COMPREHENSIVE MODEL OF THE PHOTOPHYSICS OF N-PHENYLNAPHTHALIMIDES - THE ROLE OF SOLVENT AND ROTATIONAL RELAXATION

Absorption and fluorescence spectra, fluorescence decay times, and qua ntum yields of fluorescence and triplet state formation have been dete rmined 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-waveleng th (SW) fluorescence, similar to the luminescence emitted by the N-alk yl derivatives, and/or a considerably red shifted long,wavelength (LW) luminescence are observed, and the ratio of the SW and LW fluorescenc e components is found to depend on substitution and on solvent propert ies. A striking characteristic of the N-phenylnaphthalimides (in contr ast to the N-alkyl derivatives) is the very efficient internal convers ion which results in short fluorescence decay times and in low fluores cence and triplet yields. On the basis of the experimental results, it is suggested that solvent and geometrical relaxation of the Franck-Co ndon state yields two emitting excited states, the SW and LW states, w hich emit the short-wavelength and long-wavelength fluorescence, respe ctively. 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 extend ed conjugation comprising the phenyl and naphthalimide moieties, attri buted to the coplanar geometry, together with the charge transfer char acter endows the LW excited state with an extra stability. Solvent cag e and geometrical (twisting) relaxation induces efficient internal con version by virtue of pseudo-Jahn-Teller coupling of the two low-lying excited states (''proximity effect'') as well as by the decrease of th e energy gap between the LW excited state and ground state (''energy g ap law'').