Role of Structural Changes in the Triplet−Triplet Energy Transfer Process to Oxime Derivatives

The triplet energy accepting properties of bridged hydroxy and methyl derivatives of acetophenone oxime were examined. Sandros and Agmon−Levine−Balzani (ALB) models were used to deduce the oxime triplet energies,, and the reorganizational intrinsic barriers, Δ G # (0), associated with the reaction. It is found for the first time that these compounds exhibit different degrees of nonvertical energy transfer (NVET) behavior, depending on their chemical structure. These structural properties have been investigated at both the AM1 and the B3LYP/6-31+G(d) levels of theory. The flexibility of the ground-state molecule and the structural changes occurring during the S 0 → T 1 sensitized transition seem to play crucial roles. It appears that two important rearrangements are involved, namely, rotation about the C−N double bond and a π system flattening associated with a torsion about the formal C−C single bond. To relate the NVET character to these structural changes, the energy variations associated with these latter were evaluated and successfully related to Δ G # (0). Moreover, the role of the thermal activation of the molecular coordinates is evidenced through the calculations of the ground and triplet potential energy surfaces (PESs) at DFT and CIS levels. A simple model, based on the Sandros equation, was derived to include the influence of such torsional modes on the energy-transfer behavior. Finally, the experimental quenching data were fitted to this model, leading to the conclusion that, although both C−N double bond and C−C single bond torsions occur during the sensitization, the thermal activation of C−C single bond torsion plays the major role in the NVET character of oxime derivatives.