Quinones undergo a variety of photochemical reactions. Quinoline-5,8-diones are expected to have photochemical reactivity. I synthesized quinoline-5,8-dione and its 2-methyl derivative. They were found to react with a variety of aldehydes under irradiation. An addition product was isolated as a major product of the photochemical reaction. The reactivity of PQQ, a coenzyme of soluble glucose dehydrogenase, is increased by illumination. As suggested through the study of methanol dehydrogenase, Ca<sup>2+</sup> ion is expected to form a complex with PQQ at its 5-carbonyl, 6-nitrogen atom, and 7-carboxylic acid group. Quinoline-5,8-dione derivatives have been reported to complex at the 1-nitrogen atom, and 8-carbonyl group. Given quinoline-5,8-dione’s photochemical and metal ion binding potential a 2-carboxylic acid derivative of quinoline-5,8-dione is expected to be an effective model of PQQ.