Artificial Photosynthesis is a promising strategy for conversion of solar energy into chemical feedstocks. In this context, Dye-Sensitized Photoelectrochemical Cells (DS-PECs) have achieved a key role as devices able to drive solar light-promoted water splitting. Indeed, due to its abundance and low toxicity, water is identified as the most appealing substance to be exploited in large-scale devices.

The core target of these modular devices is the photo-driven reduction of naturally abundant water to H₂, the most appealing energy-dense vector. The Hydrogen Evolution Reaction (HER, E⁰_H+/H2 = 0 V vs the Reversible Hydrogen Electrode, RHE) is typically coupled to the oxidation of water to dioxygen, providing the reducing equivalents to feed H₂ fuel synthesis. A key design principle to maximize photon-to-fuel output therefore relies on effective water oxidation catalysts (WOCs), able to overcome the thermodynamically demanding (E⁰_O2/H2O = 1.23 V vs RHE) and kinetically limiting 4e^–/4H⁺ Oxygen Evolution Reaction (OER) from water, requiring competent management of proton-coupled electron transfer (PCET) steps to selectively funnel photogenerated holes into multi-electron WOC activation at low overpotentials.^[1,2]

The essential structure of DSPEC photoanodes is composed of a semiconductor, often a nanostructured n-type semiconducting metal oxide (SCO) porous film, interfaced with a transparent conducting oxide (TCO) and integrating a WOC. The SCO is a high-band gap semiconductor (typically SnO₂ or TiO₂), incorporating a molecular light harvesting unit (i.e., a dye) that extends its light absorption by direct or indirect excited state sensitization.^[2,3]

We studied photoactive hybrid dyads composed by organic dyes and Co₃O₄ nanoparticles as WOC, with the goal of applying them on meso-SnO₂ photoanodes. The dyes belong to the unique class of KuQuinones (KuQ), polyquinoid chromophores characterized by a wide absorption in the visible and a highly oxidizing excited state (E_{1*KuQ/KuQ•–} > 2 V vs RHE), able to manage PCET events.^[4–6] KuQ was bound through a phosphonate anchor to Co₃O₄^heptOH, 3 nm spherical particles stabilized by a 1-heptanol shell.^[7] Co₃O₄^heptOH particles, synthesized via a finely controllable organometallic approach, have been reported as a rare metal-free WOC in electro- and photocatalytic systems.^[8,9] The ability of the dyads to evolve O₂ under visible light irradiation was associated with fast singlet excited state (¹*KuQ) emission quenching via electron transfer to Co₃O₄, owing to the highly oxidizing character of ¹*KuQ and to the direct chemical bond between the dye and the WOC. The hybrid dyads were then studied under visible light irradiation upon deposition on meso-SnO₂ films. The photocurrent response was associated to selective O₂ evolution through generator-collector chronoamperometric experiments. Indeed, the hybrid nanomaterials have been proven competent in photoelectrochemical OER, with Faradaic efficiency (FE_O2) close to 90%. As a comparison, “unbound” photoanodes based on KuQ-sensitized SnO₂ and Co₃O₄^heptOH, lacking a direct dye-WOC interaction, displayed similar photocurrent densities, albeit with a poorly reproducible and lower (30–50%) FE_O2. Furthermore, the photoaction spectrum, reaching 0.44% incident photon-to-current conversion efficiency (IPCE) at 510 nm, confirmed the role of KuQ as light-harvesting molecular component.

Finally, ex-situ resonance Raman and X-ray Absorption Spectroscopy (XAS) experiments allowed to map the evolution of the hybrid dyads under photoelectrochemical stress. While an expected overall oxidation state increase of the Co₃O₄ catalytic units was observed, indicating Co^IIIOOH as steady-state intermediate in the OER, overall chemical stability of the dyads was also assessed. Remarkably, KuQ was proven unaffected with respect to phosphonate bond hydrolysis and oxidative damage to the chromophore core.

These findings support the applicability of the novel dyads to DS-PECs and reinforce the superior approach of managing the directionality of the electron transfer chain by means of chemical bonds as a key design principle for Artificial Photosynthesis.