Introduction of the year 2022

The present volume is the 51st in the series ‘Photochemistry’ of the Specialist Periodical Reports (SPR) published by the Royal Society of Chemistry and, according to a structure well established since vol. 47, it consists of three different sections.

The first part of the book includes a series of critical reviews on the recent advances in the field of inorganic photochemistry, time-resolved spectroscopy and environmental photocatalysis, including the photophysics of transition metal complexes (V. Amendola, University of Pavia), the investigation of light-driven processes in cryogenic matrices (R. Fausto, University of Coimbra), the application of time-resolved infrared (A. Mezzetti, Sorbonne University, Paris) and UV (M. Curti, Leibniz Universität Hannover) spectroscopy in the elucidation of photochemical processes, and the development of photocatalytic systems for water splitting and hydrogen production (S. Ghosh, University of Paris-Sud). The other critical reviews included are focused on the behaviour and the application of coloured molecules as dyes (S. Seixas de Melo, University of Coimbra) and on the investigation of photo-oxidation processes by means of a combined preparative and spectroscopic approach (S. M. Bonesi, Universidad de Buenos Aires).

The highlights section comprises reports on different emerging research fields, including, the recent advances in polaritonic photochemistry (J. Fregoni, Universidad Autónoma de Madrid), the development of synthetic strategies based on halogen atom transfer (F. Julia, University of Murcia) and for the preparation of nitrogen-based heterocycles (W. Petersen, University of Cape Town), the potential of metal-based chromospheres as water oxidation systems (G. La Ganga, University of Messina), the preparation and application of photo-switchable peptides (O. Vasquez, Philipps-Universität Marburg), the adoption of radical-polar crossover strategies for enantioselective photocatalytic transformation (P. Wang, University of Science and Technology of China), the recent advances in the synthesis of Artemisinin (Z. Amara, Université Paris 2) and the use of indigoids and dihydropyrenes as photochemical switches (Z. Pianowski, Karlruhe Institute of Technology). Since vol. 45, the Specialist Periodical Reports hosts a contribution from the winner of the European Photochemistry Association PhD Award, that has been assigned, this year, to Dr L. Casimiro (University of Bologna).

Finally, the SPR lectures in photochemistry of this years have been authored by Profs. Jeremy Kodanko and Claudia Turro (The Ohio State University) and J. Sivaguru (Bowling Green State University).

The prestigious Porter Medal was assigned in 2022 to Professor Prashant Kamat (University of Notre Dame), in recognition of his efforts in investigating the charge transfer phenomena that take place in nanostructured assemblies for solar energy conversion.¹  The Photocatalyst Unit of Sharp Corporation received the 2022 iF Design Award, for the efforts to create a cleaner and safer workplace environment by exploiting the potentialities of photocatalytic materials with antimicrobial, deodorizing and antifouling effects.² 

Prof. Vivian W.-W. Yam, from the University of Hong Kong was awarded by the Josef Michl ACS Award in Photochemistry, for her contribution in the design of luminescent metal complexes and the application of the so-generated supramolecular assemblies in the preparation of OLED, sensing and in solar energy storage.³  In the same year, prof. Yam was also awarded by the InnoStars Award by Our Hong Kong Foundation.

The Asian and Oceanian Photochemistry Association Prize for Young Scientist has been assigned, in 2022, to Pengfei Duan (Beijing University),⁴  Sarit Agasti (University of Bangalore)⁵  and Hajime Shigemitsu (University of Osaka).⁶ 

Dr Basile Curchod (University of Bristol) has been named winner of the Marlow Award prize, by the Royal Society of Chemistry (RSC), for the development of computational protocols to predict the reactivity of excited states of organic compounds (in particular atmospheric molecules).⁷ 

The RSC Environment, Sustainability and Energy Division Early Career Award has been assigned to Dr Anabel Lanterna (University of Nottingham) in view of her efforts in developing efficient and reusable heterogeneous earth-abundant elements-based photocatalysts for synthesis and energy storage.⁸ 

The paper “Aryl dechlorination and defluorination with an organic super-photoreductant”,⁹  of which Prof. Oliver Wenger (University of Basel) is the corresponding authors has been awarded with the Photochemical and Photobiological Sciences as the most cited PPS paper in the last three years.

The Frans Habraken Best Paper Award was assigned, in 2022 to “TiO₂ Facet-dependent reconstruction and photocatalysis of CuOx/TiO₂ photocatalysts in CO₂ photoreduction” (first author: Fei Fang, University of Science and Technology of China).^10a  Finally, the European Society for Photobiology awarded Juliana Guerra Pinto (Universidade do Vale do Paraíba, Brasil) with the Giulio Jori Scholarship for her studies in the field of antimicrobial photodynamic therapy.^10b 

Prof. J. C. Scaiano recently published the volume Photochemistry Essentials, an introduction for photochemistry practitioners and students who aim to learn the essential principles of photochemistry as well to apply such principles to their research.¹¹ 

Profs. D. W. Bahnemann and A. O. T. Patrocinio recently edited the Springer Handbook of Inorganic Photochemistry,¹²  a collection of 63 chapters that aim to offer a comprehensive overview of the field of inorganic photochemistry, ranging from its historical development and the main experimental techniques employed in such field to the emerging application of photoactive inorganic compounds.

The recent advances in the design and application of Graphene Oxide composites (in several research field ranging from hydrogen production to lithium ion batteries) has been resumed. In the handbook Graphene Oxide-Metal Oxide and other Graphene Oxide-Based Composites in Photocatalysis and Electrocatalysis edited by J. Yu, L. Zhang, and P. Kuang.¹³ 

Quantum Dots Fundamentals, Synthesis and Applications (Editors: R. Ameta, J. Bhatt, S. Ameta) aims to be a practical guidance on the synthesis and applications of quantum dots, as well as a collection of key-use cases, including photocatalysis, energy cells and medical imaging.¹⁴ 

Recent Developments in Functional Materials for Artificial Photosynthesis is a volume edited by S. Ghosh and Q. Wang¹⁵  that aims to provide readers a comprehensive overview of recently developed, multifunctional materials (including, among the others, oxy sulphide and oxy nitrides and bismuth oxyhalides) employed as visible light-responsive catalysts for solar energy conversion.

The handbook Photodynamic Therapy Methods and Protocols collects both well-enstablished and protocols employed in photodynamic therapy (PDT) for a wide range of applications, ranging from antiviral to anticancer. The volume included, among the 39 chapters, a contribution on oxygen-independent photosensitizers as well as the most recent photosensitization strategies reported in literature.¹⁶ 

The 5th anniversary of ChemPhotoChem in 2022 was celebrated with a collection of invited contributions from the research groups of the members of the Editorial Advisory Board and Early Career Advisory Boards.¹⁷  In this collection, Bourgonje, Heyne and Anikovskiy continue the current debate on the validity of the names of carbon and graphene quantum dots,¹⁸  a class of fluorescent species that inherited this term from semiconductor nanocrystals whose size is correlated with optical properties. The authors add evidence from a photophysical study, including fluorescence correlation spectroscopy, that the emissive centres of carbon-based “quantum dots” are not located in nanoparticles but rather in small organic fluorophores.

The editors of ChemPhotoChem presented a dedicated Special Collection of invited research articles and reviews revolving around chiral emissive compounds and materials, since the field has regained momentum in recent years.¹⁹  Arrico, Benetti, and Di Bari present circularly polarised light-emitting terbium and europium complexes that are easily prepared from the commercially available enantiopure PyBox (2,6-bis(oxazolinyl)pyridine).²⁰  Both the chirality and the sensitization of the luminescence from the lanthanide ion (i.e. the antenna effect) originate from the PyBox ligands, making it possible to tune the chiroptical response using substituent effects.

Chemistry Europe’s journal ChemPhotoChem is supported and co-owned by several chemical societies, one of which is the German Gesellschaft Deutscher Chemiker. To celebrate this relationship, a Special Collection from guest editors Benjamin Dietzek, Axel Griesbeck, Bernd Strehmel, and Maria Wächtler was published featuring results from many researchers from the large German photochemistry community.²¹  An experimental procedure based on the combination of variable temperature NMR spectroscopy with in situ illumination and photon-counting is presented by Wilcken et al. as a versatile tool for the characterisation of short-lived intermediates.²²  While the authors applied the online detection scheme to the identification of the rotation cycle of a hemithioindigo-based molecular motor, they do further emphasise the expected use in photochemistry, photoredox catalysis, photoswitching, and photoresponsive materials. A noteworthy essay on teaching photochemistry was contributed by Bohrmann-Linde et al.,²³  in which the authors give insight into the variety of experimental approaches and teaching materials that have been employed in teaching photochemistry to students at schools and universities, in particular open educational resources in the forms of videos, animations, and e-books.

The journal of Photochemical and Photobiological Sciences has dedicated a special issue to Professor Angelo Albini on the occasion of his 75th birthday,²⁴  with contributions from colleagues, former students, and particularly friends covering the full range of current research topics in the photochemical community. Bassan et al. demonstrate that placing an iodine atom in the ortho position of a BODIPY derivative preserves both the efficient population of the triplet state and the significant positive shift of the reduction potential compared to derivatives with different substitution patterns.²⁵  The authors subsequently employ TD-DFT and spin–orbit coupling calculations to explain the subtle effects of positioning on the electronic and photophysical properties. Mezzetti et al. advocate a more widespread use of time-resolved infrared absorption spectroscopy, and hope that their tutorial review helps all readers understand the capabilities of this technique for the study of photo-induced reactions.²⁶ 

This topical issue was released in honour of Klaus Brettel, who greatly advanced the technology needed for time-resolved absorption spectroscopy – a powerful tool that has since been used in countless laboratories to unravel the electron and proton transfer mechanisms at play in biological systems.²⁷  In this spirit, Hutchison et al. designed and built an open-hardware microsecond dispersive transient absorption spectrometer from inexpensive commercially sourced components.²⁸  The instrument sacrifices some time resolution, but in its place provides a linear optical response and, notably, the full visible spectral region.

Silvia Braslavsky as a person and her research in the natural and life sciences are celebrated by Photochemical & Photobiological Sciences on occasion of her 80th birthday, with a special issue ringing in the start of a series of contributions from former colleagues and students published over the course of 2022.²⁹  In this issue, Knorr et al. characterised a new class of neutral super-photoacids in detail and found experimental evidence for excited-state proton transfer (ESPT) in acetone.³⁰  The combined results from time-correlated single-photon counting and femtosecond transient absorption spectroscopy shine light on the dynamics of solvation, rotational diffusion, and radiative relaxation processes of the photoacid under these adverse conditions for proton transfer.

A topical collection on the status of the field of antimicrobial photodynamic therapy (PDT) was gathered by Photochemical & Photobiological Sciences with the hopes of attracting the attention of everyone involved, from researchers to governments, to emphasise once more the imminent threat of antibiotic resistance in bacteria.³¹  Since curcumin, one of the potential candidates for use as a photosensitizer in PDT, unfortunately suffers from low solubility at physiological conditions, Silva et al. investigated two possible solutions for this issue: dissolution in deep eutectic solvents (DES), and complexation with cyclodextrin.³²  They found that a DES of choline chloride:glycerol has the most phototherapeutic potential as a vehicle for curcumin targeting localised superficial infections.

A special issue in honour of professor Anunay Samanta focuses on the new developments in the synthesis and characterisation of compounds with exciting excited state properties.³³  Santra, Mora, and Nath studied a julolidine-based molecular switch with a carboxylate substituent and found that the photoisomerization pathway in the excited state of the deprotonated form gets suppressed upon protonation.³⁴  This results in the emergence of a barrier-less ultrafast bond-twisting process, consequently making the molecule highly sensitive to the viscosity of medium.

In a virtual issue on the 16th International Congress on Photobiology in Córdoba, the journal Photochemistry & Photobiology reminisces about the first time that this conference was held in the Southern Hemisphere with a collection of invited reviews containing data that was presented at the time.³⁵  The theoretically and experimentally determined existence of “collective” excited states in DNA – delocalised over at least two bases – in the beginning of the 21st century greatly increased our current understanding of the role that electronic excited states play in DNA damage caused by direct absorption of UV radiation. In her invited review, Markovitsi explains their importance in the absorption of photons, the subsequent redistribution of the excited state energy within DNA, as well as the eventual formation of dimeric pyrimidine photoproducts.³⁶  The author emphasises that experimental observations of natural DNA are in full agreement with the current mechanism derived from numerous studies on model systems, in particular the boost in the collective character of the electronic excitations with the decrease in amplitude of conformational motions of the DNA bases.

In a virtual issue on Photosensory Biology, Photochemistry & Photobiology editors Francesco Lenci and Jean Cadet have collected relevant papers on the diverse areas within this topic from the last five years.³⁷  The emphasis in this field lies on the importance of light, not as a source of energy, but as an environmental stimulus for all sorts of living beings. One such example is the signalling in plants by cryptochromes: blue light receptors that are responsible for various light-induced responses in plants (and certain animals). Li and Yang have reviewed the recent advances made in the elucidation of the function of cryptochromes in plants, for instance in dicots such as pea and tomato and lower plants such as moss and fern, as well as the underlying mechanisms.³⁸ 

Ashwini Ghogare and Alexander Greer have collected 20 articles published in the journal Photochemistry & Photobiology between 2006 and 2012 that investigate type I (electron transfer) and type II (singlet oxygen) sensitized photooxygenation reactions and critically discuss the distinction between them.³⁹  The virtual issue contains, for example, a mechanistic study by Douki et al. on the combined exposure of isolated DNA to a polycyclic aromatic hydrocarbon (benzo[a]pyrene, or BaP) and UVA radiation, which showed that the majority of photo-oxidation reactions occurring in the double helix is due to type 1 photosensitization by the diol epoxide metabolite (BPDE) of BaP.⁴⁰ 

The most recent advances in organic and inorganic photoredox catalysis were published by ACS Organic & Inorganic Chemistry Au in a broad selection of reviews and articles, some focusing on the development of new reactions using known photocatalysis, and some presenting the preparation of new photocatalysts.⁴¹  Zhao et al. show that it is possible to synthesise 3-sulfonylindoles from NH-tosyl anilines, involving cyclisation and 1,3-sulfonyl migration, in the presence of either a gold(i) or silver(i) salt under blue light irradiation.⁴²  Notably, for the silver catalysis to work an iridium photocatalyst is required, from which the authors conclude that the generation of a sulfonyl radical is involved.

A special collection on perovskite materials and devices organised by both ChemPlusChem and the European Journal of Inorganic Chemistry contains a variety of research articles by scientists who are leading the field in aspects ranging from the synthesis and characterisation of perovskite materials to the application and fundamental studies of perovskite devices.^43,44  In a minireview, Ozturk et al. discuss the new-generation of perovskite solar cells employing operationally unstable organic hole-transporting layers and compare these with the currently more stable and low-cost, but less efficient, inorganic-based materials.⁴⁵  Saleh et al. present an alternative to the aforementioned issues in the form of a perovskite solar cell free of any hole-transport material.⁴⁶  Together with the lack of gold required for the back contact, their simple and fully printable mesoporous structure shows potential for upscaling and application at a large scale.

The cleavage and formation of carbon–carbon bonds have emerged as powerful tools for structural modifications in organic synthesis. Although transition-metal-catalyzed decarbonylation of unstrained diaryl ketones provides a viable protocol to construct biaryl structures, the use of expensive catalyst and high temperature (>140 °C) have greatly limited their universal applicability. Moreover, the direct activation of two inert C–C bonds in diaryl ketones without the assistance of metal catalyst has been a great challenge due to the inherent stability of the nonpolar C–C bonds. A new method using light as a direct driving force has recently been published by Cao et al., which is proposed to proceed through a dioxy radical intermediate formed by single electron transfer (SET) from the photochemically excited diaryl ketone.⁴⁷  The group reports mild reaction conditions, safe reagents, and excellent functional group tolerance.

Hydroborations are frequently used, for example, to introduce a reactive functional group for further cross-coupling reactions. While many catalytic systems exist to perform these transformations, they rarely distinguish between multiple potentially reactive sites. To fill this gap in available hydroboration methods, Bergamaschi et al. have developed a method to distinguish between ketones and carboxylic acids using only the presence or absence of visible light, respectively, and a Co(i) hydride complex (see Fig. 1).⁴⁸ 

Collins et al. have demonstrated a further exploitation of the photochemical reactivity of imine to an excited state diyl, with the development of a photochemical method for the formal [3+2] cycloaddition between cyclopropylimines and substituted alkenes.⁴⁹  The first reagent, a Schiff base of the cyclopropylamine, functions as a “masked” N-centred radical which is generated upon irradiation with visible light, and the cycloaddition products from subsequent reaction with substituted alkenes were directly converted, in the same vessel, to N-functionalised aminocyclopentanes via solvolysis and N-acylation.

Introduction of unnatural amino acids can significantly improve the binding affinity and stability of peptides, but commercial availability of such amino acids is poor. A new method was reported by Openy et al. for the conversion of lysine residues to Katritzky salts directly on support.⁵⁰  These salts are subsequently used for a photochemical deaminative Giese reaction in which a bond is formed between two sp³ carbons. The authors show the applicability of this late-stage modification route with various relevant substrates, among which all natural amino acids.

Another synthetic procedure for unnatural amino acids, in this case methionine analogues, was developed by Knowles et al. using the photocatalytic properties of riboflavin (vitamin B₂).⁵¹  The authors show that their method is suitable both for the synthesis of novel methionine analogues – by radical addition of simple sulphides to unsaturated amino acids – and for the chemoselective modification of peptide side chains, such as serine.

The photochemistry of gas-phase sulphuric acid has been accurately determined for the 11–14 eV energy range by Zhang and Lin et al. using a synchrotron radiation source for double imaging photo-electron photo-ion coincidence (i²PEPICO) spectroscopy.⁵²  In combination with high-level calculations, the authors were able to assign the three electronic states of [H₂SO₄]⁺, as well as determine the adiabatic ionisation energy towards its cationic ground state. Higher photon energies result in dissociation of the sulphuric acid cation into [HSO₃]⁺ and OH fragments.

A photochemical study on phosphenic chloride (ClPO₂) has for the first time been performed. Jiang et al. were able to first generate the compound in the gas phase by high-vacuum flash pyrolysis of 2-chloro-1,3,2-dioxaphospholane and subsequently isolate it in matrices of N₂, Ar, and CO at 10 K.⁵³  Irradiation at 193 nm gave different results depending on the applied matrix: isomerisation to the novel chlorine metaphosphate (ClOPO) occurs in the N₂ and Ar matrices – notably reversible upon subsequent 266 nm irradiation – while in solid CO ice, instead, the Cl–P bond cleavage is irreversible which leads to CO trapping of the generated chlorine atoms to form the caged radical pair ClCO˙⋯˙PO₂. These new data are compared with the photochemical behaviour of the analogue ClNO₂ in the N₂-matrix.

A photocatalytic one-pot dual-catalysis method was reported by Guo et al. for the efficient C–C cross-coupling using a ligand-free palladium salt Pd(OAc)₂ and a photo-active coordination polymer, where there is a notable significance of the one-pot approach – a reference experiment using the pre-synthesised composite of Pd/CP composite proved unproductive.⁵⁴  Mechanistic studies demonstrate the importance of interfacial electron transfer from the photochemically excited coordination polymer to generate the catalytically active Pd(0) species near its surface.

A photochemically generated palladium(ii) metallonitrene – that is, Pd(ii)–N – was identified as the species responsible for nitrogen atom transfer into aldehyde C–H bonds, which, after subsequent reaction with trimethylsilyl azide, furnishes an easy route to primary amides after deprotection.⁵⁵  The reactive metallonitrene complex is generated from the azide precursor Pd(ii)–N₃ and could be identified both crystallographically, by irradiation of a single crystal at 100 K while recording X-ray diffraction data, and magnetically, by in situ photolysis of a micro-crystalline sample inside a superconducting quantum interference device (SQUID) magnetometer at 10 K.

Zhu, Yue, and Rueping have enlisted the use of electrochemistry, photocatalysis, and photo-electrochemistry to perform a three-component cross-coupling reaction for the arylalkylation of alkynes under stereoselective control.⁵⁶  Conducted with electricity and graphite/nickel foam as the anode/cathode, respectively, the E-isomer of the trisubstituted alkene is obtained exclusively, whereas high stereoselectivity towards the Z-isomer is observed when a photoredox catalysis approach is applied using both Ir and Ni complexes. Notably, the method without Ir photocatalyst and with application of electricity generated selectively Z-trisubstituted alkenes as well.

The addition of haloalkanes to unsaturated hydrocarbons by means of visible light-mediated atom transfer radical addition (ATRA) has shown its use in recent years, but the method is limited to incorporation of (pseudo-)halides due to the radical chain propagation mechanism. Bian et al. have addressed this limitation by taking inspiration from Nature, specifically cytochrome P450 hydroxylase and non-heme iron-dependent oxygenases, and have reported the dual functionalisation of inactivated alkenes via manganese-catalysed radical ligand transfer (RLT).⁵⁷  The authors developed a modular method using two catalysts: an iridium photocatalyst and an earth-abundant manganese complex, the latter forming a high-valent species capable of delivering various nucleophiles to the carbon-centred radical formed after initial radical addition of a fluoroalkyl group. Analogously to the radical rebound step in metalloenzymes, the RLT step involves the rebounding of a manganese-coordinated nucleophile to the carbon-centred radical to form a new C–X bond.

Late-stage diversification of scaffolds by means of photocatalytic hydrogen atom transfer (HAT) is a convenient strategy, and therefore Capaldo et al. have applied a decatungstate-catalysed HAT transformation, followed by a Horner–Wadsworth–Emmons (HWE) olefination, to the allylation of aliphatic C(sp³)–H bonds.⁵⁸  This combination of two steps has furthermore been successfully performed in a “telescoped” flow protocol, requiring no purification of intermediates, which the authors state should aid the scale-up from laboratory to industrial settings.

The conversion of methane into longer-chain alkanes is an important process that can be performed, for example, via non-oxidative coupling of methane (NOCM), by which hydrogen is produced at the same time. Oxide-based photocatalysis allows NOCM under mild conditions, but the lattice oxygen causes overoxidation of methane, leading to subpar selectivity and durability. To combat this, Zhang et al. have developed a new strategy for the engineering of heteroatoms in the commonly used TiO₂ photocatalyst.⁵⁹  The authors added Pd–O₄ units to the surface of the photocatalyst, effectively reducing the contribution of oxygen sites to the valence band. With this construction, the limitations regarding overoxidation were overcome and 94.3% selectivity towards ethane was achieved along with stoichiometric production of hydrogen.

2-Substituted 1,3-oxazolidines have been employed by Luguera Ruiz et al. as precursors to the release of various radicals (tertiary, α-oxy, and α-amido) under organic photo-redox catalytic conditions.⁶⁰  These radicals, generated by previously unreported C–C cleavage from the radical cation of the heterocyclic derivatives, are then used for the conjugate radical addition to Michael acceptors and vinyl (hetero)arenes.

The incorporation of singlet oxygen (¹O₂) into organic compounds represents an attractive oxidation strategy using photosensitisers under mild conditions. One potential functional group for this reaction, enamine, unfortunately suffers from fragmentation issues, which is why Lei et al. have employed the help of Lewis acids to intercept the iminoketone intermediate formed upon oxidation with ¹O₂.⁶¹  Subsequent reaction with a second enamine in a [2+2]-cycloaddition manner affords pyrrolin-4-ones that were produced for the first time by cross-dimerisation of two different enamines (see Fig. 2).

An electrophotochemical metal-catalysed method has been developed by Wang, Li, and Fu for the radical decarboxylative functionalisation of aliphatic carboxylic acids.⁶²  This protocol works without any chemical oxidants nor azido-group transfer reagents, and the light and electrical energy can be applied separately using a modular flow cell that allows large-scale synthesis.

Strong reducing agents have been widely employed in organic chemistry for a whole range of useful transformations. The common stoichiometric metallic reductants, however, are associated with severe issues and limitations regarding cost, ease of use, safety, and generated waste. Recent explorations have harnessed the energy from (multiple) photons to attain reduction potentials equal to Li metal, where the challenge still lies in stabilising the catalytic intermediates. Widness et al. have now reported CdS nanocrystal quantum dots (QDs) with reduction potentials of up to −3.4 V vs. SCE, under irradiation with blue light, that are able to reduce aryl chlorides and phosphate esters and remain stable with turnover numbers up to 47 500.⁶³  Mechanistic experiments point towards the generation of a thermally excited electron by means of Auger recombination in an excited anionic QD, after this has been generated by two consecutive photochemical excitations with intermediate reductive quenching, and this transient highly energetic electron is deemed responsible for the observed reductions.

The synthesis of C₁₀ cycloalkane jet fuels has been shown to be feasible from carbon dioxide via isoprene using a combination of photobiological and photochemical routes.⁶⁴  For the first step, Rana et al. employed photosynthetic cyanobacteria to produce isoprene from carbon dioxide; in the subsequent photochemical step, the isoprene was dimerised into limonene, paradiprene, and isomeric C₁₀H₁₆ hydrocarbons (monoterpenes) under either near-UV irradiation, or simulated or natural sunlight. After hydrogenation of the (mixture of) isoprene dimers, and subsequent thermal conversion into [4+2] and [4+4] dimers of the hydrogenated [2+2] dimers that have the undesired effect of lowering the flashpoint below the threshold of 38 °C, the resulting monoterpenoids fully satisfy the criteria for drop-in jet fuels.

A new dual reactor coil developed by Crawford et al. is able to drive two consecutive photochemical reactions using a single LED light source in continuous flow mode.⁶⁵  In a demonstration of the advantage of this approach, two distinct photochemical reactions in the synthesis and functionalisation of quinolines were performed without interruption (see Fig. 3).

Schmid et al. propose a new physicochemical concept to be used in photo-redox chemistry for the simultaneous performance of two chemical reactions with a single catalyst in a liquid–liquid biphasic system: “{2-phases 2-reactions 1-catalyst}”.⁶⁶  The authors demonstrate the concept using a commercially available α-Keggin polyoxometalate (POM) photocatalyst for the oxidation of a hydrophobic compound in an organic phase, after which the reduced POM diffuses into the aqueous phase where it is then able to reduce a hydrophilic compound. Having regained the initial oxidation state, the POM again diffuses back into the organic phase to start the catalytic cycle anew. The concept was successfully applied to various basic reactions, such as the oxidation of an alcohol to an aldehyde, the degradation of organic dyes, the reduction of metal ions, and the in situ generation of metal–organic complexes.

The field of photocatalysis suffers from difficulties in reproducibility and comparability of experiments performed in different research groups. To combat this, Kowalczyk et al. have developed and thoroughly characterised and documented a modular and adaptable platform for a photoreactor.⁶⁷  The authors performed extensive chemical actinometry and radiometry, and provide documentation of the incident photon fluxes and homogeneities of irradiation in the various (multi-) reactor setups.

Electron donor–acceptor (EDA) complexes are species that absorb light at higher wavelengths than their constituent parts, hereby providing a visible light initiation pathway for radical reactions involving the individual substrates. In their review on the recent advances in the use of catalytic donors and acceptors, Tasnim, Ayodele, and Pitre emphasise the need for this catalytic approach to EDA complex formation.⁶⁸  When either the donor or acceptor is employed catalytically, the complexation and photogeneration of radicals are decoupled from the functionalisation of the substrate – a limitation that is inherent to the stoichiometric approach.

Meng et al. discuss our need to develop methods to accomplish multi-electron and multi-proton photochemical transformations, including efforts towards light harvesting, consecutive electron transfer methods, and tandem catalysis.⁶⁹  The authors feel that it is time to take better advantage of the photons in our sunlight, by moving past the single-electron reactions that have brought us so much already, and invest research efforts into the design of molecules and materials, mechanistic understanding, and theoretical modelling for the development of synthetic procedures that involve multiple electrons and protons.

While many concepts from the broad knowledge of asymmetric induction in chemical transformations have been adopted by the field of photo-redox catalysis, for example, from polar two-electron reactions and radical chemistry, the catalytic methods involving asymmetric ion-pairing make out only a small part of the light-catalysed reactions performed today. Schirmer and König give some perspective on the relatively small number of currently reported stereoselective synthetic procedures at the interface of ion-pairing and photo-redox catalysis.⁷⁰ 

The combination of light and electricity – photochemistry and electrochemistry – for the catalysis of chemical reactions has over the past years seen an increase in successful attempts. Huang, Steiniger, and Lambert have therefore highlighted the recent advances in this area termed electrophotocatalysis and give their perspective on the potential areas of focus for the future.⁷¹  The advantages of electrophotocatalysis are clear: the use of electrochemical potential as the electron source or sink means that there is no need for (super)stoichiometric chemical oxidants and reductants; and the generation of open-shell photocatalysts – which are usually exceptionally strong oxidants or reductants – is readily compatible, affording potent and selective redox reactions under mild conditions.

When photo-responsive single crystals are irradiated with light at a wavelength offset from the absorption maximum of the chromophore, both the “direct” effect – generation of a different excited state – and the “indirect” effect – altered spatial propagation and extent of photoreaction due to differing light penetration depth – play a possible role. Ahmed et al. have reported a method to determine these effects independently from each other, using the nitro–nitrito isomerisation of a cobalt complex that causes macroscopic deformation of the single crystal.⁷²  From a quantitative comparison of the bending curvature and crystal elongation dynamics for different irradiation wavelengths, the authors hypothesise that different excited states and even an additional photochemical reaction occur upon irradiation at different locations on the same absorption band (see Fig. 4).

Solution-processed semiconductor materials possess unique spin properties that are promising for use in spintronic and quantum information technologies. While applications in these areas are still limited due to the very short (picosecond) spin lifetimes, Liu et al. propose that these materials can already find use in molecular photochemistry that works with spin-relaxed triplet states.⁷³  The authors use a system of rhodamine B molecules anchored to CsPbBr₃ nanocrystals to demonstrate high yields of molecular triplet formation through the recombination of charges separated by excitation of either the nanocrystal or the molecule, followed by a rapid spin-flip of the generated hole in the nanocrystal. Examples of applications in triplet-fusion photon upconversion and singlet oxygen generation were demonstrated.

Recently, it has become more attainable to use different wavelengths of light for independent control over different orthogonal photochemical reactions in polymer networks, enabling high-precision tuning of material properties. In a concept article, Truong et al. highlight how recent progress in visible light photochemistry has opened opportunities for the design of polymer networks with dynamically adjustable properties, such as hydrogels and 3D-printed structures.⁷⁴  The authors propose certain adaptive properties to strive for, such as the switching of conductance, hydrophobicity, fluorescence, and cell adhesion.

Gruber et al. fine-tuned the photochemistry of a synthetic retinal chromophore to achieve similar photochemical properties as found for the retinal protonated Schiff base in the protein environments of microbial and animal rhodopsin.⁷⁵  By synthetically locking the retinal derivative at the C₁₁═C₁₂ double bond in the trans configuration, the excited-state decay is considerably sped up: from picosecond to ultrafast sub-picosecond times. Furthermore, the authors show the photoswitch undergoes unidirectional rotation in two steps by two consecutive absorptions of a 600 nm photon, opening opportunities for this molecular design to be used for red-light-driven ultrafast molecular rotary motors.

After the interaction of light with matter, migration of electrons takes place – the latter is the essence of chemistry. He et al. have now been able to follow the ultrafast dynamics of these electrons by letting a machine learning algorithm analyse high-order harmonics generated by two-color laser pulses of single molecules that were fixed in space.⁷⁶  The approach allowed the researchers to construct movies of the electron migration after tunnel ionisation of nitrogen and carbon dioxide with time steps as small as 50 attoseconds. This work may aid in the continuous efforts of controlling photochemical reactions with femtosecond lasers.

A new multifunctional photoswitchable compound has been reported by Kodura et al.: pyridinepyrene, whose pH-active moiety furnishes the compound with distinct photochemical behaviour under basic and acidic conditions.⁷⁷  At neutral and high pH, blue light induces a [2+2]-photocycloaddition yielding a cyclobutene-core structure in a reversible manner. At the low pH conditions, the absorption band is red-shifted and repulsive interactions prevent the intermolecular reaction, leaving only trans–cis isomerisation (see Fig. 5).

Another example of the recent use of lead halide perovskites for photosensitisation of molecular triplets is reported by Liu et al., in this case for light-driven isomerisation and cycloaddition reactions.⁷⁸  The authors show both a method involving grafting of the compound onto nanocrystal surfaces, and the more generally applicable method using triplet-acceptor ligands as energy relay. The latter method was then used for isomerisation of stilbenes, ring-closing of diarylethenes, and cycloaddition of acenaphthylene.

Photolysis of pyrimidine nucleobases, such as uracil, is known to result in the formation of a hydrate under aqueous conditions. While several hypotheses have been proposed, for example, involving zwitterionic or “hot” ground-state species, the mechanism of this photohydration remains unclear. Recently, a theoretical investigation by Park et al. involving nonadiabatic simulations of the uracil photodynamics has revealed the possible formation of a kinetically stable intermediate.⁷⁹  Although the species is high energy, with its half-chair puckered pyrimidine ring and twisted double bond in the ring, it should survive sufficiently long to react with a nearby water molecule.

The photo-responsive behaviour of phytochromes, responsible for sensing light in plants, fungi, and bacteria, starts with the photochemical isomerisation of the bilin chromophore in the binding pocket of the protein. An extensive range of experimental and computational efforts have been applied to study the chromophore–protein interactions, but still the mechanism and timescale of the process remains to be clarified. Salvadori et al. employed a combination of non-adiabatic surface hopping trajectories and adiabatic molecular dynamics simulations focusing on the molecular scale of photoisomerization in a bacteriophytochrome.⁸⁰  Their resulting simulations show that isomerisation of the chromophore proceeds via a hula-twist mechanism for which the kinetics are dominated by hydrogen bonding with a near-by histidine.

New chemical reactivity of β-carotene has been reported by Zbyradowski et al. which challenges the prevailing view of ¹O₂-driven deactivation of β-carotene to the endoperoxide degradation products.⁸¹  The authors experimentally show that the endoperoxides are formed in the absence of singlet oxygen via a non-concerted triplet–triplet reaction between oxygen and β-carotene, and draw the conclusion that these degradation products are not chemical markers of ¹O₂ and oxidative stress, but rather a signal of the overproduction of dangerous chlorophyll triplets in photosystems.

The non-adiabatic conical intersections have been found to be ubiquitous in molecular photochemistry: degeneracies between electronic states that provide an ultra-fast non-radiative relaxation channel. Direct experimental observation of processes close to conical intersections is challenging due to the aforementioned ultra-fast dynamics and the complex nuclear and electronic degrees of freedom. In an article providing an overview of new spectroscopic techniques, using X-ray and extreme-UV light, that could be capable of providing detailed information on conical intersections, Schnappinger et al. discuss the theoretical side of how these ultra-short pulses provide the spectral and temporal resolution needed for resolving the ultra-fast dynamics.⁸² 

The future development of Brønsted acid-mediated photocatalysis is discussed in a review by Baburaj et al. which focuses on photochemical reactions that are controlled and mediated by hydrogen bonds.⁸³  The authors highlight how excited state processes, both in the solid state and in isotropic media, are controlled by certain hydrogen-bonding templates, such as diols, thioureas, and organometallic complexes.

Another step towards the complete control of the motion of matter at the smallest scales was taken by Bach et al. who achieved mechanical threading of a tetraethylene glycol chain through a macrocycle attached to a molecular motor.⁸⁴  The photochemically driven unidirectional rotation of the hemithioindigo motor is hereby converted into unidirectional translation of the chain through the ring (see Fig. 6).

Hou et al. show once more that liquid crystal networks are a prime candidate for use as (photo-)responsive materials with complex, programmable functions. The authors embedded both racemic and enantiomerically pure molecular motors – acting as cross-linkers, actuators, and chiral dopants – into liquid crystal networks with defined orientations, resulting in the emergence of multiple types of motion on the macroscopic scale, such as bending, waving, and helical motion with different chiralities.⁸⁵ 

Ramamurthy has summarised the studies that are carried out in the author’s lab concerning photochemical reactions in octa acid, a synthetic container that forms a capsule in water that – notably – remains closed during the excited state lifetime of encapsulated molecules.⁸⁶  As an expert in the field of photochemistry in constrained environments, Ramamurthy explains the role that free space plays on the dynamics of molecules in the excited state that are confined within an inflexible cavity.

The development of a scissor-shaped azobenzene dyad, containing strongly aggregating cholesterol units at both ends of its framework, has provided Suda et al. with the first successful light-driven modulation of supramolecular polymorphism.⁸⁷  Under strong visible light irradiation, the authors were able to produce nanotoroids, while at low light intensity they obtained one-dimensional fibres.

The majority of photosensitizers that are used in photodynamic therapy for the treatment of cancer produce singlet oxygen via energy transfer from the excited triplet state to molecular oxygen, i.e. the type-II mechanism. Due to the dependence on local oxygen concentration, these possess low efficacy in treating hypoxic tumours. For this reason, Teng et al. developed a supramolecular method for the preparation of type-I photosensitizers, consisting of an electron acceptor bound to an existing type-II photosensitizer, that simultaneously generate strong oxidising cationic radicals as well as superoxide radicals, the latter via electron transfer from the anionic radical of an acceptor to molecular oxygen.⁸⁸ 

A new method for the synthesis of asymmetric azo compounds from nitroarenes has been developed by Yang et al. using a cobalt-based molecular octahedron loaded with a dye as a microenvironment photocatalyst.⁸⁹  The inner volume of the capsule, the so-called “pocket”, is able to contain more than one nitroarene molecule, which makes a condensation reaction possible between an azanol and a nitroso species – both formed in situ through photoreduction – to yield the azo product (see Fig. 7).

Photocatalytic conversion of lignocellulose, also called photo-biorefining, is an ecologically friendly method of lignin depolymerisation that shows promise for application at larger scales. Xu et al. discuss the current challenges regarding selectivity and stability of the photocatalysts in use, starting in their critical review with studies on representative model compounds that provide insight into the mechanism, and ending with the remaining hurdles to be overcome before large-scale photocatalytic lignin valorisation will be economically feasible.⁹⁰ 

Bertin et al. have reported a copper-based photocatalytic method for breaking the β-O-4 linkage in lignin model compounds.⁹¹  The reaction conditions, using a biphasic THF/water system and catalytic amounts of a nicotinamide-derived hydrogen atom donor, represent a step towards a greener process compared to current analogous methods utilising rare metals and stoichiometric amounts of hydrogen atom donors.

Copper was also the metal of choice for Ji et al. for their light-driven Ullmann coupling of phenols and aryl halides using the combined effect of plasmonic copper nanoparticles (Cu NPs) and carbon nanotubes (CNTs).⁹²  Notably, the CNTs were not only acquired from commercial sources but also from various types of plastic waste, with the latter also performing excellently as the support structure for Cu NPs (see Fig. 8).

Metal is not necessary for light-driven conversion of biomass, as evidenced by Yang et al. who prepared a metal-free photocatalyst consisting of carbon quantum dots (CQDs) anchored on 1,2,3,5-tetrakis(carbazole-9-yl)-4,6-dicyanobenzene (4CzIPN).⁹³  The authors report excellent catalytic activity of CQDs@4CzIPN for the photocatalytic conversion of a range of biomass-derived monosaccharides to lactic acid, due to sensitization by the quantum dots.

Natural wood is considered a cheap and environmentally friendly source to use for the fabrication of carbon materials, for example, by surficial carbonisation. Zhang et al. used this method in their new approach for the synthesis of a bi-functional photothermal and photochemical material for water treatment, showing both high solar-to-vapour energy efficiency and high photo-degradation efficiency of organic compounds in water.⁹⁴ 

The conversion of ethanol, one of the most important biomass platform molecules, into a promising fuel additive was achieved by Betts et al. using UV-A irradiation under ambient conditions in the presence of TiO₂. The product 1,1-diethoxyethane (trivial name acetal) is formed from the reaction of ethanol with acetaldehyde initially formed photocatalytically from ethanol.⁹⁵ 

Finding a less energy-intensive alternative to the well-known Bosch–Meiser process for the production of urea, an essential agricultural fertiliser, is a priority in the global transition to sustainable agriculture. An intriguing alternative that Xia et al. have put forward is the use of sunlight to power the reaction of NH₃ with CO₂ catalysed by Pd nanoparticles loaded on molecular sieves.⁹⁶  While the metal nanoparticles are responsible for the catalytic dissociation of NH₃ and for providing the photothermal energy needed for the subsequent reaction with CO₂, absorption of H₂O by the molecular sieves removes this by-product from the equilibrium and drives the production of urea.

A new structural configuration of materials consisting of two-dimensional WO₃ and transition metal dichalcogenides was theoretically investigated by Guo et al. in the search for more efficient photo(electro)-catalysts for the reduction of nitrogen.⁹⁷  The proposed vertical heterostructures of WO₃ with, among others, MoS₂, MoSe₂, WS₂, and WSe₂, all show reduction of N₂ to NH₃ via a distal mechanism with onset potentials lower than 0.50 V, the photocatalytic mechanism for which was elucidated by means of time-dependent ab initio non-adiabatic molecular dynamics simulations.

Mimicking the processes of Nature is a favourite pastime (and source of inspiration) for many researchers, in particular concerning photosynthesis and related systems. Using combination of a hetero-dinuclear photocatalyst with an inverted E. coli vesicle, Mengele et al. were able to emulate the cofactor-delivering function of an active chloroplast, namely the simultaneous photocatalytic generation of reduced nicotinamide adenine dinucleotide (NADH) and enzymatic production of adenosine triphosphate (ATP).⁹⁸ 

Purification of plutonium involves changes of its oxidation state during separations. Many of the chemical redox agents used today are deemed unsafe, therefore DiMucci et al. have investigated the use of photochemistry as an alternative.⁹⁹  The authors show that photoreduction of Pu⁴⁺ and (UO₂)²⁺ yield HCl (aq) and HNO₃ (aq) as side products, and that the photogenerated Pu³⁺ and U⁴⁺ can subsequently be separated using anion exchange chromatography (see Fig. 9).

In the same vein, Wang et al. showed that the photo-induced reduction of soluble U⁶⁺ into insoluble U⁴⁺, previously only possible under inert conditions, can be replaced by photo-induced formation of insoluble uranium peroxides under ambient conditions.¹⁰⁰  Notably, even without an external photocatalyst, the uranyl ion itself produces hydrogen peroxide under visible light irradiation, and the subsequent reaction between the two species produces the precipitating uranium peroxide under ambient conditions (see Fig. 10).

Productive activity in the “dark” state is a prominent feature of the photosynthetic apparatus in Nature. Artificial photocatalyst analogues that have been explored so far have lacked this ability to drive reactions after removal of the light source. In that light, a report by Pan et al. exhibits their achievement of synthesising a photocatalytic titanium-based metal–organic framework that is capable of producing hydrogen in the dark after having been irradiated.¹⁰¹  The essential factor is the generation and sufficient lifetime of the Ti³⁺ intermediate, which is responsible for the reduction of water to hydrogen.

Molecular solar thermal (MOST) systems combine solar energy conversion, storage and release in a single molecule. One class of compounds being investigated for this purpose, is azothiophene, which undergoes photochemical isomerisation from its E- to its Z-isomer (the latter being the higher energy state).¹⁰²  Triggering the energy release electrochemically enabled direct control of the release rate and provided good cyclability.

A new three-component procedure was reported by Xu et al. for the synthesis of various amines from an aldehyde, a carboxylic acid, and a primary amine, using titanium dioxide nanoparticles (25 nm) as photocatalyst.¹⁰³  The authors investigated the reaction using kinetic studies and radical-trapping experiments, which point to a single-electron-transfer mechanism involving the photochemically excited TiO₂ that reacts with the carboxylic acid, leading to decarboxylation and the formation of an alkyl radical.

Field observation and experimental simulation point towards black carbon playing a remarkable role in the catalytic oxidation of SO₂ to form atmospheric sulphate. However, with the light-irradiated mechanism being elusive, a systematic investigation of this mechanism is needed. Experimental work by Zhang et al. shows that black carbon under irradiation significantly promotes the heterogeneous conversion of SO₂ to H₂SO₄, by photo-induced OH radical reacting with SO₂.¹⁰⁴  It suggests that the OH radical formation is closely related to the abstraction and transfer of electrons in black carbon and the formation of reactive superoxide radical (O₂˙^–) intermediate.

Scientists trying to emulate the photosynthetic capabilities of natural protein structures run into the problem that these protein complexes have gone through billions of years of evolution which has left a legacy of complexity and fragility. Protein re-engineering efforts are hereby encumbered, as is the search for the underlying design rules for light-driven charge separation. Ennist et al. have chosen a different path to artificial photosynthesis, namely synthetic biology, and have developed and thoroughly characterised a simplified photochemical reaction centre that consists of a multi-cofactor protein.¹⁰⁵  Notably, transient absorption spectroscopy measurements show that the – highly stable and modular – artificial protein framework sustains charge separation lifetimes exceeding 100 ms, which is in the ideal range for light-activated catalysis.

The use of light in polymer synthesis offers several advantages, compared with thermally controlled polymerisation methods, in addition to the usual sustainability-related points, namely the potential for precise control over the structure and chain length of the macromolecule. The latter especially holds for photo-controlled living radical and cationic polymerisations, on which Aydogan et al. have published a review containing recent mechanistic studies as well as applications.¹⁰⁶ 

The final distribution of the photochemical products in molecular crystals intended for photon-powered artificial actuators plays an important role in the mechanical response. With this in mind, Morimoto et al. studied the propagation and spatiotemporal distribution of the photomechanical single-crystal-to-single-crystal transformation of 2,5-distyrylpyrazine.¹⁰⁷  Surprisingly, they found that the polymerisation propagates from the edge towards the centre, for which both the surface effect and the cooperative effect are responsible (see Fig. 10).

The efficient synthesis of cyclic polymers through photochemical cycloaddition of terminal double bonds is a potentially successful method, but it remains challenging to selectively obtain only the cyclic products. Xue et al. have recently found a way to steer the polymerisation of cyano-substituted stilbenes in the right direction using hydrogen bonding of the macromonomer with a supramolecular network.¹⁰⁸  The high local concentration of the terminal olefins in the supramolecular environment makes the [2+2] photo-cycloaddition at 430 nm irradiation extremely efficient.

The use of organic photoredox catalysts to drive polymerisations is a sub-field of general photocatalytically driven chemical transformations that is receiving increased interest, for example in the shape of light-controlled organocatalysed atom transfer radical polymerisation (O-ATRP). With the aim of finding the organic moieties and compounds most active for photocatalysis, Yang et al. studied the structure–properties relationship and photocatalytic mechanism of N-unsubstituted diketopyrrolopyrrole (DPP) analogues, showing that the N-unsubstituted DPP core plays an important role in charge transfer, and in the formation of the excited states and intermediates.¹⁰⁹ 

A new, robust method for the synthesis of polymeric nanoparticles (in water) with various morphologies was developed by Wu et al., with the key feature being that this photo-induced reversible addition–fragmentation chain transfer (photo-RAFT) polymerisation works through thick barriers, such as skin.¹¹⁰  Additionally, the highly efficient and oxygen-tolerant photocatalyst, tetrasulfonated zinc phthalocyanine, does not require deoxygenation of the system.

Wavelength-dependent behaviour of photochemical polymerisation is a key element in the design of soft matter materials using reversible-deactivation radical polymerisation (RDRP). By mapping the properties of polymers synthesised by photochemically induced atom transfer radical polymerisation (photo-ATRP) versus the absorption spectrum of the copper(ii) catalyst, Nardi, Blasco, and Barner-Kowollik were able to observe a dependence of both the number-average molecular weight and the dispersity on the applied wavelength.¹¹¹ 

Barata-Vallejo, Yerien, and Postigo wrote a comprehensive review on the use of natural pigments, vitamins, and small organic compounds in photocatalysis, which are currently receiving attention as possible replacements for classical photocatalysts like organic dyes and transition metal photocatalysts.¹¹²  The authors discuss the useful synthetic procedures that have been developed for, e.g., oxidations and reductions, carbon–carbon, carbon–oxygen, and carbon–sulfur bond formation, and double-bond isomerisation.

Recently, the greatly expanded toolbox for modification of biological compounds, such as proteins and oligonucleotides, has allowed researchers to incorporate photochemically controllable molecules in DNA and RNA in order to study its structure and function upon structural changes induced by light. In a review, Tavakoli and Min have made a collection of photo-active compounds used in oligonucleotides to this end, and they summarise the most important results obtained from these studies, for example, the impact on the DNA/RNA structure and the possible biological applications.¹¹³ 

A great advance in the efficiency of reactive oxygen species (ROS) generation by donor–acceptor (D–A) type photosensitizers (PSs) was reported by Hao et al. using a deceptively simple azo-bridge to link two D–A-type PSs.¹¹⁴  Both type-I and type-II photochemical ROS generation play a role, and computational investigations suggest the increased efficiency of intersystem crossing due to a decrease in the energy gap between the singlet and triplet states. Notably, the inherent azobenzene moiety in the photosensitizer structure means that there is an additional stimuli responsiveness where the UV-induced trans-to-cis-isomerisation results in fluorescent aggregates that provide a direct method of determining the effective photodynamic bactericidal dose (see Fig. 11).

The unanswered question of the origin of single-handedness in amino acids is still under full investigation, and stellar circularly polarised light (CPL), with its dominant UV emission, is one of the possible culprits. The recently reported circular dichroism spectra of various amino acids in the gas phase by Meinert et al. indeed show the possible contribution of asymmetric photochemical reactions, induced by stellar CPL, to the eventual evolution of l-amino acids.¹¹⁵ 

The common pharmacophore isoxazole is currently being rediscovered as a photochemical cross-linker for biologically active molecules to proteins. Cheng et al. report a thorough investigation of the photo-crosslinking reactions of isoxazole, as well as its applications in protein labelling and chemoproteomics (see Fig. 12).¹¹⁶ 

Likewise, Lougee et al. demonstrate the utility of isoxazole as a photochemical crosslinker in a range of common biological experiments, such as proteomics workflows.¹¹⁷ 

In vivo metabolic studies in humans by hyperpolarized ¹³C magnetic resonance imaging (HP ¹³C MRI) are currently only performed using HP [1-¹³C]-pyruvate. While this biological compound has many advantages, such as its facile polarisation by dynamic nuclear polarisation (DNP), its rapid loss of hyperpolarization makes the procedure extremely time-sensitive. Gaunt et al. have recently introduced an alternative in the shape of α-ketoglutaric acid (α-KG), which, with its exceptionally high photochemical yield of free radicals, can be a universal polarising agent for the hyperpolarisation of, for example, [1-¹³C]lactate.¹¹⁸ 

Despite their interesting biochemical properties, the inherent instability of hydropersulfides (RSSH) has prevented their physiological role from being studied in detail. Aggarwal et al. have addressed this instability using a photo-cleavable protecting group separated from the persulfide group by a linker, resulting in the near-quantitative release of RSSH upon irradiation with UV light.¹¹⁹ 

The challenge of using low-energy near-infrared light (650–900 nm) as stimulus for the uncaging of biologically active compounds is taken up by Janeková et al. by their development of photocages from well-known cyanine dyes.¹²⁰  These are synthetically readily available on large scale, and the authors show that efficient release of carboxylic acids in aqueous media occurs upon irradiation with light up to 820 nm, also in live HeLa cells.

V. B. and S. P acknowledge support from the Ministero dell’Università e della Ricerca (MUR) and the University of Pavia through the program “Dipartimenti di Eccellenza 2023–2027”. S. C. and J. D. S. thank the Swedish Vetenskapsrådet for a starting grant (Starting Grant – 2021-00001) and the Wenner Gren foundation (UPD2022-0079).