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-functional ized zinc(II) porphyrin and a cytosine-functionalized dinitrobenzene are as sembled via base-pairing in two types of liquid crystal (LC). In the nemati c phase, selective photoexcitation of the zinc(II) porphyrin moiety yields a narrow derivative-like signal, which is not observed when the Watson-Cric k complementary dinitrobenzene unit is absent. The rise of the narrow signa l is accompanied by the decay of the broad one, which is ascribed to the lo west excited triplet state of the zinc(II) porphyrin. These findings are ra tionalized in terms of intraensemble electron transfer (ET) occurring from the lowest excited triplet state of the zinc(II) porphyrin donor to the din itrobenzene acceptor, with the narrow EPR signal being attributed to a long distance charge-separated species. The phase pattern of the derivative-lik e signal is reversed by substituting the LC with a positive magnetic anisot ropy (Delta chi > 0) for one with the opposite sign (Delta chi < 0). The ob served 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 sign al is assigned to a thermally populated SCRP.