Photoisomerization of trans-2-[4′-(dimethylamino)styryl]benzothiazole

Article abstract of DOI:10.1111/j.1751-1097.2012.01227.x

Photoreaction of trans-2-[4′-(dimethylamino)styryl]benzothiazole (t-DMASBT) under direct irradiation has been investigated in dioxane, chloroform, methanol and glycerol to understand the mechanism of photoisomerization. Contrary to an earlier report, isomerization takes place in all these solvents including glycerol. The results show that restriction on photoisomerization leads to the increase in fluorescence quantum yield in glycerol. The results are consistent with the theoretically simulated potential energy surface reported earlier using time-dependent density functional theory (TDDFT) calculations. DFT calculations on cis isomers under isolated condition have suggested that cis-B conformer is more stable than cis-A conformer due to hydrogen-bonding interaction. In the ground state, cis-DMASBT is predominantly present as cis-B. The fluorescence spectra of the irradiated t-DMASBT suggested that photoisomerization follows not the adiabatic path as proposed by Saha et al., but the nonadiabatic path. Contrary to an earlier report, the photoisomerization of trans-2-[4′-(dimethylamino)styryl]benzothiazole occurs in all the solvents including viscous glycerol and it follows not the adiabatic, but the non-adiabatic path. Relative higher fluorescence quantum yield obtained in glycerol is due to restriction on photoisomerization. 2012 Wiley Periodicals, Inc. Photochemistry and Photobiology

Full text of DOI:10.1111/j.1751-1097.2012.01227.x

Photochemistry and Photobiology, 20**, **: *–*
Research Notes
Photoisomerization of trans-2-[4′-(Dimethylamino)styryl]benzothiazole
Anasuya Mishra, Arumugam Thangamani, Soumya Chatterjee, Francis A. S. Chipem and
Govindarajan Krishnamoorthy*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati, India
Received 26 May 2012, accepted 18 August 2012, DOI: 10.1111/j.1751-1097.2012.01227.x
ABSTRACT
to be localized on the dimethylamino group (donor) is localized
on the other part of the molecule. Thus, we reinvestigated the
photochemical nature of t-DMASBT theoretically (9). Our calcu-
lations predicted that t-DMASBT is present in two conformeric
forms, trans-A and -B (Fig. 1). Their relative populations and
spectral characteristics and nature of the excited states are pre-
dicted to be nearly the same. Our calculations also suggested that
twisting of the dimethylamino group is energetically favored
compared with twisting of dimethylanilino or dimethylanilinostyryl
moieties. With torsional rotation of the dimethylamino group, the
energy of the S3 state decreases and those of the lower energy
states increase. The S3 state crosses the S2 state, but there is an
avoided crossing between S3 and S1 states. At the perpendicular
geometry, the emitting state is described by HOMO–LUMO
excitation. The HOMO and LUMO are decoupled with the
HOMO localized on the donor dimethylamino group and the
LUMO localized on the other part of the molecule. Thus, the
donor lone pair becomes available for charge transfer to the
decoupled LUMO that results in the formation of the TICT state.
t-DMASBT has been shown to act as an excellent sensor for a
variety of heterogeneous systems (10–16). It was used as a surface
probe to monitor the premicellar aggregation in sodium dodecyl-
sulfate, cetyltrimethyl ammonium bromide and Triton X 100 (10)
and brijs (11). It was also found that t-DMASBT induces the
formation of cylclodextrin nanotubular suprastructures (12–14).
Recently, Purkayastha et al. used t-DMASBT as a fluorophore to
demonstrate the targeted drug delivery of nanoparticle (15). They
also utilized t-DMASBT to study the ionic surfactant created
peripheral confined water around silver nanoparticle (16). Despite
the fact that the sensitivity of the photophysics and photochemis-
try of t-DMASBT to environment is responsible for its wide appli-
cation in various systems, the mechanism of nonradiative decay
and photoisomerism are still controversial.
Photoreaction of trans-2-[4′-(dimethylamino)styryl]benzo-
thiazole (t-DMASBT) under direct irradiation has been inves-
tigated in dioxane, chloroform, methanol and glycerol to
understand the mechanism of photoisomerization. Contrary
to an earlier report, isomerization takes place in all these sol-
vents including glycerol. The results show that restriction on
photoisomerization leads to the increase in fluorescence
quantum yield in glycerol. The results are consistent with the
theoretically simulated potential energy surface reported earlier
using time-dependent density functional theory (TDDFT) calcula-
tions. DFT calculations on cis isomers under isolated condition
have suggested that cis-B conformer is more stable than cis-
A conformer due to hydrogen-bonding interaction. In the
ground state, cis-DMASBT is predominantly present as cis-B.
The fluorescence spectra of the irradiated t-DMASBT
suggested that photoisomerization follows not the adiabatic
path as proposed by Saha et al., but the nonadiabatic path.
INTRODUCTION
Due to wide applications, fluorescing donor–acceptor substituted
push–pull styryl dyes have received considerable attention (1–5).
trans-2-[4′-(Dimethylamino)styryl]benzothiazole
(t-DMASBT,
Chart 1) is an interesting push–pull styryl dye, and was sug-
gested as a good microsensor to study the biological functions as
well as biomimicking systems (6). Fayed et al. showed that
t-DMASBT emits from an intramolecular charge transfer (ICT)
state in polar solvents (7). Later Saha et al. (8) studied the spec-
tral characteristics of t-DMASBT and suggested that it emits
from the twisted ICT (TICT) state in polar solvents. They also
proposed that the polarity of the S3 state increases by torsional
rotation of the dimethylamino group and at 90° it forms the
TICT state. In the gas phase, the energy of the TICT state is
higher than that of the planar S1 state, but polar solvents stabilize
the TICT state and make it as the emitting state. However, the
highest occupied molecular orbital (HOMO) and lowest unoccu-
pied molecular orbital (LUMO) reported by them for the twisted
geometry are not consistent with the formation of a charge trans-
fer state. There was a large overlap between the HOMO and
LUMO and in the HOMO the electron density that is supposed
Saha et al. (6) investigated the effect of temperature on the
fluorescence spectral characteristics of t-DMASBT. They hypoth-
esized that in a highly viscous polar medium, the restricted rota-
tion of the dimethylamino group causes a greater extent of
donation of charge toward acceptor, thereby stabilizing the TICT
state and the temperature-induced TICT fluorescence quenching
is observed without any isomerization. However, in low polarity
solvents the decrease in fluorescence was attributed to the tem-
perature-dependent cis–trans isomerization. According to Saha
et al., the isomerization is supposed to take place in the excited
state i.e. by adiabatic process and the so formed cis isomer
*Corresponding author email: gkrishna@iitg.ernet.in (G. Krishnamoorthy)
© 2012 Wiley Periodicals, Inc.
Photochemistry and Photobiology © 2012 The American Society of Photobiology 0031-8655/12
1

2
Anasuya Mishra et al.
S
N
N
S
ϕ4
ϕ3
CH3
CH3
CH3
CH3
ϕ1
N
ϕ2
N
trans-A
trans-B
Chart 1. Structures of different conformers of t-DMASBT.
1.2
(a)
1
1.2
(b)
1
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
280
330
380
430
480
280
330
380
430
480
Wavelength (nm)
Wavelength (nm)
1
0.8
0.6
0.4
0.2
0
0.5
0.4
0.3
0.2
0.1
0
(d)
(c)
280
330
380
430
480
280
330
380
430
480
Wavelength (nm)
Wavelength (nm)
Figure 1. UV–Visible spectra of t-DMASBT at different irradiation times (using 420 nm cutoff filter) in (a) chloroform (b) dioxane (c) methanol and
(d) glycerol (dotted line shows the spectrum of the 420 nm cutoff filter).
procured from Sigma–Aldrich. All other solvents were procured from
Rankem, India and were tested for spurious absorbance/fluorescence in
the region of spectral measurements. Irradiations were carried out in a
standard quartz cell/NMR tube at room temperature in presence of air
with light from a 350 W ozone free Xe-arc lamp. In addition to water
filter, Scotch cutoff filters were used when required. For irradiation
experiments, the concentrations of the solutions were 3 9 10À5 to
5 9 10À5 M (in 1 cm quartz cell) and 10À2 M (in NMR tube). For flu-
orescence measurement, the absorbance of the solutions was kept
below 0.1 at the wavelength of excitation. The irradiation of the solu-
tions in quartz cells and NMR tube were followed by UV–Visible
absorption and NMR spectrometers respectively. UV–Visible absorption
and fluorescence spectra were recorded on a Varian Cary 100 spec-
trometer (Varian Australia Pty Ltd, Palo Alto, Australia) and Jobin-
Yvon Spex Fluoromax 4 fluorometer (Horiba Instruments Incorporated,
should relax radiatively. On the other hand, our time-dependent
density functional theory (TDDFT) calculations predicted that
the photoisomerization of t-DMASBT occurs via a phantom state
which decays to cis and trans isomers nonradiatively (9). There-
fore, we have investigated the photochemistry of t-DMASBT in
different solvents. The main objectives are to determine whether
(1) t-DMASBT isomerizes in polar viscous media such as glyc-
erol or not and (2) the isomerization follows the adiabatic or
nonadiabatic path.
MATERIALS AND METHODS
Edison, NJ, USA) respectively. 1H NMR was recorded using
a
t-DMASBT was synthesized by following the procedure reported by Fayed
et al. (7). A solution of p-dimethylaminobenzaldehyde (0.002 mmol) in
dry dimethyl formamide (DMF) (4 mL) was added dropwise to a solution
of 2-methylbenzothiazole (0.003 mmol) and powdered KOH (0.02 mmol)
in 8 mL of dry DMF with continuous stirring. The mixture was stirred at
140°C. After 48 h the mixture was cooled to room temperature and dilute
HCl (10%) was added to it to make it weakly acidic. The compound was
extracted with dichloromethane. The compound was purified first by col-
umn chromatography and further by preparative thin layer chromatography
using a hexane–ethyl acetate mixture.
400 MHz Varian instrument (Varian Mercury plus, Palo Alto, CA,
USA).
All the calculations were carried out using the GAUSSIAN 03 pro-
gram (17). The ground state geometries were obtained by full optimiza-
tion of structural parameters using DFT employing the 6-31G(d,p) basis
set using spin-restricted shell wavefunctions (18,19). Geometry optimiza-
tions were carried out using Becke’s three-parameter hybrid functional
B3, (20) with nonlocal correlation of Lee-Yang-Parr, LYP (21), abbrevi-
ated as B3LYP. The minimum energy nature of the stationary points was
verified from vibrational frequency analysis. The excitation energies were
obtained by vertical excitations of optimized ground states using
TDDFT/B3LYP/6-31G(d,p) calculations (22,23).
Dioxane, chloroform, methanol all HPLC grade and glycerol AR
grade were used as received. CDCl3 was used for NMR experiments.
p-dimethylaminobenzaldehyde, 2-methylbenzothiazole and CDCl3 were

Photochemistry and Photobiology
3
CDCl3. Figure 2 shows the NMR spectra of t-DMASBT at dif-
ferent irradiation times. One of the signals corresponding to a
trans olefinic proton of t-DMASBT overlaps those of the aro-
matic protons, but the other olefinic proton appears as a doublet
with a coupling constant of 16.0 Hz at d 7.14 ppm. This olefinic
proton was used to monitor the reaction. Upon irradiation, the
intensity of the doublet at d 7.14 decreases, and an olefinic pro-
ton of cis isomer appears at d 6.91 (d, J = 12.0 Hz, 1H). Other
peaks corresponding to the cis isomer also emerge. This confirms
that the blueshifted photoproduct obtained is the cis isomer of
DMASBT. Figure 1 also shows the irradiation in nonviscous sol-
vents dioxane (non polar) and methanol (polar) and also in a vis-
cous polar solvent glycerol. In all the solvents, formation of cis
isomer is observed as in chloroform. Without determining the
spectrum of the cis isomer, it is difficult to predict the effect of sol-
vent on the photostationary state. However, comparison of the pan-
els in Fig. 1 suggests higher trans contents in dioxane and
glycerol. These changes may be real or may be due to spectral
shifts that change the incident light excitation-ratio of the two iso-
mers. The absorbance ratio of the sample after irradiation (i.e. the
photostationary state) and before irradiation (i.e. the trans isomer)
at absorption maxima, Aai/Abi was calculated. Aai/Abi ratio follows
the order methanol (0.43) < chloroform (0.44) < dioxane
(0.53) < glycerol (0.67). However, the absorption maxima of the
trans isomers follows the order dioxane (394 nm) < chloroform
(400 nm) < methanol (402 nm) < glycerol (416 nm). Therefore,
it may be inferred that the content of trans is larger in dioxane and
glycerol at photostaionary state due to solvent effect.
RESULTS AND DISCUSSION
t-DMASBT was irradiated in chloroform using a 420 nm cutoff
filter and the reaction was monitored with a UV–Visible absorp-
tion spectrometer (Fig. 1a). Upon irradiation, the 400 nm absorp-
tion band of t-DMASBT gradually blue shifted with
a
hypochromic effect. The photostationary state was reached in
35 s of irradiation and an isosbestic point was observed at
327 nm. The isosbestic point revealed a simple one-to-one con-
version of t-DMASBT to cis product. To confirm the formation
of cis isomer further, we followed the reaction by 1H NMR in
Figure 2. 1H NMR spectra of t-DMASBT in CDCl3 at different irradia-
tion times (using 420 nm cutoff filter; the intensity of the peaks is
expanded in the aromatic region for clarity, * denotes chloroform peak).
cis-A
cis-B
LUMO
HOMO
Figure 3. Optimized structures along with HOMO and LUMO of cis-A and -B.

4
Anasuya Mishra et al.
Table 1. Optimized geometrical parameters for cis-conformers in the
ground state.
105000
(a)
0.9
0.6
0.3
0
Parameters
cis-A
cis-B
Relative Energy†(kcal molÀ1
Transition energy (nm)
)
6.75
392
3.77
394
70000
35000
0
410
500
590
680
Oscillator strength
0.5477
4.8
0.8420
3.8
Wavelength (nm)
Dipole moment (D)
Dihedral angle‡(°)
φ1
φ2
φ3
φ4
9.4
174.4
38.2
7.4
0.0
0.0
0.1
5.2
410
490
570
650
†With respect to ground state energy of trans-A.
‡Dihedral angles defined in Chart 1.
Wavelength (nm)
0.9
90000
60000
30000
0
(b)
0.6
0.3
has to have planar geometry and the molecule has to relax from
the planar Franck–Condon state to the TICT state. Therefore,
any restriction on rotational motion is expected to hinder not
only the reverse process but also the formation of the TICT state.
Therefore, the enhancement in fluorescence quantum yield of
t-DMASBT in glycerol is due to restriction on twisting of the
olefinic double bond that leads to isomerization.
0
410
490
570
Wavelength (nm)
650
In t-DMASBT, the TICT state is formed by the rotation of
the smaller dimethylamino group, but the competing torsional
rotation around the olefinic double bond involves the rotation of
dimethylanilino group or the benzothiazole group. Fluorescence
quantum yields of trans-stilbene were also shown to be tempera-
ture and viscosity dependent due to the twisting motion about
the central bond of phenyl ring (24–33). Although the fluores-
cence quantum yield decreases with increase in temperature, it
increases with increase in viscosity. The observed marked
increase in fluorescence quantum yield of trans-stilbene in glyc-
erol on lowering of temperature was attributed to the fact that
the processes of fluorescence and isomerization are coupled with
changing temperature in highly viscous solvents (26–28). It was
also reported that the normally very weakly fluorescent cis-stilb-
enes (29–31) and the weakly fluorescent trans-stilbenes become
strongly fluorescent in highly viscous media (32). Our present
results on t-DMASBT also indicate that photoisomerization
competes with fluorescence.
410
490
570
650
Wavelength (nm)
0.9
0.6
0.3
(c)
210000
140000
70000
0
0
430
510
590
670
Wavelength (nm)
As reported earlier (9), t-DMASBT is present in two confor-
meric forms, trans-A (l = 5.2 D) and trans-B (l = 5.1 D). The
energy difference between the two conformers is only
0.2 kcal molÀ1 and thus both conformers are predicted to be
equally probable. As the difference in dipole moments between
the two conformers is negligible their relative population is
nearly the same in all the media. Such conformers equilibrate in
the ground state, but not in the excited state (34). Consequently,
the presence of the two conformers may be reflected in kexc
dependence fluorescence spectra (35). The shifts in the fluores-
cence spectra with kexc are very small (see Supporting informa-
tion). It should be noted that the molecular parameters predicted
by the theoretical calculations for both the conformers are nearly
identical and the emission energies predicted are the same (9).
trans-A and -B conformers upon one bond twist give cis-A
and -B conformers, respectively (Scheme 1). Unlike organic
glasses or other restricted media, the flexibility of the media in
our reaction conditions should allow thermal equilibration in all
these environments. The absorption spectra were recorded after
such equilibration between the conformers. As the molecular
430
510
590
670
Wavelength (nm)
Figure 4. Fluorescence spectra of t-DMASBT in (a) dioxane, (b) aceto-
nitrile and (c) glycerol (inserts show the normalized spectra) in the course
of the irradiation time (all irradiations are carried out in fluorescence
instrument with kexc = 395 nm and 10 nm slit width).
Saha et al. (6) hypothesized that in a highly viscous medium
such as glycerol the restricted rotation of the dimethylamino
group causes a greater extent of donation of charge toward the
acceptor thereby stabilizing the TICT state responsible for the
large Stokes shift in the fluorescence. Only in low polarity
solvents, where TICT emission is very weak, the molecule
undergoes
a temperature-dependent photoisomerization. The
temperature-induced TICT quenching occurs in a polar viscous
medium without any isomerization. But our studies clearly estab-
lish that t-DMASBT photoisomerizes in all the media including
glycerol. The molecule is predicted to have planar geometry in
the ground state by both our DFT calculations (9) as well as by
AM1 calculations (6,8). The corresponding Franck–Condon state

Photochemistry and Photobiology
5
rier for the cis isomers to reach the p* state. Therefore, the cis
isomer is expected to fluoresce only in a rigid environment and
to be nonfluorescent in fluid solution at room temperature. The
fluorescence spectra of t-DMASBT were recorded in nonpolar
dioxane, polar aprotic acetonitrile and polar protic viscous glyc-
erol as a function irradiation time. In all the solvents the fluores-
cence intensities decrease, but no shift is observed in the
fluorescence spectra (Fig. 4). The decreases in fluorescence
intensities indicate the formation of cis isomer. If the adiabatic
mechanism predicted by Saha et al. is correct, the radiative
decay from excited cis isomer should result in blueshift upon
irradiation. In fact, Saha et al. attributed the hypsochromic shift
in the fluorescence spectrum of t-DMASBT in dioxane upon
increasing the temperature to thermal isomerization (6). The
absence of a spectral shift in the fluorescence spectrum of
t-DMASBT upon isomerization (Fig. 4) substantiates the nonadi-
abatic path predicted by our TDDFT calculation for t-DMASBT.
Our studies clearly establish that no shift is observed in the fluo-
rescence spectra of t-DMASBT upon isomerization. Therefore,
we attribute the small 4 nm hypsochromic shift observed in the
fluorescence spectrum of t-DMASBT in dioxane by Saha et al.
upon increasing the temperature to a thermochromic shift com-
monly observed in fluorescence spectra of dyes.
N
N
CH3
CH3
S
hν
S
N
N
CH3
trans-A
cis-A
CH3
S
S
CH3
CH3
N
hν
N
N
N
CH3
trans-B
CH3
cis-B
Scheme 1. Photoisomerization of t-DMASBT.
parameters of trans conformers were already reported (9), we
calculated the molecular parameters of cis conformers under iso-
lated condition and the optimized structures are shown in Fig. 3
and the data are compiled in Table 1. The calculations predict
that cis-B is more stable than cis-A by 0.1295 eV (ca
3 kcal molÀ1). Although the trans isomer is present as both A
and B conformers, the cis isomer is predominantly present in the
B form and the population of cis-A is less than 1%, even in the
most polar environment. cis-B is more stabilized by the presence
of pseudohydrogen bonding between thiazole nitrogen and one
of the phenyl hydrogen (Fig. 3). The presence of hydrogen bond
in cis-B also planarizes the molecule (Table 1). In protic solvents
such as methanol and glycerol, this hydrogen bond may break to
form intermolecular hydrogen bond. However, the transition
energies of both conformers are nearly same and are described
by excitation of an electron from highest occupied molecular
orbital to lowest unoccupied orbital in both conformers. The
smaller oscillator strengths of the cis conformers (Table 1) com-
pared to those of the trans conformers (oscillator strength ca 1.3; 9)
explain the observed hypochromic effect of the absorption spec-
tra on photoisomerization.
Thus, the ca 23-fold increase in fluorescence quantum yield
of t-DMASBT in glycerol observed by Saha et al. may be attrib-
uted to fact that the viscous medium enhanced the torsional
barrier leading to photoisomerization. Saha et al. also reported
that the fluorescence quantum yield of t-DMASBT decreases
with increase in temperature in both dioxane and glycerol (6).
Our earlier theoretical calculations (9) had predicted a small bar-
rier for twisting in the first excited state of t-DMASBT to attain
a perpendicular geometry. Therefore, the thermally activated non-
radiative de-excitation process that competes with fluorescence
may be assigned to isomerization.
CONCLUSION
Contrary to the report of Saha et al., the trans–cis photoisomer-
ization of t-DMASBT competes with fluorescence not only in
nonpolar solvents but also in polar and polar viscous solvents
such as methanol and glycerol. The relatively high fluorescence
quantum yield of t-DMASBT in glycerol is due to restriction by
increased viscosity on the twisting of carbon–carbon double
bond that leads to isomerzation and not to restricted twisting
from the TICT state as proposed earlier. The photoisomerization
of t-DMASBT does not occur by an adiabatic path as previously
proposed, but by a nonadiabatic path and the cis isomer is non-
fluorescent in fluid solution at room temperature. DFT calcula-
tions predict that cis-B is stabilized by intramolecular hydrogen
bonding under isolated condition.
Acknowledgements—The authors thank the Department of Science and
Technology (DST), India and the Council of Science and Industrial
Research (CSIR), India for the financial support. The authors are also
thankful to the reviewers for their valuable suggestions.
SUPPORTING INFORMATION
Additional Supporting Information may be found in the online
version of this article:
Figure S1. kexc dependence emission spectra of t-DMASBT
(normalised) in chloroform (kexc value varies from 375 to
430 nm).
Figure S2. kexc dependence emission spectra of t-DMASBT
(normalised) in dioxane (kexc value varies from 375 to 430 nm).
Figure S3. kexc dependence emission spectra of t-DMASBT
(normalised) in acetonitrile (kexc value varies from 375 to
430 nm).
Figure S4. kexc dependence emission spectra of t-DMASBT
(normalised) in methanol (kexc value varies from 375 to
430 nm).
The potential energy surface constructed by Saha et al. has a
huge barrier for rotation (77 kcal) in the ground state and it is
also considerable (36 kcal) in the excited state without a phan-
tom state (6). Although their potential energy surface predict no
photoisomerism, they hypothesized that the trans isomer (t*)
undergoes torsional rotation in the first excited state to form cis
isomer (c*) in the excited state and that the cis isomer relaxes
radiatively. Contrary to the adiabatic path proposed by Saha
et al. (6), the DFT calculation carried out by us predicted a non-
adiabatic path (9). There is a small barrier for twisting in the
trans isomer in the S1 state to reach the perpendicular state (p*).
The molecule decays from the p* state by a nonradiative path to
the ground state trans and cis isomers. There is little or no bar-

6
Anasuya Mishra et al.
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Figure S5. kexc dependence emission spectra of t-DMASBT
(normalised) in glycerol (kexc value varies from 375 to 430 nm).
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