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In the field of direct production of hydrogen from solar energy and water, photocatalytic methods hold great potential especially when metal-free molecular components are preferred. In this work, we have developed a new class of calix[4]arene-based molecular photosensitizers to be used as antenna systems in the photocatalytic production of hydrogen. The structure of the dyes has a typical donor-?-acceptor molecular architecture where a calix[4]arene scaffold is used as an embedded donor. The new materials have been fully characterized in their optical, electrochemical, and photocatalytic properties. The properties conferred by the calix[4]arene donor afforded twice larger performances compared to the corresponding linear system though showing similar quantitative optical properties. The new molecular design paves the way to a new strategy for photocatalytic hydrogen production where the calix[4]arene scaffold can afford more efficient systems and can offer the potential for host-guest supramolecular effects.

The photoelectrochemical approach to photoinduced water splitting is gaining increasing interest for solar fuels generation. Dye-sensitized photoelectrochemical cells (DS-PEC) have been studied to improve the collection of the Vis range of solar radiation. Metal complexes have so far been the most investigated dyes but in recent years metal-free organic dyes have emerged as a new frontier for the evolution of the sector. This Minireview systematically describes the use of organic sensitizers and has been organized in terms of dye-sensitized photoactive electrode(s): photoanode; photocathode; tandem DS-PEC with photoanode and photocathode. The main classes of dyes are introduced and described in their key properties, device performance, and water splitting data. Thanks to the increasing number of articles, this review highlights the growing importance of organic sensitizers and their molecular design for a more efficient solar generation of clean fuels.

This review points out some of our recent advancements in dye-sensitized photocatalytic and photoelectrochemical hydrogen production. The dye sensitization of a semiconductor (SC) surface plays a central role. The dye has the task to improve the light harvesting of the system and guarantees a fast charge transfer to the SC, while avoiding charge recombination reactions that might reduce its efficiency. The introduction of different features and exploitation of molecular designs led to the development of twelve new dyes with a “push–pull” di-branched structure. The design of the molecular structure and geometry improves light harvesting, stability under long-term irradiation, and surface properties. The dyes have been investigated in a photocatalytic device, and then the most representative molecules have been studied also in photoelectrochemical cells (PEC), in the presence either of a sacrificial electron donor (SED), or of a water oxidation catalyst (WOC). The planar structure of the carbazole-based dye enhanced the photocatalytic hydrogen production activity almost tenfold compared to the phenothiazine derivative characterized by a butterfly-like structure; the latter was endowed with a better performance in the photoelectrochemical hydrogen production. Graphic abstract: [Figure not available: see fulltext.]