Photolysis of labile bonds towards radical generation: case study of alkyl verdazyls

Two series of stable verdazyl radical derivatives, specifically 1,2,4,6-substituted-1,4-dihydro-1,2,4,5-tetrazin-3(2H)-ones (also called "alkylverdazyls" or "AlkVZs"), were systematically investigated in photoactivated C-N bond homolysis. In-depth evaluation of their mechanisms indicated that several factors played a pivotal role in the generation of radicals during photon absorbance. From quantum chemical calculations, it was found that the most influential factor is the generation of charge-separated excited states, where one electron is located at the verdazyl moiety and the second one is distributed at the alkyl part. The theoretical approach also allows one to predict the reaction rate constant based on the oscillator strengths of the S₀ → S₁ transition in AlkVZs. Surprisingly, the photon density of the LED source has a strong impact on controlling the reaction direction, and decreasing the light power could lower the yield of radicals by twofold with the full conversion of the starting materials in all cases. Our results delineate prospective approaches for achieving high yields in photochemical transformations via the variation of the wavelength of light, the careful design of the molecular structure and alterations in the LED power.