Exciplex-type behavior and partition of 3-substituted indole derivatives in reverse micelles made with benzylhexadecyldimethylammonium chloride, water and benzene

The fluorescence properties of 3-methylindole (MI), 3-indoleacetic acid (IA A), 3-indoleethyltrimethylammonium bromide (IETA), L-tryptophan (Trp) and t ryptamine hydrochloride (TA) were studied in reverse micelles solutions mad e with the cationic surfactant benzylhexadecyldimethylammonium chloride (BH DC) in benzene as a function of the molar ratio water/surfactant R (= [H2O] /[BHDC]). The fluorescence quenching of the model compound MI by benzene in cyclohexane solutions and by BHDC in benzene solutions were also studied i n detail, The fluorescence of MI in benzene is characteristic of a charge-t ransfer exciplex, The exciplex is quenched by the presence of BHDC, due to the interactions of the surfactant ion pairs with the polar exciplex, In re verse micelle solutions at low R values, all the indoles show exciplex-type fluorescence, As R increases, the fluorescence behavior strongly depends o n the nature of the indole derivative. The anionic IAA remains anchored to the cationic interface and its fluorescence is quenched upon water addition due to the increases of interface's micropolarity, For IETA, TA and Trp an initial fluorescence quenching is observed at increasing R, but a fluoresc ence recovery is observed at R > 5, indicating a probe partition between th e micellar interface and the water pool, For the neutral MI, the fluorescen ce changes with R indicate the partition of the probe between the micellar interface and the bulk benzene pseudophase, A simple two-site model is prop osed for the calculation of the partition constants K as a function of R, I n all cases, the calculation showed that even at the highest R value, about 90% of the indole molecules remain associated at the micellar interface.