Spotlight collection on photoinduced redox chemistry

We are delighted to invite the readers of Dalton Transactions to explore the themed spotlight collection on photoinduced redox chemistry. Many classes of inorganic complexes and materials can participate in photoinduced redox chemistry, in which absorption of a photon generates an excited state that then undergoes an electron-transfer event with a redox partner. These compounds span a large portion of the periodic table and include several distinct types of materials, including transition-metal and f-element coordination compounds, organometallic complexes, MOFs, nanomaterials, and extended inorganic solids.

Fundamental studies of the thermodynamics and kinetics of these excited-state redox processes remain important, as they continue to reveal key insights into how ligand design, electron configuration, molecular structure, crystal structure, environment and composition affect the excited-state redox chemistry of these many categories of inorganic compounds. Moreover, photoinduced charge-transport processes involving inorganic compounds are important elementary steps in several applications, including but not limited to solar fuels, organic photoredox catalysis, dye-sensitised solar cells, and photodynamic therapy.

This spotlight collection covers various aspects of photoinduced redox chemistry in inorganic compounds, including excited-state redox processes involving new inorganic materials, the development of novel systems for studying and optimizing these processes, and studies using known compounds for applications related to photoinduced charge transport, highlighting the important roles that existing inorganic compounds can play in these areas. This collection will showcase the combined roles that synthetic and physical inorganic chemistry, including time-resolved spectroscopy and computational studies, play in driving fundamental and applied research in this continually evolving field of research.

Kromer, Castellano and co-workers report fundamental photophysical and excited state properties of dinuclear platinum(II) 8-hydroxyquinoline complexes, a class of complexes of potential interest for light harvesting applications (https://doi.org/10.1039/D3DT00348E). The authors report that, contrary to previously reported dinuclear platinum(II) complexes, which exhibit triplet metal–metal-to-ligand charge transfer (³MMLCT) excited-state character, the studied complexes instead form monomer-like triplet ligand-centred (³LC) excited states centred on the bridging hydroxyquinoline ligands, which persist on the microsecond timescale as determined from ns–μs transient absorption spectroscopy.

Polyoxometalates are an important class of polynuclear redox active species capable of participating in photoinduced electron-transfer processes. The controlled self-assembly of these materials is therefore of significant interest for the development of hybrid photofunctional assemblies. In this regard, Repp and co-workers have reported on template-directed controlled self-assembly of polyoxovanadates through the interplay of halides or oxoanions with protonated organic cyclen templates (https://doi.org/10.1039/D3DT00252G).

Fullerenes and their derivatives have important potential applications in light-harvesting and photovoltaic applications. Stasyuk and co-workers have explored photoinduced charge separation in fullerene : phosphangulene adducts through time-dependent density functional theory methods (https://doi.org/10.1039/D1DT02034J). They show that changing the oxygen bridge atoms to sulfur between the aryl rings makes charge separation more thermodynamically favourable through destabilisation of the phosphangulene’s highest occupied molecular orbital.

Interfacial charge separation is a fundamentally important process in dye-sensitised photovoltaics. The past two decades have seen significant efforts in the design of novel molecular chromophores for anchoring to mineral surfaces for application in solar cells of this type. Stagni and co-workers report on their recent work on ruthenium(II) tetrazolato complexes as thiocyanate-free solar cell sensitisers (https://doi.org/10.1039/D0DT02621B).

The modular design of metal-based chromophore complexes offers significant scope in the design of choromophoric complexes for phototherapeutic applications, with photoinduced electron transfer reactivity being essential for type I photodynamic therapy (PDT). This is essential for hypoxic tumor environments, where low oxygen concentrations preclude singlet-oxygen-sensitised type II PDT. Liu and Zhao and their team describe the rational design of ruthenium(II) triarylamine-appended photosensitisers and evaluate their PDT activity under hypoxic conditions (https://doi.org/10.1039/D0DT01684E). Zafon and co-workers report their investigation of an iridium(III) complex photosensitiser and discuss the crucial roles played by excited states in the phototherapeutic activity (https://doi.org/10.1039/D1DT03080A). Their complex was shown to mediate photoinduced oxidation of NADH in a mitochondria-targeted mechanism, which acts to greatly inhibit the replication ability of cancer cells. Wang and Zhou report a ruthenium(II) bis-biquinoline complex with a nitroanthraquinone-appended bpy ligand, which is also able to mediate NADH oxidation and leads to enhanced photocytotoxicity in cancer cell lines and 3-D multicellular spheroids (https://doi.org/10.1039/D1DT01088C). The group of Rosenthal report their work on the photophysical and electrochemical properties of palladium(II) biladiene complexes with relevance to PDT applications (https://doi.org/10.1039/D3DT00691C).

The use of metal-complex chromophores as sensitisers and mediators of photocatalytic transformations has become a highly impactful tool in synthetic organic chemistry. Catalytic photoredox mediators can facilitate efficient, cleaner, and more sustainable methods for synthetic transformations that are far more difficult than through conventional thermally driven catalysis. Schelter and co-workers report dinuclear cerium(III) cyclooctatetrenyl complexes, which exhibit long luminescence lifetimes (τ ∼ 150–200 ns) and are potent photoreductants capable of mediating C–C coupling through halogen-atom abstraction using benzyl chloride in model reactions (https://doi.org/10.1039/D3DT00351E). Gualandi and co-workers report iridium(III) biscyclometalated chromophores as dyads with MacMillan imidazoline-4-one or Maruoka triphenylmethylpyrrolidine-based organocatalysts coupled through a 1,2,3-triazole linker (https://doi.org/10.1039/D0DT02587A). MacMillan conjugated dyads are shown to be active in photocatalytic enantioselective alkylation of aldehydes. Liang and co-workers report theoretical mechanistic studies on iridium(III) photosensitised, copper-catalysed asymmetric radical decarboxylative cyanation, in which photosensitisation occurs through single-electron transfer from the photosensitiser to the copper-based catalyst (https://doi.org/10.1039/D0DT02630A). Moving beyond transition-metal complexes, McCormick and co-workers report their studies on the aerobic photoredox reactivity of main-group-derived tellurorhodamine photocatalyst dyes (https://doi.org/10.1039/D2DT03534K). Metal–organic-frameworks (MOFs) composed of metal-complex chromophore building blocks also have potential for photocatalytic applications. Li and co-workers describe recent work in their Perspective article (https://doi.org/10.1039/D0DT02143A) on the investigation of photoinduced charge-transfer reactions in MOFs, relevant not only to photocatalysis, but also luminescent sensing applications.

Photoredox catalysis is also of significant interest in combatting anthropogenic climate change. Whilst the reduction of CO₂ in plants in the Calvin cycle is not a photochemical process, the light-driven photooxidation of water to form O₂ in biological photosynthesis has inspired considerable efforts in metal-complex-mediated photoinduced reduction of CO₂ to useful chemical feedstocks. Such efforts commonly target carbonyl complexes of rhenium and manganese. In this collection, Case and co-workers describe the visible-light-induced CO₂-reduction catalysis of a rhenium(I) tricarbonyl complex bearing a phenanthroline–naphthalimide dyad ligand (https://doi.org/10.1039/D0DT04116E) whilst Tsipis and Sarantou report their computational studies on the crucial role of triethanolamine as the sacrificial electron donor in rhenium(I) CO₂-reduction photocatalysis (https://doi.org/10.1039/D1DT02188E). The group of Weinstein reported their investigations on CO₂ reduction by sterically bulky rhenium(I) and manganese(I) complexes and reveal that one of the manganese complexes mediates photoinduced catalysis in conjunction with a zinc porphyrin photosensitiser (https://doi.org/10.1039/D0DT00252F). Whilst the nature of the photocatalyst itself is important in determining the efficiency of photoinduced reduction of CO₂, solvent effects are also known to play a key role. Asai and co-workers report results on the effect of the structure of a range of ionic liquids used as solvents for the light-driven reduction of CO₂ by a rhenium(I) catalyst with photosensitisation by an iridium(III) complex (https://doi.org/10.1039/C9DT04689E). Moving beyond rhenium, Azam and co-workers describe a uranium(VI) salen complex and its reactivity for photochemical CO₂ reduction (https://doi.org/10.1039/D0DT02620D).

Photoredox catalysis for the production of solar fuels is an area of significant importance for efforts to reduce reliance on fossil fuels. Enormous attention has therefore been focused on the use of metal complexes for photocatalytic hydrogen production. Scalambra, Díaz-Ortega and Romerosa discuss recent progress on heterodinuclear photocatalytic assemblies in this area in their Frontier review article (https://doi.org/10.1039/D2DT01870E). In their contribution, Hanan and his group report a dinuclear ruthenium(II)-based complex with a dipyridylpyrimidine-based bridging ligand and its photosensitization of photocatalytic hydrogen production (https://doi.org/10.1039/D1DT00868D). Kojima and co-workers present intriguing results on a mononuclear ruthenium(II) complex bearing a tetrapyridophenazene (tpphz) ligand, which upon irradiation in the presence of an electron donor in water : methanol results in reduction of the central pyrazine moiety of the tpphz ligand to an N,N-dihydropyrazine (https://doi.org/10.1039/D0DT03546G). Under irradiation, the complex photocatalyzes production of hydrogen and the hydrogenation of organic substrates. Bernhard and his group report nickel(II) dithiolate–polyamine-based complexes with detailed structural studies, Hirschfield surface analysis and evaluation of photocatalytic hydrogen production (https://doi.org/10.1039/D1DT00352F). Hernández-Valdéz and co-workers report their investigations of highly robust rhenium(I) bisarene sandwich complexes, which can be derivatised to incorporate pendant oligopyridyl ligands. Coordination of Co(II) to these pendant ligand motifs results in heterodinuclear complexes that are competent hydrogen evolution photocatalysts (https://doi.org/10.1039/D0DT00731E). Key to the function of many photocatalytic hydrogen evolution systems is the formation of metal-hydride complexes. Dempsey and co-workers describe the photochemical formation of a tungsten hydride complex through photolysis of a dinuclear ditungsten(I) precursor (https://doi.org/10.1039/D2DT03675D). Visible light irradiation results in cleavage of the precursor W–W bond followed by disproportionation to form mononuclear W(II) and W(0) species, the latter of which reacts further to form a hydride complex product.

This spotlight collection highlights the continuing role and importance that inorganic chemistry plays in photoredox chemistry. The array of articles represented here clearly demonstrate the continued health and vitality in this area as well as the multidisciplinary impact that workers in this area make. The work in this spotlight collection gives us confidence that inorganic chemists will continue to make important contributions to the application of photoredox processes across areas of solar energy harvesting and conversion, photosensitised organic synthesis, and phototherapeutics.

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