Joseph Suhadolnik

Unexpected Electronic Effects on Benzotriazole UV Absorber Photostability: Mechanistic Implications Beyond Excited State Intramolecular Proton Transfer

Electron withdrawing groups at the 5-position of benzotriazole ultraviolet 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.

PRONOUNCED CONFORMATIONAL PREFERENCE OF THE 3?-?-CUMYL SUBSTITUENT IN 2-(2?-METHOXY-3?-?-CUMYLPHENYL)BENZOTRIAZOLE

A pronounced conformational preference of the 3′-α-cumyl substituent in 2-(2′-methoxy-3′-α-cumylphenyl)benzotriazole is suggested from the results of molecular dynamics simulations. This suggestion is supported by both solution NMR spectral and solid-state x-ray crystallographic data. The orientation of the α-cumyl substituent may have implications on the relative performance of 3′-substituted 2′-hydroxyphenylbenzotriazole light stabilizers in polar media.