Gold(II) Porphyrins in Photoinduced Electron Transfer Reactions.

Abstract

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 [ZnII (P)-AuIII (P)]+ are presented (P: tetraaryl porphyrin). Time-resolved spectroscopic studies indicated ultrafast dynamics (k >1010 s-1 ) after visible-light excitation, which finally yielded a charge-shifted state [ZnII (P⋅+ )-AuII (P)]+ featuring a gold(II) center. The lifetime of this excited state is quite long due to a comparably slow charge recombination (k ≈3×108 s-1 ). The [ZnII (P⋅+ )-AuII (P)]+ charge-shifted state is reductively quenched by amines in bimolecular reactions, yielding the neutral zinc(II)-gold(II) bis(porphyrin) ZnII (P)-AuII (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 L3 -edge X-ray absorption near edge structure (XANES) and paramagnetic 1 H NMR spectroscopy as well as by quantum chemical calculations. Finally, the gold(II) site in ZnII (P)-AuII (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.