The effect of paramagnetic molecules on the intensity of spin-forbidden absorption bands of aromatic molecules

It is shown that there are two possible mechanisms whereby a singlet-triplet absorption band of a molecule can gain intensity under the perturbing influence of a paramagnetic species. The first involves some mixing of the triplet state with the singlet states of the molecule. The second involves a mixing of the triplet state with charge-transfer states of the complex formed between the molecule and the paramagnetic species. It is shown that in the particular case of an alternant hydrocarbon, the first singlet-triplet absorption band is unlikely to gain intensity by the first of these mechanisms. It follows by elimination that the charge-transfer mechanism is the more important. The available experimental evidence supports this viewpoint.