Structural and environmental requirements for quenching of singlet oxygen by cyanine dyes

Singlet-oxygen quenching constants were measured for 19 cyanine dyes in ace tonitrile, The most efficient quenchers were 1-butyl-2-[2-[3-[(1-butyl-6-ch lorobenz[cd]indol-2(1H)-ylidene)ethylidene]-2-chloro-1-cyclohexen-1-yl]ethe nyl]-6-chlorobenz[cd]indolium and 6-chloro-2-[2-[3-(6-chloro-1-ethylbenz[cd ]indol-2(1H)-ylidene)ethylidene]-2-phenyl-1-cyclopenten-1-yl]ethenyl]-1-eth yl-benz[cd]indolium, having quenching constants with diffusion-controlled v alues of 2.0 +/- 0.1 x 10(10) and 1.5 +/- 0.1 x 10(10) M-1 s(-1), respectiv ely. There was a trend toward increased quenching constants for cyanine dye s with the absorption band maxima at longer wavelengths. However, the quenc hing constants correlated better with the oxidation potentials of the cyani ne dyes, suggesting that quenching proceeds by charge transfer rather than energy transfer. The quenching constants for 1,1',3,3,3',3'-hexamethylindot ricarbocyanine perchlorate and 1,1'-diethyl-4,4'-carbocyanine iodide were m easured in several solvents as well as in aqueous solutions of detergent mi celles. In different solvents, the quenching constants varied by as much as a factor of 50, The quenching constants were largest in solvents with the highest values on the pi* scale of Kamlet, Abboud, Abraham and Taft, This w as consistent with quenching occurring by charge transfer. Within cells, cy anine dyes concentrate in membrane-bound organelles. The quenching constant s were substantial within detergent micelles, To the extent that micelles a re models for biological membranes, cyanine dyes may be effective biologica l singlet-oxygen quenchers.