470 J. Phys. Chem. A, Vol. 104, No. 3, 2000
Singh et al.
TABLE 5: Dielectric Constants (E) and Critical Micelle
Concentration (Cmc) of Micelles As Determined Using Probe
Dienes 3 and 5-7 and Some Other Known Probes
the fluorescence λmax is smaller as compared to that in ionic
SDS and CTAB. For example, diene 3 in SDS shows fluores-
cence at 612 nm and in Triton-X-100 at 538 nm. Further, no
change in fluorescence λmax with increasing concentration of
Triton-X-100 was observed for probe dienes 3 and 5-7. These
observations suggest that the excited states of probe dienes are
affected by ionic micelles much more than the neutral micelles
like Triton-X-100. Thus, donor-acceptor dienes 3 and 5-7 can
serve as good fluorescence probes. for microenvironment of
charged micelles.
In conclusion, it can be said that 1,4-diarylbuta-1E,3E-diene
compounds with p-nitro substituent on the aromatic ring are
capable of exhibiting TICT fluorescence. Further, the fluores-
cence properties of nitro-substituted dienes can be used to probe
the microenvironment of ionic micelles and related organized
assemblies. This study has brought out interesting features of
the excited state structure and potential energy surface of linear
polyenes. It further provides new directions for the development
of fluorescence probes as sensors and reporters of microenvi-
ronments of organized assemblies.
diene
media
calculated ꢀ
cmc (M)
3
5
6
7
CTAB
SDS
Triton-X-100
CTAB
SDS
Triton-X-100
CTAB
SDS
Triton-X-100
CTAB
SDS
Triton-X-100
CTAB
SDS
Triton-X-100
CTAB
SDS
Triton-X-100
CTAB
SDS
22
28
4
9
27
8
7
45
5
16
45
10
36
40
28
36
40
28
36
25
35
4 × 10-3
3 × 10-2
9 × 10-4
9.5 × 10-3
4 × 10-3
7 × 10-3
4 × 10-3
7 × 10-3
Py-CHO
DABMN
ANS
9 × 10-4
8 × 10-3
3 × 10-4
7 × 10-4
8 × 10-3
3 × 10-4
9 × 10-4
9 × 10-3
3 × 10-4
Triton-X-100
Acknowledgment. Research grant 37/7/95-R&D-II/559 from
the Board of Research in Nuclear Sciences, Department of
Atomic Energy, Government of India, is gratefully acknowl-
edged. We are also thankful to the reviewers of this paper for
their valuable suggestions.
a Py-CHO: pyridinecarboxaldehyde. DABMN: (dimethylamino)-
benzylidene malonitrile. ANS: anilinonaphthalenesulfonic acid.
been used as model systems for studying more complex
bioaggregates.23 Despite numerous efforts in this area, the
solvent properties of the micellar environment are still poorly
understood. Among various physical methods employed, fluo-
rescence probes have been widely used because of their
simplicity, wide scope, and extreme sensitivity at very low probe
concentration24. Comparison of the spectral data in micelles with
those in homogeneous solvent systems provides information on
the micellar environment.
Supporting Information Available: IR, UV-vis absorp-
tion, and fluoresence data for 2, 3, and 5-7 in dioxane water
mixtures at 298 K. This material is available free of charge via
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Herein, fluorescent dienes 3 and 5-7 have been employed
to determine the cmc values for SDS and CTAB. In Triton-X-
100, since no change in fluorescence λmax of the four probe
dienes with increasing concentration of the surfactant was
observed, studies were done only in SDS and CTAB micelles.
The dependence of fluorescence λmax of 3 and 5-7 on surfactant
(SDS and CTAB) concentrations is shown in Figures 6 and 7.
The calculated cmc values are given in Table 5. The fluores-
cence λmax of the probe dienes in CTAB and SDS is constant
over a narrow range of concentrations, which is the critical
micellar concentration. These cmc values are in fair agreement
with the cmc values obtained by using other known probes.19-21
Below the cmc of SDS and CTAB, there is blue shift in
fluorescence λmax for all the four probe dienes (3, 5-7).
However, above the cmc of SDS and CTAB, dienes 5-7
exhibited blue-shifted fluorescence, while for diene 3, no shift
in fluorescence λmax was observed above cmc in the two
micelles. This can be attributed to the interaction between
monomeric surfactant with probe molecules and to the presence
of premicellar aggregates. Previous reports with p-(N,N-dialkyl-
amino)benzylidine malonitrile as a fluorescence probe showed
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cmc.21 Slightly above the cmc range, there is change in
fluorescence wavelength and it is observed to be constant for
very high concentrations of the surfactants used. Thus, this
change in the fluorescence wavelength can be attributed to the
change in shape of the micelle with increase in surfactant
concentration. It is well established that in the vicinity of the
cmc anionic, cationic, and nonionic surfactant micelles are
mostly approximate to spheres but with increase in total
concentration micelles become asymmetric.24 In Triton-X-100,
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