Compounds represented by general formula (I) or salts thereof, wherein R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 each independently represents hydrogen, halogeno, C 1-6 alkyl, or C 1-6 alkoxyl; R 7 and R 8 each independently represents C 1-6 alkyl or optionally substituted aryl; R 9 and R 10 each independently represents hydrogen, C 1-6 alkyl, or C 1-6 alkoxyl; and R 11 represents hydrogen or C 1-12 alkanoyL The compounds are substantially nonfluorescent and react with singlet oxygen under physiological conditions to give a fluorescent substance. They are hence useful as a reagent for singlet oxygen determination.