Figure depicts the UV-Vis electronic absorption spectra of TiO2, CNx-TiO2, CoPOM-CNx-TiO2 and CoPOM-PEI-CNx-TiO2. All CNx-containing electrodes exhibit a significant red shift compared to the optical absorption of pristine anatase TiO2 (3.2 eV, ~ 390 nm) and CNx (2.9 eV , ~ 428 nm), which we ascribe to effective sensitization of TiO2 by CNx, including formation of a charge-transfer complex between CNx and TiO2, making thus possible also the direct optical electron transfer from the HOMO of CNx to the conduction band of TiO2. The optical absorption edge of the hybrid photoanodes (~ 2.6 eV, ~ 477 nm) determined from the Tauc plots (Supporting Information, Fig. ) is larger than the value typically obtained in our previous studies (~ 2.3-2.5 eV), which can be explained by the inherent limitations of the Tauc formalism as applied for bandgap determination of hybrid materials, and to the fact that in previous studies we determined the bandgap using the Kubelka-Munk function calculated from diffuse reflectance spectra of corresponding powders, while here we use absorptance data obtained from measurements on complete photoanodes. Notably, the change in electronic absorption properties upon the deposition of the CoPOM catalyst is negligible, which indicates that the parasitic light absorption by the CoPOM catalyst is very low. This clearly highlights the advantage of using the molecular CoPOM as compared to, for example, cobalt oxide catalysts that typically show significant light absorption in the visible due to their fully developed band structure and correspondingly low bandgap, blocking thus partially the visible light absorption by the light absorber. Figure : ATR-FTIR spectra of all CNx-containing electrodes and CoPOM powder (a); UV-Vis electronic absorption spectra of the photoanodes; Abs. = absorptance (b). The non-zero baseline can be ascribed to the differences in internal reflection and scattering at the FTO/TiO2 interface in the transmittance and reflectance measurements modes. The photoelectrocatalytic properties of our photoanodes were investigated under visible light (λ > 420 nm) using an appropriate cutoff-filter, in order to effectively shut off the intrinsic UV light absorption of TiO2. The porous TiO2 layer thus serves solely as an electron-collecting scaffold that transports electrons injected under visible light irradiation from CNX into TiO2 to the underlying FTO glass support, whereas the oxidizing equivalents (i.e., photoholes) photogenerated in CNx should be ideally channeled to the water-oxidizing CoPOM catalyst to drive dioxygen evolution from water. First, potential-dependent photocurrents (Figure ) were measured for all electrodes utilizing illumination by chopped visible light (λ > 420 nm, 150 mW cm -2 ) in borate electrolyte (pH 8). As expected, no photocurrents could be detected at the pristine TiO2 substrate since anatase TiO2 cannot be excited by visible light. The photocurrents recorded for the CNx-TiO2 electrode without any CoPOM catalyst are attributed to the photocorrosion processes presumably at the CNx/TiO2 interface as no oxygen evolution was detected at this electrode under identical experimental conditions (see Figure ). In contrast, the photocurrent values significantly increased in the presence of both CNx sensitizer and CoPOM cocatalyst. Importantly, the polymeric PEI linker-containing CoPOM-PEI-CNx-TiO2 photoanode showed the highest photocurrents within the whole potential range and also the highest monochromatic quantum efficiencies (IPCE) measured at a constant bias potential of 1.12 V vs. RHE (Figure ). Importantly, the CoPOM-PEI-CNx-TiO2 photoanode exhibited also the most negative photocurrent onset potential of 0.2 V vs. RHE, which clearly indicates an improved rectifying behavior due to more effective extraction of photogenerated holes from CNx. Figure shows photocurrent transients under the same visible light irradiation conditions at a constant potential of 0.78 V vs. RHE, and indicates a relatively good short-term stability of the photocurrent response. Interestingly, for the CoPOM-free electrode (black line), the current spikes after switching on the light and negative current overshoots appearing after the light is switched off become significantly more pronounced. Such spikes and overshoots are a typical fingerprint of intense surface recombination processes, indicating that, in the absence of water oxidation catalyst, the photoholes in CNx do not undergo the desired interfacial transfer, but instead accumulate in the CNx layer and subsequently either recombine or induce photocorrosion. In contrast, the current spikes are less pronounced and the overshoots are nearly absent in both CoPOM-containing electrodes, which again indicates that the CoPOM cocatalyst can extract holes generated in the CNx layer and trigger the desired water oxidation reaction.