Solvent effect on the sensitized photooxygenation of 2,3-dihydropyrazine derivatives

Detection of O₂(¹Δ_g) phosphorescence emission, λ_max = 1270 nm, following laser excitation and steady-state methods was employed to determine the total rate constant, k_T, and the chemical reaction rate constant, k_R, for reaction between 5,6-disubstituted-2,3-dihydropyrazines and singlet oxygen in several solvents. Values of k_T ranged from 0.26 × 10⁵ M^-1 s^-1 in hexafluoro-2-propanol to 58.9 × 10⁵ M^-1 s^-1 in N,N-dimethylacetamide for 5,6-dimethyl-2,3-dihydropyrazine (DMD) and from 5.74 × 10⁵ M^-1 s^-1 in trifluoroethanol to 159.0 × 10⁵ M^-1 s^-1 in tributyl phosphate for 5-methyl-6-phenyl-2,3-dihydropyrazine (MPD). Chemical reaction rate constants, k_R, for DMD are similar to k_T in polar solvents such as propylencarbonate, whereas for MPD in this solvent, the contribution of the chemical channel to the total reaction is about of 4%. Dependence of the total rate constant on solvent microscopic parameters, α and π*, for DMD can be explained in terms of a reaction mechanism that involves formation of a perepoxide exciplex. Replacement of the methyl by a phenyl substituent enhances dihydropyrazine ring reactivity toward singlet oxygen and modifies the dependence of k_T on solvent parameters, specially on the Hildebrand parameter. These results are explained in terms of an additional reaction path, involving a perepoxide-like exciplex stabilized by the interaction of the negative charge on the terminal oxygen of the perepoxide with the aromatic π system.