The effect of micelle–water interface electric field on the intramolecular charge transfer within ionic micelle: Dual fluorescence of sodium p-dialkylaminobenzoates in cetyltrimethylammonium micelles

Abstract

Photoinduced intramolecular charge transfer (CT) of sodium p-(dimethylamino)- and p-(diethylamino) benzoates (SDMAB and SDEAB) in cetyltrimethylammonium bromide and chloride (CTAB and CTAC) micellar solutions was investigated by the CT typical dual fluorescence, in order to identify the effect of micelle–water interface electric field on the CT process occurring within ionic micelle. The CT fluorescence in CTAB and CTAC micelles was blue shifted and enhanced compared to those in pure water. The intensity ratio of the CT fluorescence to the LE fluorescence, I a/ I b, of either SDMAB or SDEAB was found to be higher in CTAC micelle than in CTAB micelle, while these fluorophores experience the same polarity in both micelle environments. It was proposed that the higher I a/ I b ratio in CTAC micelle was due to stronger electric field at CTAC micelle–water interface. This was supported by experiments on the dual fluorescence of SDEAB and SDMAB of different CT-state dipole moment in CTAB micelles of varying micelle–water interface electric field. It was demonstrated that the electric field at the ionic micelle–water interface indeed affected the CT process occurring within micelle that the CT process is promoted and the CT emission is enhanced at higher electric field. Ionic micelle was shown capable of acting as the electric field ‘mediator’ for chemo- and biosensing based on the dependence of the CT dual fluorescence on electric field.