Introduction of the year 2019 Free

The present volume is the number 48 in the series “Photochemistry” of the Specialist Periodical Reports published by the Royal Society of Chemistry and consists in three different sections. The first part includes, as usual, a series of critical reviews on the recent advancements in computational and organic photochemistry. The second part of the volume collects different highlight papers on recent topics with the aim of providing the readership with a selection of the advanced research that could be also a pleasant reading for practitioners. Finally, the “SPR Lectures in Photochemistry” section includes contributions that aim to provide academic and students in photochemistry with a brief glossary of concepts and case studies involved in the chosen topic. In order to better serve our readers this introduction chapter include, along with a list of theme issues, handbooks and reviews published in the 2019, a section on awards and prizes assigned to researchers that operate in the different sectors of photochemistry. A presentation of the quote of the series “One hundred years ago” printed on the back cover has been also included herein.

As hinted below, the reviews section is focused on the recent efforts reported in the field of computational photochemistry, and the photoreactivity of organic compounds, including (poly)olefins, aromatics and molecules bearing different functional groups. A review on photo-rearrangement reactions has been included for the first time. On the other hands, the highlights section comprises reports on different research fields including the design and the application of photo-switches in logically systems and fluorescence sensing, the development of photoresponsive organophosphorus materials and the recent efforts in metal-free photocatalytic processes. The SPR Lectures in Photochemistry have been prepared by prof. prof. Alexander Greer (Brooklyn University), David Magri (University of Malta) and prof. Petr Klàn (Masaryk University).

In 1919 Otto Warburg, a professor in Berlin, son of the physicist Emil and future Nobel Prize in Physiology (1931), published the first of his papers on the mechanism of the “assimilation”, that is the reduction and incorporation of CO₂ in organic molecules in the cells.^1,2  This was obtained by the use of rotating sectors that had to be largely employed later for determining the role of short-lived species in chemical mechanisms. The interruption of illumination was obtained by using rotating sectors, where different rate of rotation of the rotating surfaces were chosen in such a way that a half of the experiments occurred with no illumination, with different rotation rate and different numbers of rotation per minute.

The cells, green Chlorella algae, chosen because of their fast growth, immobility and relatively simple development cycle, were either suspended in the mixture of carbonates or dissolved in bud solution; in the latter case after saturation with carbon dioxide. Single cultures were initially irradiated under sterile conditions, but later this step was omitted, since it had no role. A metal vapor lamp was used to irradiate the culture vessel, while immersed in a Becker with running water for cooling at 25 °C and a slow flushing of air to which 4 volumes of CO₂ were added and further hindered the sedimentation of cells. The CO₂ concentration was so high that any corresponding change in the extent of the assimilation had no effect. Manometric measurements were carried out for determining the CO₂ content in different experiment arrangements. As for the sector experiments, the result depended on the other ones (see Table 1). By “high illumination” the extent of the assimilation occurred unchanged also when light was lessened to the half of its strength by a grey glass, while with “low illumination” the same lessening of the light caused a lessening of the assimilation down to about a half. The comparison of the results obtained by interrupted vs continuous illumination was made by comparing the assimilation at the same total illumination time.

At high intensity irradiation by a certain amount of energy caused the consumption of more CO₂ when this was administered in intervals than when in a single step installment. The increase arrived at 100% for a rotation of 8000 rpm and ca 10% or a rotation of 2 rpm. Both at a high and at a low intensity of illumination only a small fraction of the energy was translated into chemical changes and thus the first case is not in contrast with the energy law, and for the latter nothing can be predicted from that law. The latter case corresponds to the human Eye, in the fact that in the human eye, in which a certain amount of energy destroys the same amount of CO₂ according to the whether it is administered continuously or intermittently. In order to rationalize this Result, there are two possibilities, either the destruction of CO₂ further occurred in the dark periods, which would lead to think to same form of energy storing form, or in the illuminated period the reaction goes at a twice the rate.²  In the same paper, Warburg also reports the different sensitivity of the CO₂ assimilation depending on the wavelength, previously also reported by Ciamician.³ 

This authoritative paper, submitted by a well-known scientist and a future Nobel Prizes, and related reports from the same laboratory may be considered the first official reports in (quantitative) photobiology.^1,2  Notice further that some years later, Warburg reported that even in aerobic conditions, cancer cells tend to favor metabolism via glycolysis rather than the much more efficient oxidative phosphorylation pathway that is usually preferred by most other cells of the body.⁴  In tumor cells, the last product of glycolysis, pyruvate, is converted into lactate.

The 2019 Centenary Prize (Royal Society of Chemistry) was assigned to prof. David MacMillan (Princeton University) to recognize his impressive contributions to (photoredox and organo) catalysis in organic synthesis.⁵  Prof. Tehshik Yoon (University of Wisconsin-Madison) have been honored with the Arthur C. Cope Mid-Career Scholars Award for his successes in the field of photoredox catalysis.⁶  The Pedler Award was assigned to Prof. Armido Studer (University of Münster), for his contribution in the development of radical based synthetic approaches.⁷  Prof. Maurizio Fagnoni (University of Pavia) has received the Elsevier Lecturship Award from the Japanese Photochemistry Association (JPA) for his impressive effort in the development of photocatalytic C–H functionalization via Hydrogen Atom Transfer.⁸ 

The IUPAC-Zhejiang NHU International Award for Advancements in Green Chemistry was assigned to Prof. Mingxin Liu, for his contributions in developing organic transformation via photosensitizing semiconductors,⁹  and prof. Julian West, in recognition of his efforts in the design and development of synthetic transformations using earth abundant element photocatalysts.¹⁰ 

Prof. Anna Köhler (Universität Bayreuth) has been declared the winner of the 2019 Alexander Todd-Hans Krebs Lectureship in Chemical Sciences for her pioneering investigations of triplet states, exciton dissociation and intermolecular chromophore interactions in π-conjugated polymers.¹¹ 

The JPA Special Lectureship Award was assigned to prof. Masako Kato (Hokkaido University) which worked in the synthesis of photofunctional metal complexes¹²  and to prof. Tsutomu Miyasaka, (Toin University of Yokohama), the inventor of perovskite solar cells.¹³ 

In view of his groundbreaking computational studies of hybrid organic–inorganic solids prof. Aron Walsh (Imperial College London)¹⁴  was awarded by the Corday-Morgan Prize. Prof. Federico Bella (Politecnico di Torino) has received the Environment, Sustainability and Energy Division Early Career Award, in view of the photoinduced polymerization strategies for solar cells and batteries under solvent- and catalyst-free conditions developed in his research group.¹⁵ 

Prof. Dongho Kim (Yonsei University) was the recipient of the JPA Honda-Fujishima Lectureship Award,¹⁶  whereas the JPA Lectureship Award for Asian and Oceanian Photochemistry (Sponsored by Eikosha) was assigned to prof. Hao Ming Chen, (National Taiwan University).¹⁷ 

Ultra-fast carrier dynamics observed in in nanostructures and perovskite semiconductors¹⁸  have been investigated in detail by dr. Matthew C. Beard (National Renewable Energy Laboratory, Colorado, U.S.) that received the Chemical Dynamics Award (2019). In the same field, the Marlow Award was assigned to dr. Samuel Stranks (University of Cambridge) for his studies the relationships between (photo)chemical and material properties in hybrid perovskites.¹⁹  Four researchers of the National Institute of Standards and Technology (dr. Eric K. Lin, dr. Vivek M. Prabhu, dr. Christopher L. Soles, dr. Wen-li Wu) have been awarded with the American Chemical Society (ACS) Award for Team Innovation for their efforts in the preparation of efficient chemically amplified photoresists used in deep and extreme-ultraviolet lithography for semiconductor fabrication.²⁰ 

Prof. Naomi J. Halas (Rice University) received the ACS Award in Colloid Chemistry for the preparation of (in most cases aluminum- based) nanoparticles with tunable optical properties that found applications in sensing, nanomedicine and photocatalysis.²¹ 

The European Society for Photobiology (ESP) assigned two ESP Young Investigator Award to dr. Pilar Acedo (University College London)²²  and dr. Matteo Grattieri (University of Utah).²³ 

Prof. Jennifer S. Brodbelt (The University of Texas, Austin) received the Frank H. Field and Joe L. Franklin Award for the development of ultraviolet photodissociation mass spectrometry and its application in the characterization of molecules and complexes.²⁴  The Ahmed Zewail Award in Ultrafast Science & Technology was given to prof. Hrvoje Petek, (University of Pittsburgh) for his pioneering ultrafast surface science and surface femtochemistry.²⁵ 

The Asian and Oceanian Photochemistry Association (APA) Prize for Young Scientist was assigned to dr. Ekambaram Balaraman (Indian Institute of Science Education and Research, Tirupati),²⁶  dr. Christopher R. Hall (University of Melbourne)²⁷  and prof. In Seob Park (Kyushu University).²⁸ 

Handbook. A significant number of publishing initiatives of the year was focused on the application of heterogeneous photocatalysis in sustainable develolopment. The “Photocatalytic Functional Materials for Environmental Remediation” handbook (Wiley-VCH, 400 pages) edited by Alagarsamy Pandikumar and Kandasamy Jothivenkatachalam aims to be a comprehensive overview on the potential of heterogeneous photocatalysis as a technology to remove (via oxidative processes and thus mineralization) large amounts of pollutants in air, soil, and water. Particular attention has been given to nanocomposite materials, in view of their satisfactory performance and long-term stability.²⁹  The first edition of “Nano-Materials as Photocatalysts for Degradation of Environmental Pollutants” (Elsevier, 430 pages), prepared under the supervision of Pardeep Singh, Anwesha Borthakur, P.K. Mishra and Dhanesh Tiwary is focused on the photocatalytic degradation of harmful pollutants, including petrochemicals and xenobiotic pharmaceutical waste. The volume involves chemical and environmental engineers as the primary audience.³⁰  Analogously, “Visible Light Active Structured Photocatalysts for the Removal of Emerging Contaminants” (editors: Olga Sacco and Vincenzo Vaiano, Elsevier, 230 pages) defines contaminants of emerging concern and presenting, at the same time, visible active structured photocatalysts available and the reaction pathways involved in their action.³¹  The 1st edition of the book “Heterogeneous Photocatalysis” (Elsevier, 290 pages) edited by Giuseppe Marcì and Leonardo Palmisano outlines the basic aspect of both thermal- and photo-catalysis, by presenting and comparing results obtained in the presence an inorganic solid as thermal catalyst and photocatalyst for the same process, and under the same conditions. Importantly, a section reporting the preparation methods of various photocatalysts along with the common techniques used for their characterization is present.³²  The volume “Current Developments in Photocatalysis and Photocatalytic Materials (1st Edition)”, with the ambitious subtitle New Horizon In Photocatalysis was edited by Xinchen Wang, Masakazu Anpo, and Xianzhi Fu (Elsevier, 566 pages). The handbook contains a detailed description of the chemistry and activity of the latest generation of photocatalytic materials.³³  “Photoactive Inorganic Nanoparticles” (editors: Julia Pérez Prieto and María González Béjar, Elsevier, 284 pages,) would offer an overview of capping/functionalization of nanoparticles, by describing characteristics such as structure, functionality (photo)physics and possible applications.³⁴ 

In the tenth anniversary of perovskites cells, Meysam Pazoki, Anders Hagfeldt, and Tomas Edvinsson edited the volume “Characterization Techniques for Perovskite Solar Cell Materials” (Elsevier, 276 pages), that analyses and highlights strengths and weaknesses for all the characterization techniques available for perovskite cells, and discusses the device fabrication of perovskite solar cells.³⁵  “Solar Energy Capture Materials” is the title of the book edited by Elizabeth A Gibson (The Royal Society of Chemistry, 245 pages), a detailed description of the different kind of solar cells developed, including silicon and dye-sensitised cells.³⁶ 

Maurizio Fagnoni, Stefano Protti and Davide Ravelli recently edited “Photoorganocatalysis in Organic Synthesis” the volume 18 of the Catalytic Science Series (World Scientific, 600 pages), dedicated to the use of such emerging class of photocatalysts in metal-free synthetic procedures, via either photoinduced electron transfer or via hydrogen atom transfer.³⁷ 

Special issues. Photochemistry and Photobiological Sciences (with the help of Anna Spalletti, Fausto Ortica and Loredana Latterini as the guest editors) dedicated a theme issue to the memory of prof. Ugo Mazzuccato (1929–2017),³⁸  containing 22 contributions from past and present collaborators of the photochemist who spent most of his career in Perugia. In the volume, Bortolus et al. described the synthesis of a anthraquinone 1 (Fig. 1) bearing a nitroxide radical which spin state (from doublet ground state to either strongly coupled quartet+doublet or an uncoupled triplet and doublet spin state), can be tuned by visible light irradiation and DNA binding.³⁹ 

A volume of the rare metals journal was edited by Tie-Rui Zhang, Gang Liu and Yong-Fa Zhu⁴⁰  and dedicated to the development of photo(electro)chemical nanomaterials for environmental photocatalysis (hydrogen evolution,⁴¹  CO₂ reduction⁴²  and pollutant degradation⁴³ ). ChemSusChem dedicated a special issue to the topic Water Splitting: From Theory to Practice (Guest editors: David Tilley, Annabella Selloni and Takashi Hisatomi).⁴⁴  In the volume, Youn Jeong Jang and Jae Sung Lee are the authors of a minireview on the different p-Type Metal Oxide Semiconductor Photocathodes used in water splitting.⁴⁵ 

Stefanos Giannakis, Sami Rtimi, Ricardo A. Torres-Palma and Sixto Malato were the guest editor of a volume of Applied Catalysis B: Environmental focused on “light-assisted catalysis for water and wastewater treatment” to celebrate the career of prof. Cesar Pulgarin (École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland.).⁴⁶  In the volume a review on the versatile application of (photo) Fenton process was published.⁴⁷ 

A theme issue on “Photoresponsive Molecular Switches and Machines” was published by Chemphotochem (guest editors: H. Dube, I. Aprahamian and N. Tamaoki).⁴⁸  In the volume, the preparation of visible light absorbing protein–polymer conjugate containing a photoactive protein module, (Hpg5-PYP) bearing five reactive alkynyl sites was described by Kinbara and coworkers.⁴⁹  The so prepared conjugate underwent a photoinduced viscosity change upon irradiation with blue light at 25 °C (see Fig. 2).

The same journal launched a virtual issue on “Photopolymerization” edited by Robert Liska, Mao Chen and Pavel Levkin, that covers topics where light is exploited to induce and direct polymerisation processes as well as the design of light-responsive polymers. In one of the contributions, the oxime-esters 2a,b were found to react to broad wavelength range from 400 to 460 nm and undergo sequential N–O homolysis and decarboxylation, to form reactive vinyl radicals that initiate the polymerization of an acrylate resin (Fig. 3).⁵⁰ 

Among the impressive number of theme issues launched by Molecules (MDPI), we would like to bring to your attention the volume “Photocatalytic Strategies in Organic Synthesis” (guest editor: D. Ravelli)⁵¹  and “Solar Chemicals Production and Environmental Remediation with Semiconductor/Carbon Photocatalysts” (Juan Matos Lale and Alicia Gomis Berenguer).⁵² 

Selected contributions presented to the 10th European Meeting on Solar Chemistry & Photocatalysis: Environmental Applications (SPEA10) were collected in a theme issue of the Photochemical and Photobiological Sciences,⁵³  most of them focused on the application of semiconductor photocatalysts in water splitting and environmental remediation, including the degradation of acetaminophen and diclofenac by having recourse to TiO₂ photosensitization by Eosin Y and Rhodamine B dyes.⁵⁴ 

A theme issue on “Computational Photochemistry”, edited by Denis Jacquemin, Lluìs Blancafort and Young Min Rhee appeared on ChemPhotoChem,⁵⁵  and contained a detailed investigation on the one- and two-photon reactivity of One- and Two-photon photoreactivity of Iron Pentacarbonyl [Fe(CO)₅] by means of a hierarchy of linear- and quadratic-response coupled cluster (LR- and QR-CC) methodologies (Fig. 4).⁵⁶ 

In September 2019 Nature published an updated web collection celebrating the 10th anniversary from the publication of the seminal work of Tsutomu Miyasaka and coworkers on the application of hybrid halide perovskites in photovoltaics.⁵⁷  The issue is composed by selected comments, news, research papers and reviews published since January 2015 Among the most recent contributions, we point out a review summarizing the state of the art of photovoltaic solar cell technologies by Nayak and coworkers.⁵⁸  The same topic was celebrated by a virtual collection of the American Chemical Society journals.⁵⁹ 

Applied Catalysis B published a thematic volume on “Novel Materials for Photocatalytic Applications” edited by Dionysios Dionysiou, Suresh C. Pillai and Rtimi Sam. Among the several contributions, Bahnemann and coworkers summarized the recent advances in the preparation of iron-based photocatalytic and photoelectrocatalytic nano-structures.⁶⁰  The vol. 40 of the Journal of Photochemistry and Photobiology C: Reviews included a special section on “New Trends in Heterogeneous Photocatalysts”, consisting in 4 reviews on the design and application of innovative photocatalytic materials, including, among the others, inorganic (e.g. transition metal oxides and chalcogenides) nanosheet-based hybrid photocatalysts.⁶¹ 

Carlos Espiño and José Luis Capelo Martínez edited a special Dyes and Pigments volume containing several contributions from the 3rd International Caparica Conference on Chromogenic and Emissive Materials, such as the preparation of high emissive nanoparticles for cell-imaging via incorporation of tetraphenylethylene into polymeric amphiphiles described by Song and coworkers.⁶² 

In January Photochemistry and Photobiology celebrated its 55th anniversary with a commemorative issue⁶³  consisting in 46 contribution from former and current editors of the journal as well as council members of the American Society of Photobiology.

The Royal Society of Chemistry journal Photochemical and Photobiological Sciences published a theme issue on “Plant Response to UV” under the supervision of the guest editor Gareth I. Jenkins.⁶⁴  The volume collected included eighteen perspective and research papers, including an interesting review on the potentialities of fossil pollens and spores to build a record of the ancient solar-ultraviolet irradiance received by plants.⁶⁵  A Rajendra Rathore Memorial Issue was published by the Journal of Photochemistry and Photobiology A: Chemistry.⁶⁶  Among the different contributions, Wilson and coworkers described the potentialities of dihydrodioxins as the photochemical precursor of orthoquinones able to induce oxidative DNA damage.⁶⁷ 

A volume of the journal “Photodynamic Therapy (PDT) in Oncology” was recently published by Cancer (MDPI) under the supervision of Ángeles Juarranz, Yolanda Gilaberte and Salvador González.⁶⁸ 

In the last decade photoredox catalysis took over the role of leading approach in organic photochemistry. However, recently, alternative strategies emerged and enriched the panorama of proposals, so that we prefer to use the more inclusive term visible (solar) light-driven synthesis rather than photoredox catalyzed processes. The access to terminal enones was achieved via α-methylenation of aryl ketones 3a–c under visible-light irradiation by using methanol as the C1 source and Cu@g-C₃N₄ as the photoredox catalyst. The reaction occurred in 4 to 8 h and furnished the desired product 4a–c in up to 97% yield (Scheme 1).⁶⁹ 

Lei et al. described the versatile (57 examples reported) and efficient photocatalytic radical aroyl chlorination of alkenes to form β-chloroketone that, under the work-up conditions, released the corresponding enone.⁷⁰  Whereas the use of activation of tertiary amines under oxidative conditions is considered a standard procedure, functionalization of primary amines remain a challenge, in view of competitive side reactions. The site-selective α-C(sp3)-H alkylation of easily hydrolysable amides (mainly sulfonamides) with electronpoor alkenes was made possible under blue light irradiation in the presence of the photoredox catalyst Ir(dF-CH₃-ppy)₂(dtbbpy)PF₆ and quinuclidine as the base (an example in Scheme 2).⁷¹ 

A multistep protocol for the enantioselective total synthesis of alcaloid (+)-Flavisiamine F, that involved a late stage visible light photocatalyzed radical cyclization, was recently reported by the group of Xia.⁷²  The rearrangement of an aminocyclopropane moiety 7 into the corresponding 1-aminonorbornane core 8, is initiated by the photoinduced monoelectronic oxidation of the substrate by the photoexcited complex Ir(dF-CF₃-ppy)₂(dtbbpy)PF₆ and followed by C–C homolysis of the generated amine radical cation to form the key β-iminium radical intermediate. The developed methodology operates efficiently also under continuous flow conditions and allows for the gram-scale preparations of 1-aminoboranes, some of them showing a potential to serve as aniline bioisosteres (Scheme 3).⁷³ 

Zhou et al. reported a photocatalytic protocol for the synthesis of trifluoromethylated 2,3-dihydrobenzofurans from 2-vinylphenols, sulfur ylides and the Umemoto's reagent.⁷⁴  The preparation of 2,3-dihydro-4-pyridones and 4-quinolones was realized via photoredox catalyzed dehydrogenation of 4-piperidones and 2,3-dihydro-4-quinolones under aerobic conditions.⁷⁵  Analogously, the dehydrogenation of cyclic amines (including tetrahydroisoquinolines and indolines) in water was performed by having recourse to a dual [Ru(bpy)₃]²⁺ photoredox catalyst/cobalt-based proton reduction catalyst system. Notably, molecular hydrogen was observed as the sole by product of the process.⁷⁶  2019 has been also the year in which the combination of photocatalysis and electrochemistry was made possible by few pioneering works.⁷⁷  In one of the early examples, the photoelectrocatalytic Ar–H functionalization of nitrogen based heteroarenes by alkyltrifluoroborates under oxidant-free conditions was developed. In the suggested mechanism, irradiation of the organic photocatalyst Mes-Acr⁺ and following oxidative quenching of its excited state results in the formation of an alkyl radical and the acridinyl radical Mes-Acr˙, that in turn is oxidized at the anode surface to restore the catalyst. Trapping of the alkyl radical by the protonated heteroarene afford the corresponding radical cation, that upon deprotonation and oxidation (probably operated by the acridinyl radical) afford the final product (Scheme 4).⁷⁸ 

Fu et al. demonstrated that decarboxylation of N-(acyloxy)phthalimide (NPhth) can occur in the presence of triphenylphosphine and sodium iodide under 456 nm irradiation; the generated alkyl radical was then exploited for the transition metal-free alkylation of silyl enol ethers or N-heterocycles, in a Minisci-like reaction (Scheme 5).⁷⁹ 

The generation of carbanions under mild conditions is still considered a challenge, and only few photochemical procedures have been reported in literature. Monoelectronic photoxidation of phenylacetates followed by CO₂ loss and reduction of the generated benzyl radical radicals lead to a carbanion that is in turn trapped by aliphatic aldehydes to form a Grignard analogous reaction product.⁸⁰  The potentialities of Uranyl Nitrate salt (UO₂NO₃ * 6 H₂O) as a Hydrogen Atom Transfer (HAT) photocatalyst were exploited by Capaldo et al. that optimized a procedure for the direct C–H to C–C bond conversion of unactivated (cyclo)alkanes, ethers, acetals, and amides in the presence of electrophilic olefins.⁸¹ 

Visible light drive, photocatalyst-free arylations received significant attention in 2019. De Olivera and coworkers described the functionalization of pyridines and quinolines by aryl diazonium salts via in-situ formation of an electron donor–acceptor complexes (EDA) able to absorb in the visible region.⁸²  Quinoxalinones have been 3-sulfenylated via cross-dehydrogenative coupling with thiols under metal- and catalyst-free conditions by using air as the only oxidant.⁸³ 

Photodynamic therapy (PDT) has become a widely used approach for the treatment of several tumoral diseases. Liu et al. recently summarized the design and the applications of lanthanide doped upconversion nanoparticles (Ln-doped UCNP), that upon near-infrared light (NIR) irradiation undergo UV/visible/NIR upconversion luminescence (UCL) emissions, with a great potential in improving PDT treatment for solid tumors and for imaging of disease lesions.⁸⁴  Photosensitiser BDPI-lyso (12, Fig. 5), that is characterized by a negligible dark cytotoxicity, showed a pH-dependent singlet oxygen quantum yield (ΦΔ) ranging from 0.38 (pH=7) to 0.51 (pH=5).⁸⁵ 

Mao and co-workers described the synthesis of an Ir-based mitochondria-targeted PDT agent able to act, upon irradiation, as PhotoAcid Generator (PAG, 13 in Fig. 6). Furthermore, its photodegradation products act as singlet oxygen photosensitizer to cause a dual-mode (oxygen-independent and oxygen-dependent) photodynamic damage in mitochondria even under hypoxic conditions.⁸⁶ 

The use of 7-dehydrocholesterol (7-DHC) instead of cholesterol in the preparation of photosensitizer (meso tetraphenylporphyrin) encapsulated liposomes was found to enhance their anticancer activity. This is due to the oxidation of 7-DHC into its endoperoxide by photogenerated singlet oxygen that lead to a combined PDT and photoactivated chemotherapy.⁸⁷ 

A Metal Organic Framework (MOF) composed by Ti-oxo chain secondary building units and photosensitizing 5,10,15,20-tetra(p-benzoato)porphyrin (TBP) ligands have been tested for hypoxia-tolerant type I PDT. Such material is able to generate, upon visible light irradiation both singlet oxygen via an energy transfer pathway and other reactive oxygen species (superoxide, hydrogen peroxide, hydroxyl radical via electron transfer) and was tested successfully on CT26 cancer cells.⁸⁸  For the same purpose, covalent organic nanosheets (CONs) characterized by a donor–acceptor molecular heterostructure have been prepared via the co-condensation reaction of 2,3,6,7,10,11-hexahydroxytriphenylene and 5,15-bis(4-boronophenyl)-porphyrin.⁸⁹  Thionaphthalimides 14 (Fig. 7) were found to produce high amounts of ROS under both oxygenated and disareated conditions, suggesting its use as photosensitizer for PDT under hypoxia.⁹⁰ 

Diagnostic devices, according to the acronym “ASSURED”, require to be Affordable, Sensitive, Specific, User-friendly, Rapid and robust, Equipment-free, and Deliverable.⁹¹  In this context, biodegradable cellulose polymers recently emerged as promising starting materials for biocompatible tools. The immobilization of three different model oligopeptides on paper fibers was achieved by following a modular chemoenzymatic approach that relies on the merging of Diels Alder cycloaddition involving a photogenerated enol followed by site-specific sortase A-catalyzed transamidation. In order to verify the spatially control of the reaction, the peptide was decorated with a fluorescent 5(6)-carboxytetramethylrhodamine. (TAMRA) at the amino terminus (Fig. 8).⁹² 

The photorelease of 2-arachidonoylglycerol (2-AG) (a cannabinoid receptors 1 and 2 antagonist) was performed in live cells staring from a pro-lipid conjugated to a photoreactive coumarin-based moiety. Such photocaged compound allows for the overcoming of several limitations including the poor instability of 2-AG in aqueous media (Fig. 9a).⁹³ 

Photolabile protecting groups (PPGs) releasing bioactive compounds upon two-photon excitation have emerged as increasingly popular tools to control and study physiological processes. Yet the limited two-photon photosensitivity of many cages is still a critical issue for applications. Polarized extended coumarines with a large two-photon sensitivity at two complementary wavelengths in the NIR spectral region have been investigated as photolabile protecting groups for the two-photon light induced release of glycine (an example in Fig. 9b).⁹⁴ 

The role of Metal Organic Frameworks (MOFs) in pre preparation of photoresponsive materials has recently received significant attention.⁹⁵  A two-fold interpenetrated 3D MOF bearing two crystallographically distinct C–C double bonds was recently prepared. The material undergoes single crystal-to-single-crystal (SCSC), thermal reversible [2+2] photocycloaddition, that lead to an improved softness of the crystals (see the schematic representation in Fig. 10).⁹⁶ 

The long time known reversible [2+2] photodimerization of coumarins have been exploited for the preparation of photoresponsive polymer materials via wavelength selective photopolymerization. Thus, the monomers 7-(hydroxyethoxy)-4- methyl-coumarin (AECM) and tetrahydrofurfuryl acrylate, were copolymerized at 405 nm (in the presence of acylphosphinate TPO-I as the initiator, Fig. 11). The resulting polymer, upon UV light (λ>300 nm) irradiation, undergoes photo-induced [2+2] dimerization of the coumarin pendant groups; photocleavage of the cross-link to recovery back to the original structure is possible by tuning the irradiation wavelength (λ =254 nm).⁹⁷ 

Nocentini et al. summarized in a remarkable review the recent advancements of Direct Laser Writing technique for patterning of 3D photoresponsive polymers.⁹⁸  Solid-state gas sensors based on metal–oxide transistors have different drawbacks such high power consumption and poor selectivity. Recently, a light-assisted graphene-based sensor for NO₂ have been investigated by Yan et al. Interestingly, UV irradiation was found to improve the response of the sensor sevenfold with respect to the dark condition, resulting in a detection limit below 1 ppm (42.18 ppb).⁹⁹  Differently substituted dibenzo[hi,st]ovalenes (DBOV, 15a-c) that belong to the class of nanographenes have been prepared through a multistep protocol and their photophysics fully characterized revealing the potential of such derivatives as luminescent material, with an emission wavelenght located at 607–647 nm and a quantum yield emission in the 0.67–0.89 range (some esamples in Fig. 12).¹⁰⁰ 

Water splitting via heterogeneous photocatalysts is now recognized a cost-effective technology for the large-scale conversion of solar energy into hydrogen. Domen and co-workers recently took stock on this topic with a tutorial review divided in three sections, the first one devoted to the basics of photocatalytic water splitting and the others focused on the recent advancements in photocatalysis via particle suspension and immobilized particulate systems.¹⁰¹  The same group dedicated a perspective papers to the design approaches for photocatalytic semiconductor materials and technologies recently proposed in literature for hydrogen production.¹⁰²  Another exhaustive review, devoted to the use of graphitic carbon nitride (g-C₃N₄) based photocatalysts in water splitting processes under metal-free conditions has been written by Mishra et al.¹⁰³  A Z-scheme porous g-C₃N₄/Sn₂S₃-diethylenetriamine (Pg-C₃N₄/Sn₂S₃-DETA) composite was designed and applied as photocatalyst for CO₂ reduction to CH₄ and CH₃OH, with a production rate of 4.84 μmol h⁻¹ g⁻¹ and 1.35 μmol h⁻¹ g⁻¹, respectively.¹⁰⁴  An electrode for water oxidation, consisting of a light absorber (a Ru(ii) polypyridyl complex), an intermediate electron donor layer (NiO), and a water oxidation catalyst (a Ru(ii) 2,2′-bipyridine-6,6′-dicarboxylate complex) was assembled by Wang et al. Notably, the use of a NiO overlayer enhanced the performance of the electrode towards water oxidation, and upon one sun exposition in a pH=4.65 solution a 1.1 mA cm⁻² photocurrent density was measured for 2 h without any decomposition of the photoactive system.¹⁰⁵  A photocatalyst with hydrophobic surfaces that enable a three-phase contact of gaseous carbon dioxide water and photocatalyst (solid) and overcomes mass transfer limitations of CO₂ was prepared by Li et al.¹⁰⁶  The material, composed by polymeric carbon nitride nanosheet with a 1H, 1H, 2H, 2H-perfluorodecanethiol functionalized hydrophobic surface and loaded with Pt particles (Pt/o-PCN) allowed for a selectivity of the CO₂ Reduction Reaction (CRR, with respect to the Hydrogen Evolution Reaction, HER) of 87.9% (Scheme 6).¹⁰⁶