cyanine dyes , particularly indocyanine dyes , having at least 7 carbon atoms in the polymethine chain which may possess a fused cyclopentene or cyclohexene ring in the form of a tri - iodide salt exhibit a substantial reduction in the propensity to associate compared to dyes having the identical cation but different anion . the tri - iodide salts are particularly useful as sensitizers in organic photoconductor systems .