Temperature Dependence of the Photoisomerization of cis-1-(2-Anthryl)-2-phenylethene. Conformer-Specificity, Torsional Energetics and Mechanism

Abstract

Emission from cis-1-(2-anthryl)-2-phenylethene, c-APE*, in toluene is resolved into 1t-APEB* and 1c-APE* components at temperatures ranging between 4.3 and 59.3 °C. Decomposition of effective fluorescence quantum yields, φfc, into pure component fluorescence quantum yields, φft-B and φfc, shows that φft-B increases 24% with increasing temperature while φfc decreases more than 3-fold over this temperature range. On the basis of the fraction of molecules that escape the 1c-APE* potential energy minimum, 1 − φfc, the efficiency of adiabatic formation of 1t-APEB* remains remarkably temperature independent at 50.5 ± 0.7%. These results, together with photoisomerization quantum yields as a function of [c-APE] in degassed and air-saturated toluene, reveal a detailed photoisomerization mechanism. At infinite dilution and in the absence of molecular oxygen, photoisomerization of c-APE occurs predominantly via the adiabatic, conformer-specific 1c-APEB* → 1t-APEB* pathway. This torsional motion experiences a 4.44 ± ...