Intramolecular charge transfer in coumarin based donor-acceptor systems: Formation of a new product through planar intermediate

Article abstract of DOI:10.1016/j.jphotochem.2015.02.007

Fluorescence behavior and intramolecular charge transfer (ICT) property of N,N′-dimethylamino substituted anil derivative of coumarinyl amine was studied by steady state and time-resolved fluorescence spectroscopy in combination with density functional theory calculation. Quantitative analysis of the effect of solvent hydrogen bonding on several spectral parameters was done using multiple regression analysis based on Kamlet-Taft model. The present system shows an additional long wavelength fluorescence band almost instantaneously in presence of methanol in sharp contrast to the model unsubstituted analogue. This new fluorescence band is believed to be due to the formation of a new isomer (cis-type) through a planar intermediate. While there is a substantial barrier (～50 kJ mol^-1) in the ground state, the isomerization process in the excited state in polar-protic environment proceeds very fast and believed to be catalyzed by hydrogen bond formation at the imine nitrogen atom in combination with stabilization of the resulting species through ICT.

Full text of DOI:10.1016/j.jphotochem.2015.02.007

Journal of Photochemistry and Photobiology A: Chemistry 303 (2015) 67–79
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Intramolecular charge transfer in coumarin based donor-acceptor
systems: Formation of a new product through planar intermediate
Smritakshi Phukan a, Mithu Saha a, Amarta Kumar Pal a, A.C. Bhasikuttan b,
Sivaprasad Mitra a,
*
a
Center for Advanced Studies, Department of Chemistry, North-Eastern Hill University, Shillong 793 022, India
Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
b
A R T I C L E I N F O
A B S T R A C T
Fluorescence behavior and intramolecular charge transfer (ICT) property of N,N0-dimethylamino
substituted anil derivative of coumarinyl amine was studied by steady state and time-resolved
fluorescence spectroscopy in combination with density functional theory calculation. Quantitative
analysis of the effect of solvent hydrogen bonding on several spectral parameters was done using
multiple regression analysis based on Kamlet–Taft model. The present system shows an additional long
wavelength fluorescence band almost instantaneously in presence of methanol in sharp contrast to the
model unsubstituted analogue. This new fluorescence band is believed to be due to the formation of a
Article history:
Received 1 December 2014
Received in revised form 29 January 2015
Accepted 16 February 2015
Available online 17 February 2015
Keywords:
Coumarin fluorescence
Intramolecular charge transfer
DFT calculation
Solvent hydrogen bonding
Photochromic reaction
new isomer (cis-type) through a planar intermediate. While there is a substantial barrier (ꢀ50 kJ molꢁ1
)
in the ground state, the isomerization process in the excited state in polar-protic environment proceeds
very fast and believed to be catalyzed by hydrogen bond formation at the imine nitrogen atom in
combination with stabilization of the resulting species through ICT.
ã 2015 Elsevier B.V. All rights reserved.
1. Introduction
Photochromism in aromatic anils is an active field of
contemporary research mostly due to its potential and practical
Derivatives of 1,2-benzopyrone, more commonly known as
coumarins, are widely occurring in nature. It was first isolated in
1820 from a specific variety of bean, and many other coumarin
derivatives found in a wide range of plants [1–3]. Compounds of
the coumarin series are known to be active as fluorescent dyes and
also materials for dye lasers [4,5]. Coumarins exhibit interesting
fluorescence properties, including a high degree of sensitivity to
their local environment like polarity and viscosity. This sensitivity
has led to their widespread application as sensitive fluorescent
probes of a wide range of systems, including homogeneous
solvents and mixtures, and heterogeneous materials [6–8].
Coumarin derivatives can also be used for anticoagulant, antibac-
terial, antifungal, antihelminitic, hypothermal properties and
vasodilatory action [9,10]. Recent studies have shown that several
coumarin derivatives can act as efficient anticancer agents [11–15].
For example, a simple compound like 7-hydroxycoumarin inhibits
the release of Cyclin D1, which is over expressed in many types of
cancer.
applications [16–19]. The anils of salicylaldehyde (salicylideneani-
line, SA) and its derivatives are known to show additional
absorption as well as fluorescence property due to the formation
of a stable photochromic species. The process is initiated within
ultrafst time scale in the excited state; but, also known to be
occurring in the ground state in presence of alcoholic solvents
[20–22]. The mechanism of SA photochromism is controversial
with two different viewpoints (Scheme 1): one involves
a
preceding hydrogen atom transfer step followed by the formation
of cis and trans keto isomer; whereas, the other mechanism
involves the transfer of a hydrogen ion with the formation of cis
and trans zwitterionic species [23–25]. Nevertheless, in both the
cases, the presence of o-hydroxy group seems indispensable for the
formation of the photochromic species.
Although new syntheses and studies on photophysical behavior
of substituted coumarins have attracted a lot of attention in recent
times [26–29], to the best of our knowledge, there is no report on
the fluorescence properties derived from the anils of coumarinyl-
amine. Furthermore, the electron deficient coumarinyl moiety can
serve as an efficient acceptor, if coupled with suitable donor
system. With this intention, we have synthesized two anil products
of substituted para-amino benzaldehyde with 6-amino coumarin
(structures of the compounds are given in Chart 1) and
* Corresponding author. Tel.: +91 364 272 6634; fax: +91 364 255 0076.
E-mail addresses: smitra@nehu.ac.in, smitranehu@gmail.com (S. Mitra).
http://dx.doi.org/10.1016/j.jphotochem.2015.02.007
1010-6030/ã 2015 Elsevier B.V. All rights reserved.

68
S. Phukan et al. / Journal of Photochemistry and Photobiology A: Chemistry 303 (2015) 67–79
Scheme 1. Structure of different cis- and trans-photoproducts resulting from salicylideneaniline (SA) photochromism.
investigated the photophysical behavior with steady state and time
resolved fluorescence spectroscopy in combination with density
functional theory (DFT) calculation. The dimethylamino substitut-
ed anil (II) is capable to undergo intramolecular charge transfer
and is expected to show additional excited state photophysics in
comparison with its model unsubstituted counterpart, I. The
current study intends to give idea on photon induced ICT process
between covalently linked donor–acceptor groups built within
highly fluorescent coumarin framework. The moderately twisted
donor–acceptor linked coumarin (II) give high fluorescence
quantum yields with large Stoke-shift which is proposed to be
due to the formation of a final photoproduct. The rapid formation
of a new fluorescence band in polar protic solvent for II can show
promising application in designing “turn-on” fluorescence sensor
for protic environment. Furthermore, almost full charge separation
in the excited state of strongly twisted donor–acceptor provides
the possibility of electron transfer initiated photocatalysis.
2. Experimental
2.1. Synthesis and characterization of the substituted coumarin
derivatives (I–II)
The synthesis procedure of the investigated compounds is given
in Scheme 2. In brief, 6-amino coumarin (3) has been synthesized
according to the literature procedure [30] from 1 via 2. A mixture of
3 (1 mmol) and respective aryl aldehydes 4a-b (1 mmol) was taken
in a round bottle flask, 5 mL of anhydrous ethanol was added to it.
The reaction mixture was stirred at room temperature for 2 h. After
completion of the reaction (monitored by thin layer chromatogra-
phy, TLC), the solvent was removed under high vacuum and crude
reaction mass was washed with diethyl ether and hexane to get the
pure product I or II.
(E)-6-(Benzylideneamino)-2H-chromen-2-one (I): M.p. – 150–
R
152 ꢂC [31]. 1H NMR (acetone-d6, 400 MHz)
d= 8.70 (s, 1H), 8.04–
7.98 (m, 3H), 7.59–7.38 (m, 6H), 6.48 (d, J = 9.2 Hz, 1H). (E)-6-((4-
methylbenzylidene) amino)-2H-chromen-2-one (II): M.p. – 195–
N
197 ꢂC [31]. 1H NMR (CDCl3, 400 MHz)
d= 8.26 (s,1H), 7.71–7.64 (m,
3H), 7.34–7.20 (m, 3H), 6.68 (d, J = 8.8 Hz, 2H), 6.38 (d, J = 9.6 Hz,
1H), 3.00 (s, 6H). Single crystal of compound II was obtained by
careful recrystallization from methanol, and the structure was
unambiguously confirmed by X-ray crystallography. In the
supplementary section, Fig. 1S shows the ORTEP diagram of
compound II with probability ellipsoids representation (30%) and
O
O
R= H (I), N(Me)2 (II)
Chart 1. Structure of the substituted coumarin derivatives investigated in the
present study.

S. Phukan et al. / Journal of Photochemistry and Photobiology A: Chemistry 303 (2015) 67–79
69
Scheme 2. Schematic route toward the synthesis of the investigated coumarin compounds, I–II.
Table 1S gives the details of crystal data and structure refinement
information. On the other hand, Table 2S collects the important
structural parameters.
at 293 K. The relative experimental error of the measured quantum
yield was estimated within ꢄ10%.
The fluorescence decay curves were obtained using LED based
time correlated single photon counting (TCSPC) system procured
from Photon Technology International (PTI). The instrument
response function (IRF) was obtained by using a dilute colloidal
suspension of dried non-dairy coffee whitener. The experimentally
obtained fluorescence decay traces I(t), collected at the magic
angle (54.7ꢂ) to eliminate any contribution from the anisotropy
decay, were expressed as a sum of exponentials (Eq. (2)) and
analyzed by non-linear least-square iterative convolution method
based on Lavenberg–Marquardt algorithm [36] as implemented in
the data analysis software (FelixGX version 4.0) from PTI.
2.2. Solvents and reagents
Silica gel G (E-Merck, India) was used for TLC. Hexane, as
mentioned in the previous section, refers to the distilled fraction
boiling between 60 and 80 ꢂC. The organic solvents used for the
photophysical characterization were of spectroscopic grade
(>99.5%) as received from Aldrich Chemical Company. The solvent
water was obtained from Elix10 water purification system
(Millipore India Pvt. Ltd.). All the experiments were carried out
ꢀ
ꢁ
at room temperature (293 K). Sample concentration (8–10
was kept sufficiently dilute to avoid any effect from aggregation
and/or inner filter effect. Dielectric constants ( ) and refractive
mM)
P
t
ti
IðtÞ ¼ i ai ꢃ exp
ꢁ
(2)
e
Here, ai is the amplitude of the i-th component associated with
fluorescence lifetime ti such that ai = 1. The reliability of fitting
was checked by numerical value of reduced chi-square (
Durbin–Watson (DW) parameter and also by visual inspection of
residual distribution in the whole fitting range [37].
indices (n) of pure solvents were taken from literature [32].
P
x
2),
2.3. Instruments and data analysis
Melting points were determined in open capillaries and are
uncorrected. 1H NMR (400 MHz) spectra were recorded on Bruker
Advance II-400 spectrometer in CDCl3 and acetone-d6 (chemical
2.4. Theoretical calculation
shifts in
data of compound II was collected at 293 K with Mo K
= 0.71073 Å) using Agilent Xcalibur (Eos Gemini) diffractometer
d
with TMS as internal standard). The X-ray diffraction
Density functional theory (DFT) within the framework of Kohn–
Sham (KS) formalism has been used for all the calculations. To
elucidate the lowest energy conformers, full geometry optimiza-
tion was performed for all the structures both in isolated as well as
in their hydrated complexes using both Hartree–Fock (HF) as well
as several hybrid exchange-correlation functional like B3LYP, CAM-
B3LYP and B3PW91 in combination with 6-311++G(d,p) basis set as
implemented in Gaussian 09 program package [38]. Frequency
calculations were done in each stationary point to characterize the
minimum energy equilibrium structure. Vertical transition ener-
gies up to the first ten singlet excited states and corresponding
oscillator strengths for all the structures were estimated using time
dependent DFT method (TD-DFT) at the same level of calculation.
The effect of solvent on the energy parameters was incorporated by
self-consistent reaction field calculation using polarizable contin-
uum model (SCRF-PCM) as implemented in Gaussian 09 software.
a
radiation
(l
equipped with a graphite monochromator. The software used for
data collection CrysAlis PRO (Agilent, 2011), data reduction
CrysAlis PRO and cell refinement CrysAlis PRO. The structure
were solved by direct methods and refined by full-matrix least-
squares calculation using SHELXS-97 [33] and SHELXL-97 [34]
(CCDC 895026). Steady-state absorption spectra were recorded on
a PerkinElmer model Lambda25 absorption spectrophotometer.
Corrected fluorescence spectra were taken in a Hitachi model
FL4500 spectrofluorimeter. Quartz cuvettes of 10 mm optical path
length received from PerkinElmer, USA (part no. B0831009) and
Hellma, Germany (type 111-QS) were used for measuring
absorption and fluorescence spectra, respectively. In both the
fluorescence emission and excitation spectral measurements, 5 nm
bandpass was used in the excitation and emission side. Fluores-
cence quantum yields (ff) were calculated by comparing the total
fluorescence intensity (F) under the whole fluorescence spectral
range with that of quinine bisulfate in 0.5 M H2SO4 solution,
3. Results and discussion
3.1. Steady state spectral behavior of I and II
(
ffs = 0.546 [35]) using the following equation.
ꢀ
ꢁ
2
Fi 1 ꢁ 10ꢁA
ni
ns
Absorption spectra of I appear within 250–400 nm range with a
main peak at ꢀ275 nm and a shoulder at ꢀ340 nm region.
Excitation at 275 nm gives the principal fluorescence emission
peak at ꢀ445 nm along with a shoulder at 325 nm region in
different solvents (Fig. 1a). On excitation at ꢀ340 nm, the
s
ffi
¼
ffs
ꢃ
ꢃ
ꢃ
(1)
1 ꢁ 10ꢁA
i
Fs
where Ai and As are the optical densities of the sample and
standard, respectively; and ni is the refractive index of the solvent

70
S. Phukan et al. / Journal of Photochemistry and Photobiology A: Chemistry 303 (2015) 67–79
Fig. 1. (a) Absorption and fluorescence emission (lexc = 275 nm) spectra of I in some selected solvents. For absorption (left panel): 1,4-dioxane (i); acetonitrile (ii); methanol
(iii) and for emission: hexane (i); 1,4-dioxane (ii); methanol (iii). (b) Fluorescence emission (lexc = 340 nm) spectra of I in 1,4-dioxane (i); 1-butanol (ii); DMSO (iii);
acetonitrile (iv); water (v).
fluorescence peak position appears at ꢀ445 nm region in hexane
and resembles very closely with the long wavelength emission
obtained on 275 nm excitation wavelength (Fig. 1b). Interestingly,
this fluorescence emission peak position vary within the range of
400–525 nm region in a variety of solvents, details of which are
listed in Table 1. However, the solvent dependent spectral data in
this system is somewhat scattered and do not correlate with any of
the solvent parameters.
peak maxima appear at ꢀ360 nm in non-interacting solvents and
show variation depending on the hydrogen bonding property of
the solvent (Fig. 2a). For example, about ꢀ10 nm blue shift is
observed for protic solvents like methanol (lmax = 350 nm) with
predominant hydrogen bond donation ability (acidity); whereas,
ca. 6–7 nm red shift is observed for solvent with strong hydrogen
bond acceptance (basicity) property like DMSO (lmax = 367 nm).
However, no apparent correlation of the absorption peak maxi-
mum with solvent polarity parameter was obtained. Excitation at
the respective absorption gives very broad and strongly solvent
dependent emission spectral profile within 380–650 nm region
On the other hand, in case of II, the absorption spectra show a
single broad peak within 325–425 nm region. Careful observation
of the absorption profiles in different solvents indicate that the
Table 1
Photophysical parameters of I in some representative solvents.
P
ffa (10ꢁ2
)
tav (ns)
kr (106 sꢁ1
)
k
(108 sꢁ1
)
a
nr
Medium
labs (nm)
lem (nm)
Hexane
275, 335 (sh)
275, 329 (sh)
275, 330 (sh)
288, 330 (sh)
275, 329 (sh)
275, 330 (sh)
280, 350 (sh)
445
488
11.1
0.40
0.54
0.43
0.20
0.38
9.7
1.6
–
6.94
–
6.18
–
1,4-Dioxane
Methanol
DMSO
Acetonitrile
1-Butanol
Water
410, 520
410
400, 510
410, 520
430, 540
1.5
1.6
0.9
0.7
1.8
0.36
0.27
0.22
0.54
5.39
6.66
6.25
11.11
14.28
5.50
(sh) indicates the presence of a shoulder.
a
Fluorescence lifetime and quantum yields are measured for higher energy emission peak.
Fig. 2. (a) Absorption spectra of II in methanol (i); toluene (ii); acetonitrile (iii); chloroform (iv); water (v). (b) Fluorescence emission spectra of II in acetonitrile (i); hexane
(ii); 1-propanol (iii); toluene (iv); 1,4-dioxane (v).

S. Phukan et al. / Journal of Photochemistry and Photobiology A: Chemistry 303 (2015) 67–79
71
Table 2
Photophysical behavior of II in different solvents.
P
nr
No
Medium
D
f(
e
,n)a
labs (nm)
lem (nm)
tav (ns)
ff
kr (106 sꢁ1
)
k
(108 sꢁ1
)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Hexane
Benzene
Toluene
1,4-Dioxane
Chloroform
Ethylacetate
Dichloromethane
1-Butanol
DMSO
0
0
350
356
358
357
363
355
362
364
366
363
364
360
350
350
447
466
460
480
485
480
465
441, 514
427, 540
441, 530
445, 514
426, 495
441, 528
440
1.2
6.1
7.0
–
–
–
–
0.05
0.03
0.07
0.12
0.03
–
8.20
4.29
–
1.56
1.39
–
0.02
0.03
0.14
0.19
0.22
0.26
0.26
0.27
0.27
0.3
–
–
–
–
–
–
5.9
3.2
3.4
7.1
2.9
8.6
1.9
1.8
–
–
0.03
0.03
0.03
0.03
0.05
0.02
0.07
9.37
8.82
4.23
10.34
5.81
10.53
38.89
3.03
2.85
1.37
3.34
1.10
5.16
5.17
DMF
1-Propanol
Acetonitrile
Methanol
Water
0.31
0.32
a
Polarity parameter (calculated using Eq. (4)).
(Fig. 2b and Table 2). The large emission spectral shift indicates
strong interaction of excited II with the surrounding media and
needs careful analysis (see below).
solvent dependent spectral shift and large dipole moment change
upon excitation for II can be envisaged due to the efficient internal
charge transfer with or without the formation of different
conformer in the excited state, quite similar to the classic case
of para-(N,N0-dimethylamino)benzonitrile (DMABN) observed by
Lippert [42]. Following the same argument, it is preferable to
identify the fluorescing species of II as to be originated from an
intramolecular charge transfer (ICT) state; where, the charge
migration occurs from the dimethylanilino moiety toward the
coumarinyl group in the excited state. The characteristic features of
the ICT emissive specie and involvement of nitrogen lone-pair of
electrons toward the formation of this state is discussed in the
following sections.
3.2. Solvent dependent spectral shift of II
Any possible contribution from the excimer and/or exciplex
formation on the origin of the large spectral shift is neglected
considering the dilute working concentration regime of the
fluorophore in all the solvents. Therefore, the origin of large shift
in fluorescence emission spectra of II in solution may be due to the
one or more of the following effects. These include (i) general
solvent polarity effect; (ii) specific solvent effect including
hydrogen bonding, etc.; (iii) internal vibrational relaxation; and
(iv) solvent relaxation. It is rather difficult to isolate one single
specific reason for the spectral shift. Nevertheless, while it is
necessary to do the ultrafast time-resolved measurements for the
determination of the extent of vibrational and/or solvent relaxa-
tion, quantitative information of general and specific solvent effect
from the steady state spectral properties can be estimated
following the same protocol described elsewhere [39].
The excited state dipole moment (mE) for the charge transfer
state can also be calculated from Lippert–Mataga equation
(Eq. (5)), i.e., from linear variation of Stokes shift (DnST) of
fluorescence band against the solvent polarity parameter,
Df(e,n)
by considering only the dipole–dipole interaction between the
excited fluorophore and the solvent molecules (Fig. 3, upper panel)
[43].
ꢂ
ꢃ
2
2
nST
¼
Dn0ST
þ
ꢃ ðmE
ꢁ
mGÞ ꢃ
D
fðe; nÞ
(5)
ð4pe0Þðhca3Þ
3.2.1. General solvent effect on fluorescence solvatochromism of II:
estimation of excited state dipole moment
The excited state dipole moment (mE) of the fluorophore can be
The calculated value of mE from the slope of the straight line is
21.4 D and in quite good agreement with that calculated from the
fluorescence energy plot discussed above.
estimated from the slope of the plot of fluorescence energy (nEM
)
against the solvent polarity parameter,
equation [40].
Df(e,n) using the following
It is to be mentioned here that during linear fitting, the data
related to water do not correlate well and consequently, this data
point was excluded from linear regression. The large deviation of
spectral data in water medium can be a combination of several
effects like extra stabilization of the energy states and/or specific
solute–solvent interaction through hydrogen bonding. However,
considering the fact that the data in other polar protic solvents like
linear alcohols shows reasonably good correlation, the deviation in
aqueous medium is considered to be mostly due to the
stabilization of the highly solvated charge transferred species in
water cluster.
ꢂ
ꢃ
2
nEM
¼
n0EM
ꢁ
ꢃ ½mE
ðmE
ꢁ
mGÞꢅ ꢃ
D
fðe
; nÞ
(3)
(4)
ð4pe0Þðhca3Þ
where
Df(e,n) defined as
e
2
ꢁ 1
n2 ꢁ 1
D
fð ; nÞ ¼
e
ꢁ
e
þ 1 2n2 þ 1
The value of Onsager cavity radius “a” (5.74 Å) and the ground
state dipole moment (mG = 9.9 D) was obtained from DFT calcula-
tion results by using Gaussian 09 program following the same
protocol as described earlier [41]. Usually, the value of “a” is ꢀ0.5 Å
larger than the radius corresponding to the molecular volume
computed using “Density” keyword in Gaussian 09 to account for
the van der Waals radii of the surrounding solvent molecules [38].
The magnitude of mE for II calculated from the slope of the straight
line using Eq. (3) (Fig. 3, lower panel) is 18.4 D. On the other hand,
similar analysis of the absorption spectral data results the dipole
moment of the Frank–Condon (FC) excited state to be 13.0 D. The
3.2.2. Specific solvent effect on the spectral behavior of II
The involvement of specific solvent effect and hydrogen bond
formation becomes more pertinent since the molecular framework
of II contains several potential sites capable of hydrogen bonding
through solvent hydrogen bond donation (a) as well as hydrogen
bond acceptance ( ) ability of the amphoteric solvents like water
b
and alcohol, etc. To quantify this prediction further, the steady state
spectral properties of II are interpreted by means of the linear

72
S. Phukan et al. / Journal of Photochemistry and Photobiology A: Chemistry 303 (2015) 67–79
Fig. 3. Variation of fluorescence peak energy (i) and Stokes shift (ii) for II with polarity parameter. The solid line represents the linear fit of the experimental data points with
Eqs. (3) and (5), respectively. The light shaded point (for water) is excluded from the linear regression. The solvent numbering is given in Table 2.
important (corresponding to large Pa and/or Pb) in describing the
solvation energy relationship (LSER) concept using Kamlet–Taft
photophysical behavior of II.
equation (Eq. (6)) [44].
It is to be noted here that the blue shift in the absorption peak of
P ¼ P0 þ s
p
ꢆ þ a
a
þ bb
(6)
II in methanol (ꢀ350 nm, Table 2) was ascribed in Section 3.1 above
due to the specific solvent effect through solvent hydrogen bond
donation ability. However, the spectral positions in higher alcohol
do not show this blue shift. In fact, it resembles more with the data
in polar aprotic solvent like DMSO. A reasonable explanation for
this apparent contradictory observation can be put with the results
of multiple regression analysis discussed above and given in
Table 3, that shows the absorption behavior is mostly controlled by
where P is the value of the solvent dependent property to be
modeled, and P0, s, a and b are the coefficients determined from the
LSER analysis. The term
p
* indicates the measure of solvent
and are the scales of
dipolarity/polarizibility [45], whereas,
a
b
hydrogen bond donation acidity and acceptance basicity of the
solvent, respectively [46]. The corresponding parameters for all the
solvents are taken from the literature [47,48].
the solvent acidity (
Furthermore, the relative contribution from
more than one and half times in comparison with
(36%). Comparison of values for methanol and water (0.98,1.17
a
) as well as solvent basicity (
parameter (57%) is
parameter
b) parameters.
b
The results of multiple regression analysis are summarized in
a
Table 3.
A few representative correlation diagrams of the
a, b
experimental values with those calculated from multiple regres-
sion analysis using Eq. (6) are shown in Fig. 2S (Supplementary
section). A close look into the above results reveals several
interesting features for II: (i) the principal solvent contribution
toward the absorption spectral maximum (nABS) is through solvent
and 0.66, 0.47, respectively) in comparison with 1-propanol and
1-butanol (0.84, 0.90 and 0.84, 0.84, respectively) [48] indicate that
the later solvents may act more like a basic solvent under suitable
condition. In fact, the
b value for both 1-propanol and 1-butanol is
more than even for DMSO (0.76). Therefore, it is expected that the
absorption peak of II in the higher alcohols will resemble more to
that in basic solvents like DMSO, as indeed observed in the present
investigation.
hydrogen bonding ability (
a and b) corresponding to the higher
values of Pa and Pb; (ii) the positive contribution of ‘a’ parameter
(solvent hydrogen bond donation ability) indicates higher absorp-
tion energy (lower wavelength) in protic solvents in contrast to the
negative ‘b’ (solvent hydrogen bond acceptance basicity) contri-
bution, as indeed observed in Fig. 2a; (iii) although the relative
contribution of solvent polarity (‘s’ parameter) becomes large in
3.3. Time-resolved fluorescence behavior
Time-resolved fluorescence decay behavior of the investigated
systems were measured using time-correlated single photon
counting (TCSPC) method by excitation with 340 and 365 nm
the excited state (as manifested in nEM
higher dipole moment of the fluorescing species,
interference of the solvent hydrogen bonding ability seems
,
DnST
,
ff and tav) due to
strong
a
Table 3
Regression fit to solvatochromic parameters toward several spectral properties (P) of II.a
P
P0
s
a
b
Ps (%)
Pa (%)
Pb (%)
nABS (cmꢁ1
)
28083.05
21629.94
6453.11
116.88
788.39
ꢁ671.51
0.024
3.49
621.22
1187.03
ꢁ991.63
6.76
12.39
14.49
46.19
36.45
35.91
18.65
12.21
5.6
57.33
68.96
73.30
48.27
7.67
nEM (cmꢁ1
)
ꢁ4387.8
3396.17
ꢁ0.025
0.74
DnST (cmꢁ1
)
ꢁ565.81
ff
ta (ns)
n
0.0375
4.73
0.0029
ꢁ5.36
55.86
a
The regression analysis was done using Eq. (6).

S. Phukan et al. / Journal of Photochemistry and Photobiology A: Chemistry 303 (2015) 67–79
73
Fig. 4. Representative time-resolved fluorescence decay traces (expt, open circle) along with the simulated data (fit, solid line) and instrument response function (irf) for I (a)
and II (b). The upper panels and the inset show the distribution of weighted residual and autocorrelation function (AC) for different fitting function, respectively. The values of
reduced chi-square (x
2), Durbin–Watson (DW) parameters are also mentioned for respective cases.
LED for I and II, respectively and monitoring the fluorescence at the
corresponding emission maximum. It has been observed that in
none of these cases, the experimental decay profiles could be fitted
with single exponential model. In general, all the fluorescence
decay curves were analyzed by two exponential decay functions.
Some of the representative experimental decay profiles and the
fitting data along with several statistical parameters are also
shown in Fig. 4. The average decay times in different solvents,
calculated by using Eq. (7), are incorporated in Tables 1 and 2.
somewhat lower than the prescribed value for pure
p
*
p
transition, which is known to be of the order of ꢀ109 sꢁ1 [49]. So, it
is possible that the absorption of these coumarinyl compounds
contain a significant contribution from
p
* n transition also
hidden under pure absorption. Analysis of the significant
p
*
p
molecular orbitals (MOs) involved in the photo-absorption process
obtained from (TD)DFT calculation indeed predicts the same (see
below).
3.4. Time-dependent fluorescence spectral shift in II
Siait2i
Siaiti
P
tav
¼
i fiti
¼
(7)
The fluorescence spectral behavior of II shows certain
interesting properties in polar protic solvents, particularly in
methanol. On excitation at the absorption maxima (350 nm),
freshly prepared methanol solution of II gives a broad and
structured fluorescence within 410–460 nm region. However, with
time, a new but distinctive peak appears at ꢀ550 nm and the initial
band merely appears as a shoulder (Fig. 5a). Considering the
species responsible for 410 nm and 550 nm emissions as two
different isomers (A and B in Scheme 3) of excited II (see below for
further evidence on this hypothesis), the experimentally obtained
The combination of these data along with fluorescence
quantum yield (ff) gives the radiative and non-radiative decay
rate constants kr
=
ff/tf and Sk = (1 ꢁ ff)/tf, respectively for both
nr
the systems. It is interesting to note that Sk values are almost
nr
two orders of magnitude higher than the corresponding kr values;
indicating that the non-radiative decay as the major path of excited
state deactivation and also explains the lower fluorescence yield of
these systems. Further, the calculated kr (ꢀ106 sꢁ1) values are
Fig. 5. (a) Evolution of time dependent fluorescence spectra of II in methanol within 0 min (i) to 550 min (xv). Insets show the time variation of the intensity decay and rise
along with non-linear regression in terms of first-order kinetics. (b) Fluorescence spectra of II in methanol within temperature range of 298 K (i) to 333 K (viii). Inset shows the
fitting of the intensity ratio with Arrhenius type of equation.

74
S. Phukan et al. / Journal of Photochemistry and Photobiology A: Chemistry 303 (2015) 67–79
Scheme 3. Mechanistic representation and the importance of intramolecular charge transfer (ICT) toward the formation of the new product B from A in case of II.
time (t) dependent fluorescence intensity, F(t), were treated with
the standard integrated expression
alcoholic solvents due to the formation of the final photoproduct
[50], which is known to form due to the structural rearrangement
of the proton transferred keto–enol tautomer precursor. A similar
mechanism can also be proposed for II and given in Scheme 3.
From the results given above, it is reasonable to describe the
relaxation framework of II in terms of conversion of structure A to
B with two consecutive first order processes through a planar
intermediate (see below). The process is very fast in the excited
state as evidenced from the appearance of the new band within
minutes and considered to be a barrierless event. However, the
ground state conversion rate is quite slow and proceeds through an
energy barrier of ꢀ50 kJ molꢁ1 (see also the theoretical calculation
result). In fact, coumarin derivatives are well known to form
several tautomeric structures due to the formation of solvent
mediated proton transferred species in different electron rich
regions, particularly in the excited state. For example, in a recent
communication it was shown that, a fairly simpler system like
7-(hydroxyl)-4-trifluoromethyl coumarin show multiple fluores-
cence bands due to different proton transfer pathways and
tautomerization during excited state interaction with 1-methyl-
imidazole base [51].
k
t = ln{(F ꢁ F0)/(Ft ꢁ F0)} (F0
a
and F indicates the fluorescence intensity at initial and infinite
a
time range, respectively), for a first-order process using liner-least-
square analysis and found to be occurring in two distinct steps with
rate constants k1 = 1.90 ꢃ10ꢁ4 sꢁ1 and k2 = 2.92 ꢃ10ꢁ5 sꢁ1, respec-
tively (Fig. 3S in Supplementary section). On the other hand, from
Fig. 5a, it looks apparently that the intensity of 410 nm band also
increases with time. However, as the 550 nm band intensity show a
large increase with time, it is necessary to verify the intensity at
410 nm with respect to the normalized value at 550 nm. In fact, as
shown in the inset (i), the normalized intensity at 410 nm show an
exponential decay with
ulation of the time dependent exponential rise in 550 nm band
k
= (1.83 ꢄ 0.12) ꢃ 10ꢁ4 sꢁ1. Similarly, sim-
intensity (inset ii) yields the rate constant
k
= (5.75 ꢄ 0.30) ꢃ 10ꢁ5
sꢁ1
;
both these values are in close agreement with linear
regression analysis data. Further, as seen in Fig. 5b, a similar
observation could also be triggered by raising the experimental
temperature. Analysis of the temperature variation data with an
Arrhenius type of equation leads to the calculation of activation
energy Ea = 51.21 ꢄ3.04 kJ molꢁ1 for the conversion of A to B. A
similar time-dependent spectral change is also seen in other protic
solvents like higher alcohols and even in water; however, to a
lesser extent. Apparently, the formation of a new band (species B)
in presence of alcoholic solvents is quite similar to the well-known
photo- and thermo-chromic property of aromatic anils [16,19]. For
example, the spectrum of salicylideneaniline (SA) and its
derivatives show long wavelength absorption (at ꢀ430 nm) in
Interestingly, on addition of catalytic amount of acetic acid
(<1% v/v) into the methanol solution of II, the 550 nm emission
appears instantaneously. However, the intensity of this band
decreases on further increase in the acid concentration of the
solution (Fig. 4S in Supplementary section) with a concomitant
increase in 430 nm band intensity. A clear isosbestic point at
498 nm indicates the existence of two different species in

S. Phukan et al. / Journal of Photochemistry and Photobiology A: Chemistry 303 (2015) 67–79
75
equilibrium. The excitation spectral peak position corresponding
to this new band (lem = 550 nm) for II in methanol is blue shifted
by ca. 35 nm (from 363 nm to 327 nm) in comparison with that
obtained by monitoring the emission at 450 nm. A similar blue-
shift is also observed in the absorption spectral peak of II in
methanol (Fig. 5S). This observation further reaffirms the predic-
tion that the appearance of long-wavelength fluorescence band is
due to the formation of a new species, in analogy with the
photochromic product in case of SA derivatives discussed above.
Interestingly, no such time-dependent behavior is observed for
fluorescence emission of I (Fig. 6S), which clearly indicates the
importance of dimethyl amino group of II through the formation of
an ICT state on the origin of this behavior. It is already well-known
that the formation of final photochromic product in SA and its
derivatives involves an intramolecular proton migration on the
immine nitrogen from the neighboring phenolic moiety and
subsequent rearrangement of the keto species. Similar argument
can hold true even for II. The formation of H-bond on N-11 atom
(either by addition of catalytic amount of acid or through protic
solvent) is further stabilized by the ICT process from the dimethyl
amino group in the excited state. Considering the typical basicity of
imine nitrogen (pKb = 10) and acidity of methanol (pKa = 16), the
proton abstraction of II from the solvent, shown in Scheme 3,
seems ambiguous. However, it is well known that the acidity of the
phenolic proton and the basicity of nitrogen atoms increase
considerably in the excited state. For example, the protonation of
imine nitrogen in the excited state of anionic 6-hydroxy quinolone
occurs even in 12 M NaOH solution [52]. This large increase in
basicity is due to the localization of excess electron density on the
phenolic oxygen. A similar situation can be envisaged for II also,
can contribute toward H-abstraction from strong protic solvents
like methanol and/or water. Intramolecular charge transfer (ICT,
which is also an excited state phenomenon) from the dimethy-
lamino group in II may further facilitate this process. The resulting
conformer may undergo further rearrangement through N11-C12
single bond rotation, in a similar way as that in SA and its
derivatives. The formation of H-bond and subsequent stabilization
of the resulting species through ICT process seem to be the key
steps and working cooperatively toward the formation of the new
conformer B (and the time-dependent spectral shift); because no
such behavior is observed for I, the molecule which does not
contain the electron donating dimethyl amino group in the para-
position. The same mechanism prevails even in presence of
catalytic amount of acid. However, in presence of excess acid,
protonation occurs on the dimethylamino-nitrogen of II and the
formation of the ICT state becomes no longer feasible.
To further confirm the formation of a new species in solution,
we have continuously monitored the HPLC data of II in methanol
with time (Fig. 6). It is clear from the HPLC data that a new peak
with retention time (RT) of 2.4 ꢄ 0.1 min appears in addition to the
original peak at 3.0 ꢄ 0.1 min. Furthermore, the relative abundance
of the new peak (with less RT) was found to increases with time
with a concomitant decrease in the intensity of the original peak
(with higher RT). Interestingly, the magnitude of less RT obtained
in the reversed phase HPLC experiment is indicative of relatively
more polar nature for the new species, which is consistent with the
mechanism proposed in Scheme 3 for the formation of cis
conformer (B). Further, 1H NMR (400 MHz) data of fresh solution
of II in MeOD-d4 can be written as: d= 8.42 (s,1H), 7.99 (d, J = 9.6 Hz,
1H), 7.78 (d, J = 8.8 Hz, 2H), 7.49–7.44 (m, 2H), 7.38 (d, J = 8.8 Hz,1H),
6.82 (d, J = 8.8 Hz, 2H), 6.47 (d, J = 9.6 Hz, 1H), 3.07 (s, 6H). However,
similar experiment done with the old solution (24 h) of II shows
more proton peaks in aromatic region, possibly due to the
formation of a different isomer, and can be summarized as:
d
= 8.43 (s, 1H), 8.00 (d, J = 9.6 Hz, 1H), 7.82–7.76 (m, 2H), 7.50–7.45
(m, 2H), 7.39 (d, J = 8.4 Hz, 1H), 7.24 (d, J = 8.8 Hz, 2H), 7.13 (d,
J = 8.8 Hz, 2H), 6.99–6.96 (m, 1H), 6.85 (d, J = 2.4 Hz, 1H), 6.82–6.78
(m, 2H), 6.75 (d, J = 8.8 Hz, 2H), 6.47 (d, J = 9.2 Hz, 1H), 6.36 (d,
J = 9.6 Hz, 1H), 5.26 (s, 1H), 3.07 (s, 6H), 2.92 (s, 6H).
3.5. Theoretical calculation using density functional theory
3.5.1. Standardization of calculation methodology
All calculations were done using Gaussian type split-valence
triple-j basis set with polarization and diffuse function, 6-311++G
(d,p). To compare the applicability of different methods, the
Fig. 6. Time evolution of the HPLC data for II dissolved in methanol.

76
S. Phukan et al. / Journal of Photochemistry and Photobiology A: Chemistry 303 (2015) 67–79
ground state geometry of II was fully optimized both in Hartree–
Fock (HF) as well as density functional theory (DFT) using several
exchange-correlation hybrid energy functionals like Becke-3-
parameter-Lee–Yang–Parr (B3LYP), cam-B3LYP and B3PW91 etc.
as implemented in Gaussian-09 program package [38]. Frequency
calculation was done in each case to confirm the stationary state.
Important structural parameters are collected in Table 1S and
compared with the results obtained from X-ray diffraction
experiment. It is seen that the DFT calculations show somewhat
better correlation of the structural parameters with those obtained
from the experiment. It is indeed well established in the literature
that the DFT calculations are in good agreement and sometimes
better than even the results obtained from several other
computationally costlier methods like Møller–Plesset (MP2) or
coupled-cluster (CC) calculations [53–56]. Therefore, all the results
reported here are obtained by DFT calculations. Vertical transition
energies up to the first ten singlet excited states and corresponding
oscillator strengths for all the structures were estimated using
time-dependent DFT method (TD-DFT). However, considering the
limitations of B3LYP calculations in describing the excited states,
cam-B3LYP functional was used throughout, with the same basis
function. General solvent effect on the energy parameters was
incorporated by self-consistent reaction field calculation using
polarizable continuum model (SCRF-PCM) as implemented in
Gaussian 09 software. Similar methodology was successfully
applied recently for characterizing hydrogen-bonded structure
and the spectroscopic properties of large organic heterocyclic
systems [57–60].
first ten singlet excited states. The contributions of different
orbitals in the corresponding excited states within the experimen-
tally observed absorption spectral range for both the compounds.
The calculated excitation wavelengths are in good agreement with
the absorption spectra shown in Figs. 1 and 2. The calculated
transition energies and the corresponding oscillator strengths for
several singlet excitations within the experimentally observed
absorption spectral wavelength for both the compounds (Figs. 1
and 2) are given in Supplementary section (Table 3S). The excited
states for both the systems are mainly constituted from the HOMO
(H) ! LUMO (L) transition with some contributions from the other
(mainly H ꢁ 1 and L + 1) orbitals also. Careful analysis of electron
density distribution in all these orbitals confirms the
of electronic absorption for both I and II mixed with significant
contribution from the
* n transition also, as indeed mentioned
p
*
p nature
p
in the earlier section. Furthermore, the analysis of LUMO in II
clearly indicates a complete transfer of charge from the benzeniod
side toward the coumarinyl moiety in sharp contrast to that in I
(Fig. 7), confirming the hypothesis of ICT in the excited state of the
former and the involvement of the dimethylamino group toward
the formation of this state as predicted from the experimental
results discussed above.
3.5.3. Formation of the ICT species and excited state planarization in II
The fully optimized structures of II confirm the non-planar
geometrical arrangement of the benzenoid and coumarinyl
moieties in the ground state with a dihedral angle
Q(C5–C6–
N15–C16) ꢀꢁ140.3ꢂ (Table 2S). However, to form an effective ICT
state, these two ring system needs to be planar. For the
construction of potential energy curve (PEC) along this dihedral
angle as the reaction coordinate, the coordinate driven minimum
energy path (MEP) approach had been adopted, i.e., for a given
3.5.2. Characterization of the excited states: evidence of charge
migration in II
To understand the photophysical behavior, TDDFTcalculation of
both the compounds I and II were done in different solvents for the
value of
Q, all other geometrical parameters have been optimized
Fig. 7. Nature of the important molecular orbitals involved in the optical transition of I and II. H and L represents the highest occupied and lowest unoccupied molecular
orbitals (HOMO and LUMO), respectively.

S. Phukan et al. / Journal of Photochemistry and Photobiology A: Chemistry 303 (2015) 67–79
77
Fig. 8. Potential energy curves (PECs) for the rotation around C5–C6–N15–C16 dihedral angle for I,II and II specifically hydrogen bonded with water at N15 (II-H2O) both the
ground (triangle, left vertical axis) as well as in the excited (square, right vertical axis) states in gas phase (A) and water continuum (B).
in the ground state without any symmetry consideration using
cam-B3LYP/6-311++G(d,p) method. The excited state surface in the
gas phase was obtained by TDDFTcalculation at the same level and
to take the general solvent effect into account, self-consistent
isodensity polarized continuum model (SCIPCM) was used.
Representative curves for both I and II in gas phase as well as in
water solvent are shown in Fig. 8a for both S0 and S1(pp*) state. As
expected, the ground state PES show that the non-planar structure
is the most stable conformer with minimum at about ꢁ140ꢂ for
both the compounds. However, in the excited state, a single
methanol and believed to proceed through the formation of a
hydrogen bond donation on imine nitrogen from the solvent, as
depicted in Scheme 3. Therefore, it was interesting to model the
effect of specific hydrogen bonding at N15 of II through solvent HB
donation on the planarization in the excited state. For this purpose,
similar calculation as mentioned above was repeated with specific
hydrogen bonded water molecule at N15 and the results are shown
in Fig. 8b. The ground state PEC remains similar as that in case of
isolated II. The excited state PEC still shows the minimum in the
planar configuration; however, there exists a small barrier in the
conversion of Franck–Condon excited state to the planar ICT state.
This result is consistent and in good agreement with the
observation of time and/or temperature dependent change in
fluorescence spectral of II discussed earlier. Further, the ground
state barrier height for planarization of isolated II (Fig. 8a) was
found to be only ca. 5 kJ molꢁ1 even with or without considering
the general solvent effect of water (as continuum through PCM
method). However, the calculated barrier increased substantially
(ꢀ16 kJ molꢁ1) by considering the specifically hydrogen bonded II-
water cluster in isolated condition and further increased to almost
ꢀ33 kJ molꢁ1 by considering the solvent polarity effect also. These
results are in line with our observation of about 50 kJ molꢁ1 barrier
height obtained experimentally during the conversion of A to B for
II in methanol discussed earlier and further reaffirms the
importance of both solvent hydrogen bonding and polarity on
photophysical behavior of II, as indeed predicted in the earlier
sections.
minimum is observed in the planar conformation (
Q
ꢀꢁ180ꢂ) for II
in sharp contrast to that in I, where the most stable structure is still
the non-planar structure with
Q
ꢀꢁ155ꢂ in the gas phase and
ꢀꢁ160ꢂ in water. Apparently no significant solvent dependence is
observed in the minimum position for II, only except little
difference in the shape of S1 PEC. For example, the formation of the
planar ICT state proceeds through a relatively shallower PEC in case
of water compared with that in gas phase; although in both cases,
the process seems to be barrierless in nature. These results further
confirmed the facile ICT state formation in case of II and the
involvement of dimethyl amino group toward this process. The
calculation results are in good agreement with the mechanism
proposed earlier in Scheme 3 for the conversion of A to B, which is
believed to proceed through the formation of a planar state. The
planarization (and subsequent formation of B) is very fast in the
excited state due to the barrierless potential energy surface;
however, in the ground state, it involves a barrier and consequently
proceeds in a relatively slower process. On the other hand, in case
of I, the planarization is not feasible even in the excited state and
therefore, the formation of B can be ruled out, as indeed evidenced
from the absence any time-dependent spectral shift for this
system.
4. Conclusions
The photophysical behavior of differently substituted anil
derivatives of coumarinyl amine were studied in detail using
steady state and time-resolved fluorescence spectroscopy in
combination with density functional theory calculation. Efficient
intramolecular charge transfer in the case of dimethylamino
substituted derivative (II) is confirmed from the high dipole
moment as well as analysis of electron density distribution in the
excited state. Linear regression of solvent dependent spectral shift
It is already noted in the earlier section that the spectral
behavior of II is strongly modulated by solvent hydrogen bonding
ability. This is indeed in line with the similar observations for
coumarin series of compounds reported in literature [61–64].
Further, the time and/or temperature dependent isomerization in
this system is prominent in presence of protic solvent like

78
S. Phukan et al. / Journal of Photochemistry and Photobiology A: Chemistry 303 (2015) 67–79
data using Kamlet–Taft model indicates the importance of solvent
hydrogen bonding on the excited state behavior of II.
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by intramolecular charge transfer in case of II leads to the
formation of a new product. The process seems to be instantaneous
in the excited state; however, involves a barrier of ꢀ50 kJ molꢁ1 in
the ground state. The rapid formation of the new fluorescence band
in dimethylamino substituted coumarin compound can be
effectively manipulated to use as a “turn-on” fluorescent sensor
for monitoring the protic environment in biological samples.
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Financial support through research project 2009/37/26 from
the Board of Research in Nuclear Sciences (BRNS), Government of
India is gratefully acknowledged. The TSCPC system and the XRD
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Appendix A. Supplementary data
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