Influence of structure on the unusual spectral behavior of 4-dialkylamino-1,8-naphthalimide

Spectral behavior of 4-amino-1,8-naphthalimide derivatives differing in their amino functionality has been studied in different solvents with varying polarity. The results reveal that despite the +I effect of the methyl group, the charge-transfer absorption band maximum of N-ethyl-4-dimethylamino-1,8-naphthalimide (3) is blue-shifted with respect to that of the un-substituted and monoethyl-substituted derivatives, 1 and 2, respectively. AM1 calculations indicate a lower ground state dipole moment of 3 compared to 1 or 2. The single crystal structure of 3 suggests that low dipole moment and blue-shift of its CT maximum are due to twisting of the dimethylamino moiety relative to the 1,8-naphthalimide ring.