Energy gap dependence of the nonradiative decay rate constant of 1-anilino-8-naphthalene sulfonate in reverse micelles

Steady-state and picosecond time-resolved fluorescence spectroscopies were conducted to study excited state properties of 1-anilino-8-naphthalene sulfonate (ANS) in reverse AOT (sodium bis(2-ethylhexyl)sulfosuccinate)/isooctane micelles. The maximum wavelength, quantum yield, and lifetime of ANS fluorescence were determined as a function of the micelle size. Detailed analyses of these photophysical parameters demonstrated that the energy gap between the ground and excited states of ANS decided the nonradiative decay rate constant (k(nr)). On the basis of both energy gap dependence of k(nr) and micelle size effects on the photophysical parameters of ANS, the characteristic fluorescence behavior of ANS in the reverse AOT micelles was discussed in terms of the structures of water molecules: bound- and free-water.