Diminished photoisomerization of active ruthenium water oxidation catalyst by anchoring to metal oxide electrodes

Surface-binding of molecular water oxidation catalysts through phosphonated ligands offers a promising strategy for attaching homogeneous catalysts onto conductive or semiconductive oxide surfaces for heterogeneous catalysis. In this work, the highly active [Ru(tpy)(pynap)OH2]2+ (tpy = 2,2′:6′,2″-terpyridine; pynap = 2-(pyrid-2′-yl)-1,8-naphthyridine) water oxidation catalyst is attached onto metal oxide electrodes through a phosphate group. Electrochemical and photoelectrochemical results confirm that ruthenium oxidation chemistries and water oxidation proficiency remain largely unaffected by phosphonation. Surface-binding reveals minimal photoisomerization of the active d-form and allows us to evaluate photoelectrochemical and mechanistic properties of the catalyst. Spectroelectrochemical experiments support the evolution of multiple ruthenium oxidation states in agreement with Pourbaix diagrams. Although photoisomerization of d-[Ru(H2PO3-tpy)(pynap)OH2]2+ is considerably hindered when the catalyst is attached onto a rigid oxide electrode, surface desorption remains a major challenge.