A series of new Ruthenium(II) complexes based on the (1,10phenanthrolin-5-yl)diethyl phosphonate ligand L2 and their corresponding phosphonic acid along with reference compounds are synthesized in order to evaluate the influence of the phosphonate substitution on the catalytic performance of such molecules. UV-vis absorption and emission spectroscopy show, that introduction of a phosphonate moiety into the periphery of the phenanthroline ligand does not alter the photophysical properties of the complex. In contrast, the electrochemical features of all compounds are affected upon respective functionalization compared to the reference molecule [Ru(bpy) 3 ](PF 6 ) 2 in aqueous medium. Photostability tests in water or in water/persulfate show a dependency of the stability towards photodecomposition on quenching efficiency with persulfate anion. The lower quenching efficiency thus leads to increased stability based on the negative charge of the phosphonate moiety in 2 and P1 in aqueous medium following a quenching efficiency trend of [Ru(bpy) 3 ](PF 6 ) 2 > 1 > 2 > P1. Photocatalytic water oxidation using [Ru(dpp)(pic) 2 ](PF 6 ) 2 reveals that compound 1 and [Ru(bpy) 3 ](PF 6 ) 2 doe not exhibit any activity under the utilized conditions, P1 shows a negligible TON of 7, whereas compound 2 reaches a TON of 140 after 1 h. This fact highlights that the redox potential is not the sole driver in the catalytic cycle and accentuates the importance of different criteria determining the suitability of photosensitizers in light-driven water oxidation, such as light absorption, redox potential of the Ru III /Ru II event, photostability towards and quenching efficiency with the sacrificial agent.

The synthesis of a new Ru II -based water oxidation catalyst is presented, in which a nitrophenyl group is introduced into the backbone of dpp via a pH-sensitive imidazole bridge (dpp = 2,9-di-(2′-pyridyl)-1,10-phenanthroline). This modification had a pronounced effect on the photophysical properties and led to the appearance of a significant absorption band around 441 nm in the UV–vis spectrum upon formation of the monoprotonated species under neutral conditions. Theoretical investigations could show that the main contributions to this band arise from transitions involving the imidazole and nitrophenyl motif, allowing us to determine the p K a value (6.8 ± 0.1) of the corresponding, twofold protonated conjugated acid. In contrast, the influence of the nitrophenyl group on the electrochemical properties of the catalytic center was negligible. Likewise, the catalytic performance of Ru(dppip-NO 2 ) and its parent complex Ru(dpp) was comparable over the entire investigated pH range (dppip-NO 2 = 2-(4-nitrophenyl)-6,9-di(pyridin-2-yl)-1 H -imidazo[4,5- f ][1,10]phenanthroline). This allowed the original catalytic properties to be retained while additionally featuring a functionalized ligand scaffold, which provides further modification opportunities as well as the ability to report the pH of the catalytic solution via UV–vis spectroscopy.