Abstract
The formation of so-called solar fuels from abundant low-energetic compounds, such as carbon dioxide or water, relies on the chemical elementary steps of photoinduced electron transfer and accumulation of multiple redox equivalents. The majority of molecular systems explored to date require sacrificial electron donors to accumulate multiple electrons on a single acceptor unit, but the use of high-energetic sacrificial redox reagents is unsustainable. In recent years, an increasing number of molecular compounds for reversible light-driven accumulation of redox equivalents that do not need sacrificial electron donors has been reported. Those compounds are the focus of this mini review. Different concepts, such as redox potential compression (achieved by proton-coupled electron transfer, Lewis acid-base interactions, or structural rearrangements), hybrids with inorganic nanoparticles, and diffusion-controlled multi-component systems, will be discussed. Newly developed strategies to outcompete unproductive reaction pathways in favor of desired photoproduct formation will be compared, and the importance of identifying reaction intermediates in the course of multiphotonic excitation by different time-resolved spectroscopic techniques will be discussed. The mechanistic insights gained from molecular donor-photosensitizer-acceptor compounds inform the design of next-generation charge accumulation systems for solar energy conversion.