Synthesis and characterization of a trinuclear iridium(III) based catalyst for the photocatalytic reduction of CO2.

A trimetallic Ir(III) based complex (3) was synthesized and fully characterized by spectroscopic and electrochemical methods. A detailed comparison to its mono-(1) and bimetallic (2) analogue regarding the photocatalytic reduction of CO2 is outlined. In particular, the effect of intramolecular quenching, provided by ethyl tethers, was investigated. Moreover, the relationship between the photophysical properties, the lifetime of the excited state, the quenching efficiency and the catalytic performance is presented. Notably, the covalent linkage of the Ir(III) moieties within the three-armed ligand structure (complex 3) leads to a twofold increase of the turn over number (TON) compared to its monometallic analogue 1. Taking in account the quantum efficiency of 10% and the TONCO = 60 (per Ir(III) center), complex 3 is a highly active Ir(III) based photocatalyst.