Intramolecular photoinduced electron transfer in a hydrogen bonded zinc(II) porphyrin-dinitrobenzene complex by time-resolved electron paramagnetic resonance spectroscopy

A time-resolved electron paramagnetic resonance (TREPR) study was performed on a hydrogen bonded donor-acceptor complex, in which a guanine-functionalized zinc(II) porphyrin and a cytosine-functionalized dinitrobenzene are assembled via base-pairing in two types of liquid crystal (LC). In the nematic phase, selective photoexcitation of the zinc(II) porphyrin moiety yields a narrow derivative-like signal, which is not observed when the Watson-Crick complementary dinitrobenzene unit is absent. The rise of the narrow signal is accompanied by the decay of the broad one, which is ascribed to the lowest excited triplet state of the zinc(II) porphyrin. These findings are rationalized in terms of intraensemble electron transfer (ET) occurring from the lowest excited triplet state of the zinc(II) porphyrin donor to the dinitrobenzene acceptor, with the narrow EPR signal being attributed to a long distance charge-separated species. The phase pattern of the derivative-like signal is reversed by substituting the LC with a positive magnetic anisotropy (Delta chi > 0) for one with the opposite sign (Delta chi < 0). The observed narrow signal is assigned to a spin correlated radical pair (SCRP). In the isotropic phase at higher temperatures, a narrow, net absorptive EPR signal is observed regardless of the type of LC employed. This latter signal is assigned to a thermally populated SCRP.