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 [Zn^II (P)-Au^III (P)]⁺ are presented (P: tetraaryl porphyrin). Time-resolved spectroscopic studies indicated ultrafast dynamics (k ${}_{\mathrm{ET}{}_1}$ >10¹⁰ 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 (k ${}_{\mathrm{BET}{}_2}$ ≈3×10⁸ 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 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 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.

Keywords: electron transfer; gold; paramagnetic NMR spectroscopy; photoinduced electron transfer; porphyrins.