Proton transfer reaction of 4-methyl-2,6-diamidophenol in alcoholic solvents at room temperature and 77 K

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

www.elsevier.nl/locate/theochem

* 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


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


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.


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,


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.


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


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


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


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.

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Documents

Proton transfer reaction of 4-methyl-2,6-diamidophenol in alcoholic solvents at room temperature and 77 K