Photophysics and halide quenching of a cationic metalloporphyrin in water.

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.