The looming energy crisis due to limited stocks of fossil fuels makes the search for environmentally clean and renewable energy and fuel sources indispensable. One approach to address these problems is to use photocatalytic systems, which harvest sunlight and split water, producing molecular oxygen and hydrogen. The latter can be either stored and utilized as a transportable fuel or converted into energy-rich organic molecules, to cope with the intermittent character of the solar radiation. The emerging concept of supramolecular photocatalysts (SPCs) utilizes strategies known from molecular design. An SPC is comprised of a photoactive center, where ultrafast photoinduced charge generation occurs, an intramolecular electron relay to transport the charge, and photoactivated catalytically active centers for water splitting. Additional light-harvesting antennas can be integrated into the molecular structure to enhance the absorption cross-section of the photocatalyst and to transfer energy to the photoactive primary electron donor. This design differs from the concept of quasi-homogeneous catalysis (which has been intensively studied over the last two decades), in which a photoactive primary electron donor, a molecular electron relay, and a colloidal catalyst are mixed in solution. Here, the electron-transfer processes, which are initialized by absorption of light, rely on collisions between the individual molecular functional units present in solution. As a consequence, principles of molecular design to improve the function-determining charge transfer processes cannot be applied readily. In SPCs, on the other hand, alterations of the molecular framework directly enable tuning its catalytic activity by (i) controlling the electron transfer from the photoactive electron donor to the catalytically active center, (ii) prohibiting the back-electron transfer, (iii) controlling the stability of the charge-separated state and (iv) protecting the ligand framework against decomposition pathways under both reductive/oxidative and acid/alkaline conditions.

Optimization of SPCs for water splitting requires characterization of the photoinduced processes by spectroscopic, spectroelectrochemical and theoretical means. Understanding the photoinduced reaction steps proceeding on an exceptionally broad femtosecond-to-minutes time scale and ultimately leading to the catalytically active species, requires interplay between spectroscopy and theory to derive correlations between the photoinduced steps, structural elements, their organization, the molecular environment, and catalytic activity. Design principles for SPCs must be optimized for rapid and efficient energy and electron transfer, based on inspiration both from theoretical simulations and nature: e.g. the high level of organization or the use of coherent energy transfer. To cope with these tasks, PERSPECT-H2O addresses the photophysics and photochemistry of SPCs aiming at a spectroscopy/theory-guided design of novel water splitting SPCs. The process towards such an ambitious paradigm presents a truly interdisciplinary task that requires collaboration over both discipline and national boundaries.