The triplet state of anthracene photodimers and the wavelength dependence of the photodissociation process

Photodimers of anthracene and 9-methylanthracene photodissociate around 300 K mainly via the excited singlet state, around 77 K exlusively via the triplet state. As an intermediate an electronically excited “tight complex” (Xs) is proposed, of which only a small fraction rearranges to an excimer. The quantum yield of excimer and triplet formation is wavelength dependent.