Investigation of the Excited-State Electron Transfer and Cage Escape Yields Between Halides and a Fe(III) Photosensitizer

Abstract

Excited-state quenching and reduction of [Fe(phtmeimb) 2 ] + , where phtmeimb is phenyl[tris(3-methyl-imidazolin-2ylidene)]borate, with iodide, bromide, and chloride were studied in dichloromethane, acetonitrile, and acetonitrile/water 1:1 mixture by means of steady-state and time-resolved spectroscopic techniques. Quenching rate constants were almost diffusion-limited in dichloromethane and acetonitrile and followed the expected periodic trend, i.e., I − > Br − > Cl − . Confirmation of excited-state reductive electron transfer was only unambiguously obtained when iodide was used as a quencher. The cage escape yields, i.e., the separation of the geminate radical pair formed upon bimolecular excited-state electron transfer, were determined. These yields were larger in dichloromethane (0.079) than in acetonitrile (0.017), and no photoproduct could be observed in acetonitrile/water 1:1. This study further emphasizes that solvents with low dielectric constant are more suited for productive excited-state electron transfer using Fe(III) photosensitizers with 2 LMCT excited state.

Photoinduced One-Electron Chloride Oxidation in Water Using a Pentacationic Ir(III) Photosensitizer

“…These yields are low compared to values recorded in acetonitrile or other organic solvents for photosensitizers where X 2 •– was clearly observed, where values range 0.25–0.96. ,, The yields are however in line with the low cage-escape yields recently obtained with Ir­(III) binuclear complexes for halide oxidation in acetonitrile/water mixtures . Note that low cage-escape yields around 0.04 with observation of I 2 •– in acetonitrile have been reported using Ru­(II) and Fe­(III) photosensitizers. , …”