Intramolecular charge transfer in rigidly linked naphthalene–trialkylamine compounds

Abstract

The photophysical properties of two rigidly linked naphthalene–trialkylamine compounds have been examined in a series of solvents using transient absorption spectroscopy and time-resolved spectrofluorimetry. In alkane solvents, excitation of either compound populates a locally excited (naphthalene-like) singlet state (LESS) which fluoresces strongly and which retains a relatively long lifetime. In polar aprotic solvents, the lifetime of the LESS is substantially reduced owing to formation of an intramolecular charge-transfer state (CTS), which corresponds to full electron transfer across the molecule. The rate of formation of the CTS, under such conditions, is extremely fast and comparable to the reorientation time of the solvent. Deactivation of the CTS, which occurs on the nanosecond timescale, involves fluorescence, population of a locally excited triplet state, and charge recombination to restore the ground state. The rate of formation of the CTS is markedly slower in alkanol solvents that can hydrogen bond to the N atom on the donor and, in such cases, charge transfer involves an additional activation energy of ca. 0.12 eV. Under these conditions, it appears that the controlling feature involved in formation of the CTS concerns breakage of a hydrogen bond, whereas in aprotic solvents the intrinsic barrier is likely to be associated with the modest structural changes that might accompany charge transfer. The rates of formation and deactivation of the CTS are discussed briefly in terms of current electron-transfer theory.