Absolute rate constants for 5-deazariboflavin triplet excited state in one-electron transfer processes

Laser-flash photolysis was employed to determine absolute rate constants for quenching of 5-deazariboflavin triplet excited state by amines and phenols in methanol solution. The observed reactivity order for triplet quenching by aliphatic amines parallels very well the corresponding trend in oxidation potentials for these reducing agents; quenching by aromatic amines is significantly more efficient. In the case of phenols, quenching rate constants render a Hammett reaction constant (ρ) value of −1.2 ± 0.1. This negative ρ value is in accord with a mechanism in which hydrogen-atom transfer from phenols to 5-deazariboflavin triplet excited state takes place via an electron–proton transfer sequence.