No abstract

The production of hydrogen from sunlight and water is finding an increasingly important role in the production of clean fuels from sustainable and abundant energy sources. In this process, commonly referred to as artificial photosynthesis, the role of the dye sensitizer is critical for optimizing the harvesting of visible light and triggering the reduction reaction at the catalytic active site. In recent decades organometallic sensitizers have mainly been studied, often requiring the use of scarce and, in some cases, toxic elements. This microreview describes the state of the art in the use of metal-free organic sensitizers, highlighting advantages over their organometallic counterparts. The main design and synthetic strategies, specific properties, and device performances are presented. Thanks to recent advances and lower manufacturing costs, organic sensitizers seem set to be of increasing importance for next-generation clean fuels

Dibranched donor-(π-acceptor)2 dyes, where phenothiazine is the donor core, cyanoacrylic acid is the acceptor/anchoring group, and π is represented by mono- and poly-cyclic simple and fused thiophene derivatives, were tested as photosensitizers in the photocatalytic production of H2 , in combination with a Pt/TiO2 catalyst. The optical and electrochemical properties of the dyes were investigated, showing that careful design of the thiophene-based π spacer afforded enhanced optical properties. In the H2 production over 20 h, the new thiophene-based sensitizers revealed improved stability after longer irradiation times and enhanced performances, in terms of H2 production rates and light-to-fuel efficiencies, after an initial activation period, which were for the first time associated with enhanced stability under photocatalytic production of H2 and the absence of critical dye degradation.

Dye‐sensitized solar cells (DSSCs) that use an aqueous (40 % w/w water content) choline chloride based deep eutectic solvent as an electrolyte medium have been investigated. The joint combination of the eutectic mixture and an appropriate hydrophilic sensitizer afforded a DSSC with a power conversion efficiency comparable to that using the same electrolyte composition but with conventional, toxic, and volatile solvents as the medium, which thereby paves the way to a new generation of eco‐friendly, nature‐inspired, low‐cost solar devices.

A novel heteroleptic ruthenium complex carrying a heteroaromatic-4,4'-pi-conjugated 2,2'-bipyridine [Ru(II)LL'(NCS)(2)] (L = 4,4'-bis[(E)-2-(3,4-ethylenedioxythien-2-yl)vinyl]-2,2'-bipyridine, L' = 4,4'-(dicarboxylic acid)-2,2'-bipyridine) was synthesized and used in dye-sensitized solar cells, yielding photovoltaic efficiencies of 9.1% under standard global AM 1.5 sunlight.

Multi‐branched multi‐anchoring metal‐free dyes as photosensitizers for dye‐sensitized solar cells (DSSCs) are reviewed. The article outlines design strategies, main synthetic routes and optical and photovoltaic properties of two families of multi‐branched sensitizers: (a) D–(π–A)n (D = donor, π = π‐spacer, A = acceptor/anchoring functionality) structures containing arylamine, carbazole, phenothiazine or phenoxazine derivatives as D groups, and (b) multi‐donor multi‐anchoring architectures from interconnected mono‐branched D–π–A arms, together with X‐ and Y‐shaped dyes. Although this class has been reported only in the last five years, a variety of molecular architectures, donors, and π‐spacers have been used and combined in multi‐branched molecules. The multi‐branched geometry induces distinctive features including enhanced qualitative and quantitative optical properties, increased currents and quantum efficiencies, and control of molecular aggregation.

A thiophene-based donor-acceptor phenothiazine dye has been functionalized with a peripheral glucose unit (PTZ-GLU) to bust its affinity to water and enhance dye-sensitized photogeneration of hydrogen. Compared to the corresponding alkyl derivative (PTZ-ALK), as well as the common hydrophilic triethylene glycol substitution (PTZ-TEG), the sugar derivative shows a lower contact angle; PTZ-GLU performed twice more efficient than PTZ-TEG in the photogeneration of hydrogen in terms of evolved gas and turnover number.

The first example of a heteroarylvinylene π-conjugated quaterpyridine Ru(II) sensitizer (N1044) was synthesized and used in dye-sensitized solar cells; the dye has an effective panchromatic absorption band, covering the entire visible spectrum up to the NIR region, and superior electrochemical characteristics (HOMO/LUMO and bandgap energies) with respect to previous representative Ru(II) bi- and quaterpyridine sensitizers. A record IPCE curve ranging from 360 to 920 nm has been measured with a maximum of 65% at 646 nm and still 33% efficiency at 800 nm; this leads to substantially increased photocurrent (19.2 mA cm(-2)) when compared to the prototype N719 Ru(II) sensitizer.