m, 2 H), 7.38 (bs, 6 H). 13C NMR (acetone-d6,
125 MHz): ı 142.91, 137.73, 135.68, 132.15, 131.52, 130.12, 128.77,
128.17, 128.12, 127.76, 127.66, 127.57, 127.38, 127.00, 126.70,
126.60, 123.76. HR MS (ESI) m/z calcd. for C29H19BrN2: 474.0732;
Found: 474.0721.
2.2.3.2. 2-(Anthracen-9-yl)-1,4,5-tris(4-methoxyphenyl)-1H-
imidazole (4b). Cream powder, yield 63%. 1H NMR (CDCl3,
400 MHz): ı 8.42 (s, 1 H), 7.92–7.94 (m, 2 H), 7.81–7.83 (m, 2
H), 7.62–7.64 (m, 2 H), 7.37–7.43 (m, 4 H), 7.16–7.19 (m, 2 H),
6.79–6.82 (m, 4 H), 6.70–6.71 (m, 2 H), 6.26–6.29 (m, 2 H), 3.78 (s,
6 H), 3.47 (s, 3 H). 13C NMR (CDCl3, 125 MHz): ı 159.09, 158.37,
144.80, 137.77, 132.31, 132.28, 131.06, 129.27, 128.88, 128.69,
128.54, 128.36, 128.17, 127.66, 126.26, 125.60, 125.15, 123.29,
113.97, 113.63, 113.40, 55.24, 55.16, 54.99. HR MS (ESI) m/z calcd.
for C38H30N2O3: 562.2256; Found: 562.2242.
2.2.2. General procedure for the butylation of
4,5-diphenyl-2-substituted-1H-imidazoles
A
mixture
of
4,5-diphenyl-2-substituted-1H-imidazole
(8 mmol), 1-bromobutane (1.64 g, 12 mmol), BTEAC (0.50 g),
15 mL of 50% sodium hydroxide and benzene (5 mL) (CAUTION:
Benzene is a known carcinogen) was refluxed for 36 h. After a
clear organic layer formed, the reaction mixture was poured into
beaker containing hot water. It was allowed to stand overnight in a
hood. The solid formed was collected by filtration and thoroughly
wash with liberal amounts of water. Finally, the solid residue
2.2.3.3. 1,4,5-Tris(4-methoxyphenyl)-2-(pyren-1-yl)-1H-imidazole
(4c). Colorless solid, yield 74%. 1H NMR (CDCl3, 400 MHz): ı 8.46

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D. Kumar, K.R.J. Thomas / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 162–173
Scheme 1. Synthesis of the fluorophore containing imidazoles.
(d, J = 9.2 Hz, 1 H), 8.16–8.18 (m, 2 H), 8.04–8.08 (m, 2 H), 7.98–8.01
(m, 3 H), 7.82 (d, J = 7.9 Hz, 1 H), 7.61–7.64 (m, 2 H), 7.13–7.16
(m, 2 H), 6.79–6.84 (m, 6 H), 6.44–6.48 (m, 2 H), 3.79 (s, 6 H),
3.56 (s, 3 H). 13C NMR (CDCl3, 125 MHz): ı 159.15, 158.48, 158.40,
146.33, 137.74, 132.37, 131.41, 131.20, 130.95, 130.87, 129.69,
129.20, 129.01, 128.75, 128.58, 128.06, 127.62, 127.29, 126.04,
125.85, 125.66, 125.33, 124.79, 124.51, 124.02, 123.30, 113.97,
113.65, 55.23, 55.17, 55.12. HR MS (ESI) m/z calcd. for C40H30N2O3:
586.2256; Found: 586.2244.
3. Results and discussion
3.1. Synthesis and electronic spectra
The synthetic procedures employed for obtaining the fluo-
rophore enriched imidazole derivatives are outlined in Scheme 1.
The three-component reaction involving a diketone, an aldehyde
and a nitrogen source (ammonium acetate) was performed to
obtain the parent imidazoles which were alkylated subsequently by
a phase transfer catalytic method. For obtaining the N-aryl deriva-
tives, we used the four-component reaction including an additional
nitrogen source in the form of aromatic amine besides ammonium
acetate. The reaction was smooth with less sterically demand-
ing aniline derivatives; however no N-aryl derivatives could be
obtained when using 1-naphthylamine or 9-aminoanthracene.
Later reactions proceeded to yield the parent NH-imidazoles
(2a–2c) rather than the expected N-arylated derivatives. The imi-
dazole derivatives containing pyrene or anthracene are yellow in
color. The N-butylated (3a–3c) and N-arylated (4a–4c) derivatives
are soluble in common organic solvents, while the unsubstituted
imidazoles (2a–2c) are highly soluble in DMF and methanol only.
UV–Vis absorption spectra of the compounds were recorded
in a series of solvents of different polarities viz. cyclohexane,
toluene, dichloromethane, N,N-dimethylformamide, acetonitrile
and methanol at the concentration of 2 × 10−5 M; corresponding
data are presented in Table 1. Due to poor solubility, quantita-
tive data for 2c was gathered in DMF and methanol and for other
solvents only qualitative data are presented.
2.3. Theoretical investigations
The ground state geometry of the compounds at the gas phase
were optimized using the density functional theory method with
the B3LYP functional in conjugation with the basis set 6-31G(d,p) as
implemented in the Gaussian 09 package [28]. The default options
for the tight self-consistent field (SCF) convergence and threshold
limits in the optimization were used. The electronic transitions
were calculated using the time-dependent DFT (B3LYP) theory
and the 6-31G (d,p) basis set. At least 10 excited states were
calculated for each molecule. Excited-state structures were opti-
mized using configuration interaction singles (CIS) at the same
level of theory and fluorescence energies were obtained by using
TDDFT by following this procedure. Firstly, the geometry of each
imidazole derivative was optimized at its first singlet excited
state using CIS/B3LYP/6-31G(d,p). Second, the electronic transi-
tions were calculated by using the CIS optimized excited-state
geometry and following TDDFT/B3LYP/6-31G(d,p). Even though
the properties calculated for the geometry optimized using a
different theory may not be accurate, the trends may be used
for meaningful insights. Calculations of the vibrational modes
revealed no imaginary frequencies for the final equilibrium geome-
tries.
The absorption spectra of the imidazole derivatives recorded in
N,N-dimethylformamide are displayed in Fig. 1. All the derivatives
display a complex absorption profile characteristic of the pres-
ence of multiple chromophores. In general, the anthracene based
imidazoles exhibit red-shifted absorption when compared to the

D. Kumar, K.R.J. Thomas / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 162–173
165
Table 1
Absorption properties of imidazole derivatives containing anthracene and pyrene.
Compound
ꢀabs, nm (ε, ×103 M−1 cm−1
)
Cyclohexane
Toluene
DCM
DMF
ACN
Methanol
2a
3a
2b
3b
4a
365 (9.0), 384
(10.9)
348 (5.9), 365 (9.2),
384 (9.9)
377 (10.5), 397
(13.1)
358 (6.4), 377
(10.7), 397 (12.1)
346 (6.3), 364 (9.3),
384 (9.5)
350.5 (5.3), 367.5
(8.4), 387 (9.7)
351 (5.5), 368 (8.6),
387 (8.8)
361 (5.3), 379.5
(9.2), 400 (10.9)
360.5 (6.2), 379
(10.5), 400 (11.4)
349.5 (6.0), 367.5
(8.9), 387.5 (8.8)
368 (9.2), 386.5
(10.4)
349 (5.5), 368.5
(8.5), 387.5 (9.9)
351.5 (6.1), 368
(9.3), 387.5 (9.2)
360.5 (5.2), 379
(8.7), 399 (10.6)
359.5 (6.5), 379
(10.8), 400 (11.7)
349.5 (6.5), 368
(9.6), 387 (9.3)
365.5 (8.7), 384
(9.7)
349 (6.1), 366 (9.2),
385 (9.0)
358 (5.9), 376
(10.0), 396 (11.7)
357.5 (7.5), 376.5
(12.2), 397 (13.0)
332 (3.6), 348.5
(6.3), 366 (9.4), 385
(9.0)
348 (6.3), 364 (9.6),
383 (12.4)
349 (6.8), 366
350.5 (6.5), 368
(9.8), 387 (9.6)
360 (6.3), 378.5
(10.6), 398.5 (12.4)
359.5 (6.8), 378.5
(11.2), 399 (11.9)
350 (6.5), 367.5
(9.6), 387 (9.3)
(10.0), 385 (9.3)
356.5 (6.3), 375
(10.5), 395 (11.4)
357.5 (7.8), 376
(13.1), 397 (13.4)
333 (3.7), 348.5
(6.8), 366 (10.2),
385 (9.4)
4b
346.5 (0.07), 364.5
(0.1), 384.5 (0.09)
349.5 (5.7), 367.5
(8.5), 387 (8.4)
350 (6.0), 368 (8.9),
387 (8.6)
349.5 (6.1), 367.5
(9.0), 387 (8.8)
331.5 (3.5), 348
(5.9), 366 (8.7), 385
(8.4)
333 (3.3), 348 (6.2),
366 (9.2), 385 (8.5)
2c
3c
308, 323, 344, 354a
308 (14.8), 323
(18.0), 342 (23.9)
328 (19.3), 342.5
(18.4), 361.5 (22.3)
377a
372a
328 (20.0), 344
(29.0)
329 (19.3), 344
(23.2)
379 (29.8)
345 (24.9)
371a
357.5 (28.9)
328 (22.0), 342
(33.0)
327 (20.5), 342
(28.1)
327.5 (17.2), 345.5
(24.7)
330 (17.8), 359.5
(20.1)
342 (27.4)
4c
328.5 (18.6), 344
(22.3)
342 (23.5)
a
Extinction coefficient could not be obtained due to poor solubility.
analogous pyrene derivatives. However, the molar extinction coef-
ficients of the pyrene derivatives are significantly higher than those
observed for the anthracene derivatives.
led to a blue-shift for the longer wavelength peak when compared
to that of 2c. Probably, N-alkylation inhibits the conjugation exten-
sion into the imidazole segment such a reduction in conjugation
might blue shift the absorption wavelength [33]. Despite the more
pronounced non-planarity between the imidazole and the pyrene
segments the red-shifted absorption observed for 4c clearly indi-
cate the involvement of the phenyl groups in the extension of
conjugation by delocalization of ␲-electrons when compared to the
N-butyl derivative (3c).
The absorption spectra of the compounds were unaffected by
the solvent polarity which indicates that the interaction of the dyes
with the solvents in the ground state is less significant [34]. A slight
blue shift observed for the imidazoles (2a, 2b and 2c) in ACN and
methanol when compared to those recorded in nonpolar solvents
such as cyclohexane and toluene probably stems from the hydrogen
bonding interactions between the N-H unit and the solvents [35].
The anthracene based imidazoles exhibit absorption spectra fea-
turing ␲–␲* transitions arising from the anthracene segment in
the range 349–400 nm. This band showed vibrational fine struc-
ture characteristic of isolated anthracene chromophore [29,30].
The intensity ratio of the three sharp peaks is typically 1:1.7:1.8.
Another interesting feature is that the incorporation of bromine
on anthracene unit (2b and 3b) leads to a slight red shift and
hyperchromism for the ␲–␲* transition when compared to the
non-brominated analogs (2a and 3a). However, the N-alkylation
of the imidazole unit has no effect on the absorption features of the
anthracene based imidazoles.
Pyrene based imidazoles exhibited broad and comparatively
less structured absorption profiles. Two absorption bands peak-
ing at ∼280 and 328–379 nm were observed. The red-shifted peak
is assigned to have major contribution from the ␲–␲* transition
while the shorter wavelength absorption may arise from a pyrene
localized ␲–␲* transition [31,32]. Interestingly, N-alkylation (3c)
3.2. Emission spectra
Emission spectra of the compounds were measured in
a
series of solvents of different polarities viz. cyclohexane,
toluene, dichloromethane, N,N-dimethylformamide, acetonitrile
and methanol and the relevant parameters are presented in Table 2.
For example, the emission spectra recorded for the compounds
in cyclohexane are displayed in Fig. 2. It may be appropriate to
discuss the emission spectra of the compounds in three different
categories: N-H imidazoles, N-alkyl imidazoles and N-aryl imida-
zoles. Within a class, the nature of the aryl substituent on the C-2
position plays the major role in positioning the emission peak.
Among the anthracene containing imidazoles the N-H imidazoles
(2a and 2b) show the red-shifted emission when compared to the
corresponding N-alkyl (3a and 3b) and N-aryl (4a and 4b) imida-
zoles. On the contrary, no such effect was observed for the pyrene
containing imidazoles (2c and 3c) and they showed the most blue-
shifted emission profile among the series. This is opposite to the
trend observed in the absorption spectra (vide supra). It may be
reasoned that the anthracene containing N-H imidazoles (2a and
2b) in the excited state assumes the more planar structure when
compared to the N-alkyl imidazoles (3a and 3b). Obviously the
attainment of planarity is hindered in the latter compounds due
to the presence of N-alkyl unit. Very large Stokes shifts observed
for the compounds 2a and 2b when compared to 3b and 3c also
supports this suggestion. However, as the pyrene based imidazoles
Fig. 1. Absorption spectra for the imidazole derivatives (2a–2c, 3a–3c and 4a–4c)
recorded in N,N-dimethylformamide solutions.

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D. Kumar, K.R.J. Thomas / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 162–173
Table 2
Emission properties of imidazole derivatives containing anthracene or pyrene segments.
Dye
ꢀem, nm (˚F, %)
Cyclohexane
Toluene
DCM
DMF
ACN
MeOH
Film
a
2a
3a
2b
3b
4a
4b
2c
3c
4c
489 (51)
433, 453 (50)
506 (36)
465 (48)
459 (53)
459 (55)
430 (81)
432 (86)
428,444 (87)
492 (48)
456 (53)
510 (33)
470 (55)
467 (35)
467 (33)
437, 456 (75)
438 (72)
452 (75)
495 (58)
466 (64)
512 (18)
490 (48)
499 (14)
500 (14)
458 (76)
453 (71)
477 (69)
494 (49)
482 (44)
526 (11)
517 (8)
524 (6)
520 (6)
472 (77)
463 (73)
492 (67)
492 (49)
482 (39)
515 (6)
515 (9)
526 (2)
498 (7)
467 (59)
463 (60)
494 (48)
475 (52)
458 (53)
497 (22)
483 (28)
500 (6)
467
a
483
450
502 (6)
441
a
457 (76)
445 (63)
477 (59)
461
470
a
No emission.
substituted derivatives in the excited state may originate from
the more dipolar interactions with the solvents. This attribution
seems reasonable as the N-substituted derivatives (3a, 3b, 4a, and
4b) possess larger dipole moment in the ground state than the
unsubstituted imidazoles (2a and 2b) (vide supra). Interestingly,
all the pyrene derivatives (3a, 3b and 3c) showed a positive solva-
tochromism. These trends were also manifested in the correlation
of Stokes shift with the polarity scale (Table 3). On increasing the
polarity of the solvent the Stokes shift increased monotonously
except for minor variations observed for the DMF and methanol
solutions which may be ascribed to the specific interactions
such as hydrogen bonding or molecular aggregations (see below)
[36].
The emission of the N-H imidazoles (2a, 2b and 2c) is completely
quenched in the solid state and those of N-alkyl derivatives are blue
shifted when compared to those observed in polar solvents. Emis-
sion quenching in the solid state for the N-H imidazoles may be
attributed to the formation molecular assemblies [37] and the blue
shift of the N-alkyl derivatives may originate due to the change in
the dielectric constant of the medium of measurement [38]. For-
mation of aggregated dimers in the excited state is more likely as
the emission intensity of the N-H imidazoles (2a–2c) dramatically
decreased on increasing the concentration in polar solvents such as
dimethylformamide (see for example Fig. 3). The solid state photo-
luminescence maxima of the anthracene based N-aryl derivatives
(4a and 4b) are blue shifted than those observed in cyclohexane
(Figs. 4 and 5 and Table 2) may be attributed to the formation of
H-aggregates.
Fluorescence quantum yield was determined by using
coumarin-1 as reference (˚F = 0.99 in ethyl acetate) [39,40].
In general, the pyrene based imidazoles (2c, 3c and 4c) showed
emission efficiency higher than those of the anthracene based
derivatives (2a, 2b, 3a, 3b, 4a, and 4b). Also, the quantum yield
was significantly affected by the solvent polarity. On increasing
the polarity of the solvent, the quantum efficiency decreased
dramatically except for in methanol. In methanol, all the deriva-
tives displayed reasonable emission intensity which probably
suggests that the emissive states do not involve appreciable charge
transfer.
Fig. 2. Emission spectra for the imidazole derivatives (2a–2c, 3a–3c and 4a–4c)
recorded in cyclohexane.
are already assuming more planar structure (see below theoreti-
cal calculations) structural reorganization is less significant for the
pyrene derivatives. A similar reason can also be assigned for the
red-shifted emission observed for the N-aryl derivative 4c. In keep-
ing with these observations, the Stokes shift calculated for the N-H
imidazoles (2a and 2b) containing anthracene moiety are larger
than those of the corresponding N-alkyl and aryl imidazoles (3a,
3b, 4a and 4b). Very large Stokes shifts generally indicate an exten-
sive structural reorganization in the excited state [35]. While for
the pyrene based imidazoles a reverse order was noticed. Solvent
polarity played no significant role on the emission peak of the N-
H imidazoles containing anthracene unit (2a and 2b). However,
the N-alkyl and aryl derivatives (3a, 3b, 4a, and 4b) exhibited a
positive solvatochromism indicative of a stabilized excited state
in the polar solvents. The difference in the behavior of the N-
Table 3
Stokes shift observed for the imidazole derivatives in different solvents and the parameters used for the correlation.
Solvent
ET(30)
ꢁ*
Orientation polarizability
Stokes shift (cm−1
)
2a
3a
2b
3b
4a
4b
2c
3c
4c
Cyclohexane
Toluene
DCM
DMF
ACN
30.9
33.9
40.7
43.2
45.6
55.4
0.00
0.54
0.82
0.88
0.75
0.60
−0.001
0.013
0.219
0.275
0.306
0.309
5592
5515
5671
5564
5716
5057
2947
3910
4381
5060
5227
4140
5426
5392
5563
6051
5835
5196
3684
3723
4654
5658
5771
4485
4255
4393
5800
6756
6963
5974
4221
4427
5840
6609
5894
6054
4993
3642
5048
4993
5541
6090
6092
6113
6995
6092
7641
6768
4298
5693
8105
4298
8997
8275
MeOH

D. Kumar, K.R.J. Thomas / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 162–173
167
Fig. 3. (a) Emission spectra of 2a recorded in DMF at different concentrations. (b) Variation of emission intensity for 2a–2c in DMF with concentration.
3.3. Solvatochromism
Solvent dependent spectral properties were interpreted with
Lippert–Mataga equation [41–43], ET(30) parameter [44] and
Kamlet–Taft equation [45,46]. Lippert–Mataga equation provides
a measure for the change in the dipole moment upon excitation of
molecule. In this method a plot of Stokes’ shift versus orientation
polarizability (ꢂf) was performed to fit the equation given below
ꢀ
ꢁ
2
ε − 1
n2 − 1 (ꢃE − ꢃG)2
v¯A − v¯F
=
−
+ constant
a3
hc 2ε + 1
2n − 1
where h is Planck’s constant (6.6256 × 10−27 erg), c is the velocity
of light (2.9979 × 1010 cm/s), a is radius of cavity in which molecule
resides, v¯A and v¯F are the wave numbers (cm−1) of the absorption
and emission, respectively; ε is dielectric constant and n is refrac-
tive index of the solvent. The orientation polarizability is defined
as
ꢀ
ꢁ
2
ε − 1
n2 − 1
2n − 1
ꢂf =
−
hc 2ε + 1
Fig. 4. Emission spectra of 4a recorded in series of solvents and film.
Representative plots showing the variation of Stokes shift with ori-
entation polarizability for the compounds 2a, 2c and 4c are shown
in Fig. 6. Anthracene derivatives (2a, 2b, 3a, 3b and 4a) showed
deviations from linearity in methanol while the pyrene derivatives
(2c, 3c and 4c) deviated from linearity in cyclohexane or dimethyl-
formamide. The anomalous behavior of the anthracene containing
dyes in methanol clearly indicates the presence of non-specific
interactions such as hydrogen bonding or molecular aggregation
and it must be noted here that the Lippert–Mataga correlation
does not account for the hydrogen bonding effects and formation
of molecular complexes. Since the ␲–␲ interactions are gener-
ally facilitated in the non-polar solvents such as cyclohexane, the
deviation observed for 2c in cyclohexane may be attributed to the
aggregation of the dye molecules by hydrogen bonding or stacking
interactions is speculated.
The plot of Stokes shifts versus the ET(30) parameter shown
for 2c and 3a in Fig. 7 confirmed the exhibition of positive sol-
vatochromism by the compounds. Again the anthracene based
imidazoles (2a, 2b, 3a, 3b, 4a and 4b) deviated significantly in
methanol while the pyrene containing imidazoles (2c, 3c and 4c)
showed deviation for cyclohexane or methanol and dimethylfor-
mamide. Even though this correlation effectively maps the charge
Fig. 5. Emission spectra recorded for the spin cast films of 3a–3c and 4a–4c.

168
D. Kumar, K.R.J. Thomas / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 162–173
Fig. 6. Lippert–Mataga plots of the dyes (a) 2a, (b) 2c and (c) 4c.
Fig. 7. Correlation of ET(30) parameter with Stokes shift for the compounds (a) 2c and (b) 3a.

D. Kumar, K.R.J. Thomas / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 162–173
169
Fig. 8. Kamlet–Taft plots for the compounds (a) 2b and (b) 2c.
transfer in a molecule with the range of solvents, the polarizabil-
ity in protic solvents is often misinterpreted. For this reason the
Stokes shift observed in methanol or dimethylformamide did not
fall in the straight line. The deviation observed for 2c in cyclohexane
again reiterates the formation of aggregates.
Finally, the emission maxima observed in the different solvents
were plotted against Kamlet–Taft solvent parameter (ꢁ*). Selected
trends observed for 2b and 2c is shown in Fig. 8. The Kamlet–Taft
solvent parameters are the hydrogen bond donation ability ˛,
hydrogen bond acceptor ability ˇ and the dipolarity-polarizability
ꢁ*; the linear equation relating the Kamlet–Taft parameters with
the emission maximum is generalized as
while in 3c and 4c it is due to the hydrogen bonding interactions
with the solvents However, the anthracene containing derivatives
(2a, 2b, 3a, 3b, 4a and 4b) displayed scattering in polar solvents
such as methanol, acetonitrile and dimethylformamide indicating
a molecular aggregation which is not modeled in these correlations.
From the above discussion it is clearly evident that in these
molecules besides the solvent parameters such as polarity and
polarizability the secondary effects such as hydrogen bonding or
molecular aggregation affects the emission properties. Thus, the
non-specific interactions between the dye molecules and the sol-
vents or neighboring molecules significantly alter the relaxation of
molecules in the excited states.
v¯mf ax = 23.36 − 6.70ꢁ∗ − 0.64˛ + 0.54ˇ
3.4. Theoretical studies
Strikingly, the Kamlet–Taft plots showed better linear fit for the
pyrene containing derivatives 2c, 3c and 4c. Because this scale
accounts hydrogen bonding interactions also, it may be safely
assumed that deviations observed for these derivatives in the cor-
relations of orientation polarizability and ET(30) arise mainly due
to hydrogen bonding interactions. As the NH imidazole 2c showed
anomalous behavior for the earlier correlations in cyclohexane it is
suggested to be arising from the intermolecular hydrogen bonding
To gain more insight into the photophysical behavior of
the imidazole derivatives, DFT calculations were performed for
the compounds. The unconstrained geometries of the com-
pounds in gas phase were optimized by DFT using the Becke’s
three-parameter functional hybridized with the Lee–Yang–Parr
correlation functional and the 6-31G(d,p) basis set. Optimized
geometries of the anthracene derivatives (2a, 3a and 4a) showed
Table 4
Calculated inter-planar angles (◦) formed by imidazole unit with other aromatic segments in the optimized geometries of the compounds.
Compound
Ground state (DFT/B3LYP/6-31G(d,p))
Excited state (CIS/B3LYP/6-31G(d,p))
C-Phenyl
C-Phenyl
N-Phenyl
Anthracene/pyrene
C-Phenyl
C-Phenyl
N-Phenyl
Anthracene/pyrene
2a
3a
4b
2c
3c
4c
27.98
20.71
24.05
28.97
20.55
25.43
40.71
62.79
55.39
40.57
68.00
55.68
NA
NA
61.65
NA
NA
51.27
70.23
75.55
28.01
53.94
45.78
38.21
28.70
30.15
40.85
28.46
31.65
41.47
64.77
61.58
34.36
72.78
60.10
NA
NA
64.67
NA
NA
33.26
65.06
69.42
9.28
37.42
30.78
61.81
64.82
Table 5
Computed dipole moments, absorption (TDDFT/B3LYP/6-31G(d,p)) and emission wavelengths (CIS/B3LYP/6-31G(d,p)//TDDFT B3LYP/6-31G(d,p)), oscillator strengths and
Stokes shifts of the compounds.
Compound
ꢃg, Debye
ꢀabs, nm
f
Assignment
ꢃe, Debye
ꢀem, nm
Stokes shift, cm−1
2a
3a
4b
2c
3c
4c
3.20
4.19
4.41
2.99
4.10
4.63
431.6
429.1
458.9
405.1
391.2
420.2
0.20
0.10
0.01
0.64
0.29
0.28
HOMO → LUMO (96%)
2.90
4.07
4.58
2.73
4.00
4.98
520.6
465.1
481.0
445.9
416.3
444.2
3961
1804
1001
2259
1541
1285
HOMO → LUMO (92%) HOMO-1 → LUMO (8%)
HOMO → LUMO (99%)
HOMO → LUMO (95%)
HOMO → LUMO (88%)HOMO-1 → LUMO (10%)
HOMO → LUMO (96%)

170
D. Kumar, K.R.J. Thomas / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 162–173
Fig. 9. Frontier molecular orbitals (HOMO: left; LUMO: right) of the compounds 2a (top), 3a (middle) and 4b (bottom).
significant deviation from co-planarity between the anthracene
and the imidazole rings (Table 4), while in the pyrene derivatives
(2c, 3c and 4c) pyrene unit was less tilted from the imidazole
plane (Table 4). The non-planarity between the imidazole and the
anthracene/pyrene units increased on N-substitution of the imida-
zole. In the excited state N-H imidazoles (2a and 3a) assumed more
planar configuration between the imidazole and the anthracene
or pyrene unit. It is also notable that among the anthracene and
pyrene derivatives (2a, 3c, 3a and 3c), the pyrene containing imi-
dazoles (3a and 3c) formed relatively more planar arrangement
between the imidazole and the aromatic segments. Another inter-
esting feature is the negligible change in the dipole moment for
the dyes in the gaseous excited state. This points that the struc-
ture in the excited state is less polarized when compared to that
in the ground state. This is quite reasonable as there are no triv-
ial donor–acceptor structural elements in the present structures.
However, the positive solvatochromism observed for all the com-
pounds suggests a more polarized excited state. This must result
in a higher dipole moment in the excited state. The disagreement
between the computed dipole moments of the dyes in the gaseous
phase and the observed positive solvatochromism probably points
that the solvation and the induced dipole moments of the dyes play
a major role in dictating the excited state properties of the dyes in
solution phase.
The lowest unoccupied molecular orbital (LUMO) of the com-
pounds were invariably localized on the anthracene or pyrene unit
(Figs. 9 and 10). The highest occupied molecular orbital of the com-
pounds were generally contributed by the entire molecule with the
exception of 4b in which the anthracene unit did not participate in
the LUMO construction. The calculated ground state dipole moment
increased on N-substitution which suggests stronger donor abil-
ity for the imidazole on N-substitution (Table 5). Although the
Table 6
Thermal and electrochemical properties of imidazole derivatives.
Compound
Td (◦C)
Tonset (◦C)a
Eox (V)
HOMO (eV)
E0–0
LUMO (eV)
2a
3a
2b
3b
4a
4b
2c
3c
4c
450
432
422
347
430
430
416
460
473
376
355
318
344
370
343
367
393
428
1.036
1.176
1.084
1.244
1.168, 1.068
1.156, 1.060
0.468
1.132, 0.724
1.012, 0.580
5.836
5.976
5.884
6.044
5.868
5.860
5.268
5.524
5.380
2.86
2.96
2.76
2.86
2.91
2.90
2.96
3.09
2.97
2.976
3.016
3.124
3.184
3.058
3.056
2.308
2.842
2.842
a
Temperature corresponding to 10% weight loss.

D. Kumar, K.R.J. Thomas / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 162–173
171
Fig. 10. Frontier molecular orbitals (HOMO: left; LUMO: right) of the compounds 2c (top), 3c (middle) and 4c (bottom).
trend in the lowest energy electronic excitations experimentally
3.6. Electrochemical study
observed for the compounds are consistent with the calculated
ones, their oscillator strengths and absorption positions vary sig-
nificantly. Such deviations may be arising due to the long range
intermolecular interactions or solvent specific contributions which
cannot be treated effectively by TDDFT alone [47]. The trend in
the emission peak positions calculated by the theory for the com-
pounds 2a and 3a follows grossly the trend exhibited by the
dyes in the solution. But for the compounds 2c and 3c the same
trend predicted by theory for the emission peak is not realized
in cyclohexane. It may be due to the broadening (see Fig. 2) of
the emission profile which led to the incorrect estimation of the
emission position. It may be possible that the poorly resolved
vibronic pattern contributed to the broadening of the emission
profile.
The electrochemical propensity of the derivatives was examined
by using cyclic voltammetric (CV) and differential pulse voltam-
metric (DPV) techniques. The redox potentials were calibrated
using ferrocene as internal standard in each measurement. The data
are collected in Table 6. All the compounds showed one irreversible
oxidation peak. However, 4a, 4b and 3c displayed two irreversible
oxidation peaks and 4c exhibited two quasi-reversible peaks. It
appears that the electron-rich methoxy and pyrene segments
3.5. Thermal studies
The thermal stability of the imidazole derivatives was examined
by thermogravimetric analysis and the relevant data are presented
in Fig. 11 and Table 6. All compounds showed excellent thermal
stability and their thermal decomposition temperatures are in the
range 347–473. The lowest Td is observed for 3d and the highest
for 4c. In general, the pyrene derivatives are more thermally stable
than the corresponding anthracene analogs. Pronounced thermal
stability for the pyrene based organic amorphous materials is well
documented in the literature [48,49]. Among the anthracene imi-
dazoles, the highest thermal stability was observed for the N-H
imidazole (2a). Alkylation (3a and 3b) and arylation (4a and 4c)
diminishes the thermal decomposition temperature slightly. But, in
the case of pyrene based imidazoles the thermal stability assumes
the reverse order 2c < 3c < 4c.
Fig. 11. Thermogravimetric traces of the compounds.

172
D. Kumar, K.R.J. Thomas / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 162–173
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2010.12.018.
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Acknowledgments
We thank the Council of Scientific and Industrial
Research (CSIR), New Delhi for financial support (grant no.
01(2111)/07/EMR-II). DK acknowledges CSIR for the research
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