Concentration Increase of the Singlet-Oxygen Generation Quantum Yield by an Indotricarbocyanine Dye

Regularities in the generation of singlet oxygen sensitized by the indotricarbocyanine dye HITC with I- and ClO4 (-) anions in highly and slightly polar solvents were studied as functions of dye concentration (C-d). The yield of singletoxygen formation (gamma(Delta)) by HITC (I-) increased in slightly polar solvents as its concentration increased. It was shown that HITC with C-d < 10(-7) M in CHCl3 and < 5 center dot 10(-6) M in o-dichlorobenzene with any anion was found primarily in the form of free ions. It was established that the concentration increase of singlet-oxygen generation by HITC (I-) was due to the heavy-atom effect of the anion as a result of their existence as contact ion pairs and an increase of the fraction of such pairs in a solution with an increased photosensitizer concentration. The quantum yield of singletoxygen formation for HITC with ClO4 (-) counterion was stable in various solvents as C-d was varied over a wide range.