Perturbations between Electronic States in Aromatic and Heteroaromatic Molecules

The low‐temperature electronic spectra of several aromatic azines in regions of $\mathrm{n\pi *}$ and $\mathrm{\pi \pi *}$ 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 $\mathrm{\pi \pi *}$ states overlap and aromatic carbonyl compounds where $\mathrm{n\pi *}$ and $\mathrm{\pi \pi *}$ 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.