The comparison of several butadiyne and thiophene derivatives to 8-methoxypsoralen and methylene blue as singlet oxygen sensitizers

The conversion of adamantylidene adamantane into adamantanone was used to compare the relative singlet oxygen sensitizing ability of 8-methoxypsoralen (8-MOP) a-terthienyl (2,2': 5',2"-terthiophene), 2,5-diphenylthiophene, 5-phenyl-2,2'-bithiophene, 1,4-diphenylbutadiyne, l-phenyl-4-(2-thienyl) butadiyne, 2,5-di-(2-thienyl)- butadiyne, and l-phenyl-l,3,5-heptatriyne with that of methylene blue. Although the main mechanism of phototoxicity for 8-MOP is usually considered to be oxygen independent, this compound nevertheless was 5% as efficient as methylene blue in sensitizing oxygen in dichloromethane solution. The other compounds tested were 0.85 to 2.35 times more efficient than methylene blue. Both/J-carotene and p-benzo- quinone were very effective quenchers with all the sensitizers. In all the cases, p-benzoquinone was found to quench a reactive excited state rather than a radical intermediate. ^-Carotene, on the contrary, quenched the formation of singlet oxygen. A reinvestigation disclosed that the three 2,5-disubstituted thiophenes and their butadiyne congeners required the presence of oxygen in order to be phototoxic toward Saccharomyces cerevisiae.