Absorption and emission spectra of isomeric tolunitriles

Absorption and emission characteristics ofo-, m- andp-tolunitriles in polar and non-polar solvents under different conditions have been investigated in detail. Solvatochromic shifts of band origin of these molecules in non-polar solvents show that their dipolemoments in the first excited singlet state are almost the same while its value in the second excited singlet is larger in the metathan in the para-isomer. Vibronic analyses of the low temperatures absorption, fluorescence and phosphorescence spectra of all the three molecules have provided evidence that these molecules are slightly distorted in the first excited singlet state while such distortion in the phosphorescence emitting triplet state is larger. The data on fluorescence and phosphorescence quantum yield and phosphorescence lifetime of the tolunitriles are reasonably interpreted as showing that in these molecules, particularly m-andp- tolunitriles, the internal conversion rate from the first excited singlet to the ground state is probably small and that the charge transfer character of the triplet state in thep-isomer is larger than that in the meta.