Indian Journal of Chemistry
Vol. 34A, November 1995, pp. 850-856
Ground and excited state proton transfer of 4-methyl- 2,6-diacetylphenol
in some highly polar aprotic solvents: Interaction with base
Ranjan Das, Sivaprasad Mitra, Debnarayan Nath & Samaresh Mukherjee"
Department of Physical Chemistry, Indian Association for the Cultivation of Science,
Jadavpur, Calcutta 700 032, India
Received 13 February 1995; revised and accepted 6 June 1995
The proton transfer processes of the ground and excited states of 4-methyl-2, 6-diacetylphenol
(MAOH) have been investigated by means of absorption, emission, nanosecond and picosecond
transient emission spectroscopy at different temperatures. The fluorescence quantum yields are
found to be dependent on temperature, excitation energy and added base. Unlike 4-methyl-2, 6-diformylphenol (MFOH), MAOH .shows a single fluorescence band at room temperature in dimethyl
sulphoxide and N,N-dimethylformamide and at 77K it does not exhibit any phosphorescence emission. The fluorescence lifetime and decay. rates of MAOH are temperature dependent and are relatively faster than those of MFOH. The solute-solvent interaction appears to play an important role in
the proton transfer reaction of MAOH.
The proton transfer processes of intramolecularly
hydrogen bonded molecules are topics of current
research activity 1-10. Solvent effects on proton
transfer to solvent raise interesting problems concerning static and dynamic processes. Particularly
intriguing is the question, when the solvent, which
is also the proton acceptor, behaves as a continuous medium or whether its molecular properties
indispensable in understanding
proton-transfer
and proton solvation dynamics. Several spectroscopic studies support the hypothesis that compounds like o-hydroxybenzaldehyde
(OHBA),
methyl salicylate (MS) derivatives and related
compounds are hydrogen bonded to the solvent
in hydrogen bonding solvents":". The proton
transfer of aromatic molecules which are hydrogen bonded in the ground state undergoes
changes markedly depending on the kind of substitution on the aromatic ring or carbonyl group.
It has been noted in the present study that both
the absorption and emission spectra of MAOH
are considerably different from that of MFOH despite the similarity in the molecular structure. Replacement of the hydrogen atoms of the formyl
group in MFOH by methyl groups seems to
change the nature and position of the absorption,
emission spectra and decay rates as wen. In a preceding paper!" we reported dual emission for
MFOH in dimethyl sulphoxide (DMSO) and N,Ndimethylformamide (DMF) at 460 and 530 nm.
In this paper we have examined the spectral properties of MAOH at room temperature and 77K
in relation to that of MFOH. We have also observed blue shift both in the absorption and emission spectra of MAOH compared to that of
MFOH. We have observed reduced lifetimes and
relatively faster decay rates in the case of MAOH.
Materials and Methods
Like MFOH,MAOH
was also prepared in the
laboratory'V". Both the compounds were recrystallized from alcohol and dried before use and
both exhibit a single spot on the TLC plate. Spectroscopic grade dimethyl sulphoxide (DMSO),
N,N-dimethylformamide
(DMF),
acetonitrile
(ACN), carbon tetrachloride (CCI4) (Aldrich) were
freshly distilled before use. Triethylamine (TEA)
was used as received from F1uka. Quantum yields
( tP) of fluorescence were determined by comparison with a standard (quinine sulphate in 0.1 (N)
H2SO4-acid, tPf = 0.54) as described earlier n.lO.
Fluorescence lifetimes were measured by using
time correlated single photon counting (TCSPC)
set up. The sample was excited either with nitrogen flash lamp'? or using a CW mode locked NDYAG laser driven rhodamine 6G dye laser system 18. The emission was monitored at the magic
angle (54.7°) in the latter case to eliminate any
contribution from the decay of anisotropy the
TCSPC system was coupled to. a multichannel
plate photomultiplier (model 2809U, Hamamatsu
Corp.). The half width of the excitation response
function was 280 ps 18. All other experimental details are same as described earlier!". TEA is used
851
DAS et al.: GROUND & EXCITED STATE PROTON TRANSFER OF 4-METHYL-2,6-DIACETYLPHENOL
as base throughout.
4.5 x 10-5 mol dm-3.
[MAOH] is maintained
reactions with TEA. The values of the equilibrium constants (Ko) in the ground state for the
reaction of MAOH with TEA in DMSO and
DMF are 1.9 x 102 dm ' mol- I and 1.1 x 102 dm '
mol-I, respectively, by Ketelaar's method as described earlier!"?", The presence of oxygen lone
pair and electron releasing groups in DM~O and
DMF are responsible for their stronger mteraction capability. In DMSO and DMF the ele~tron
lone pair are much more localised and available
than nitrogen lone pair in ACN. However, we
were unable to evaluate Ko in ACN since no
measurable change in absorption spectra was detected even on the addition of TEA upto a concentration
- 2.0 x 10-2 mol dm-J. This also
shows that intramolecularly
hydrogen-bonded
closed conformertl) of MAOH is the main absorbing species in ACN in the ground state as in
n-hexane and CCI4. On the otherhand, MFOH in
ACN exhibits two absorption bands, at 350 nm
and 440 om respectively.
at
Results and Discussion
Absorption study
Like MFOHI\ MAOH exhibits two absorption
bands, one at 350 om and another relatively weak
band at 450 nm in DMSO and DMF. On the addition of a strong base like TEA the 450 om
band grows up without any change in its position
(Fig. 1). The long wavelength absorption band of
MFOH in DMSO and DMF appeared a 460480 om region in addition to the 350 om band.
Like MFOHI4 the 350 and 450 nm band of
MAOH may be assigned to the intramolecularly
hydrogen-bonded
closed conformer and phenolate anion (I and III in Scheme 1), respectively.
However, only the 350 nm band occurred in
ACN solution of MAOH and does not show any
change even on the addition of TEA ([TEA] =
1.6 x 10 - 2 mol dm - 3). This observation shows
that DMSO and DMF are much stronger proton
accepting solvents than ACN. This is considerably
supported by the equilibrium constants for the
Emission study at room temperature
Unlike MFOH, MAOH shows a single emission
band at 510-520 om region in all the solvents
studied here (Fig. 1). On addition of TEA: the intensity of the 510 nm band is found to increase
without any change in its position in DMF and
DMSO. The excitation spectra of the 510 nm
emission appear at 460 nm in DMSO and DM.F
both in the presence and absence of TEA. ThIS
excitation spectra is indeed similar to the 450 om
D ~F
~\:\
,\1\
I
:
I
,\
I . I
. I \
I
,
\
.~
c
.!'!
c
,,
,
I
,,
I
,,
\
\
0-6
\
\
••
I
v
a
I
\
.tl
\
o
..
O·l.
Ul
.tl
\
<{
0·2
I
I
600
500
I
l.OO
I
300
300
350
",nm
l.OO
450
",nm
J
Fig. I-Absorption
spectra of MAOH in DMSO: [MAOHj = 4.5 x 1{1-5 ~ol.dm-3 for all the cases. [TEAl.x ~0-2. mol dm.- =A
(0.0), B (S.O), C (4.5). Emission spectra (D) of MAOH in DMF and excitation spectra (E) of 520 nm errussion In ACN and In
DMSO (F) at room temperature.
c~'
tilC"
c
1/
o
*
~.
C
0
........CH1
~
'H---s
II
Clos~d
conrormer
Op~n Conformer
(S & Solvent)
111
Anion
852
INDIAN J CHEM. SEe. A, NOVEMBER
1995
absorption band (attributed to the MAO - ion)
and cannot explain the ground state closed conformer. This observation clearly indicates that in
DMSO and DMF, MAO- (III) ion is the fluorescing species as shown in Scheme 2. The anion (III)
fluorescence can result from excitation of III, resulting from ground state intermolecular proton
transfer (GIMPT) and also through a solvent mediated proton transfer in the excited state (excited
state intermolecular proton transfer, EIMPT) as
shown in Scheme 2. The quantum yield (~f) of
fluorescence emission is found to increase sharply
in DMSO and DMF with increase in excitation
wavelength (Am) (Fig. 2a). This increase in ~I
with increase in excitation wavelength is due to
the increase in population of the MAO - ion
TEMPERATURE(K)
50
0"
120
•
""
",
t
" ,#-
(Hi
"T
0
....•
.'
Acac
"
iii
;:
"~
~
::;)
::;)
c:J
l
04
0
II>
~
'" "- ,
"" ~
~ 0.•
<C
EXCITATION
WAVELENGTH
0
(a)
O'OL--_~_---L--_~--""
350
360
370
liD
390
(nm)
TEMPERATURE (K)
50
120
190
260
0·35~--"----r--"---"'-'
\
.\
\
030
\.
(III )
Scheme
2
Scheme 2-A schematic energy-state diagram for the dynamic
processes of the major species of MAOH in dimethyl sulphoxide (DMSO) and N,N-dimethylformamide
(DMF). The
straight and wavy lines represent radiative and nonradiative
processes respectively.
which fluoresces at 510 nm. However, MFOH exhibits dual emission bands at 460 and 530 nm respectively in both DMSO and DMF. The 460 and
530 nm bands were attributed to intermolecularly
hydrogen bonded open conformer and corresponding phenolate anion respectively .
The fluorescence spectra of MAOH shows a
single emission band at 520 nm and it does not
show any measurable change in ACN even on addition of base.· This observation is quite similar to
that in non-polar, non-hydrogen bonding solvents
like CCI4 and n-hexane. The excitation spectra of
the 520 nm emission band for MAOH in ACN is
quite different from that in DMSO and DMF and
agree reasonably well with the 350 nm absorption
band. The excitation spectra for MAOH in ACN
shows a single band at 360 nm corresponding to
330
260
190
(I)
330
.
\
'\"
025
\
\
o
...•
\
~ 020
~
~
>-
Me
\
Cl
"'
T
u
\).exc
<[
\.
~ 0·15
~
Me"f'YC,~
\
CY
n
010
I
z
o
".\
0\
0·05
~O"~
/C-:::::-
"-.
\
000 L-._.l.--_..i-_..L
__ --1-_(-:-b~)
340 350 360 370 380 390
E xc. WAVELENGTH (nm)
Fig. 2-Variation
of fluorescence quantum yield (Illr) with
(a) excitation wavelength (Aexc) (-)
and temperature (---) in
DMSO and (b) excitation wavelength (Am) (•.) and temperature (0) in ACN.
MI
o
1001 tautomlT
MAOH,IV
Schlml
3
Scheme 3-A schematic energy-state diagram for the dynamic
processes of the major species of MAOH in acetonitrile
(ACN). The straight and wavy lines represent radiative and
nonradiative processes respectively where kf, kr"' are respective radiative and nonradiative decay rate constants for
MAOH fluorescence.
DAS et al.: GROUND & EXCITED
STATE PROTON TRANSFER
a monitoring wavelength = 520 nm, hoth in the
presence and absence of TEA (Fig. lE). This implies that the species responsible for the 520 nm
emission in ACN originates from the ground state
closed conformer (I). This indicates that enol tauto mer (IV), (Scheme 3) is the fluorescing species
resulting from an ultrafast excited state intramolecular proton transfer (ESIP'I) of (I). In ACN, <Pr
is found to decrease with the increase of A~xc
(Fig. 2b). This is due to the decrease in population of the fluorescing species which originates
ftom
the
intramolecularly
hydrogen-bonded
closed conformer (I) of MAOH. However, MFOH
shows dual emission at 460 and 520 nm and
reacts considerably with TEA even in ACN both
in the ground and excited states to form the
corresponding phenolate anion, MFO -. Based on
all these observations the following statements
can be made:
(1) The species responsible for appearance of
510-520 nm fluorescence band is the MAO- anion (III, Scheme 1) in DMF, DMSO and enol tautomer (IV) in ACN.
(2 ) Ground/excited state equilibria/kinetics operative for MAOH in ACN are different from
those in DMSO and DMF. This cannot be explained on the basis of dielectric constant of the
solvent.
(3) The population of the species in the excited
state responsible for 510 nm emission is a function of excitation energy and added base in
DMSO, DMF and same emitting species is present at all excitation energies and in the presence
of base.
(4) The blue shift in absorption (450 nm band)
and emission spectra (510-520 nm band) of
MAOH from that of MFOH can be regarded as
due to the stronger intramolecular
hydrogen
bonding in MAOH.
The basic discrepancy of the dissimilar behaviour of MAOH and MFOH in ACN is attributed
to the competition between intramolecular hydrogen bond of the molecule and intermolecular
hydrogen bonding with solvent. In hydrogen
bonding environment, a significant population of
the anionic species (MAO -) is expected to be
present depending on the strength of interaction
with the solvent. This is particularly expected in
the presence of base which promotes the reaction
and may give rise to an increased concentration
and intensityZ1.22 of the phenolate anion. In the
case of MAOH the intramolecular hydrogen bond
is much stronger (than in MFOH) so that interaction with solvent cannot disrupt this band. As a
853
OF 4-METHYL-2,6-DIACETYLPHENOL
result ESIPT of (I), takes place in ACN (Scheme
3) and enol tautomer (IV) is the fluorescing species. But in the case of MFOH intermolecular interaction with solvent disrupt the comparatively
weaker intramolecular H-bond and intermolecular
proton transfer (IPT) process resulting in MFOion occurs in ACN. Theoretical evaluation using
AM 1 technique."
of bond length and bond
strength data also supports the above argument.
In Fig. 3 the 0 - H, C = 0 and O---H bond
lengths in MAOH (Y) in the ground state are
0.97 A, 1.24 A and 1.94 A respectively. But in
MFOH (X) they are 0.97 A, 1.23 A and 2.0 A respectively. The higher O---H intramolecular hydrogen bond length in the case of MFOH
(2.00 A) than MAOH (1.92 A) indicate that O---H
bond in MAOH is much stronger than O---H
bond in MFOH. This speculation is more convincingly proved by the O---H bond strength data.
The O---H bond strength in MAOH and MFOH
in the ground state are 4.45 and 4.13 kcallmol respectively. But in DMSO and DMF the stronger
solute-solvent interaction overcomes. intramolecular hydrogen-bond strengths in both MAOH
and MFOH and disrupts the intra Hvbonds, As a
result IPT occurs in both MFOH and MAOH.
Emission study at 77 K
It has been found that by lowering the temperature at 77K the <Pf increases sharply without any
change in position of the band in all the solvents.
The relative <Pr are found to be 0.72 and 0.66 in
DMSO and DMF, respectively (Am = 360 nm),
This shows that population of the species responsible for 510 nm emission is also a function of
temperature. It is quite interesting to note that the
excitation spectra obtained by monitoring the
emission at 510 nm show a single band at 360 nm
at 77K in DMSO and DMF. Accordingly, the
main species in the ground state at 77K is the
closed conformer (I) and the species responsible
for the 510 nm emission is anion (III) which is
9
2'0:5
;1 ~o"
o
·~c/
1·235A
'1
c
I
It
MAOH(Y)
"'FOH{X)
Fig. ~- The intramolecularly hydrogen bonded closed conformers of MFOH (X) and MAOH (Y) where O---H is the intramolecular hydrogen bond.
854
INDIAN J CHEM. SEC. A, NOVEMBER 1995
formed on excitation (EIMPT, Scheme 2) at 17K.
Thus, we come to the conclusion that base
(TEA), excitation energy and temperature act
similarly in the intermolecular proton transfer
reaction of MAOH. as the temperature is raised,
the fluorescence intensity and ,pc gradually quench
(Fig. 2) reflecting an increase in the nonradiative
decay rate.
In ACN a single fluorescence band at 520 nm
is observed with enhancement in intensity relative
to that at room temperature. The corresponding
excitation spectra in ACN exhibit a single band at
360 nm. Therefore, in ACN the single fluorescence band at 520 nm originates from photoexcitation of the closed conformer (I) followed by an
ultrafast ESIfT
(Scheme 3). At 17K unlike
MFOH17
and
o-hydroxybenzaldehyde
(OHBA)1l,12 we are unable to detect any phosphorescence emission on irradiation in the case of
MAOH. In the case of o-hydroxybenzaldehyde
(OHBA)11,12. Nagaoka et al. suggested that phosphorescence occurs from open conformer by the
rotation of the formyl group and suggested two
structural forms for OHBA. On the other hand,
only one structural form (II) for the open conformer of MAOH may be proposed since rotation of
the )C = 0 group is inhibited in MAOH although
we have failed to observe any emission due to II.
This must be due to the presence of two .methyl
groups in MAOH. Moreover, our results also support the earlier report" that the compounds in
which the formyl group is unable to rotate will
not emit blue or UV band on photoexcitation-v".
~
c
~
o
u
We failed to observe the blue band in MAOH ,
which was found to occur in the case of MFOHl4
even when excited at such a wavelength where the
intensity of emission is expected to be highest. So,
the presence of II is discarded in MAOH for all
these solvents.
Fluorescence decay study at different temperatures
The fluorescence decays of MAOH in. all the
solvents show single exponential decays at all the
temperatures studied here. A typical decay profile
is shown in Fig. 4. The radiative (k{) and nonradiative (kc"r) decay rates are obtained from ,pc and
lifetimes (1'c) values as described earlier'? and are
displayed in Table 1. We have failed to observe a
risetime in the decay profiles (e.g. Fig. 4). From
these observations we. conclude that- III is formed
from photoexcited II by an ultrafast proton transfer
process where II can be intermediate involved in
theEIMPT
EIMPT
(MAOH* + S ""- MAO - * + SH +, as shown
Scheme 2) process as
MAOH*
in
+ S ..• MAOH*---S'" MAO - * + SH+
photoexcited I photoexcited II photoexcited III
It is evident that the decay rates are relatively
slower at 17K from those obtained at room temperature. An interesting point to note in Table 1
is that the k[ values are higher than kr at 17K
except in ACN. However, kr are always dominant at room temperature. The decay rates obtained in ACN are an order of magnitude higher
than that obtained in DMSO and DMF and are
very similar to those obtained in a non-hydrogen
bonding solvent CCl4 (Table 1). This cannot be
explained on the basis of dielectric properties of
the solvent, rather the intermolecular solute-solvent interaction appears to play a major role in
the deactivation of the excited state of MAOH
that fluorescences at 510 nm. The ,pc and 1'c in
ACN reflect a relatively weaker interaction of
MAOH with this solvent medium and considerable increase of kc"r as shown in Table 1.
Table l=-Lifetime (l'f)' quantum yield of fluorescence ('1)' radiative (len and nonradiative (Ie!",) decay rate constant for
MAOH at room temperature and 77K·
Fig. 4- Typical decay profile (A.em= 510 run) of MAOH in
DMSO at 77K. The lamp profile is denoted by broken lines
(resolution = 0.167 ns/channeI). The solid curve represents
the best computer fit of experimental points to a single exponential decay (x2 = 1.03, Durbin Watson (D.W) parameter e= 1.82).
Solvent
'1
I'f
10-9 k{,s-I
DMSO
0.28 (0.82)
4.3 (6.8)
0.07 (1.2)
1.6 (0.3)
DMF
0.21 (0.76)
3.3 (5.1)
0.6 (1.5)
2.4 (0.4)
ACN
CCI4
0.08 (0.18)
0.06
0.3 (3.2)
0.4
3.0 (0.8)
1.5
30 (3.7)
24
"The values at 77K are given in the parenthesis.
10-9 ktnr,s-I
DAS et al: GROUND & EXCITED STATE PROTON TRANSFER
10
8L2.5
~~1----~1~----~
3.0
3.5
4.0
lOOO/T
(K-1)
10~'--------~'-~---------------(b-')
-, -,
..
c:_
.>t:
9
8~
2.8
-,
.'",-
~~
'"
,
~~~
3.3
3.6
1
1000/ T (K- )
Fig. 5-(a) Plot of log k, versus lOOOIT for MAOH in ACN
(---) and DMSO (-).
(b) Plot of log kr"' versus lOOOIT for
MAOH in ACN (---) and DMSO (-).
The nonradiative deacy rate constants. (kr"') of
MAOH is found to be strongly temperature dependent and is given by (1 )
kr"'( T) = kc"r( ex:.) exp( - EAIRT)
... (1)
where EA denotes activation energy. In Fig. 5a
and 5b the temperature dependence of k, and
kr"'( T) for MAOH are shown. The plots of log
kc"r( T) versus liT are linear for MAOH in both
the solvents DMSO and ACN, but the values of
pre-exponential factors, kc"r( ec ) as well as EA are
remarkably different. In DMSO, we obtained
E A = 0.87 kcal/mol and kc"r( ec ) = 7.6 X 108 s - I.
On the other hand, in ACN, EA == 7.1 kcal/mol
and kr( ec ) = 4.1 X 1012 s -I. This activation energy in ACN is greater than that obtained for similar other molecules like o-hydroxybenzaldehyde
(OHBA), methyl salicylate (MS)11.12in nonpolar,
non-hydrogen bonding solvents. This observation
shows that E A depends considerably on the group
attached to the carbonyl carbon which indicates
the importance of the carbonyl torsion in inducing the nonradiative decay.
OF 4-METHYL-2,6-DlACETYLPHENOL
855
The foregoing results on MAOH in these polar
aprotic solvents lead to the following conclusions:
(1) The variation of l"r with temperature and
added base can be linked to change in 1>r·
(2) The thermal effect on 1>r is mainly the consequence of changes in the rates of the decay processes.
(3) The temperature and added base act in a similar way on the emission and decay properties
ofMAOH in DMSO and DMF.
(4) The results in DMSO (£=46.7),
DMF
(£=36.7)
and ACN (£=37.5)
cannot be explained on the basis of dielectric properties (s) of
the solvent. The solute-solvent interaction seems
to play an important role in the proton transfer
reaction of MAOH. In the case of MAOH, ACN
does not behave like a hydrogen bonding solvent
like DMSO and DMF. In this respect ACN belongs to the category of non-hydrogen bonding
solvents like CCl4 and hexane. The absorption
spectra, higher 1>r and slower decay rates in
DMSO and DMF imply stronger interaction with
MAOH in comparison to that with ACN. These
results appear to indicate that the intramolecular
hydrogen bond in MAOH is stronger compared
to that of MFOH.
(5) The activation energy for the nonradiative
process is dependent on the group attached to the
carbonyl carbon and indicates the importance of
the carbonyl torsion in the nonradiative decay.
Acknowledgement
The authors RD and SM are grateful to the
CSIR and UGC, New Delhi for providing them
with the senior research fellowships. We express
our sincerest thanks to the Chemical Physics
group of Prof. N. Periasamy of Tata Institute of
Fundamental Research (Bombay) for measurement of lifetime values in their picosecond instrument. We are also thankful to Prof. K. Nag of the
inorganic chemistry department of this institute
for his help in preparation of MAOH.
References
I Swinney T C & Kelley D F, J phys
(1991)
2
(1991)
3
4
5
6
Chern, 95
10369.
Tolbert M & Nesselroth S M, J phys Chern, 95
10331.
Schwartz B Y, Peteanu L A & Harries C B, J phys
96 (1992) 3591.
Frey W, Larmer F & Elsasser T, J phys Chern, 95
10391.
Ita A, Fujiwara Y & Itoh M, J chem fhys, 96
7474.
Takagoshi K & Hikichi K, J Am chem Soc, 115
9747.
Chern,
(1991)
(1992)
(1993)
856
7
8
9
10
11
12
13
14
15
INDIAN J CHEM. SEC. A, NOVEMBER
Catalan J & del Valle J C, } Am chem Soc, 115 (199:»
4321.
Nakamura H, Terazirna M & Hirota N, J phys Chern, 97
(1993)8952.
Mavni J, Berensendsen A J C & Van gunstenen W F, J
phys Chern, 97 (1993) 13469.
Yanagimachi M, Tarnai N & Masahara H, Chem Phys
Lett; 201 (1993) 115.
Nagaoka S, Hirota N, Sumitani M & Yoshihara K, J Am
chem Soc, 105 (1983) 4220.
Nagaoka S, Hirota N, Sumitani M, Yoshibara K, Kochany
E L & Ewamura H, JAm chem Sac, 106 (1984) 6913.
Roy R & Mukherjee S, Indian J Chern, 28A(1989) 545.
Das R, Mitra S & Mukherjee S, J Photochem Photobiol
A: Chern, 76 (1993) 33.
Adhikary B, Mandai S K & Nag K, J chem Soc; Dalton
Trans, (1988) 935.
1995
16 Mandai S K & Nag K, J org Chern, 51 (1986) 3900.
17 Das R, Mitra S & Mukherjee S, Bull chem Soc. Japan, 66
(1993) 2492.
18 Periasamy N, Doraiswami S, Maiya G B & Venkataraman
B, J chem Phys, 88 (1988) 1638.
19 Mukherjee S, Bull chem Soc, Japan, 60 (1987) 1119.
20 Ketelaar J AA, Reel Trav Chim; 71 (1952) 1104.
21 Woolfe G J & ThistIetwaite P J, J Am chem Soc. 102
(1980)6'117.
22 Klopffer W, Adv Photochem; 10 (1977) 311.
23 Mitra S, Das R & Mukherjee S, (to be communicated).
24 Acuna A U, Catalan J & Toribio F, J phys Chern, 85
(1981)241.
25 Catalan J, Toribio F & Acuna A U, J phys Chern, 86
(1982) 303.
26 NagaokaS& Nagashima U, Chem Phys, 136 (1989) 153.