A free-base cofacial bisporphyrin, H(4)DPOx, forms a pi-complex with acridinium ion (AcH+) by pi-pi interaction in benzonitrile (PhCN). Formation of the H(4)DPOx-AcH+ pi-complex was probed by UV-vis and NMR spectroscopy. The binding constant between AcH+ and H(4)DPOx is determined as 9.7 x 10(4) M-1. Photoinduced electron transfer (ET) from the H(4)DPOx to the AcH+ moiety occurred efficiently in the d-complex to form the ET state (H(4)DPOx(center dot+)-AcH center dot). The ET state is successfully detected by laser flash photolysis. The lifetime of the ET state is 18 mu s in PhCN at 298 K, and the quantum yield of the ET state is 90%. The temperature dependence of the ET state lifetime has been examined in the range from 273 to 353 K. The ET state lifetime exhibited a large temperature dependence, and the linear plot of In(k(BET) T-1/2) vs T-1, in accordance with the Marcus equation, affords the ET reorganization energy (0.54 eV). As a result, a remarkably long-lived ET state has been attained at low temperature, and virtually no decay of the ET state was observed at 77 K. Such an extremely long-lived ET state is indeed detected by steadystate UV-vis absorption spectroscopy.