Tunable single and dual emission behavior of imidazole fluorophores based on D-π-A architecture

Article abstract of DOI:10.1016/j.jphotochem.2014.04.008

A series of three D-π-A molecules containing 4,5-diphenyl-1H-imidazole moiety as the donor (D) with cyanoacetic acid, rhodanine-3-acetic acid and 4-nitrophenyl acetonitrile as the acceptor (A) connected by a biphenyl bridge have been synthesized and characterized. Crystallography of BIC shows C4-phenyl group to be twisted nearly perpendicular to the imidazole unit and the remaining groups are in plane. The absorption spectra of the compounds are dominated by intramolecular charge transitions that arise from the imidazole core to the acceptor groups. The compounds display distinct dual and single emission behavior in different solvent environments, namely locally excited (LE ～ 430 nm) and intramolecular charge transfer (ICT ～ 550 nm). The Lippert-Mataga analysis suggested that the excited state dipole moment of ICT band is higher than the LE band. The influence of general and specific solvent effects on photophysical properties of the fluorophores was discussed with Catala?n four parameter and Kamlet-Taft three parameter solvent scales. These results suggest that the major influence from solvent dipolarity takes part in a prominent role on changing the emission behavior of the compounds. The quantum yield of the compounds ranged between 0.07 and 0.55 in different solvents and it was shown to depend on the substitution pattern most notably that based on acceptor groups. All the compounds exhibit two lifetimes in the singlet excited state corresponding to LE state (short lived) and ICT state (long lived). In addition, the triplet state absorption properties of the compounds are also investigated by transient absorption measurement. The geometric structure and electronic structure of the molecules in the ground state is studied with DFT methods, whereas the energies of the lowest singlet excited states are calculated by employing TD-DFT methods. The results show that most prominent bands in acetonitrile solvent with higher oscillator strength for BIC, BIN and BIR are due to LE.

Full text of DOI:10.1016/j.jphotochem.2014.04.008

Journal of Photochemistry and Photobiology A: Chemistry 284 (2014) 36–48
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Tunable single and dual emission behavior of imidazole fluorophores
based on D-␲-A architecture
N. Nagarajan, G. Velmurugan, P. Venuvanalingam∗, R. Renganathan∗
School of Chemistry, Bharathidasan University, Tiruchirappalli 620024, Tamil Nadu, 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 three D-␲-A molecules containing 4,5-diphenyl-1H-imidazole moiety as the donor (D) with
cyanoacetic acid, rhodanine-3-acetic acid and 4-nitrophenyl acetonitrile as the acceptor (A) connected
by a biphenyl bridge have been synthesized and characterized. Crystallography of BIC shows C4-phenyl
group to be twisted nearly perpendicular to the imidazole unit and the remaining groups are in plane.
The absorption spectra of the compounds are dominated by intramolecular charge transitions that arise
from the imidazole core to the acceptor groups. The compounds display distinct dual and single emission
behavior in different solvent environments, namely locally excited (LE ∼ 430 nm) and intramolecular
charge transfer (ICT ∼ 550 nm). The Lippert–Mataga analysis suggested that the excited state dipole
moment of ICT band is higher than the LE band. The influence of general and specific solvent effects on
photophysical properties of the fluorophores was discussed with Catalán four parameter and Kamlet–Taft
three parameter solvent scales. These results suggest that the major influence from solvent dipolarity
takes part in a prominent role on changing the emission behavior of the compounds. The quantum yield
of the compounds ranged between 0.07 and 0.55 in different solvents and it was shown to depend on the
substitution pattern most notably that based on acceptor groups. All the compounds exhibit two lifetimes
in the singlet excited state corresponding to LE state (short lived) and ICT state (long lived). In addition,
the triplet state absorption properties of the compounds are also investigated by transient absorption
measurement. The geometric structure and electronic structure of the molecules in the ground state is
studied with DFT methods, whereas the energies of the lowest singlet excited states are calculated by
employing TD-DFT methods. The results show that most prominent bands in acetonitrile solvent with
higher oscillator strength for BIC, BIN and BIR are due to LE.
Received 17 December 2013
Received in revised form 3 April 2014
Accepted 10 April 2014
Available online 21 April 2014
Keywords:
Imidazole
Dual emission
ICT
D-␲-A
DFT/TDDFT
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
because of the potential tuning of the material properties by form-
ing highly polar excited states and large charge separation [10]
A great deal of research efforts have been devoted on organic
molecules featuring intramolecular charge transfer (ICT) in both
photochemistry and biochemistry because of their notable applica-
tions in the field of sensors [1–3], molecular electronics, non-linear
optics [4], dye sensitized solar cells [5,6] and pharmacology [7,8].
The electronic coupling between the locally excited (LE) state and
the charge transfer (CT) state results in excited-state electron trans-
fer in organic donor–acceptor compounds which in turn leads to
the increased dipole moment in excited state, large Stokes shift
and pronounced solvatochromic effect [9]. Organic ␲-conjugated
molecules possessing electron donor and acceptor groups exhibit-
ing the phenomenon of dual fluorescence are of special interest
that can be manipulated using external stimuli. Several mod-
els have been studied to explain dual emission bands [11,12]
observed in some electron donor–acceptor compounds. The energy
dissipation of electronically excited state is mainly governed by
hydrogen bonding and solvent polarity [13]. Considerable attention
has been paid toward synthesis of diverse classes of donor–acceptor
type fluorescent materials, which possess brilliant photophysical
behavior, some typical examples being squarins [14], BODIPY [15],
oligophenylenevinylenes [16], fluorescein [17], cyanine [18], rho-
damine [19], pyridine [20], etc.
Imidazole, a biologically active compound, is well known in
the medicinal field [21–24] as a drug with anticancer, antimicro-
bial, antibacterial, antifungal activities and as an antioxidant. In
recent years, researchers have been employing imidazole com-
pounds in optoelectronic (DSSC [25,26], OLEDs [27–29] and NLO
[30]) applications because of its attractive properties. For instance,
∗
Corresponding authors. Tel.: +91 431 2407053; fax: +91 431 2407045.
E-mail address: rrengas@gmail.com (R. Renganathan).
http://dx.doi.org/10.1016/j.jphotochem.2014.04.008
1010-6030/© 2014 Elsevier B.V. All rights reserved.

N. Nagarajan et al. / Journal of Photochemistry and Photobiology A: Chemistry 284 (2014) 36–48
37
it can function both as an electron donor and acceptor. Moreover
the rigidity of imidazole may help to reduce the reorganization
energy during charge transfer and introduction of an auxiliary
electron donor at positions 4 and 5, and its conjugation as well
as electron acceptors at position 2 of imidazole favors the for-
mation of conjugated dipolar compounds. Through appropriate
modification of the conjugated chain, it is possible to tune the
charge-transfer transition of dipolar molecules [31]. Continuous
efforts have been put forward to enhance the performance of
optoelectronic device by using various acceptor groups, viz., car-
boxylic acid [32,33], cyanoacetic acid [34,35], rhodanine-3 acetic
acid [35], 4-nitrophenyl acetonitrile [36]. However, despite this
effort, the influence of these different acceptor groups on its photo-
physical properties is not yet completely understood. Presumably,
these groups are very sensitive to the solvent environments
which drastically changes its absorption and emission behavior. In
view of this, we synthesized 4,5-diphenyl-1H-imidazole coupled
with three different acceptor moieties such as cyanoacetic acid,
rhodanine-3 acetic acid and 4-nitrophenyl acetonitrile, respec-
tively. Interestingly, we observed both dual emission (LE + ICT)
and single emission behavior, which was very sensitive to its sol-
vent environments. Effects of solvents on its excited state were
probed by lifetime measurements. To verify the influence of differ-
ent functional groups, we also synthesized D-␲-D type compound
by replacing acceptor groups with tertiary butyl group and ana-
lyzed its solvatochromic behavior. The general and specific solvent
effects on optical properties of these compounds were discussed
with the help of Lippert–Mataga, Catalán and Kamlet–Taft param-
eters.
where ˚R is the fluorescence quantum yield of reference, I is the
area under emission spectrum, OD is the optical density at the
excitation wavelength and n is the refractive index of solvent. Flu-
orescence lifetime measurements were carried out in a picosecond
time correlated single photon counting (TCSPC) spectrometer. The
excitation source is the tunable Ti-sapphire laser (Tsunami, Spec-
tra Physics, USA). The compounds were excited at their maximum
absorption (ꢀmax) and monitored at longer wavelength emission
(ICT band) in the respective solvents. The fluorescence decay was
analyzed by using the software provided by IBH (DAS-6). Tran-
sient absorbance to detect triplet properties was performed on
air-free solutions if not otherwise specified. The experiments were
carried out using nanosecond laser flash photolysis (applied pho-
tophysics). The third harmonic (355 nm) of a Q-switched Nd:YAG
laser (Quanta-Ray, LAB 150, spectra physics) with 8 ns pulse width
and 150 mJ pulse energy was used to excite the samples. The
transients were probed using a 150 W pulsed Xenon lamp, a Czerny-
Turner monochromator, and Hamamatsu R-928 photomultiplier
tube as detector. The transient signals were captured with an
Agilent infiniium digital storage oscilloscope, and the data were
transferred to the computer for further analysis [38,39].
The multiple linear regression approach of Kamlet–Taft [40] and
Catalán [41] new generalized solvent polarity scale (four parame-
ter) has been used to correlate UV–vis absorption and emission
energies with an index of the solvent dipolarity/polarizability
which is a measure of the solvent’s ability to stabilize a charge or
dipole through nonspecific dielectric interactions (ꢁ* or SP/SdP),
indices of the solvent’s hydrogen-bond donor strength (˛ or SA)
and hydrogen-bond acceptor strength (ˇ or SB) according to Eqs.
(2) and (3):
yA,E = y0 + aꢁ × ꢁ∗ + b˛ × ˛ + cˇ × ˇ (Kamlet–Taft equation)
∗
2. Experimental
(2)
2.1. Reagents and instruments
All the chemicals were commercially available and they were
used without further purification. 1H NMR and 13C NMR were
recorded on a Bruker Avance 400 (400 MHz) NMR spectrom-
eter. Dimethylsulfoxide (DMSO-d6) was used as solvent and
tetramethylsilane (TMS) as internal standard. X-ray crystallo-
yA,E = y0 + aSA × SA + bSB × SB + cSP × SP + dSdP
× SdP (Catalá n equation)
(3)
where yA,E denotes a solvent-dependent physicochemical property
of absorption and emission in a given solvent and y0 the statis-
tical quantity corresponding to the value of the property in the
gas phase. SP (solvent polarisability), SdP (solvent dipolarity), SA
and SB represent independent solvent parameters accounting for
various types of solute–solvent interactions. aSA, bSB, cSP and dSdP
are adjustable coefficients that reflect the sensitivity of physical
property y in a given solvent to the various solvent parameters.
graphic diffraction data were collected on
a Bruker SMART
APEX-II CCD diffractometer at room temperature using graphite-
˚
monochromated Mo K␣ radiation (ꢀ = 0.71073 A). Integration and
cell refinement were carried out using Bruker SAINT. The struc-
tures were solved by direct methods (SHELXS 86/SHELXS 97) and
refined by full-matrix least squares on F2 using SHELXL 97. All
non-hydrogen atoms were refined anisotropically. MERCURY was
used for all graphical representation of the results. CCDC reference
number 854109 contains the supplementary crystallographic data
for the crystal of BIC. UV–vis spectra were recorded at room tem-
perature in quartz cuvettes using JASCO V360 spectrophotometer
for hexane, chloroform (CHCl3), ethylacetate (EtOAc), tetrahy-
drofuran (THF), acetone, acetonitrile (MeCN), methanol (MeOH),
N,N-dimethylformamide (DMF) solutions. Fluorescence spectra
were obtained from PerkinElmer-LS55 Spectrofluorometer in the
above mentioned solvents. Due to poor solubility, qualitative data
were only collected in hexane, CHCl3 and EtOAc solvents. Emis-
sion spectra were recorded by exciting the molecules in their
absorption maximum unless otherwise mentioned. All the exper-
iments were performed at room temperature at a concentration
of 10−5 M. Quantum yield of the dyes was calculated by following
standard procedure using 9,10-diphenylanthracene [37] (˚f = 0.95
in ethanol) as reference by using the following equation:
2.2. Synthesis and characterization
As shown in Scheme 1, all the four compounds BIC, BIR, BIN and
BIT were synthesized in three steps involving imidazole construc-
tion with benzil, ammonium acetate and 4-bromo benzaldehyde
in acetic acid followed by Suzuki–Miyaura cross-coupling reac-
tion catalyzed by Pd[PPh3]4 with 4-formylphenyl boronic acid and
4-tertiary butyl phenyl boronic acid (BIT) followed by Knoeve-
nagel condensation of resulted aldehyde with cyanoacetic acid,
rhodanine-3 acetic acid and 4-nitrophenyl acetonitrile and the
products obtained in moderate to high yields were characterized
well by NMR, MS, and molecular structure of one of the com-
pound (BIC) was confirmed by Single crystal X-ray diffraction
analysis. The structures of synthesized compounds are shown in
Scheme 2.
2.2.1. Synthesis
Is × ODR × n2s
2.2.1.1. 2-(4-Bromophenyl)-4,5-diphenyl-1H-imidazole (1). A mix-
ture of benzil (1.0 g, 4.7 mmol), 4-bromo benzaldehyde (0.87 g,
˚f = ˚R
(1)
IR × ODs × n2R

38
N. Nagarajan et al. / Journal of Photochemistry and Photobiology A: Chemistry 284 (2014) 36–48
Scheme 1. General synthetic route employed for the preparation of target compounds.
4.7 mmol), ammonium acetate (1.44 g, 18.8 mmol) and glacial
acetic acid (30 mL) was heated at 120 ◦C for 3 h. After cooling to
room temperature, the reaction mixture was poured into distilled
water with stirring. The separated solid was filtered off, washed
with water and dried to give the expected product in good yields.
White powder (91%), 1H NMR (DMSO-d6): ı 7.28 (1H, m), 7.30 (2H,
t, 7.2 Hz), 7.37 (1H, m), 7.44 (2H, t, 8 Hz), 7.52 (2H, m), 7.68 (2H, m),
8.02 (2H, m).
2.2.1.3. 2-(4ꢀ-Tert-butylbiphenyl-4-yl)-4,5-diphenyl-1H-imidazole
(BIT). White powder (78%), 1H NMR (DMSO-d6): ı 1.32 (9H, s), 7.28
(1H, t, 7.2 Hz), 7.31 (2H, t, 7.4 Hz), 7.38 (1H, t, 7.2 Hz), 7.45 (2H, t,
7.2 Hz), 7.51 (4H, m), 7.64 (2H, d, 7.6 Hz), 7.67 (2H, d, 8.4 Hz), 7.77
(2H, d, 8.4 Hz), 8.15 (2H, d, 8.4 Hz), 12.72 (1H, S). 13C NMR: 31.04,
34.20, 125.68, 126.17, 126.61, 128.36, 129.11, 139.54, 145.28 and
150.01.
2.2.1.4. Synthesis of BIC, BIN and BIR. A 15 mL acetonitrile solution
of compound 2 (1 mmol), corresponding acceptor groups (1 mmol)
and piperidine (0.1 mmol) was refluxed for 12 h under nitrogen
atmosphere. The formed precipitate was collected by filtration,
washed with acetonitrile and dried under vacuum and then the
solid was stirred in water (10 mL) and aq. HCl (1 M, 10 mL) mix-
ture for 1 h. The solid was collected by filtration and dried under
vacuum.
2.2.1.2. 4ꢀ-(4,5-Diphenyl-1H-imidazol-2-yl)biphenyl-4-
carbaldehyde (2). A solution of compound 1 (1 g, 2.66 mmol),
4-formylphenylboronic acid (0.38 g, 3.2 mmol), Pd(PPh3)4 (0.15 g,
0.13 mmol) and K2CO3 (1.1 g, 7.9 mmol) in THF (50 mL) and water
(10 mL) was heated to reflux in an N2 atmosphere for 12 h. The
solution was cooled to room temperature and extracted with
dichloromethane. The extracts were dried with anhydrous Na2SO4
and concentrated by rotary evaporation. The residue was purified
by column chromatography (petroleum ether:EtOAc, 5:1) to obtain
the pure product. White powder (75%), 1H NMR (DMSO-d6): ı 7.23
(1H, t, 7.6 Hz), 7.31 (2H, t, 7.6 Hz), 7.38 (1H, m), 7.45 (2H, d, 8.8 Hz),
7.45 (2H, t, 7.6 Hz), 7.54 (2H, m), 7.92 (2H, d, 8.8 Hz), 8.02 (4H, t,
7.6 Hz), 8.22 (2H, d, 8.8 Hz), 10.06 (1H, s), 12.8 (1H, s). 13C NMR:
125.78, 127.11, 127.21, 127.38, 127.76, 128.39, 130.13, 130.28,
135.08, 138.11, 144.90, 145.08, 192.60.
2.2.1.5. 2-Cyano-3-[4ꢀ-(4,5-diphenyl-1H-imidazol-2-yl)-biphenyl-4-
yl]-acrylic acid (BIC). Yellow solid (82%), 1H NMR (DMSO-d6): ı 7.32
(2H, d, 7.2 Hz), 7.38 (4H, t, 8 Hz), 7.54 (4H, d, 7.6 Hz), 7.93 (2H, d,
8.4 Hz), 8.01 (2H, d, 8.4 Hz), 8.16 (2H, d, 8.4 Hz), 8.22 (2H, d, 8.4 Hz),
8.38 (1H, s); 13C NMR (DMSO-d6): 103.32, 116.33, 125.79, 127.08,
127.23, 127.77, 128.41, 130.15, 130.60, 131.38, 132.79, 137.94,
Scheme 2. Structures of the synthesized compounds.

N. Nagarajan et al. / Journal of Photochemistry and Photobiology A: Chemistry 284 (2014) 36–48
39
143.56, 144.89, 153.52, 163.35, 147.13. HRMS (ESI) calculated for
31H21N3O2 [M+H]+ 468.1712. Found m/z: 468.1759.
3.2. UV–vis absorption spectra
C
To gain insight into the photophysical processes in these com-
pounds, efforts have been taken to investigate their absorption and
emission behaviors in different solvents. The spectral data of all
the compounds in various solvents are reported in Table 1. From
Fig. 2A, the absorption band at 279 nm in BIT is attributed to the
␲–␲* transition arising from the phenyl groups attached at C4 and
C5 position of imidazole ring. The band at 329 nm is attributed to the
␲–␲* transition arising from biphenyl group attached to C2 posi-
tion [52]. Moreover, the effect of solvent polarity (from nonpolar
to polar) on the absorption behavior is very minimal about 10 nm
(Fig. 2A). On the other hand, replacing tertiary butyl moiety in BIT
with various acceptor moieties results in enhanced bathochromic
shift; in addition, the spectral behavior is very sensitive to sol-
vent polarity. In BIC, tuning of solvent polarity from hexane to
THF results in a large bathochromic shift (Fig. 2B) of about 55 nm.
Conversely, the negative solvatochromism observed in MeOH and
MeCN could be attributed to the hydrogen bonding interaction of
solvent molecules with imidazole ring. This typical behavior can
be ascribed to the hydrogen-bonding interactions, which proba-
bly retard the reorientation of the fluorophore by hindering the
rotation of the aryl unit [53,54]. The highly sensitive peaks around
330–390 nm in different solvents suggest that the band arises
because of intramolecular charge transfer between the donor imid-
azole moiety to the acceptor cyanoacrylic acid. Further, it can be
evidenced from the results obtained from binary solvent absorption
spectral measurements. For instance, upon titration of THF solu-
tion of BIC with water, interestingly we have observed a significant
blue shift of 30 nm (for 20% water) as shown in Fig. 2C. On further
titration, no remarkable change has been observed in the absorp-
tion wavelength. From the observed result, it can be evidenced
that the increasing percentage of water content in the THF solution
induces the hydrogen bonding interaction which may diminish the
electron withdrawing nature of acceptors groups and/or retard the
reorientation of the molecule by hindering the rotation of the aryl
unit.
2.2.1.6. 2-(5-((4ꢀ-(4,5-Diphenyl-1H-imidazol-2-yl)biphenyl-4-
yl)methylene)-4-oxo-2-thioxo thiazolidin-3-yl)acetic acid (BIR).
Yellowish red solid (73%), 1H NMR (DMSO-d6): ı 4.76 (2H, s), 7.40
6H, m), 7.54 (4H, d, 7.6 Hz), 7.80 (2H, d, 7.6 Hz), 7.92 (5H, m), 8.24
(2H, d, 7.6 Hz). 13C NMR: 43.64, 125.83, 127.05, 127.32, 127.77,
128.41, 129.96, 131.31, 132.10, 138.10, 141.39, 145.07, 166.74,
193.00, 209.00, 147.13. HRMS (ESI) calculated for C33H23N3O3S3
[M+H]+ 574.1259, Found m/z: 574.1275.
2.2.1.7. 3-[4ꢀ-(4,5-Diphenyl-1H-imidazol-2-yl)-biphenyl-4-yl]-2-
(4-nitro-phenyl)-acrylonitrile (BIN). Orange solid (76%), 1H NMR
(DMSO-d6): ı 7.24 (1H, d, 7.2 Hz), 7.31 (2H, t, 7.4 Hz), 7.39 (1H, d,
7.2 Hz), 7.45 (2H, t, 7.4 Hz), 7.51 (2H, d, 7.2 Hz), 7.56 (2H, d, 7.2 Hz),
7.92 (2H, d, 8.4 Hz), 7.99 (2H, d, 8.4 Hz), 8.05 (2H, d, 8.8 Hz), 8.12
(2H, d, 8.4 Hz), 8.20 (2H, d, 8.4 Hz), 8.34 (3H, t, 8.8 Hz), 12.79 (1H, s).
13C NMR: 107.72, 117.41, 124.21, 125.70, 126.57, 126.80, 126.84,
127.00, 127.10, 127.79, 127.79, 128.16, 128.35, 128.61, 129.54,
130.02, 130.32, 130.87, 132.18, 135.00, 138.01, 140.03, 142.01,
144.98, 145.61, 147.13. HRMS (ESI) calculated for C36H24N4O2
[M+H]+ 544.1899. Found m/z: 544.1878.
2.2.2. Computational methods
To elucidate the optical and electronic properties of the
molecule, theoretical calculations have been performed on the
imidazole fluorophore. The geometries of the molecule were fully
optimized at the DFT level using the B3LYP [42–45] functional
(Becke’s three-parameter hybrid functional using the LYP corre-
lation functional) at the 6-311G(d,p) basis set. The excited-state
geometries were optimized by the ab initio configuration inter-
action singles method (CIS) [46]. These fully optimized stationary
points were further characterized by harmonic vibrational fre-
quency analysis to ensure that all the structures are minima on
the potential energy surface. The electronic absorption and emis-
sion spectra, both in vacuum and in solvent, were carried out using
the time-dependent density functional theory (TDDFT) [47,48]
method CAM-B3LYP/6-311G(d,p) [49] using the optimized ground
and excited structures, respectively. The solvent effect has been
included by the polarized continuum model (PCM) [50]. The pro-
gram used to perform the calculations was the Gaussian 09W [51].
By the same token, replacement of cyanoacrylic acid in BIC
by other acceptors such as rhodanine-3-acetic acid (BIR) and 4-
nitrophenylacetonitrile (BIN) leads to bathochromic shift of ꢀmax
by 56 nm and 27 nm (SI Figs. 2 and 3) while changing the polarity
of the environment from non-polar to polar. The charge trans-
fer efficiency of the compounds based on ꢀmax follows the order
BIC < BIN < BIR. It could be attributed to the presence of two strong
electron withdrawing groups ( CN and NO2) in BIN and addi-
tional electron withdrawing groups such as ketone and thione
with COOH in BIR results in very high bathochromic shift. The
extinction coefficients of the charge-transfer bands (Table 1) in
these compounds are moderate to high ranging from ∼11,000 to
3. Results and discussion
3.1. Crystal structure
The molecular structure of BIC was determined by single crystal
X-ray crystallography. Figure S1 illustrates the molecular packing
feature of BIC crystal, and the ORTEP view of the crystal is shown
in Fig. 1. Interestingly, the molecule is arranged in a zig-zag like
structure along c axis (Fig. S1). The compound is crystallized in a
monoclinic crystal system in the P21/c space group. The torsion
angle between the imidazole ring and C4-phenyl ring (77.2)◦ sug-
gests that the phenyl ring is more twisted. On the other hand, the
torsional angle values of phenyl ring at C5 (6.6)◦ and C2 (32.0)◦ sug-
gest that the molecular frame work of the phenyl groups (C2 and
C5) are twisted slightly; hence, the ␲-conjugation will not be much
affected (Fig. 1). Similarly, the spacer biphenyl group (42.2)◦ and
the phenyl group attached with acceptor group (21.03)◦ also less
twisted. Hence, the ␲-conjugation from imidazole moiety toward
the acceptor moiety will not be disturbed. Likewise, the bond angles
of benzene ring and five member ring are close to 120◦ and 108◦
suggest that the ␲ electrons in the whole molecule are delocalised.
The relevant crystal data of BIC are presented in SI Table 1.
∼60,000 M−1 cm−1
.
3.3. Fluorescence emission and excitation spectra
More notably, the emission spectra of all the compounds show
remarkably strong solvatochromic red shifts on going from nonpo-
lar hexane to polar DMF (Table 1). It is noticeable that the effect
of polarity of the medium on the emission behavior is more pro-
nounced than that of the absorption behavior. This observation
suggests that the emitting states of the compounds are more polar
than the ground state. This effect is typical for an ICT transition
within the fluorophores [55]. As shown in Fig. 3A, BIC exhib-
ited single emission behavior in hexane, THF, acetone, CHCl3 and
DMF and dual emission in MeOH, EtOAc, MeCN solutions. In hex-
ane, Stokes shift is very small, about 4213 cm−1 which could be
attributed to the LE → S0 transition and the emission behavior is
independent of excitation wavelength. On increasing the polarity

40
N. Nagarajan et al. / Journal of Photochemistry and Photobiology A: Chemistry 284 (2014) 36–48
Fig. 1. X-ray molecular structure of BIC showing thermal ellipsoids of 50% probability. Solvent DMSO also present in the crystal structure.
Table 1
Relevant data from absorption and emission curves of the compounds in solvents with different polarity.
abs
em
ꢀ
nm
ε × 103 M−1 cm−1
ꢀ
nm
ꢂꢃ (cm−1
)
max
max
BIC
BIN
BIR
BIC
BIN
BIR
BIC
BIN
BIR
BIC
BIN
BIR
Hexane
CHCl3
EtOAc
THF
Acetone
MeOH
MeCN
DMF
332
358
382
387
369
357
352
361
–
374
394
398
392
381
386
401
357
398
408
413
408
400
400
406
–
–
–
37.5
54.0
51.9
11.9
43.7
–
–
–
60.4
60.5
–
–
–
–
54.0
12.6
43.4
35.0
46.2
386
443
428, 555
554
–
443
493
494
444, 518
424, 509
436, 528
453, 520
401
445
572
570
432, 506
446, 516
457, 508
532
4213
5359
8159
7789
7869
8927
9469
8470
–
3073
2653
7027
6669
4746
5620
5314
5833
4164
5096
4882
6205
6600
6967
5706
520
415, 524
424, 528
524
40.7
39.9
of the medium, ICT transition is more prominent than the LE tran-
sition which resulted in large red shifted emission. Furthermore,
the emission behavior is also independent of different excitation
wavelengths (SI Figs. 15 and 16). On the contrary, the observed
dual emission behavior, peak intensity and peak position are very
sensitive to the excitation wavelength in MeOH, EtOAc and MeCN
(for example, see SI Figs. 8 and 9). LE transition around 424 nm
is more prominent on higher energy excitation while ICT transi-
tion around 550 nm is more prominent on lower energy excitation.
Comparing the results, the Stokes shift increases on increasing the
polarity of the solvents and decreases rapidly in polar protic sol-
vents. The observed results suggest that the Stokes shift of the
compound in different solvents is governed by three factors: sol-
vent polarity, hydrogen bonding and ICT. It is expected that the
compounds can have both the hydrogen-bonding interaction as
well as ICT in MeOH and MeCN and the latter is more prominent
in other solvents. The emission spectra of BIC in binary solvent
mixture (water in THF) also confirm the above discussions. From
Fig. 3B, we observed significant change on increasing the percent-
age of water in THF solution of BIC. In pure THF, the emission
from ICT has only been observed. On increasing the percentage
of water, the ICT emission around 530 nm was diminished while
the new emission peak around 430 nm was generated. At 20%
of water in THF solutions, the major emission from LE transition
was only observed. The observed result supports the dual emis-
sion behavior is completely governed by the solvent environment.
Subsequently, this remarkable feature has also been evidenced
by comparing the emission behavior of BIT. Fig. 3C depicts the
emission spectra of BIT in different solvents with varying polarity.
Single emission behavior with the independent nature of emis-
sion spectra on different excitation wavelength suggests that the
observed band is attributed to the normal LE → S0 transition. The
bathochromic shift of 26 nm observed on varying polarity of the
solvents can be ascribed to the general solvent effect. Moreover, no
ICT emission was observed which is due to the absence of acceptor
groups.
As evidenced earlier for BIC, BIN also exhibited the excitation
energy dependant dual emission (SI Fig. 10A) and ICT behavior
in MeCN and EtOAc solutions. In DMF, a peak was broadened on
higher energy excitation; however, no distinct dual emission was
observed. The emission spectrum of BIR in different solvents is
shown in SI Fig. 10B. The justification for the dual emission (in
MeOH and MeCN) and ICT behavior of BIR in different solvents
can be correlated with the discussion of BIC and BIN. However,
excluding hexane and CHCl3, the emission behavior is highly
dependent on excitation wavelength in other polar solvents. It
can be attributed to the availability of more number of functional
groups that will enhance the possibility to form more rigid solvation
cage due to hydrogen bonding with solvents. Hence, it can experi-
ence two different local environments in the ground state itself. It
can be analyzed from emission wavelength dependant excitation
spectra measurements. In which, we observed two different exci-
tation spectral behavior on changing the emission wavelength (SI
Fig. 11) from higher energy to lower energy confirms the presence
of two different local environments.
3.4. Optimized geometry and frontier molecular orbital analysis
The optimized ground-state geometries of the synthesized com-
pound are shown in SI Fig. 14. Selected bond lengths compared
with the experimental data are shown in SI Table 2. The C C bond
˚
length in all the compounds is about 1.34–1.48 A, suggests that

N. Nagarajan et al. / Journal of Photochemistry and Photobiology A: Chemistry 284 (2014) 36–48
41
Fig. 2. Normalized absorption spectra of BIT (A) and BIC (B) in solvents with different
polarity. Absorption spectra of BIC in THF–water (C) solvent system.
Fig. 3. Normalized emission spectra of BIC (A) and BIT (C) in solvents with different
polarity. Emission spectra of BIC in THF–water (B) solvent system.
the ␲ electrons are delocalized over the entire molecule. For the
sake of characterizing the optical and electronic properties of the
molecule, it is useful to examine the highest occupied molecular
orbitals (HOMOs), lowest unoccupied molecular orbitals (LUMOs)
and band gaps (HOMO–LUMO gap). The relative ordering of the
HOMO and LUMO orbitals provide a realistic indication in the exci-
tation properties. From Fig. 4, it is interesting to note that the HOMO
of BIC, BIN and BIR is localized on the donor part, while the LUMO is
mainly centered on the acceptor part. On the other hand LUMO+1,
LUMO+2, HOMO−1 orbitals are predominantly localized on the ␲-
spacer and acceptor unit, while HOMO−2 of BIC, BIN is localized
on the donor phenyl part and BIR is an acceptor part. This indicates
that the HOMO → LUMO transition bear a significant ICT charac-
ter and the other absorption bands have dominant contributions
HOMO → LUMO+1 and HOMO → LUMO+2 and are mainly ␲ → ␲*
character. The energy gaps of compounds BIC, BIN and BIR are found
to be 2.7, 2.53 and 2.63 eV, respectively. The donor centered HOMO
levels for compounds BIC, BIN and BIR are observed in the range
of −5.61, −5.62 and −5.55 eV, respectively. The acceptor centered
LUMO levels for them are in the range of −2.87, −3.09 and −2.92 eV.
The increasing order of band gap is as follows: BIN < BIR < BIC.
In addition to the above, the molecular orbitals are often used
to relate the spectral properties of molecules and provide decisive
clues for synthesizing newer molecules. Therefore it is essential
to identify and understand the nature of various segments of
the molecule and their individual contributions toward HOMOs
and LUMOs. Hence the contributions of various fragments of the
molecules have been computed using QMForge [56] program. The
whole molecule has been segmented into three fragments, namely
donor, ␲-spacer and acceptor units and their corresponding per-
centage contributions are summarized in Table 2. As evident from
the molecular orbital diagrams, HOMOs are majorly centralized
by donor (80%) and interestingly LUMOs are majorly stabilized by
acceptor (78%). This indicates that the donor and acceptor parts
in BIC, BIN and BIR are responsible for the intramolecular charge
transfer upon excitation.

42
N. Nagarajan et al. / Journal of Photochemistry and Photobiology A: Chemistry 284 (2014) 36–48
Fig. 4. FMO pictures of BIC, BIN and BIR.
3.5. Computed absorption and emission spectra
and the contributing configurations. The calculated wavelengths
from absorption and emission spectra, excitation wavelength, main
transition configurations and oscillator strengths for the most rel-
evant singlet excited states of BIC, BIN and BIR are summarized in
Tables 3 and 4, respectively.
From Table 3 and the MOs in Fig. 4, the experimental band
found at 352 nm for BIC corresponds to the transition predicted
at 361 nm, and this originates from HOMO → LUMO transition
with ICT character (62%). Another band exhibited at 265 nm
mainly originates from HOMO → LUMO+2, HOMO → LUMO+1 and
HOMO → LUMO, which assigned as ␲ → ␲* transition (37%, 29%
and 18%, respectively). As for BIN, the experimental absorption
band of 386 nm at longer wavelength is of ICT character that cor-
responds to the transition calculated at 371 nm, which originates
from HOMO → LUMO (41%). Another band exhibited at 286 nm
mainly originates from HOMO → LUMO+2, HOMO → LUMO+3 and
To understand electronic transitions of the synthesized com-
pounds, TDDFT calculations on the absorption and emission spectra
in both vacuum and solvent (MeCN) were performed. The elec-
tronic absorption and emission spectra were calculated using the
TD-DFT method to rationalize the nature of electronic transitions
Table 2
Molecular orbital composition (%) in the ground state for BIC, BIN and BIR.
Molecule
HOMO
Donor
LUMO
Donor
␲-Spacer
Acceptor
␲-Spacer
Acceptor
BIC
BIN
BIR
79.89
78.74
75.20
18.61
19.20
20.38
1.49
2.06
4.42
3.41
1.45
2.14
40.56
20.43
25.97
56.02
78.12
71.90

N. Nagarajan et al. / Journal of Photochemistry and Photobiology A: Chemistry 284 (2014) 36–48
43
Table 3
Electronic transition data obtained by TD-DFT (ꢀabs) for BIC, BIN and BIR at the CAM-B3LYP/6-311G(d,p) optimized geometry.
Molecule
States
Expt. ꢀabs (nm)
Cal. ꢀabs (nm)
Oscillator strength f
E (eV)
Major contribution
354.6
1.6077
3.50
HOMO → LUMO (65%)
HOMO−1 → LUMO (20%)
HOMO → LUMO+1 (10%)
HOMO → LUMO+2 (48%)
HOMO → LUMO+1 (22%)
HOMO → LUMO (12%)
HOMO → LUMO (62%)
HOMO−1 → LUMO (24%)
HOMO → LUMO+1 (9%)
HOMO → LUMO+2 (37%)
HOMO → LUMO+1 (29%)
HOMO → LUMO (18%)
Gas-phase
BIC
262.3
361.1
264.8
0.3734
1.7687
0.4644
4.73
3.43
4.68
352
MeCN
366.0
287.7
370.9
286.2
2.0214
0.3405
2.2191
0.3394
3.39
4.31
3.34
4.33
HOMO → LUMO (49%)
HOMO−1 → LUMO (30%)
HOMO → LUMO+1 (9%)
HOMO → LUMO+2 (41%)
HOMO → LUMO (23%)
HOMO → LUMO+3 (9%)
HOMO → LUMO (41%)
HOMO−1 → LUMO (36%)
HOMO → LUMO+1 (11%)
HOMO → LUMO+2 (35%)
HOMO → LUMO+3 (27%)
HOMO → LUMO (21%)
Gas-phase
MeCN
BIN
386
381.0
269.9
1.5968
0.4365
3.25
4.59
HOMO−1 → LUMO (39%)
HOMO → LUMO (37%)
HOMO−2 → LUMO (10%)
HOMO → LUMO+3 (36%)
HOMO → LUMO+1 (21%)
HOMO → LUMO (10%)
HOMO → LUMO+2 (10%)
HOMO−1 → LUMO (50%)
HOMO → LUMO (42%)
HOMO → LUMO+3 (31%)
HOMO → LUMO+1 (23%)
HOMO → LUMO (14%)
Gas-phase
BIR
400
390.8
273.5
1.9800
0.4107
3.17
4.53
MeCN
HOMO → LUMO (35%, 27% and 21%, respectively), which can
be assisted as ␲ → ␲* transition. Similarly, concerning BIR the
experimental band found at 400 nm corresponds to the tran-
sition calculated at 391 nm with larger oscillator strengths is
ICT character of HOMO → LUMO (50%) transition. Another band
exhibited at 273 nm is ␲ → ␲* transition, and originates from
HOMO → LUMO+3, HOMO → LUMO+1 and HOMO → LUMO (31%,
25% and 14%, respectively). Theoretical emission spectra for imid-
azole fluorophores based on optimized excited-state geometries
are presented in Table 4. The emission peaks in MeCN solvent, with
the largest oscillator strength for the compounds BIC, BIN and BIR
are due to LUMO → HOMO transition. The calculated absorption
and emission bands are in good agreement with the experimental
results.
Overall, the DFT and TDDFT calculations reveal deeper insights
into the electronic structures, and optical properties and the nature
of transition of the synthesized molecules are well explored by our
theoretical methods.
3.6. Solvatochromic measurement and dipole moments
The extent of charge separation on electronic excitation of com-
pounds were quantified by measuring the change in the dipole
moment (ꢂꢄ = ꢄe − ꢄg) utilizing the shift between the absorp-
tion and emission maxima ꢂv= (vabs − vem) as a function of solvent
polarity. According to the Lippert–Mataga [57] equation (4).
2
hca3
vabs − vem = ꢂv =
(ꢄe − ꢄg)2ꢂf + Constant
(4)
Table 4
Emission data obtained by TD-DFT for BIC, BIN and BIR at the CIS/6-31G(d,p) optimized geometry.
Molecule
BIC
States
Expt. ꢀem (nm)
Cal. ꢀem (nm)
Oscillator strength f
E (eV)
Major contribution
Gas-phase
414.6
1.9400
2.0606
2.99
LUMO → HOMO (82%)
LUMO → HOMO−1 (10%)
LUMO → HOMO (81%)
LUMO → HOMO−1 (12%)
MeCN
424
432.1
2.87
Gas-phase
MeCN
432.4
447.3
2.2166
2.3241
2.87
2.77
LUMO → HOMO (70%)
LUMO → HOMO−1 (23%)
LUMO → HOMO (69%)
LUMO → HOMO−1 (23%)
BIN
BIR
436
457
Gas-phase
MeCN
468.0
487.0
1.5113
1.6119
2.65
2.55
LUMO → HOMO (71%)
LUMO → HOMO−1 (24%)
LUMO → HOMO (67%)
LUMO → HOMO−1 (28%)

44
N. Nagarajan et al. / Journal of Photochemistry and Photobiology A: Chemistry 284 (2014) 36–48
Table 5
Estimated from Eq. (3) coefficients (yo, aSA, bSB, cSP, dSdP), their correlation coefficients (r) for the multiple linear regression analysis of E(A), E(F) and E(S) of the compounds
as a function of the Catalán four-parameter solvent scale.
y
yo (kcal/mol)
aSA
3.86( 7.34)
13.08( 12.75)
−9.21( 14.24)
bSB
cSP
dSdP
0.39( 5.40)
r
BIC(LE)
Eabs
Eemi
Est
95.03( 15.32)
106.4( 26.61)
12.95( 29.73)
−13.28( 7.14)
−21.26( 12.41)
7.98( 13.86)
−13.77( 23.03)
−49.0( 40.0)
35.23( 44.23)
0.8119
0.8767
0.7045
−3.78( 9.38)
4.17( 10.48)
BIC(ICT)
BIN(LE)
Eemi
Est
66.15( 18.84)
26.94( 10.16)
6.21( 9.03)
−2.34( 4.87)
−20.76( 8.79)
10.59( 28.32)
−24.36( 15.27)
−11.62( 6.64)
0.9359
0.9582
7.48( 4.74)
12.01( 3.58)
Eabs
Eemi
Est
79.35( 1.88)
53.37( 7.41)
25.98( 5.57)
4.75( 0.63)
8.58( 2.49)
−3.83( 1.87)
−8.23( 0.64)
−10.91( 2.54)
2.67( 1.91)
−2.09( 2.27)
3.37( 8.96)
−5.47( 6.74)
−1.59( 0.73)
14.05( 2.90)
−15.64( 2.18)
0.9956
0.9834
0.9853
BIN(ICT)
BIR(LE)
Eemi
Est
54.90( 6.0)
24.44( 4.73)
5.44( 2.01)
−0.69( 1.38)
−7.33( 2.05)
−0.90( 1.41)
12.60( 7.25)
−24.69( 4.99)
−13.06( 2.43)
0.9880
0.9915
11.47( 1.61)
Eabs
Eemi
Est
86.88( 9.75)
98.61( 26.36)
11.73( 23.22)
1.71( 4.67)
7.38( 12.63)
−5.67( 11.12)
−6.51( 4.55)
−31.82( 12.29)
25.30( 10.83)
12.78( 14.66)
−45.30( 39.63)
32.52( 34.90)
−5.60( 3.43)
6.29( 9.29)
−11.89( 8.19)
0.9103
0.8745
0.8316
BIR(ICT)
Eemi
Est
70.24( 18.36)
16.63( 11.98)
5.66( 8.79)
−3.95( 5.74)
−25.20( 8.56)
0.12( 27.59)
−12.90( 18.01)
−5.17( 6.47)
−0.41( 4.22)
0.9285
0.9204
18.68( 5.59)
where
3.7. Multi-linear regression analysis
ε − 1
n2 − 1
Multi-parameter correlation is an excellent and preferable
method to analyze the various factors affecting the photophysical
properties of the compounds and which has been applied suc-
cessfully to various physicochemical parameters [59]. The most
frequently used solvent scales are those of Kamlet–Taft, and
Catalán. The new four-parameter Catalán solvent scale has an
advantage over other solvent scales. Hence, herein the spectral
and photophysical parameters of the compounds studied in all sol-
vents used will be discussed based on the four-parameter Catalán
solvent scale (Table 5). However, the correlations obtained using
Kamlet–Taft solvent scale are also presented (Table 6) and will be
compared with each other.
ꢂf =
−
(5)
2n2 + 1
2ε + 1
where ꢂꢃ is the Stokes shift, ꢄg and ꢄe are ground and excited
state dipole moments, c is the speed of light, h is the Planck’s
constant, and a is the Onsager cavity radius swept out by the fluo-
rophore. A plot of ꢂv versus ꢂf gives ꢂꢄ from the slope. Using
the mole inspiration software, the volume of the compounds is
˚
measured. By using the volume, Onsager cavity radius 4.65 A (BIC),
˚
˚
4.88 A (BIN) and 4.88 A (BIR) has been calculated. From the DFT cal-
culations at the B3LYP/6-311G(d,p) level, the ground state dipole
moment (ꢄg) is obtained. The values of ꢄg for compounds BIC,
BIR and BIN are 6.87, 6.04 and 9.29 D, respectively. The positive
slope of the Lippert–Mataga plot in the solvents studied suggests
that general solvent effects are responsible for the observed posi-
tive solvatochromic shifts in emission. The dipole moment change
on excitation is estimated from the slope of Eq. (4) to be around
19.21, 16.65 and 21.39 D for BIC, BIR and BIN (for ICT band), respec-
tively, which is higher than the ꢄe (LE band) values (15.90, 10.36
and 19.43 D for BIC, BIR and BIN, respectively). Substantially, high
values of ꢄe (ICT) than ꢄe (LE) thus obtained from plot suggest that
the fluorescence states of all these compounds are of strong ICT
character [58].
Table 5 compiles the estimated regression coefficients yo, aSA
,
bSB, cSP, dSdP and the correlation coefficients (r) for the multilinear
regression analysis of the maxima of absorption (Eabs) and emission
(Eemi) of the compounds according to Eq. (3) for all the solvents
studied. Use of the solvent parameters set (SA, SB, SP and SdP) gives
fair fits to Eabs using r as a goodness-of-fit criterion (r = 0.8119 for
BIC, 0.9956 for BIN and 0.9103 for BIR). Moreover, the analysis of
the Eabs data of the compounds to the popular Kamlet–Taft solvent
scales (Table 6) using the (˛, ˇ, ꢁ*) parameters showed fair fits (but
slightly lower than Catalán treatment) as assessed by the values of
r (0.7605 for BIC, 0.6912 for BIN and 0.9172 for BIR).
Table 6
Correlation of E(A), E(F) and E(S) with Taft’s ꢁ* (solvent dipolarity/polarisability), ˛ and ˇ (hydrogen bond donating and accepting ability of the solvent) values based on
multiple linear regression analysis (Kamlet–Taft).
y
yo (kcal/mol)
a˛
4.64( 4.17)
14.19( 7.00)
−9.54( 7.58)
bˇ
cꢁ*
r
BIC(LE)
Eabs
Eemi
Est
84.48( 2.90)
74.70( 4.87)
9.78( 5.27)
−16.11( 11.05)
−31.14( 18.54)
15.03( 20.06)
−1.80( 7.93)
0.7605
0.8562
0.6744
−0.32( 13.30)
2.13( 14.40)
BIC(ICT)
BIN(LE)
Eemi
Est
71.10( 5.16)
13.38( 3.01)
−1.36( 7.42)
−10.17( 19.65)
−5.94( 11.46)
−15.58( 14.10)
0.8244
0.8677
6.01( 4.33)
17.39( 8.22)
Eabs
Eemi
Est
78.65( 4.77)
55.62( 7.27)
23.01( 3.28)
2.66( 2.38)
8.75( 3.63)
−6.09( 1.64)
−6.75( 6.59)
−10.85( 10.04)
4.09( 4.53)
−3.35( 6.74)
17.03( 10.26)
−20.44( 4.64)
0.6912
0.8376
0.9508
BIN(ICT)
BIR(LE)
Eemi
Est
68.58( 8.94)
9.05( 4.32)
−4.05( 4.48)
3.75( 12.39)
−10.51( 5.97)
−20.33( 12.67)
0.7061
0.9083
6.72( 2.16)
16.99( 6.10)
Eabs
Eemi
Est
78.49( 1.60)
70.15( 6.10)
8.34( 5.45)
1.31( 2.30)
12.30( 8.77)
−10.99( 7.84)
−7.25( 6.09)
−34.63( 23.21)
27.37( 20.76)
−6.38( 4.37)
6.31( 16.66)
−12.70( 14.90)
0.9172
0.7529
0.6437
BIR(ICT)
Eemi
Est
69.15( 5.31)
9.33( 4.32)
1.73( 7.64)
−0.42( 6.21)
−18.64( 20.23)
−6.93( 14.52)
0.7761
0.5632
11.38( 16.43)
0.55( 11.79)

N. Nagarajan et al. / Journal of Photochemistry and Photobiology A: Chemistry 284 (2014) 36–48
45
Table 7
Quantum yield (˚f), lifetime (ꢅ), average lifetime (ꢁꢅꢂ), radiative (kr) and nonradiative (knr) rate constants of the compounds in different solvents.
BIC
BIN
THF
BIR
THF
Acetone
MeOH
MeCN
DMF
Acetone
MeOH
MeCN
DMF
THF
Acetone
MeOH
MeCN
DMF
0.79
28
1.97
72
ꢅ1 (ns)
(%)
ꢅ2 (ns)
1.46
21
3.21
79
1.07
2.09
0.49
2.33
2.43
1.39
44
2.48
56
1.19
2.04
0.50
2.45
2.45
1.15
39
4.05
61
1.22
3.59
0.15
0.41
2.36
1.19
42
1.78
58
1.18
1.58
0.40
2.52
3.78
1.12
37
1.16
63
1.19
1.45
0.55
3.77
3.08
1.47
25
2.33
75
0.93
2.18
0.11
0.50
4.08
1.48
16
2.53
84
0.92
2.42
0.18
0.74
3.38
1.08
48
2.95
52
1.08
2.47
0.07
0.28
3.75
0.90
26
2.39
74
1.15
1.59
0.15
0.94
5.32
1.32
48
2.50
52
1.22
1.99
0.17
0.85
4.16
0.85
40
2.01
60
1.00
1.75
0.18
1.02
4.67
1.05
41
2.47
59
1.07
31
3.35
69
0.82
32
2.13
68
˛
1
˛
(%)
2
2
1.10
2.14
0.19
0.88
3.78
1.14
2.40
0.09
0.37
3.78
1.24
1.92
0.17
0.88
4.31
1.21
1.80
0.22
1.21
4.32
ꢁꢅꢂ (ns)
˚
f
kr (108 s−1
)
knr (108 s−1
)
It is striking that the notable solvent-dependant spectral shifts
can be described so well by the new Catalán solvent scales. The
advantage of this generalized treatment of the solvent effect is
that it allows one to split up the relative contribution of dipolarity,
polarizability, acidity and basicity of the medium. By contrast, in
the Kamlet–Taft approach, the solvent parameter ꢁ* contains both
(solvent dipolarity and polarizability) effects, and hence this latter
methodology can never be used to entangle solvent polarizability
and dipolarity effects.
Herein, the Catalán methodology is used to unravel which sol-
vent properties are primarily responsible for the observed spectral
shifts. The estimated high bSB (13.28 for BIC and 6.51 for BIR) and
cSP (13.77 for BIC and 12.78 for BIR) values compared to the aSA
and dSdP in the analysis of Eabs of BIC and BIR indicate that the small
change in Eabs principally reflected a change in basicity and polariz-
ability of the environments. Conversely, for BIN, the change in Eabs
arises not solely from solvent basicity (bSB is 8.23) of the environ-
ment. Similar dependences also observed from the correlation of
(Eabs) with the three-parameter solvent polarity scales, where the
contribution of solvent basicity (Table 6) is higher than the other
parameters.
quantum yields are found to be increased with the increase in
solvent polarity. This clearly indicates a highly polar emissive
state. However, it is decreased in MeOH, which may be due to
protic solvent–solute interactions [60]. ˚f values are decreased
with increase in the strength of acceptor groups. For instance, BIC
showed ˚f values of around 0.40–0.55 exclude MeOH. But, BIN and
BIR showed significant decrease in ˚f values to be 0.09–0.22.
To interpret the photophysical properties in a more intuitive
manner, the fluorescence lifetimes of the compounds were mea-
sured in different solvents. Thus, fluorescence decay traces of the
compounds (Fig. 5A–C) in different solvents were collected as
Next we examined which solvent characteristics account for
the shift of Eemi in LE and ICT band. As observed for Eabs, fair fits
were also found for the multilinear analysis of Eemi (for both LE and
ICT) data of the compounds accordingly to Eq. (3). Among them,
Eemi (ICT) fits much better than Eemi (LE). Indeed, the r values for
Eemi (LE) found were 0.8767 for BIC, 0.9834 for BIN, 0.98745 for
BIR which is higher for Eemi (ICT) like 0.9359 for BIC, 0.9880 for
BIN, 0.9285 for BIR. The Kamlet–Taft treatment has much lower fit
values ranging from 0.7529 to 0.8562 for the studied compounds.
The relatively large bSB and cSP value of BIC and BIR in Eemi (LE)
band reveals that the solvent basicity and polarisability plays a
major role in governing LE band of emission spectra. Conversely, for
Eemi (ICT), remarkable contribution from solvent dipolarity (dSdP
)
implies that the observed large bathochromic shift is due to the
change in the solvent dipolarity of the medium. Obviously, sol-
vent dipolarity plays major role on excited state of the molecules
than the polarizability does, which is a reason for strong red shift
[59]. For BIN, solvent dipolarity (both LE and ICT) solely plays a
major role on shifting the spectral behavior in excited state than
the acidity, basicity and polarisability. As evidenced from Catalán
approach, Kamlet–Taft methodology also revealed that the dipolar-
ity/polarizability (ꢁ*) of the environment is the major factor which
affects Eemi values of the compounds (Table 6).
3.8. Quantum yield and time-resolved fluorescence analysis
Fluorescence quantum yields (˚f) of the compounds were mea-
sured in solvents of different polarity and are presented in Table 7.
Because of high polar nature, the compounds are not completely
soluble in less polar solvents (hexane, CHCl3, EtOAc). Hence, the
quantum yields in these solvents are not calculated. Fluorescence
Fig. 5. Typical fluorescence decay curves of compounds BIC (A), BIN (B) and BIR (C)
associated with lamp profile in different solvents.

46
N. Nagarajan et al. / Journal of Photochemistry and Photobiology A: Chemistry 284 (2014) 36–48
a function of emission wavelength ꢀem using the single photon
counting technique. The full line is the calculated fit assuming an
exponential decay convoluted with the instrument response func-
tion. Simultaneous curve fitting of the time resolved fluorescence
histograms of the compounds as a function of emission wavelength
with ꢅ linked over ꢀem, revealed that the decays are biexponential.
The results of lifetime measurements of the compounds are listed
in Table 7. All the compounds have two lifetimes; among them the
lifetime of major components (52–84%) are long lived one which
ranges from 1.16 to 4.05 ns for BIC, 2.33 to 2.95 ns for BIN and 1.97
to 3.35 ns for BIR, and the lifetime of minor component (16–48%)
are short lived one which ranges from 1.12 to 1.49 ns for BIC, 0.90
to 1.32 ns for BIN and 0.79 to 1.07 ns for BIR. From Table 7, the short
lived lifetime can be assigned to LE transition and the long lived
lifetime can be assigned to ICT transition.
BIR and BIC exhibited two absorption peaks in the transient
spectra among them, and the peak at 450 nm is very stable and
long lived over various delay times. However, the peak at 510 nm
is less stable and disappeared immediately within few delay times.
BIN showed the triplet state absorption peaks at 480 and 530 nm;
conversely the long wavelength peak is more stable toward differ-
ent delay times. Further, the contribution from TSA [64] and GSB
has only been observed in the transient spectra and no contribu-
tion from stimulated emission has been observed at this time scale
[65,66]. The triplet lifetime of the compounds obtained from the
kinetic traces at 510 nm for BIC, 480 nm and 540 for BIN are 0.52,
1.29 and 0.99 ␮s, respectively. Unfortunately, we are unable to cal-
culate the triplet state lifetime of BIR and BIC (at 440 nm) because
of long lifetime of the compounds, which are not decayed within
the measured time domain.
Using the experimental fluorescence quantum yield (˚f) and
averaged lifetime (ꢁ␶ꢂ), the rate constants of radiative (kr) and non-
radiative (knr) decay were calculated according to Eqs. (6) and (7).
5. Conclusions
In summary, a series of D-␲-A architecture based on 4,5-
diphenyl-1H-imidazole moiety as a donor termini connected
to biphenyl groups as central spacer with various acceptor
groups such as cyanoacetic acid, rhodanine-3-acetic acid and 4-
nitrophenyl acetonitrile were synthesized. It was demonstrated
that their absorption properties were governed by intramolecular
charge separation from the donor to the acceptor fragments. Ther-
mal stability of the compounds analyzed by optical measurements
was consistent with the results obtained from thermogravimet-
ric analysis. From Lippert–Mataga treatment, it was proved that
the electronically excited state of the fluorophores is substantially
more polar than the ground state, which was affirmed by the
large solvent-sensitive fluorescence behavior of the compounds.
Structure to spectral response of different acceptor groups was
also explained by relating the results with BIT which does not
have any acceptor groups. Moreover, the origins of the fascinating
photophysical properties were also discussed with four-parameter
Catalan and Kamlet–Taft approaches. The results of multilinear
regression analysis suggested that the emitting state was gov-
erned primarily by solvent dipolarity. The structural effect on
their optical and electrochemical properties is investigated by
DFT, TD-DFT methods and the results give better understanding
of electronic structure and absorption spectra of the synthesized
molecule. The calculated absorption and emission wavelength of
the molecules were relatively in accordance with the experimental
results. Contributions of donor and acceptor units toward the elec-
tronic transitions were explained with the help of Molecular orbital
coefficients analysis. Overall, the studied photophysical proper-
ties of imidazole derivatives are of vast significance in the further
development of novel molecules in connection with predetermined
photophysical properties that will find extensive applications in
research fields of current interests.
˚
ꢅ
f
kr =
(6)
(7)
1 − ˚f
knr
=
ꢅ
From Table 7, BIC has higher ˚f values results almost equal radiative
(kr) and nonradiative (knr) constants. But, BIN has very low ˚f val-
ues results very high nonradiative rate constant than the radiative
rate constant. This can be ascribed to the presence of NO2 group
which enhances the inter system crossing [61] and internal con-
version [62] to the ground state. BIR also exhibits low ˚f values
which can be attributed to the increased nonradiative decay and
solvent–solute interactions.
4. Transient absorption
Having characterized in detail the photophysical properties of
the compounds in singlet excited state, we performed laser flash
photolysis from nano- to the millisecond range in order to obtain
information on triplet excited state dynamics. The transient absorp-
tion spectra of compounds in DMF solution measured at several
delay times from 1 ␮s to 10 ␮s after excitation at 355 nm is shown
in Fig. 6 (BIR) and SI Figs. 12 and 13 (BIC and BIN, respectively).
According to the following Eq. (8), the transient absorption spec-
trum has contributions from the ground state bleaching (GSB),
stimulated emission (SE), excited state absorption (ESA) and triplet
state absorption (TSA) [63].
ꢂA = ꢂAESA + ꢂATSA − ꢂAGSB − ꢂASE
(8)
0.020
0.015
0.010
0.005
0.000
0.2
0.4
0.6
0.8
1
Supporting information
Absorption and emission spectra of BIN and BIR in various sol-
vents, excitation energy dependant emission spectra in various
solvents, transient absorption spectra, TGA, optimized geometry
and crystallographic information data.
1.5
2
350
400
450
500
550
600
650
700
750
Wavelength (nm)
Acknowledgements
-0.005
-0.010
-0.015
N.N. thank DST-INSPIRE program for the financial assistant as
a JRF (IF10495). R.R. and N.N. thank DST, India, for their financial
support in the form of a research grant (Ref. No. SR/S1/PC-12/2011,
dt. 20/09/2011). G.V. thanks University Grants Commission
(UGC), India for the financial support in the form of Science
Fig. 6. Transient absorption spectra of BIR in DMF solution (in various ␮s domains)
after excitation at 355 nm.

N. Nagarajan et al. / Journal of Photochemistry and Photobiology A: Chemistry 284 (2014) 36–48
47
Meritorious Student Fellowship. P.V. thanks UGC for the award
of emeritus fellowship for the Year 2012-13 (Ref. No. F.6-6/2012-
13/EMERITUS-2012-13-GEN-1171/(SA-II). We are thankful to Prof.
P. Ramamurthy for providing TCSPC and Laser flash photolysis
instrumental facilities.
[22] C. Congiu, M.T. Cocco, V. Onnis, Design, synthesis, and in vitro antitumor activity
of new 1,4-diarylimidazole-2-ones and their 2-thione analogues, Bioorg. Med.
Chem. Lett. 18 (2008) 989–993.
[23] L. Nagarapu, A. Satyender, B. Rajashaker, K. Srinivas, P.R. rani, K. Radhika, G.
Subhashini, Synthesis and antimicrobial activity of novel C-linked imidazole
glycoconjugates, Bioorg. Med. Chem. Lett. 18 (2008) 1167–1171.
[24] R.C. Smith, J.C. Reeves, Antioxidant properties of 2-imidazolones and 2-
imidazolthiones, Biochem. Pharmacol. 36 (1987) 1457–1460.
[25] L. Zhou, C. Jia, Z. Wan, Z. Li, J. Bai, L. Zhang, J. Zhang, X. Yao, Triphenylamine-
based organic dyes containing benzimidazole derivatives for dye-sensitized
solar cells, Dyes Pigm. 95 (2012) 743–750.
Appendix A. Supplementary data
[26] D. Kumar, K.R.J. Thomas, C.-P. Lee, K.-C. Ho, Novel pyrenoimidazole-based
organic dyes for dye-sensitized solar cells, Org. Lett. 13 (2011) 2622–2625.
[27] S. Park, J.E. Kwon, S.Y. Park, Strategic emission color tuning of highly fluorescent
imidazole-based excited-state intramolecular proton transfer molecules, Phys.
Chem. Chem. Phys. 14 (2012) 8878–8884.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
jphotochem.2014.04.008.
[28] Y. Yuan, D. Li, X. Zhang, X. Zhao, Y. Liu, J. Zhang, Y. Wang, Phenanthroimidazole-
derivative semiconductors as functional layer in high performance OLEDs, New
J. Chem. 35 (2011) 1534–1540.
References
[29] N. Nagarajan, G. Velmurugan, A. Prakash, N. Shakti, M. Katiyar, P. Venu-
vanalingam, R. Renganathan, Highly emissive luminogens based on imidazo[1
2-a]pyridine for electroluminescent applications, Chem. Asian J. (2013),
http://dx.doi.org/10.1002/asia.201301061.
[30] J. Yu, Y. Cui, J. Gao, Z. Wang, G. Qian, Enhanced optical nonlinearity and
improved transparency of inorganic–organic hybrid materials containing ben-
zimidazole chromophores, J. Phys. Chem. B 113 (2009) 14877–14883.
[31] M. Velusamy, Y.-C. Hsu, J.T. Lin, C.-W. Chang, C.-P. Hsu, 1-Alkyl-1H-imidazole-
based dipolar organic compounds for dye-sensitized solar cells, Chem. Asian J.
5 (2010) 87–96.
[1] Q. Li, M. Peng, H. Li, C. Zhong, L. Zhang, X. Cheng, X. Peng, Q. Wang, J. Qin, Z.
Li, A. New, Turn-on naphthalenedimide-based chemosensor for mercury ions
with high selectivity: successful utilization of the mechanism of twisted intra-
molecular charge transfer, near-IR fluorescence, and cell images, Org. Lett. 14
(2012) 2094–2097.
[2] K. Hanaoka, Y. Muramatsu, Y. Urano, T. Terai, T. Nagano, Design and synthe-
sis of a highly sensitive off–on fluorescent chemosensor for zinc ions utilizing
internal charge transfer, Chem. Eur. J. 16 (2010) 568–572.
[3] L. Xue, C. Liu, H. Jiang, A ratiometric fluorescent sensor with a large Stokes shift
for imaging zinc ions in living cells, Chem. Commun. (2009) 1061–1063.
[4] A.C. Arias, J.D. MacKenzie, I. McCulloch, J. Rivnay, A. Salleo, Materials and appli-
cations for large area electronics: solution-based approaches, Chem. Rev. 110
(2010) 3–24.
[32] C.-H. Chang, Y.-C. Chen, C.-Y. Hsu, H.-H. Chou, J.T. Lin, Squaraine-arylamine
sensitizers for highly efficient p-type dye-sensitized solar cells, Org. Lett. 14
(2012) 4726–4729.
[33] J.-Y. Li, C.-Y. Chen, W.-C. Ho, S.-H. Chen, C.-G. Wu, Unsymmetrical squaraines
incorporating quinoline for near infrared responsive dye-sensitized solar cells,
Org. Lett. 14 (2012) 5420–5423.
[34] Y. Wu, W. Zhu, Organic sensitizers from D-[small pi]-A to D-A-[small pi]-A:
effect of the internal electron-withdrawing units on molecular absorption,
energy levels and photovoltaic performances, Chem. Soc. Rev. 42 (2013)
2039–2058.
[35] M. Liang, J. Chen, Arylamine organic dyes for dye-sensitized solar cells, Chem.
Soc. Rev. 42 (2013) 3453–3488.
[36] M. Singh, R. Kurchania, J.A. Mikroyannidis, S.S. Sharma, G.D. Sharma, An A-D-
A small molecule based on the 3,6-dithienylcarbazole electron donor (D) unit
and nitrophenyl acrylonitrile electron acceptor (A) units for solution processed
organic solar cells, J. Mater. Chem. A 1 (2013) 2297–2306.
[5] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Dye-sensitized solar cells,
Chem. Rev. 110 (2010) 6595–6663.
[6] H. Tian, X. Yang, R. Chen, R. Zhang, A. Hagfeldt, L. Sun, Effect of different dye
baths and dye-structures on the performance of dye-sensitized solar cells based
on triphenylamine dyes, J. Phys. Chem. C 112 (2008) 11023–11033.
[7] D. Srikun, E.W. Miller, D.W. Domaille, C.J. Chang, An ICT-based approach to
ratiometric fluorescence imaging of hydrogen peroxide produced in living cells,
J. Am. Chem. Soc. 130 (2008) 4596–4597.
[8] S. Soldevila, F. Bosca, Photoreactivity of fluoroquinolones: nature of aryl cations
generated in water, Org. Lett. 14 (2012) 3940–3943.
[9] M.A. Ei-Kemary, Photoinduced intramolecular charge transfer of 3-cyano-
4-furyl-6-phenyl-2-(9-anthralylidene)-pyridine, J. Photochem. Photobiol. A:
Chem. 137 (2000) 9–14.
[10] A.H. Younes, L. Zhu, Tunable dual fluorescence of 3-(2 2ꢀ-bipyridyl)-substituted
iminocoumarin, ChemPhysChem 13 (2012) 3827–3835.
[37] J.V. Morris, M.A. Mahaney, J.R. Huber, Fluorescence quantum yield determina-
tions. 9,10-Diphenylanthracene as a reference standard in different solvents, J.
Phys. Chem. 80 (1976) 969–974.
[38] N. Dhenadhayalan, C. Selvaraju, P. Ramamurthy, Photoionization and time-
dependent Stokes shift of coumarin 307 in soft matter: solvation and
[11] F.S. Santos, T.M.H. Costa, V. Stefani, P.F.B. Gonc¸ alves, R.R. Descalzo, E.V.
Benvenutti, F.S. Rodembusch, Synthesis, characterization, and spectroscopic
investigation of benzoxazole conjugated Schiff bases, J. Phys. Chem. A 115
(2011) 13390–13398.
[12] A. Demeter, S.I. Druzhinin, S.A. Kovalenko, T.A. Senyushkina, K.A. Zachari-
asse, Dual fluorescence and ultrafast intramolecular charge transfer with
6-N,N-dialkylaminopurines. A two-state model, J. Phys. Chem. A 115 (2011)
1521–1537.
radical-ion pair recombination dynamics, J. Phys. Chem.
10892–10902.
B 115 (2011)
[39] R. Kumaran, P. Ramamurthy, Denaturation mechanism of BSA by urea deriva-
tives: evidence for hydrogen-bonding mode from fluorescence tools, J. Fluoresc.
21 (2011) 1499–1508.
[40] M.J. Kamlet, J.L.M. Abboud, M.H. Abraham, R.W. Taft, Linear solvation energy
relationships. 23. A comprehensive collection of the solvatochromic param-
eters, pi, alpha, and beta, and some methods for simplifying the generalized
solvatochromic equation, J. Org. Chem. 48 (1983) 2877–2887.
[41] J. Catalán, Toward a generalized treatment of the solvent effect based on four
empirical scales: dipolarity (SdP, a new scale), polarizability (SP), acidity (SA),
and basicity (SB) of the medium, J. Phys. Chem. B 113 (2009) 5951–5960.
[42] A.D. Becke, Density-functional thermochemistry. III. The role of exact exchange,
J. Chem. Phys. 98 (1993) 5648–5652.
[13] N. Mataga, T. Kubota, Molecular Interactions and Electronic Spectra, M. Dekker,
New York, 1970.
[14] K.T. Arun, D.T. Jayaram, R.R. Avirah, D. Ramaiah, ␤-Cyclodextrin as a photosen-
sitizer carrier: effect on photophysical properties and chemical reactivity of
squaraine dyes, J. Phys. Chem. B 115 (2011) 7122–7128.
[15] A.B. Nepomnyashchii, M. Bröring, J. Ahrens, A.J. Bard, Synthesis, photophysical,
electrochemical, and electrogenerated chemiluminescence studies. Multiple
sequential electron transfers in BODIPY monomers, dimers, trimers, and poly-
mer, J. Am. Chem. Soc. 133 (2011) 8633–8645.
[43] J.P. Perdew, Density-functional approximation for the correlation energy of the
inhomogeneous electron gas, Phys. Rev. B 33 (1986) 8822–8824.
[44] A.D. Becke, Density-functional exchange-energy approximation with correct
asymptotic behavior, Phys. Rev. A 38 (1988) 3098–3100.
[16] C. Vijayakumar, V.K. Praveen, K.K. Kartha, A. Ajayaghosh, Excitation
energy migration in oligo(p-phenylenevinylene) based organogels:
structure–property relationship and FRET efficiency, Phys. Chem. Chem.
Phys. 13 (2011) 4942–4949.
[45] C. Lee, W. Yang, R.G. Parr, Development of the Colle–Salvetti correlation-
energy formula into a functional of the electron density, Phys. Rev. B 37 (1988)
785–789.
[46] J.B. Foresman, M. Head-Gordon, J.A. Pople, M.J. Frisch, Toward a systematic
molecular orbital theory for excited states, J. Phys. Chem. 96 (1992) 135–149.
[47] R.E. Stratmann, G.E. Scuseria, M.J. Frisch, An efficient implementation of time-
dependent density-functional theory for the calculation of excitation energies
of large molecules, J. Chem. Phys. 109 (1998) 8218–8224.
[17] T. Egawa, Y. Koide, K. Hanaoka, T. Komatsu, T. Terai, T. Nagano, Development
of a fluorescein analogue, TokyoMagenta, as a novel scaffold for fluorescence
probes in red region, Chem. Commun. 47 (2011) 4162–4164.
[18] K.J. Zanotti, G.L. Silva, Y. Creeger, K.L. Robertson, A.S. Waggoner, P.B. Berget,
B.A. Armitage, Blue fluorescent dye–protein complexes based on fluorogenic
cyanine dyes and single chain antibody fragments, Org. Biomol. Chem. 9 (2011)
1012–1020.
[19] B. Bag, A. Pal, Rhodamine-based probes for metal ion-induced chromo-
/fluorogenic dual signaling and their selectivity towards Hg(ii) ion, Org. Biomol.
Chem. 9 (2011) 4467–4480.
[48] E. Runge, E.K.U. Gross, Density-functional theory for time-dependent systems,
Phys. Rev. Lett. 52 (1984) 997–1000.
[49] T. Yanai, D.P. Tew, N.C. Handy, A new hybrid exchange–correlation functional
using the Coulomb-attenuating method (CAM-B3LYP), Chem. Phys. Lett. 393
(2004) 51–57.
[50] S. Miertusˇ, E. Scrocco, J. Tomasi, Electrostatic interaction of a solute with a con-
tinuum. A direct utilization of AB initio molecular potentials for the prevision
of solvent effects, Chem. Phys. 55 (1981) 117–129.
[20] M. Sarma, T. Chatterjee, S. Ghanta, S.K. Das, D-␲-A-A-␲-D prototype
2,2ꢀ-bipyridine dyads exhibiting large structure and environment-sensitive flu-
orescence: synthesis, photophysics, and computation, J. Org. Chem. 77 (2011)
432–444.
[21] N. Nagarajan, G. Vanitha, D.A. Ananth, A. Rameshkumar, T. Sivasudha, R.
Renganathan, Bioimaging, antibacterial and antifungal properties of imidazole-
pyridine fluorophores: synthesis, characterization and solvatochromism, J.
Photochem. Photobiol. B: Biol. 127 (2013) 212–222.
[51] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato,

48
N. Nagarajan et al. / Journal of Photochemistry and Photobiology A: Chemistry 284 (2014) 36–48
X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada,
[59] K. Guzow, M. Czerwinska, A. Ceszlak, M. Kozarzewska, M. Szabelski, C.
Czaplewski, A. Lukaszewicz, A.A. Kubicki, W. Wiczk, Photophysical properties
M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, T. Vreven, J.A. Montgomery, J.E. Peralta, F. Ogliaro, M. Bearpark,
J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K.
Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega,
J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W.
Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador,
J.J. Dannenberg, S. Dapprich, A.D. Daniels, Farkas, J.B. Foresman, J.V. Ortiz, J.
Cioslowski, D.J. Fox, Gaussian 09, Revision B.01, Gaussian, Inc., Wallingford, CT,
2009.
of 3-[2-(N-phenylcarbazolyl)benzoxazol-5-yl]alanine derivatives
– exper-
imental and theoretical studies, Photochem. Photobiol. Sci. 12 (2013)
284–297.
[60] E. Krystkowiak, K. Dobek, A. Maciejewski, An intermolecular hydrogen-bonding
effect on spectral and photophysical properties of 6-aminocoumarin in protic
solvents, Photochem. Photobiol. Sci. 12 (2013) 446–455.
[61] C.E. Crespo-Hernández, G. Burdzinski, R. Arce, Environmental photochem-
istry of nitro-PAHs: direct observation of ultrafast intersystem crossing in
1-nitropyrene, J. Phys. Chem. A 112 (2008) 6313–6319.
[52] Y. Zhang, S.-L. Lai, Q.-X. Tong, M.-F. Lo, T.-W. Ng, M.-Y. Chan, Z.-C. Wen, J. He,
K.-S. Jeff, X.-L. Tang, W.-M. Liu, C.-C. Ko, P.-F. Wang, C.-S. Lee, High efficiency
nondoped deep-blue organic light emitting devices based on imidazole-␲-
triphenylamine derivatives, Chem. Mater. 24 (2011) 61–70.
[62] V.M. Farztdinov, R. Schanz, S.A. Kovalenko, N.P. Ernsting, Relaxation of opti-
cally excited p-nitroaniline: semiempirical quantum-chemical calculations
compared to femtosecond experimental results, J. Phys. Chem. A 104 (2000)
11486–11496.
[53] D. Kumar, K.R.J. Thomas, C.-C. Lin, J.-H. Jou, Pyrenoimidazole-based deep-
blue-emitting materials: optical, electrochemical, and electroluminescent
characteristics, Chem. Asian J. 8 (2013) 2111–2124.
[54] M. Wächtler, M. Maiuri, D. Brida, J. Popp, S. Rau, G. Cerullo, B. Dietzek, Utiliz-
ing ancillary ligands to optimize the photophysical properties of 4H-imidazole
ruthenium dyes, ChemPhysChem 14 (2013) 2973–2983.
[55] B. Ramachandram, A. Samanta, Modulation of metal-fluorophore communi-
cation to develop structurally simple fluorescent sensors for transition metal
ions, Chem. Commun. (1997) 1037–1038.
[63] H. Imahori, S. Kang, H. Hayashi, M. Haruta, H. Kurata, S. Isoda, S.E. Canton, Y.
Infahsaeng, A. Kathiravan, T.R. Pascher, P. Chábera, A.P. Yartsev, V. Sundstrom,
Photoinduced charge carrier dynamics of Zn–Porphyrin–TiO2 electrodes: the
key role of charge recombination for solar cell performance, J. Phys. Chem. A
115 (2010) 3679–3690.
[64] A.I. Ciuciu, K. Skonieczny, D. Koszelewski, D.T. Gryko, L. Flamigni, Dynam-
ics of intramolecular excited state proton transfer in emission tunable,
highly luminescent imidazole derivatives, J. Phys. Chem.
791–803.
C 117 (2013)
[56] A.L. Tenderholt, QMForge, Version 2.1, Standford University, Standford, CA,
USA, 2007.
[57] D. Kumar, K.R.J. Thomas, Optical properties of pyrene and anthracene con-
taining imidazoles: experimental and theoretical investigations, J. Photochem.
Photobiol. A: Chem. 218 (2011) 162–173.
[65] L. Zedler, S. Kupfer, I.R. de Moraes, M. Wächtler, R. Beckert, M. Schmitt, J. Popp,
S. Rau, B. Dietzek, Trapped in imidazole: how to accumulate multiple photo-
electrons on a black-absorbing ruthenium complex, Chem. Eur. J. 20 (2014)
3793–3799.
[66] M. Wächtler, S. Kupfer, J. Guthmuller, S. Rau, L. González, B. Dietzek, Structural
control of photoinduced dynamics in 4H-imidazole-ruthenium dyes, J. Phys.
Chem. C 116 (2012) 25664–25676.
[58] S.S. Bag, R. Kundu, Installation/modulation of the emission response via click
reaction, J. Org. Chem. 76 (2011) 3348–3356.