SPECTROSCOPIC PROPERTIES OF AROMATIC DICARBOXIMIDES .1. N-H AND N-METHYL-SUBSTITUTED NAPHTHALIMIDES

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 2b, 2.8 D for 3b and < 2 D for 4b, 1 D approximate to 3.33564 x 10(-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 dicarboximide 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, k(f), increases. This difference is found to arise from the energy gap between the lowest (1)(pi,pi*) singlet excited state and the upper 3(n,pi*) 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,pi* and pi,pi* 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 2.1 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.