Singlet Oxygen Production in Water: Aggregation and Charge-Transfer Effects

Abstract

Precise quantum yields for generation of singlet molecular oxygen, O2(1Δg), have been measured for tetrakis(4-sulfonatophenyl)porphyrin (TSPP), closely-related water-soluble porphyrin derivatives, and tris(2,2‘-bipyridyl)ruthenium(II) (bpy3Ru2+) in O2-saturated water. Under the experimental conditions, TSPP is present in an aggregated state, but the measured quantum yield for formation of O2(1Δg), ΦΔ = 0.51, remains on the same order as that of the corresponding monomer in methanol, ΦΔ = 0.70. Comparison with the quantum yield for formation of the triplet state indicates that quenching of the triplet by O2 gives O2(1Δg) with an efficiency of ca. 80%. There is a small but significant increase in ΦΔ when D2O is used in place of H2O. For the other porphyrin derivatives, the ability of the aggregate to sensitize formation of O2(1Δg) appears to depend on the total electronic charge resident on the molecule, decreasing with decreasing negative charge. A strong solvent dependence noted for O2(1Δg) production with (bpy3Ru2+) is explained in terms of competition between triplet energy transfer and photoinduced electron (or partial charge) transfer. Similar behavior could be responsible for the relatively low ΦΔ value observed for TSPP.