A free-base cofacial bisporphyrin, H4DPOx, forms a π-complex with acridinium ion (AcH+) by π-π interaction in benzonitrile (PhCN). Formation of the H4DPOx-AcH+ π-complex was probed by UV-vis and NMR spectroscopy. The binding constant between AcH + and H4DPOx is determined as 9.7 × 104 M-1. Photoinduced electron transfer (ET) from the H4DPOx to the AcH+ moiety occurred efficiently in the π-complex to form the ET state (H4DPOx.+-AcH.). The ET state is successfully detected by laser flash photolysis. The lifetime of the ET state is 18 μs 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(kBETT1/2) 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 steady-state UV-vis absorption spectroscopy.