Comprehensive model of the photophysics of N-phenylnaphthalimides: The role of solvent and rotational relaxation

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 N-phenylnaphthalimides (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 pseudo-Jahn-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'').