Determination of the Activation Barrier to Energy Transfer from 3 pi pi* to Charge Transfer Levels via Steady State and Transient Luminescence Measurements on Bis(4-Chlorothiophenol)(1,10-Phenanthroline)Zinc(II).

Recent studies on mixed ligand chelates of M(II)(BS)2(N-N) M(II) = Zn(II), Cd(II); BS = anion of a substituted benzenethiol; N-N = dinitrogen-heterocycle reveal the existence of multiple level emissions originating from different electronic configurations. The 6.5 K thermally modulated emission of Zn(4-Cl-PhS)2(phen) showed that the population of the emitting levels could be redistributed via heat pulses, a result that suggested an energy barrier between two accessible excited configurations. We report here an initial investigation of the temperature-dependence of both luminescence lifetimes and intensities of Zn(4-Cl-PhS)2(phen) in an attempt to measure the activation barrier between electronic configurations of dissimilar orbital parentages. The effect of temperature on the luminescence intensity and excited state lifetimes of the title compound is interpreted in terms of a barrier to the transfer of energy from the lowest 3 Pi Pi level to lower-lying charge-transfer levels. Fits of an Arrhenius expression to both lifetimes and relative intensities yield an activation energy and preexponential factor of approx. 140/cm and approx. 100,000/s, respectively.