Gold(II) Porphyrins in Photoinduced Electron Transfer Reactions

In the context of solar-to-chemical energy conversion, inspired by natural photosynthesis, the synthesis, electrochemical properties and photoinduced electron-transfer processes of three novel zinc(II)-gold(III) bis(porphyrin) dyads [Zn-II(P)-Au-III(P)](+) are presented (P: tetraaryl porphyrin). Time-resolved spectroscopic studies indicated ultrafast dynamics (kET1 >10(10) s(-1)) after visible-light excitation, which finally yielded a charge-shifted state [Zn-II(P+)-Au-II(P)](+) featuring a gold(II) center. The lifetime of this excited state is quite long due to a comparably slow charge recombination (kBET2 approximate to 3x10(8) s(-1)). The [Zn-II(P+)-Au-II(P)](+) charge-shifted state is reductively quenched by amines in bimolecular reactions, yielding the neutral zinc(II)-gold(II) bis(porphyrin) Zn-II(P)-Au-II(P). The electronic nature of this key gold(II) intermediate, prepared by chemical or photochemical reduction, is elucidated by UV/Vis, X-band EPR, gold L-3-edge X-ray absorption near edge structure (XANES) and paramagnetic H-1 NMR spectroscopy as well as by quantum chemical calculations. Finally, the gold(II) site in Zn-II(P)-Au-II(P) is thermodynamically and kinetically competent to reduce an aryl azide to the corresponding aryl amine, paving the way to catalytic applications of gold(III) porphyrins in photoredox catalysis involving the gold(III/II) redox couple.