We developed an optical system to detect singlet oxygen produced by chlorin-ringed chlorophyll (Chl) species, i.e., Chl a, Chl b, Chl d, and di-vinyl-Chl a, with a high sensitivity and examined the relationship between molecular structures and reaction rates. Chl a in acetone was the lowest producer of singlet oxygen and the most effective quencher; in contrast, Chl b exhibited the opposite properties. These results showed that replacement of side chain(s) from a methyl group to a formyl group on the R7 position on a chlorin ring induced a higher production and lower quenching of singlet oxygen in Chl molecules. (C) 2009 Elsevier B. V. All rights reserved.