Spectroscopic properties of aromatic dicarboximides .3. Substituent effect on the photophysical properties of N-phenyl-2,3-naphthalimides

In the present article we describe the effect of the substitution on the photolysis properties of a series of N-phenyl-2,3-naphthalimides. It is found that the decrease in the electron-donating character of the substituent on the N-phenyl ring changes the fluorescence emission from a weak, broad and short-lived long-wavelength (LW) emission into a structured and long-lived fluorescence localized at the short wavelengths (SW) which behaves like that of the unsubstituted 2,3-naphthalimide. Both SW and LW emissions can be observed in some cases. The different results can be explained on the basis of a three-level scheme where the vibrationally relaxed Franck-Condon state populates two different excited states, the one (SW) emitting at short wavelengths, and the other (LW) emitting at long wavelengths. The substitution on the phenyl ring influences mainly the energy of the LW excited state. The nature of the substituent on the N-phenyl ring has a determining influence also on the internal conversion process by virtue of the solvent- and rotation-induced pseudo-Jahn-Teller coupling of the two excited states.