Intramolecular charge transfer emission of trans-2-[4′- (dimethylamino)styryl]benzimidazole: Effect of solvent and pH

Article abstract of DOI:10.1016/j.jphotochem.2013.03.012

The intramolecular charge transfer (ICT) exhibiting fluorophore trans-2-[4′-(dimethylamino)styryl]benzimidazole (t-DMASBI) is synthesized and its spectral characteristics are investigated using absorption and fluorescence techniques. The solvatochromic studies reveal that the molecule is emitting from the locally excited state in non-polar and from an ICT state in polar solvents. Photoisomerization competes with fluorescence, and in viscous solvents restriction on twisting of olefinic bond increases the fluorescence. Theoretical calculations predict that t-DMASBI exists in two confomeric forms. trans-B conformer is more stable than trans-A. The cis isomer also exists as two conformers. However, the relative stability of conformer B further increased in cis isomer. The spectral characteristics of both the conformers of trans isomer are nearly same. The longest wavelength transition of trans isomer has ππ* character and is due to HOMO-LUMO single excitation. The theoretical calculated excitation and emission energies of trans conformer differ very little from each other and are in excellent agreement with the absorption and fluorescence spectral maximum in cyclohexane. The prototropic study indicates that unlike other benzazole derivatives t-DMASBI forms only one kind of monocation upon protonation at benzimidazole nitrogen in water. But at lower pH the second protonation occurs at dimethylamino nitrogen to form dication.

Full text of DOI:10.1016/j.jphotochem.2013.03.012

Journal of Photochemistry and Photobiology A: Chemistry 260 (2013) 50–58
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Intramolecular charge transfer emission of
ꢀ
trans-2-[4 -(dimethylamino)styryl]benzimidazole: Effect of solvent and pH
∗
Anasuya Mishra, Soumya Chaterjee, G. Krishnamoorthy
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India
a r t i c l e i n f o
a b s t r a c t
ꢀ
trans-2-[4 -
Article history:
The
(
intramolecular
charge
transfer
(ICT)
exhibiting
fluorophore
Received 13 December 2012
Received in revised form 5 March 2013
Accepted 13 March 2013
dimethylamino)styryl]benzimidazole (t-DMASBI) is synthesized and its spectral characteristics
are investigated using absorption and fluorescence techniques. The solvatochromic studies reveal that
the molecule is emitting from the locally excited state in non-polar and from an ICT state in polar
solvents. Photoisomerization competes with fluorescence, and in viscous solvents restriction on twisting
of olefinic bond increases the fluorescence. Theoretical calculations predict that t-DMASBI exists in
two confomeric forms. trans-B conformer is more stable than trans-A. The cis isomer also exists as two
conformers. However, the relative stability of conformer B further increased in cis isomer. The spectral
characteristics of both the conformers of trans isomer are nearly same. The longest wavelength transition
of trans isomer has ␲␲* character and is due to HOMO-LUMO single excitation. The theoretical calculated
excitation and emission energies of trans conformer differ very little from each other and are in excellent
agreement with the absorption and fluorescence spectral maximum in cyclohexane. The prototropic
study indicates that unlike other benzazole derivatives t-DMASBI forms only one kind of monocation
upon protonation at benzimidazole nitrogen in water. But at lower pH the second protonation occurs at
dimethylamino nitrogen to form dication.
Available online 25 March 2013
Keywords:
Solvatochromism
Push–pull styrene
Prototropic equilibrium
Styrylbenzimidazole
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
Though several aromatic hydrocarbon derivatives based
push–pull aromatic systems are studied, heterocycle based
push–pull aromatic systems are scarily reported [19–24]. Ho et al.
investigated the effect of substitution on the ICT characteris-
tics of styrylheterocycles [19] and suggested that strong donor
N,N-dimethylamino group can lead to ICT on those molecules.
Fayed et al. studied benzothiazole and benzoxazole derived
push–pull styrenes (Chart 1) [21,22] and butadienes [23] with
N,N-dimethylaminophenyl group as donor. They revealed that
benzothiazole and benzoxazole based push–pull systems exhibit
ICT emission and interesting prototropic equilibria. Fayed et al.
established the utility of benzazole derived push–pull buta-
dienes to probe the micelles [25]. On the other hand Saha
Intramolecular charge transfer (ICT) has gained a substantial
attention because of its application in making photoelectronic and
nonlinear optical devices [1,2], chemical sensing [3] and under-
standing photochemical and photobiological processes [4–7]. ICT
property is exhibited by molecules having both donor and acceptor
moieities linked through spacer. ‘Push–pull’ aromatic olefins (D-
␲
-A type molecules) also contain both donor and acceptor linked
through spacer. Owing to their unique structural characteristic,
these styryl dyes show very high first-order hyperpolarizability,
which in turn makes these dyes very efficient materials for appli-
cations in molecular electronics [8,9]. Styryl dyes are also known
to have very good photoelectroconversion efficiency [10,11]. Their
photophysics and photochemistry are also of key interest to many
researchers [12–18]. Decay through twisting of the single bond
linking electron donor and electron acceptor groups result ICT
state formation. On the other hand, the double bond twisting leads
to trans–cis isomerization. These properties widen their potential
application as sensor and probes.
ꢀ
et al. and Purkayastha et al. extend studies on trans-2-[4 -
(dimethylamino)styryl]benzothiazole (t-DMASBT, Chart 1) [26,27]
and elaborated the utility of it as fluorescent probe for various
microheterogeneous systems [28,29]. However, despite the fact
that benzimidazole is also an important member of benzazole
family and it can also act as an acceptor, still now no benizimi-
dazole based push–pull aromatic systems was investigated. It is
found in the literature, the nature of heterocyclic ring affects the
spectral characteristics and the protropic equilibria of benzazole
ꢀ
derivatives. For e.g., in case of 2-(2 -hydroxyphenyl)benzazoles
∗ Corresponding author. Tel.: +91 3612582315; fax: +91 3612584315.
E-mail addresses: gkrishna@iitg.ernet.in,
the excited state proton transfer was shown to dependent on
ꢀ
gkrishna 2000@yahoo.com (G. Krishnamoorthy).
the hetero atom of azole ring [30]. In 4 -aminophenylbenzazoles
1
010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2013.03.012

A. Mishra et al. / Journal of Photochemistry and Photobiology A: Chemistry 260 (2013) 50–58
51
X
N
states because this method includes both long-range correction
and dispersion correction [39–42]. But unfortunately, in Gaussian
03 package it is not possible to perform calculation using ␻B97X-
D method. Therefore, we are not able to test ␻B97X-D method.
However, the literature report also indicates that though for other
system ␻B97X-D results are better than B3LYP results, for 4-N,N-
dimethylaminobenzonitrile (the molecule which also have charge
transfer state with dimethylamino group as donor like the present
system) B3LYP result is nearer to the reference than the ␻B97X-D
result [42]. The excitation energies obtained by B3LYP/6-31G(d,p)
level for t-DMASBI are in good agreement with experimental values
CH
CH
X = S (t-DMASBT)
(t-DMASBO)
= NH (t-DMASBI)
N
=
O
Monocations
X
CH
3
+
N
N
H
CH
CH
3
3
(
see later). Hence, B3LYP/6-31G(d,p) method is functioning good at
X
N
ꢀ
least for trans-2-[4 -(dimethylamino)styryl] benzimidazole. Vibra-
tional frequency analyses were performed to verify the minimum
energy nature of the stationary points. The excitation energies were
obtained by vertical excitations of optimized ground states using
TDDFT/B3LYP/6-31G(d,p) calculations. Since the geometries as well
as molecular properties obtained at the CIS level are quite rea-
sonable and correct, at least as a first approximation for a variety
of molecules [43,44], we optimized the structure of the molecule
in the excited state by CIS method. TDDFT calculations were per-
formed on these optimized geometries to obtain the emission
energies. The computations were performed with a developed ver-
sion of Gaussian 03 [45].
+
N
H
CH
3
ꢀ
Chart 1. Structures of trans-2-[4 -(dimethylamino)styryl]benzazoles and their pos-
sible monocations.
the site of protonation was found to be dependent on azole
ring [31–34]. Therefore, we synthesized and investigated the
photophysics as well as the prototropic equilibrium of trans-2-[4 -
ꢀ
(
dimethylamino)styryl]benzimidazole (t-DMASBI, Chart 1).
3. Results and discussion
2
. Materials and methods
3.1. Absorption spectra
t-DMASBI was synthesized by heating the equimolar mixture of
-N,N-dimethylaminocinnamic acid (Sigma Aldrich Chemicals) and
Absorption spectra of t-DMASBI were recorded in solvents of
4
1
different polarity, viscosity and hydrogen bonding capacities and
the data are complied in Table 1. The absorption spectrum of t-
DMASBI in any given solvent is blue shifted compared to t-DMASBT
,2-diaminobenzene (Sigma Aldrich Chemicals) in polyphoshporic
◦
acid at 220 C for 4 h by following the procedure reported in the lit-
erature for the synthesis of similar molecules [35]. The compound
was first purified by column chromatography followed by thin layer
and t-DMASBO [21,22]. However it is red shifted compared to
ꢀ
2
-[4 -(dimethylamino)phenyl]benzimidazole [33]. The absorption
1
chromatography. The compound was confirmed by H NMR spec-
spectrum is bathochromically shifted with increase in polarity and
hydrogenbondingcapacityof the solvent. But wateris an exception,
where it is blue shifted compared to that in acetonitrile. This can
be explained as follows. The absorption spectrum undergoes a red
shift when hydrogen bonding or protonation occurs at imidazole
nitrogen and a blue shift if hydrogen bonding or protonation occurs
at dimethylamino group [33]. The observed blue shift in water sug-
gests that the reduction in conjugation of nitrogen lone pair by
hydrogen bonding of water with dimethylamino group dominates
over other factors those favor the red shift.
The solvent effects on spectral characteristics are studied by
empirical solvent parameters and are often useful to determine
the predominant interactions. Kamlet Taft solvatochromic method
46] is one of the most preferred approaches. This multi-parametric
approach separates polarity effect from hydrogen bonding effects.
troscopy and mass spectrometry as follows: 1H NMR (400 MHz,
CDCl , ppm) ␦ 7.62 (d, J = 7.6 Hz, 1H); 7.50 (d, J = 16.4 Hz, 1H); 7.43
3
(
d, J = 8 Hz, 1H); 7.17 (d, J = 5.2 Hz, 2H); 7.07 (dd, J = 8, 17.2 Hz, 2H);
6.79 (d, J = 16.4 Hz, 1H), 6.45 (d, J = 8, 2H), 5.73 (brs, 1H), 2.85 (s,
6H). From HRMS the molecular mass (M+1) was found to be m/z:
264.035.
Except glycerol and glycol (AR grade) all other solvents used for
the spectral measurement are of HPLC grade and all the solvents
were procured from Rankem or Merck, India. All the solvents were
further tested for spurious fluorescence in the region of spectral
measurements before use. Millipore water was used for the prepa-
ration of aqueous solutions. The pH of the solution was adjusted by
using dilute solutions of phosphoric acid and potassium hydroxide.
The pH of different solutions were measured using Jenway (model
No 3510) pH meter. The pH meter was calibrated by using three
different standard buffer solutions (pH 4, pH 7 and pH 10) within
a range of ± 0.02 pH units before any measurement. A Varian Cary
[
Table 1
ab
fl
1
00 UV–visible spectrophotometer and a Horiba Jobin Yon Spex
Absorption maxima (ꢁmax, nm) and fluorescence maxima (ꢁmax, nm) and fluores-
cence quantum yield (ꢀf) of t-DMASBI in different solvents.
Fluoromax 4 instrument were used to record the absorption and
1
the fluorescence spectra, respectively. H NMR was recorded using
ab
fl
Solvent
ꢁ
, nm
ꢁ
, nm
ꢀf
max
max
a 400 MHz Varian instrument. The fluorescence quantum yields
were measured at ␭exc 375 nm with respect to quinine sulfate in
Cyclohexane
Dioxane
Ethylacetate
Dimethyl formamide
367
371
369
376
372
374
372
375
384
385
363
399, 421
432
436
457
449
452
459
462
471
0.12
0.18
0.16
0.28
0.13
0.15
0.13
0.08
0.35
0.88
1
N sulfuric acid (ꢀ = 0.55) [36]. For fluorescence measurements the
f
absorbance of the solutions is kept ∼0.1 at the absorption maxima.
The molecular geometries in the ground state were optimized
by employing gradient corrected exchange functional of Becke [37]
and the correlation functional of Lee, Yang and Paar (LYP) [38]
1-Propanol
Ethanol
Acetonitrile
Methanol
Glycol
Glycerol
Water
(
B3LYP) using density functional theory (DFT) method. 6-31G(d,p)
473
490
basis set is used in the calculation. It is reported in the liter-
ature that ␻B97X-D method is suited to deal with the excited

5
2
A. Mishra et al. / Journal of Photochemistry and Photobiology A: Chemistry 260 (2013) 50–58
ꢀ
Fig. 1. Optimized structures of different conformers of trans- and cis-2-[4 -(dimethylamino)styryl]benzimidazole.
The spectral band energy is correlated using the multiple linear
regression analysis approach as shown below:
the hydrogen bonding of the solvent with dimethylamino group
decreases its conjugation thereby destabilize the system. The blue
shift in water compared to methanol is consistent with this fact.
Alike other benzazole analogs, t-DMASBI is also present in two
conformeric forms, trans-A and trans-B and the optimized struc-
tures are shown in Fig. 1. The data were summarized in Table 2. The
relative stability of trans-B is more in t-DMASBI than that of trans-
A (Table 2) and, the barrier for conversion of trans-A to trans-B is
plotted in Fig. 3. But the spectral characteristics of both conform-
ers are very close to each other. The dipole moment of t-DMASBI
∗
E = E + sꢂ + a˛ + bˇ
(1)
0
where ꢂ* is the polarity/polarizability parameter of the solvents, ␣
is an index of hydrogen bond donating ability of the solvent and ␤ is
an index of hydrogen bond accepting capacity of the solvent. The s,
a and b coefficients are measure of the sensitivity to each individual
contributing parameter. A fit for absorption spectral data resulted
in following equation
(
Table 2) is higher than that of benzothiazole analog [26]. The
∗
E = 27440 − 357 ꢂ + 71 ˛ − 722 ˇ
(2)
molecule is nearly planar except the dimethylamino group, which
◦
with a 0.60 regression, but the E0 is in fair agreement with the
experimental value in cyclohexane. The negative values of ‘s’ and
b’ show the stabilization of the electronic state. The positive value
is ∼7 out of the molecular plane. The longest wavelength transi-
tion of t-DMASBI is described by HOMO-LUMO single excitation.
The frontier molecular orbitals (Fig. 2) suggest that the transi-
tion is ␲␲* in nature. The vertical excitation energy obtained is in
‘
of ‘a’ indicates the destabilization and is due to the fact that
Table 2
Optimized parameters for trans- and cis-DMASBI in S0 and S1 states.
Parameters
trans-A
trans-B
S0
S1
S0
S1
Relative Energya (eV)
T. E (nm)
0.0
3.3655
401 (399)
1.5831
7.8
−0.0178
365 (367)
1.4357
5.9
3.3215
406 (399)
1.5024
6.8
b
364 (367)
1.4451
6.3
Oscillator strength (f)
ꢃ (D)
◦
Dihedral angles ( )
ϕ1
ϕ2
ϕ3
ϕ4
7.3
0.8
180
180
0.1
0.4
180
180
7.2
0.9
180
0.21
1.0
0.0
180
0.01
cis-A
cis-B
S0
S1
S0
S1
Relative energya (eV)
T. E (nm)
0.1393
354
3.4298
479
0.1126
372
2.5788
481
b
Oscillator strength (f)
ꢃ (D)
Dihedral angles ( )
0.6202
6.8
0.6554
8.1
0.9199
2.1
0.6163
3.9
◦
ϕ1
ϕ2
ϕ3
ϕ4
9.5
42.3
8.0
4.6
9.6
43.8
4.9
7.2
0.1
0.1
0.1
1.3
9.6
38.8
4.3
9.0
a
With respect to ground state energy of trans-A.
T. E., transition energy and the values in parentheses are experimental value for t-DMASBI in cyclohexane.
b

A. Mishra et al. / Journal of Photochemistry and Photobiology A: Chemistry 260 (2013) 50–58
53
◦
◦
Fig. 2. Optimized structure and the isosurface plot of frontier molecular orbitals obtained by TDDFT calculation for planar (ϕ1 = 0 ) and twisted (ϕ1 = 90 ) conformers of
trans-A (corresponding plots of trans-B conformer is similar and not shown).
excellent agreement with the experimental absorption maxima
obtained in cyclohexane.
state in nonpolar and polar solvents respectively [21,22]. It may
also be noted that the Stokes shift observed in more polar acetoni-
trile is higher than that of protic methanol. This clearly indicates
the dipolar interaction predominate over the hydrogen bonding
interaction and it may be due to ICT nature of the excited state.
Despite the fact that the spectra of t-DMASBI are hypsocromically
shifted compared to corresponding oxazole and thiazole analogs,
the Stokes shifts of benzimidazole analog are comparable to that
of other azoles. Since ICT emission has different orbital origin than
absorption, the dipole moment of the ICT state can be calculated
using fluorescence data from the following equation [5]
3.2. Fluorescence spectra
Fluorescence spectra of t-DMASBI are plotted in Fig. 4 and data
are summarized with absorption spectral data in Table 1. The
geometry of the relaxed excited S1 state was obtained by full opti-
mization of the molecule in the excited state and the data are
presented in Table 2. The emission maxima for both the conform-
ers obtained by TDDFT are agreed well with experimental value in
cyclohexane (Table 2). As expected the molecule is completely pla-
narized in the relaxed excited state. Other than the small reduction
2
ꢃe(ꢃe − ꢃg)
ꢀ
o
f
ꢄ¯ f = −
ꢅf − ꢄ¯
(3)
3
hca
◦
in angle (∼7 ) between the dimethylamino group and other part
where ꢄ¯ f is the fluorescence maximum, ꢃg and ꢃe are the dipole
moments in ground state and excited state respectively, a is
of the molecule, there is no major change in the structure of the
molecule in the relaxed excited state. This is consistent with the
small Stokes shift observed in nonpolar solvents such as cyclohex-
ꢀ
Onsager cavity radius and solvent parameter ꢆf is defined as
−1
ane (3495 cm ). In nonpolar solvents the fluorescence spectrum
is structured. In polar solvents the band is broad and structure-
less. The Stokes shifts observed in polar solvents are also very high
2
ε − 1
n − 1
ꢀ
ꢅf =
−
(4)
2
2
ε + 1
2(2n + 1)
−1
−1
(
5022 cm in methanol and 5095 cm in acetonitrile). All these
where ε and n are solvent dielectric constant and refractive index
feature are same as that of t-DMASBT and t-DMASBO, where the
respectively. Attempt to fit a single straight line gave a poor fit.
molecule is reported to emit from locally excited state and ICT
1.2
1
10
1
0
0
0
0
.8
.6
.4
.2
0
385
435
485
535
585
635
Wavelength (nm)
Fig. 4. Normalized fluorescence spectra of t-DMASBI in (1) cyclohexane, (2) dioxane,
(3) ethylacetate, (4) propanol, (5) ethanol, (6) acetonitrile, (7) dimethylforamide, (8)
methanol, (9) glycol, (10) glycerol (ꢁexc = 375 nm).
Fig. 3. Ground state potential energy surfaces for the conversion of trans-A to trans-
B.

5
4
A. Mishra et al. / Journal of Photochemistry and Photobiology A: Chemistry 260 (2013) 50–58
2
2
2
2
2
2
2
2
2
4000
3500
3000
2500
2000
1500
1000
0500
0000
4.1760 eV
3.5925 eV
S1
3
.1356 eV
0
.4544 eV
0
.3367 eV
0.3751 eV
φ = 90°
1
φ = 90°
φ = 90° S
2
4
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
Fig. 6. Energy level diagram for twisted conformers of trans-A. Similar results have
been determined for trans-B (not shown).
f'
ꢀ
Fig. 5. The plot fluorescence maxima, ꢄ¯ flu against solvent polarity parameter, ꢆf .
the dimethylamino group (ϕ ), the dimethylanilino ring (ϕ ), the
1
2
◦
dimethylaminostyryl ring (ϕ ) to 90 and calculated the molecular
4
energies of the twisted state thus formed. The twisting of dimethy-
However two different plots, one for non polar and other for polar
solvent gave a good fit (Fig. 5). This substantiates the prediction
that depending on the solvent, t-DMASBI emitting from two dif-
ferent states, the locally excited state in non-polar solvent and the
ICT state in polar solvents. The dipole moment calculated for the
ICT state from this solvatochromic plot is 16.7 D and this suggests
that the emitting state may be a twisted ICT state (see later). This
is consistent with Cogan et al. [47] prediction of a larger dipole
moment for the perpendicular geometry of the ICT state than the
planar one of the ICT state. The dipole moment is higher than the
dipole moment calculated for the ICT of t-DMASBT (11.26 D) by
Saha et al. [26]. Therefore, the ICT emission of t-DMASBI has to be
more sensitive to environmental polarity than that of correspond-
ing benzothiazole.
Unlike, DMABN, t-DMASBI emits single ICT emission like its
other benzazole analogs [21,22,26]. Recently Rettig et al. [48]
reported that m-cyano-N-phenylpyrrole emits single emission
from locally excited state in nonpolar solvent and single emission
from TICT emission in polar solvents. They suggested that locally
excited state of m-cyano-N-phenylpyrrole has lower energy than
that of its ICT state in nonpolar medium and the order is reversed
in polar solvents.
lanilino ring (ϕ ) and the dimethylaminostyryl ring (ϕ ) are high
2
3
energy processes compared to that of dimethylamino group (Fig. 6).
The potential energy surface was constructed for the rotation of
dimethylamino group (Fig. 7 and Table 3). The energy of S3 state
decreases and that of the ground and the first two excited states
increases with rotation of donor moiety. The S3 state and S2 state
crosses each other. On the other hand there is an avoided crossing
between S3 and S1 states. The HOMO and LUMO are decoupled at
twisted geometry (Fig. 2).
3.4. Effect of viscosity and photoisomerization
The fluorescence quantum yield of t-DMASBI increases in vis-
cous solvents compared to non viscous solvents (Table 1). In most
viscous glycerol, the fluorescence quantum yield is more than 11
times higher than that of methanol. Similar viscosity effect on
fluorescence quantum yield is reported for t-DMASBO by Fayed
et al. [21] and for t-DMASBT by Saha et al. [27]. There are two
twisting processes (i) rotation around single bond that leads to for-
mation of TICT state (ii) torsional relaxation around olefinic bond
that leads to isomerization. Saha et al. suggested that in a highly
viscous polar medium, there is a restricted rotation of the dimethy-
lamino group and it causes a greater extent of donation of charge
toward acceptor and therefore the TICT state is stabilized in vis-
cous medium. They hypothesized that the temperature-induced
TICT fluorescence quenching is observed without any isomerization
Following equation is obtained from the multiple linear regres-
sion analysis of the fluorescence data with good correlation
coefficient of 0.94
∗
E = 23791 − 2047 ꢂ − 955 ˛ + 94 ˇ
(5)
Significant enhancement in the value of ‘s’ compared to absorp-
tion spectra suggests the greater stabilization of the system by
dipole-dipole interaction due to its charge transfer nature. The neg-
ative value of ‘a’ indicates stabilization by hydrogen bond donating
capacity of the solvent. Due to charge transfer, the electron density
on the dimethylamino nitrogen decreases, thereby the interaction
of the solvent with the dimethylamino nitrogen decreases. On the
other hand the charge transfer increases the electron density on the
imidazole nitrogen therefore the hydrogen bonding with imidazole
nitrogen increases.
S
3
4
3
2
1
0
1
S
2
S
1
S
0
3.3. Nature of ICT state
-
According to Grabowski’s TICT model [5], the ICT state is
0
10
20
30
40
50
60
70
80
90
formed by the rotation of the electron donating group from within
the whole molecular plane to a position perpendicular to other
moiety of the molecule is accompanying charge transfer from
electron donor to electron acceptor. To search the TICT state
we have first twisted different possible single bonds by rotating
(o
)
1
Fig. 7. Simulated potential energy surfaces for the formation of TICT state obtained
by the torsional motion of dimethylamino group in trans-A (similar plot was
obtained for trans-B, not shown).

A. Mishra et al. / Journal of Photochemistry and Photobiology A: Chemistry 260 (2013) 50–58
55
Table 3
Transition character of different energy levels on the pathway to the TICT state in t-DMASBI A.
Dihedral
angle(␸1
S1 State
S2 State
S3 State
MO
◦
)
MO
T. E. (f)a
MO
T. E. (f)a
T. E. (f)a
0
LUMO ← HOMO (100%)
3.088 (1.58)
LUMO ← HOMO-2 (35.4%)
LUMO ← HOMO-1 (64.6%)
3.871 (0.02)
LUMO ← HOMO-2 (44.6%)
LUMO ← HOMO-1 (21.3%)
LUMO+1 ← HOMO (14.9%)
LUMO+2 ← HOMO (19.2%)
LUMO ← HOMO-2 (43.9%)
LUMO ← HOMO-1 (22.6%)
LUMO+1 ← HOMO (14.4%)
LUMO+2 ← HOMO (19.0%)
LUMO ← HOMO-2 (42.1%)
LUMO ← HOMO-1 (26.6%)
LUMO+1 ← HOMO (12.6%)
LUMO+2 ← HOMO (18.6%)
LUMO ← HOMO-2 (39.0%)
LUMO ← HOMO-1 (33.8%)
LUMO+1 ← HOMO (9.2%)
LUMO+2 ← HOMO (18.0%)
LUMO ← HOMO-2 (37.7%)
LUMO ← HOMO-1 (45.1%)
LUMO+2 ← HOMO (17.2%)
LUMO ← HOMO-2 (21.5%)
LUMO ← HOMO-1 (62.5%)
LUMO+2 ← HOMO (16.0%)
4.056 (0.001)
1
2
3
0
0
0
LUMO ← HOMO (100%)
LUMO ← HOMO (100%)
LUMO ← HOMO (100%)
3.085 (1.58)
3.077 (1.57)
3.063 (1.56)
LUMO ← HOMO-2 (37.0%)
LUMO ← HOMO-1 (63.0%)
3.869 (0.02)
3.862 (0.02)
3.846 (0.02)
4.048 (0.001)
4.025 (0.001)
3.982 (0.01)
LUMO ← HOMO-2 (41.5%)
LUMO ← HOMO-1 (58.5%)
LUMO ← HOMO-2 (48.8%)
LUMO ← HOMO-1 (51.2%)
4
0
0
LUMO ← HOMO (100%)
LUMO ← HOMO (100%)
3.041 (1.52)
3.009 (1.42)
LUMO ← HOMO-2 (56.7%)
LUMO ← HOMO-1 (43.3%)
3.820 (0.02)
3.803 (0.02)
3.907 (0.02)
3.771 (0.09)
5
LUMO ← HOMO-2 (68.2%)
LUMO ← HOMO-1 (21.1%)
LUMO+3 → HOMO (10.7%)
6
7
8
0
0
0
0
LUMO ← HOMO-1 (22.5%)
LUMO ← HOMO (77.5%)
LUMO ← HOMO-1 (69.8%)
LUMO ← HOMO (30.2%)
LUMO ← HOMO-1 (51.5%)
LUMO ← HOMO (48.5%)
LUMO ← HOMO-1 (100%)
2.958 (1.18)
3.402 (0.76)
2.743 (0.18)
2.681 (0.00)
LUMO ← HOMO-2 (100%)
LUMO ← HOMO-2 (100%)
3.764 (0.01)
3.739 (0.01)
3.722 (0.01)
3.717 (0.02)
LUMO ← HOMO-1 (83.8%)
LUMO ← HOMO (16.2%)
LUMO ← HOMO-1 (69.8%)
LUMO ← HOMO (16.2%)
LUMO ← HOMO-1 (52.3%)
LUMO ← HOMO (47.7%)
LUMO ← HOMO-2 (100%)
3.599 (0.30)
3.402 (0.76)
3.276 (1.27)
3.242 (1.44)
LUMO ← HOMO-1 (62.9%)
LUMO ← HOMO (37.1%)
LUMO ← HOMO (100%)
9
a
T. E., transition energy in eV; f, oscillator strength.
in polar solvents and in non-polar solvents such as dioxane tem-
perature dependent isomerization occurs. However, the arguments
of Saha et al. are not convincing in the sense that any rotation
on dimethylamino group should restrict not only the reverse pro-
cess, but also the forward process of formation of TICT state. In
addition, considering the fact that N,N-dimethylaminobenzonitrile
t-DMASBO [21] and butadiene derivatives [23]. To identify the path
of isomerization in t-DMASBI we constructed the potential energy
surface for the isomerization. The salient features of the poten-
tial energy surface are the photoisomerization occurs via phantom
state in a nonradiative way and there is small barrier for rotation
of the trans isomers to reach the perpendicular state, little or no
barrier for cis isomers to obtain the perpendicular geometry (Fig. 8).
Our calculation predicted that same as that of trans isomer in cis
isomer of DMASBI also conformer B is more stable than conformer A
(Table 2). In cis-A conformer the dimethylanilino ring is pushed out
of plane. But cis-B conformer is more planar. It may be the pseudo
intramolecular hydrogen bond between the azole nitrogen and one
of the phenyl hydrogen planarizes the molecule (Fig. 1).
(
DMABN) emits TICT emission in solid matrices [49,50], rotation
of the dimethylamino group of t-DMASBT in glycerol is not unex-
pected. It may also be noted that although t-DMASBI has a bulky
acceptor (benzimidazole) moiety compared to the cyano group
of DMABN, as with DMABN, t-DMASBI also forms a TICT state
by torsional rotation of dimethylamino group. Since the rotat-
ing dimethylamino group is quite small as in DMABN, the ICT
emission was found to be less dependent on viscosity [51]. Only
for molecules those have bulky donor groups, for example (N,N-
dimethylaminophenyl)-diphenylphosphine-oxide which has the
bulky anilino group as donor, the increase in viscosity retards the
TICT process even at moderate viscosity [52]. In contrary to Saha
et al. arguments [27] it was shown that t-DMASBT undergoes isom-
erization not only in non polar solvents, but also in viscous polar
solvent including glycerol even at room temperature [53,54]. Since
the formation of TICT state in t-DMASBI also involves rotation of
small dimethylamino group, the viscosity of the solvent is expected
to have less effect on the TICT processes. On the other hand the
isomerization involves the rotation of bulky group like dimethy-
laminophenyl or benzazole ring. Therefore, the effect of viscosity
will be much higher on the twisting of carbon-carbon double bond
6
S
3
5
4
3
2
1
0
1
S
2
1
S
S0
-
[
55,56] than on carbon–nitrogen bond. In other words, the pho-
180
150
120
90
o)
60
30
0
toisomerization is restricted in viscous medium rather than the
TICT process. This will reduce the photoisomerization and lead to
increase in fluorescence quantum yield. Fayed et al. also suggested
that the photosiomerization is complementary to emission in
(
Fig. 8. Potential energy surfaces simulated for photoisomerization of trans-A to cis-
A.

5
6
A. Mishra et al. / Journal of Photochemistry and Photobiology A: Chemistry 260 (2013) 50–58
+NH
H
N
CH3
CH3
N
H
N
N
H
N
+
CH3
CH
3
Chart 2. Resonance structures of monocation of t-DMASBI.
0
0
0
.16
.14
.12
It is well established [31–34,57] that the protonation of
dimethylamino nitrogen causes a blue shift, as the lone pair of
electrons on this nitrogen no longer involves in conjugation. On
the other hand the protonation at electron withdrawing imidazole
nitrogen will enhance the conjugation. Therefore it can be inferred
from the red shifted absorption and fluorescence spectra the for-
mation of monocation by protonation of imidazole nitrogen in
both ground and excited states. To substantiate further the assign-
ment of monocation, we have performed TDDFT calculations on the
ground state optimized structures to obtain the excitation. The pre-
dicted vertical transition energy, 427 nm is close to that of 411 nm
obtained experimentally.
pH
pH
2
2
2
3
3
.7
.8
.9
.3
.7
8.8
7.9
7.6 (solid
7.2
lines)
6.6
6
5
.0
.3
0.1
4
4
5
6
6
7
7
7
.2
.3
.3
.0
.6
.2
.6
.9
4.3
4.2
0
0
0
0
.08
.06
.04
.02
0
3.7
3.3 (dotted
2.9 lines)
2.8
8.8
2
.7
Single absorption and emission spectra indicate the replace-
ment of hetero atom strongly affects the prototropic equilibrium.
For t-DMASBT in water the equilibrium favored the more polar
dimethylamino protonated monocation over the less polar ring
nitrogen protonated product. But in the present molecule the
protonation of the dimethylamino nitrogen was completely sup-
pressed to favor the red shifted monocation. This can be explained
as follows: In all three molecules charge flow from dimethy-
lamino nitrogen to benzazoles decreases the charge density on
dimethylamino nitrogen and increases the electron density in
azole nitrogen. This favors the protonation of azole nitrogen than
dimethylamino nitrogen. In t-DMASBT and t-DMASBO, the pres-
ence of sulfur and oxygen atom, respectively, decreases the charge
on the azole nitrogen due to their electron withdrawing nature.
Owing to decrease in the electron density the protonation occurs at
lower pH and is responsible for protonation at both dimethylamino
and azole nitrogen sites. On the other hand replacing sulfur/oxygen
atom with >NH group should increase the charge on azole nitrogen.
Therefore azole nitrogen is solely protonated at a higher pH. This is
evident from the lower pKa of monocation-neutral equilibrium of t-
DMASBO and t-DMASBT (Table 4) [21,26]. However at higher acid
concentration, protonation at dimethylamino group of t-DMASBI
also occurs to form dication as indicated by the blue shift in the
absorption (Fig. 9) and the fluorescence spectra (Fig. 10).
2
70
320
370
420
470
520
Wavelength (nm)
Fig. 9. UV–visible absorption spectra of t-DMASBI in water at different pH. pH range
.8–4.2 are shown as solid line and pH range 3.7–2.7 are shown as dotted lines.
8
3
.5. Effect of pH
The effect of pH on t-DMASBT and t-DMASBO were studied by
Fayed et al. [21,22]. They found that two kinds of monocations, one
by protonation of the benzazole nitrogen and other by protona-
tion of the dimethylamino nitrogen, are formed (Chart 1). To find
the effect of replacement of hetero atom by ‘NH’ we have studied
both absorption and fluorescence characteristics as a function of
pH. On decreasing the pH a new peak started to appear on the red
side of the neutral molecule’s absorption band indicating the pro-
tonation of the dye molecule (Fig. 9). The absorbance of the new
band increases with increase in acid concentration. At pH 4.30,
the first protonation of the neutral molecule was completed; fur-
ther decrease in pH the spectrum was blue shifted suggesting the
formation of dication. The pKa value for the neutral-monocation
equilibrium and monocation-dication equlibria are 6.04 and 3.41,
respectively. At pH 4.30 single red shifted emission of the proton-
ated species was found (Fig. 10). This substantiates the formation
of single monocation.
The pKa in the excited state (pKa*) for neutral-monocation equi-
librium is calculated by Forster cycle method [58] and is compared
with that of azole nitrogen of t-DMASBO and t-DMASBT (Table 4).
1
0
0
0
.2
.9
.6
.3
0
The positive value of pK * obtained for azole nitrogen is consistent
with the fact that its basicity increases upon excitation due to flow
charge from dimethylamino group. Interestingly the pKa*–pKa dif-
ference is more in case of t-DMASBI than its analogs. It may also be
noted that as mentioned earlier the excited state dipole moment of
t-DMASBI is higher than that of t-DMASBT.
a
(c)
(a)
(b)
Though the molecule carries a charge in the protonated form,
−
1
the Stokes shift observed for the monocation is only 4800 cm
,
Table 4
The pKa and the pKa* obtained by Forster cycle method for azole nitrogen.
Molecule
pKa
pKa*
pKa* − pKa
t-DMASBI
t-DMASBO
t-DMASBT
6.04
2.6
4.8
10.87
7.15
8.47
4.83
4.55
3.67
3
70
450
530
Wavelength (nm)
610
690
a
b
a
Fig. 10. Fluorescence spectra of (a) neutral (b) monocation and (c) dication of t-
DMASBI in water.
From Ref [21].
From Ref [26].
b

A. Mishra et al. / Journal of Photochemistry and Photobiology A: Chemistry 260 (2013) 50–58
57
which is much smaller than the Stokes shift calculated for the neu-
by picosecond transient absorption: comparison with electron transfer in pho-
tosynthesis, Journal of Physical Chemistry 94 (1990) 4550–4559.
7] F.A.S. Chipem, A. Mishra, G. Krishnamoorthy, The role of hydrogen bonding
in excited state intramolecular charge transfer, Physical Chemistry Chemical
Physics 14 (2012) 8775–8790.
−1
tral molecule in water (7140 cm ). This substantiates the earlier
conclusion that the molecule emits from TICT state. As the molecule
has higher dipole moment in the TICT state than locally excited
state, it is expected to stabilize the system by a greater extent.
The absence of TICT emission in monocation may be due to reso-
nance stabilization (Chart 2), that is supposed to increase the double
bond character of carbon-nitrogen bond that twists to form the
TICT state as observed in benzoxazole and benzothiazole analogs
[
[
8] U. Abraham, An introduction to ultrathin organic films: from Langmuir-
Blodgett to self-assembly, Academic Press, Boston, 1991.
[
9] J. Messier, F. Kajzar, P. Prasad (Eds.), Organic Molecules for Nonlinear Optics
and Photonics, Klumer Academic Publishers, Dordrecht, Boston, 1991.
[10] Z.S. Wang, F.Y. Li, C.H. Huang, L. Wang, M. Wei, L.P. Jin, N.Q. Li, Photoelectric
conversion properties of nanocrystalline TiO2 electrodes sensitized with hemi-
cyanine derivatives, Journal of Physical Chemistry B 104 (2000) 9676–9682.
[11] F. Li, J. Zheng, C. Huang, L. Jin, J. Zhuang, J. Guo, X. Zhao, T. Liu, Novel multifunc-
tional umbrella molecule material combining photoelectric conversion and
second-order optical nonlinearities in Langmuir-Blodgett monolayers, Journal
of Physical Chemistry B 104 (2000) 5090–5095.
[
21,22]. This is supported by the shortened carbon–nitrogen bond
distance in the monocation (1.359 A˚ in monocation and 1.384 A˚ in
neutral molecule).
[
12] G.G. Gurzadyan, H. Görner, Picosecond transient absorption spectroscopy
of trans-1-(9-anthryl)-2-(4-R-phenyl)ethylenes with electron-donating and -
accepting substituents, Chemical Physics Letters 359 (2002) 146–152.
4
. Conclusion
[
13] J.-S. Yang, C.-K. Lin, A.M. Lahoti, C.-K. Tseng, Y.-H. Liu, G.-H. Lee, S.-M. Peng,
Effect of ground-state twisting on the trans-cis photoisomerization and TICT
state formation of aminostilbenes, Journal of Physical Chemistry A 113 (2009)
ꢀ
Effect of hetero atom replacement in azole ring of trans-2-[4 -
dimethylamino)styryl]benzazoles is investigated by synthesizing
(
4
868–4877.
14] B. Wandelt, P. Turkewitsch, B.R. Stranix, G.D. Darling, Effect of temper-
ature and viscosity on intramolecular charge-transfer fluorescence of
-p-dimethylaminostyrylpyridinium salt in protic solvents, Journal of the
Chemical Society, Faraday Transactions 91 (1995) 4199–4205.
t-DMASBI. Theoretical calculations predicted that hetero atom
replacement significantly affects the relative stability of the
conformers. From the solvatochromic effect of absorption and flu-
orescence spectra it can be inferred that t-DMASBI emits from two
different excited states in nonpolar and polar solvents. The emitting
state in polar solvent is an ICT state. This is supported by greater
contribution of dipolar interaction to stabilize the excited state. The
dipole moment of the ICT state in t-DMASBI is higher than that of its
benzothiazole analog. Therefore it may be more sensitive to polar-
ity than t-DMASBT. Theoretical calculations suggest that rotation
of the dimethylanilino ring and the dimethylaminostyryl ring are
higher energy processes compared to rotation of dimethylamino
group. The fluorescence quantum yield of t-DMASBI increases with
increase in viscosity of the medium. This reveals that the torsional
rotation of carbon-carbon double bond that leads to isomerization
can act as a major non-radiaive decay channel. The theoretical cal-
culation predicts that the photoisomerization that competes with
fluorescence follows the non-adiabatic path and occurs via perpen-
dicular state. Unlike, its benzoxazole or benzothiazole derivative,
in t-DMASBI, the protonation occurs only at azole nitrogen to form
monocation. This is due to greater charge density in azole nitrogen
of benzimidazole derivative than those of benzoxazole or benzoth-
iazole derivative. This conclusion is substantiated by the higher pKa
of the protropic equilibrium of t-DMASBI. However at lower pH, the
dimethylamino nitrogen of t-DMASBI is also protonated to form
dication.
[
a
4
[
15] I. Grabchev, T. Philipova, Photophysical and photochemical properties of
some triazine-stilbene fluorescent brighteners, Dyes and Pigments 44 (2000)
1
75–180.
[
16] A. Mokdad, J.L. Belof, S.W. Yi, S.E. Shuler, M.L. McLaughlin, B. Space, R.W.
Larsen, Photophysical studies of the trans to cis isomerization of the push–pull
molecule: 1-(pyridin-4-yl)-2-(N-methylpyrrol-2-yl)ethene (mepepy), Journal
of Physical Chemistry A 112 (2008) 8310–8315.
17] W. Akemann, D. Laage, P. Plaza, M.M. Martin, M. Blanchard-Desce, Pho-
toinduced intramolecular charge transfer in push–pull polyenes: effects of
solvation, electron-donor group, and polyenic chain length, Journal of Physical
Chemistry B 112 (2008) 358–368.
18] J.-S. Yang, K.-L. Liau, C.-Y. Hwang, C.-M. Wang, Photoinduced single- versus
double-bond torsion in donor-acceptor-substituted trans-stilbenes, Journal of
Physical Chemistry A 110 (2006) 8003–8010.
[19] S.-L. Wang, T.-I. Ho, Substituent effects on intramolecular charge-transfer
behaviour of styrylheterocycles, Journal of Photochemistry and Photobiology
A: Chemistry 135 (2000) 119–126.
[
[
[
20] M. Shaikh, J. Mohanty, P.K. Singh, A.C. Bhasikuttan, R.N. Rajule, V.S. Satam, S.R.
Bendre, V.R. Kanetkar, H. Pal, Contrasting solvent polarity effect on the photo-
physical properties of two newly synthesized aminostyryl dyes in the lower
and in the higher solvent polarity regions, Journal of Physical Chemistry A 114
(
2010) 4507–4519.
[21] T.A. Fayed, Intramolecular charge transfer and photoisomerization of 2-
p-dimethylaminostyryl)benzoxazole: a new fluorescent probe, Journal of
(
Photochemistry and Photobiology A: Chemistry 121 (1999) 17–25.
22] T.A. Fayed, S.S. Ali, Protonation dependent photoinduced intramolecular charge
transfer in 2-(p-dimethylaminostyryl)benzazoles, Spectroscopy Letters 36
[
[
[
(
2003) 375–386.
23] S.E.-D.H. Etaiw, T.A. Fayed, N.Z. Saleh, Photophysics of benzazole derived
push–pull butadienes: a highly sensitive fluorescence probes, Journal of Pho-
tochemistry and Photobiology A: Chemistry 17 (2006) 238–247.
24] M. Sowmiya, A.K. Tiwari, Sonu, S.K. Saha, Study on intramolecular
Acknowledgement
ꢀ
charge
transfer
fluorescence
properties
of
trans-4-[4 -(N,N-
dimethylamino)styryl]pyridine: effect of solvent and pH, Journal of
Photochemistry and Photobiology A: Chemistry 218 (2011) 76–86.
25] T.A. Fayed, S.E.-D.H. Etaiw, N.Z. Saleh, Micellar effects on the molecular aggre-
gation and fluorescence properties of benzazole-derived push–pull butadienes,
Journal of Luminescence 121 (2006) 431–440.
26] S.K. Saha, P. Purkayastha, A.B. Das, Photophysical characterization and effect
of pH on the twisted intramolecular charge transfer fluorescence of trans-
2-[4-(dimethylamino)styryl]benzothiazole, Journal of Photochemistry and
Photobiology A: Chemistry 195 (2008) 368–377.
The authors are thankful to Department of Science and Tech-
nology (DST), India and Council of Science and Industrial Research
[
[
(
CSIR), India for the financial support.
References
[
27] S.K. Saha, P. Purkayastha, A.B. Das, Excited state isomerization and effect
of viscosity- and temperature-dependent torsional relaxation on TICT
flu-orescence of trans-2-[4-(dimethylamino)styryl]benzothiazole, Journal of
Photochemistry and Photobiology A: Chemistry 199 (2008) 179–187.
[
1] Z.R. Grabowski, K. Rotkiewicz, A. Siemiarczuk, D.J. Cowley, W. Baumann,
Twisted intramolecular charge transfer states (TICT). A new class of excited
states with a full charge separation, Nouveau Journal de Chimie 3 (1979)
4
43–454.
ꢀ
[28] S.S. Jaffer, S.K. Saha, G. Eranna, A.K. Sharma, P. Purkayastha, Intramolecular
charge transfer probe induced formation of ␣-cyclodextrin nanotubular
suprastructures: a concentration dependent process, Journal of Physical Chem-
istry C 112 (2008) 11199–11204.
[
[
[
[
[
2] W. Tan, D. Zhanga, D. Zhu, 4-N-Methyl-N -(2-dihydroxyboryl-benzyl)amino
benzonitrile and its boronate analogue sensing saccharides and fluoride ion,
Bioorganic and Medicinal Chemistry Letters 17 (2007) 2629–2633.
ꢀ
3] A. Mielniczak, B. Wandelt, S. Wysocki, 4-(4 -Dimethylaminostyryl)pyridinium
29] A. Maity, S.S. Jaffer, T. Das, P. Ghosh, P. Purkayastha, Orientation of a TICT probe
trapped in the peripheral confined water created by ionic surfactant envelope
around silver nanoparticles, Langmuir 27 (2011) 4068–4075.
derivative: a solvent viscosity-and polarity-sensitive fluorescent sensor, Mate-
rials Science 20 (2002) 59–68.
[
4] R.A. Marcus, Electron transfer reactions in chemistry: theory and experiment,
Reviews of Modern Physics 65 (1993) 599–610, and related references therein.
5] Z.R. Grabowski, K. Rotkiewicz, W. Rettig, Structural changes accompanying
intramolecular electron transfer: focus on twisted intramolecular charge-
transfer states and structures, Chemical Reviews 103 (2003) 3899–4302.
6] H. Luck, M.W. Winsor, W. Rettig, Pressure dependence of the kinetics of
30] M.A. Ros, M.C. Ros, Ab initio study of the hydrogen bond and proton transfer in
ꢀ
ꢀ
2
-(2 -hydroxyphenyl)benzothiazole and 2-(2 -hydroxyphenyl)bezimidazole,
[
Journal of Physical Chemistry A 102 (1998) 1560–1567.
31] J.K. Dey, S.K. Dogra, Solvatochromism and prototropism in 2-(aminophenyl)
benzothiazoles, Bulletin of the Chemical Society of Japan 64 (1991)
3142–3152.
[
ꢀ
photo- induced intramolecular charge separation in 9,9 -bianthryl monitored

5
8
A. Mishra et al. / Journal of Photochemistry and Photobiology A: Chemistry 260 (2013) 50–58
[
32] A.K. Mishra, S.K. Dogra, Effect of solvents and pH on the spectral behavior of
Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov,
G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A.
Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B.G. John-
son, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, GAUSSIAN 03 (Revision E.01),
Gaussian, Inc., Wallingford, CT, 2004.
ꢀ
2
-(4 -aminophenyl)benzimidazole, Bulletin of the Chemical Society of Japan 85
(
1985) 3587–3592.
33] J.K. Dey, S.K.
dimethylamino)phenyl)benzothiazole and its benzimidazole analogue:
effects of solvents and pH on electronic spectra, Journal of Physical Chemistry
8 (1994) 3638–3644.
ꢀ
2-(4 -N,N-
[
Dogra,
Dual
fluorescence
of
(
[46] M.J. Kamlet, J.L.M. Abboud, M.H. Abraham, R.W. Taft, Linear solvation
energy relationships. 23. A comprehensive collection of the solvatochromic
parameters, ␲*, ␣, and ␤, and some methods for simplifying the gener-
alized solvatochromic equation, Journal of Organic Chemistry 48 (1983)
2877–2887.
9
ꢀ
[
34] G. Krishnamoorthy, S.K. Dogra, Dual fluorescence of 2-(4 -N,N-
dimethylaminophenyl)benzoxazole: effect of solvent and pH, Chemical
Physics 460 (1999) 119–124.
[
35] D.W. Hein, R.J. Alheim, J.J. Leavitt, The use of polyphosphoric acid in the syn-
thesis of 2-aryl- and 2-alkyl-substituted benzimidazoles, benzoxazoles and
benzothiazoles, Journal of the American Chemical Society 79 (1957) 427–429.
36] A. Crosby, J.N. Demas, Measurement of photoluminescence quantum yields.
Review, Journal of Physical Chemistry 75 (1971) 991–1024.
[47] S. Cogan, S. Zilberg, Y. Haas, The electronic origin of the dual fluorescence
in donor–acceptor substituted benzene derivatives, Journal of the American
Chemical Society 128 (2006) 3335–3345.
[
[
[
[48] S. Murali, W. Rettig, TICT formation in para- and meta-derivatives of N-
phenylpyrrole, Journal of Physical Chemistry A 110 (2006) 28–37.
[49] C. Cazeau-Dubroca, A. Peirigua, S.A. Lyazidi, G. Nouchi, Twisted intramolecular
charge transfer fluorescence of para-N,N-dimethylamino benzonitrile in rigid
matrix at room temperature, Chemical Physics Letters 98 (1983) 511–514.
[50] C. Cazeau-Dubroca, A. Peirigua, M.B. Brahim, G. Nouchi, Specific hydrogen
bonding interaction versus free volume model for TICT molecules in polymeric
matrices, Chemical Physics Letters 157 (1989) 393–397.
[51] J. Hicks, M. Vandersall, Z. Baraogic, K.B. Eisenthal, The dynamics of barrier cross-
ings in solution: the effect of a solvent polarity-dependent barrier, Chemical
Physics Letters 116 (1985) 18–24.
[52] P. Changenet, P. Plaza, M.M. Martin, Y.H. Meyer, Role of intramolecular torsion
and solvent dynamics in the charge-transfer kinetics in triphenylphosphine
oxide derivatives and DMABN, Journal of Physical Chemistry A 101 (1979)
8186–8194.
37] A.D. Becke, Density-functional thermochemistry. 3. The role of exact exchange,
Journal of Chemical Physics 98 (1993) 5648–5652.
38] C.T. Lee, W. Yang, R.G. Parr, Development of the Colle-Savetti correlation-
energy formula into a functional of the electron-density, Physical Review B
3
7 (1988) 785–789.
[
39] J.-D. Chai, M. Head-Gordon, Long-range corrected hybrid density functionals
with damped atom–atom dispersion corrections, Physical Chemistry Chemical
Physics 10 (2008) 6615–6620.
[
[
[
40] P.C. Hariharan, J.A. Pople, Influence of polarization functions on MO hydrogena-
tion energies, Theoretical Chemistry Accounts 28 (1973) 213–222.
41] P.C. Hariharan, J.A. Pople, Accuracy of AHn equilibrium geometries by single
determinant molecular orbital theory, Molecular Physics 27 (1974) 209–214.
42] N. Mardirossian, J.A. Parkhilla, M. Head-Gordon, Benchmark results for empir-
ical post-GGA functionals: difficult exchange problems and independent tests,
Physical Chemistry Chemical Physics 13 (2011) 19325–19337.
[53] A. Mishra, A. Thangamni, S. Chatterjee, F.A.S. Chipem, G. Krishnamoorthy,
ꢀ
Photoisomerization of trans-2-[4 -(dimethylamino)styryl]benzothiazole, Pho-
[
43] J.F. Stanton, J. Gauss, N. Ishikawa, M. Head-Gordon, A comparison of sin-
gle reference methods for characterizing stationary points of excited-state
potential-energy surfaces, Journal of Chemical Physics 103 (1995) 4160–4174.
44] K.B. Wiberg, Y.-G. Wang, A.E. de Oliveira, S.A. Perera, P.H. Vaccaro, Comparison
of CIS- and EOM-CCSD-calculated adiabatic excited-state structures. Changes
in charge density on going to adiabatic excited states, Journal of Physical Chem-
istry A 109 (2005) 466–477.
tochemistry and Photobiology 39 (2013) 247–252.
[54] F.A.S. Chipem, S. Chatterjee, G. Krishnamoorthy, Theoretical study on
ꢀ
photochemical behavior of trans-2-[4 -(dimethylamino)styryl]benzothiazole,
[
Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 121–127.
[55] J. Saltiel, J.T.D. Agostino, Separation of viscosity and temperature effects on
the singlet pathway to stilbene photosiomerization, Journal of the American
Chemical Society 94 (1972) 6445–6456.
[
45] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheese-
man, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P.
Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Strat-
mann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J. Ochterski, P.Y. Ayala, K.
Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich,
A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B.
[56] J. Saltiel, Y.-P. Sun, The intrinsic potential energy barrier for twisting in the
trans-stilbene S1 state in hydrocarbon solvents, Journal of Physical Chemistry
93 (1989) 6246–6250.
[57] N. Dash, F.A.S. Chipem, R. Swaminathan, G. Krishnamoorthy, Hydro-
ꢀ
gen bond induced twisted intramolecular charge transfer in 2-(4 -N,N-
dimethylaminophenyl)imidazo[4,5-b]pyridine, Chemical Physics Letters 460
(2008) 119–124.
[58] T. Forster, Die pH-Abhangigkeit der fluoreszene von napthalinderivaten,
Zeitschrift fur elektrochemie 54 (1950) 531–535.