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

Emission from cis -1-(2-anthryl)-2-phenylethene, c -APE*, in toluene is resolved into 1 t -APE B * and 1 c -APE* components at temperatures ranging between 4.3 and 59.3 °C. Decomposition of effective fluorescence quantum yields, φ fc, into pure component fluorescence quantum yields, φ ft -B and φ fc, shows that φ ft -B increases 24% with increasing temperature while φ fc decreases more than 3-fold over this temperature range. On the basis of the fraction of molecules that escape the 1 c -APE* potential energy minimum, 1 − φ fc, the efficiency of adiabatic formation of 1 t -APE B * 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 1 c -APE B * → 1 t -APE B * pathway. This torsional motion experiences a 4.4 4 ± 0.1 4 kcal/mol barrier probably located at the perpendicular, 3 p*, geometry. Since 12% of 1 t -APE B * intersystem cross to 3 t -APE B *, the known triplet state quantum chain process enhances photoisomerization quantum yields at higher [ c -APE]. Triplets formed directly from 1 c -APE* also contribute to this pathway. In air-saturated solutions, oxygen eliminates the quantum chain process by reducing the lifetime of 3 t -APE*. However, the quenching of 1 c -APE* by O 2 gives 3 c -APE*, thus enhancing photoisomerization quantum yields via rapid 3 c -APE* → 3 t -APE* adiabatic torsional displacement. No photoisomerization of 1 c -APE A * need be postulated to account for our observations. The enthalpy difference between ground state conformers, Δ H AB, favors c -APE B by 0.92 ± 0.02 kcal/mol.