Influence of the structure of the amino group and polarity of the medium on the photophysical behavior of 4-amino-1,8-naphthalimide derivatives

Photophysical behavior of two series of 4-amino-1,8-naphthalimide derivatives differing in the amino functionality has been studied as a function of the polarity of the medium. The location of the lowest singlet state of the systems, intramolecular charge transfer (ICT) in nature, is largely controlled by the conformation of the 4-amino group rather than its electron-donating ability. The fluorescence properties of the systems are strongly dependent on the nature of the amino group as well as on the polarity of the media. An increase in the length of the dialkyl groups connected to the amino nitrogen or an increase in the size of the ring containing the amino nitrogen enhances the nonradiative deactivation of the fluorescent state of the molecule. The nonradiative relaxation process is facilitated by an increase in the polarity of the media. The present results suggest that the radiationless deactivation of the fluorescent state of the systems is dependent on the nitrogen inversion rate at the amino group.