Perturbations between Electronic States in Aromatic and Heteroaromatic Molecules

The low-temperature electronic spectra of several aromatic azines in regions of nπ* and ππ* overlap are presented and compared. It is shown that transitions to the higher-energy state are generally quite diffuse compared with those to the lower. Comparisons are made with aromatic hydrocarbons where ππ* states overlap and aromatic carbonyl compounds where nπ* and ππ* states overlap. The factors which determine the severity of broadening of higher states are discussed. It is concluded that when two states of different orbital type are nearby, there is a more severe breakdown of the Born–Oppenheimer approximation. This breakdown is less severe if the two states are far apart or if the molecule has a similar geometry in both states. The presentation includes accounts of the low-temperature singlet–triplet and/or singlet–singlet crystal spectra or mixed-crystal spectra of pyrazine, 2.6-dimethylpyrazine, tetramethyl pyrazine, pyrimidine, pyridazine, quinoxaline, phenazine, and diazaphenanthrene. The spectra of derivatives of benzene, naphthalene acetophenone, and benzophenone are discussed.