Spectroscopic properties of aromatic dicarboximides. Part 2.—Substituent effect on the photophysical properties of N-phenyl-1,2-naphthalimide

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Abstract
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 (λmax f= 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 S1(ππ*) and S2(nπ*) excited states (pseudo-Jahn–Teller effect) which is enhanced by solvent relaxation and twisting of the phenyl ring towards a coplanar geometry.