Lactim-lactam tautomerism is favoured over enol-keto tautomerism in 2-hydroxy-5-(4-fluorophenyl)nicotinic acid: Experimental and quantum chemical approach

Article abstract of DOI:10.1016/j.jphotochem.2010.06.028

A model compound, 2-hydroxy-5-(4-fluorophenyl)nicotinic acid (HFPNA) has been synthesized and its ground and excited-state properties towards tautomerisation via possible two ways of proton transfer process have been elaborately examined by means of steady-state absorption, emission and time-resolved emission spectroscopy and quantum chemical calculations. The theme issue of the present contribution is to illustrate a direct competition between two potential excited-state intramolecular proton transfer (ESIPT) pathways within the same molecule. By a direct comparison of spectral characteristics of HFPNA with those of salicylic acid (SA) and 2-hydroxypyridine (2HP) under similar experimental conditions it has been demonstrated that lactim-lactam tautomerisation dominates over enol-keto tautomerisation in HFPNA. Experimental evidences and structural calculation at Density Functional Theory (DFT) (B3LYP/6-31G**) and Hartree-Fock (6-31G**) levels predict the existence of both lactim and lactam-forms in the ground state. The ground and excited-state theoretical potential energy surfaces (PES) of HFPNA along both the ways of proton transfer coordinate at DFT level reveal that PES of HFPNA resemble well with that of 2HP while there are prominent differences from that of SA. We also report on the possibility of application of the studied molecule as a sensor of medium-pH following its sensitive response towards pH of the medium.

Full text of DOI:10.1016/j.jphotochem.2010.06.028

Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 203–214
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Lactim–lactam tautomerism is favoured over enol–keto tautomerism in
2
-hydroxy-5-(4-fluorophenyl)nicotinic acid: Experimental and quantum
chemical approach
∗
Bijan Kumar Paul, Anuva Samanta, Samiran Kar, Nikhil Guchhait
Department of Chemistry, University of Calcutta, 92 A.P.C. Road, Kolkata 700009, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A model compound, 2-hydroxy-5-(4-fluorophenyl)nicotinic acid (HFPNA) has been synthesized and its
ground and excited-state properties towards tautomerisation via possible two ways of proton transfer
process have been elaborately examined by means of steady-state absorption, emission and time-
resolved emission spectroscopy and quantum chemical calculations. The theme issue of the present
contribution is to illustrate a direct competition between two potential excited-state intramolecular
proton transfer (ESIPT) pathways within the same molecule. By a direct comparison of spectral character-
istics of HFPNA with those of salicylic acid (SA) and 2-hydroxypyridine (2HP) under similar experimental
conditions it has been demonstrated that lactim–lactam tautomerisation dominates over enol–keto tau-
tomerisation in HFPNA. Experimental evidences and structural calculation at Density Functional Theory
(DFT) (B3LYP/6-31G**) and Hartree–Fock (6-31G**) levels predict the existence of both lactim and lactam-
forms in the ground state. The ground and excited-state theoretical potential energy surfaces (PES) of
HFPNA along both the ways of proton transfer coordinate at DFT level reveal that PES of HFPNA resemble
well with that of 2HP while there are prominent differences from that of SA. We also report on the pos-
sibility of application of the studied molecule as a sensor of medium-pH following its sensitive response
towards pH of the medium.
Received 8 March 2010
Received in revised form 19 May 2010
Accepted 25 June 2010
Available online 22 July 2010
Keywords:
Competition between lactim–lactam and
enol–keto tautomerism
2
-Hydroxy-5-(4-fluorophenyl)nicotinic
acid
Absortion and emission spectroscopy
DFT
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
this phenomenon has thus, quite naturally, received immense stim-
ulate not only because of its attractive photophysics but also for its
conquer of a commanding position in applied research.
The pioneering works of Weber [1], Förster [2] and Weller [3]
envisaged a new arena of research in the field of photochemistry
in the middle of the last century through the unique observation
of Excited-State Intramolecular Proton Transfer (ESIPT) reaction
in methyl salicylate. Since then this phenomenon has undergone
an explosive growth by virtue of its promising roles in an exten-
sive arena of research, providing the wide scope of important
applications of ESIPT reaction. The unique four-level photophysical
scheme of PT reaction normally allows the total exclusion of self-
absorption (through rupture of mirror image symmetry between
absorption/excitation and emission spectra) whereby promoting
its application as ultraviolet stabilizers [4], laser dyes [5,6], radi-
ation hard scintillators and so forth [7]. Furthermore, the ESIPT
phenomenon has been exploited as optical probes for biomolecules
and in the investigation of various biomimicking, biological and
supramolecular microenvironments [8,9]. The attention focused on
Salicylic acid (SA), the benchmark molecule studied vastly
by different groups, shows ESIPT reaction via the pre-existing
intramolecular six member hydrogen bond (IMHB) in the ground
state. There are systems where the size of the hydrogen bonded ring
is different from six member ring and sometimes intermolecular
solvent mediated hydrogen bonding can assist the proton transfer
process. An asymmetric double well type potential is modeled to
describe the feasibility of proton transfer reaction for the ground
and excited states for several prototype systems [10]. The proton
transferred (PT) or the keto tautomer enormously differs from its
corresponding enol tautomer in structure, charge density distri-
bution and dipole moment and hence gives rise to a large Stokes
shifted fluorescence, which is recognized as the “spectroscopic sig-
nature” of the process. The nature of the entire ESIPT process is
found to be controlled by the barrier energy. Theoretically it is
reported that low barrier or barrierless proton transfer process is
the governing factor behind such ultrafast ESIPT reaction [10–12].
Similarly, another ESIPT system that has received huge attention
over the years in the scientific community is 2-hydroxypyridine
(2HP), which exhibits two stable tautomeric forms, the lactim-form
∗ Corresponding author. Tel.: +91 33 2350 8386; fax: +91 33 2351 9755.
E-mail addresses: nguchhait@yahoo.com, nikhil.guchhait@rediffmail.com
(
N. Guchhait).
1
010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.06.028

2
04
B.K. Paul et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 203–214
2
-hydroxypyridine (2HP) and the lactam-form 2-pyridone (2PY).
spectroscopic techniques. The molecule is an important synthetic
precursor for the synthesis of compounds having anxiolytic activ-
ity. Thus quite naturally a detailed photophysical characterization
of the molecule is expected to exert positive impetus to its future
applications through an improved understanding of its behaviour.
Apart from this, a great many number of nicotinic acid deriva-
tives has found enormous medicinal and biochemical applications
since years [37a–c]. So quite naturally, studies designed to delve
into the photophysical characterization of such important nico-
tinic acid derivatives demand relevance as well as significance
with a view to improve their future applications through a more
scrupulous characterization of their photophysics and photochem-
istry. Also the presence of the fluorine atom in HFPNA (Scheme 1)
induces some quite critical but interesting modifications to the
fundamental photophysics of the molecule through rendering
the 4-fluorophenyl moiety electron deficient which subsequently
paves way for explicit investigation on photoinduced electron
transfer (PET) mechanism and its perturbation by various external
agents [37d,e]. Modulations of photophysical properties of HFPNA
imparted by external agents like protons, metal ions, etc. dic-
tate its viable nature towards development of a new fluorescent
chemosensor. However, with an emphasis on the target of our
present work we avoid to comment on this topic in the present
contribution. The structure of the molecule is crucial and quite
interesting as it consists of two proton transfer sites suitable for tau-
tomerisation, one resembles to SA and another resembles to 2HP.
The co-existence of two very well known and suitable sites for ESIPT
reaction in HFPNA paves the way for direct competition between
the two routes of ESIPT making its photophysics quite compli-
cated and challenging. The possibility of photoinduced electron
A large volume of experimental and theoretical works have been
pursued so far on the well-known ESIPT system of 2HP and the
existence of both the tautomers at the ground state has been doc-
umented with the fact that 2HP is more stable than 2PY [13–34].
In fact, 2HP ↔ 2PY tautomerisation reaction has been enormously
studied in various phases [13–21], in matrixes [22,23], while a
massive volume of theoretical investigation has also been ded-
icated to the study of this system with special emphasis being
driven to the concept of mimicking the system for mutation in DNA
base pairs [24–28]. Experiments reveal that the enthalpy difference
between 2HP and 2PY is 2–3 kJ/mol and hence 2HP form favours
over 2PY-form and the barrier for conversion is 40–46 kcal/mol
[
29,30]. However, the latter form is known to predominate in polar
solvents [29,30]. It is reported that the 2HP to 2PY tautomerisation
via ESIPT process is controlled by the nature of the barrier, but it
is quite interesting to note that DFT methods predict that in the
ground state 2PY is more stable than 2HP [29–31]. Only methods
including high electron correlation effects have been reported to
be able to reproduce the experimental findings correctly [31–33].
A report of DFT energetic results indicates that 2PY is more stable
than 2HP by 0.47 kcal/mol at DFT//B3LYP/6-311++G** level [31]. In
spite of such discrepancy with the experimental results, DFT/B3LYP
method has been quite appreciably exploited by several authors
for the study of potential energy surface (PES), barrier height for
2
HP ↔ 2PY transformation reaction with commendable efficiency
and fruition [24–29,35,36].
In thepresent work, wehave synthesizedanew modelmolecule,
2
-hydroxy-5-(4-fluorophenyl)nicotinic acid (HFPNA) and carried
out a detailed photophysical study of the molecule based on
Scheme 1. Schematic presentation of ESIPT reaction (lactim–lactam tautomerisation) and different absorbing and emitting species of HFPNA.

B.K. Paul et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 203–214
205
transfer (PET) from the parent pyridine unit to electron deficient
fluorophenyl unit can also have a possibility to compete with the PT
reaction. As long as the occurrence of ESIPT reaction in both SA [38]
and 2HP [29,39] is established, the study of photophysics of HFPNA
will throw light into the direct competition between the two well-
known ESIPT routes. Ground and excited-state properties have
been explored using steady-state absorption and emission spec-
troscopy and time-resolved emission spectroscopy. At the same
time quantum chemical calculations have also been performed in
order to provide a theoretical support to our experimental findings.
Structures, energies and potential energy curve (PEC) along the PT
coordinate by quantum chemical calculations for the ground and
excited states have been performed at Density Functional Theory
(MeOH) were purchased from Spectrochem (India) and used
after proper distillation. Sulfuric acid (H SO ), sodium hydroxide
2
4
(NaOH), triethylamine (TEA), trifluoroacetic acid (TFA), phospho-
ric acid (H PO ) and ethanol (EtOH) were obtained from E-Merck
3
4
and used as received. Triple distilled water was used for preparing
aqueous solutions.
2.3. Instrumentations and procedures
The absorption and emission measurements were done by
Hitachi UV-Vis U-3501 spectrophotometer and Perkin-Elmer LS-
0B spectrophotometer, respectively. In all measurements the
5
sample concentration was maintained in the micromolar range in
order to avoid aggregation and reabsorption effects. All spectral
measurements have been carried out with freshly prepared solu-
(
DFT) and Hartree–Fock (HF) levels of theory.
With a view to the swelling volume of reports available in litera-
ture concerning ground- and excited-state proton transfer reaction
in numerous molecular systems, the present contribution seems to
demand novelty in view of the critical but unique molecular archi-
tecture of the title compound HFPNA which subsequently leads to
focus on an interesting issue: competition between two potential
ESIPT pathways within the same molecule. As far as our knowledge
goes, this is the first report of its kind.
◦
tions at ambient temperature of 27 C, unless otherwise specified.
Fluorescence lifetimes were obtained from
Correlated Single Photon Counting (TCSPC) spectrometer
time-resolution = 1.2 ns) using nanoLED-07 (IBH, UK) as the
a
Time-
(
light source at 280 and 340 nm to trigger the fluorescence of
HFPNA The observed fluorescence intensities were fitted by
using a non-linear least-squares fitting procedure to a function
ꢀ
t
ꢀ
ꢀ
(
X(t) =
E(t )R(t − t ) dt) comprising the convolution of the IRF
0
ꢁ
N
2
. Experimental
t/ꢀi
(
E(t)) with a sum of exponentials (R(t) = A +
Bi e ) with
i=1
pre-exponential factors (B ), characteristic lifetime (ꢀ ) and a
i
i
2.1. Synthesis of HFPNA
background (A). Relative contribution of each component was
obtained from a biexponential fitting finally was expressed by the
following equation:
To a solution of the Vilsmeier reagent prepared from DMF and
◦
POCl at 10 C, 4-fluorophenyl acetic acid was added and then the
3
◦
Bn
N
i=1
mixture was stirred and heated at 70 C for 8 h. After cooling to
an = ꢁ
(1)
room temperature the mixture was added slowly to a mixture of ice
Bi
◦
and water (temperature <10 C) and then a solution of Na CO was
2
3
The quality of the fits have been assessed over the entire decay
range, including the rising edge, and tested with a visual inspec-
tion of the residuals of the fitted functions to the actual data and
other statistical parameters, e.g. ꢁ criteria and Durbin–Watson
DW) parameter. A value of ꢁ close to unity and value of DW >1.8
added slowly until pH 11 was achieved. Toluene was added to the
alkaline mixture and the resulting mixture was refluxed for 1.5 h.
After cooling to room temperature the separated aqueous layer
was extracted with toluene. The combined organic extracts were
washed with water and dried Na SO and toluene was evaporated
2
2
(
2
4
ascertain quality fit obtained in each case.
in vacuo. The solid residue was recrystallized from the mixture of
DCM and n-heptane to yield yellow crystals of 3-dimethylamino-
The mean (average) fluorescence lifetimes for the decay curves
were calculated from the decay times and the relative contribution
of the components using the following equation [10]:
2
-(4-fluorophenyl)propenal.
To a solution of sodium methoxide in methanol, cyanoac-
ꢁ
etamide and 3-dimethylamino-2-(4-fluorophenyl)propenal were
added. The mixture was stirred at room temperature for 1.5 h and
then refluxed for 10 h. During this time a yellow solid precipitated.
The reaction mixture was diluted with water, and acidified with
2
aiꢀ
i
i
ꢁꢀꢂ = ꢀavg = ꢁ
(2)
iaiꢀi
Solutions of definite pH values were prepared (by varying the com-
1
0% HCl. The yellow solid was filtered off, washed with water,
ethanol and diethyl ether and then with n-hexane. This afforded
-hydroxy-5-(4-fluorophenyl)nicotinonitrile as a yellow solid. The
position of H PO and NaOH in water) by Systronics ␮pH System
3
4
3
61 pH-meter.
2
above yellow solid product was added to a mixture of acetic acid
and conc. HCl. The reaction mixture was refluxed for 18 h, diluted
with water and cooled under stirring. The solid was filtered off,
washed with water and 50% ethanol. This afforded 2-hydroxy-5-(4-
fluorophenyl)nicotinic acid (HFPNA) as a light gray solid. 1H NMR
2
.4. Theoretical calculations
Theoretical calculations have been performed to calculate
structures and potential energy surfaces along the isomerisation
coordinates according to the “distinguished coordinate approach”
(
300 MHz, CDCl ) ı: 14.7 (brs, 1H, COOH), 8.59 (brd, J = 3.00 Hz, 1H,
3
[
10,40,41]. The ground state structures for all the possible confor-
ArH), 7.65 (brd, J = 2.70 Hz, 1H, ArH), 7.31 (ddd, J = 11.70, 8.70 and
mations were computed using Hartree–Fock (HF) and DFT methods
by GUASSIAN 03 suit of programme [42]. Excited-state structure
of lactim-form has been optimized at CIS level and the geome-
2
.10 Hz, 2H, ArH), 7.02 (t, J = 8.70, 8.40 Hz, 2H, ArH).
2.2. Materials
try of the lactam-form has been optimized by fixing the O –H
d
distance (here O7–H8 bond in Chart 1) at Na· · ·H distance (here
2
HP was obtained from Aldrich Chemical Company (USA) and
N · · ·H distance in Chart 1) at HF level, since for otherwise the
3
8
used as received and SA was obtained from SRL, India and used
after repeated crystallization from MeOH. Spectroscopic grade
solvents such as n-hexane (HEX), methylcyclohexane (MCH), ace-
tonitrile (ACN), tetrahydrofuran (THF), dioxane (DOX), chloroform
more stable lactim-form will be reverted back in calculation [10].
Ground state intramolecular proton transfer (GSIPT) curves were
elucidated using the energies of the B3LYP/6-31G** fully optimized
geometry at fixed O –H distances over the range of 0.95–1.8 Å.
d
(
CHCl ), n-butanol (BuOH), iso-propanol (iPrOH), and methanol
The excited-state potential energy curves (PECs) were obtained by
3

2
06
B.K. Paul et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 203–214
Fig. 1. Normalized absorption spectra of HFPNA in different media ([HFPNA] =
.5–2.0 ␮M).
1
of the closed conformer of SA (Scheme S1 of Supplementary Infor-
mation [44]). Bisht et al. studied the excitation spectra of SA under
supersonic nozzle expansion condition and found the S –S1 tran-
0
sition origin at 335.34 and 311.52 nm which are assigned to the
closed conformer of SA and its rotamer, respectively (Scheme S1)
[
38]. In condensed phase spectra, however, we did not observe such
two distinctly resolved bands due to the possibility of overlapping
of rotamer absorption band within the broad band. Solute–solvent
interaction in solution may be responsible for such broadening of
the spectrum.
Fig. 1 and Table 1 display the absorption spectra and the corre-
sponding spectroscopic parameters for HFPNA recorded in different
solvents at room temperature. (The absorption spectra in Fig. 1, S1
and S2 have been normalized with respect to peak absorbance so
as to allow a clear visual comparison of the absorption maxima
positions. The actual absorbance values are not of much signifi-
cance here.) As seen in the figure, HFPNA exhibits two absorption
bands – one at ∼250 nm and another at ∼350 nm. A direct com-
parison reveals that the appearance and shape of the absorption
spectra of HFPNA are closure to those of 2HP while are distinctly
different from those of SA. Hence, the ∼250 and ∼350 nm absorp-
tion bands are assigned to the lactim-form and lactam-form of
HFPNA, respectively (Scheme 1). Additionally, a hump is observed
at ∼317 nm only in polar protic solvents; with 2HP, however, no
additional hump was observed in protic media. This may be due
to the anionic species produced upon deprotonation of the –COOH
group of HFPNA (Scheme 1). However, the possibility of formation
of intermolecular hydrogen bonded clusters of HFPNA with protic
Chart 1. Schematic of computed ground state low energy structures of HFPNA
obtained by DFT//B3LYP/6-31G** method (energies with respect to lactim-form are
given and the values within parenthesis indicate energies at HF/6-31G** level).
calculating the Franck-Condon transition energies for the B3LYP/6-
3
1G** ground state optimized structures at fixed O –H distances at
d
the TDDFT/6-31G** level. The Franck–Condon curves for the pro-
ton transfer process were generated by adding the TDDFT/6-31G**
excitation energies to the corresponding GSIPT curves. This type of
approach has been successfully reported in several recent papers
for studying ESIPT reaction [4,10,40,41,43].
3
. Results and discussions
3.1. Absorption study
Structurally HFPNA molecule has two basic units capable for
exhibiting proton transfer reaction and these are well studied
systems – 2-hydroxypyridine (2HP) and salicylic acid (SA). There-
fore it is worth to discuss spectral properties of SA and 2HP
briefly. In all solvents, 2HP exhibits two absorption bands –
one at the range of ∼227–230 nm and another at ∼297–305 nm
Table 1
Spectroscopic parameters of HFPNA obtained from absorption and emission spectra
in different solvents at room temperature.
Solvent
Absorbance
Emission,
ꢂ1 (nm)
Quantum
yield, ˚f
[
17] (Figure S1, Table S1 of Supplementary Information [44]). A
large number of reports on the tautomerisation reaction between
ꢂ1 (nm)
ꢂ2 (nm)
2
HP ↔ 2PY confirm about the presence of two stable tautomeric
MCH
HEX
CHCl3
ACN
275
273
256
252
255
255
255
255
255
256
262
384
384
349
349
348
352
352
317, 351
317, 353
334
422
420
422
415
419
419
419
419
415
424
413
0.113
0.112
–
0.235
–
0.511
0.462
0.451
0.260
0.159
–
forms, the lactim-form (2HP) and the lactam-form (2PY) in the
ground state [13–21,24,29,45–48]. Comparing the observed spectra
in different solvents (Figure S1) with available data and consider-
ing the magnitude of the barrier height, the shorter wavelength and
the longer wavelength absorption bands have been assigned to 2HP
and 2PY, respectively [17,29].
In case of salicylic acid, a broad and structureless absorp-
tion band appears at 300–312 nm wavelength region (Figure S2,
Table S1 of Supplementary Information [44]) due to ␲–␲* transition
DOX
BuOH
iPrOH
EtOH
MeOH
Water
DMSO
353

B.K. Paul et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 203–214
207
teristics are noticeably parallel to those of 2HP while at the same
time the differences with SA are also prominent. The well-known
dual emission of SA is explained on the basis of the presence of two
conformers viz., closed form and the rotamer (Scheme S1) of which
the former is capable of undergoing ESIPT [10,31]. But the lack of
dual emission in HFPNA hints the absence of similar type of closed
conformer and rotamer as in SA, rather the emission spectral profile
of HFPNA can be explained according to Scheme 1 (which parallels
to Scheme S2 of Supplementary Information [44]), i.e. excitation
at the longer wavelength (ꢂex ∼350 nm, lactam-form of HFPNA)
produces the local emission of the lactam-form only. But following
excitation at the shorter wavelength (ꢂex ∼250 nm, lactim-form of
HFPNA) also, the local emission of the lactam-form is obtained due
to the operation of an ultrafast lactim- to lactam-changeover on the
excited-state surface.
Insets of Fig. 2 reveal that the excitation spectra obtained in
all solvents assayed are authentic replicates of the corresponding
absorption spectra indicating that the origin of the lactam-form
emission is through excitation of the lactim-form, i.e. the afore-
mentioned interpretation of emission characteristics of HFPNA in
connection with resemblance to those of 2HP is taken over a rein-
forcing avenue through the excitation spectral properties of HFPNA.
3.3. Spectral modulations as a function of variation of
medium-pH
The pH variation experiments on the absorption and emis-
sion spectral profiles resulted in some interesting modulations
which emerge to play vital roles in construing the ground- and
excited-state photophysics of HFPNA with a deeper insight. A care-
ful scrutiny of the spectral modifications with pH-change leads us
to unravel the presence of different absorbing and emitting species
in solution under various circumstances.
Fig. 2. Fluorescence emission spectra of HFPNA (1.5–2.0 ␮M) in different media: (a)
ꢂex = 255 nm and (b) ꢂex = 350 nm. Insets show the excitation spectra.
Upon addition of NaOH to a methanolic solution of HFPNA, the
∼317 nm hump is found to persist and experience an increment
in absorbance, while the ∼350 nm band disappears (Fig. 3a). At the
same time as seen in Fig. 3b, addition of sulfuric acid to a methanolic
solution of HFPNA results in a reverse effect, i.e., suppression of
the ∼317 nm hump and persistence of only ∼350 nm band. This
observation is in line with previous assignment that the ∼317 nm
hump is more likely to be due to the anion (rather than hydrogen
bonded clusters) which thus disappears in the presence of acid and
persists in basic medium whereas the neutral form is responsible
for the ∼350 nm band (Scheme 2).
solvents cannot be ruled out at this stage. Also a slight blue shift of
the lower energy absorption band of HFPNA in water (from ∼350
to ∼335 nm) may be due to deprotonation of the –COOH group
in neutral water as deprotonation of –COOH functional moiety is
known to give rise to blue shifted absorption [49].
3.2. Emission and excitation study
The emission spectra of 2HP in different solvents are displayed
Similar observations are found for addition of acid TFA and base
TEA in a polar aprotic solvent ACN (Fig. 4a and b). The hump at
∼317 nm (anion, Scheme 2) in ACN is, however, generated only after
addition of TEA. Furthermore, addition of TEA to an ACN solution
of SA generates the anion band at ∼300 nm (Figure S5 of Supple-
mentary Information [44]); this is close to 317 nm of HFPNA. Thus
it seems safe to conclude that ∼317 nm absorption band of HFPNA
is attributable to the anionic species formed upon deprotonation
in Figure S3 and the results are collected in Table S1 of Supplemen-
tary Information [44]. The most interesting feature in the emission
properties of 2HP is that excitation at either of the two absorption
maxima produces emission maxima at the same position. This is
explained as follows: excitation at ꢂ ma b as x of 2PY-form (lower energy
max
band) shows the local emission of 2PY. But excitation at ꢂabs of 2HP
(
higher energy band) too produces the local emission of 2PY and not
−
of2HP, because theprocessof emissionin this case is supposedtobe
preceded by an ultrafast PT process in the excited state. A schematic
presentation of the phenomenon is given in Scheme S2 of Supple-
mentary Information [44]. On the other hand, SA exhibits distinct
dual emission in all solvents (Figure S4 of Supplementary Informa-
tion [44]). The higher energy emission band is due to the rotamer of
SA while the large Stokes shifted band is attributed to the PT-form
of the –COOH group. Whereas, addition of base (OH or TEA) to
2HP has been found to cause no significant changes at all (figures
not given), which may be due to repulsive interaction between the
localized lone pair on N-atom of 2HP and the negative charge of
−
OH anion. That the –COOH group might be ionized in neutral water
is again supported by the fact that in the presence of dilute sulfu-
ric acid the ∼335 nm band in aqueous medium shifts to ∼350 nm
(Fig. 5) which is ascribed to the neutral species (Scheme 2).
[
10,38] (Scheme S1 of Supplementary Information [44]).
Fig. 2a and b illustrates the emission spectral profiles of HFPNA
As in case of absorption spectra, the pH induced modifications of
the emission profile of HFPNA are also noteworthy. Fig. 6a reveals
the red shift of emission maxima along with intensity enhancement
with decrease of pH of a methanolic solution of HFPNA. On the other
hand, increase of pH leads to intensity enhancement coupled with
blue shift of the emission maxima (Fig. 6b). Such consistent shift-
as assayed in different solvents at room temperature and the rele-
vant spectroscopic parameters are compiled in Table 1. As seen in
the figures, excitation at either of the two absorption maxima of
HFPNA leads to no change in the position of the emission maxima.
No dual emission is observed for HFPNA and the spectral charac-

2
08
B.K. Paul et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 203–214
Fig. 3. Effect of variation of pH on the absorption profile of HFPNA (1.5–2.0 ␮M) in
methanolic medium: (a) increasing pH (addition of base (NaOH)) and (b) decreasing
pH (addition of acid (H2SO4)).
Fig. 4. Effect of addition of (a) acid (TFA) and (b) base (TEA) on the absorption spectra
of HFPNA (1.5–2.0 ␮M) in aprotic (ACN) medium.
ing of the emission maxima positions with variation of pH seems
to reinforce the assignments of pH variation experiments on the
absorption profile. In protic solvent the –COOH group may remain
ionized and with decrease of pH the neutral species is formed
whereas in an alkaline solution the –OH proton is also likely to
undergo ionization producing a dianionic species (Scheme 2). The
insets of Fig. 6a and b depict the excitation spectra with pH varia-
tion which are found to be in excellent agreement with absorption
spectra, i.e., a hump at ∼309 nm (due to anion) is found in pure
MeOH but disappears in acidic pH leaving only the ∼345 nm band
Very similar observations are found for pH variation experiments
in a polar aprotic solvent (ACN) also (figures not given).
The observation of considerable intensity enhancement with
variation of pH afforded us the impetus to go into the search for any
correlation between the magnitude of emission intensity variation
and pH of the medium so as to verify the possibility of exploiting
HFPNA as a pH sensor. For this purpose the emission spectra of
HFPNA have been recorded in a series of solutions of varying pH
(prepared from varying the composition of H PO and NaOH) and
3
4
the magnitudes of emission intensity are plotted as a function of
pH (Fig. 6c). The obtention of a linear regression points towards the
probability that HFPNA can be employed as a sensor to determine
(
due to the neutral species). Similarly in alkaline pH the ∼345 nm
band disappears and the band at ∼309 nm (due to anion) persists.
Scheme 2. Protonation/deprotonation reaction in HFPNA as a function of pH of the medium.

B.K. Paul et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 203–214
209
Fig. 5. Effect of addition of acid (H2SO4) on the absorption spectra of HFPNA
1.5–2.0 ␮M) in aqueous medium.
(
the pH of the medium. However, that at higher pH the linearity
is lost seems attributable to the complicacies of situation as an
alkaline pH may induce the formation of dianionic species.
3.4. Fluorescence quantum yield measurements
The fluorescence quantum yields of the PT species of all three
compounds studied have been estimated in solvents of different
polarities using secondary standard method. Bovine Serum Albu-
min (ꢂabs ∼280 nm and ˚ = 0.13 in Tris–HCl buffer of pH = 7.4) is
f
the secondary standard for 2HP and SA and for HFPNA this pur-
pose is served by anthracene (ꢂabs ∼350 nm and ˚ = 0.27 in MeOH).
f
The quantum yield values have been calculated on the basis of the
following equation [40,43,50,51]:
2
I · OD · ꢃ
S
S
˚
= ˚S
2
S
I · OD · ꢃ
where ꢃ is refractive index of the medium, OD is absorbance, I is
area under the fluorescence curve and ˚ is quantum yield and
subscript “S” stands for the corresponding parameters of the sec-
ondary standard. The calculated values of quantum yields of the
PT species in different solvents at room temperature for all three
compounds have been tabulated in Table 1 (HFPNA) and S1 (2HP
and SA). The quantum yields are found to be quite appreciable and
this observation seems to receive good theoretical verification from
the magnitude of the computed oscillator strength in the excited
state, which is quite high for the lactam (PT) form in comparison
to that for the lactim-form (Section 3.6). Additionally, for polar
protic solvents the quantum yield values are seen to be reduced
with increase in H-bonding efficiency of the solvents. This obser-
vation is rationalized on the ground that increased efficiency of
intermolecular H-bonding between solute and solvent accompa-
nies with it the activation of radiationless decay channels which
in turn induces reduction on radiative decay route as manifested
through consequent lowering of quantum yields [40,43].
Fig. 6. Effect of variation of pH on the emission profile (ꢂex = 350 nm) of HFPNA
(
1.5–2.0 ␮M) in methanolic medium: (a) decreasing pH (addition of acid (H2SO4))
and (b) increasing pH (addition of base (NaOH)). (c) Plot of emission intensity (I430 nm)
of HFPNA as a function of pH of the medium (ꢂex = 350 nm).
3
.5. Fluorescence lifetime measurements
istry whence with a view to fathom deeper into the excited-state
photophysics of HFPNA, fluorescence lifetime has been measured.
The molecule is excited at ꢂex = 280 and 340 nm with a view to
excite the lactim- and lactam-forms, respectively (ideally the lac-
tim and lactam-forms should have been excited at ꢂex ∼250 and
350 nm respectively, but we were restricted by the limitations of
the available light sources). The time-resolved fluorescence decay
profiles in polar aprotic medium ACN (Fig. 7a) are best fitted to
monoexponential functions as evident from the data compiled
in Table 2. For ꢂex = 340 nm it is reasonable to presume that the
Because of its inherent sensitivity towards excited-state affairs,
fluorescence lifetime measurements have long been recognized as
a sensitive indicator of the local environment of a fluorophore, apart
from being applied in monitoring sensitive issues like differential
degrees of solvent relaxation around a fluorophore, presence of
more than one chemical entity in a solution and so forth [10,40,43].
The supreme sensitivity of this technique has acquired it a position
of enormous significance and erudition in the realm of photochem-

2
10
B.K. Paul et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 203–214
Fig. 7. Time-resolved emission decay profile of HFPNA in (a) ACN (ꢂem = 415 nm) and (b) MeOH (ꢂem = 415 nm). The excitation wavelengths (ꢂex = 280 nm and ꢂex = 340 nm
to trigger the fluorescence of lactim and lactam forms, respectively) are indicated in the figure legends. The lower panel shows the residual plot of the fitted functions to the
respective data.
lactam-form is selectively excited since the lactim-form does not
absorb at this wavelength (Fig. 1; Section 3.1) when the corre-
sponding time-constant conform to the excited-state lifetime of the
lactam-form. However, we encountered a more crucial observation
during monitoring the excited-state fluorescence lifetime of HFPNA
in ACN solvent at ꢂex = 280 nm which leads to a monoexponential
decay pattern with a lifetime value intimately comparable to that
obtained in the previous case. This result seems to strongly sub-
stantiate our steady-state spectral findings in the sense that upon
selectively exciting the lactim-form (ꢂex = 280 nm; the lactam-form
does not absorb at the higher energy wavelength, see Fig. 1 and Sec-
tion 3.1) the resultant excited-state lifetime corroborates to that
of the lactam-form. This result can be readily connected to the
operation of an ultrafast ESIPT (lactim → lactam) process over the
excited-state PES. However, it is ethical to point out that ideally a
sequential kinetics should have been observed with a rising com-
ponent for the lactam-form having a time-constant comparable in
magnitude to that corresponding to the decay component of the
lactim-form. This sort of an observation would have established
the occurrence of an ultrafast lactim → lactam transformation on
the PES beyond doubts. That we are not fortunate enough to com-
ply with such an observation seems attributable to the limitation
of the time-resolution of our instrument (1.2 ns). Indeed, it is not
irrational to consider that the lactim-form is endowed with an
ultrafast excited-state lifetime in view of the fluorescence decay
characteristics of the parent molecule 2HP, as mentioned below.
An attempt to measure the fluorescence lifetime in a protic
solvent (MeOH), however, appeared with a little more compli-
cacy in form of a biexponential decay pattern (for both ꢂex = 280
and 340 nm) with a major contributing slower component and a
short-lived minor component (Fig. 7b, Table 2). Since the time-
constant for the slower component (major) is strikingly similar
for each choice of excitation wavelength (280 and 340 nm) and is
comparable to that for excited-state lifetime of the lactam-form
obtained in ACN, it seems justified to ascertain the correspond-
ing time-constant to the lactam-form. Obtention of a biexponential
decay might seem a little puzzling at the first glance, but is inter-
preted with attention to protic character of the solvent which
can thereby induce the formation of solute–solvent intermolec-
ular hydrogen bonded clusters and/or anion of HFPNA which is
subsequently becoming responsible for the short-lived minor com-
ponent. A detailed analysis of the steady-state spectral properties
of HFPNA in foregoing sections strongly substantiates such pos-
sibilities (Sections 3.1 and 3.3). Indeed, the substantial impact of
solute–solvent intermolecular hydrogen bonding interactions on
the fluorescence decay properties of fluorophores are quite well
Table 2
Fluorescence lifetimes of HFPNA in different solvents at room temperature.
Medium
ꢂex (nm)
ꢀ1 (ns)
ꢀ2 (ns)
a1 (%)
a2 (%)
ꢀavg (ns)
ꢁ2
DWa
ACN
ACN
MeOH
MeOH
280
340
280
340
3.977
3.973
5.242
5.357
–
–
100.0
100.0
74.1
–
–
25.9
16.9
–
–
0.99
1.00
1.00
1.03
1.95
1.93
1.93
1.98
1.848
1.343
4.870
5.162
83.1
a
Durbin–Watson parameter.

B.K. Paul et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 203–214
211
known and profusely discussed in the literature [52,53]. Thus the
steady-state and time-resolved spectral assignments seem to be
efficient complements of each other in construing the photophysics
of the studied molecule. Additionally, the present findings also
advocate for the commendable sensitivity of the technique of fluo-
rescence lifetime measurement towards excited-state interactions
by virtue of which the impact of solute–solvent interaction on
the photophysics of HFPNA is also indicated (differential decay
behaviour in polar aprotic and protic media).
Unfortunately a direct comparison of the lifetime data of
HFPNA with those of SA and 2HP was prohibited because of the
extremely fast decay of 2HP falling beyond the measuring limit of
the instrument. Typical decay profiles with corresponding fitting
parameters for SA are provided in the supplementary information
the ground state, but, our experimental results point towards the
existence of two conformers viz., lactim- and lactam-forms at the
ground state. Therefore, for further calculation we shall start with
the lactim-form.
The changes in the structural parameters (Table 3) as a result
of transition from the ground state (GS) to the excited state (ES),
such as increase of O7–H8 bond length from 0.9474 Å to 0.9482 Å,
decrease of N –O7 distance from 2.2501 Å to 2.2498 Å and decrease
3
◦
◦
of ∠N –H –O7 bond angle from 78.133 to 77.570 support the
possibility of translocation of the proton from O7 to N3 in the ES.
Additionally, the process of ESIPT should result in increment of dou-
3
8
ble bond character of C –O7 bond with simultaneous decrement of
4
the same in N –C bond, as manifested in the corresponding bond
3
4
distances (Table 3).
[
44] (Figure S6 and Table S2).
Examination of the computed negative charge distribution
according to Mulliken scheme at HF/6-31G** level reveals an
increase in negative charge distribution on N-atom (from −0.643 in
GS to −0.649 in ES) with simultaneous decrease of the same on O-
atom of OH group (from −0.681 in GS to −0.672 in ES) from ground
to the excited state. Such modulation of electronic charge density
stand in line with translocation of the proton following photoex-
citation [40,43]. It is probable that on excitation HFPNA attains a
delocalized state, which then relaxes to the proton transferred con-
figuration with the transfer of the proton from –OH group to the
ring N-atom.
3
3
.6. Theoretical calculation
.6.1. Structures, energies and changes in geometrical parameters
during ESIPT reaction
Conformational landscapes of HFPNA molecule are explored in
the electronic ground state by Density Functional Theory (DFT)
method using B3LYP hybrid functional and 6-31G** basis set and
the structural parameters for the lactim and lactam-forms are pre-
sented in Table 3. We have also calculated the structures, energies
of different forms of HFPNA at HF level of theory, employing the
same 6-31G** basis set so that comparison of the results of two
theoretical methods does not suffer from the shortcomings due to
change of basis set. Schematic of different low energy computed
structures with respect to the position of the hydroxyl and car-
boxyl groups for the ground state are shown in Chart 1. Out of four
possible structures, three have stabilizing intramolecular hydrogen
bond, the lack of which is making structure-B a high energy con-
former. Although, calculations at either level of calculation (DFT
or HF) predicts the intramolecularly hydrogen bonded structure-
A (six member H-bond) to be the lowest energy conformer in
3.6.2. Elucidation of potential energy curve (PEC) for ESIPT
reaction in HFPNA
A more vivid insight into the mode of tautomerisation reaction
in HFPNA can be obtained from the examination of the PEC across
both the probable PT pathways. In fact, the most critical assessment
of an intrinsic proton transfer process occurring within a molecule
can be achieved by scrutiny of PEC across the reaction coordinate.
At a first glance ESIPT reaction in HFPNA was expected to occur
via the route that resemble that of SA because of its involvement
with a more stable six member H-bond, although the experimental
Table 3
Relevant structural parameters for the optimized ground and excited states of HFPNA at HF and CIS levels and at DFT level with B3LYP hybrid functional and 6-31G** basis
set.
Parametersa
HF level
Lactim-form
GS
DFT level/B3LYP
Lactim-form
GS
Lactam-form
GS
Lactam-form
GS
ES
N3–C4
C4–C5
1.296
1.397
1.502
1.320
0.947
1.874
0.947
1.341
–
1.306
1.442
1.491
1.323
0.946
1.885
0.948
1.336
–
1.359
1.446
1.499
1.314
0.956
1.842
–
1.214
0.995
2.067
117.123
121.485
116.449
111.986
146.616
–
95.533
109.235
100.606
–
179.779
−0.379
0.076
0.536
1.325
1.407
1.506
1.345
0.974
1.815
0.971
1.363
–
1.400
1.428
1.501
1.340
0.988
1.690
–
1.234
1.026
C5–C10
C10–O11
O11–H12
H12–O7
H8–O7
C4–O7
N3–H8
N3–O7
2.250
2.249
2.283
2.093
∠
∠
∠
∠
∠
∠
∠
∠
∠
∠
∠
∠
∠
∠
N3–C4–O7
C4–C5–C10
C5–C10–O11
C10–O11–H12
O11–H12–O7
C4–O7–H8
C4–O7–H12
H8–N3–C4
N3–H8–O7
N3–C4–O7–H8
N3–C4–O7–H12
O7–H12–O11–C10
O11–C10–C5–C4
O11–H12–O7–C4
116.440
126.316
118.329
113.166
142.314
108.855
100.242
–
78.133
−0.059
179.714
−0.720
0.997
117.505
126.843
118.473
113.00806
142.640
108.83713
101.082
–
116.219
126.125
117.821
110.882
145.845
106.988
99.882
–
79.659
−0.046
−179.661
0.462
114.830
119.289
115.146
110.820
151.473
–
87.473
106.748
101.037
–
−178.175
−0.187
−0.289
−7.627
77.570
0.115
−179.026
0.182
−0.634
−0.634
−0.344
0.997
−0.702
a
◦
Bond lengths in Å and angles in degree ( ).

2
12
B.K. Paul et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 203–214
Fig. 9. Variation of O7· · ·N3 distance (pink circle) and ∠O7· · ·H8· · ·N3 angle (blue
square) with the proton transfer coordinate (O7–H8 bond axis) in the S0 state (cal-
culated at DFT/B3LYP/6-31G** level) in course of proton translocation in HFPNA
according to Scheme S2.
(
Fig. S7). Therepulsivenature ofthe S surfacetogetherwithits high
1
energy barrier and lack of minimum for the keto-form indicates the
inoperativeness of an ESIPT reaction along this route (Scheme S1).
Therefore, the theoretical results support our assignments of the
experimental findings that in HFPNA, ESIPT reaction follows the
route shown in Scheme S2, rather than that of Scheme S1.
In the above case, the PECs have been constructed by observing
the change of energy along the reaction coordinate (RC), which in
our case has been specified as to be the coordinate along which the
proton undergoes translocation from O7 to O9 (as in structure-A
of Chart 1)/N3 (as in lactim-form of Chart 1), i.e. the R
dis-
O7−H8
tance. The R
distance was varied in 0.025 Å increment and the
O7−H
8
rest of the geometry was optimized for each choice of R
tance. Fig. 9 shows that the process of proton transfer also involves
significant modulation of other geometrical parameters, mainly
dis-
O7−H8
∠
O7· · ·H · · ·N angle and O7· · ·N3 distance, reflecting the inher-
8
3
Fig. 8. Potential energy curves for S0 and S1 states along the proton transfer coor-
dinate of HFPNA according to (a) Scheme S1 and (b) Scheme S2. Calculations at
DFT//B3LYP/6-31G** level for the S0 state and at TDDFT(TD-B3LYP/6-31G**) level
for S1 state.
ent complex nature of the phenomenon. The figure dictates that
midway through the proton transfer process the O7· · ·N length
8
contracts to a minimum and the ∠O7· · ·H · · ·N angle increases to
8
3
a maximum before relaxing to the lactam-form. A crossover point
between lactim- and lactam-form is found at around O7–H8 dis-
tance of ∼1.4 Å. This mirror image plot points towards the error that
would be inevitably incorporated in the calculation by freezing all
the coordinates during evaluation of the PEC.
findings are in line with those of 2HP. At this point, theoretical
calculations, more particularly the evaluation of PECs, have been
exploited with a view to afford critical information for deciphering
the photophysics of HFPNA.
Fig. 10a shows the three-dimensional potential energy sur-
faces (PESs) for the proton transfer reaction of HFPNA according
to Scheme S2, i.e. change of energy with simultaneous variation of
O7–H and O7· · ·N distances are depicted for both ground (S ) and
For a better comparison, the PECs for SA and 2HP for the ESIPT
reaction are constructed at the same level of calculation and given
respectively in Figure S7 and S8 of Supplementary Information [44].
Fig. 8a and b displays the PECs of HFPNA for PT reaction accord-
ing to Scheme S1 and S2 (in Supplementary Information [44]),
respectively. Clearly, a comparison of the PECs reflects remark-
able dissimilarities between HFPNA (Fig. 8a) and SA (Figure S7),
but side-by-side the PECs in Fig. 8b (i.e. ESIPT reaction in HFPNA
according to Scheme S2) happen to bear noticeable similarities
with those of Figure S8 (2HP ↔ 2PY). The S1 surface (Figure S7)
for SA along the proton transfer coordinate predicts a barrierless
process of ESIPT while the inoperativeness of a GSIPT process is
dictated by the repulsive nature and high barrier height at the S0
surface (also no minimum for the keto form at S0 PEC). In HFPNA
the operation of GSIPT (along the route depicted in Scheme S1) is
negated based on similar arguments, i.e. repulsive nature of the S0
surface coupled with considerably high magnitude of the energy
barrier (Fig. 8a). But interestingly an ESIPT process (according to
Scheme S1) in HFPNA also seems improbable following the qual-
itative similarity in appearance of the S1 surface to the S0 surface
8
3
0
excited states (S ) and Fig. 10b shows the PESs with simultaneous
1
variation of O7–H and O7· · ·O distances, i.e. PT reaction of HFPNA
8
9
according to Scheme S1. These results provide further confirma-
tion to the conclusions derived from the analysis of Fig. 8, i.e. for PT
reaction in HFPNA as per Scheme S1, the barrier still persists in the
S1 state as against the case of SA (Figure S11) while for a PT reaction
according to Scheme S2, the PESs of HFPNA are strikingly similar
to those of 2HP (Figure S11). Since the course of ESIPT reaction in
HFPNA is already documented to strongly perturb other geomet-
rical parameters like ∠O7· · ·H · · ·N angle and O7· · ·N distance,
8
3
3
apart from O7–H8 bond, the three-dimensional PES (variation of
energy with simultaneous variation of two important geometrical
parameters viz. O7–H8 and O7· · ·N3 distances) stand as a strong
complement for the two dimensional PECs.
Fig. 11 shows the variation of oscillator strength with R
O7−H8
distance on going from lactim-form to the lactam-form in the
S1 state. Enhancement of the calculated oscillator strength from
lactim-form to the lactam-form on S1 surface complies with the
(
Fig. 8a), i.e. carrying strong contradiction to the observations for SA

B.K. Paul et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 203–214
213
Fig. 10. Three-dimensional potential energy surfaces for proton transfer in HFPNA showing the variation of energy as a function of (a) O7–H8 and O7· · ·O9 distances according
to Scheme S1 ((i) GS; (ii) ES) and (b) O7–H8 and O7· · ·N3 distances according to Scheme S2 ((iii) GS; (iv) ES).
posed to be preceded by an ultrafast ESIPT reaction on the S1
surface. Such emission spectral properties remarkably comply with
those of 2HP and are not in agreement with those of SA which
produces dual emission characteristic of a rotamer and the PT-
form.
Theoretical calculations at DFT and HF levels predict the
existence of intramolecularly H-bonded stable lactim and lactam-
forms. The structural changes and charge distribution at the proton
translocation site are found to be in favour of ESIPT reaction. Poten-
tial energy curves for an ESIPT reaction in HFPNA via the route that
resemble to SA are found to be in strong incongruity with those of
SA. In SA, the phenomenon of PT on the S1 surface is a barrierless
process whereas in HFPNA the barrier is quite high (∼0.08 kcal/mol
at DFT/B3LYP/6-31G(d,p) level). Also it is found that the nature and
shape of the S surface in HFPNA is very similar to that of the S sur-
1
0
face, as against the case for SA. On the other extreme, PECs for an
ESIPT reaction in HFPNA via the route that resemble to 2HP are sim-
ilar in nature. Additionally, the presence of two tautomeric forms
in the ground state is also verified in the lactim–lactam surface but
not in the enol–keto surface.
Fig. 11. Variation of the oscillator strength with O7–H8 distance in the S1 state for
ESIPT reaction according to Scheme S2 (at DFT/B3LYP/6-31G** level).
In a nut-shell, the properties and the feasibility of proton trans-
fer process and its mechanism in HFPNA in the excited and ground
states inferred from the experimental data are well complemented
from theoretical calculations.
high fluorescence quantum yield of the proton transferred species
(
lactam-form) (as seen in Table 1).
4
. Conclusion
In this paper, the photophysical properties of HFPNA have
been investigated from experimental and theoretical perspectives
on the basis of absorption, emission and time-resolved emission
spectroscopy and quantum chemical calculations, respectively.
The absorption and emission spectra of HFPNA are found to be
similar to those of 2HP and distinctly different from those of
SA. Spectral characteristics confirm the existence of lactim and
lactam-forms in the ground state and excited-state lactim ↔ lactam
tautomerisation reaction. Excitation of lactam-form produces its
local emission whereas excitation of the lactim-form too shows
emission of lactam-form, i.e., emission from lactim-form is sup-
Acknowledgements
N.G. acknowledges C.S.I.R., India (Project no. 01(2161)07/EMR-
II) and D.S.T., India (Project no. SR/S1/PC/26/2008) for financial
supports. B.K.P. and A.S. are also grateful to C.S.I.R., New Delhi
for Senior Research Fellowship. Prof. T. Ganguly, Department of
Spectroscopy, Indian Association for the Cultivation of Science,
Jadavpur, Calcutta 700032, India is acknowledged for allowing us to
use the TCSPC instrument for fluorescence lifetime measurements.

2
14
B.K. Paul et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 203–214
Appendix A. Supplementary data
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