The photophysical properties of the N-H and N-methyl derivatives of 1,2-, 2,3-and 1,8-naphthalimides 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 26, 2.8 D for 36 and <2 D for 46, 1 D z 3.33564 x C m, and 26, 36 and 46 are N-methyl-1,2naphthalimide, N-methyl-2,3-naphthalimide and N-methyl-1 ,&naphthalimide). However, important modifications of the photophysical properties are observed which depend on the relative position of the dicarboximide moiety on the naphthalene ring: the intersystem crossing rate constant of 46 increases dramatically by three orders of magnitude compared with that of 26 ; simultaneously, the fluorescence quantum yield decreases from 0.77 to 0.03, although the corresponding rate constant, k,, increases. This difference is found to arise from the energy gap between the lowest '(n,n*) singlet excited state and the upper 3(n,n*) triplet state, which is of the order of 9 kcal mol-' for 26 and less than 2 kcal mol-' for 46 in acetonitrile solution. Protic solvents increase the energy difference between the n,n* and n,n* states thus decreasing the mixing of the two levels; as a consequence, the lifetime of 46 is increased, i.e. from <60 ps in hexane to 2.1 ns in trifluoroethanol. A triplet-triplet annihilation process occurs with the N-methyl derivatives 36 and 46 which leads to a monomer delayed fluorescence with the former, and mainly to a delayed excimer emission with the latter.

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Absorption and fluorescence spectra, fluorescence decay times, and quantum yields of fluorescence and triplet state formation have been determined for N-phenyl and substituted N-phenyl derivatives of 1,2-, 2,3-, and 1,8-naphthalimides, using stationary irradiation and laser flash excitation methods. The effects of substituents on the N-phenyl group on solvent polarity and viscosity have been studied. A short-wavelength (SW) fluorescence, similar to the luminescence emitted by the N-alkyl derivatives, and/or a considerably red shifted long-wavelength (LW) luminescence are observed, and the ratio of the SW and LW fluorescence components is found to depend on substitution and on solvent properties. A striking characteristic of the Nphenylnaphthalimides (in contrast to the N-alkyl derivatives) is the very efficient internal conversion which results in short fluorescence decay times and in low fluorescence and triplet yields. On the basis of the experimental results, it is suggested that solvent and geometrical relaxation of the Franck-Condon state yields two emitting excited states, the SW and LW states, which emit the short-wavelength and long-wavelength fluorescence, respectively. The geometry of the SW state is similar to that of the ground state, while twisting of the phenyl group toward a coplanar geometry is assumed to be required in the formation of the LW state. The extended conjugation comprising the phenyl and naphthalimide moieties, attributed to the coplanar geometry, together with the charge transfer character endows the LW excited state with an extra stability. Solvent cage and geometrical (twisting) relaxation induces efficient internal conversion by virtue of pseudoJahn-Teller coupling of the two low-lying excited states ("proximity effect") as well as by the decrease of the energy gap between the LW excited state and ground state ("energy gap law").

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