CHALCOGENAPYRYLIUM DYES AS POTENTIAL PHOTOCHEMOTHERAPEUTIC AGENTS - SOLUTION STUDIES OF HEAVY-ATOM EFFECTS ON TRIPLET YIELDS, QUANTUM EFFICIENCIES OF SINGLET OXYGEN GENERATION, RATES OF REACTION WITH SINGLET OXYGEN, AND EMISSION QUANTUM YIELDS

Chalcogenapyrylium dyes of general structure 1 were examined in methanolic solution with respect to the generation of singlet oxygen. Heavy atom effects are pronounced with quantum yields of singlet oxygen generation [Φ(1O2)] increasing from 0.0004 for pyrylium dye (la) to 0.12 for tellurapyrylium dye (Ij). Oxygen concentration studies gave triplet yields (ΦT) of 0.18 for Ij and 0.11 for li and estimated triplet lifetimes (τT) of 0.3 and 0.8 μs, respectively, in methanol. Similar lifetimes were determined in water. Selenapyrylium dye (If) was found to have τT of > 10 μs such that ΦT and Φ(1O2)should be equivalent. Variations in rate constants for intersystem crossing were found to be the dominant factor in controlling triplet yields. Values of Φ(1O2)were found to be sensitive to methyl substitution in the hydrocarbon backbone. Increased steric interactions were found to lower values of Φ(1O2)in methyl-substituted dyes 2 and 4 relative to unsubstituted dyes 1 and 3. The same methyl-substituted compounds showed lower quantum yields for fluorescence, suggesting increased rates of radiationless decay in both S1 and T1 states. Tellurapyrylium dyes were much more reactive with singlet oxygen than the other chalcogenapyrylium dyes, giving products 5 derived from formal oxidative addition of hydrogen peroxide across tellurium. Singlet oxygen was determined to be the oxidant under photooxidation conditions of tellurapyrylium dyes based on the similarity of reactivities measured with rose bengal and methylene blue as singlet oxygen sensitizers, on increased rates of reaction in deuterated solvents, on much slower rates of reaction of hydrogen peroxide with tellurapyrylium dyes, and on the formation of different products upon the reaction of superoxide with tellurapyrylium dyes. © 1990, American Chemical Society. All rights reserved.