The 9-mesityl-10-methylacidinium cation (Acr+-Mes) was incorporated into Na+-exchanged mesoporous aluminosilicate (AlMCM-41) by cation exchange. Incorporation of Acr+-Mes into AlMCM-41 has made the lifetime of the electron-transfer (ET) state (Acr•-Mes•+) produced upon photoexcitation of Acr+-Mes extremely long (e.g., 10 s) even at high temperatures (e.g., 373 K). The formation and decay of the ET state can be repeated by on-off photoirradiation cycles. Both intra- and intermolecular back electron-transfer processes of the ET state of Acr+-Mes were observed by electron spin resonance (ESR) and time-resolved nanosecond diffuse reflectance absorption spectra, respectively. The ET state has both strong oxidizing and reducing abilities to be able to oxidize neighboring 1-pyrenemethylammonium and reduce the hexyl viologen dication incorporated into AlMCM-41 by cation exchange, respectively.