Effects of paramagnetic lanthanide ions on decay process of photogenerated radical pairs in porphyrin-viologen linked compounds at reversed micellar surfaces

Radical pairs were generated in reversed micelles by laser excitation of zinc tetraphenylporphyrin-viologen linked compounds (ZP nV) with a polymethylene spacer (-(CH₂)_n-; n=4, 6 and 8). The optical transient-absorption and chemically induced dynamic electron polarization (CIDEP) spectra indicated that paramagnetic lanthanide ions invariably accelerated the decay process of the radical pairs at 0.3 T. The typical E/A/E/A spectra for the ZP6V and ZP8V systems were explained as due to S-T₀ mixing and electron-spin relaxation between Zeeman-splitted triplet sublevels. In the case of ZP4V, the paramagnetic Gd³⁺ ion accelerated the radical decay process even at zero-magnetic field, and strong emissive CIDEP spectra evolved with the elapsed time after laser excitation. Appreciable contribution of S-T level crossing in the radical decay process was suggested to account for the novel features of the ZP4V system.