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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In order to clarify mechanisms of excited state interactions in hydrogen-bonded pairs, we have studied the kinetics of dynamic quenching of singlet and triplet fluorenone by a series of alcohols, phenols, and related compounds, in which hydrogen-bonding power, redox potential, and acidity are systematically varied. In addition, effects of solvent basicity or polarity and deuteration help identify the role of hydrogen-bonding in physical or chemical quenching processes. Alcohols and weak acids, with high oxidation potentials, do not quench the triplet, but quench the singlet at rates which parallel hydrogen-bonding power. This is attributed to a physical mechanism, involving vibronic coupling to the ground state via the hydrogen bond. This is much stronger in the excited state than in the ground state, and provides efficient energy dissipation in the radiationless transition. Phenols, with hydrogen-bonding power comparable to that of the alcohols but with much lower oxidation potentials, quench both singlet and triplet by electron or H-atom transfer, depending on potentials, acidities, and solvent polarity, as shown by formation of anion or neutral fluorenone radicals from the triplet. Rates increase with both decreasing oxidation potential of the phenol and increasing acidity of the incipient cation radical. Quenching proceeds via a hydrogen-bonded complex and is facilitated by proton transfer contributions to the effective excited state redox potential.

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