Structure dependence of electron spin polarization in Zn-porphyrin-quinone ensembles oriented in a liquid crystal

Photoinduced intramolecular electron transfer was studied by time-resolved electron paramagnetic resonance spectroscopy in two photosynthetic model systems oriented in a nematic liquid crystal. The donor-acceptor assemblies consisted of differently substituted zinc porphyrins covalently linked to a quinone with different spacers, i.e., triptycyl and cyclohexylene. The orientation of the guest molecules with respect to the director of the liquid crystal was determined by line shape analysis of their triplet spectra and by molecular modeling. It is demonstrated that the polarized pattern of the radical pair spectrum is sensitive mainly to the spacer structure, but to some extent also to the peripheral substituents.