An aged Ru complex has been successfully employed for the photochemical monoreduction of NAD⁺ achieving mediator-free NADH formation with the only aid of l-cysteine and visible light.

The reversible and fatigue-resistant photoswitching of azobenzene chromophores is preserved at the surface of silicon nanocrystals; a photosensitized Z → E photoisomerization of azobenzene is observed upon selective excitation of the silicon core.

Photoredox catalysis often relies on excited-state quenching data to rationalize performance, yet such metrics can obscure the impact of solvent cage escape on overall efficiency. We report a systematic study of the effect of electrostatic interactions on the excited-state quenching, cage escape, and back-electron transfer processes in a benchmark system comprising methyl viologen (MV²⁺) and differently carboxylated ruthenium polypyridyl complexes with net charges from 2+ to 4–. Increasing electrostatic attraction between photosensitizer and MV²⁺ enhances the quenching rate constant (k _q ) up to the diffusion limit but simultaneously suppresses cage escape quantum yields, resulting in an inverse correlation between k _q and photochemical MV^•+ production. Transient absorption spectroscopy confirms that cage escape, rather than quenching or back-electron transfer, governs the quantum yield of product formation. Protonation of carboxylate groups to yield uniformly 2+ complexes equalizes quenching rates and substantially increases cage escape efficiency for the originally anionic species. These results establish electrostatic control of charge separation as a decisive factor in photoredox catalysis and challenge the practice of predicting yields solely from quenching experiments. Consideration of both the initial and post-electron-transfer charges of the photocatalyst/quencher pair emerges as a general design principle for maximizing cage escape and, consequently, photoredox reaction efficiency.

Photocatalytic H₂ generation holds promise in the green production of alternative fuels and valuable chemicals. Seeking alternative, cost-effective, stable, and possibly reusable catalysts represents a timeless challenge for scientists working in the field. Herein, commercial RuO₂ nanostructures were found to be a robust, versatile, and competitive catalyst in H₂ photoproduction in several conditions. We employed it in a classic three-component system and compared its activities with those of the widely used platinum nanoparticle catalyst. We observed a hydrogen evolution rate of 0.137 mol h^-1 g^-1 and an apparent quantum efficiency (AQE) of 6.8% in water using EDTA as an electron donor. Moreover, the favorable employment of l-cysteine as the electron source opens possibilities precluded to other noble metal catalyst. The versatility of the system has also been demonstrated in organic media with impressive H₂ production in acetonitrile. The robustness has been proved by the recovery of the catalyst by centrifugation and reusage alternatively in different media.

Solar-driven hydrogen production is a cornerstone of sustainable energy systems, yet its implementation remains intrinsically constrained by reliance on continuous illumination, limiting temporal control and compatibility with intermittent renewable sources. This perspective articulates the emerging concept of on-demand solar hydrogen generation, in which photon absorption is intentionally decoupled from hydrogen evolution through reversible charge storage and stimuli-responsive catalytic activation. We introduce a systematic classification of on-demand approaches across molecular, semiconductor, and device-level platforms, highlighting how these architectures enable programmable hydrogen release triggered by electrical, chemical, or thermal stimuli and sustained operation beyond illumination periods. Moving beyond a descriptive survey, we propose key performance metrics, including Switching Efficiency, Response Time, and Cycle Fidelity, to enable consistent evaluation and comparison of on-demand systems. Recent advances demonstrate substantial progress in charge storage, catalytic reversibility, and dynamic control, directly addressing the intermittency limitations of conventional photocatalytic and photoelectrochemical technologies. While challenges remain in kinetic synchronization, durability, and scalability, on-demand hydrogen concepts establish a coherent design framework for flexible and dispatchable solar fuels. By enabling integration with variable renewable inputs, this paradigm points toward adaptive and intelligent solar-fuel systems applicable from grid stabilization to off-grid and extraterrestrial environments.