INTRAMOLECULAR CHARGE-TRANSFER IN RIGIDLY LINKED NAPHTHALENE-TRIALKYLAMINE COMPOUNDS

The photophysical properties of two rigidly linked naphthalene-trialky lamine compounds have been examined in a series of solvents using tran sient absorption spectroscopy and time-resolved spectrofluorimetry. In alkane solvents, excitation of either compound populates a locally ex cited (naphthalene-like) singlet state (LESS) which fluoresces strongl y and which retains a relatively long lifetime. In polar aprotic solve nts, the lifetime of the LESS is substantially reduced owing to format ion of an intramolecular charge-transfer state (CTS), which correspond s to full electron transfer across the molecule. The rate of formation of the CTS, under such conditions, is extremely fast and comparable t o the reorientation time of the solvent. Deactivation of the CTS, whic h occurs on the nanosecond timescale, involves fluorescence, populatio n of a locally excited triplet state, and charge recombination to rest ore the ground state. The rate of formation of the CTS is markedly slo wer in alkanol solvents that can hydrogen bond to the N atom on the do nor and, in such cases, charge transfer involves an additional activat ion energy of ca. 0.12 eV. Under these conditions, it appears that the controlling feature involved in formation of the CTS concerns breakag e of a hydrogen bond, whereas in aprotic solvents the intrinsic barrie r is likely to be associated with the modest structural changes that m ight accompany charge transfer. The rates of formation and deactivatio n of the CTS are discussed briefly In terms of current electron-transf er theory.