The oxidation of water to molecular oxygen is the key step to realize water splitting from both biological and chemical perspective. In an effort to understand how water oxidation occurs on a molecular level, a large number of molecular catalysts have been synthesized to find an easy access to higher oxidation states as well as their capacity to make O−O bond. However, most of them function in a mixture of organic solvent and water and the O−O bond formation pathway is still a subject of intense debate. Herein, we design the first amphiphilic Ru-bda (H₂bda=2,2′-bipyridine-6,6′-dicarboxylic acid) water oxidation catalysts (WOCs) of formula [Ru^II(bda)(4-OTEG-pyridine)₂] (1, OTEG=OCH₂CH₂OCH₂CH₂OCH₃) and [Ru^II(bda)(PySO₃Na)₂] (2, PySO₃⁻=pyridine-3-sulfonate), which possess good solubility in water. Dynamic light scattering (DLS), scanning electron microscope (SEM), critical aggregation concentration (CAC) experiments and product analysis demonstrate that they enable to self-assemble in water and form the O−O bond through different routes even though they have the same bda²⁻ backbone. This work illustrates for the first time that the O−O bond formation pathway can be regulated by the interaction of ancillary ligands at supramolecular level.

The oxidation of water to molecular oxygen is the key step to realize water splitting from both biological and chemical perspective. In an effort to understand how water oxidation occurs on a molecular level, a large number of molecular catalysts have been synthesized to find an easy access to higher oxidation states as well as their capacity to make O−O bond. However, most of them function in a mixture of organic solvent and water and the O−O bond formation pathway is still a subject of intense debate. Herein, we design the first amphiphilic Ru-bda (H₂bda=2,2′-bipyridine-6,6′-dicarboxylic acid) water oxidation catalysts (WOCs) of formula [Ru^II(bda)(4-OTEG-pyridine)₂] (1, OTEG=OCH₂CH₂OCH₂CH₂OCH₃) and [Ru^II(bda)(PySO₃Na)₂] (2, PySO₃⁻=pyridine-3-sulfonate), which possess good solubility in water. Dynamic light scattering (DLS), scanning electron microscope (SEM), critical aggregation concentration (CAC) experiments and product analysis demonstrate that they enable to self-assemble in water and form the O−O bond through different routes even though they have the same bda²⁻ backbone. This work illustrates for the first time that the O−O bond formation pathway can be regulated by the interaction of ancillary ligands at supramolecular level.

Bicarbonyl-substituted sulfur ylide is a useful, but inert reagent in organic synthesis. Usually, harsh reaction conditions are required for its transformation. For the first time, it was demonstrated that a new, visible-light photoredox catalytic annulation of sulfur ylides under extremely mild conditions, permits the synthesis of oxindole derivatives in high selectivities and efficiencies. The key to its success is the photocatalytic single-electron-transfer (SET) oxidation of the inert amide and acyl-stabilized sulfur ylides to reactive radical cations, which easily proceeds with intramolecular C−H functionalization to give the final products.

Square-planar polypyridyl platinum(II) complexes possess a rich range of structural and spectroscopic properties that are ideal for designing artificial photosynthetic centers. Taking advantage of the directionality in the charge-transfer excitation from the metal to the polypyridyl ligand, we describe here diplatinum(II)–ferrocene dyads, open-butterfly-like dyad 1 and closed-butterfly-like dyad 2, which were designed to understand the conformation and orientation effects to prolong the lifetime of charge-separated state. In contrast to the open-butterfly-like dyad 1, the closed-butterfly-like dyad 2 shows three-times long lifetime of charge separated state upon photoexcitation, demonstrating that the orientation in the rigid structure of dyad 2 is a very important issue to achieve long-lived charge separated state.

Nature uses hydrogenase enzyme to catalyze proton reduction at pH 7 with overpotentials and catalytic efficiencies that rival platinum electrodes. Over the past several years, [FeFe]-hydrogenase ([FeFe]-H₂ase) mimics have been demonstrated to be effective catalysts for light-driven H₂ evolution. However, it remains a significant challenge to realize H₂ production by such an artificial photosynthetic system in neutral aqueous solution. Herein, we report a new system for photocatalytic H₂ evolution working in a broad pH range, especially under neutral conditions. This unique system is consisted of branched polyethylenimine (PEI)-grafted [FeFe]-H₂ase mimic (PEI-g-Fe₂S₂), MPA-CdSe quantum dots (MPA=mercaptopropionic acid), and ascorbic acid (H₂A) in water. Due to the secondary coordination sphere of PEI, which has high buffering capacity and stabilizing ability, the system is able to produce H₂ under visible-light irradiation with turnover number of 10 600 based on the Fe₂S₂ active site in PEI-g-Fe₂S₂. The stability and activity are much better than that of the same system under acidic or basic conditions and they are, to the best of our knowledge, the highest known to date for photocatalytic H₂ evolution from a [FeFe]-H₂ase mimic in neutral aqueous solution.

The direct and controlled activation of a C(sp³)H bond adjacent to an O atom is of particular synthetic value for the conventional derivatization of ethers or alcohols. In general, stoichiometric amounts of an oxidant are required to remove an electron and a hydrogen atom of the ether for subsequent transformations. Herein, we demonstrate that the activation of a CH bond next to an O atom could be achieved under oxidant-free conditions through photoredox-neutral catalysis. By using a commercial dyad photosensitizer (Acr⁺-Mes ClO₄⁻, 9-mesityl-10-methylacridinium perchlorate) and an easily available cobaloxime complex (Co(dmgBF₂)₂⋅2 MeCN, dmg=dimethylglyoxime), the nucleophilic addition of β-keto esters to oxonium species, which is rarely observed in photocatalysis, leads to the corresponding coupling products and H₂ in moderate to good yields under visible-light irradiation. Mechanistic studies suggest that both isochroman and the cobaloxime complex quench the electron-transfer state of this dyad photosensitizer and that benzylic CH bond cleavage is probably the rate-determining step of this cross-coupling hydrogen-evolution transformation.

A visible-light-induced hydrogen evolution system based on a CdSe quantum dots (QDs)–TiO₂–Ni(OH)₂ ternary assembly has been constructed under an ambient environment, and a bifunctional molecular linker, mercaptopropionic acid, is used to facilitate the interaction between CdSe QDs and TiO₂. This hydrogen evolution system works effectively in a basic aqueous solution (pH 11.0) to achieve a hydrogen evolution rate of 10.1 mmol g⁻¹ h⁻¹ for the assembly and a turnover frequency of 5140 h⁻¹ with respect to CdSe QDs (10 h); the latter is comparable with the highest value reported for QD systems in an acidic environment. X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and control experiments demonstrate that Ni(OH)₂ is an efficient hydrogen evolution catalyst. In addition, inductively coupled plasma optical emission spectroscopy and the emission decay of the assembly combined with the hydrogen evolution experiments show that TiO₂ functions mainly as the electron mediator; the vectorial electron transfer from CdSe QDs to TiO₂ and then from TiO₂ to Ni(OH)₂ enhances the efficiency for hydrogen evolution. The assembly comprises light antenna CdSe QDs, electron mediator TiO₂, and catalytic Ni(OH)₂, which mimics the strategy of photosynthesis exploited in nature and takes us a step further towards artificial photosynthesis.

Quantum dots (QDs) offer new and versatile ways to harvest light energy. However, there are few examples involving the utilization of QDs in organic synthesis. Visible-light irradiation of CdSe QDs was found to result in virtually quantitative coupling of a variety of thiols to give disulfides and H₂ without the need for sacrificial reagents or external oxidants. The addition of small amounts of nickel(II) salts dramatically improved the efficiency and conversion through facilitating the formation of hydrogen atoms, thereby leading to faster regeneration of the ground-state QDs. Mechanistic studies reveal that the coupling reaction occurs on the QD surfaces rather than in solution and offer a blueprint for how these QDs may be used in other photocatalytic applications. Because no sacrificial agent or oxidant is necessary and the catalyst is reusable, this method may be useful for the formation of disulfide bonds in proteins as well as in other systems sensitive to the presence of oxidants.

Natural photosynthesis offers the concept of storing sunlight in chemical form as hydrogen (H₂), using biomass and water. Herein we describe a robust artificial photocatalyst, nickel-hybrid CdS quantum dots (Ni_h-CdS QDs) made in situ from nickel salts and CdS QDs stabilized by 3-mercaptopropionic acid, for visible-light-driven H₂ evolution from glycerol and water. With visible light irradiation for 20 h, 403.2 μmol of H₂ was obtained with a high H₂ evolution rate of approximately 74.6 μmol h⁻¹ mg⁻¹ and a high turnover number of 38 405 compared to MPA-CdS QDs (mercaptopropionic-acid-stabilized CdS quantum dots). Compared to CdTe QDs and CdSe QDs, the modified CdS QDs show the greatest affinity toward Ni²⁺ ions and the highest activity for H₂ evolution. X-ray photoelectron spectroscopy (XPS), inductively-coupled plasma atomic emission spectrometry (ICP-AES), and photophysical studies reveal the chemical nature of the Ni_h-CdS QDs. Electron paramagnetic resonance (EPR) and terephthalate fluorescence measurements clearly demonstrate water splitting to generate ⋅OH radicals. The detection of DMPO-H and DMPO-C radicals adduct in EPR also indicate that ⋅H radicals and ⋅C radicals are the active species in the catalytic cycle.

Quantum dots (QDs) offer new and versatile ways to harvest light energy. However, there are few examples involving the utilization of QDs in organic synthesis. Visible-light irradiation of CdSe QDs was found to result in virtually quantitative coupling of a variety of thiols to give disulfides and H₂ without the need for sacrificial reagents or external oxidants. The addition of small amounts of nickel(II) salts dramatically improved the efficiency and conversion through facilitating the formation of hydrogen atoms, thereby leading to faster regeneration of the ground-state QDs. Mechanistic studies reveal that the coupling reaction occurs on the QD surfaces rather than in solution and offer a blueprint for how these QDs may be used in other photocatalytic applications. Because no sacrificial agent or oxidant is necessary and the catalyst is reusable, this method may be useful for the formation of disulfide bonds in proteins as well as in other systems sensitive to the presence of oxidants.

Irradiation of cyclohexyl phenyl ketone (1) results in either intra- or intermolecular hydrogen abstraction to afford compounds 1-phenylhept-6-en-1-one (2) and α-cyclohexyl benzyl alcohol (3) as photoproducts, in which 3 has a pair of enantiomers. Herein, two types of chiral hexagonal liquid crystals were prepared to direct the enantioselective photochemical reaction of 1. One of the hexagonal liquid crystals is composed of 1-tetradecyl-3-methylimidazolium bromide/p-xylene/H₂O with the modification by a chiral inductor. The other one is formed by chiral (S)-3-hexadecyl-1-(1-hydroxy-propan-2-yl)-imidazolium bromide/p-xylene/H₂O. The product analysis shows that the latter one can be used as a microreactor to achieve stereoselective photochemical reaction, while the former one produced compound 3 with no enantioselectivity at all.

2-Ureido-4(1H)-pyrimidinone-bridged ferrocene–fullerene assembly I is designed and synthesized for elaborating the photoinduced electron-transfer processes in self-complementary quadruply hydrogen-bonded modules. Unexpectedly, steady-state and time-resolved spectroscopy reveal an inefficient electron-transfer process from the ferrocene to the singlet or triplet excited state of the fullerene, although the electron-transfer reactions are thermodynamically feasible. Instead, an effective intra-assembly triplet–triplet energy-transfer process is found to be operative in assembly I with a rate constant of 9.2×10⁵ s⁻¹ and an efficiency of 73 % in CH₂Cl₂ at room temperature.

Thanks to the superior redox potential of platinum(II) complex compared with that of Ru(bpy)₃²⁺ in the excited state, an efficient and selective visible-light-induced CDC reaction has been developed by using a catalytic amount (0.25 %) of 1. With the aid of FeSO₄ (2 equiv), the corresponding amide could not be detected under visible-light irradiation (λ=450 nm), but the desired cross-coupling product was exclusively obtained under ambient air conditions. A spectroscopic study and product analysis revealed that the CDC reaction is initiated by photoinduced electron-transfer from N-phenyltetrahydroisoquinoline to the complex. An EPR (electron paramagnetic resonance) experiment provides direct evidence on the generation of superoxide radical anion (O₂^−.) rather than singlet oxygen (¹O₂) under irradiation of the reaction system, in contrast to that reported in the literature. Combined, the photoinduced electron-transfer and subsequent formation of superoxide radical anion (O₂^−.) results in a clean and facile transformation.

With visible-light irradiation, a mild, simple, and efficient metal-free photocatalytic system for the facile construction of sp³–sp³ CC bonds between tertiary amines and activated CH bonds has been achieved. Spectroscopic study and product analysis demonstrate for the first time that photoinduced electron transfer from N-aryl tetrahydroisoquinolines to eosin Y bis(tetrabutylammonium salt) (TBA-eosin Y) takes place to generate TBA-eosin Y radical anion, which can subsequently react with nucleophiles and molecular oxygen. More strikingly, electron spin resonance (ESR) measurements provide direct evidence for the formation of superoxide radical anions (O₂^−.) rather than singlet oxygen (¹O₂) during visible-light irradiation. This active species is therefore believed to be responsible for the large rate of acceleration of the aerobic photocatalytic reactions.

To mimic [FeFe] hydrogenases (H₂ases) in nature, molecular photocatalysts 1a, 1b, and 1c anchoring rhenium(I) complex S to one of the iron cores of [FeFe]-H₂ases model complex C, have been constructed for H₂ generation by visible light in homogeneous solution. The time-dependence of H₂ evolution and a spectroscopic study demonstrate that the orientation of S and the specific bridge in 1a, 1b, and 1c are important both for the electron-transfer step from the excited S* to the catalytic C, and the formation of unprecedented long-lived charge separation for 1a (780 μs), 1b, and 1c (>2 ms) in [FeFe]-H₂ases mimics. The fast forward electron-transfer step from the excited S* to the catalytic C but the slow back electron-transfer step of the charge-recombination in the designed photocatalysts 1a, 1b, and 1c are reminiscent of the behavior of [FeFe]-H₂ases in nature.

Reactions of carboxyl-functionalized cubane-like photocyclodimer 2 with a series of aromatic amines 3 were investigated for understanding the reactivity of this unique system. Spectroscopic and X-ray crystal structural analyses demonstrate that N,N′-dicyclohexylcarbodiimide (DCC) is reactive toward 2 affording stable 4 with a well-retained cubane-like structure. With the assistance of DCC, mono- and diacyl-substituted photocyclodimers of 2-sustituted naphthalene (5 and 6) have also been achieved in reasonable yields in addition to the formation of 4. Subtle change in the electronic properties of the series of aromatic amines 3 modulates the reactivity and product distribution remarkably.