Time‐Resolved Photoacoustic Calorimetry of Tri‐ and Tetraphenylethylenes. Do S₀ and T₁ Energy Surfaces Cross?*

Abstract

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 TI energy surfaces of arylalkenes do not always cross.

Characteristics of the Electronic Structures of Diabatically and Adiabatically Z/E-Isomerizing Olefins in the T1 State

“…It is not surprising that 1,1,2,2-tetraphenyl ethylene has a very curved T 1 PES that only admits diabatic Z/E-isomerizations. 40 The data from 1-4 clarifies that phenyl substitution on its own cannot bring an adiabatically Z/E-isomerizing olefin.…”

“…Thus, 1,1-diphenyl substitution does not lead to more shallow T 1 PES because of the attenuated delocalization of the triplet biradical from the isomerizing CC bond. It is not surprising that 1,1,2,2-tetraphenyl ethylene has a very curved T 1 PES that only admits diabatic Z/E -isomerizations . The data from 1 − 4 clarifies that phenyl substitution on its own cannot bring an adiabatically Z/E -isomerizing olefin.…”