Participation of chromophore pairs in photoinduced intramolecular electron transfer for a naphthalimide spermine conjugate

Photophysical and electron transfer properties of the conjugate of 1,8-naphthalimide and spermine (BNS) and related chromophore system including, N-propyl-1,8-naphthalimide (NAP), have been investigated. Absorption and emission spectra for the BNS-spermine derivative revealed a ground state intramolecular complexation of the two naphthalimide chromophores, particularly for aqueous media. Fluorescence quenching studies of BNS using an electron donor, 1,4-diazabicyclo[2,2,2]octane (DABCO), yielded quenching rate constants near that anticipated for a diffusion-controlled process. Transients observed on laser flash photolysis (λexc=355 nm) corresponding to the naphthalimide radical anion (λmax=410 nm) and the triplet state of BNS (λmax=470 nm) have been identified. Observation of a lower fluorescence quantum yield (in comparison to NAP) and the appearance of the radical anion intermediate on laser flash photolysis in the absence of any external electron donor (e.g., for BNS in its free base form) suggests a mechanism of intramolecular electron transfer involving either (a) interaction of two naphthalimide chromophores or (b) interaction of naphthalimide groups and a dialkyl amine moiety.