Homogeneous photocatalytic H2 production using a RuII bathophenanthroline metal-to-ligand charge-transfer photosensitizer

Abstract

The prototypical [Ru(bpy)3]2+ (bpy=2,2′-bipyridine) photosensitizer has been previously demonstrated to be labile in aqueous photocatalytic solutions, especially in the presence of coordinating electron donors. Here, an alternative RuII molecular sensitizer, [Ru(dpp)3]2+ (dpp=4,7-diphenyl-1,10-phenanthroline or bathophenanthroline), is described, which is considerably more stable than its bpy congener, allowing enhanced photocatalysis metrics in conjunction with a cobalt glyoxime ([Co(dmgH)2pyCl], dmgH=dimethylglyoxime, py=pyridine) water reduction catalyst and N,N-dimethyl-p-toluidine (DMT) as the sacrificial donor in a 1:1 mixture of CH3CN/H2O. Photoluminescence studies revealed that DMT reductively quenches the excited state of [Ru(dpp)3]2+ with a bimolecular rate constant of kq=4.9×109 m−1 s−1. The rate constant measured for electron transfer from the reduced sensitizer to the [Co(dmgH)2pyCl] was found to be near the diffusion limit, kCo=2.4×109 m−1 s−1. H2 production by photocatalysis was independently monitored by using a high-throughput photochemical reactor equipped with pressure transducers, gas chromatogram, and a mass spectrometer for detection; this illustrated that the composition yields high turnover numbers (TONs), approaching 10 000 (H2/Ru) with respect to the sensitizer and deuteration studies using D2O confirmed that H2 is primarily produced from protons derived from water in these systems.