Lowest π, π* Triplet State of Acetophenone

Abstract

The characteristic short-lived n, π* phosphorescence from acetophenone in hydrocarbon glass is replaced by a long-lived (∼1 sec) phosphorescence in very polar and especially hydrogen-bonding solvents. From the absorption and emission spectra in various solvents together with the ESR spectrum of the phosphorescent state, it is possible to assign the latter as the perturbed 3La state. In nonpolar solvents this state lies between the 1n, π* and 3n, π* states. In all probability, intersystem crossing in acetophenone proceeds from the 1n, π* state to the 3La state. On the basis of lifetime data and polarization studies it is suggested that the 3La→1A transition derives intensity through both mechanisms shown below:[Complex chemical formula]