Photoinduced Electron Transfer in Porphyrin- and Fullerene/Porphyrin-Based Rotaxanes as Studied by Time-Resolved EPR Spectroscopy

Photoinduced intramolecular electron-transfer (ET) and energy-transfer (EnT) processes in two rotaxanes, one containing both zinc porphyrin and C-60 fullerene moieties incorporated around the Cu(I) bisphenanthroline core [(ZnP)(2)-Cu(I)(phen)(2)-C-60] and a second complex lacking the fullerene [(ZnP)(2)-Cu(I)(phen)(2)], were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz (X-band) combined with a selective photoexcitation of the rotaxane moieties. The experiments were carried out in isotropic toluene and ethanol and in anisotropic. nematic liquid-crystal (E-7) media over a wide range of temperatures corresponding to the different states of the solvents. The TREPR results are compared with those obtained previously by optical methods in dichloromethane at room temperature. It is demonstrated that the efficiencies and pathways of the light-driven ET and EnT processes in both rotaxanes strongly depend on the properties of their microenvironment, resulting in the formation of distinct charge-separated states under different experimental conditions. The complementary results revealed by the optical and TREPR techniques are attributed to the relatively high conformational mobility of the mechanically interlocked rotaxane systems. Because of the solute-solvent interactions, the rotaxanes are able to change conformation in different microenvironments, which affects the parameters of the photoinduced processes occurring in these systems.