Photophysics and halide quenching of a cationic metalloporphyrin in water

Abstract

In this work, the steady state S₀–S₁ and S₀–S₂ absorption and emission behaviour of the water-soluble tetrakis(N-methyl-4-pyridyl)porphyrin zinc(II) tetrachloride (ZnTMPyP) in media of constant and high ionic strength, both with and without iodide ions as a fluorescence quencher, was measured. The quenching of the ZnTMPyP S₁ state by iodide ions proceeds primarily through diffusion-limited interaction in an encounter pair but the formation of a loose association between the ZnTMPyP S₁ state and iodide ions also provides a minor quenching pathway. The ZnTMPyP S₂ state was quenched minimally by iodide, likely through an electron transfer mechanism at an average donor–acceptor distance of ∼0.7 nm. The results presented here highlight the notion that significant iodide quenching of the ZnTMPyP S₁ state can be a source of inefficiencies in porphyrin-based dye-sensitized solar cells. The minimal quenching of the S₂ state indicates that no significant loss of efficiency will be introduced by using iodide as component of the electrolyte system in upconversion by triplet–triplet annihilation (UC-TTA) enhanced solar cells.