Spectroscopic effects of vibronic coupling between nearby 3nπ* and 3ππ* states of dimethylbenzaldehydes in a durene matrix

A vibrational analysis is presented of high resolution ππ* phosphorescence spectra of the 2,4-, 2,5- and 3,4-dimethylbenzaldehyde-1h₁ and ?1d₁, guests in durene host. As previously reported for the fully hydrogenated molecules, the energy gaps between the zero-point levels of the ³ngp*, and ³ππ* states of the guests deuterated on the aldehydic group are determined from the temperature dependence of their phosphorescence spectra and lifetimes. These vibronic energy gaps tend to be larger in the deuterated species.The existence of the vibronic coupling between ³nπ* and ³ππ* states of dimethylbenzaldehydes is suggested by the important activity, in the ππ* phosphorescence spectra, of nontotally symmetric modes involving the aldehydic group and the aromatic ring. Among these vibrations, the most active are the aldehydic CH and CD wagging modes which also show overtone activity. The intensities of the induced bands show a strong deuterium effect which in the 2,4 and 2,5 compounds is anomalous in that the deuterated species shows the higher induced intensity. The observation of phosphorescence spectra from each molecule in two different sites allows an assessment of the crystal field effect which is found to be moderately strong. These observations can be understood qualitatively on the basis of weak pseudo Jahn-Teller-like coupling between the ³ππ* and ³nπ* states which is subject to interference from Herzberg-Teller coupling involving at least one additional state. No convincing evidence is found for nonplanar distortions of the guest molecules.