Photophysics and photochemistry ofo-xylene solutions III. Photoisomerization

Abstract The photoisomerization of o -xylene, in dilute hydrocarbon solutions, to the meta isomer, and to a slight extent to the para -form, has been studied as a function of temperature. The quantum yields are small, e . g . for formation of m -xylene Φ m = 2.5 × 10 −3 at 30°C and formation of p -xylene Φ p , is less than 10% of Φ m . Using measured values of the fluorescence lifetimes a value for the energy of activation for isomerization is obtained. The effects of added xenon and butene-2 show that the first excited state is involved in the isomerization. A transient diradical of the “pre-fulvene” or “Jano” type may give rise to benzvalene or produce direct isomerization. Isomerization appears to result from a vibrational level of S 1 higher than that which is involved in internal conversion. The mechanism of isomerization is discussed.