EPR and ENDOR on Chlorin Anion and Cation Radicals: Electronic Structure of Models for Photosynthetic Electron Transfer

Porphyrins are widely used as electron donors in model systems for electron transfer in primary processes of photosynthesis. More recently, chlorins, whose π electron system is more similar to that of chlorophylls, have become available in model compounds. In this study, the electronic structure of 5,10,15,20-tetraphenylchlorinatozinc (ZnTPC) was investigated using electron paramagnetic resonance and electron nuclear double resonance. The orbitals involved in the charge separation and recombination process were studied by performing measurements on the anion and cation radical, respectively. The experimental results were compared with semi-empirical RHF-INDO/SP calculations. On the basis of the results, suggestions for the optimal design of model systems with long-lived charge-separated states are made. The influence of the linking position on the efficiency of charge separation is shown. Ratios of 30:1 are calculated for the efficiencies expected for different linking positions.