Light-driven reduction of aromatic olefins in aqueous media catalysed by aminopyridine cobalt complexes

A catalytic system based on earth-abundant elements that efficiently hydrogenates aryl olefins using visible light as the driving-force and H₂O as the sole hydrogen atom source is reported. The catalytic system involves a robust and well-defined aminopyridine cobalt complex and a heteroleptic Cu photoredox catalyst. The system shows the reduction of styrene in aqueous media with a remarkable selectivity (>20 000) versus water reduction (WR). Reactivity and mechanistic studies support the formation of a [Co–H] intermediate, which reacts with the olefin via a hydrogen atom transfer (HAT). Synthetically useful deuterium-labelled compounds can be straightforwardly obtained by replacing H₂O with D₂O. Moreover, the dual photocatalytic system and the photocatalytic conditions can be rationally designed to tune the selectivity for aryl olefin vs. aryl ketone reduction; not only by changing the structural and electronic properties of the cobalt catalysts, but also by modifying the reduction properties of the photoredox catalyst.

A dual catalytic system based on earth-abundant elements reduces aryl olefins to alkanes in aqueous media under visible light. Mechanistic studies allow for rational tunning of the system for the selective reduction of aryl olefins vs ketones and vice versa.     

Ultra-small PbS nanocrystals as sensitizers for red-to-blue triplet-fusion upconversion

Photon upconversion is a strategy to generate high-energy excitations from low-energy photon input, enabling advanced architectures for imaging and photochemistry. Here, we show that ultra-small PbS nanocrystals can sensitize red-to-blue triplet-fusion upconversion with a large anti-Stokes shift (ΔE = 1.04 eV), and achieve max-efficiency upconversion at near-solar fluences (I_th = 220 mW cm⁻²) despite endothermic triplet sensitization. This system facilitates the photo-initiated polymerization of methyl methacrylate using only long-wavelength light (λ_exc: 637 nm); a demonstration of nanocrystal-sensitized upconversion photochemistry. Time-resolved spectroscopy and kinetic modelling clarify key loss channels, highlighting the benefit of long-lifetime nanocrystal sensitizers, but revealing that many (48%) excitons that reach triplet-extracting carboxyphenylanthracene ligands decay before they can transfer to free-floating acceptors—emphasizing the need to address the reduced lifetimes that we determine for molecular triplets near the nanocrystal surface. Finally, we find that the inferred thermodynamics of triplet sensitization from these ultra-small PbS quantum dots are surprisingly favourable—completing an advantageous suite of properties for upconversion photochemistry—and do not vary significantly across the ensemble, which indicates minimal effects from nanocrystal heterogeneity. Together, our demonstration and study of red-to-blue upconversion using ultra-small PbS nanocrystals in a quasi-equilibrium, mildly endothermic sensitization scheme offer design rules to advance implementations of triplet fusion, especially where large anti-Stokes wavelength shifts are sought.

We demonstrate the use of ultra-small PbS quantum dots as endothermic sensitizers for red-to-blue triplet-fusion upconversion, achieving nanocrystal-sensitized upconversion photochemistry.     

Regiodivergent sulfonylarylation of 1,3-enynes via nickel/photoredox dual catalysis

Catalytic difunctionalization of 1,3-enynes represents an efficient and versatile approach to rapidly assemble multifunctional propargylic compounds, allenes and 1,3-dienes. Controlling selectivity in such addition reactions has been a long-standing challenging task due to multiple reactive centers resulting from the conjugated structure of 1,3-enynes. Herein, we present a straightforward method for regiodivergent sulfonylarylation of 1,3-enynes via dual nickel and photoredox catalysis. Hinging on the nature of 1,3-enynes, diverse reaction pathways are feasible: synthesis of α-allenyl sulfones via 1,4-sulfonylarylation, or preparation of (E)-1,3-dienyl sulfones with high chemo-, regio- and stereoselectivity through 3,4-sulfonylarylation. Notably, this is the first example that nickel and photoredox catalysis are merged to achieve efficient and versatile difunctionalization of 1,3-enynes.

A mild reaction protocol for regiodivergent sulfonylarylation of 1,3-enynes via dual nickel and photoredox catalysis has been developed, which led to efficient synthesis of α-allenyl sulfones or 1,3-dienyl sulfones.     

Understanding the mechanism of direct visible-light-activated [2 + 2] cycloadditions mediated by Rh and Ir photocatalysts: combined computational and spectroscopic studies

The mechanism of [2 + 2] cycloadditions activated by visible light and catalyzed by bis-cyclometalated Rh(iii) and Ir(iii) photocatalysts was investigated, combining density functional theory calculations and spectroscopic techniques. Experimental observations show that the Rh-based photocatalyst produces excellent yield and enantioselectivity whereas the Ir-photocatalyst yields racemates. Two different mechanistic features were found to compete with each other, namely the direct photoactivation of the catalyst–substrate complex and outer-sphere triplet energy transfer. Our integrated analysis suggests that the direct photocatalysis is the inner working of the Rh-catalyzed reaction, whereas the Ir catalyst serves as a triplet sensitizer that activates cycloaddition via an outer-sphere triplet excited state energy transfer mechanism.

The mechanism of [2 + 2] cycloadditions activated by visible light and catalyzed by bis-cyclometalated Rh(iii) and Ir(iii) photocatalysts was investigated, combining density functional theory calculations and spectroscopic techniques.     

Controlling the optical and catalytic properties of artificial metalloenzyme photocatalysts using chemogenetic engineering

Visible light photocatalysis enables a broad range of organic transformations that proceed via single electron or energy transfer. Metal polypyridyl complexes are among the most commonly employed visible light photocatalysts. The photophysical properties of these complexes have been extensively studied and can be tuned by modifying the substituents on the pyridine ligands. On the other hand, ligand modifications that enable substrate binding to control reaction selectivity remain rare. Given the exquisite control that enzymes exert over electron and energy transfer processes in nature, we envisioned that artificial metalloenzymes (ArMs) created by incorporating Ru(ii) polypyridyl complexes into a suitable protein scaffold could provide a means to control photocatalyst properties. This study describes approaches to create covalent and non-covalent ArMs from a variety of Ru(ii) polypyridyl cofactors and a prolyl oligopeptidase scaffold. A panel of ArMs with enhanced photophysical properties were engineered, and the nature of the scaffold/cofactor interactions in these systems was investigated. These ArMs provided higher yields and rates than Ru(Bpy)₃²⁺ for the reductive cyclization of dienones and the [2 + 2] photocycloaddition between C-cinnamoyl imidazole and 4-methoxystyrene, suggesting that protein scaffolds could provide a means to improve the efficiency of visible light photocatalysts.

Artificial metalloenzyme visible light photocatalysts possess enhanced optical properties and are competent towards single electron and energy transfer organic transformations.     

Unleashing non-conjugated polymers as charge relay mediators

The core factors affecting the efficiency of photocatalysis are predominantly centered on controllable modulation of anisotropic spatial charge separation/transfer and regulating vectorial charge transport pathways in photoredox catalysis, yet it still meets with limited success. Herein, we first conceptually demonstrate the rational design of unidirectional cascade charge transfer channels over transition metal chalcogenide nanosheets (TMC NSs: ZnIn₂S₄, CdS, CdIn₂S₄, and In₂S₃), which is synergistically enabled by a solid-state non-conjugated polymer, i.e., poly(diallyldimethyl ammonium chloride) (PDDA), and MXene quantum dots (MQDs). In such elaborately designed photosystems, an ultrathin PDDA layer functions as an intermediate charge transport mediator to relay the directional electron transfer from TMC NSs to MQDs that serve as the ultimate electron traps, resulting in a considerably boosted charge separation/migration efficiency. The suitable energy level alignment between TMC NSs and MQDs, concurrent electron-withdrawing capabilities of the ultrathin PDDA interim layer and MQDs, and the charge transport cascade endow the self-assembled TMC/PDDA/MQD heterostructured photosystems with conspicuously improved photoactivities toward anaerobic selective reduction of nitroaromatics to amino derivatives and photocatalytic hydrogen evolution under visible light irradiation. Furthermore, we ascertain that this concept of constructing a charge transfer cascade in such TMC-insulating polymer-MQD photosystems is universal. Our work would afford novel insights into smart design of spatial vectorial charge transport pathways by precise interface modulation via non-conjugated polymers for solar energy conversion.

Electron relay of interim polymer layer boosts photocatalytic organic transformation.     

Supramolecular photocatalysts fixed on the inside of the polypyrrole layer in dye sensitized molecular photocathodes: application to photocatalytic CO2 reduction coupled with water oxidation

The development of systems for photocatalytic CO₂ reduction with water as a reductant and solar light as an energy source is one of the most important milestones on the way to artificial photosynthesis. Although such reduction can be performed using dye-sensitized molecular photocathodes comprising metal complexes as redox photosensitizers and catalyst units fixed on a p-type semiconductor electrode, the performance of the corresponding photoelectrochemical cells remains low, e.g., their highest incident photon-to-current conversion efficiency (IPCE) equals 1.2%. Herein, we report a novel dye-sensitized molecular photocathode for photocatalytic CO₂ reduction in water featuring a polypyrrole layer, [Ru(diimine)₃]²⁺ as a redox photosensitizer unit, and Ru(diimine)(CO)₂Cl₂ as the catalyst unit and reveal that the incorporation of the polypyrrole network significantly improves reactivity and durability relative to those of previously reported dye-sensitized molecular photocathodes. The irradiation of the novel photocathode with visible light under low applied bias stably induces the photocatalytic reduction of CO₂ to CO and HCOOH with high faradaic efficiency and selectivity (even in aqueous solution), and the highest IPCE is determined as 4.7%. The novel photocathode is coupled with n-type semiconductor photoanodes (CoO_x/BiVO₄ and RhO_x/TaON) to construct full cells that photocatalytically reduce CO₂ using water as the reductant upon visible light irradiation as the only energy input at zero bias. The artificial Z-scheme photoelectrochemical cell with the dye-sensitized molecular photocathode achieves the highest energy conversion efficiency of 8.3 × 10⁻²% under the irradiation of both electrodes with visible light, while a solar to chemical conversion efficiency of 4.2 × 10⁻²% is achieved for a tandem-type cell using a solar light simulator (AM 1.5, 100 mW cm⁻²).

A novel dye-sensitized molecular photocathode with polypyrrole networks exhibits high efficiency and durability for photocatalytic CO₂ reduction by using water as reductant and visible light as energy.     

Molecular crystalline capsules that release their contents by light

Here, we present single crystalline capsules of a photoresponsive molecule produced by simple recrystallization from organic solutions without direct human processing. During the crystal growth process, a movie was taken of the capsule taking in the organic solution. The capsules responded rapidly (<1 s) to the UV light stimuli and released the captured solution or solute. In principle, they can take in any substance dissolved in organic solvents, and their size can be controlled. Moreover, the capsule can be broken by multi-photon excitation using a near-infrared laser within the biological window. Furthermore, because the molecular packing in the crystal is unidirectional, the response can be controlled by the polarization of the light. This study shows the new potential of photoresponsive molecules.

A novel diarylethene formed “crystalline capsules” containing liquid inclusions, and chemicals trapped in the capsules were released by photoinduced breaking. Operations by multiphoton and linear polarized light were demonstrated.     

NHC and visible light-mediated photoredox co-catalyzed 1,4-sulfonylacylation of 1,3-enynes for tetrasubstituted allenyl ketones

The modulation of selectivity of highly reactive carbon radical cross-coupling for the construction of C–C bonds represents a challenging task in organic chemistry. N-Heterocyclic carbene (NHC) catalyzed radical transformations have opened a new avenue for acyl radical cross-coupling chemistry. With this method, highly selective cross-coupling of an acyl radical with an alkyl radical for efficient construction of C–C bonds was successfully realized. However, the cross-coupling reaction of acyl radicals with vinyl radicals has been much less investigated. We herein describe NHC and visible light-mediated photoredox co-catalyzed radical 1,4-sulfonylacylation of 1,3-enynes, providing structurally diversified valuable tetrasubstituted allenyl ketones. Mechanistic studies indicated that ketyl radicals are formed from aroyl fluorides via the oxidative quenching of the photocatalyst excited state, allenyl radicals are generated from chemo-specific sulfonyl radical addition to the 1,3-enynes, and finally, the key allenyl and ketyl radical cross-coupling provides tetrasubstituted allenyl ketones.

Unprecedented NHC and photocatalysis co-catalyzed radical 1,4-sulfonylacylation of 1,3-enynes has been realized, providing structurally diversified tetrasubstituted allenyl ketones via allenyl and ketyl radical cross-coupling.     

Decarboxylative 1,4-carbocyanation of 1,3-enynes to access tetra-substituted allenes via copper/photoredox dual catalysis

We disclose herein the first example of merging photoredox catalysis and copper catalysis for radical 1,4-carbocyanations of 1,3-enynes. Alkyl N-hydroxyphthalimide esters are utilized as radical precursors, and the reported mild and redox-neutral protocol has broad substrate scope and remarkable functional group tolerance. This strategy allows for the synthesis of diverse multi-substituted allenes with high chemo- and regio-selectivities, also permitting late stage allenylation of natural products and drug molecules.

An efficient synthesis of multi-substituted allenes by metallaphotoredox-catalyzed decarboxylative 1,4-carbocyanation of 1,3-enynes is described.     

Porphyrin-based donor–acceptor COFs as efficient and reusable photocatalysts for PET-RAFT polymerization under broad spectrum excitation

Covalent organic frameworks (COFs) are crystalline and porous organic materials attractive for photocatalysis applications due to their structural versatility and tunable optical and electronic properties. The use of photocatalysts (PCs) for polymerizations enables the preparation of well-defined polymeric materials under mild reaction conditions. Herein, we report two porphyrin-based donor–acceptor COFs that are effective heterogeneous PCs for photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT). Using density functional theory (DFT) calculations, we designed porphyrin COFs with strong donor–acceptor characteristics and delocalized conduction bands. The COFs were effective PCs for PET-RAFT, successfully polymerizing a variety of monomers in both organic and aqueous media using visible light (λ_max from 460 to 635 nm) to produce polymers with tunable molecular weights (MWs), low molecular weight dispersity, and good chain-end fidelity. The heterogeneous COF PCs could also be reused for PET-RAFT polymerization at least 5 times without losing photocatalytic performance. This work demonstrates porphyrin-based COFs that are effective catalysts for photo-RDRP and establishes design principles for the development of highly active COF PCs for a variety of applications.

Porphyrin-based donor–acceptor COFs are effective heterogeneous photocatalysts for photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT), including for aqueous polymerizations and under red-light excitation.     

Photomediated core modification of organic photoredox catalysts in radical addition: mechanism and applications

Dihydrophenazines and their analogues have been widely used as strong reducing photoredox catalysts in radical chemistry, such as organocatalyzed atom transfer radical polymerization (O-ATRP). However, when dihydrophenazines were employed as organic photoredox catalysts (OPCs) to mediate O-ATRP, the initiator efficiency was nonquantitative due to cross-coupling between dihydrophenazines and radical species. Here, a new kind of core modification for dihydrophenazines, phenoxazines and phenothiazines was developed through this cross-coupling process. Mechanistic studies suggested that the radical species would be more likely to couple with OPC'' radical cations rather than the ground-state OPC. Core modification of OPCs could stabilize the radical ions in an oxidative quenching catalytic cycle. Significantly, core modifications of OPCs could lower the energy of light required for photoexcitation. Compared with their noncore-modified counterparts, all the core-modified dihydrophenazines and phenoxazines exhibited efficient performance in controlling O-ATRP for the synthesis of poly(methyl methacrylate) with higher initiator efficiencies under the irradiation of simulated sunlight.

Photomediated core modification of organic photocatalysts through a radical/radical cation cross-coupling process enables the ability to stabilize radical ions and prevent undesirable side reactions.     

Cyanine-based near infra-red organic photoredox catalysis

Direct metal-free near infra-red photoredox catalysis is applied to organic oxidation, photosensitization and reduction, involving cyanines as photocatalysts. This photocatalyst is competitive with conventional reactions catalyzed under visible light. Kinetic and quenching experiments are also reported. Interestingly, these systems are compatible with water media, opening perspective for various applications.

Direct metal-free near infra-red photoredox catalysis is applied to oxidation, reduction and photosensitization, involving cyanines as photocatalysts. Mechanistic insights through kinetic and quenching experiments are also reported.     

Stereoselective synthesis of unnatural α-amino acid derivatives through photoredox catalysis

A protocol for stereoselective C-radical addition to a chiral glyoxylate-derived N-sulfinyl imine was developed through visible light-promoted photoredox catalysis, providing a convenient method for the synthesis of unnatural α-amino acids. The developed protocol allows the use of ubiquitous carboxylic acids as radical precursors without prior derivatization. The protocol utilizes near-stoichiometric amounts of the imine and the acid radical precursor in combination with a catalytic amount of an organic acridinium-based photocatalyst. Alternative mechanisms for the developed transformation are discussed and corroborated by experimental and computational studies.

A protocol for stereoselective C-radical addition to a chiral glyoxylate-derived N-sulfinyl imine was developed through visible light-promoted photoredox catalysis, providing a convenient method for the synthesis of unnatural α-amino acids.     

Manipulation of charge carrier flow in Bi4NbO8Cl nanoplate photocatalyst with metal loading

Separation of photoexcited charge carriers in semiconductors is important for efficient solar energy conversion and yet the control strategies and underlying mechanisms are not fully established. Although layered compounds have been widely studied as photocatalysts, spatial separation between oxidation and reduction reaction sites is a challenging issue due to the parallel flow of photoexcited carriers along the layers. Here we demonstrate orthogonal carrier flow in layered Bi₄NbO₈Cl by depositing a Rh cocatalyst at the edges of nanoplates, resulting in spatial charge separation and significant enhancement of the photocatalytic activity. Combined experimental and theoretical studies revealed that lighter photogenerated electrons, due to a greater in-plane dispersion of the conduction band (vs. valence band), can travel along the plane and are readily trapped by the cocatalyst, whereas the remaining holes hop perpendicular to the plane because of the anisotropic crystal geometry. Our results propose manipulating carrier flow via cocatalyst deposition to achieve desirable carrier dynamics for photocatalytic reactions in layered compounds.

An efficient spatial separation of photoexcited carriers is demonstrated in a layered-oxyhalide nanoplate by controlling the direction of carrier flows upon Rh cocatalyst loading, which leads to drastic photocatalytic H₂ evolution activity.     

De novo design and synthesis of dipyridopurinone derivatives as visible-light photocatalysts in productive guanylation reactions

Described here is the de novo design and synthesis of a series of 6H-dipyrido[1,2-e:2′,1′-i]purin-6-ones (DPs) as a new class of visible-light photoredox catalysts (PCs). The synthesized DP1–5 showed their λ_Abs(max) values in 433–477 nm, excited state redox potentials in 1.15–0.69 eV and −1.41 to −1.77 eV (vs. SCE), respectively. As a representative, DP4 enables the productive guanylation of various amines, including 1°, 2°, and 3°-alkyl primary amines, secondary amines, aryl and heteroaryl amines, amino-nitrile, amino acids and peptides as well as propynylamines and α-amino esters giving diversities in biologically important guanidines and cyclic guanidines. The photocatalytic efficacy of DP4 in the guanylation overmatched commonly used Ir and Ru polypyridyl complexes, and some organic PCs. Other salient merits of this method include broad substrate scope and functional group tolerance, gram-scale synthesis, and versatile late-stage derivatizations that led to a derivative 81 exhibiting 60-fold better anticancer activity against Ramos cells with the IC₅₀ of 0.086 μM than that of clinical drug ibrutinib (5.1 μM).

A novel visible-light photocatalyst was designed and its photocatalytic efficacy in the guanylation of amines overmatched common metal-core and organic photocatalysts.     

Chemical synthesis of stimuli-responsive guide RNA for conditional control of CRISPR-Cas9 gene editing

CRISPR-Cas9 promotes changes in identity or abundance of nucleic acids in live cells and is a programmable modality of broad biotechnological and therapeutic interest. To reduce off-target effects, tools for conditional control of CRISPR-Cas9 functions are under active research, such as stimuli-responsive guide RNA (gRNA). However, the types of physiologically relevant stimuli that can trigger gRNA are largely limited due to the lack of a versatile synthetic approach in chemistry to introduce diverse labile modifications into gRNA. In this work, we developed such a general method to prepare stimuli-responsive gRNA based on site-specific derivatization of 2′-O-methylribonucleotide phosphorothioate (PS-2′-OMe). We demonstrated CRISPR-Cas9-mediated gene editing in human cells triggered by oxidative stress and visible light, respectively. Our study tackles the synthetic challenge and paves the way for chemically modified RNA to play more active roles in gene therapy.

Conditional control of CRISPR-Cas9 activity by reactive oxygen species and visible light is achieved using stimuli-responsive guide RNA synthesized by a general method based on RNA 2′-O-methylribonucleotide phosphorothioate.     

Asymmetric β-arylation of cyclopropanols enabled by photoredox and nickel dual catalysis

The enantioselective functionalization and transformation of readily available cyclopropyl compounds are synthetically appealing yet challenging topics in organic synthesis. Here we report an asymmetric β-arylation of cyclopropanols with aryl bromides enabled by photoredox and nickel dual catalysis. This dual catalytic transformation features a broad substrate scope and good functional group tolerance at room temperature, providing facile access to a wide array of enantioenriched β-aryl ketones bearing a primary alcohol moiety in good yields with satisfactory enantioselectivities (39 examples, up to 83% yield and 90% ee). The synthetic value of this protocol was illustrated by the concise asymmetric construction of natural product calyxolane B analogues.

An asymmetric β-arylation of cyclopropanols with aryl bromides was enabled by enantioselective photoredox and nickel dual catalysis.