Homogeneous Photocatalytic H 2 Production Using a Ru II Bathophenanthroline Metal‐to‐Ligand Charge‐Transfer Photosensitizer

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 Ru II 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) 2 pyCl], dmgH=dimethylglyoxime, py=pyridine) water reduction catalyst and N , N ‐dimethyl‐ p ‐toluidine (DMT) as the sacrificial donor in a 1:1 mixture of CH 3 CN/H 2 O. Photoluminescence studies revealed that DMT reductively quenches the excited state of [Ru(dpp) 3 ] 2+ with a bimolecular rate constant of k q =4.9×10 9   m −1  s −1 . The rate constant measured for electron transfer from the reduced sensitizer to the [Co(dmgH) 2 pyCl] was found to be near the diffusion limit, k Co =2.4×10 9   m −1  s −1 . H 2 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 (H 2 /Ru) with respect to the sensitizer and deuteration studies using D 2 O confirmed that H 2 is primarily produced from protons derived from water in these systems.