Intramolecular charge transfer processes in confined systems. Nile red in reverse micelles

Abstract

Intramolecular charge transfer (ICT) processes of the neutral fluorescence probe, nile red, I, confined in the water pool of aerosol-OT (AOT) reverse micelle in n-heptane, is studied using picosecond emission spectroscopy. Utilizing the solvatochromism of nile red, only those probe molecules inside the reverse micelle are selectively excited. It is observed that while in aqueous solutions the lifetime (t_f) of nile red is 650 ps, inside the reverse micelles t_f increases to 3.73 ns in reverse micelle and to 2.06 ns at the highest water content of the microemulsion (w_o = 32). With increase in the water-to-surfactant ratio, w_o, as the water pool swells in size, the lifetime and quantum yield of emission decrease and the rate of the ICT process of nile red increases. However, the magnitude of the change (at most 8 times) in the rate of the ICT process of nile red compared to that of ordinary water is much smaller than the several thousandfold decrease observed in the solvation dynamics of water in the water pool relative to bulk water. It is proposed that while the solvation dynamics in the water pool is governed by the dielectric relaxation time, dynamics of the ICT process is controlled by the static polarity of the medium.