A photochemical study of the triplet excited state of pyrene-4,5-dione and pyrene-4,5,9,10-tetrone derivatives

The present study details the use of nanosecond laser transient absorption spectroscopy to investigate the reactivity of the triplet excited states of pyrene-4,5-dione and pyrene-4,5,9,10-tetrone and the respective 2,7-di-t-butyl derivatives. The quinone triplet excited states were quenched by 2-propanol and 1,4-cyclohexadiene with quenching rate constants (k(q)) of the order 10(7) L mol(-1)s(-1), as well as by phenols, amines, and N-acetyl tryptophan methyl ester (model biological compounds) with near diffusion limit experimental rate constants (k(q) > 10(9) L mol(-1) s(-1)) and very small kinetic isotopic effects for phenolic H/D abstraction. Hydrogen abstraction from the quenchers gave rise to the corresponding radical pairs. The intermediates were spectroscopically characterized. The quinones can also act as type II photosensitizers as they can generate singlet oxygen with quantum yields of up to 0.90. Finally, steady-state photolysis of the compounds demonstrated that pyrene-4,5-diones undergo a one-step reduction whilst pyrene-4,5,9,10-tetrones are reduced in two sequential stages. Both the diones and the tetrones are reduced to the respective aromatic catechols. Notably, the changes are reversible indicating that the compounds can act as photo-redox catalysts for reactions involving hydrogen atom abstraction (HAT).