Time-resolved photoacoustic calorimetry of tri- and tetraphenylethylenes. Do S-0 and T-1 energy surfaces cross?

We have studied triphenylethylene, tetraphenylethylene and one derivative of the latter employing time-resolved photoacoustic calorimetry (PAC). We report relaxed triplet energies and triplet lifetimes. These experiments show that a fourth independent parameter can be extracted from PAC data by appropriate experimental design. The results with tetraphenylethylene are compared to data for thermal geometric isomerization of substituted tetraphenylethylenes. The comparison shows that So and T-1 energy surfaces of arylalkenes do not always cross.