Solvent modulated photophysics of 9-methyl anthroate: Exploring the effect of polarity and hydrogen bonding on the emissive state

Article abstract of DOI:10.1016/j.saa.2013.04.059

Photophysical properties of an anthracene derivative 9-methyl anthroate (9-MA) have been investigated using absorption and emission spectroscopy, in combination with quantum chemical calculations. Solvatochromic effects on the Stokes shifted emission band clearly demonstrate the highly polar character of the excited state, which is also supported by the enhancement of dipole moment of the molecule upon photoexcitation. The emission band has been found to be dependent on polarity and hydrogen-bonding ability of the solvents. Multiple linear regression analysis method has been utilized to rationalize the effect of hydrogen bonding interaction on the emissive state, which was further confirmed by the analysis of the non-radiative decay constants and urea induced H-bonding disruption study. The experimental results correlate well with theoretical predictions obtained via density functional theory (DFT).

Full text of DOI:10.1016/j.saa.2013.04.059

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 237–244
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Solvent modulated photophysics of 9-methyl anthroate: Exploring the effect
of polarity and hydrogen bonding on the emissive state
⇑
Aniruddha Ganguly, Sankar Jana, Soumen Ghosh, Sasanka Dalapati, Nikhil Guchhait
Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
Solvatochromic behavior of 9-MA
exploited by fluorimetry.
Evidence of the emissive state to be
the Franck–Condon state.
Multiple linear regression analysis to
quantify the effect of hydrogen
bonding.
ꢀ
ꢀ
Urea induced H-bonding disruption
study.
Theoretical calculations to support
the experimental findings.
a r t i c l e i n f o
a b s t r a c t
Article history:
Photophysical properties of an anthracene derivative 9-methyl anthroate (9-MA) have been investigated
using absorption and emission spectroscopy, in combination with quantum chemical calculations. Solva-
tochromic effects on the Stokes shifted emission band clearly demonstrate the highly polar character of
the excited state, which is also supported by the enhancement of dipole moment of the molecule upon
photoexcitation. The emission band has been found to be dependent on polarity and hydrogen-bonding
ability of the solvents. Multiple linear regression analysis method has been utilized to rationalize the
effect of hydrogen bonding interaction on the emissive state, which was further confirmed by the analysis
of the non-radiative decay constants and urea induced H-bonding disruption study. The experimental
results correlate well with theoretical predictions obtained via density functional theory (DFT).
Ó 2013 Elsevier B.V. All rights reserved.
Received 19 January 2013
Received in revised form 5 April 2013
Accepted 10 April 2013
Available online 22 April 2013
Keywords:
9-Methyl anthroate
Dipole moment
Multiple linear regression
Hydrogen bonding
Non-radiative decay constants
Introduction
malian cells [5]. Anthracene derivatives are widely studied because
of their enormous ability to sense biologically relevant ions, espe-
Anthracene based fluorophores stand out owing to their simple
structure, facile synthetic procedure and high fluorescence quan-
tum yields [1,2]. Generally, fused ring hydrocarbons are referred
to as potential carcinogens, but surprisingly a recently reported
anthracene based metabolite, Alterporriol L, isolated from a marine
fungus, is found to show cytotoxicity against different types of can-
cer cells [3,4], whereas as obvious some derivatives are known to
be uniquely mutagenic and cytotoxic in both bacterial and mam-
cially alkaline earth and transition metal ions, not only through
specific binding but also via inhibition of photoelectron transfer
[6–10]. Anthracene derivatives also have potential applications in
the manufacturing of OLEDs and field-effect transistors which are
frequently used in flat panel displays and smart cards [11,12],
and are effective singlet oxygen producer from the first excited sin-
glet and triplet states [13]. There are even reports of anthracene
based fluorophores being used as membrane probes [14]. Photodi-
merization of anthracene derivatives, especially of esters has also
been exhaustively studied, even in solid state [15,16]. It has been
first noticed by Werner et al. that substitution on the anthracene
⇑
Corresponding author. Tel.: +91 3323508386; fax: +91 3323519755.
E-mail address: nguchhait@yahoo.com (N. Guchhait).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.04.059

238
A. Ganguly et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 237–244
nucleus has a marked influence on its photophysical properties
[17], which was also confirmed by Schuster via 13C NMR spectros-
copy [18]. Werner and co workers have also investigated the fluo-
rescence characteristics of 9-anthroic acid and some ester
derivatives of it in mixed solvents (ethanol–water and dioxane–
water) as well as in a few pure solvents, though the studies are
not so detailed and lack any theoretical support [19,20]. Thus, ex-
plicit photophysical studies on such derivatives are still a require-
ment from scientific point of view.
Spectroscopic grade solvents such as n-hexane (HEX), cyclohexane
(CY), carbon tetrachloride (CCl4), dioxane (DOX), tetrahydrofuran
(THF), chloroform (CHCl3), dichloromethane (DCM), dimethylform-
amide (DMF), dimethyl sulfoxide (DMSO), acetonitrile (ACN), iso-
proanol (iPrOH), n-butanol (BuOH), ethanol (EtOH) and methanol
(MeOH) were purchased from Spectrochem, India and were used
after proper distillation as required. Urea was procured from E-
Merck, India and was recrystallized from methanol before use. Tri-
ple distilled water was used for preparing the aqueous solutions.
Solvation studies of fluorophores, especially those containing
polar substituents on the aromatic ring/s draw considerable atten-
tion to researchers, owing to the influence of solvents on the corre-
sponding fluorescence spectra and quantum yields, which may
arise from the perturbations due to solvent refractive index and
dielectric constant or from intermolecular hydrogen bonding be-
tween the fluorophore and the solvent or even from complexation
between the fluorophore and the solvent [21,22]. These factors
modulate the energy gap between the ground (S0) and the first ex-
cited singlet (S1) state and thus cause a shifting in the emission
spectra and/or affect quantum yields. The polarity and the hydro-
gen bonding ability of the solvents are known to be the key factors
in modulating the routes of energy dissipation from the electronic
excited state, especially when the emissive state is polar [22–24].
Also, as proposed by Lim, solvent-moderated shifts of the energy
levels may enhance or inhibit non-radiative transitions to the
ground state [25].
Our present work aims towards a detailed photophysical study
of 9 methyl anthroate (Scheme 1), which is known to inhibit Clꢁ
conductance across the epithelial membrane and fatty acid incor-
poration into phospholipids in human airway epithelial cells [26],
in various pure solvents differing in polarity and hydrogen bonding
ability by using absorption and emission spectroscopy. The target
of the study is to explore the effect of polarity and hydrogen bond-
ing on the radiative transitions of this compound from the excited
singlet state explicitly. Finally, the results obtained from the exper-
imental studies have been rationalized with quantum chemical
calculations.
Instrumental details
The absorption and emission spectral measurements were done
by Hitachi UV–vis U-3501 spectrophotometer and Perkin–Elmer
LS-55 Luminescence spectrophotometer, respectively. In all mea-
surements the sample concentration was maintained in the range
of ꢂ10ꢁ6 M in order to avoid aggregation and reabsorption effects.
All absorption and emission spectral measurements have been per-
formed at room temperature with proper background corrections
and with air-equilibrated, freshly prepared solutions only.
Fluorescence lifetimes were measured by the method of Time
Correlated Single-Photon counting (TCSPC) using a HORIBA Jobin
Yvon Fluorocube-01-NL fluorescence lifetime spectrometer. The
sample was excited using a picosecond laser diode at 375 nm
and the signals were collected at the magic angle of 54.7° to elim-
inate any considerable contribution from fluorescence anisotropy
decay [21,28,29]. The typical time resolution of our experimental
set-up is ꢂ100 ps. The decays were deconvoluted using DAS-6 de-
cay analysis software. The acceptability of the fits was judged by v2
criteria and visual inspection of the residuals of the fitted function
to the data. Mean (average) fluorescence lifetimes were calculated
using the following equation [28,29]:
P
ais2i
aisi
P
sav
¼
in which ai is the pre-exponential factor corresponding to the ith
decay time constant, si
.
Fluorescence quantum yield (Uf) was determined using quinine
sulfate as the secondary standard (Uf = 0.54 in 0.1 M H2SO4) using
the following equation [30,31],
Materials and methods
Materials
AS ðAbsÞR g2S
US
¼
U
ꢃ
ꢃ
gR2
R AR ðAbsÞS
9-Methyl anthroate (Scheme 1) was synthesized and purified
according to the literature procedure [27]. For a detailed descrip-
tion of synthesis and purification, see Supporting Information.
where ‘‘A’’ terms denote the integrated area under the fluorescence
curve, ‘‘Abs’’ denote absorbance, the refractive index of the med-
g
ium and U, the fluorescence quantum yield. Subscripts ‘‘S’’ and
‘‘R’’ stand for denoting respective parameters for the studied sample
and reference, respectively.
Computational details
All theoretical calculations except the excited-state optimiza-
tion of the ester were performed in Gaussian 03W suite of pro-
grams [32] whereas the latter has been done in Gaussian 09W
suite of programs [33]. The global minimum energy optimized
structure of the ester in vacuuo was obtained by optimizing the
geometry at DFT (density functional theory) level using B3LYP hy-
brid functional (which comprises of Becke’s three parameter hy-
brid exchange functional in conjunction with the nonlocal
correlation functional of Lee, Yang and Parr). For this purpose 6-
311++G(d,p) basis set has been chosen because this basis set is of
triple-f quality for valence electrons with diffuse functions which
are useful in calculations for anions and structures with lone pair
electrons [34,35]. To get the optimized structure of the molecule
in solvents of different polarity, the Polarizable Continuum model
Scheme 1. Ground state optimized structure of 9-MA obtained at DFT (B3LYP/6-
311++G(d,p)) level.

A. Ganguly et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 237–244
239
(PCM) method was used applying the corresponding keywords
implemented in the software. In this method, the solute, treated
quantum chemically, is placed in a cavity surrounded by the sol-
vent. The latter is considered as a continuum characterized by its
bulk properties, such as dielectric constant or polarity [35]. Highest
Occupied Molecular Orbital (HOMO) and Lowest Unoccupied
Molecular Orbital (LUMO) diagrams were also constructed from
the optimized structure.
ular hydrogen bonding interaction between the ester and these
protic solvents [39]. This argument seems justified on the ground
of efficient hydrogen bonding ability of methanol and water and
is in harmony with several reports with aromatic molecules con-
taining polar groups [37,39]. The minute red shift of the band posi-
tion in water as compared to methanol may be attributed to
greater polarity of water than methanol. Although it is the solvent
polarity which is most often invoked to account for the medium
polarity-dependent emissions, the role of specific properties of sol-
vent such as hydrogen bonding ability, dielectric constant, etc.
might be instrumental in some cases in governing the photophys-
ics of such molecules [37]. The impact of such specific interactions
in governing the overall photophysics of 9-MA has been clearly
Results and discussions
Absorption spectra
demonstrated through
parameters in the forthcoming sections.
a detailed analysis of solvatochromic
Absorption spectra of 9-MA recorded in various solvents show a
structured band with a maximum near 360 nm (Fig. 1a). The spec-
tral band maxima observed in different solvents are presented in
Table 1. This broad absorption band is assigned to the S1 S0 tran-
sition of the anthracene chromophore [17,19]. High absorption
Emission and excitation spectra
intensity of the band is indicative of its being a
p–
pꢄ type transi-
The single emission band of the molecule recorded in various
solvents by excitation at the respective absorption maxima are
shown in Fig. 1b and the position of the emission band maxima
are presented in Table 1. In n-hexane, a broad emission band is ob-
served with a maximum at ꢂ440 nm. The emission maximum is
found to be gradually red-shifted with increasing polarity of the
solvent. This solvatochromic red-shift of about 40 nm (from
ꢂ440 nm in n-hexane to ꢂ480 nm in water) supports for the polar
character of the emissive state. It is anticipated that greater stabil-
ization of the polar photoexcited state in solvents of higher polarity
produces the observed solvatochromic red-shift of the emission
band. That there exists only a single species in the ground state
and the observed phenomenon is exclusively an excited state
tion, which is also supported by the indifference of the absorption
spectra towards the addition of protic acid. The absorption band
exhibits slight dependence on solvent polarity. A slight red shift
(ꢂ5 nm) of the absorption maxima with increasing solvent polarity
is rationalized in anticipation with progressively greater degree of
stabilization of the excited state in polar environments. This obser-
vation relates well with the signature of a polar excited state (pꢄ
state is more polar and thus more stabilized in polar solvents
resulting such red shift) and is consistent with literature reports
[36–38]. A slight blue shift of the band position in methanol and
water in comparison to less polar solvents DMSO might seem a
little puzzling but is not unlikely owing to the possible intermolec-
0.12
HEX
HEX
40 nm
(a)
(b)
CY
CCl
4
CCl
4
THF
THF
0.08
0.04
0.00
ACN
ACN
DMSO
MeOH
Water
DMSO
BuOH
MeOH
Water
330
360
390
420
400
450
500
550
600
Wavelength(nm)
Wavelength(nm)
Fig. 1. (a) The room temperature absorption spectra of 9-methyl anthroate in different solvents as indicated in the figure legend. (b) Fluorescence emission spectra of 9-
methyl anthroate in different solvents as indicated in the figure legend (exciting at the corresponding absorption band maxima).
Table 1
Spectroscopic parameters and quantum yield of 9-methyl anthroate in solvents of different polarity at room temperature.
ꢁ1
ꢀ
m(cm
Solvents
ET (30)
kmax (abs) in nm
kmax (em) in nm
D
)
Quantum yield (Uf)
HEX
CY
CCl4
DOX
THF
CHCl3
DCM
DMF
DMSO
ACN
30.9
31.2
32.5
36.0
37.4
39.1
41.1
43.8
45.0
46.0
48.6
50.2
51.9
55.5
63.1
360
360
361
363
363
362
364
363
363
362
361
363
362
361
363
440
443
445
452
450
448
455
456
455
455
462
463
465
468
480
5050
5204
5000
5424
5326
5638
5494
5618
5583
5646
6056
5950
6119
6333
6043
0.201
0.193
0.251
0.221
0.205
0.122
0.303
0.121
0.133
0.106
0.058
0.075
0.043
0.021
0.010
iPrOH
BuOH
EtOH
MeOH
H2O

240
A. Ganguly et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 237–244
Water
MeOH
process, is confirmed by a good juxtaposition of the fluorescence
excitation spectra in various solvents assayed (Fig. S1 in the Sup-
porting information) with the absorption spectra.
(a)
6500
6000
5500
5000
EtOH
PrOH
i
BuOH
Polarity and H-bonding effect on the emissive state of 9-MA
DMF
ACN
DMSO
DCM
THF
DOX
To get a quantitative idea about the polarity of the emissive
state, the dipole moment of the emissive state has been calculated
using the method proposed by Lippert [23,36]. According to his
proposition, the dipole moment of the emissive state can be calcu-
lated from the solvatochromism of the UV absorption and the fluo-
rescence spectra using the following relations,
CY
HEX
35
30
40
45
50
55
60
65
E
T
(30) (kcal/mol)
0.30
0.25
0.20
0.15
0.10
0.05
0.00
(b)
CCl
4
ð
lFC
ꢁ
lg
Þ
THF
DOX
ꢀ
ma
¼
lg
l
D
fðer
;
g
Þ þ Const
2pe
0hcq3
DMSO
ACN
DMF
i
ð
lss
ꢁ
lg
Þ
PrOH
ꢀ
mf
¼
D
fðer
;
g
Þ þ Const
EtOH
SS 2
pe
0hcq3
Water
60
MeOH
h
i
h
i
g2ꢁ1
er ꢁ1
30
40
50
where
D
fðer
;
g
Þ ¼
ꢁ
g2þ1
2er þ1
2
E
(30) (kcal/mol)
T
ꢀ
ꢀ
In this equation, ma and mf are the positions of the absorption
=cmꢁ1) vs. polarity parameter (ET (30)/kcal molꢁ1
ꢀ
Fig. 3. (a) Plot of Stokes shift ðDm )
and emission band maxima in cmꢁ1, respectively, whereas
D
f is
for 9-methyl anthroate, as obtained from spectroscopic measurements in various
solvents. (b) Plot of quantum yield of the compound against ET (30) parameter of
the solvents.
the solvent polarity parameter. The symbols h, c, 0 and repre-
q,
e
g
sent Planck’s constant, speed of light, Onsager cavity radius, per-
mittivity of vacuum and refractive index of the medium,
respectively. The terms lg, lFC and lss are the dipole moments cor-
gen bonding ability of a solvent. The correlation of Stokes shift (in
cmꢁ1) with the solvent dependent ET (30) parameter [42] is shown
in Fig. 3a where two distinct lines are observed, one for the non-
hydrogen-bonding solvents (non-polar and polar aprotic) and the
other for the polar protic solvents. This indicates that both electro-
static dipolar and hydrogen-bonding interactions influence the
emission of the compound of interest.
responding to the ground state, Franck–Condon excited state and
solvent stabilized excited state of the system. Now, for the opti-
mized global minimum structure of 9-MA (calculated at DFT level
of theory using B3LYP hybrid functional and 6-311++G(d,p) basis
set), the value of ‘
respectively. From the slope of ma vs.
q
’ and ‘
l
g’ are found to be 5.15 Å and 1.77 D,
ꢀ
Df plot (Fig. S2a in the Sup-
porting information), the dipole moment of the Franck–Condon
state is calculated to be 3.32 D. Such an increase in dipole moment
of the compound from the ground to the excited state further cor-
roborates to the enhanced polarity of the emissive state and hence
justifies the red-shift of the emission maxima with polar solvents
through solvent stabilization. Interestingly, the dipole moment of
the solvent stabilized excited state, calculated from the slope of
ꢀ
Such H-bonding interaction might also open up non-radiative
decay paths in protic solvents. This possibility is indeed reflected
in the lowering of quantum yields observed in solvents of increas-
ing hydrogen bonding characters. The measured fluorescence
quantum yields of the compound are presented in Table 1. The
quantum yield values show a remarkable decrease with increasing
polarity of the solvents up to the polar aprotic limit and then in the
solvents with H-bonding capability the rate of decrement drops.
Such an observation may be attributed to differential contribution
of electrostatic dipolar and hydrogen bonding interactions [43,44].
However, as mentioned before, increasing intermolecular hydro-
gen-bonding ability of the solvents makes way for the accessibility
of some non-radiative decay channels which deplete the quantum
yield of the ester (9-MA) by almost an order of magnitude than in
the corresponding polar aprotic solvents. The significant lowering
of quantum yield of the emission from acetonitrile to water is
hence explained on the basis of the increasing hydrogen bonding
strength of water. The plot of quantum yield against ET (30) param-
eter of the solvents (Fig. 3b), also reflects the fact where two dis-
tinct lines with different slopes are observed.
mf vs.
Df plot (Fig. S2b in the Supporting information) is found to
be 3.48 D which is almost same as that of the FC state, which
strongly suggests that the Franck–Condon state is the emissive
one and thereby wipes off the presence of any charge transfer phe-
nomenon. However, for both the cases, the deviation from linearity
in the plot for polar protic solvents (not shown in the figures) indi-
cates that solute–solvent hydrogen-bonding interactions have a
substantial effect on the emission band [40,41]. In this context it
is worth to mention that the plot of the emission maxima (in
cmꢁ1) against hydrogen-bonding parameter ‘
a’ is found to be linear
(Fig. 2), which further substantiate the effect of solute–solvent
hydrogen-bonding interactions and thus points towards the poten-
tial applicability of the aforesaid probe as a quantifier of the hydro-
In order to get acumen into the different modes of solvation
determining the absorption and fluorescence energies, the multi-
ple linear regression analysis approach, proposed by Abraham
et al. [45] has been used, which is represented by the following
equation:
21800
i
PrOH
21600
EtOH
MeOH
BuOH
21400
21200
21000
20800
P ¼ Po þ a
a
þ bb þ spꢄ
Water
1.20
where P is the value of the solvent dependent property to be mod-
eled, Po, a, b and s are the coefficients determined from the regres-
sion analysis. The parameter pꢄ, is an index of the dielectric effects
0.75
0.90
1.05
α
exerted by the solvent,
a and b parameters [46] represent the
Fig. 2. Correlation of emission maxima (in cm-1) of 9-methyl anthroate in different
protic solvents with the H-bonding parameter of the solvents.
hydrogen bond donating and accepting ability of the solvent,
respectively. Correlations of E(A) and E(F), i.e. absorption and fluo-
a

A. Ganguly et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 237–244
241
rescence band maxima in cmꢁ1, respectively, were calculated
against the above mentioned parameters. The following regression
equations were obtained for 9-MA with good correlation factors
(0.986 and 0.997 for E(A) and E(F) respectively);
10
CCl
4
DCM
DMF
(a)
HEX
DMSO
THF
8
6
4
2
0
þ 100:5b ꢁ 59:5pꢄ
BuOH
EðAÞ ¼ 27544 þ 57:1
a
i
PrOH
ꢁ 180:9b ꢁ 814:3pꢄ
EtOH
EðFÞ ¼ 22779 ꢁ 831:1
a
MeOH
The ratio of the regression coefficients of the parameters
a and b de-
30
40
50
60
notes the relative importance of hydrogen bond donation over
acceptance by the solvents towards the fluorophore [37,45]. The ra-
tio is 0.57 in case of E(A), indicating relatively weak hydrogen bond-
ing interaction in the ground state, whereas for E(F), the ratio is
ꢂ4.6, indicating that the ability of the solvent to act as a hydrogen
bond donor plays a crucial role in the excited state. On the other
hand, the high value of the regression coefficient corresponding to
the solvent dielectric function (pꢄ) in the excited state also reveals
a substantial effect of solvent polarity on the excited fluorophore,
which is quite likely owing to the enhanced dipole moment of the
emissive state.
E
(30) (kcal/mol)
T
MeOH
EtOH
BuOH
(b)
1.50
i
1.25
1.00
0.75
0.50
prOH
ACN
DMF
DMSO
HEX
30
THF
CCl
4
35
40
45
50
55
E
T
(30) (kcal/mol)
Fluorescence lifetime study
Fig. 5. (a) Plot of fluorescence lifetime (ns) vs. polarity parameter (ET (30)/
kcal molꢁ1) for 9-methyl anthroate, as obtained in various solvents. (b) Variation of
log (knr/kr) as a function of polarity parameter (ET (30)/kcal molꢁ1) of solvents.
Representative time-resolved decay profiles and corresponding
residuals of 9-MA in different solvents are displayed in Fig. 4 and
Prompt
HEX
the relevant data are summarized in Table 2. The compound is
found to exhibit a single-exponential decay pattern in all the as-
sayed solvents except water where a bi-exponential decay is ob-
served. From the correlation between lifetime of the excited state
and solvent dependent ET (30) parameter (Fig. 5a), it’s evident that
the lifetime of the emissive state changes very little from non-polar
to polar aprotic solvents, but a significant change is observed in po-
lar protic solvents. For instance, the lifetime of the compound in
polar protic isopropanol (ꢂ4.12 ns) is found to be much lower than
the corresponding lifetime in polar aprotic DMSO (ꢂ8.18 ns) which
in turn closely resembles the lifetime in non-polar n-hexane
(ꢂ9.27 ns). Again, the steep change of lifetime from isopropanol
to methanol (ꢂ1.72 ns) is noteworthy. Such observations can be
explained on the basis of opening of parallel non-radiative chan-
nels operative in polar protic solvents via formation of intermolec-
ular hydrogen-bonding between the ester and the solvent which
leads to depletion of the fluorescence lifetime [38,43]. In case of
1000
100
10
ACN
iPrOH
EtOH
MeOH
0
10
20
30
40
50
Time (ns)
4
2
MeOH
EtOH
0
-2
-4
4
2
0
-2
-4
4
iPrOH
ACN
HEX
2
0
-2
-4
4
2
0
-2
-4
4
2
0
water, the average lifetime (
the average of a lower amplitude component with a lifetime of
6.04 ns ( = 0.032) and another faster component (360 ps) with a
higher amplitude ( = 0.971). Considering the trend of depletion
sav) is found to be ꢂ1 ns, which is
-2
-40 10 20 30 40 50 60
Time (ns)
a
a
Fig. 4. Typical fluorescence decay profiles of 9-methyl anthroate associated with
the lamp profile in different solvents and the plot of residuals corresponding to the
fluorescence decays (top and bottom respectively); kexc = 375 nm.
of lifetime of this compound with increasing H-bonding ability of
the solvents, the faster component can be unambiguously assigned
to the free ester (structure without any imposed rigidity) in water,
whereas the other component with a considerably longer lifetime
may arise from rigid intermolecular H-bonded cluster of the ester,
which is possibly formed in the excited state by virtue of small size
and excellent efficiency of H-bond formation of water [35]. This
type of cluster formation may induce an added degree of rigidity
to the entire molecular architecture and thereby reducing the
vibrational degrees of freedom, a direct consequence of which
may be the enhancement of fluorescence lifetime [37].
Table 2
Fluorescence decay parameters along with radiative and non-radiative rate constants
of 9-methyl anthroate in various solvents.
Solvents
ET (30)
Lifetime (ns) v2
kr (ꢃ107)/sꢁ1
knr (ꢃ107)/sꢁ1
8.616
8.674
8.308
8.997
HEX
CY
CCl4
THF
30.9
31.2
32.5
37.4
41.1
43.8
45.0
46.0
48.6
50.2
51.9
55.5
9.27
9.29
9.01
8.83
8.66
8.26
8.18
5.26
4.11
4.43
2.89
1.72
1.04
2.170
2.084
2.789
2.326
3.509
1.467
1.627
2.024
1.410
1.700
1.518
1.270
1.12
1.06
1.10
1.09
1.09
1.03
1.03
1.04
1.05
1.04
1.05
To rationalize the effect of solvents on the dynamics of the
DCM
DMF
DMSO
ACN
8.039
excited state, the radiative [kr = Uf/
(1 ꢁ Uf)/ f] rate constants were calculated using fluorescence
quantum yields (Uf) and lifetimes ( f) of the compound in different
sf] and non-radiative [knr =
10.629
10.587
16.971
22.890
20.845
32.982
56.822
s
s
iPrOH
BuOH
EtOH
MeOH
solvents. The values of kr and knr in different solvents are presented
in Table 2. The non-radiative rate constant values for 9-MA show
significant sensitivity towards solvent polarity. The variation of
log (knr/kr) with ET (30) has been depicted in Fig. 5b. From a close

242
A. Ganguly et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 237–244
scrutiny of the figure, it is observed that with increasing solvent
polarity, the log (knr/kr) value of the compound increases gradually
and shows a steep rise in case of polar protic solvents, which in
turn strengthens our proposition of opening of deactivating non-
radiative channels in protic solvents owing to solute–solvent
hydrogen bonding.
The presence of urea is, however, found to induce no significant
modification to the qualitative appearance of the absorption profile
compared to that in the absence of the same (Figure not shown).
The impact of added urea is more conspicuously seen through
quantitative perusal of the emission intensity as shown in Fig. 6,
where a substantial increase in intensity as well as in quantum
yield is observed, without any observable spectral shift. The corre-
sponding quantum yield values are presented in Table 3. The
increasing value of quantum yield of the ester with increasing con-
centration of urea advocates for the decrement of non radiative de-
cays owing to urea induced disruption of H-bonds.
Urea induced hydrogen bonding disruption study
Urea has long been known as a reagent which can efficiently
perturb hydrogen bonding [47,48]. Though the exact mechanism
is still a matter of debate, till date two mechanisms have been pop-
ular to explain such observations. One is an indirect mechanism in
which urea is believed to act only at the level of the solvent, alter-
ing the structure of water in a way that facilitates dissolution of
hydrophobic probes, i.e. by invoking the capacity of urea as ‘‘water
structure breaker’’ [49].
By virtue of its sensitivity to the associated environment of a
probe, fluorescence lifetime serves as an indicator to explore the
microenvironment around a fluorophore [28,47]. Fig. 7 illustrates
the typical time-resolved fluorescence decay profiles of the probe
in aqueous medium in presence of various amounts of urea as
indicated in the figure legends and the corresponding fitting
parameters are collected in Table 3. As stated earlier, 9-MA shows
a bi-exponential decay in aqueous medium, a lower amplitude
The description on the classical hydrophobic effect given by
Sinanoglu [50], states that when a nonpolar solute is dissolved in
aqueous media, the dissolution process starts with the formation
of a ‘‘cavity’’ in the solvent network in order to accommodate the
solute. Breslow [51,52] in his work documented that the presence
of urea results in an increase of the energetic cost associated with
the cavity formation. Hence the direct mechanism of action of urea
came into operation in which urea was proposed to participate in
the solvation of the hydrophobic species by replacing some water
molecules from the hydration shell of the solute. Thus an intrinsic
interaction between the chaotrope and the hydrophobic substrate
must exist to overcome the enhanced thermodynamic difficulties
associated with the cavity formation. Herein, we have designed
our experiment in a manner so as to delve into the effect of addi-
tion of urea on the hydrogen bonded probe in aqueous medium.
For this purpose, the absorption and emission spectral properties
of the probe has been monitored with increasing urea concentra-
tion in aqueous medium. Furthermore, with a view to establish
the impact of externally added chaotrope on the H-bonded probe,
time resolved emission study has also been done.
component with a lifetime of ꢂ6 ns (
a
= 0.032) and another faster
= 0.971), and we
component (360 ps) with a higher amplitude (
a
assigned the faster component to the free ester in water, whereas
the other component was assumed to be arising from the rigid
intermolecular H-bonded cluster of the ester. Interestingly, with
the addition of urea the lifetime assigned to the rigid cluster signif-
icantly reduces whereas the other corresponding to the free probe
does not substantially alter (although an increment is observed,
which might be due to the depletion of non radiative decay rates
in presence of urea) and the individual contributions of both the
species remain almost constant and thus a depletion of the average
lifetime of the ester is observed. The above mentioned observation
can be rationalized considering the efficacy of urea to participate in
the solvation of the hydrophobic species by replacing some water
molecules from the hydration shell of the solute and thereby pro-
moting the rupture of the formed H-bonded cluster, which results
in a decrement of the corresponding lifetime due to negation of the
imposed rigidity.
Analysis of quantum chemical calculations
10 M Urea
8 M Urea
6 M Urea
4 M Urea
2 M Urea
0 M Urea
The ground state global minimum geometry of 9-MA in vacuuo
at DFT level (B3LYP/6-311++G(d,p)) shows that the ester group is
twisted and out of plane of the anthryl ring by ꢂ56° (Scheme 1, an-
gle represented by h). As mentioned earlier, the calculated dipole
moment for the ground state structure is found to be 1.76 D at
the same level of theory. The pictures of the HOMO and LUMO of
the compound in the ground state are shown in Fig. 8, which reveal
[Urea]
a more or less uniform distribution of the
p cloud density over the
entire system for the optimized structure of the ester for both the
molecular orbitals. This is a corroboration of the presence of an
400
450
500
550
600
extensive delocalization in the ground state through the entire
p
Wavelength (nm)
network of the system and predicts the
p
–
pꢄ (allowed) nature of
Fig. 6. Fluorescence emission spectra of 9-methyl anthroate in aqueous medium in
presence of various concentrations of urea as indicated in the figure legend.
(Exciting at the corresponding absorption band maximum).
the transition, also further supported by high oscillator strength
value of 0.2556, obtained by TDDFT method using the same basis
set. TDDFT calculation also predicts the first vertical transition
S1 S0 of the ester in vacuuo, at wavelength 335.77 nm which is
in good agreement with the experimental values obtained via
absorption spectroscopy. To gather information about the effect
of solvent polarity on the compound in the ground state, the struc-
ture has been optimized in presence of solvent using the PCM
(Polarizable Continuum Model) method and the results are sum-
marized in Table S1 of the Supporting information. As expected,
in the ground state with increasing polarity of the solvent there
is an increase in dipole moment which leads to stabilization in po-
lar media and an increase in the twisting angle (h) is also observed
Table 3
Quantum yield values and fluorescence decay parameters of 9-methyl anthroate in
aqueous medium having various concentrations of urea.
(Urea) in
M
Quantum yield
(U)
a1
a2
s1
(ns)
s2
(ns)
sav
(ns)
v2
0
4
8
0.010
0.017
0.028
0.035
0.970 0.032 0.360 6.04
0.984 0.016 0.402 4.94
0.988 0.012 0.573 3.72
0.958 0.043 0.628 1.39
2.384
1.160
0.805
0.698
1.03
1.07
1.08
0.99
10

A. Ganguly et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 237–244
243
which might be argued to be the reason behind the observed
enhancement in dipole moment.
Prompt
0 M Urea
4 M Urea
8 M Urea
10 M Urea
1000
100
10
The excited state global minimum geometry of 9-MA in vacuuo
obtained at TD-DFT level of theory (using the same basis set)
shows that here also the ester group is twisted and out of plane
of the anthryl ring by ꢂ34° (Table S1 in the Supporting informa-
tion). The calculated dipole moment for the aforesaid structure is
found to be 2.04 D at the same level of theory. The simulated emis-
sion spectra obtained from the optimized excited state predicts the
emission maximum to be at ꢂ460 nm having an oscillator strength
of 0.0938, which fairly agrees with the experimental results.
Conclusion
0
10
20
30
40
Time (ns)
The title compound 9-methyl anthroate (9-MA) was synthe-
sized and its photophysical behavior has been studied spectroscop-
ically as well as theoretically by means of quantum chemical
calculations. Solvatochromic measurements predict the existence
of a polar excited state. Higher dipole moment of the emissive FC
state compared to the ground state is the sole reason for solvent
polarity dependent red shifted emission band of the compound.
On the other hand, the hydrogen-bonding interaction with the sol-
vents is also responsible for the stability of the emissive state as is
evident from the ET (30) plots. Fluorescence quantum yields and
lifetime data, along with the regression analysis also support the
fact that in protic solvents, non-radiative decay of the emissive
state is prominent owing to intermolecular hydrogen bonding
interaction with the solvent. Theoretical studies also substantiate
the presence of a Franck–Condon emissive state and accounts well
for the polarity dependence of the emission spectra.
4
2
0 M Urea
0
-2
-4
4
4 M Urea
8 M Urea
2
0
-2
-4
4
2
0
-2
-4
4
10 M Urea
30
2
0
-2
-4
0
10
20
Time (ns)
Acknowledgements
Fig. 7. Typical fluorescence decay profiles of 9-methyl anthroate associated with the
lampprofileinaqueousmediumdifferinginureaconcentrationsandtheplotofresiduals
corresponding to the fluorescence decays (top and bottom respectively); kexc = 375 nm.
NG acknowledges Department of Science and Technology, India
for financial assistance through Project No. SR/S1/PC-26/2008. AG
gratefully acknowledges CSIR, New Delhi, India for Junior Research
Fellowship. SJ, SG and SD would like to thank UGC for Research
Fellowships.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2013.04.059.
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