Journal of the Chemical Society - Faraday Transactions p. 411 - 422 (1994)
Update date:2022-08-10
Topics:
Wintgens, Veronique
Valat, Pierre
Kossanyi, Jean
Biczok, Laszlo
Demeter, Attila
Berces, Tibor
The photophysical properties of the N-H and N-methyl derivatives of 1,2-, 2,3- and 1,8-naphtalimides have been studied.The shift of the fluorescence emission position as a function of the solvent polarity indicates only a weak variation of dipole moment for the excited state compared with the corresponding value in the ground state (5.7 D for 2b, 2.8 D for 3b and <2 D for 4b, 1 D ca. 3.33564E-30 C m, and 2b, 3b and 4b are N-methyl-1,2-naphthalimide, N-methyl-2,3-naphthalimide and N-methyl-1,8-naphthalimide).However, important modifications of the photophysical properties are observed which depend on the relative position of the carboximide moiety on the naphthalene ring: the intersystem crossing rate constant of 4b increases dramatically by three orders of magnitude compared with that of 2b; simultaneously, the fluorescence quantum yield decreases from 0.77 to 0.03, although the corresponding rate constant, kf, increases.This difference is found to arise from the energy gap between the lowest 1(?,?*) singlet excited state and the upper 3(n,?*) triplet state, which is of the order of 9 kcal mol-1 for 2b and less than 2 kcal mol-1 for 4b in acetonitrile solution.Protic solvents increase the energy difference between the n,?* and ?,?* states thus decreasing the mixing of the two levels; as a consequence, the lifetime of 4b is increased, i.e. from < 60 ps in hexane to 6.1 ns in trifluoroethanol.A triplet-triplet annihilation process occurs with the N-methyl derivatives 3b and 4b which leads to a monomer delayed fluorescence with the former, and mainly to a delayed excimer emission with the latter.
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