Water-induced fluorescence quenching of aniline and its derivatives in aqueous solution

Nonradiative deactivation processes of excited aniline and its derivatives in aqueous solution were investigated by steady-state and time-resolved flu orescence measurements to reveal characteristic solvent effects of water on the relaxation processes of excited organic molecules. The magnitude of no nradiative rate (k(nr)) of excited aniline derivatives increased significan tly in water compared to that in organic solvents (cyclohexane, ethanol, an d acetonitrile). The fluorescence lifetime measurements in organic solvent/ H2O mixed solvents suggested that the fluorescence quenching in water was n ot due to exciplex formation but due to interactions with a water cluster. From temperature effect experiments on the fluorescence lifetime and quantu m yield of aniline, N-methylaniline, and N,N-dimethylaniline. the apparent activation energies for the nonradiative deactivation rate in water were de termined as 21, 30, and 41 kJ mol(-1), respectively. Upon substitution of h ydrogen atoms in the aromatic ring of aniline derivatives for deuterium ato ms resulted in normal deuterium isotope effect in cyclohexanel i.e. k(nr) d ecreased by deuterium substitution, while in water the same deuterium subst itution led to an increase in k(nr) (the inverse isotope effect). The inver se isotope effects implied that a direct internal conversion to vibrational ly higher excited states in the electronically ground state is not a domina nt mechanism but the transition to a close-lying energy level, e.g. the rel axation to charge transfer to solvent (ctts) state, would be associated wit h the quenching mechanism in water.