title: Does excited-state proton-transfer reaction contribute to the emission behaviour of 4-aminophthalimide in aqueous media?
abstract: 4-Aminophthalimide (AP) is an extensively used molecule both for fundamental studies and applications primarily due to its highly solvent-sensitive fluorescence properties. The fluorescence spectrum of AP in aqueous media was recently shown to be dependent on the excitation wavelength. A time-dependent blue shift of its emission spectrum is also reported. On the basis of these findings, the excited-state solvent-mediated proton-transfer reaction of the molecule, which was proposed once but discarded at a later stage, is reintroduced. We report on the fluorescence behaviour of AP and its imide-H protected derivative, N-BuAP, to prove that a solvent-assisted excited-state keto-enol transformation does not contribute to the steady-state and time-resolved emission behaviour of AP in aqueous media. Our results also reveal that the fluorescence of AP in aqueous media arises from two distinct hydrogen-bonded species. The deuterium isotope effect on the fluorescence quantum yield and lifetime of AP, which was thought to be a reflection of the excited-state proton-transfer reaction in the system, is explained by considering the difference in the influence of H(2)O and D(2)O on the nonradiative rates and ground-state exchange of the proton with the solvent.
authors.affiliation: School of Chemistry, University of Hyderabad, Hyderabad 500 046 (India); Current address: Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106 (Taiwan).
authors.forename: Dinesh Chandra
authors.initials: DC
authors.lastname: Khara
authors.forename: Sanghamitra
authors.initials: S
authors.lastname: Banerjee
authors.forename: Anunay
authors.initials: A
authors.lastname: Samanta
keywords: fluorescence spectroscopy; hydrogen bonding; isotope effects; keto-enol transformations; solvent effects