SPECTROSCOPIC PROPERTIES OF AROMATIC DICARBOXIMIDES .2. SUBSTITUENT EFFECT ON THE PHOTOPHYSICAL PROPERTIES OF N-PHENYL-1,2-NAPHTHALIMIDE

Absorption and fluorescence spectra, fluorescence decay times, fluorescence quantum yields and triplet yields have been determined for N-phenyl-1,2-naphthalimide and its phenyl-substituted methyl derivatives in different solvents. N-Phenyl-1,2-naphthalimide emits long-wavelength fluorescence in hexane (lambda(f)(max) = 550 nm) which is red shifted by methyl substitution at the meta and para positions of the phenyl ring and by using a solvent of higher polarity. The fluorescence decays on the sub-nanosecond timescale. When the N-phenyl-1,2-naphthalimide has o-methyl substituents in the phenyl group, they emit dual fluorescence. The location of the short-wavelength component is constant while that of the long-wavelength component is blue shifted as a result of o-methyl substitution. Ortho substitution also increases the decay time of the long-wavelength fluorescence. The results are explained in terms of vibronic interaction between the S-1(pi pi*) and S-2(n pi*) excited states (pseudo-Jahn-Teller effect) which is enhanced by solvent relaxation and twisting of the phenyl ring towards a coplanar geometry.