Cooperative Photocatalysis Research Articles

Zirconium metal-organic framework photocatalysis with TEMPO for blue light-powered aerobic sulfoxidation.

A zirconium metal-organic framework (MOF), NU-1003, along with TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), is constructed as a system of cooperative photocatalysis for blue light-powered aerobic sulfoxidation. The incorporation of 4 mol% of TEMPO enhances the hole transfer and thereby aerobic sulfoxidation by NU-1003 photocatalysis, demonstrating the pivotal role of a redox mediator in transforming an established MOF.

Leveraging cooperative photocatalysis for the concurrent production of solar fuels and value-added chemicals: mediated by a metal-free porphyrin-based polymeric framework

Porphyrin-based Porp-Tz porous polymeric framework for the concurrent production of solar-fuel and value-added fine chemicals: an economic approach towards sustainability.

Cooperative photocatalysis of hafnium-based metal–organic framework and TEMPO for selective oxidation of sulfides

Cooperative photocatalysis of hafnium-based metal–organic framework and TEMPO for selective oxidation of sulfides

Cooperative photocatalysis and l-/d-proline catalysis enables enantioselective oxidative cross-dehydrogenative coupling of acyclic benzylic secondary amines with ketones

We developed an enantioselective cross-dehydrogenative coupling of acyclic benzylic secondary amines with ketones by combining photocatalysis and l-/d-proline catalysis.

Cooperative photocatalysis of dye-Ti-MCM-41 with trimethylamine for selective aerobic oxidation of sulfides illuminated by blue light.

Ti-incorporated mesoporous materials have widespread applications in photocatalysis. Their adjustable pores could accommodate dyes like alizarin red S (ARS) to circumvent the lack of visible light response. Herein, Ti-MCM-41 was obtained to anchor visible light-capturing ARS, forming ARS-Ti-MCM-41. The ARS-Ti-MCM-41 was screened for the selective photocatalytic oxidation of organic sulfides. To improve the stability of the anchored ARS, electron transfer was orchestrated by a mediator trimethylamine (TMA, 2mol%) illuminated by blue LEDs. Phenyl methyl sulfide could be almost entirely converted into phenyl methyl sulfoxide with 99% of selectivity within 18min. In addition, Ti-MCM-41 was beneficial for the anchored ARS, which in turn guaranteed good recycling performance of ARS-Ti-MCM-41. The solvent trifluoroethanol enabled the stability of TMA and facilitated the highly selective formation of the target sulfoxides. This work sheds light on the vast possibility for visible light photocatalysis of dye-mesoporous materials.

Visible light-driven selective oxidation of amines by cooperative photocatalysis of niobium oxide nanorods with an electron–proton transfer mediator

Cooperative photocatalysis has gained increasing interest to better an existing redox reaction with a mediator. Herein, an ingenious system is constructed for the visible light-driven selective oxidation of a variety of amines by merging niobium oxide (Nb2O5) photocatalysis with an electron–proton transfer mediator, 2,2,6,6-tetramethylpiperidine-1-oxoammonium hexafluorophosphate (TEMPO+PF6−). The visible light-driven dissociative adsorption of amine over Nb2O5 nanorods gives rise to an amine–Nb2O5 complex with a weak response to visible light (λ < 460 nm). With adding 4 mol% of TEMPO+PF6−, the conversion of benzylamine over Nb2O5 is approximately 2.5 times that without TEMPO+PF6−. The proton and electron transfer between the amine–Nb2O5 complex and TEMPO+PF6− is the key to assuring cooperative photocatalysis. Besides, the oxidant dioxygen is predominantly reduced to superoxide anion in affording the imine products. This work highlights it is feasible to ameliorate selective organic conversions over metal oxide photocatalysts with a suited mediator.

Integrating TEMPO into a Metal–Organic Framework for Cooperative Photocatalysis: Selective Aerobic Oxidation of Sulfides

Integrating TEMPO into a Metal–Organic Framework for Cooperative Photocatalysis: Selective Aerobic Oxidation of Sulfides

Combining Brønsted base and photocatalysis into conjugated microporous polymers: Visible light-induced oxidation of thiols into disulfides with oxygen

Numerous applications in visible light photocatalysis have been found over conjugated microporous polymers (CMPs) whose function could be rationally designed at the molecular level. In this context, the oxidation of thiols into disulfides entails proton and electron transfer and thus requires both Brønsted base and photocatalysis, which could be both combined into CMPs. With carbazole as a Brønsted base and an electron donor, CMPs were constructed to implement the synergistic deprotonation and oxidation of thiols into disulfides in ethanol (C2H5OH). Gratifyingly, the bifunctional CMPs could activate molecular oxygen (O2) to superoxide anion (O2•–) and promote the blue light-induced selective oxidation of thiols into symmetrical disulfides with high efficiency in C2H5OH. More remarkably, the highly selective formation of unsymmetrical disulfides could also be achieved without adding a Brønsted base. This work highlights the feasibility of combining cooperative photocatalysis into CMPs for versatile chemical transformations.

Photocatalytic Multielectron Reduction of Nitroarenes to Anilines by Utilizing an Electron-Storable Polyoxometalate-Based Metal-Organic Framework.

A powerful approach to generate photocatalysts for the highly selective reduction of nitrobenzene using light as the driving force is a combination of photosensitizers and electron-storable components in a cooperative photocatalysis fashion. Herein, a new precious metal-free photocatalyst, {ZnW-TPT}, was prepared by incorporating a Zn-substituted monovacant Keggin polyanion [SiZnW11O39]6- and a photoactive organic bridging link 2,4,6-tri(4-pyridyl)-1,3,5-triazine (TPT) into a framework. In this structure, the direct coordination bond between [SiZnW11O39]6- and the TPT ligand and the π-π interactions between TPT molecules help separate and migrate photogenerated carriers, which improves the photocatalytic activity of {ZnW-TPT}. The photoelectrochemical properties of {ZnW-TPT} were well studied by solid UV-vis absorption, fluorescence, transient photocurrent response, and electrochemical impedance spectroscopy tests. {ZnW-TPT} efficiently converts using hydrazine hydrate with 99% conversion and 99% selectivity for anilines under mild conditions.

Designing a Polyoxometalate-Incorporated Metal-Organic Framework for Reduction of Nitroarenes to Anilines by Sequential Proton-Coupled Electron Transfers.

Developing new photocatalysts for reduction of nitroarenes to anilines under mild conditions is very significant. Herein, a new polyoxometalate-based metal-organic framework (POMOF), {[Co(H2O)]2[Co2(H2O)6(TPT)][Co(TPT)PW11O39]}·3H2O·TPT (namely, CoW-TPT, TPT = 2,4,6-tri(4-pyridyl)-1,3,5-triazine), was prepared by incorporating Co(II)-substituted Keggin-type anions [PCoW11O39]5- and a photosensitizer (TPT) into a framework. In this structure, the direct coordination bond between [PCoW11O39]5- and TPT ligand and π···π interactions between TPT molecules are beneficial for the separation and migration of photogenerated carriers, thus improving the photocatalytic activity of CoW-TPT. The combination of both photosensitizer TPT and the electron-storable component [PCoW11O39]5- in a cooperative photocatalysis fashion is conducive to the photocatalytic multielectron reduction of nitroarenes. CoW-TPT provided a high conversion of 94.71% in the photocatalytic reduction of nitroarenes to anilines utilizing triethanolamine as the proton source and electron donor by sequential proton-coupled electron transfers.

Hydroxynaphthalene–Nb2O5complex photocatalysis for selective aerobic oxidation of amines induced by blue light

Aerobic oxidation of benzylamine to imine by cooperative photocatalysis of DHNS–Nb2O5and TEMPO is viable.

Facile synthesis of 2D covalent organic frameworks for cooperative photocatalysis with TEMPO: The selective aerobic oxidation of benzyl amines

2D (two-dimensional) covalent organic frameworks (COFs) have recently emerged as versatile photoactive functional materials. Herein, three β-ketoenamine linked 2D COFs were assembled with 2,4,6-triformylphloroglucinol (Tp) and 4,7-bis(4-aminophenyl)-2,1,3-benzothiadiazole (BTD) as the building blocks at 150, 120, and 25 ℃ respectively, leading to Tp-BTD-150, Tp-BTD-120, and Tp-BTD-25. In particular, the facile synthesis at 25 ℃ afforded Tp-BTD-25 that featured superior activity for the photocatalytic selective aerobic oxidation of benzylamine to the other two assembled at higher temperatures. Most importantly, both the stability and activity of Tp-BTD-25 photocatalyst were augmented with exogenous 5 mol% TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) to ease the transfer of electrons and protons during the oxidation of benzyl amines. Preeminently, the selective aerobic oxidation of benzyl amines into imines was established by cooperative photocatalysis of Tp-BTD-25 with TEMPO irradiated by blue LEDs (light-emitting diodes). This work underscores that crystalline 2D COFs via facile synthesis can cooperatively work with a redox mediator to steer photocatalytic selective reactions.

Extending aromatic acids on TiO2 for cooperative photocatalysis with triethylamine: Violet light-induced selective aerobic oxidation of sulfides

Designing visible light photocatalysts with a metal oxide semiconductor as the starting material could expand a new horizon for the conversion and storage of solar energy. Here, the benchmark photocatalyst TiO2 was used to pursue this goal by anchoring aromatic acids. Extending the aromatic acid was strategically deployed to design TiO2 complexes with violet light-induced selective aerobic oxidation of sulfide as the probe reaction. With benzoic acid (BA) as the initial molecule, horizontally extending one or two benzene rings furnishes 2-naphthoic acid (2-NA) and 2-anthracene acid (2-AA). Moreover, triethylamine (TEA), an electron transfer mediator, was introduced to maintain the integrity of the anchored aromatic acids. Notably, there was a direct correlation between the π-conjugation of aromatic acid ligand and the selective aerobic oxidation of sulfides. Among the three aromatic acids, 2-AA delivered the best result over TiO2 due to the most extensive π-conjugated system. Ultimately, violet light-induced selective aerobic oxidation of sulfides into corresponding sulfoxides was conveniently realized by cooperative photocatalysis of 2-AA-TiO2 with 10 mol% of TEA. This work affords an extending strategy for designing the next-generation ligands for semiconductors to expand visible light-induced selective reactions.

Dual-Single-Atom Tailoring with Bifunctional Integration for High-Performance CO2 Photoreduction.

Single-atom photocatalysis has been demonstrated as a novel strategy to promote heterogeneous reactions. There is a diversity of monoatomic metal species with specific functions; however, integrating representative merits into dual-single-atoms and regulating cooperative photocatalysis remain a pressing challenge. For dual-single-atom catalysts, enhanced photocatalytic activity would be realized through integrating bifunctional properties and tuning the synergistic effect. Herein, dual-single-atoms supported on conjugated porous carbon nitride polymer are developed for effective photocatalytic CO2 reduction, featuring the function of cobalt (Co) and ruthenium (Ru). A series of in situ characterizations and theoretical calculations are conducted for quantitative analysis of structure-performance correlation. It is concluded that the active Co sites facilitate dynamic charge transfer, while the Ru sites promote selective CO2 surface-bound interaction during CO2 photoreduction. The combination of atom-specific traits and the synergy between Co and Ru lead to the high photocatalytic CO2 conversion with corresponding apparent quantum efficiency (AQE) of 2.8% at 385nm, along with a high turnover number (TON) of more than 200 without addition of any sacrificial agent. This work presents an example of identifying the roles of different single-atom metals and regulating the synergy, where the two metals with unique properties collaborate to further boost the photocatalytic performance.

Cooperative Photocatalysis with 4-Amino-TEMPO for Selective Aerobic Oxidation of Amines over TiO2 Nanotubes.

Attaching π-conjugated molecules onto TiO2 can form surface complexes that could capture visible light. However, to make these TiO2 surface complexes durable, integrating 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) or its analogues as a redox mediator with photocatalysis is the key to constructing selective chemical transformations. Herein, sodium 6,7-dihydroxynaphthalene-2-sulfonate (DHNS) was obtained by extending the π-conjugated system of catechol by adding a benzene ring and a substituent sodium sulfonate (-SO3 - Na+ ). The DHNS-TiO2 showed the best photocatalytic activity towards the blue light-induced selective aerobic oxidation of benzylamine. Compared to TEMPO, 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) could rise above 70% in conversion of benzylamine over the DHNS-TiO2 photocatalyst. Eventually, a wide range of amines could be selectively oxidized into imines with atmospheric O2 by cooperative photocatalysis of DHNS-TiO2 with 4-amino-TEMPO. Notably, superoxide (O2 •- ) is crucial in coupling the photocatalytic cycle of DHNS-TiO2 and the redox cycle of 4-amino-TEMPO. This work underscores the design of surface ligands for semiconductors and the selection of a redox mediator in visible light photocatalysis for selective chemical transformations.

Blue light-powered hydroxynaphthoic acid-titanium dioxide photocatalysis for the selective aerobic oxidation of amines

Solar photocatalysis is the key to resolve many environmental challenges but is usually hard to achieve over a metal oxide semiconductor. Therefore, assembling π-conjugated molecules onto semiconductors becomes an efficient approach to solar conversion via ligand-to-metal charge transfer. Here, a rational design of ligands for titanium dioxide (TiO2) is presented to produce robust visible light photocatalysts. Three hydroxynaphthoic acids (HNAs) were selected as ligands by extending an extra benzene ring of salicylic acid (SA) at 3,4 or 4,5 or 5,6 positions. These ligands could regulate the performance of TiO2 in which 2-hydroxy-1-naphthoic acid (2H1NA) endows the best outcome. In detail, blue light-powered cooperative photocatalysis of 2H1NA-TiO2 with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, 5 mol%) inaugurates the expeditious formation of imines by oxidation of amines with atmospheric oxygen (O2). Interestingly, the increase of the O2 pressure from 1 atm to 0.4 MPa promoted the selective oxidation of benzylamine but thereafter declined with a further boost to 0.6 MPa. Notably, an electron transfer between the oxidatively quenched 2H1NA-TiO2 and TEMPO is established, offering a new pathway for environmental applications. This work presents a strategy in designing cutting-edge visible light photocatalysts via altering semiconductors with surface ligands.

Cooperative photocatalysis of dye-TiO2 nanotubes with TEMPO+BF4− for selective aerobic oxidation of amines driven by green light

Visible light photocatalysis could offer an eco-friendly alternative for selective transformation of organic molecules. Herein, a cooperative system is created with alizarin red S (ARS), TiO2 nanotubes (TNTs), and an extra redox mediator. The electron transfer between oxidatively quenched ARS-TNTs and the redox mediator is the key to secure cooperative photocatalysis. The selective aerobic oxidation of primary and secondary amines was constructed by cooperative photocatalysis of ARS-TNTs with 2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate (TEMPO+BF4−) driven by green light. Specifically, the activity of anatase TNTs could reach about 2.2 times that of the P25 (Aeroxide P25) TiO2 precursor. Due to increased polarity, TEMPO+BF4− serves more efficiently for electron transfer than (2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO), conferring above 1.5 times of activity that of TEMPO for the selective aerobic conversion of amines. This work features the potential of designing redox mediators in amplifying cooperative photocatalysis driven by visible light.

A 2D porphyrin-based covalent organic framework with TEMPO for cooperative photocatalysis in selective aerobic oxidation of sulfides

This work reports the cooperative photocatalysis of a two-dimensional covalent organic framework with an electron transfer mediator in a selective organic transformation.

Visible Light‐Driven Cooperative DPZ and Chiral Hydrogen‐Bonding Catalysis

What is the most favorite and original chemistry developed in your research group?The visible light photosensitizer DPZ.How did you get into this specific field? Could you please share some experiences with our readers?Since I began my postdoctoral work at NUS, chiral hydrogen‐bonding catalysis, an important branch of asymmetric organocatalysis, has become the general area of focus of my research. To broaden the applications of this strategy, I proposed to exploit photocatalysis to generate highly reactive radical intermediates, thus overcoming the current limitations in both reaction and substrate types stemming from the low energy of hydrogen‐bonding interactions. The development of highly efficient organophotocatalysts and explorations of transition metal‐free cooperative photocatalysis and chiral hydrogen‐bonding catalysis have thereforebeen my research focus since 2013. My students and I strive to follow the saying “stick to the research direction, thoroughly understand the scientific challenges, and face those challenges with optimism and determination”.What is the most important personality for scientific research?To do independent, original, ground‐breaking and useful chemistry.What are your hobbies?Playing basketball, slow long distance running and reading.How do you maintain a balance between research and family?Having the understanding and support of my family is the most important thing since scientific research requires a large investment of time.Who influences you the most in your life?My parents.Chiral H‐bonding catalysis is a powerful platform for asymmetric synthesis. Nevertheless, the rather low energy of hydrogen‐bonding interactions has inspired the development of generic platforms with high reactivity, thus effectively expanding the utility of this platform by enabling the use of more kinds of readily accessible feedstocks as substrates and providing new, direct and mild synthetic approaches. To this end, we have developed a transition metal‐free cooperative photocatalysis and chiral hydrogen‐bonding catalysis platform by devising a highly efficient organophotosensitizer, DPZ. A series of important asymmetric transformations, in which the stereocentres were formed via either ionic‐type or radical‐ type bond‐forming pathways, have been developed. The successful development of several novel enantioselective protonation reactions and the construction of all‐carbon quaternary stereocentres via highly reactive radical coupling reactions robustly demonstrate the excellent catalytic ability of both DPZ as a photocatalyst and the flexible dual catalyst system.

Cooperative TiO2 photocatalysis with TEMPO and N-hydroxysuccinimide for blue light-driven selective aerobic oxidation of amines

TiO2 has been the focus of attention in semiconductor photocatalysis for several decades because it can potentially settle the grand energy and environmental issues with earth-abundant elements of Ti and O. However, because of its wide band gap, TiO2 can only collect UV light, hindering its practical applications under the illumination of sunlight. In view of this, an interesting phenomenon of light-driven adsorption of amines onto TiO2 to form a visible light-absorbing complex was adapted to assemble smart photocatalysis. The endurance of this complex was eminently refurbished by blue light-driven continuous adsorption of amines. This in turn promoted a vital selective chemical transformation, blue light-driven selective oxidation of amines into imines with atmospheric dioxygen (O2). More importantly, the inclusion of TEMPO and N-hydroxysuccinimide (NHS) into the smart photocatalytic system could cooperatively expedite the blue light-driven selective aerobic oxidation of amines into imines through dual independent reaction channels, resembling that of enzymatic catalysis. This work underscores the importance of manoeuvring multiple reaction channels by cooperative photocatalysis during selective chemical transformations.