Deuterium isotope effect on 4-aminophthalimide in neat water and reverse micelles

Abstract

Picosecond time-resolved emission of 4-aminophthalimide (4-AP) in neat water and D₂O and in the water pool of aerosol OT reverse micelles in n-heptane is reported. It is proposed that in protic solvents 4-AP undergoes ultrafast solvent-mediated proton transfer and the emission originates from the proton-transferred species. The abrupt decrease in the emission quantum yield and lifetime of 4-AP in protic solvents is attributed to the reduction in the S₁-S₀ energy gap in the proton-transferred species. Nearly 4-fold increase in the emission quantum yield (φ _f) and lifetime (τ_f) of 4-AP is observed in neat D₂O compared to water. This is ascribed to the retardation of the nonradiative S₁ → S₀ internal conversion processes on deuteration of the imide proton. In neat water or D₂O the solvation dynamics is too fast to be detected using our picosecond setup. However, in the water pool of reverse micelles the relaxation of the water (or D₂O) slows down appreciably. The initial component of the solvation dynamics in reverse micelles, in the case of D₂O, is observed to be 1.5 times slower than that in the case of H₂O.