Journal of Photochemistry and Photobiology A: Chemistry 199 (2008) 179–187

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Journal of Photochemistry and Photobiology A:
Chemistryjournal homepage: www.elsevier.com/locate/jphotochem

Excited state isomerization and effect of viscosity- and temperature-dependent
torsional relaxation on TICT ﬂuorescence of trans-2-[4-(dimethylamino)
styryl]benzothiazole
Subit Kumar Saha a,∗ , Pradipta Purkayastha a,∗ , Asim Bikas Das b , Surajit Dhara cabcChemistry Group, Birla Institute of Technology and Science, Pilani 333031, Rajasthan, India
Biological Sciences Group, Birla Institute of Technology and Science, Pilani 333031, Rajasthan, India
School of Physics, University of Hyderabad, Hyderabad 500046, AP, Indiaa r t i c l e

i n f oArticle history:
Received 5 December 2007
Received in revised form 11 April 2008
Accepted 17 May 2008
Available online 27 May 2008
Keywords:
TICT
Fluorescence
Viscosity
Temperature
Torsional relaxationa b s t r a c t
Effect of viscosity and temperature on twisted intramolecular charge transfer (TICT) ﬂuorescence oftrans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) have been studied. TICT ﬂuorescence quantum yield in glycerol solution is found to be ∼23-fold greater than that in ethylacetate as a non-viscous
solvent. For high-viscosity solvent, the ﬂuorescence quantum yield increases at low temperature dueto the decrease in free-volume of the solvent, which favors the decrease in torsional relaxation of themolecule that induces the radiationless decay. However, the free-volume concept is not meaningful at atemperature much above the glass transition temperature when the free-volume is highly abundant. The
temperature-dependent phenomenon provides a more accurate description compared to the viscositydependent study in a series of solvents of varying viscosity at a given temperature. The stabilization of the
TICT state, as a consequence of the restricted motion of the N(CH3 )2 group in DMASBT, results in a large
Stokes shifted ﬂuorescence band. The TICT ﬂuorescence characteristics of DMASBT are found to be different from that of molecular rotors. In solvents of low polarity, where TICT is practically zero, the molecule
exhibits excited state temperature-induced cis–trans isomerization showing ﬂuorescence emissions fromboth the isomers. However, temperature-induced TICT ﬂuorescence quenching is observed in polar viscous medium without any isomerization. Results indicate that DMASBT can be a potential microsensor
for biomimicking as well as real biological systems.
© 2008 Elsevier B.V. All rights reserved.

1. Introduction
Multichromophoric molecules displaying characteristic features of charge separation are well-suited model systems
demonstrating charge transfer [1,2]. These molecules having
intramolecular charge transfer (ICT) and twisted intramolecularcharge transfer (TICT) characteristics show highly Stokes shifted
ﬂuorescence bands that depend on the chemical and physicalproperties of the medium [3]. Highly Stokes shifted ﬂuorescencethat appears in addition to the normal ﬂuorescence as a result of
twisting of the donor part around the single bond connector ina donor–acceptor system is called twisted intramolecular chargetransfer (TICT) ﬂuorescence as ﬁrst suggested by Grabowski et al.
[3].

∗ Corresponding authors. Tel.: +91 1596 245073x238; fax: +91 1596 244183.
E-mail addresses: sksaha@bits-pilani.ac.in (S.K. Saha), ppurkayastha@bitspilani.ac.in (P. Purkayastha).
1010-6030/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2008.05.020

Effect of viscosity and temperature on TICT ﬂuorescence has
been an interesting subject of research from long before [2,4].
There are recent reports on effect of solvent polarity and viscosity on TICT properties of various molecular systems such as
coumarins, N,N -dimethylaminobenzonitrile (DMABN), a groupof molecules called ﬂuorescent molecular rotors [4], 9-(N,Ndimethylamino)anthracene (9-DMA) [5], etc. Although solvent
polarity causes a bathochromic shift of the emission band in allthese compounds, this shift is smallest in the case of molecularrotors. Peak intensity is inﬂuenced strongly by solvent viscosity in
DMABN and the molecular rotors, but polarity and viscosity inﬂuences cannot be separated with DMABN [4]. Coumarins, on theother hand are not sensitive to viscosity. The dependence of TICT
ﬂuorescence on viscosity of the medium has been widely reported
and is thought to occur by a time-dependent intramolecular reorientation process [2,6,7]. Dey and Warner [5] have noticed strong
dependence of ﬂuorescence peak intensity of 9-DMA on solvent
polarity and viscosity similar to DMABN. Mielniczak et al. [2] havestudied on a solvent viscosity and polarity sensitive ﬂuorescent

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S.K. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 199 (2008) 179–187

sensor, 4-(4-dimethylaminostyryl)pyridinium (DMASP) derivative.
They explain that the enhancement of the TICT ﬂuorescence intensity with the increase in the viscosity of the medium surrounding
the ﬂuorophores is due to the reduction of the non-radiativetransitions induced by intramolecular reorientation and diffusional collisions with the solvent molecules and justiﬁed by the
Debye–Stokes–Einstein (DSE) hydrodynamic model [7]. Haidekker
et al. [4] have explained the dependency of the emission intensityof molecular rotors by studying viscosity-dependent phenomenain the solvent of varying viscosities at a constant temperature
through a power law relation proposed by Förster and Hoffmann [8]suggesting that the relaxation process is much deviated from theDSE diffusion in the high-viscosity region. In the present work on
DMASBT it has been observed that although DMASBT is structurally
similar to DMASP but the relaxation process is dependent on thetemperature- and viscosity-dependent free-volume of the solvent
proposed by Loutfy and Arnold [7] rather than the DSE hydrodynamic model. Moreover, temperature-dependent phenomena
provides better description of the torsional relaxation rather thanviscosity-dependent study at a constant temperature which is incontrary to the study on some molecular rotors by Haidekker et al.
[4]. The free-volume concept [7] rather than the DSE hydrodynamic
model gives more satisfactory result to explain the high quantum
yield in the highly viscous solvents at low temperature. Recently,
Dey and Warner [5] have ruled out the association of large Stokes
shifted ﬂuorescence with the TICT process in the viscous medium
without any speciﬁc reason. But this work suggests that TICT ﬂuorescence can be highly Stokes shifted in viscous medium and theprobable reason for the same has been discussed. It has also been
observed in this study that not only the ﬂuorescence quantum yieldof DMASBT is dependent on viscosity but also the ﬂuorescence bandis highly red shifted in a polar solvent [9], which is in contrary to themolecular rotors. The present report on DMASBT shows that unlike
coumarins and molecular rotors, both solvent polarity and viscosity
have their own effects on emission intensity as well as on emission
wavelength. It has been demonstrated that in case of DMASBT thiseffect is not as complex as DMABN [4]. It can be mentioned thatmany cellular and organismal functions are related to the viscosityof their environment, and the alteration in cell membrane viscosity
is well known to be responsible for several diseases, e.g. cell malignancy, hypercholesterolemia, atherosclerosis and diabetes [4].
In view of the development of a method that could be applicable for the measurements of viscosity in biological systems, and inparticular real-time measurements of viscosity changes on a microscopic scale, the present study aims to get a ﬂuorescent moleculethat can act as a microsensor for biomimicking as well as real biological systems. We chose DMASBT because of high sensitivity of its
ﬂuorescence properties on polarity and pH [9] of the environment.
The present work is mainly focused on to see the sensitivity of TICT
ﬂuorescence of DMASBT on the viscosity of the medium. Although,some success to measure the viscosity in biological systems has
been accomplished through the use of molecular rotors by meansof their viscosity-dependent ﬂuorescence quantum yields, but the
micropolarity of the biological systems can also get changed due tothe intercalation of water molecules. The molecular rotors cannot
sense this too effectively. It is also worth mentioning here that conventional mechanical viscometers are cumbersome to use and areincapable to perform real-time viscosity measurements.
Another important excited state process is photoisomerization.
A system of excited state charge transfer coupled with isomerization under a certain condition can be an interesting model for
important natural photoprocesses like photosynthesis and vision. It
is now quite well known that the molecular properties get changed
remarkably on photoisomerization [10–15]. In the present work,
temperature-dependent excited state isomerization process has

also been studied to see how excited state isomerization affects the
charge transfer phenomenon. DMASBT has been reported to havenon-genotoxicity [16].
2. Experimental
2.1. Materials and methods
DMASBT was procured from Aldrich Chemical Company, WI,
USA. The methods of recrystallization and purity check are mentioned elsewhere [9]. All the solvents used are of spectroscopic
grade and procured from Spectrochem Chemical Company, India.
Triple distilled water was used for the preparation of the aqueous solutions. To record the UV–vis absorption and ﬂuorescence
spectra of DMASBT in pure solvents, a stock solution of DMASBT(1.001 × 10−3 M) was prepared in pure methanol, 0.1 mL of which
was poured in a 10 mL volumetric ﬂask and left for a few hours
for complete evaporation of methanol and then the compound
was dissolved in respective solvents to make the ﬁnal volume to
10 mL. For the preparation of solutions in different percentages of
the glycerol–water mixtures, 0.05 mL (2.002 × 10−3 M) of DMASBT
solution in methanol was added to a mixture of glycerol andwater and the ﬁnal volume of it was adjusted to 10 mL. Methanol
was added due to low solubility of DMASBT in water as well as
glycerol–water mixture. The percentage of methanol present inglycerol–water mixture is only 0.5. The concentration of DMASBT
in all the experimental solutions used for spectroscopic measurements was 1.001 × 10−5 M. The ﬂuorescence quantum yields were
determined with respect to that of quinine sulfate in 0.1N H2 SO4 as
0.55.
The absorption spectra were recorded using a Jasco V570 UV–vis
spectrophotometer. Fluorescence measurements were performed
using a Shimadzu RF-5301PC scanning spectroﬂuorimeter. Fluorescence lifetimes were determined from time-resolved intensity
decay by the method of time-correlated single-photon counting
using a picosecond diode laser at 370 nm (IBH, U.K., nanoLED-07)
as light source. The typical response time of this laser system was
50 ps. The ﬂuorescence decays were deconvoluted using IBH DAS6
software. Mean (average) ﬂuorescence lifetimes for biexponential
iterative ﬁttings were calculated from the pre-exponential factors
and the decay times using the following equation [17]:<  >= a1 1 + a2 2(1)

The steady-state ﬂuorescence anisotropy measurements were performed with the same steady state spectroﬂuorimeter ﬁtted witha polarizer attachment. The excitation and emission bandwidths
used for the anisotropy measurements were 5 nm each. The steadystate anisotropy, r can be represented as [18]r=

IVV − GIVH
IVV + 2GIVH(2)where IVH and IVV are the intensities obtained from the excitation
polarizer oriented vertically and the emission polarizer oriented
in horizontal and vertical positions, respectively. The factor G isdeﬁned as
G=

IHV
IHH

(3)

where I terms refer to parameters same as mentioned above forthe horizontal position of the excitation polarizer. All measurements other than the study of temperature effect were done at room
temperature (25 ◦ C).
To study the temperature dependence of the ﬂuorescence of
DMASBT, a simple on–off temperature controller was designed. A
suitable heater made of aluminium block was fabricated for holding

S.K. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 199 (2008) 179–187

the cuvette. The temperature controller is based on a Wheatstonebridge balance. Adjustment of the resistance of the potentiometer(Rset ) can provide the desired temperature. The bridge was balanced
when the sample attained the desired temperature. A thermistor,
which forms a part of the bridge, is placed in the heater. The temperature dependence of the non-linear resistance was calibrated withthe help of a thermocouple. A digital display was connected to theheater to monitor the temperature. The accuracy of the temperature measurement was within 0.1◦ . The temperature of the sample
was kept ﬁxed during the ﬂuorescence scan.
Hyperchem package version 6.01 (Hypercube Inc., Canada) has
been used for the theoretical calculations. The ground state (S0geometry) of the compound was optimized by AM1 method. AM1SCI (singly excited conﬁguration interaction) was performed to get
the energy (Eg ) and dipole moment in the ground state and the transition energies (Ei→j ), and dipole moments of different excitedstates. We have taken care of all the singly excited conﬁgurations
within an energy window of 11 eV from the ground state. Ei→jcorresponds to the excitation of an electron from the orbital i(occupied in the ground state) to the orbital j (unoccupied in the
ground state). The total energy of the excited state (Ej ) was thencalculated as Ej = Eg + Ei→j . The CI wavefunction has been used togenerate g orbitals and one-electron density matrices, which wereused to calculate the dipole moments of the excited states of thecompounds.
3. Results and discussion
3.1. Absorption and emission characteristics of DMASBT in
different solvents
Table 1 summarizes the spectral properties of DMASBT in aseries of solvents studied earlier [9]. Both absorption and emission
band maxima are red shifted with increasing the polarity of the solvents. Structured absorption and ﬂuorescence bands in non-polar
and/or less polar solvents progressively become structureless with
the increase in the polarity of the solvent media. The mirror image
relationship between absorption and emission bands in non-polar
solvents conﬁrmed similar geometries of absorbing and emitting
Table 1ﬂaAbsorption peak position (abmax (nm)), ﬂuorescence peak position (max (nm)) , ﬂuorescence quantum yields (ϕf ) and Stokes shift (ss ) of DMASBTb in different solvents
along with their ET (30) (kcal mol−1 ) values
Solvents

ET (30)abmax (nm)ﬂmax (nm)ss (cm−1 )ϕf (±0.0001)c

Cyclohexane
HeptaneHexaneDiethyl ether
1,4-Dioxane
THFd
Ethylacetate
DMFe
DMSOf
Acetonitrile
Isopropanol
Ethanol
Methanol
GlycerolWaterg

30.9
31.1
32.4
34.5
36.0
37.4
38.1
43.6
44.8
45.6
48.4
51.9
55.4
56.7
63.1

384
387
384
390
393
396
391
402
405
394
394
396
397
409
402

447
446
445
460
467
482
477
504
513
504
493
498
507
530
516

3671
3418
3604
3902
4032
4506
4611
5035
5198
5540
5097
5173
5465
5582
5496

0.00100.00140.00180.00200.00220.00300.00530.00700.00780.00660.00480.00450.00300.12100.0012ex = 395 nm.
[DMASBT] = 1.001 × 10−5 M.
cFluorescence quantum yields at ex = 395 nm.
dTetrahydrofuran.
eDimethylformamide.
fDimethylsulfoxide.
g
At pH 7.2.

181

Scheme 1. Structure trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT).

species. The appearance of broad structureless Stokes shifted bands
in polar solvents and progressive increase in ﬂuorescence quantum
yields with increasing the polarity of the solvents suggested thecharge transfer (CT) character of the emitting state in polar media[9]. Emissions in non-polar solvents are described to be originatedfrom the locally excited (LE) state. Upon excitation, transfer of anelectronic charge from the donor (D) part to the acceptor (A) siteof the molecule leads to a charge separation. According to quantum chemical AM1 calculations [9] on DMASBT, the dimethylaminogroup ( N(CH3 )2 ) acts as the donor part and the acceptor part is the
2-styrylbenzothiazole moiety (Scheme 1). The donor and acceptor
moieties are ﬂexibly linked by a C–N bond. The torsional angle ()(Scheme 1) related to the rotation of donor with respect to acceptor
around C N bond is equal to zero when the amino lone pair orbitalis perpendicular to the molecular plane of the acceptor subsystem.
A twisting with torsional angle,  = 90◦ leads to full intramolecular charge transfer (ICT) in the polar medium at the excited state
known as TICT state. Theoretical calculations [9] show that the S3state is responsible for the polarity-induced TICT emission. The calculated dipole moments of STICT
state and the ground singlet TICT
3
)
are
12.5
and
0.8
Debye,
respectively. Therefore, greater
state (STICT
0
state
comparedto the STICT
state by the inﬂustabilization of STICT
30ence of polar medium
results
intoadecreaseinthe energy gap,

− STICTwith a consequent red shift in the TICT emission.
E STICT
30
As a result of that the rate of internal conversion from STICT
to STICTis30higher in water than that in less polar solvents. This fact is in favor oflower TICT ﬂuorescence quantum yield in the aqueous medium ascompared to that in non-polar medium [19]. Moreover, TICT emissions in less polar solvents are also increased due the decrease in
the rate of intersystem crossing from the STICT
state to triplet TICT
3
state [20]. The detailed explanations of the results obtained fromthe absorption and ﬂuorescence study of DMASBT in pure solventsof different polarities have been given elsewhere [9]. Fluorescencequantum yields and Stokes shifts of DMASBT in the solvents of different polarities are listed in Table 1. The variation of ﬂuorescencequantum yields and Stokes shift with the solvent polarity parameter, ET (30) have been shown in Fig. 1 and its inset, respectively.
Fig. 1 shows that the ﬂuorescence quantum yield increases, reaches
maximum and then decreases with increasing the polarity of thesolvents. This can be explained by the fact that initially ﬂuorescencefrom charge transfer state is increased due to the stability of thisstate with respect to the LE state, but later it is decreased in highlypolar solvents due to the high possibility of non-radiative processes,
internal conversion and inter system crossing [9]. As a matter offact Stokes shifts are also substantially changed with increasing thepolarity of the medium (Fig. 1(inset)). Although ﬂuorescence quantum yields are low in most of the solvents, but both quantum yieldand Stokes shift of the molecule are highest in glycerol as solvent.

ab

3.2. Fluorescence study in glycerol–water mixture
As mentioned above, both ﬂuorescence quantum yield andStokes shift of DMASBT are highest in glycerol (Table 1). As depicted
in Fig. 2(a), the ﬂuorescence intensity increases with increasing
the percentage of glycerol in glycerol–water mixture with a sub-

182

S.K. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 199 (2008) 179–187

Fig. 1. Plot of ﬂuorescence quantum yields of DMASBT vs. ET (30) of pure solvents. (Inset) Plot of Stokes shift (cm−1 ) of DMASBT vs. ET (30) (kcal mol−1 ) of
pure solvents: () cyclohexane; () heptane; (䊉) hexane; () diethyl ether; ()1,4-dioxane; () THF; () ethylacetate; (♦) DMF; () DMSO; () acetonitrile; ()isopropanol; ( ) ethanol; ( ) methanol; () glycerol; ( ) water. ex = 395 nm[DMASBT] = 1.001 × 10−5 M.

stantial bathochromic shift of the ﬂuorescence band up to 60%
glycerol–water mixture. Above this, although increase in intensity
continues but the shift of peak maxima plateaus. Increased energy
gap between STICTand STICT
states in glycerol due to lower polarity
30
compared to water may be a reason for the increased TICT ﬂuorescence but in that case it demands a hypsochromic shift withincreasing percentage of glycerol in the mixture. The red shiftedband cannot be due to exciplex, because in that case ﬂuorescencebands for monomer as well as exciplex are expected [18]. The ﬂuorescence quantum yield in glycerol is much greater than that in
non-viscous solvent. As for example, it is almost 23-fold greaterin glycerol than that in ethylacetate as a non-viscous solvent. Fluorescence quantum yield in pure glycerol solution is almost 15
times greater than that in 20% glycerol–water mixture, whereas
the same in case of 4-(4-dimethylaminostyryl)pyridinium derivative, reported by Mielniczak et al. [2] got increased by six times
only. The presence of shoulder at lower wavelength for the ﬂuorescence band of DMASBT in solution of low glycerol percentages
could be due to the contribution of LE ﬂuorescence as a result of
introduction of non-polarity by the addition of glycerol in water.
This disappears at high percentage of glycerol due to the higher
population and stabilization of TICT state as discussed later.
The extent of restriction imposed by the medium in which the
ﬂuorophore is dissolved can be described by the study of ﬂuorescence anisotropy of DMASBT in glycerol. The plot for variation of
ﬂuorescence anisotropy as a function of weight percentage of glycerol in glycerol–water mixture is shown in Fig. 2(b). Fluorescenceanisotropy (em = 550 nm) of DMASBT increases with increasing theviscosity of the medium to a very high anisotropy value (∼0.38)
in pure glycerol solution. This can be attributed to the signiﬁcant
restriction of rotational diffusion of the molecule in a mediumof high viscosity and can be correlated with the increase in ﬂuorescence intensity. Loutfy et al. [7] studied the effect of viscosity
and temperature on ﬂuorescence quantum yields of molecularrotors extensively. To compare the characteristics of DMASBT withthose of molecular rotors, the effect of temperature on ﬂuorescencequantum yields of DMASBT in dioxane (Fig. 3(a)) and in glycerol(Fig. 3(b)) has been studied. It is observed that in case of dioxane
ﬂuorescence quantum yield decreases with increasing temperature

Fig. 2. (a) Plot of variation of ﬂuorescence spectra of DMASBT with the variationof percentage of glycerol in glycerol–water mixture. The broken line represents 0%
glycerol, i.e., pure aqueous solvent (ex = 367 nm). (b) Plot of variation of ﬂuorescence anisotropy (em = 550 nm) of DMASBT as function of the composition of theglycerol–water mixture (ex = 367 nm). [DMASBT] = 1.001 × 10−5 M.

accompanied by a substantial blue shift of the wavelength maxima at a temperature higher than 45 ◦ C, whereas in case of glycerol
ﬂuorescence quantum yields are decreased without any change inwavelength maxima. Effect on ﬂuorescence quantum yields andwavelength maxima are explained separately in the following sections.
The DSE hydrodynamic model and the free-volume-dependent
torsional relaxation dynamics of probe proposed by Loutfy et al.
[7] have been tested to describe the relaxation processes. Försterand Hoffmann [8] derived an equation, f = B x which was foundto hold good for various dyes. In this equation, f and are ﬂuorescence quantum yield and viscosity coefﬁcient, respectively, B isa constant and x lies between 1/3 and 2/3. The ﬂuorescence quantum yields of DMASBT in different composition of glycerol–water
mixtures are measured at 25 ◦ C and are listed in Table 2 along withthe values of viscosities of the mixtures. Fig. 4(a) shows the plotof log f against log . A reasonably good straight line (regression
coefﬁcient = 0.94) is obtained with a slope much less than unity(0.38). This suggests that DSE diffusion in high-viscosity region is

S.K. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 199 (2008) 179–187

not a suitable model for relaxation process. To see the effect of temperature in high-viscosity solvent, ﬂuorescence quantum yields of
DMASBT in glycerol have been determined at different temperatures. The values of ﬂuorescence quantum yields in glycerol alongwith the viscosity values of glycerol at different temperatures arealso listed in Table 2. Fig. 4(b) shows the variation of ﬂuorescencequantum yields on /T. The data displayed in Fig. 4(b) show thatﬂuorescence quantum yield (f ) is proportional to ( /T)x , where
x is equal to 0.49 for temperatures between 24 and 80 ◦ C. The
much better regression coefﬁcient (0.996) of this plot comparedto the plot of variation of quantum yields with viscosity at a given
temperature, suggests that rather than the viscosity-dependent
phenomenon in a series of solvents at a given temperature, thetemperature-dependent studies give more consistent data and provide less ambiguous tests of hydrodynamic predictions. To test
the importance of the temperature- and viscosity-dependent freevolume of the viscous solvent on the molecular relaxation processof the excited state (in case of DMASBT it is S3 ) the following equations have been used [7]. The free-volume of a solvent is dependent
on the temperature as follows:f = fg + ˛(T − Tg )(4)

where ˛ is the thermal expansion coefﬁcient, f and fg are the freevolume fractions at given temperatures T and Tg , respectively. The
constants ˛ = 4.4 × 10−4 deg−1 , fg = 0.025 and Tg = 187 K for glycerol[7].
The viscosity of liquid is dependent on the free-volume fraction
as follows [21]:= A exp

f 
o

(5)

f

where f and fo are free- and occupied-volume fractions, respectively. This dependence is controlled by the value of A of liquid.
This relationship between viscosity and free-volume fractions has
shown more accuracy in the determination of viscosity of nheptadecane at different temperatures compared to that obtainedby using Andrade temperature equation [21]. The torsional motion
induced non-radiative decay rate constant, knr can be related to thefree-volume fraction of solvent, f according to the equation:oexp
knr = knr

 −xf o(6)

fo is the rate ofwhere x is a constant for the particular molecule and knr
orientation of the free-rotor. The non-radiative decay rate constant,183

knr is related to the ﬂuorescence quantum yield and radiative rate
constant as follows:
knr = kr1
f − 1(7)

Therefore, from Eqs. (6) and (7), the following equation is obtained
which relates ﬂuorescence quantum yield with the free-volume
fraction:
f =

k 
r
o
knr

exp

 xf 
o(8)fTherefore,ln f = ln

 k   xf 
r
o
o
knr

+

(9)

f

The free-volume fractions (f) of glycerol at different temperatures
are calculated and tabulated in Table 2. Fig. 5 represents the plot ofln f against 1/f. This plot shows the linear dependence with regression coefﬁcient, 0.99. The slope of this plot (=xfo , Eq. (9)) =0.598.
Fig. 5 (inset) is the plot of ln against 1/f. The slope of this plot isfound to be 1.18 (regression coefﬁcient = 0.99), which is the valueof fo Eq. (5). From the values of these last two slopes x is calculated to be 0.51. The slight deviation from the linear dependence ofln f on 1/f at high temperature as explained by Loutfy et al. [7] isdue to the high abundance of free-volume available for the rotation
dependent relaxation of the molecule.
Combining Eqs. (5) and (8) the relation found is as follows:f = B

 x

(10)

T
x

o )(T/A) . The variation of log  with log( /T) has
where B = (kr /knrfalready been shown in Fig. 4(b). The slope value, i.e., the valueof exponent x (0.49) agrees well with those calculated above(0.51). This result is consistent with the fact that the rotation
dependent non-radiative decay rate increases with increasing thefree-volume and decreasing the viscosity of the solvent. Therefore,
the viscosity-dependent ﬂuorescence quantum yield of DMASBT
can be explained on the basis of the dependence of viscosity coefﬁcient ( ) on the free-volume of solvents whose free-volume isvery small to cause high viscosity. At temperature much above theglass transition temperature, the fraction of free-volume is so largethat the free-volume dependence of ﬂuorescence quantum yield
becomes inappropriate [7].
To provide additional information on the effect of viscosity
on the non-radiative decay rate, the ﬂuorescence decay proﬁlesTable 2
Fluorescence quantum yields (f ) of DMASBT in different percentage of glycerol–water mixture at constant temperature (25 ◦ C), viscosities ( ) of glycerol–water mixtures at
25 ◦ C, ﬂuorescence quantum yields (f ) in pure glycerol at different temperatures, viscosities ( ) and free-volume fractions (f) of pure glycerol at different temperatures
Constant temperature study with glycerol–water mixture% of glycerol in mixtures (v/v)
24
32
40
48
56
64
71
75
90
100

a

a

(cP)

1.80
2.38
3.18
4.97
7.59
15.46
27.91
36.43
267.00934.00Temperature-dependent study with pure glycerol

Quantum yield (f )b , cTemperature (K)

0.013
0.016
0.017
0.019
0.029
0.039
0.055
0.062
0.112
0.121

298.15303.15308.15313.15318.15323.15328.15333.15338.15343.15348.15353.15d(cP)

934.00637.28449.95317.96220.03156.17109.33
75.27
49.73
36.95
28.44
15.67

Quantum yield (f )b , c , d

f × 102

0.121
0.115
0.098
0.079
0.062
0.050
0.044
0.035
0.030
0.025
0.021
0.018

7.391
7.611
7.831
8.051
8.271
8.491
8.711
8.931
9.151
9.371
9.591
9.811

Ref. [2] (viscosity data were obtained by extrapolation of dependence of viscosity on (v/v) % of glycerol–water mixture).
ex = 395 nm.
cStandard deviation ±0.0001.
dRef. [7] (viscosity data were obtained by extrapolation of dependence of viscosity on temperature in Kelvin scale) [DMASBT] = 1.001 × 10−5 M.
b184

S.K. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 199 (2008) 179–187

Fig. 3. (a) Variation of ﬂuorescence spectra of DMASBT in dioxane as a function oftemperature of the solution. Inset: plot of the ﬂuorescence intensities against temperature. (b) Variation of ﬂuorescence spectra of DMASBT in glycerol as a functionof temperature of the solution. Inset: plot of the ﬂuorescence intensities against
temperature. ex = 395 nm, and [DMASBT] = 1.001 × 10−5 M.

Fig. 4. (a) Plot of ﬂuorescence quantum yields (f ) of DMASBT against viscosities
of different composition of glycerol–water mixtures at 25 ◦ C. (b) Plot of ﬂuorescence quantum yields (f ) of DMASBT in glycerol against /T. ex = 395 nm, and[DMASBT] = 1.001 × 10−5 M.

of DMASBT have been measured in 20% glycerol–water and pureglycerol solutions and are shown in Fig. 6(a) and (b), respectively.
The calculated lifetimes are listed in Table 3. Since the lifetime of
DMASBT in water is very low (<50 ps), instrumental limitation did
not allow us to calculate it. The raw experimental data are suitably
ﬁtted by double exponential ﬁtting equation. The contributions ofdifferent species in terms of pre-exponential factors, 2 and standard deviations are also mentioned in Table 3. The slow decay of
ﬂuorescence of both the species with increasing the viscosity ofthe medium supports the restricted orientational reorganizationof solvent molecules in viscous environments. Better value of 2

Table 3
Lifetimes () and pre-exponential factors (a) of DMASBT in different percentages ofglycerol in glycerol–water mixtures% of glycerola1 1 (ps)a2

 2 (ps)

20
100

0.76
0.32

130 ± 3257 ± 30.24
0.68

706 ± 51100 ± 6

2

1.47
0.99

Fig. 5. Plot of ln f against 1/f. (Inset) Plot of ln

against 1/f.

S.K. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 199 (2008) 179–187

185

Table 4
Fluorescence quantum yields (ϕf ), average lifetimes (  ), radiative (kr ) and nonradiative (knr ) rate constants of DMASBT in different percentages of glycerol inglycerol–water mixtures% of glycerol
20
100

ϕf
0.008
0.121

 (ps)

kr (s−1 )knr (s−1 )

268
830

2.99 × 10
1.46 × 108

73.70 × 109
1.06 × 109

that contributions of different fractions are reversed when percentage of glycerol increases (Table 3). The average lifetime values of
DMASBT in different percentages of glycerol–water mixtures calculated using Eq. (1) are tabulated in Table 4. The decay parameters
are affected by the increased percentage of glycerol, with the average lifetime,  changing from 268 ps in 20% glycerol to 830 ps inpure glycerol. The increased viscosity of medium is reﬂected in theincrease in excited state lifetime. From calculated values of ϕf and , we calculated the radiative and non-radiative rate constants for
the TICT process using the following equations [22]:Fig. 6. Lifetime decay proﬁles of DMASBT in (a) 20% glycerol, and (b) pureglycerol solutions. The gray dots represent the signal for the prompt and theblack ones represent that for DMASBT. The solid line is the ﬁt to the raw data.
ex = 370 nm, em = 500 nm for 20% glycerol, and em = 550 nm for pure glycerol.
[DMASBT] = 1.001 × 10−5 M. Instead of the residuals of the ﬁts, the 2 values havebeen mentioned in Table 3 that carries the same signiﬁcance.

in high percentage of glycerol is in favor of relatively low probability of formation of multiple species (more than 2) in a mediumof high viscosity. The restricted orientational reorganization of solvent molecules in viscous environments is consistent with the highvalues of ﬂuorescence anisotropy and ﬂuorescence quantum yield.
The large Stokes shift or the bathochromic shift of the ﬂuorescence band in viscous medium can be due the restricted rotationalmotion of N(CH3 )2 group attached to DMASBT. The decrease in
free-volume with increasing the viscosity of the solvent causes the
restricted motion of N(CH3 )2 group thereby increasing the TICT
ﬂuorescence quantum yield as well as the bathochromic shift of
the ﬂuorescence band because the proper orientation of N(CH3 )2
group is related not only to the population of the TICT molecules
but also to the stabilization of the TICT state. The plateau in the shiftof band maxima above 60% glycerol–water mixture could be due tothe fact that stabilization of TICT state as a result of proper orientation of N(CH3 )2 group is complete at this viscosity. The continuity
in increase in ﬂuorescence quantum yields for solutions above 60%
glycerol is as a result of decrease in the free-volume with increasing viscosity. The biexponential decay could be due to two different
populations of excited species where one experiences smaller fraction of free-volume giving slower decay, whereas other gets larger
fraction of free-volume resulting in faster decay. This is the reason

kr =

ϕf


(11)

knr =

1
− kr


(12)

It is apparent from Table 4 that radiative rate constant (kr ) is
increased by 4.9 times whereas non-radiative rate constant (knr ) is
decreased by 3.5 times with increasing the concentration of glycerol in glycerol–water mixture. The decrease in non-radiative decayrate as explained above are due to the decrease in free-volume andincrease of viscosity of the solvent that causes rotational restrictionin the molecule.
It is well known that ﬂuorescence anisotropy may change due
to rotational diffusion of the molecule as well as the ﬂuorescence
lifetime. To make sure that the observed change in steady stateanisotropy of DMASBT in different percentages of glycerol–water
mixture is not due to any change in the ﬂuorescence lifetime,
Perrin’s equation Eq. (13) [18] has been used to calculate the approximate values of the average rotational correlation times:
c =

 r
ro − r(13)

where r, ro and  are the steady-state anisotropy, limiting
anisotropy and mean lifetime of DMASBT, respectively. As mentioned above, the calculated values are approximate; there are a
few reasons for that. Although Perrin’s equation is not strictly valid
for the present case yet the equation is considered to be valid dueto the use of mean values of ﬂuorescence lifetime. Other reasons
are the unavailability of exact value of ro as the time resolved
anisotropy could not be done and the lifetime of species less than
50 ps could not be measured. In the present calculation, the valueof ro is taken as 0.38 in case of glycerol–water mixture, where 0.38
is the anisotropy value of DMASBT in pure glycerol and ro = 0.18 incase of aqueous medium (taken from the literature [17]). The lifetime value in aqueous medium is considered to be 50 ps. With thislifetime value, although the exact value of  c is not found, but atleast the correct trend of  c values in the medium of different viscosities could be predicted. The values of  c calculated using Eq. (13)are found to be ∼25 and ∼71 ps in aqueous and 20% glycerol–water
mixtures, respectively taking experimentally determined r values
as 0.06 and 0.08 for water and 20% glycerol–water mixtures, respectively. If the lifetime of DMASBT in aqueous medium was lower
than 50 ps (greater value is not expected because then we could
have calculated lifetime with our instrument),  c value could befurther lower than 25 ps according to Eq. (13). The increase of  cwith increasing viscosity of environments deﬁnitely suggests that

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S.K. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 199 (2008) 179–187

the observed changes in steady state ﬂuorescence anisotropy values (Fig. 2(b)) are not due to lifetime-induced phenomena but aspredicted earlier that rotational restriction imposed on molecule
increases with increasing the viscosity of the medium.
Contrary to the argument made by Dey and Warner [5] regarding the absence of TICT ﬂuorescence of 9-DMA in viscous medium
due to the restricted motion of N(CH3 )2 group, many groups havereported the high yield of TICT ﬂuorescence in viscous medium[2,4]. Compared to these, the simpler and greater dependenceof TICT ﬂuorescence quantum yields of DMASBT on viscosity isobserved. Unlike coumarin derivatives and molecular rotors (e.g.,
9-(dicyanovinyl)-julolidine, DCVJ) [4], ﬂuorescence parameters of
DMASBT are dependent on both viscosities and polarities of thesolvents. When emission wavelengths of coumarin derivatives are
only inﬂuenced by solvent polarities without any effect of solvent
viscosities on ﬂuorescence parameters, the TICT ﬂuorescence intensities of molecular rotors are sensitive only to viscosities with low
sensitivity towards solvent polarities [4]. The ﬂuorescence intensity of DMASBT is increased by 15-fold as compared to the ﬁvefold
increase in DCVJ with increasing the percentage of glycerol from
20 to 100. Mielniczak et al. [2] explained the dynamics of reorientation relaxations of the 4-(4-dimethylaminostryryl)pyridinium(DMASP) derivatives in solution with high friction between therotating probe and solvent molecules, which is already provenfor many compounds exhibiting ICT properties including DMABN[6,7,23–25]. This is in support to the argument that dependency of
TICT ﬂuorescence intensity on viscosity of medium is a well-known
phenomenon. Moderately high ﬂuorescence quantum yields with
substantial bathochromic shifts of ﬂuorescence bands in DMF andDMSO as solvents can also be explained by their restricted orientational reorganizations because of viscosity.
3.3. Temperature-dependent ﬂuorescence study of DMASBTThe effects of temperature on the ﬂuorescence properties of
DMASBT in dioxane and glycerol as solvents are represented by
Fig. 3(a) and (b), respectively. On increasing the environment temperature of the system the ﬂuorescence quantum yield decreases,
which is a quite common observation that happens due to theenhanced movement of the molecule and thus opening more andmore non-radiative decay routes [26,27]. Interestingly, in dioxane
as solvent there is no wavelength shift of ﬂuorescence band maxima
up to a temperature of ∼45 ◦ C, but once this temperature is crossed
there is a clear hypsochromic shift of ∼4 nm followed by freezing of
any further shift with a gradual quenching of the ﬂuorescence. Thisappears to be a probable conﬁgurational change of the molecularsystem. It is also pertinent to note that unlike many cases [13], thechanged isomer is also ﬂuorescent. Since the molecule was initiallyin the trans form, therefore at a temperature of ≥45 ◦ C the mostprobable change in conﬁguration is due to its conversion to the cis
form. However, no such hypsochromic shift is observed in case ofglycerol as solvent, although ﬂuorescence is quenched as usual withincreasing the temperature. The inset of Fig. 3(a) shows the natureof the decrease in the ﬂuorescence intensity of DMASBT in dioxane

Fig. 7. Conﬁguration interaction calculations using AM1 method showing the potential energy surface (PES) diagram at different twist angles (25-10-11-26, vide
Scheme 2).

with increasing temperature. The decline follows a segmented linearity with two different slopes. On the other hand, the same in thecase of glycerol involves an exponential decay (inset of Fig. 3(b)).
This may be attributed to the fact that in case of glycerol the quenching is simply due to the increase in the rate of non-radiative decay atmoderately high temperature; however, at very high temperatures
much above the glass transition temperature of the solvent when
the free-volume is plentiful and free-volume concept becomes useless, the decrease in ﬂuorescence quantum yield is probably due
to the increase in singlet–triplet intersystem crossing [28]. On theother hand in case of dioxane, the quenching is deﬁnitely due tothe increase in the rate of non-radiative processes but in addition
to that there is a proposed conﬁgurational change after a certain
temperature (45 ◦ C). Conﬁguration interaction calculations using
AM1 method can be efﬁciently used to get the potential energy
surface (PES) diagrams (Fig. 7) at different torsion angles (25-10-1126, vide Scheme 2) [29,30]. Solid lines indicate the PES for DMASBT
in vacuum and the broken lines constitute those for the solvatedmolecule in dioxane (relative permittivity = 2.22). The ﬁgure clearlyshows the change in the magnitude of the energy barrier in the twoelectronic states, viz., S0 (ground state) and S1 (ﬁrst excited singlet
state). In both the states, trans isomer of DMASBT is the most stableone. The potential energy surface constructed by rotating the aromatic moieties on both sides of the double bond connecting C10
and C11 (vide Scheme 2) clearly shows that the trans geometry ismore stable than the cis form in the ground state. An energy barrierof 77 kcal mol−1 separates the two isomers in the ground state.
The next question that arises is whether the molecule retains its
trans geometry even after excitation. AM1-SCI calculations showthat the energy barrier between the cis and trans geometries

Scheme 2. Optimized geometry of trans isomer of DMASBT in the ground state.

S.K. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 199 (2008) 179–187

decreases remarkably to 36 kcal mol−1 . The structure of DMASBT isquite similar to that of stilbene derivatives those are well known
for their photoisomerization [13]. The ground state conﬁguration changes on excitation. The peculiarity in the present caseis that unlike stilbene, DMASBT retains its trans geometry andshows solvent polarity-dependent TICT in the excited state. Therefore, temperature-induced quenching study of the ﬂuorescence of
DMASBT in a solvent of low polarity (dioxane in the present study)
so as to rule out the interference of any TICT seemed to be a very
appropriate method of investigation.
4. Conclusion
DMASBT has been found to be highly sensitive to solvent polarity. TICT ﬂuorescence characteristics also depend on the viscosityof the environment profusely. In high viscous solvent such as glycerol, the free-volume concept proposed by Loufty et al. has beenfound to provide the accurate description of the solvent’s viscosity
and temperature-dependent torsional relaxation of DMASBT. The
torsional motion in the molecule responsible for inducing the nonradiative decay processes is hindered by the viscous drag of thesolvent that results in an increase in ﬂuorescence quantum yieldin highly viscous solvents. The rotation dependent radiationlessdecay rate gets reduced thereby increasing the ﬂuorescence quantum yield due to the decrease in the free-volume and increase ofviscosity of the solvent. The free-volume of the solvent decreases
with an increase in viscosity and decrease in temperature of the solvent, which in turn reduces the torsional relaxation in the molecule
resulting into an increase in the ﬂuorescence quantum yield. At
temperature much above the glass transition temperature, thisdependence is meaningless as it is known that the temperaturedependent viscosity of the solvent will then be related to thespeciﬁc details of the molecular structure of the solvent rather
than the free-volume. Results also show that the temperaturedependent studies provide more accurate description than theviscosity-dependent phenomena in the medium of varying viscosity at a constant temperature. Therefore, DMASBT can be used asa potential microscopic probe molecule that can provide valuable
information about the hydrodynamic interaction in various solvents. The restricted rotation of N(CH3 )2 group causes greater
extent of donation of charge towards acceptor thereby stabilizing the TICT state responsible for large Stokes shifted ﬂuorescencein highly viscous medium at low temperature. The temperaturedependent cis–trans isomerism has also been observed in solventsof low polarity where there is no TICT. Once the molecule getsthe suitable environment for TICT, it prefers to undergo that said
process. The characteristic of DMASBT is therefore, found to behighly polarity and viscosity dependent. The remarkably high ﬂuorescence quantum yield in polar viscous medium compared to
the non-viscous medium can make the molecule a useful microviscosity sensor to study the biological systems where many cellularand organismal functions are dependent on the viscosity of theirenvironments. There are a very few ﬂuorescent probes available
for real-time measurements of viscosity changes on a microscopic
scale. In this regard DMASBT can be a useful candidate not only due
to its high sensitivity towards viscosity change, but also because

187

of its high sensitivity towards polarity and pH [9]. Therefore, thepresent study provides a molecular probe which can behave as a
microsensor to study biological functions as well as biomimicking systems. Moderately high ﬂuorescence quantum yields in polar
aprotic and viscous solvents like DMF and DMSO can be explainedon the basis of restricted molecular rotation.
Acknowledgements
Financial support and facilities provided by Birla Institute ofTechnology and Science, Pilani, India are gratefully acknowledged.
The authors also extend their acknowledgement to Professor Nitin
Chattopadhyay and Dr. Arabinda Mallick of Department of Chemistry, Jadavpur University, Calcutta, India, for allowing us to use
the time-resolved ﬂuorescence set-up and the HyperChem software. Reviewers of this paper are also greatly acknowledged forproviding valuable suggestions. One of the authors (PP) gratefully
acknowledges the ﬁnancial support from the DST, Govt. of India forthe project number SR/FTP/CS-114/2005 under the SERC Fast Trackscheme.
References[1] W. Rettig, Angew. Chem. Int. Ed. Engl. 25 (1986) 971.
[2] A. Mielniczak, B. Wandelt, S. Wysocki, Mater. Sci. 20 (2002) 59, and references
cited therein.
[3] Z.R. Grabowski, K. Rotkiewicz, W. Rettig, Chem. Rev. 103 (2003) 3899.
[4] M.A. Haidekker, T.P. Brady, D. Lichlyter, E.A. Theodorakis, Bioorg. Chem. 33(2005) 415, and references cited therein.
[5] J. Dey, I.M. Warner, J. Phys. Chem. A 101 (1997) 4872.
[6] Z.R. Grabowski, Pure Appl. Chem. 64 (1992) 1249.
[7] R.O. Loutfy, B.A. Arnold, J. Phys. Chem. 86 (1982) 4205.
[8] Th. Förster, G. Hoffmann, Z. Phys. Chem. (Frankfurt am Main) 75 (1971) 63.
[9] S.K. Saha, P. Purkayastha, A.B. Das, J. Photochem. Photobiol. A: Chem. 195 (2008)
368–377.
[10] A. Cembran, F. Bernardi, M. Olivucci, M. Garavelli, Proc. Nat. Acad. Sci. U.S.A. 102(2005) 6255.
[11] D.H. Waldeck, Chem. Rev. 91 (1991) 415.
[12] L. Ji, L. Liu, W. Wang, L. Xu, Chem. Phys. Lett. 406 (2005) 268.
[13] S. Arzhantsev, K.A. Zachariasse, M. Maroncelli, J. Phys. Chem. A 110 (2006) 3454.
[14] Y.V. Il’ichev, W. Kuhnle, K.A. Zachariasse, Chem. Phys. 211 (1996) 441.
[15] Y.V. Il’ichev, K.A. Zachariasse, Ber. Bunsen-Ges. Phys. Chem. 101 (1997) 625.
[16] H. Li, Y. Xue, C.T. Ung, C.W. Yap, Z.R. Li, Y.Z. Chen, Chem. Res. Toxicol. 18 (2005)
1071.
[17] A. Mallick, B. Haldar, N. Chattopadhyay, J. Phys. Chem. B 109 (2005) 14683.
[18] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Kluwer Academic/Plenum Publishers, New York, 1999.
[19] A. Mallick, S. Maiti, B. Haldar, P. Purkayastha, N. Chattopadhyay, Chem. Phys.
Lett. 371 (2003) 688.
[20] P. Avouris, W.M. Gelbert, M.A. El-Sayed, Chem. Rev. 77 (1977) 793.
[21] A.K. Doolittle, J. Appl. Phys. 22 (1951) 1471.
[22] S.K. Saha, S.M. Santra, S.K. Dogra, J. Mol. Struct. 478 (1999) 199.
[23] R. Lapouyade, K. Czeschka, W. Majenz, W. Rettig, E. Gilabert, C.J. Rulliere, Phys.
Chem. 96 (1992) 9643.
[24] B. Wandelt, P. Turkewitsch, B.R. Stranix, G.D. Darling, J. Chem. Soc., Faraday.
Trans. 91 (1995) 4199.
[25] M.S.A. Abdel-Mottaleb, A.M.K. Sherief, L.F.M. Ismaiel, F.C. De Schryver, M.A. Van
der Auweraer, J. Chem. Soc., Faraday Trans. 85 (1989) 1779.
[26] P. Bojarski, A. Matczuk, C. Bojarski, A. Kawski, B. Kuklinski, G. Zurkowska, H.
Diehl, Chem. Phys. 210 (1996) 485.
[27] P. Bojarski, H. Grajek, G. Zurkowska, B. Kuklinski, B. Smyk, R. Drabent, J. Fluoresc.
9 (1999) 391.
[28] R. Das, D. Guha, S. Mitra, S. Kar, S. Lahiri, S. Mukherjee, J. Phys. Chem. A 101(1997) 4042.
[29] P. Purkayastha, N. Chattopadhyay, Phys. Chem. Chem. Phys. 2 (2000) 203.
[30] P. Purkayastha, N. Chattopadhyay, Int. J. Mol. Sci. 4 (2000) 335.