Excited state lactim to lactam type tautomerization reaction in 5-(4-fluorophenyl)-2-hydroxypyridine: Spectroscopic study and quantum chemical calculation

Article abstract of DOI:10.1007/s10895-010-0692-3

The photophysical properties of 5-(4-fluorophenyl)-2-hydroxypyridine (FP2HP) have been studied by steady state and time resolved spectroscopy in combination with quantum chemical calculations. The molecule FP2HP exists as lactim and lactam form in the gro

Full text of DOI:10.1007/s10895-010-0692-3

J Fluoresc (2011) 21:95–104
DOI 10.1007/s10895-010-0692-3
ORIGINAL PAPER
Excited State Lactim to Lactam Type Tautomerization
Reaction in 5-(4-Fluorophenyl)-2-Hydroxypyridine:
Spectroscopic Study and Quantum Chemical Calculation
Anuva Samanta & Bijan Kumar Paul & Samiran Kar &
Nikhil Guchhait
Received: 28 January 2010 /Accepted: 22 June 2010 /Published online: 8 July 2010
#
Springer Science+Business Media, LLC 2010
Abstract The photophysical properties of 5-(4-fluorophenyl)-
2-hydroxypyridine (FP2HP) have been studied by steady state
and time resolved spectroscopy in combination with quantum
chemical calculations. The molecule FP2HP exists as lactim
and lactam form in the ground state due to small stability
difference but does not undergo lactim to lactam isomerisation
due to high barrier energy. Whereas in the excited state the
lactim form undergoes tautomerization producing red shifted
emission of the lactam tautomer along with the local emission
of the lactim form. In polar protic solvents, the barrier for
lactim-lactam tautomerization rapidly decreases forming the
lactam tautomer only. Temperature has pronounced effect on
the excited state tautomerization equilibrium and is clearly
reflected in the measured equilibrium constant (Ktau0) and free
energy change (ΔG0). Structural calculations at Hartree Fock
and Density Functional Theory levels, calculated stability of
the isomers in different solvents using Polarized Continuum
Model and the theoretical potential energy surfaces for the
ground and excited states along the proton transfer coordinate
are reported for the tautomeric equilibrium of FP2HP.
Introduction
For long time the proton transfer (PT) reaction is found to
be one of the most encountered photoinduced process
studied both experimentally and theoretically due to its
immense application in various fields of scientific research.
The molecules capable of tautomerization via proton
transfer reaction are used as fluorescence probes for the
measurement of solvation dynamics [1] and biological
environments [2], fluorescence recording [3], ultraviolet
stabilizers [4], the development of laser dyes [5–7], metal
ion sensors [8], radiation hard-scintillator [9] and organic
light emitting devices [10].
Out of diversified molecular systems capable of PT
reaction, studies on photoinduced proton transfer reaction
in nitrogen heterocycles of pyridine class with a proton
bridging between nitrogen and oxygen atom are one of the
interesting problem [11, 12]. In this context, the photo-
induced 2-hydroxypyridine (2HP) to 2-pyridone (2PY)
tautomerism by proton transfer (PT) process [13–15] has
drawn tremendous attention to biologists for many decades
because this process plays an important role in the mutation
of DNA [16, 17] and it is the representative of a large
number of heterocyclic molecules that are relevant to
biological function [18]. The tautomerism of purine and
pyrimidine bases occur naturally in nucleic acids,
neucleotides which play a key role in mutagenesis [19].
The molecule 2HP exhibits two stable tautomeric forms:
the lactim form 2HP and the lactam form 2PY. This
tautomeric equilibria has been extensively studied
experimentally in condensed and gas phases [13–15] as
well as theoretically. In the 2HP-2PY equilibrium the
energy difference between the two tautomers is sufficiently
small and hence both species observed in the ground state,
but proton transfer does not proceed in the ground state
.
Keywords Tautomerization 5-(4-fluorophenyl)-2-
.
.
hydroxypyridine Fluorescence Density Functional
.
Theory (DFT) Time Dependent Density Functional
Theory (TDDFT)
:
:
:
A. Samanta B. K. Paul S. Kar N. Guchhait (*)
Department of Chemistry, University of Calcutta,
92 A. P. C. Road,
Kolkata 700009, India
e-mail: nguchhait@yahoo.com

96
J Fluoresc (2011) 21:95–104
because of a high energy barrier (16,000–20,000 cm−1) [13,
20–23], whereas in polar solvents the lactam- form
predominates over the lactim- form [24, 25]. Experiment
in supersonic jet expansion [13] reveals that the S0-S1 band
origin were observed at 36,123 cm−1 and 29,831 cm−1
which were identified as a π → π* transition for the 2HP
and 2PY form, respectively [15]. In the ππ* excited singlet
state, the large exothermicity (≈6,500 cm−1) of the lactim to
lactam reaction should decrease the barrier for proton
transfer, thus allowing lactim to lactam transformation after
photoexcitation [26].
HEX
MCH
CHCl3
THF
DOX
ACN
MeOH
H2O
DMSO
0.4
0.3
0.2
0.1
0.0
In the present work, we have synthesized 5-(4-
fluorophenyl)-2-hydroxypyridine (FP2HP), quite similar
to that of 2HP having one substitution at fifth position and
carried out its photophysical studies by steady state
absorption and emission in combination with time resolved
spectroscopy. The molecule is an important synthetic
precursor for the synthesis of compounds having anxiolytic
activity [27]. Interestingly, this molecule is suitable for
enol-keto tautomerization along the five member hydro-
gen bonding ring. Therefore, tautomerization of this
synthetic precursor could be important for both the ground
and excited states. The para substituted fluorophenyl unit
(electron deficient center) can be used as electron acceptor
from the parent pyridine unit (electron donor) and may
influence the hydrogen bond strength at the proton transfer
site and thus affect the isomerisation reaction. The
possibility of photo-induced electron transfer (PET) from
the parent pyridine unit to electron deficient fluorophenyl
unit can compete with the PT reaction. Lastly, this
molecule can be used as multi-dentate ligand binding
sites for making complex with metal ions so that this
molecule can be used as transition metal ions sensor. The
effect of temperature on the tautomerization reaction has
been explored by following fluorescence spectra. Quantum
mechanical calculations have been performed for structural
and potential energy curve calculations along the proton
transfer coordinate for the ground state and first excited
singlet state at Density Functional Theory (DFT) and Time-
Dependent Density Functional (TDDFT) levels of theory to
correlate the experimental results with theoretical data. The
effect of solvents on lactim-lactam equilibrium has also
been explored theoretically for a correlation with the
experimental results.
260
280
300
320
340
360
380
Wavelength (nm)
b
350
300
250
200
150
100
50
Hex(λex=268nm)
MCH(λex=268nm)
CHCl3(λex=268nm)
DOX(λex=268nm)
THF(λex=268nm)
ACN(λex=265nm)
iPrOH(λex=260nm)
MeOH(λex=260nm)
H2O(λex=260nm)
300 325 350 375 400 425 450 475 500 525
Wavelength (nm)
c
HEX (385nm)
MCH (384nm)
CHCl3 (408nm)
DOX (406nm)
THF (404nm)
ACN (407nm)
ACN (338nm)
iPrOH (394nm)
MeOH (394nm)
H2O (392nm)
400
300
200
100
225
250
275
300
325
350
375
400
Experimental section
Wavelength (nm)
Fig. 1 a Absorption spectra of FP2HP in different solvents at room
temperature, b Fluorescence emission spectra of FP2HP in solvents of
differing polarity at room temperature. c Fluorescence excitation
spectra of FP2HP at different emission maxima
To a solution of the Vilsmeier reagent at 10°C, 4-
fluorophenyl acetic acid was added. The mixture was
stirred and heated at 70°C for 7 h. After cooling to room
temperature, the mixture was added slowly to a mixture of
ice and water and then a solution of Na2CO3 was added

J Fluoresc (2011) 21:95–104
97
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 Na2SO4 and toluene was evaporated in vacuum. The
solid residue was recrystallized from the mixture of
dichloromethane and n-heptane to yield yellow crystals
of 3-dimethylamino-2-(4-fluorophenyl)-proplactim. To a
solution of sodium methoxide in methanol cyanoaceta-
mide, the above product was added. The mixture was
stirred at room temperature for 1.5 h and then refluxed for
10 h. During this time, a yellow solid precipitate was
formed. The reaction mixture was diluted with water and
acidified with 10% HCl. The yellow solid was filtered off,
washed with water, ethanol and diethyl ether and then with
n-hexane. This afforded 2-hydroxy-5-(4-fluorophenyl)-
nicotinonitrile as a yellow solid. This product was added to a
mixture of acetic acid and conc. HCl and refluxed for 18 h,
diluted with water and cooled under stirring. The solid was
filtered off, washed with water and then 50% ethanol. This
afforded 2-hydroxy-5-(4-fluorophenyl)-nicotinic acid as a
light gray solid. A mixture of the above product and freshly
distilled quinoline was stirred and heated to 215°C for 14 h
under a nitrogen atmosphere. The reaction mixture was cooled
to room temperature and n-heptane were added. The solid was
filtered off and washed with n-heptane and then recrystallized
from a mixture of dichloromethane and n-heptane to get pure
5-(4-fluorophenyl)-2-hydroxypyridine.
The absorption and emission spectra of FP2HP have
been taken by Hitachi UV-Vis (Model U-3501) spectro-
photometer and Perkin Elmer (Model LS-50B) fluorimeter,
respectively. Temperature variation fluorescence spectra
have been measured by keeping the emission cell compart-
ment at a chosen temperature using variable temperature
chiller. In all measurements, the sample concentration has
been maintained within the range 10−5–10−6 mol/dm3.
Fluorescence lifetime measurement has been done by
Time Correlated Single Photon Counting (TCSPC) tech-
nique using nanoLED-07 (IBH UK) [28]. The light source
used in the TCSPC set up is a nano LED of wavelength
295 nm.
All calculations have been performed with the Gaussian
03 package [29]. All possible ground state conformers of
FP2HP have been optimized at Hartree Fock (HF) and
Density Functional Theory (DFT) level using B3LYP
functional and 6–31G** basis set. The optimization of the
FP2HP in different solvents has been carried out using
Polarized Continuum Model (PCM) at Hartree Fock (HF)
level [30, 31]. The ground state potential energy surfaces
along the proton transfer coordinate have been done using
relaxed scan [30, 31]. The excited state behaviors of FP2HP
are obtained in the light of Time Dependent Density
Functional Theory (TDDFT).
Results and discussions
The solvents methylcyclohexane (MCH), cyclohexane
(CH), tetrahydrofuran (THF), 1,4-dioxane (DOX), carbon
tetrachloride (CCl4), acetonitrile (ACN), dimethylsulfoxide
(DMSO), isopropanol (Iso-prop), 1-butanol (BuOH) and
methanol (MeOH) were purchased from Spectrochem and
the purity of solvents have been checked in the wavelength
range used. Triple distilled water was used for the
preparation of aqueous solutions.
Absorption spectra
Figure 1a depicts the absorption spectra of FP2HP in
different solvents at room temperature. The FP2HP molecule
exhibits two absorption bands irrespective of the nature of
the solvent such as H-bonding ability and polarity. Compar-
ing with the absorption spectra of 2-hydroxypyridine having
similar absorption bands at ~227–230 nm and at ~297–
Table 1 Spectroscopic parameters measured by absorption and fluorescence spectra of FP2HP molecule at room temperature
Solvents
λAbs (nm)
λEm (nm)
ꢀf (FP2HP- form) (×102)
ꢀf (FP2PY- form) (×102)
(λex=265 nm)
ꢀf (FP2PY- form) (×102)
(λex=310 nm)
CYC
268, 312
268, 312
269, 325
268, 325
266, 325
273, 330
265, 322
262, 320
260, 318
259, 310
335, 387
335, 387
352, 403
346, 405
338, 407
342, 403
395
0.104
0.207
0.054
1.432
0.244
0.035
–
3.649
5.516
0.90
1.50
2.23
3.23
2.47
4.61
6.20
6.12
4.44
9.25
MCH
THF
2.814
DOX
ACN
DMSO
BuOH
iPrOH
MeOH
H2O
9.317
10.924
4.121
23.477
22.745
16.785
33.309
392
–
396
–
394
–

98
J Fluoresc (2011) 21:95–104
1.4
0.7
Δ H 0=-5.88kcal/mol
305 nm, the higher energy band of FP2HP at ~260 nm and
the lower energy band at ~315 nm are ascribed to the
absorption band of the lactim and lactam form of FP2HP,
respectively. In the ground state, structurally aromatic
lactim form may predominate over the lactam form due
to higher stability. Later on by theoretical calculation we
have found that the relative stability of two forms
depends on the nature of the solvent. However, the red
sided absorption band with low intensity of the lactam
form may be due to less S0–S1 energy gap and low
oscillator strength than the lactim form. The position of
the bands are solvent dependent, but the effect of solvents
on the lactam form is more prominent than that of lactim
one. From Table 1 it is clear that the low energy
absorption band is red shifted with polarity of the aprotic
solvents. From now onwards the lactim form is ascribed as
FP2HP and the lactam form as 5-(4-fluorophenyl)-2-
pyridone (FP2PY). The position of lactim form is blue
shifted with increasing H-bond forming ability of solvents
due to intermolecular H-bonding interaction with the protic
solvents. But the spectral characteristics of the lactam form
indicate stabilizing dipolar interaction between the solvent
dipole and the solute. However, it is important to note that the
absorption spectra of FP2PY undergoes a detectable red shift
on going from nonpolar solvent, e.g. CYC, MCH to polar
protic solvents e.g. MeOH, but is again blue shifted from
methanol to water. Such solvatochromic dependency of the
absorption spectra of the presently investigated ESIPT probe
seems to bear considerable consistency with the spectroscopic
behavior of another closely related molecular system, 2,6-
diaminopyridine, recently reported [32], and thus further
substantiates the solvent effect on the spectroscopic properties
of FP2HP.
180
150
120
90
Δ S0= -0.019kcal/mol.K
0.0
-0.7
333K
273K
0.0030
0.0033
0.0036
(1/T)/(10-3) Kt-1
60
30
0
275 300 325 350 375 400 425 450 475 500 525
Wavelength (nm)
140
b
ΔH0=-4.9kcal/mol
2.8
2.1
1.4
ΔS0=-0.012kcal/mol.K
120
100
80
60
40
20
0
273K
333K
0.0030
0.0033
(1/T)/(10-3) K-1
0.0036
300 325 350 375 400 425 450 475 500 525 550
Wavelength (nm)
c
MCH
ACN
2
0
-2
-4
-6
Emission and excitation spectra
It is well known that in case of 2HP, the energy difference
between the two tautomers is sufficiently small and hence
both the forms exist in non-polar solvents. But the energy
barrier for PT reaction in the ground as well as in the
excited electronic states is large [33]. On excitation at any
of the absorption band of 2HP either at ~230 nm or
~300 nm, single emission at ~350–370 nm has been
observed that was assigned as the local emission of 2PY
form. In case of FP2HP, on 260 nm excitation, the molecule
shows two emission bands: one at ~338 nm and another at
~407 nm (Fig. 1b) and it shows only one emission at
~407 nm on 315 nm excitation. Though the PT barrier in
the ground state is quite high but this barrier for PT reaction
from lactim to lactam, i.e. FP2HP to FP2PY, decreases in
the ππ* excited singlet state due to large exothermicity.
Single emission band at ~395 nm is observed in case of
polar protic solvents. Here PT reaction is more feasible due
100
75
50
25
270
285
300
315
330
Temperature(K)
Fig. 2 Fluorescence emission spectra (λext=260 nm) of FP2HP as a
function of temperature in (a) MCH and (b) ACN solvent, (inset; Plot
of lnKtau vs 1/T). c Plot of ΔG0 and % of Lactam form vs
0
temperature for FP2HP in MCH and ACN solvent

J Fluoresc (2011) 21:95–104
99
Table 2 Experimental tautomeric equilibrium constants (Ktau0) and standard free energy change (ΔG0) (kcal/mol) of FP2HP at different
temperatures obtained from Fig. 2a,b and Eqs. 1, 2, 3 and 4
Temp (K)
MCH solvent
ACN solvent
ΔG0 (using Eq. 2)
ΔG0 (using Eq. 4)
Ktau
ΔG0 (using Eq. 2)
ΔG0 (using Eq. 4)
0
0
Ktau
273
283
293
303
313
323
333
3.012
2.425
1.731
1.176
0.837
0.607
0.462
−0.598
−0.498
−0.320
−0.098
0.111
−0.655
−0.464
−0.273
−0.082
0.109
17.938
15.868
10.120
9.243
7.022
4.668
3.607
−1.566
−1.555
−1.348
−1.339
−1.212
−0.989
−0.849
−1.626
−1.506
−1.386
−1.266
−1.146
−1.027
−0.907
0.320
0.300
0.511
0.491
to lowering of exothermicity as well as lowering of barrier
energy in an environment capable of making intermolecular
hydrogen bonding with solvent molecules. Excitation
spectra for 338 nm emission band show that this emission
is originated from the species absorbing at 260 nm
(Fig. 1c). So we have assigned this band (~338 nm) to LE
of the FP2HP form. The excitation spectra monitored at
407 nm originates two bands that are quite similar to that of
the absorption spectra of the molecule. Therefore, red
shifted band arises not only from the LE emission of the
lactam form, but from the lactim form due to ESIPT
reaction.
The data presented in Table 1 reveals that the emission
band maxima position of FP2HP shows some solvent
dependency in the form that the emission maxima is
comparatively red shifted with increasing solvent polarity.
Such an effect, in analogy to literature, may be interpreted
to be the result of charge migration from the hydroxyl
oxygen to the heterocyclic ring part of the molecule upon
photoexcitation [30–32, 34]. This, in turn, will decrease the
charge density of the hydroxyl oxygen and hence its proton
donor ability [30–32, 34]. However, in comparison to the
recent report on 2,6-diaminopyridine [32], the observed
solvatochromic effect in our case is not so large, and seems
not surprising since an amino functional moiety can
function as a better charge donor than a hydroxyl group.
It is, indeed, noteworthy that the absorption and fluores-
cence solvatochromic shifts of FP2HP are somewhat
greater than the model compound 2-hydroxypyridine. This
might be an indication towards the presence of intramolec-
ular hydrogen bond and excited state intramolecular proton
transfer in FP2HP [32]. The occurrence of ESIPT in FP2HP
has, however, been elaborately discussed in the upcoming
sections.
Lamp
ACN (340nm)
MCH (335nm)
1000
100
10
0
10
20
Time (ns)
30
b
Lamp
MCH (387nm)
ACN (410nm)
Water (410nm)
1000
100
10
0
20
30
10
Time (ns)
Fluorescence quantum yield
Fig. 3 a Fluorescence decay profiles of lactim tautomer of FP2HP in
MCH (λmon=335 nm) and in ACN (λmon.=340 nm), b Fluorescence
decay plots of lactam tautomer of FP2HP in MCH (λmon.=387 nm),
ACN (λmon.=410 nm) and H2O (λmon.=410 nm)
The fluorescence quantum yield of FP2HP and FP2PY
forms in solvents of different polarities are measured

100
J Fluoresc (2011) 21:95–104
relative to Tryptophan ( ꢀR=0.13) in Tris-HCl buffer and
β-napthol ( ꢀR=0.23) in MCH as the secondary standard,
respectively (Table 1) [35]. The quantum yield values are
calculated using the following equation,
increases with increase of temperature. So the atoms
comprising the FP2HP molecule oscillate and rotate much
faster, hence the intramolecular hydrogen bond O-H...N is
weakened thereby reducing the possibility of intramolecular
proton transfer. The temperature dependent lactim → lactam
reaction is characterized by tautomeric equilibrium constant
(Ktau0) which can be calculated using the relation given
below;
AS:ODR:n2S
AR:ODS:n2R
ΦS ¼ ΦR:
Where, ꢀS and ꢀR are the quantum yields, AS and AR
are the integrated fluorescence areas, ODS and ODR are the
absorbance values and nS and nR are the refractive indices
for sample and reference molecule, respectively.
ILactam
0
Ktau
¼
ð1Þ
ILactim
Where ILactam and ILactim represent the emission intensity
of lactam (FP2PY form) and lactim (FP2HP form) form,
respectively. As seen in Table 2, Ktau value decreases with
As shown in Table 1, it is clear that lactim form
undergoes excited state PT reaction indicated by the large
value of quantum yield of lactam form than lactim form.
Comparing the quantum yield of FP2PY form for different
excitation, it is clear that for 260 nm excitation, the
corresponding value is higher than that for 315 nm
excitation. For the former one, lactim form tautomerises
to lactam through intramolecular proton transfer mechanism
and the corresponding value gives the total of lactam form
as well as tautomerized lactam form. On the other hand,
315 nm excitation gives only the emission of the lactam
form, so this quantum yield value corresponds only to
lactam form of FP2HP. This comparison clearly indicates
lactim-lactam tautomerism in the excited state.
0
increasing temperature thereby favoring the lactim formation.
The standard free energy ΔG0 is related to Ktau as,
0
0
ΔG0 ¼ ꢀRT lnðKtau
Þ
ð2Þ
The standard enthalpy (ΔH0) and entropy (ΔS0) can be
computed as
ΔH0 ΔS0
0
lnðKtau Þ ¼ ꢀ
þ
ð3Þ
RT
R
So from the plot of ln(Ktau0) vs 1/T as shown in inset of
Fig. 2a and b, ΔH0 and ΔS0 can be determined for the
lactim-lactam equilibrium. The ΔG0 value thus can be
recalculated for different temperature using the value of
ΔH0 and ΔS0 using the following equation
Effect of temperature
The effect of temperature variations on the spectral properties
of FP2HP in non-polar and polar aprotic solvents are
presented in Fig. 2a and b. In both the cases of MCH and
ACN solvent with increase of temperature the emission
intensity of FP2HP form increases with concomitant de-
crease of the intensity of FP2PY form. However, the increase
in intensity for FP2HP form and the decrease in intensity of
FP2PY form are more pronounced in non-polar solvent
MCH than polar aprotic ACN solvent. This indicates that
temperature influences the position of FP2HP-FP2PY equi-
librium. As shown in Fig. 2c, the percentage of lactam form
decreases rapidly in MCH than ACN solvent with increase
of temperature. Temperature is essentially a measure of
overall kinetic energy. Kinetic energy as well as its
components of vibrational and rotational energy also
ΔG0 ¼ ΔH0 ꢀ TΔS0
ð4Þ
As shown in Table 2, this recalculated ΔG0 value is
generally the same as it is calculated using Eq. 2. The
feasibility of lactim formation is established from the plot
of ΔG0 vs T depicted in Fig. 2c where ΔG0 value increases
with increase of temperature.
Fluorescence lifetime
Fluorescence decay profiles for FP2HP have been measured
in different solvents (Fig. 3, Table 3). FP2HP molecule
undergoes biexponential decay due to the presence of
lactim-lactam tautomers. The average fluorescence lifetime
of FP2HP monitored at ~340 nm in MCH and ACN are
Table 3 Fluorescence lifetime data of FP2HP in non-polar, polar aprotic and polar protic solvents at room temperature
Solvents
λmon (nm).
A1
τ1 (ns)
A2
τ2 (ns)
τavg. (ns)
χ2
MCH
MCH
ACN
ACN
H2O
335
387
340
410
410
0.537
0.976
0.569
0.850
0.960
2.71
0.74
1.58
0.45
0.76
0.463
0.024
0.431
0.150
0.039
7.85
4.56
6.34
2.80
2.81
5.09
0.83
3.63
0.80
0.84
1.01
1.20
1.04
1.06
1.02

J Fluoresc (2011) 21:95–104
101
5.09 ns and 3.63 ns, respectively, while that for FP2PY are
0.83 ns, 0.80 ns and 0.84 ns in MCH, ACN and water,
respectively. From the lifetime data analysis, it is clear that
decay of proton transferred excited state, i.e. decay of
FP2PY is faster than the FP2HP tautomer. On the other
hand, decay time of FP2PY form is not so much affected on
the nature of the solvents. Unfortunately a direct comparison
of the lifetime of FP2HP with 2HP was prohibited due to
extremely fast decay of 2HP falling beyond the measuring
limit of the instrument.
2.30
2.25
2.20
2.15
2.10
105
100
95
90
85
80
75
Quantum chemical results
0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
The global minimum structure of FP2HP has been obtained
by ground state optimization of different low energy
computed structures (Fig. 5) at Hartree Fock (HF) level
and Density Functional Theory (DFT) level using B3LYP
functional and 6–31G** basis set. Considering all possible
structures of FP2HP, the lactim form (FP2HP) is most
stable than any of the other forms in the gas phase.
Intramolecular hydrogen bonded lactam form (FP2PY) is
also more stable than open form. As the energy difference
between the lactim and lactam form is sufficiently small
(~0.267 kcal/mol at DFT level), these two forms may exist
in the ground state. In the excited state the FP2PY form is
stabilized by ~12.343 kcal/mol (DFT level) than FP2HP
form as depicted in Fig. 4b. The optimized parameters for
the lactim and lactam tautomers in the ground state at DFT
level and HF level are presented in Table 4. Calculation
predicts possible hydrogen bond between the two electro-
negative centers in both the lactim and lactam form. The
DFT calculations using B3LYP functional at 6–31G** basis
set were performed with the variation of torsional angle in
order to calculate the strength of intramolecular hydrogen
bonding estimated by the difference in energy between the
closed conformer (FP2HP-from) and the open conformer.
The calculated intramolecular hydrogen bond energy
(EIMHB) at DFT level for the closed conformer is
determined to be 5.52 kcal/mol. Intramolecular strained
four member hydrogen bonded ring may be cause of weak
hydrogen bond energy. Interestingly the calculated dipole
moment for the FP2HP is very low, but the corresponding
dipole moment for the FP2PY is high (Table 4). Therefore,
solvent polarity induced stabilization of the lactam form is
expected to be high. That may be the cause of solvent
polarity dependent variation of absorption band for the
lactam form but not for the lactim form.
R
O-H (Å)
b
135
120
105
25.52kcal/mol
106.8kcal/mol
S1
30
20
10
0
93.97kcal/mol
35.84kcal/mol
S0
0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
RO-H (Å)
c
140
120
100
18.1kcal/mol
119kcal/mol
S1
20
0
S0
33.3kcal/mol
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
RO-H (Å)
Fig. 4 a Variation of Od...Na bond distance
and Od-H1-Na bond
angle (-●-) with RO-H distance for FP2HP. Calculated potential energy E
(kcal/mol) vs OH distance for the ground (S0) as well as first excited
singlet state (S1) in the gas phase at DFT level using B3LYP functional
and 6–31G** basis set (performing full geometry optimization at every
step) for (b) FP2HP and (c) 2HP
Theoretical study on the lactim-lactam tautomerism of
FP2HP has been performed in a series of solvents with
increasing dielectric constant (ε). We have adopted five
solvents including CH (ε=2.015), CCl4 (ε=2.228), THF (ε=
7.58), ACN (ε=37.5) and DMSO (ε=46.7) and carried out
PCM calculations at HF level using 6–31G** basis set to

102
J Fluoresc (2011) 21:95–104
Table 4 Structural parameters of FP2HP in the ground state at HF level
using 6–31G** basis set and DFT level using B3LYP functional and 6–
31G** basis set
F
F
Parameters
HF level
DFT level
ESIPT
FP2HP-
form
FP2PY-
form
FP2HP-
form
FP2PY-
form
N
N
H
Od–H1 (Å)
0.946
1.334
1.307
2.258
2.263
78.245
0.605
2.417
1.203
1.380
0.996
2.247
68.237
3.022
0.971
1.352
1.331
2.245
2.293
80.467
0.505
2.445
1.226
1.412
1.013
2.289
69.126
2.828
O
H
O
C2 - Od (Å)
C2 - Na (Å)
Na - H1 (Å)
Od - Na (Å)
∠ Od – H1 – Na (0)
μ (D)
λem=338nm λem=410nm
hν
hν
F
F
F
obtain the geometries and its effect on lactim-lactam
equilibrium. On going from gas phase to polar aprotic solvent
with increasing dielectric constant of the medium, energy
difference between the lactam and lactim tautomer increases
favoring the lactam formation which is consistent with the
larger dipole moment (μ) of the lactam tautomer than the
lactim one as presented in Table 5. The calculated dipole
moment in polar aprotic solvents is larger than that in the gas
phase. It is noticeable that in polar aprotic solvent, the
enhancement of dipole moment for the lactam tautomer is more
than that of lactim tautomer. This increase in μ can be explained
by the structural change of lactam and lactim form. As shown in
Table 5, C2-Od bond length in lactam form is lengthened and
C2-Na bond distance is contracted thereby increasing the
dipole moment from gas to solvent with high ε value. The
Na...H1 bond length for the lactim form is also increased
from gas to DMSO by about 0.042Å due to weakening of
the intramolecular H-bond and favoring solute-solvent H-
bonding. The Od-H1 bond distance and Od-C2-Na bending
angle of lactim tautomer are slightly increased (0.015Å and
N
N
N
a
2
H
H
O
O
H
O
1
d
Open form
FP2PY-form
abs=310nm
FP2HP-form
abs=270nm
λ
λ
Fig. 5 Possible structures of FP2HP in the ground as well as excited
state and their excitation and emission scheme
0.610) in polar solvents which enhances the solute dipole
moment. These calculations clearly indicate how the solvent
polarity affect the relative stability of two tautomers and the
results correlate well with the experimental observation.
From the structural calculation (Table 4) it is found that
both the lactim and lactam form may form four-member
intramolecular hydrogen bonded ring at the proton translo-
cation site. It is expected that the strained four member ring
Table 5 Optimized geometries of lactim (FP2HP) and lactam (FP2PY) tautomers in different solvents calculated at HF/6–31G** level using
PCM model
Parameters
Solvents
Gas (ε=1.0)
CH (ε=2.015)
CCl4 (ε=2.228)
THF (ε=7.58)
ACN (ε=37.5)
DMSO (ε=46.7)
ΔEa (kcal/mol)
−1.898
0.605
−0.017
0.7445
3.517
0.207
0.762
2.099
0.912
2.904
0.980
2.947
0.985
μ (FP2HP) (D)
μ (FP2PY) (D)
3.022
3.577
4.098
4.330
4.352
C2 - Od (FP2PY) (Å)
C2 - Na (FP2PY) (Å)
Na …H1 (FP2HP) (Å)
Od –H1 (FP2HP) (Å)
∠Od-C2-Na (FP2HP) (0)
1.203
1.209
1.210
1.217
1.221
1.221
1.380
1.375
1.374
1.368
1.366
1.366
2.258
2.275
2.276
2.292
2.299
2.300
0.946
0.951
0.952
0.958
0.961
0.961
117.983
118.220
118.251
118.480
118.586
118.596
a ΔE = (EFP2PY-EFP2HP
)

J Fluoresc (2011) 21:95–104
103
may relax during the course of proton transfer process, i.e.,
proton transfer reaction is multi-dimensional in nature [36,
37]. Interestingly the plot of the variation of Od – H1
distances with Od...Na distances and Od-H1-Na angles shows
a systematic change during proton transfer process (Fig. 4a).
As the PT process proceeds, the Od...Na distance decreases
and attains a minima at 1.47Å, then relaxes back to its
lactam form. Simultaneously, Od-H1-Na angle increases to a
maximum at 1.42Å with increase of RO-H distance and then
returns with the formation of lactam form. This clearly
indicates that proton transfer process is multi-dimensional in
nature [36, 37] and change in Od – H1 distance is coupled
with two other structural parameters, i.e. Od...Na distance and
Od-H1-Na angle. Figure 4b and c show the potential energy
versus RO-H distance for FP2HP and 2HP in the relaxed
calculation where we have performed geometry optimization
at each stage of OH bond length. In both cases the energy of
S0 and S1 states increases with RO-H distance and both the
states exhibit double minima potential. In case of FP2HP,
with respect to the global minimum lactim form the
tautomerization reaction in the ground state has a barrier of
35.84 kcal/mol (Fig. 4b). It is worth to point out here that, in
the gas phase, proton transfer process for transformation
from 2HP to 2PY in the ground state does not proceed at room
temperature and the calculated barrier height at the same level
of theory is 33.3 kcal/mol (Fig. 4c). Comparing the calculated
barrier of FP2HP with 2HP we can easily say that proton
transfer in FP2HP does not occur in the ground state but both
the forms exist because of their very small energy difference.
In the first excited state, the energy barrier for FP2HP* to
FP2PY* transformation along the proton transfer coordinate
is 25.5 kcal/mol and this tautomerization reaction is inferred to
be exothermic one. Very similar results are obtained in case of
2HP. The calculated barrier to the tautomerization of 2HP in
the S1 pathway is 18.1 kcal/mol and is lower than in the
ground state. Therefore, intramolecular proton transfer may
be expected in the excited state for both the molecules in the
S1 surface through the 4-member intramolecular H-bond. The
calculated S0–S1 transition energy for the lactim and lactam
form obtained from the PES (Fig. 4b and c) are found to be
106.80 kcal/mol and 93.97 kcal/mol, respectively, and are
consistent with the observed absorption band for lactim and
lactam forms at 268 nm (106.68 kcal/mol) and 312 nm
(91.64 kcal/mol), respectively. The lower absorption intensity
of the lactam from can be established from the calculated low
oscillator strength for FP2PY form (f=0.0395) than FP2HP
form (f=0.0597) (Fig. 5).
and time resolved spectroscopy and quantum chemical
calculations. The polarity and hydrogen bonding ability of
the solvents have prominent influence on the stability of
lactam form rather than the lactim form in the ground state
and their subsequent spectral properties. Spectral signature
predicts the evidence of PT reaction but not of PET
reaction. Thermodynamics parameters such as Ktau0, ΔG0,
ΔH0 and ΔS0 for keto-enol equilibrium obtained by
temperature variation study support the weakening of
four-member intramolecular H-bond with increase of
temperature thereby favoring keto to enol formation.
Quantum chemical calculations at Hartree Fock and
Density Functional Theory levels predict the existence of
lactim and lactam tautomers due to comparable stability in the
ground state. Calculations predict that with increasing polarity
of the solvents, the equilibrium shifted towards lactam tautomer
in the ground state and exhibiting more influential effect of the
polarity of the solvent on the lactam tautomer. The potential
energy curve shows the feasibility of proton transfer in the
excited state leading to keto-lactim tautomerization due to
lowering the barrier energy than in the ground state. The
spectral properties obtained from experimental and theoretical
studies of FP2HP are found to be very similar to that of its parent
molecule 2-hydroxypyridine and the intramolecular H-bonding
is weaker in FP2HP than 2-hydroxypyridine which may be the
cause of higher PT barrier in FP2HP than 2-hydroxypyridine.
Acknowledgement NG acknowledges DST, India (Project No.
Project No. SR/S1/PC/26/2008) and CSIR, India (Project no. 01
(2161)07/EMR-II) for financial support and AS and BKP thanks
CSIR, New Delhi, for Senior Research Fellowship. The authors are
thankful to Dr. Nitin Chattopadhyay of Jadavpur University, Kolkata,
for allowing them to use fluorescence lifetime measurement set-up.
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