Influence of the S0−T1 structural changes on the triplet–triplet sensitization of dienes

The triplet–triplet energy transfer properties of different dienes were examined through time-resolved laser spectroscopy, and the different structural changes between ground and triplet states were investigated by molecular modeling. It was found that these compounds exhibit very different behaviors depending on their molecular structure. The calculations of ground- and triplet-state potential energy surfaces (PESs) at the B3LYP/6-31+G(d) level clearly evidenced the crucial role of the thermal activation of a single bond torsion in order to decrease the energetic requirement for the transition. A recently developed model, that included the influence of such a torsional mode, was successfully used to support this hypothesis.