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\0) and 1/[H2O]n (taking different values of n) is
not linear (not shown). This suggests that differ-
ent kind of solvated MCs of DMAPPI are in
equilibrium. Presence of different kind of solvated
molecules either in neutral or MCs have been
shown earlier also by noting the red shifts ob-
served in the fluorescence excitation spectra with
increase in uem [21,32]. This behaviour is different
from that observed by Belletete et al. [46] for
2-(4%-N,N-dimethylaminophenyl)
reverse micelles and this process is also quite fast
in these microenvironments. The decrease in the
fluorescence quantum yield of MC2 on increasing
w0 is due to the solvent induced quenching as the
magnitude of fluorescence quantum yield de-
creases with increase of water content.
The large red shift in the fluorescence spectrum
of MC3, even at the smallest amount of water
(w0=3.1) has been assigned to the TICT state of
MC3 and can be explained as follows. In neutral
DMAPPI, [32] the TICT emission is only ob-
served in polar/protic solvents suggesting that the
hydrogen bonding plays the major role in the
TICT emission. It has also been shown earlier
that the hydrogen bonding of the solvents with PI
ring tends the PI ring to be more planar with the
benzene ring, thereby facilitating the charge flow
from dimethylaminophenyl ring to the PI ring.
Thus it was suggested that it was the twisting of
the ꢀNMe2 group and planarity of the PI ring
with benzene ring leads to the TICT emission. In
MC3, the electron withdrawing nature of the PI
ring increases due to the protonation and addition
of water leads the PI ring and benzene ring to be
more planar. This will lead to greater charge flow
from ꢀNMe2 group to PI ring and thus enhance-
ment of TICT emission at the expense of normal
emission. Further addition of water will also
quench the normal emission as observed in neat
water. As observed earlier, formation of TICT
state is an excited state phenomenon and has the
same ground state precursor as for the LE emis-
sion. A small red shift (ꢀ6 nm) observed in the
fluorescence excitation spectra when monitored at
500 nm is due to the solvation of the MC3 of
DMAPPI to different extent as suggested earlier.
All the MCs are present in the bound water
region near the polar head group of the surfac-
tants, which is less polar than the free water in the
entrapped pool. This conclusion is based on the
-3,3-dimethyl-3H-indole and water molecule in
AOT, as the latter molecule contains only one
basic centre compared with three in the present
case. Thus they found out that the probe molecule
and the water molecule form a 1:1 association
complex. Further based on their temperature de-
pendent study, they have shown that the complex
formed is a hydrogen bonded complex between
the probe and the water molecules. Although we
have not carried out a similar study, based on the
following results (i.e. small red shifts observed in
ab
max
the u , red shift observed in the 400 nm fluores-
cence band and the TICT band, see below) a
similar hydrogen bonded complexes containing
different kinds of stochiometric complexes of
probe with water molecules can also be suggested
in our case.
Two points are worth mentioning about the
results observed when water is added to AOT/n-
heptane system. The first, a large blue shift is
observed in the fluorescence excitation spectra
monitored at 380 nm and the second a large red
shift is observed in the fluorescence spectra ob-
served at uexc=430 nm. As pointed out earlier
[21] (Scheme 2) the MC1 is highly polar (vg=20.7
D) and the MC2 is the least (vg=2.8 D). It is also
shown by Belletete et al. [29] that at w0=0, the
effective dielectric constant at the interface of the
AOT reverse micelles is 2.2 and it increases with
increase in the water amount. Thus at w0=0,
both MC2 and MC3 are present and are confi-
rmed by absorption, fluorescence excitation and
fluorescence spectra. With the increase in the wa-
ter content, the least polar MC2 will be trans-
formed into MC1 or MC3. As shown earlier, the
MC2 in neat water is formed in the S1 state from
the MC1, similar phenomenon is also responsible
for the formation of MC2 from MC1 in the
following arguments. (i) The of both the emis-
fl
sion bands decrease very sharply on addition of
small amount of water (Fig. 5) and then remains
nearly independent with further addition of water.
Similar sharp decrease in is also observed for
fl
4-aminonapthalimide in AOT/n-heptane/water
[47] and 8-anilinonapthalene sulphonic acid in
TX-100/hexanol/water in cyclohexane [48] reverse
Krishnamoorthy
