An electron donor-acceptor dyad is provided that can provide a charge-separated state with longevity and not only high oxidizing power but also high reducing power. A compound of the present invention is a quinolinium ion derivative represented by the following formula (I), a stereoisomer or tautomer thereof, or a salt thereof: ##STR00001## where R.sup.1 is a hydrogen atom or an alkyl group, and Ar.sup.1 to Ar.sup.3 each are a hydrogen atom or an electron-donating group. The compound of the present invention has the above-mentioned structure and therefore can provide a charge-separated state with longevity and not only high oxidizing power but also high reducing power and can be used for various products such as photocatalysts, photosensitizers, dyes, oxidants, reductants, dye-sensitized solar cells, and organic EL devices.