Multichannel photoinduced intramolecular electron-transfer excitations in a bis-naphthalimide spermine conjugate by time-dependent density functional theory.

Density functional theory was applied to the investigation of photoinduced electron transfer (ET) and the absorption spectrum for a bis-naphthalimide spermine conjugate. The multichannel feature of ET excitation in this system was focused on because four groups may act as electron donors and acceptors. The segment in this conjugate, N-(N-methylpropyl)-1,8-naphthalimide, which contains one donor and acceptor pair, was studied at first. Through theoretical calculation, the absorption band at 340 nm was assigned to the pi-->pi* transition. For the whole system involving four chromophores, this work suggested three types of ET. From the theoretical investigation, the naphthalimide radical anion turned out to be formed via intramolecular ET between the two terminal naphthalimide groups, rather than via the electron transfer between the dialkylamine moiety and the naphthalimide one. Furthermore, the electronic coupling matrix elements according to the generalized Mulliken-Hush theory were estimated and the detailed analyses showed that the strongest absorption was due to the local excitation of the naphthalimide chromophore.