FLUORESCENCE PROBING AND PROTON-TRANSFER EQUILIBRIUM REACTIONS IN WATER, SDS, AND CTAB USING 3,3-DIMETHYL-2-PHENYL-3H-INDOLE

The effect of acid and base concentrations on 3,3-dimethyl-2-phenyl-3H -indole (1) in water has yielded four species, namely, the neutral, th e monoanion, the monocation, and the dication, whereas in SDS it gives neutral and monocation in both the ground and excited singlet states. It was shown that the large spectral shifts observed in the monocatio n relative to the neutral species are caused mainly by an increase in the planarity of the molecule rather than by a large solvent relaxatio n. The number of ground-state conformers are possibly less for the cha rged species as suggested from k(F)(t) values. From the nu(F)BAR corre lation with the dielectric constants, it has been possible to approxim ately determine an effective polarity corresponding to e values of 42 and 33 for SDS and CTAB, respectively, at pH 9.5. The dependence of th e fluorescence quantum yield on the viscosity of the medium has enable d us to determine viscosity values of 3.6 and 6.6 cP for SDS and CTAB, respectively. These observations show that molecule 1 is located at t he micellar-water interface (Stern layer) with limited exposure to wat er molecules. The cmc values for SDS and CTAB at pH 9.5 have been calc ulated to be (7.4 +/- 5) X 10(-3) and (8.2 +/- 0. 5) X 10(-4) M, respe ctively. The cmc values are decreased to almost-equal-to 1.5 X 10(-3) M at pH 1. Molecule 1 shows more binding affinity for CTAB than SDS at pH 9.5, as is evident from binding constant data. The fluorometric ti tration method has yielded ground-state pK(a) values both in pure wate r and SDS for the monocation-neutral equilibrium. This shows the proto n-transfer equilibrium is not established during the lifetime of the e xcited species. Forster cycle calculations have shown that the probe m ight become more basic upon excitation in SDS micelles. An increase in pK(a) values in SDS (4.75) compared to pure water (3.25) for this equ ilibrium is consistent with the pseudophase ion-exchange (PIE) model.