UV absorber photostability

Electron withdrawing groups at the 5-position of benzotriazole ultraviolet uv absorbers (UVA) surprisingly improve photopermanence despite weakening the intramolecular hydrogen bond believed to be responsible for photostability. These results, encountered in coatings, thermplastics, and solvents, contradict the assumption from the widely accepted view of UVA photostability that a stronger H-bond would generate a more stable UVA. Calculations suggest the involvement of a conical intersection in benzotriazole excited state deactivation and support a significant role for charge transfer. Modifications to the traditional model of UVA photostability are discussed which more fully consider electronic factors and molecular motions other than H-transfer.