FT-ESR Investigation of Photo-Induced Charge Transfer in the ZnTeraphenylporhyrin/Duroquinone System

Abstract

Photochemical reactions in liquids inevitably lead to anamalous populations of electron spin sublevels of radical intermediates. This transient deviation of populations from Boltzmann values can be described very well by invoking the Radical Pair (RPM) and Triplet Mechanism (TM), which predict diagonal elements of the spin density matrix in terms of radical parameters (Δg, am_I). In recent publications /l, 2/ it was demonstrated that FT-ESR is particularly suited to explore the short time behaviour of this transient polarization, resulting from the fact that radicals can be time-labeled with 10 ns precision and subsequently observed via their FID, which is governed by the unperturbed spin hamiltonian.

In addition to the order of magnitude improvement in time resolution as compared to transient nutation experiments, the high phase resolution of the apparatus allowed unambigously the observation of dispersive signal components, originating from the time dependence of spin hamiltonian parameters during the short life time of the radical pair intermediate. The build-up and decay constants of the radical pair concentration could be measured for the first time with a time resolution of 10 ns and are discussed in terms of an attractive Coulomb potential. This attractive potential leads to a noticable delay in the cage escape of the radical pair constituents. In ethanol at 240 K the decay rate was found to be 3 10⁶ s^-1 only. The observed temperature dependence of the thermally activated detrapping was in good agreement with a recent theoretical model /3/.