Proton transfer reaction of 4-methyl-2,6-diamidophenol inalcoholic solvents at room temperature and 77 K
D. Guhaa, A. Mondala, D. Natha, S. Mitrab, N. Chattopadhyayc, S. Mukherjeea,*
aDepartment of Physical Chemistry, Indian Association for the Cultivation of Science, 2A and B Raja S.C. Mullick Road, Jadavpur,
Calcutta 700 032, IndiabLaboratory for Molecular Dynamics and Spectroscopy, Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F,
BE-3001 Heverlee, BelgiumcDepartment of Chemistry, Jadavpur University, Calcutta 700 032, India
Received 12 June 2000; accepted 22 September 2000
Abstract
Ground and excited state proton transfer reaction of 4-methyl-2,6-diamidophenol (MDOH) has been studied in alcoholic
solvents, using steady state and nanosecond spectroscopy, at room temperature (RT) and 77 K both in presence and absence of
triethylamine (TEA). Solute±solvent interaction appears to play a major role in determining the nature of the absorbing and
¯uorescing species. The emission properties of MDOH have been examined in relation to those of 4-methyl-2,6-diformylphenol
(MFOH). At 77 K the emission due to the open conformer is markedly suppressed and consists of phosphorescence both in
presence and absence of TEA. The ¯uorescence decay rates are relatively slow in alcoholic solvents compared to those in non-
polar solvents and non-radiative decay rates are always found to be dominant over the radiative rates. Generation of the
trajectories for the intramolecular proton transfer reaction in the ground (S0) and the lowest excited singlet (S1) states using
the semiempirical AM1-SCI method demonstrates that the intramolecular proton transfer process is favored in the S1 state both
thermodynamically and kinetically. The activation energy for the excited state intramolecular proton transfer (ESIPT) process
has been calculated in various alcoholic solvents differing in polarity assuming Onsager's dielectric continuum model. q 2001
Elsevier Science B.V. All rights reserved.
Keywords: Proton transfer reaction; 4-Methyl-2,6-diamidophenol; Alcoholic solvents
1. Introduction
Much attention has been directed, from both theo-
retical and experimental points of view, to the ground
and excited state proton transfer (ESPT) of hydrogen
bonded molecules [1±11]. Two types of proton trans-
fer reactions have been characterized in literature, viz.
excited state intermolecular proton transfer (ESPT)
and excited state intramolecular proton transfer
(ESIPT), depending upon the structure of the probe
molecule and the nature of the environment [12±16].
In the case of ESIPT the extent of changes in the
geometry is very small and is con®ned to the immedi-
ate vicinity of the reaction site within the molecule. In
the other case, electronic excitation of the species
causes rapid charge distribution making the proton-
donor group present in the molecule more acidic in the
excited state compared to the ground state. The
Journal of Molecular Structure (Theochem) 542 (2001) 33±45
0166-1280/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S0166-1280(00)00816-2
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* Corresponding author. Tel.: 191-33-473-4971;
fax: 191-33-473-2805.
E-mail address: [email protected] (S. Mukherjee).
increased acidity in the excited state results in the
release of a proton to an external base thus giving
rise to the formation of an anion.
ESIPT plays an important role in chemistry and has
practical applications. A molecule undergoing ESIPT
can act as a potential candidate for the production of
optical switches [17] and memories [18]. Excited state
proton transfer are important in many chemical and
biological processes ranging from UV photochemical
reactions in plants to tautomeric interconversion of
nucleic acid bases [19,20]. The molecules showing
ESPT are also used as laser dyes [21] and polymer
stabilizers [22].
Understanding of the mechanism of proton transfer
in a molecule can lead to important insights into the
behavior of complex systems. The proton transfer
reaction of intramolecularly hydrogen bonded
molecules is in¯uenced remarkably by the kind of
D. Guha et al. / Journal of Molecular Structure (Theochem) 542 (2001) 33±4534
Fig. 1. A schematic diagram for the dynamic processes of the major species of MDOH in normal and ¯uoroalcohols, in the ground state, and at
77 K.
substitution on the aromatic ring or side groups as
well as the nature of the environment [1±6].
Solvent effects on proton transfer to solvent raise
interesting problems concerning static and dynamic
processes. Several spectroscopic studies have
shown that the compounds like o-hydroxybenzalde-
hyde (OHBA), methylsalicylate (MS) and related
molecules are hydrogen bonded to the solvent
when the solvent is a proton acceptor [7,8].
OHBA and 4-methyl-2,6-diformyl phenol (MFOH)
are simple prototypical examples of aromatic
molecules with an intramolecular hydrogen bond
involving a carbonyl group. Nagaoka et al. [7]
have studied the structures and dynamic processes
of the excited states of OHBA and its derivatives
in different environments. From the dependence of
¯uorescence decay rate constant of OHBA on the
dielectric constant of the solvent, they suggest that
large Stokes shifted ¯uorescence is due to ESIPT
and formation of enol tautomer. It is also proposed
that out-of-plane bending motion involving the
carbonyl carbon is important in the non-radiative
decay processes of such compounds. OHBA
shows a dual ¯uorescence in ethanol and methanol
as reported by them. They have shown that OHBA
predominantly exists in the form of intermolecu-
larly hydrogen-bonded open conformer in alcohols
at room temperature. Smith and Kaufmann have
shown that the rate of ESIPT in MS is faster
than 1011 s21 even at low temperature solid matrix.
They proposed formation of zwitterionic structure
due to ESIPT and the process is strongly tempera-
ture dependent. They observed dual ¯uorescence
for zwitterion and normal closed conformer and
concluded that the two excited state forms of MS
are not in equilibrium [23]. Pimentel et al. [11]
have determined the structure of the non-hydrogen
bonded photorotamer of OHBA by infrared spectro-
scopy. The S1 state of the closed form of OHBA
has been identi®ed as an n, pp hydrogen atom
transfer state and the S2 state is assigned to the
p,pp proton-transfer state with a very strong intra-
molecular hydrogen bond.
In some preceding papers [9,10] we have discussed
the structures of the prototropic species and the
dynamics of the prototropic process of the ground
and the excited states of MFOH and 4-methyl-2,6-
diacetylphenol (MAOH) (Fig. 1) in various solvents.
The following salient features have been obtained in
the alcoholic solvents:
1. In alcoholic solvents, two absorbing species, the
intramolecularly hydrogen bonded closed confor-
mer and the phenolate anion are detected. In the
excited state, however, two ¯uorescing species, the
anion and the intermolecularly hydrogen bonded
open conformer have been identi®ed.
2. Solvent properties are the predominant factor in
controlling the rate and the extent of the prototropic
reaction for both MFOH and MAOH.
3. The ¯uorescence decay rates are relatively slow in
alcoholic solvents compared to those in non-polar
solvents.
4. At 77 K, the ¯uorescence is replaced by phosphor-
escence after irradiation in the case of MFOH,
whereas MAOH does not show any phosphores-
cence. The non-radiative rates are always dominant
over the proton transfer processes and are found to
decrease by lowering the temperature.
It has been proposed that the occurrence of
phosphorescence in the case of MFOH is from the
open conformer associated with a rotation of the
formyl group. It has been suggested that due to
the presence of bulky alkyl group, attached to the
carbonyl group, the acetyl groups are unable to rotate
in the case of MAOH and do not show any phosphor-
escence [24]. Nagaoka et al. showed that this replace-
ment of ¯uorescence by phosphorescence does not
take place in the case of o-hydroxyacetophenone
(OHAP) and o-hydroxypropiophenone (OHPP) [7]
in non-polar solvents but it occurs in the case of
OHBA [7]. However, it has also been observed that
OHAP and OHPP show phosphorescence in polar
protic solvents, although this process is very slow
compared to that of OHBA. We are unable to detect
the phosphorescence of MAOH even in alcoholic
solvents [10]. In the present paper, we have extended
our studies to 4-methyl-2,6-diamidophenol (MDOH)
in different alcoholic solvents. To get complementary
evidence spectroscopic measurements have also been
made in a non-polar solvent, n-heptane.
Although ab-initio calculations involving extended
basis sets with extensive con®guration interactions
(CI) have been successful in explaining structures,
energetics and reactivities of small molecules in
D. Guha et al. / Journal of Molecular Structure (Theochem) 542 (2001) 33±45 35
different electronic states, such reports on the proton
transfer process are still limited in number principally
because of the large size of the molecular systems.
However, semiempirical molecular orbital methods
have established their wide utilities in this respect.
The methods provide acceptable approximations to
give results which are quite close to the experimental
®ndings [14,25±27]. From the fact that AM1 method
describes the energetics and topographies of the
H-bonded systems fairly accurately [28], we have
endeavored to apply the calculation on the prototropic
reaction for MDOH. In this paper, we have looked at
the ESIPT reaction of the ¯uorophore through the
semiempirical AM1-SCI model, via the generation
of the potential energy surfaces (PES) on which the
proton transfer process is supposed to take place.
2. Experimental
Similar to MFOH and MAOH, MDOH was
prepared following Adison's technique [29]. The
compounds were recrystallized from alcohol and
dried before use. Ethanol, methanol, propanol, isopro-
panol, 1-butanol, 1-pentanol, 1-hexanol, tri¯uoroetha-
nol, penta¯uoro-1-propanol, hepta¯uoro-1-butanol
and n-heptane were of spectroscopic grade (E.
Merck) and were all dried and distilled before use.
The solvents were checked for purity by both steady
state ¯uorescence and nanosecond apparatus. Triethy-
lamine (TEA) was procured from E. Merck, Germany
and was used as received.
Absorption and ¯uorescence spectra were recorded
in JASCO 7850 spectrophotometer and Perkin-Elmer
MPF 44B spectro¯uorimeter, respectively. The
relative quantum yields were measured from the
area under the emission curves as described earlier
[24,30,31] taking MFOH, in ethanol, as standard
�ff � 0:46�: The low temperature emission spectra
were recorded on a Hitachi spectro¯uorimeter
F4500. Phosphorescence lifetimes were measured
from the decay of the phosphorescence with time.
The ¯uorescence lifetimes, at room temperature,
were measured in a SP-70 nanosecond spectrometer
(Applied PhotoPhysics Ltd, UK) adopting the time-
correlated single-photon counting (TCSPC) technique
and using a pulsed nitrogen lamp. The system has a
pump response of about 2 ns and is free of RF modu-
lation. The linearity of the TAC has also been tested.
All other experimental details are same as earlier [30].
The concentration of the sample was maintained at
,5 £ 1025 mol dm23. For the theoretical calculations
D. Guha et al. / Journal of Molecular Structure (Theochem) 542 (2001) 33±4536
Fig. 2. Absorption spectra of MDOH in pure ethanol and in presence of TEA. Range of [TEA] �0 2 15� � 0 to 6:5 £ 1025 mol dm23 TEA.
�MDOH� � 7:5 £ 1025 mol dm23:
we have used the commercial software package
`Hyperchem-5.0' obtained from Hypercube Inc.,
Canada.
3. Results and discussion
3.1. Absorption, emission and excitation spectra
3.1.1. In normal alcohols
The absorption spectra of MDOH show three bands
in all the pure alcoholic solvents studied here; one
stronger band at 325 nm, one relatively weak band
at 360 nm and another shoulder at 420 nm region.
By the addition of a strong base like TEA, the
intensity of the 360 nm band increases at the expense
of 325 and 420 nm bands (Fig. 2). A single band with
a maximum at 360 nm appeared at the highest concen-
tration of TEA (,1022 mol dm23) used here. It is
pertinent to mention here that the absorption spectra
of MDOH in n-heptane appeared at 325 nm. Accord-
ingly, the 325 nm band can safely be assigned to the
intramolecularly hydrogen bonded closed conformer
of MDOH as is suggested in the case of MFOH [9]
(Fig. 1). It is also noted that MDOH shows a single
broad absorption band at 360 nm in aqueous alkaline
medium. Hence, 360 nm absorption band, in the
alcoholic solution, is due to the anion of MDOH
(i.e. MDO2). Since a red shift is expected due to the
formation of solvent separated ion pair, 420 nm band
may be due to this conformer [32].
The emission spectra of MDOH show a single band
at 465 nm when excited by 330 nm light in all the
alcoholic solvents studied here. This large Stokes
shifted (,9000 cm21) emission band can be assigned
as due to the ESIPT and formation of enol tautomer
(Fig. 3). The point to mention here is that we observed
emission spectra of MDOH in n-heptane at 470 nm
and this band is assigned as due to ESIPT. However,
this emission spectra (465 nm) is clearly excitation
wavelength (l exc) dependent. By increasing the l exc
the 465 nm band shifted to 440±450 nm with forma-
tion of a broad band (Fig. 3). By the addition of a base
like TEA the intensity of 440±450 nm band is seen to
increase gradually and this band is found to be
independent of any l exc. The excitation spectrum of
the 465 nm emission gave a maximum at 330 nm,
where as the excitation spectrum of 440±450 nm
D. Guha et al. / Journal of Molecular Structure (Theochem) 542 (2001) 33±45 37
Fig. 3. Emission (1±6) and excitation (7,8) spectra of MDOH in glycerol in: (A) presence of TEA (,1022 mol dm23); and (B) absence of TEA
at different excitation (l exc) and monitoring emission (lmon) wavelengths:(1±6), lexc � 330 2 380 nm; 7, lmon at 430 nm for A and B; 8, lmon
at 465 nm for B only.
emission shows another weak band at 370 nm in
addition to the 330 nm band (Fig. 3). The excitation
spectrum of 440±450 nm emission, at the maximum
concentration of TEA, shows a single band at 370 nm.
The 330 and 370 nm excitation bands reasonably
agree with the absorption spectra for the closed
conformer and anion, respectively. These observa-
tions indicate the presence of more than one species
in the ground and excited states. The ESIPT and
formation of the enolic tautomer of MDOH is thus
evidenced by a large Stokes shifted (,9000 cm21)
¯uorescence, at a selected excitation energy which
originates directly from the ground state closed
conformer. It may be noted that in case of MFOH
we observed only anionic species in the excited
state [9] and we were unable to detect ESIPT in protic
solvents.
Mention may be made here that the emission
maximum of MDOH in aqueous alkaline medium
occurs at 430 nm. Thus, in the case of MDOH, both
the tautomer and anion are likely to be present, in the
excited state, in equilibrium in pure alcoholic
solvents, where the tautomer or the anion formation
is incomplete. This shows that MDOH can undergo
ESIPT reaction yielding the characteristic large
Stokes shifted ¯uorescence of the phototautomer
even in alcoholic solvents by exciting with low energy
light. In the presence of TEA, anion formation is
complete, resulting in a single emission.
3.1.2. In ¯uoroalcohols
Unlike in normal alcohols, the absorption spectra of
MDOH in 2,2,2-tri¯uoroethanol (TFE), 2,2,3,3,3-
penta¯uoro-1-propanol (PFP) and 2,2,3,3,4,4,4-hepta-
¯uoro-1-butanol (HFB) give a band at 325 nm
(corresponding to the closed conformer) with a
shoulder at 420 nm region (Fig. 4). The 420 nm
absorption band may be due to the hydrogen bonded
ion pair or solvent-separated ion pair, as mentioned
above The emission spectra showed a single band
appearing at 450 nm. It is interesting to note that,
unlike normal alcohol, the intensity of the ¯uores-
cence band decreases gradually with an increase in
the excitation energy without any change in position
of the band. The observed static ¯uorescence quench-
ing is due to the decrease in population of the species
present in the excited state. By the addition of TEA
the emission spectrum is shifted to 435 nm and the
position of this band found independent of any excita-
tion energy. The 450 nm band is assigned to the
intermolecularly hydrogen bonded open conformer
which is formed due to the solute±solvent interaction.
D. Guha et al. / Journal of Molecular Structure (Theochem) 542 (2001) 33±4538
Fig. 4. Absorption (a), emission (b) and excitation (c) spectra of MDOH in TFE in presence (broken lines) and absence (solid lines) of TEA at
different excitation (l exc) and monitoring emission (lmon) wavelengths: (a) in absence (1) and presence (2) of TEA (,1022 mol dm23); (b)
lexc; nm � 330 to 380 (1±6) and 330 to 360 (7±10); and (c) lmon; nm � 410 to 460 (1±6) and 420 to 460 (7±11).
The excitation spectrum of the open conformer agrees
well with the absorption spectrum with a 330 nm band
at all monitoring wavelengths examined here (Fig. 4).
This indicates that the open conformer originates from
the ground state closed conformer. In the presence of
TEA, emission and excitation spectra appeared at 435
and 360 nm, respectively.
All these observations indicate that intermolecular
solute±solvent interaction is stronger in the ¯uoro-
alcohol environments. However, anion formation is
not possible in pure ¯uoroalcohols. A promoter base
is necessary for the complete transfer of proton in both
normal and ¯uoroalcohol media.
3.2. Emission and excitation spectra of MDOH at
77 K
Like MFOH [9], the spectra of MDOH consist of
phosphorescence in all the alcoholic solvents studied
here (Fig. 5). The phosphorescence peak appears at
,468 nm. Contrary to the room temperature ¯uores-
cence, an increase in l exc results in an increase in the
low temperature emission intensity, keeping the peak
position unchanged (Fig. 5). The presence of phos-
phorescence indicates the presence of intersystem
crossing (ISC) during the ESPT process of MDOH.
This is characteristic of a 3ppp of aromatic carbonyl
of benzaldehyde type of molecules in alcoholic
solvents. According to the suggestion of Nagaoka et
al. [7], both the open conformer IIA and IIB can give
rise to 3ppp type of phosphorescence in these
alcoholic solvents (Fig. 1). However, this conversion
of ¯uorescence into phosphorescence is slow in the
case of MDOH compared to that of MFOH that is
revealed from the larger lifetime values �t 0f� of
MDOH (6.8 ns) than that of MFOH (5.9 ns). The
slower conversion of MDOH re¯ects that the rotation
of the formyl group is perturbed by the larger size of
the amido group of MDOH compared to that of the
hydrogen atom attached to the carbonyl groups in
MFOH (Fig. 1). This seems to indicate that the
rotation of the formyl group is involved in the
occurrence of phosphorescence [7]. It is also observed
that the open conformer emission �lexc � 330 nm� is
markedly suppressed.
We have examined the emission spectra of acetyl
derivative, MAOH in alcoholic solvents. In this case,
we were unable to detect any phosphorescence in pure
solvents. However, in the presence of a strong base
like TEA, we observed phosphorescence spectra even
in the case of MAOH in alcohols. This phenomenon is
relatively slower compared to those of MDOH and
MFOH. Replacement of the hydrogen atom of the
formyl group by the bulky ±NH2 or ±CH3 groups
slows down the rotation of the carbonyl group,
thereby, increasing the rate of intersystem crossing.
These observations indicate that intramolecular
hydrogen bonding is stronger in MAOH than in the
case of MFOH and rupture of the intramolecular bond
is necessary for the rotation of the carbonyl group and
appearance of phosphorescence spectra. In the case of
MAOH, a promoter base is necessary to rupture the
intramolecular bond. The phosphorescence excitation
spectrum agrees well with the absorption spectrum,
D. Guha et al. / Journal of Molecular Structure (Theochem) 542 (2001) 33±45 39
Fig. 5. Fluorescence (a,1±4), phosphorescence (b) �lexc � 330 nm� and phosphorescence excitation (c) spectra of MDOH in ethanol at 77 K:
shutter control corrected spectra; l exc, nm: 330 (1), 340 (2), 350 (3) and 360 (4). Phosphorescence measured with chopper.
D. Guha et al. / Journal of Molecular Structure (Theochem) 542 (2001) 33±4540
Fig. 6. Typical decay pro®le of 450 nm emission of MDOH in ethanol at 298 K. lexc � 334 nm; resolution� 0.089 ns/channel; and reduced
chi-square �x2R� � 1:1:
Table 1
Quantum yield (f f), ¯uorescence lifetime (t f), phosphorescence lifetime (tp), radiative �krf � and non-radiative �knr
f � decay rates of MDOH in
different solvents (Values in parenthesis indicate t in presence of TEA.)
Solvent ffa t f
b (ns) t p (s) krf £ 1027 (s21) knr
f £ 1027 (s21)
MeOH 0.63 5.8 (5.0) 0.83 (0.66) 10.8 6.5
Ethanol 0.65 4.9 (4.7) 0.77 (0.60) 13.4 7.2
Isopropanol 0.43 6.0 (5.2) ± 7.2 9.4
Propanol 0.43 5.8 (5.2) ± 8.1 10.6
Butanol 0.45 5.1 (4.8) ± 8.9 10.7
Pentanol 0.49 4.9 (4.6) ± 10.0 10.4
Hexanol 0.46 5.7 (5.2) ± 8.1 9.5
TFE 0.41 5.6 (5.3) 0.69 (0.43) 7.3 10.5
n-heptane 0.11 3.2 (3.0) 0.54 (0.40) 3.4 27.9
a Estimated for the highest energy band.b Monitoring wavelength is 470 nm in n-heptane and 450 nm in alcoholic solvents.
indicating that the main species present is the intra-
molecularly hydrogen bonded closed conformer
before irradiation which is converted to the open
conformer after irradiation.
3.3. Fluorescence decay
We have examined the ¯uorescence decay of
MDOH by time correlated single photon counting
(TCSPC) technique using nanosecond ¯ashlamp. A
typical decay pro®le for MDOH in ethanol is given
in Fig. 6. The ¯uorescence decay can be well analyzed
by a single exponential ®t in all the alcoholic solvents
by monitoring at a particular wavelength. The plot of
the weighted residuals corresponding to a single
exponential ®t is also shown. In the presence of
TEA, the ¯uorescence can be ®tted to a single
exponential decay, indicating that only anion exists
in the excited state in the presence of the base. The
¯uorescence decay rate constants (kf) are given by the
sum of the radiative �krf� and the non-radiative �knr
f �decay rates. From the nanosecond measurements and
quantum yields of ¯uorescence we have estimated the
decay rates of the ¯uorescing species, using the
following equations:
1
tf
� krf 1 knr
f ; krf � f
tf
�1�
The values of the rate constants are depicted in Table
1. It can be seen from Table 1 that the rate constants
are lower in magnitude from those of MFOH [9]. The
low values of the rate constants clearly indicate that
both the radiative and non-radiative processes in
MDOH are much slower than in MFOH. This slow
process points to the fact that the excited state proton-
transfer process of MDOH is not essentially barrier
free. Compared to MFOH, the ¯uorescence quantum
yields (f f) are relatively higher in MDOH. Unlike
MFOH, where �knrf � values are always higher than
�krf�for a particular alcoholic solvent [9], in MDOH
D. Guha et al. / Journal of Molecular Structure (Theochem) 542 (2001) 33±45 41
Table 2
Comparison of AM1 optimized geometries of MDOH and MFOH in the S0 electronic state
O1±H14 0.973 0.974
O1±C2 1.362 1.363
C2±C7 1.418 1.410
C7±C12 1.486 1.465
C12±O19 1.258 1.237
O19±H14 1.878 2.005
C2±C3 1.420 1.419
C3±C8 1.496 1.473
C8±O13 1.256 1.234
O1±O13 4.163 3.982
Bond lengths are in angstroms
E (kcal/mol) 2 2569.9780 2 2227.3223
m (Debye) 4.34 3.70
the decay rate constants are of nearly the same
magnitude.
3.4. Theoretical work
3.4.1. Calibration of the methodology used
The semiempirical AM1 method is known to
describe energetics and topographics of H-bonded
systems fairly accurately [14]. We have therefore
optimized the ground state geometry of MDOH and
MFOH using the AM1 method. The optimized
parameters are reported in Table 2 for comparison.
Relevant structural data on MFOH is also available
in literature [14]. The presence of electron releasing
NH2 group in MDOH have quite expectedly
increased CyO bond lengths and the increase in
electron density of the carbonyl oxygen atoms
have resulted in a shorter equilibrium O19±H14
distance (by < 0.13 AÊ ) relative to that in MFOH.
The other structural features remain more or less
comparable.
3.4.2. Energetics of proton transfer in S0 and S1 states
of MFOH
The calculated activation barrier is quite high in the
S0 state and therefore the proton transfer rate in this
state is expected to be very low particularly in a
non-interacting medium. This corroborates our
experimental observation that intramolecular proton
transfer does not take place at all in the S0 state of
MDOH [33].
3.4.3. Proton transfer energy and the reaction path
To construct the reaction path representing the
proton transfer in MDOH, the O19±H14 distance (r)
has been chosen as the coordinate. As the proton
translocation distance of the mobile hydrogen atom
is considered to be the key parameter for the construc-
tion of the ESPT potential, the O19±H14 distance is
varied between what is normal for the primary and
what is known to be the equilibrium tautomeric
O19±H14 distance.
The maximum in the S0 state is a true saddle point
on the PES and occurs at a proton transfer distance of
1.13 AÊ . Table 3 shows the exo (endo) thermicities and
activation energies for the tautomerization process of
MDOH in both the states. It turns out that the reaction
is appreciably endothermic in the S0 state and the
activation energy (DEact) for the transfer is also quite
high. Accordingly, proton transfer in the S0 state is
unlikely to occur. On the other hand, in the S1 state,
the proton transfer reaction is predicted to be slightly
exothermic in the polar alcoholic environments. A
relatively small proton transfer barrier in the S1 state
is indicative of a rather shallow well characterizing
the primary form. It is not deep enough to contain an
appreciable number of bound vibrational levels, so
that the potential is effectively of the anharmonic
single well type for all intents and purposes.
The fact that we have observed experimentally only
the tautomeric emission in non-polar solvent, and both
the tautomeric and anionic (or open conformer)
emission from the S1 state can now be explained in
light of the nature of PES obtained theoretically,
which points to a strongly asymmetric double-well
potential in which the proton moves. Since Frank-
Condon excitationfrom the S0 state could result in
vibrational excitation on the S1 surface, we would
like to explore whether such an excitation would
effectively reduce the barrier height operative in the
proton transfer process on the excited S1 surface
further. It appears from our results that the vibrational
excitation would take the system almost over the
barrier and eventually in the potential well represent-
ing the tautomeric form explaining why we have
observed only the tautomer emission from the S1
state in non-polar solvent. This does not essentially
mean that S1 surface has a single potential well in all
the solvents studied here. It may be said therefore that
the proton transfer process on the S1 state may well be
a vibrationally assisted over-barrier process.
For methylsalicylamide, a compound structurally
similar to MDOH, Smith and Kaufmann [1], proposed
D. Guha et al. / Journal of Molecular Structure (Theochem) 542 (2001) 33±4542
Table 3
Energetics of MDOH Activation (DEact) and Tautomerization (DET)
in S0 and S1 states (The dielectric constant values (e ) are at 258C.
The energy values are in kcal/mol)
Solvent e DES0act DE
S0
T DES1act DE
S1
T
Vacuum 0 19.19 12.57 12.70 2.38
n-heptane 1.97 18.32 11.24 11.42 0.88
Ethanol 24.3 14.72 6.87 9.71 21.18
Tri¯uoroethanol 26.7 14.69 6.83 9.70 21.19
Methanol 32.63 14.63 6.75 9.67 21.23
Water 80 14.46 6.54 9.60 21.33
a zwittterionic structure for the ¯uorescing state. From
our results we have shown that both MDOH and
MFOH ¯uorescence does not correlate well with the
solvent dielectric constant [34] negating the possible
existence of zwitterionic structure. The ¯uorescing
state of MDOH can be regarded as less ionic. In
fact, the transfer process seems to be more like a
ªhydrogen transferº rather than a purely proton
transfer process.
The experimental ®ndings reveal that ESIPT
reaction is one of the competing paths for the excited
¯uorophore. We have, therefore, tried to see the
reason why the tautomer is not present in the ground
state but its formation becomes feasible in the S1 state.
The ground state structure of MDOH has been
optimized using the AM1 method and the structural
data, along with those for MFOH, have been
presented in Table 2 for comparison. The ground
state energies and dipole moments are also given in
Table 2.
The S1 state energy has been calculated considering
the vertical transition only; i.e. there is practically no
geometric distortion of the S1 state as compared to that
of the S0 state. However, the electronic redistribution
takes place resulting in a modi®cation in the energy as
well as the dipole moment of the same.
The PES for the intramolecular proton transfer
reaction has been generated through the calculation
of the energies of the con®gurations with varying
O19±H14 distance. While this distance has been
D. Guha et al. / Journal of Molecular Structure (Theochem) 542 (2001) 33±45 43
Fig. 7. Variation of Potential Energy during proton transfer of an MDOH molecule in isolation (A0, A1), in n-heptane (B0, B1) and in ethanol
(C0, C1) in the S0 and S1 states, respectively.
restrained to a particular value for a single con®gura-
tion, all other structural parameters have been totally
optimized. Fig. 7 represents the simulated PES for the
ESIPT process of an isolated MDOH molecule in S0
and S1 states. The ®gure re¯ects that while the ESIPT
process is endothermic in the S0 state, the same is
exothermic in the S1 state. Hence the reaction is
thermodynamically unfavorable in the S0 state but is
favorable in the S1 state. Solvent stabilizations of the
electronic states in alcoholic solvents with different
polarities have been estimated assuming Onsager's
continuum model [35]. The generated PES, corre-
sponding to the solvent stabilized species, in both S0
and S1 states, thus, gives a theoretical estimate of the
activation energy (kinetic parameter for the reaction)
as well as the energy change for the process (thermo-
dynamic parameter for the reaction) in the solution
phase. The basic nature of the PES does not change
remarkably from that in the isolated condition. The
PES for the solvated molecule has been exhibited in
Fig. 7. The activation energies (DEact) and energies of
reaction (DEr) in different alcoholic solvents have
been tabulated in Table 3. From Table 3 it is evident
that in the ground state, the keto form of MDOH is the
most stable form and intramolecular proton transfer is
improbable in this state because of both thermo-
dynamic (DEr is positive, endothermic) and kinetic
(DEact is high) factors. However, consideration of
the same factors suggests that the ESIPT reaction is
feasible in the S1 state in all the alcoholic solvents.
4. Conclusions
Presence of some coexisting species in the ground
state results in the complex nature of the emission of
MDOH in alcoholic solvents. These species may origi-
nate from the corresponding ground state forms, or may
be produced upon electronic excitation. In normal alco-
hols, solute±solvent interaction results in a number of
isomers. The closed and anion conformers appear in the
ground state whereas the emission spectra indicate
presence of anionic species in the excited state. In ¯uor-
oalcohols, the open conformer is observed in absence of
TEA. In all cases the presence of TEA results in only a
single species in solution, the anion. This is quite
contrary to the observation in MFOH where, in normal
alcohols, only anion form exists but in pure TFE both the
open and anion forms were observed. All these facts
point to a more stable intramolecular hydrogen bond
in MDOH, due to presence of NH2 groups, which
make it more dif®cult to rupture. Theoretical calcula-
tions also corroborate this fact. The AM1 calculations
on the S0 and S1 states tend to suggest the following
scenario for the proton transfer process of MDOH.
The process is thermodynamically endothermic in the
ground state and also encounters high activation barrier.
In the S1 state the process becomes exothermic with low
barrier height.
Acknowledgements
The authors thank Prof. S.P. Bhattacharyya of IACS
for his suggestions. The spectroscopy department of
IACS is acknowledged for the low temperature
measurements. N.C. acknowledges the ®nancial
support from CSIR, Govt. of India. D. Guha is a
recipient of a senior research fellowship from CSIR.
A.M. thanks IACS for providing him with a junior
research fellowship.
References
[1] K.K. Smith, K.J. Kaufmann, J. Phys. Chem. 85 (1981) 2895.
[2] A. Grabowska, J. Sepiol, C.J. Rulliene, J. Phys. Chem. 95
(1991) 10 493.
[3] T.P. Smith, K.A. Zaklika, K. Zhaker, G.C. Walker, K.
Tominga, P.F. Barbara, J. Phys. Chem. 95 (1991) 10 465.
[4] M. Weichmann, H. Pont, F. Larner, W. Frey, T. Elssaeser,
Chem. Phys. Lett. 165 (1990) 28.
[5] J. Catalan, J. Palomar, J.L.G. de Paz, J. Phys. Chem., A 101
(1997) 7914.
[6] M.A. Rios, M.C. Rios, J. Phys. Chem., A 102 (1998) 1560.
[7] S. Nagaoka, N. Hirota, K. Yoshihara, E.L. Kochang, H.
Ewamura, J. Am. Chem. Soc. 106 (1984) 6913.
[8] M. Yanagimachi, N. Tamai, H. Masuhara, Chem. Phys. Lett.
201 (1993) 115.
[9] R. Das, S. Mitra, S. Mukherjee, Bull. Chem. Soc. Jpn 66
(1993) 2492.
[10] R. Das, S. Mitra, D.N. Nath, S. Mukherjee, J. Chim. Phys. 93
(1996) 458.
[11] M.A. Morgan, E. Orton, G.C. Pimental, J. Phys. Chem. 94
(1990) 7927.
[12] G.L. Arnaut, J.S. Formosinho, J. Photochem. Photobiol. A:
Chem. 75 (1993) 1.
[13] E.M. Koswer, D. Huppert, Annu. Rev. Phys. Chem. 37 (1986)
127.
[14] S. Mitra, R. Das, S.P. Bhattacharyya, S. Mukherjee, J. Phys.
Chem., A 101 (1997) 293.
D. Guha et al. / Journal of Molecular Structure (Theochem) 542 (2001) 33±4544
[15] S. Mitra, R. Das, S. Mukherjee, Chem. Phys. Lett. 221 (1994)
368.
[16] S. Nagaoka, Y. Shinde, K. Mukai, J. Phys. Chem., A 101
(1997) 3061.
[17] R.C. Haddon, F.H. Stillinger, in: F.L. Carter (Ed.), Molecular
Electronic Devices, Marcel Dekker, New York, 1982 (chap.
II).
[18] H. Sixl, D. Higelin, in: F.L. Carter (Ed.), Molecular Electronic
Devices, Marcel Dekker, New York, 1987 (Part 1±2).
[19] J.S. Kwiatkowski, T.J. Zielinski, R. Rein, Adv. Quantum
Chem. 18 (1986) 85.
[20] M.D. Topall, J.R. Fresco, Nature 263 (1976) 285.
[21] S. Nagaoka, M. Fujita, T. Takemura, H. Baba, Chem. Phys.
Lett. 123 (1986) 489.
[22] T. Werner, G. Woessner, A.E.H. Kramer, in: S.P. Pappas, F.H.
Weinslow (Eds.), Photodegradation and Photostabilization of
Coatings, ACS Symposium Series 151American Chemical
Soceity, Washington, DC, 1981, p. 1.
[23] K.K. Smith, K.J. Kaufmann, J. Phys. Chem. 82 (1978) 2286.
[24] S. Mitra, R. Das, S. Mukherjee, Chem. Phys. Lett. 228 (1994)
393.
[25] T.A. Engeland, T. Bultmann, N.P. Ersting, M.A. Rodriguez,
W. Theil, Chem. Phys. 163 (1992) 43.
[26] B. Dick, J. Phys. Chem. 94 (1990) 5752.
[27] J. Catalan, F. Faberio, M.S. Guijarro, R.M. Claramunt, M.D.
Santa Marxcla, M.C. Foces-Foces, F.H. Cano, J. Elguero, R./
fnm . Sastre, J. Am. Chem. Soc. 112 (1990) 747.
[28] M.J.S. Dewar, E.G. Zoebisch, E.F. Healy, J.J.P. Stewart, J.
Am. Chem. Soc. 107 (1985) 3902.
[29] A.W. Adison, Inorg. Nucl. Chem. Lett. 12 (1976) 899.
[30] S. Mitra, R. Das, S. Mukherjee, Chem. Phys. Lett. 202 (1993)
549.
[31] P.R. Bangal, S. Lahiri, S. Kar, S. Chakravarti, J. Lumin. 69
(1996) 49.
[32] H. Baba, H. Matsayama, H. Kokubun, Spectrochim. Acta 25A
(1969) 1709.
[33] D. Guha, S. Mukherjee, Chem. Phys. Lett. 307 (1999) 177.
[34] S. Mitra, R. Das, S. Mukherjee, Spectrochim. Acta 50A (1994)
1301.
[35] C.J.F. Bottcher, Theory of Electronic Polymerization, 1,
Elsevier, Amsterdam, 1983.
D. Guha et al. / Journal of Molecular Structure (Theochem) 542 (2001) 33±45 45