Design and Synthesis of a Triazine-Based sp2 Carbon-Conjugated Covalent Organic Framework for Blue Light Photocatalysis.

Fully conjugated covalent organic frameworks (COFs) can exhibit great potential in semiconductor photocatalysis. But their syntheses remain elusive due to the low reversibility of vinylene linkage. Herein, by tuning the amount of base and temperature, a novel triazine-based sp2  carbon-conjugated COF (TA-sp2 c-COF) is successfully constructed over Cs2 CO3 . Besides, the influence of modulating factors on the chemical and optoelectronic properties of TA-sp2 c-COF is thoroughly investigated. TA-sp2 c-COF adopts an eclipsed AA stacking structure with uniform micropores (1.4 nm). The blue light photocatalysis of the highly crystalline TA-sp2 c-COF is established for the selective oxidative coupling of amines with oxygen, and the predominant role of superoxide is identified in forming imines. This work foretells that meticulous modulation of reaction conditions is the key to constructing sp2  carbon-conjugated COFs toward solar photocatalysis.

Surface Engineering of Conjugated Polybenzothiadiazoles and Integration with Cobalt Oxides for Photocatalytic Water Oxidation.

Conjugated polymers feature promising structure and properties for photocatalytic water splitting. Herein, a hydrolysis strategy was demonstrated to rationally modulate the surface hydrophilicity and band structures of conjugated poly-benzothiadiazoles. High hydrophilicity not only enhances the dispersions of polymeric solids in an aqueous solution but also reduces the absorption energy of water molecules. Besides, both theoretical and experimental results reveal that a more positive valence band potential is generated, which contributes to enhancing the photocatalytic water oxidation performance. Accordingly, the surface-modified conjugated polymers show largely promoted photocatalytic water oxidation activities by deposition of cobalt oxides as cocatalysts.

Superoxide generated by blue light photocatalysis of g-C3N4/TiO2 for selective conversion of amines.

Reactive oxygen species (ROS) are remarkably reactive chemical oxygen-containing molecules that not only occupy critical positions in cell signaling and homeostasis for regulating aerobic living organism's growth and development but also broadly participate in the environmental management as extraordinary oxidizing agents. Inspired by the behaviors of ROS, we designed an artful visible light photocatalytic system for the selective conversion of amines due to the activation of oxygen (O2) to superoxide (O2-) over g-C3N4/TiO2. Here, blue light was manipulated as a light source to circumvent the initiation of the strong nonselective hydroxyl radical (OH) that is often generated by valence band holes (hvb+) of TiO2. Aerial O2 was employed to achieve the long-lived, exclusive ROS, O2-, while acetonitrile, an aprotic solvent, was utilized to prolong the lifetime of O2-. Importantly, the g-C3N4/TiO2 possesses an exceptional capability for the generation of O2-. Based on the synergistic effect of two ingredients of the g-C3N4/TiO2 photocatalyst, the highly selective conversion of amines was achieved with superior conversions in comparison with the pristine TiO2 and g-C3N4. Furthermore, a mechanism dominated by O2- was proposed according to the kinetic studies, electron paramagnetic resonance (EPR), and ROS quenching experiments. This work highlights the importance of ROS in defining the desirable outcomes over semiconductor photocatalysts.

Cooperative smart TiO2 photocatalysis and TEMPO catalysis: Visible light-mediated selective aerobic oxidation of amines.

Here, we report a highly efficient cooperative photocatalytic system for the selective oxidation of amines with air. The visible light-assisted adsorption of amines gives rise to a visible light complex on TiO2 that can be self-repaired to safeguard smart photocatalysis. Smart TiO2 photocatalysis works cooperatively with TEMPO catalysis that can perform the visible light-mediated selective oxidation of amines in a swift and recyclable manner. This discovery provides an alternative for addressing environmental challenges by reducing pollutants at the source for oxidative chemical reactions.

2D sp2 Carbon-Conjugated Porphyrin Covalent Organic Framework for Cooperative Photocatalysis with TEMPO.

2D covalent organic frameworks (COFs) are receiving ongoing attention in semiconductor photocatalysis. Herein, we present a photocatalytic selective chemical transformation by combining sp2 carbon-conjugated porphyrin-based covalent organic framework (Por-sp2 c-COF) photocatalysis with TEMPO catalysis illuminated by 623 nm red light-emitting diodes (LEDs). Highly selective conversion of amines into imines was swiftly afforded in minutes. Specifically, the π-conjugation of porphyrin linker leads to extensive absorption of red light; the sp2 -C=C- double bonds linkage ensures the stability of Por-sp2 c-COF under high concentrations of amine. Most importantly, we found that crystalline framework of Por-sp2 c-COF is pivotal for cooperative photocatalysis with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). This work foreshadows that the outstanding hallmarks of COFs, particularly crystallinity, could be exploited to address energy and environmental challenges by cooperative photocatalysis.

2D and 3D Porphyrinic Covalent Organic Frameworks: The Influence of Dimensionality on Functionality.

The construction of 2D and 3D covalent organic frameworks (COFs) from functional moieties for desired properties has gained much attention. However, the influence of COFs dimensionality on their functionalities, which can further assist in COF design, has never been explored. Now, by selecting designed precursors and topology diagrams, 2D and 3D porphyrinic COFs (2D-PdPor-COF and 3D-PdPor-COF) are synthesized. By model building and Rietveld refinement of powder X-ray diffraction, 2D-PdPor-COF crystallizes as 2D sheets while 3D-PdPor-COF adopts a five-fold interpenetrated pts topology. Interestingly, compared with 2D-PdPor-COF, 3D-PdPor-COF showed interesting properties, including 1) higher CO2 adsorption capacity; 2) better photocatalytic performance; and 3) size-selective photocatalysis. Based on this study, we believe that with the incorporation of functional moieties, the dimensionality of COFs can definitely influence their functionalities.

Designed Synthesis of a 2D Porphyrin-Based sp2 Carbon-Conjugated Covalent Organic Framework for Heterogeneous Photocatalysis.

The construction of stable covalent organic frameworks (COFs) for various applications is highly desirable. Herein, we report the synthesis of a novel two-dimensional (2D) porphyrin-based sp2 carbon-conjugated COF (Por-sp2 c-COF), which adopts an eclipsed AA stacking structure with a Brunauer-Emmett-Teller surface area of 689 m2 g-1 . Owing to the C=C linkages, Por-sp2 c-COF shows a high chemical stability under various conditions, even under harsh conditions such as 9 m HCl and 9 m NaOH solutions. Interestingly, Por-sp2 c-COF can be used as a metal-free heterogeneous photocatalyst for the visible-light-induced aerobic oxidation of amines to imines. More importantly, in comparison to imine-linked Por-COF, the inherent structure of Por-sp2 c-COF equips it with several advantages as a photocatalyst, including reusability and high photocatalytic performance. This clearly demonstrates that sp2 carbon-linked 2D COFs can provide an interesting platform for heterogeneous photocatalysis.

Al(2)O(3) Surface Complexation for Photocatalytic Organic Transformations.

The use of sunlight to drive organic reactions constitutes a green and sustainable strategy for organic synthesis. Herein, we discovered that the earth-abundant aluminum oxide (Al2O3) though paradigmatically known to be an insulator could induce an immense increase in the selective photo-oxidation of different benzyl alcohols in the presence of a large variety of dyes and O2. This unique phenomenon is based on the surface complexation of benzyl alcohol (BnOH) with the Brønsted base sites on Al2O3, which reduces its oxidation potential and causes an upshift in its HOMO for electron abstraction by the dye. The surface complexation of O2 with Al2O3 also activates the adsorbed O2 for receiving electrons from the photoexcited dyes. This discovery brings forth a new understanding on utilizing surface complexation mechanisms between the reactants and earth abundant materials to effectively achieve a wider range of photoredox reactions.

Cooperative photoredox catalysis.

Visible-light photoredox catalysis has been experiencing a renaissance in response to topical interest in renewable energy and green chemistry. The latest progress in this area indicates that cooperation between photoredox catalysis and other domains of catalysis could provide effective results. Thus, we advance the concept of cooperative photoredox catalysis for organic transformations. It is important to note that this concept can bridge the gap between visible-light photoredox catalysis and other types of redox catalysis such as transition-metal catalysis, biocatalysis or electrocatalysis. In doing so, one can take advantage of the best of both worlds in establishing organic synthesis with visible-light-induced redox reaction as a crucial step.

Visible-Light-Induced Photoredox Catalysis of Dye-Sensitized Titanium Dioxide: Selective Aerobic Oxidation of Organic Sulfides.

TiO2 photoredox catalysis has recently attracted much interest for use in performing challenging organic transformations under mild reaction conditions. However, the reaction scheme is hampered by the fact that TiO2 can only be excited by UV light of wavelengths λ shorter than 385 nm. One promising strategy to overcome this issue is to anchor an organic, preferably metal-free dye onto the surface of TiO2. Importantly, we observed that the introduction of a catalytic amount of the redox mediator TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] ensured the stability of the anchored dye, alizarin red S, thereby resulting in the selective oxidation of organic sulfides with O2. This result affirms the essential role of the redox mediator in enabling the organic transformations by visible-light photoredox catalysis.

Selective aerobic oxidation mediated by TiO(2) photocatalysis.

TiO2 is one of the most studied metal oxide photocatalysts and has unparal-leled efficiency and stability. This cheap, abundant, and non-toxic material has the potential to address future environmental and energy concerns. Understanding about the photoinduced interfacial redox events on TiO2 could have profound effect on the degradation of organic pollutants, splitting of H2O into H2 and O2, and selective redox organic transformations. Scientists traditionally accept that for a semiconductor photocatalyst such as TiO2 under the illumination of light with energy larger than its band gap, two photocarriers will be created to carry out their independent reduction and oxidation processes. However, our recent discoveries indicate that it is the concerted rather than independent effect of both photocarriers of valence band hole (hvb(+)) and conduction band electron (ecb(-)) that dictate the product formation during interfacial oxidation event mediated by TiO2 photocatalysis. In this Account, we describe our recent findings on the selective oxidation of organic substrates with O2 mediated by TiO2 photocatalysis. The transfer of O-atoms from O2 to the corresponding products dominates the selective oxidation of alcohols, amines, and alkanes mediated by TiO2 photocatalysis. We ascribe this to the concerted effect of both hvb(+) and ecb(-) of TiO2 in contribution to the oxidation products. These findings imply that O2 plays a unique role in its transfer into the products rather than independent role of ecb(-) scavenger. More importantly, ecb(-) plays a crucial role to ensure the high selectivity for the oxygenation of organic substrates. We can also use the half reactions such as those of the conduction band electron of TiO2 for efficient oxidation reactions with O2. To this end, efficient selective oxidation of organic substrates such as alcohols, amines, and aromatic alkanes with O2 mediated by TiO2 photocatalysis under visible light irradiation has been achieved. In summary, the concerted effect of hvb(+) and ecb(-) to implement one oxidation event could pave the way for selective oxofunctionalization of organic substrates with O2 by metal oxide photocatalysis. Furthermore, it could also deepen our understanding on the role of O2 and the elusive nature of oxygen species at the interface of TiO2, which, in turn, could shed new light on avant-garde photocatalytic selective redox processes in addressing the energy and environmental challenges of the future.

Heterogeneous visible light photocatalysis for selective organic transformations.

The future development of chemistry entails environmentally friendly and energy sustainable alternatives for organic transformations. Visible light photocatalysis can address these challenges, as reflected by recent intensive scientific endeavours to this end. This review covers state-of-the-art accomplishments in visible-light-induced selective organic transformations by heterogeneous photocatalysis. The discussion comprises three sections based on the photocatalyst type: metal oxides such as TiO2, Nb2O5 and ZnO; plasmonic photocatalysts like nanostructured Au, Ag or Cu supported on metal oxides; and polymeric graphitic carbon nitride. Finally, recent strides in bridging the gap between photocatalysis and other areas of catalysis will be highlighted with the aim of overcoming the existing limitations of photocatalysis by developing more creative synthetic methodologies.

UV-assisted removal of inactive peroxide species for sustained epoxidation of cyclooctene on anatase TiO2.

Epoxidation of olefins with H2O2 is one of the most important reactions in organic synthesis. We found that anatase TiO2 can be a good catalyst for the epoxidation of cyclooctene with H2O2 at room temperature. However, the catalyst deactivated quickly in the presence of excess amount of H2O2 because of the formation of inactive side-on Ti-η(2)-peroxide species on the surface of TiO2, the presence of which was confirmed by isotope-labelled resonance UV Raman spectroscopy and kinetics studies. Interestingly, the epoxidation reaction could be dramatically accelerated under irradiation of UV light with λ≥350 nm. This phenomenon is attributed to the photo-assisted removal of the inactive peroxide species, through which the active sites on the surface of anatase TiO2 are regenerated and the catalytic epoxidation of cyclooctene with H2O2 is resumed. This finding provides an alternative for sustained epoxidation reactions on TiO2 at room temperature. Moreover, it also has significant implications on the deactivation pathway and possible solutions in Ti-based heterogeneous catalysis or photocatalysis.

Tailoring ß-ketoenamine covalent organic framework with azo for blue light-driven selective oxidation of amines with oxygen.

Covalent organic frameworks (COFs) present bright prospects in visible light photocatalysis with abundant active sites and exceptional stability. Tailoring an established COF with photoactive group is a prudent strategy to extend visible light absorption toward broad photocatalysis. Here, a ß-ketoenamine COF, TpBD-COF, constructed with 1,3,5-triformylphloroglucinol (Tp) and 4,4'-biphenyldiamine (BD), is tailored with azo to validate this strategy. The insertion of azo into BD affords 4,4'-azodianiline (Azo); TpAzo-COF is successfully constructed with Tp and Azo. Intriguingly, the insertion of azo enhances π-conjugation, thereby facilitating visible light absorption and intramolecular electron transfer. Moreover, TpAzo-COF, with an appropriate electronic structure and impressive specific surface area of 1855 m2 g-1, offers substantial active sites conducive to the reduction of oxygen (O2) to superoxide. Compared with TpBD-COF, TpAzo-COF exhibits superior performance for blue light-driven oxidation of amines with O2. Superoxide controls the selective formation of product imines. This work foreshadows the remarkable capacity of tailoring COFs with photoactive group toward broad visible light photocatalysis.

Expanding Olefin-Linked Covalent Organic Frameworks toward Selective Photocatalytic Oxidation of Organic Sulfides.

Olefin-linked covalent organic frameworks (COFs) have exhibited great potential in visible-light photocatalysis. In principle, expanding fully conjugated COFs can facilitate light absorption and charge transfer, leading to improved photocatalysis. Herein, three olefin-linked COFs with the same topology are synthesized by combining 2,4,6-trimethyl-1,3,5-triazine (TMT) with 1,3,5-triformylbenzene (TFB), 1,3,5-tris(4-formylphenyl)benzene (TFPB), and 1,3,5-tris(4-formylphenylethynyl)benzene (TFPEB), namely, TMT-TFB-COF, TMT-TFPB-COF, and TMT-TFPEB-COF, respectively. From TMT-TFB-COF to TMT-TFPB-COF, expanding phenyl rings provides only limited expansion for π-conjugation due to the steric effect of structural twisting. However, from TMT-TFPB-COF to TMT-TFPEB-COF, the insertion of acetylenes eliminates the steric effect and provides more delocalized π-electrons. As such, TMT-TFPEB-COF exhibits the best optoelectronic properties among these three olefin-linked COFs. Consequently, the photocatalytic performance of TMT-TFPEB-COF is much better than those of TMT-TFB-COF and TMT-TFPB-COF on the oxidation of organic sulfides into sulfoxides with oxygen. The desirable reusability and substrate compatibility of the TMT-TFPEB-COF photocatalyst are further confirmed. The selective formation of organic sulfoxides over TMT-TFPEB-COF under blue light irradiation proceeds via both electron- and energy-transfer pathways. This work highlights a rational design of expanding the π-conjugation of fully conjugated COFs toward selective visible-light photocatalysis.

Benzothiadiazole covalent organic framework photocatalysis with an electron transfer mediator for selective aerobic sulfoxidation.

Covalent organic frameworks (COFs) are promising visible light photocatalysts for aerobic oxidation reactions. However, COFs usually suffer from the assault of reactive oxygen species, leading to hindered electron transfer. This scenario could be addressed by integrating a mediator to promote photocatalysis. Starting with 4,4'-(benzo-2,1,3-thiadiazole-4,7-diyl)dianiline (BTD) and 2,4,6-triformylphloroglucinol (Tp), TpBTD-COF is developed as a photocatalyst for aerobic sulfoxidation. Adding an electron transfer mediator 2,2,6,6-tetramethylpiperidine-1­oxyl (TEMPO), the conversions are radically accelerated, over 2.5 times of that without TEMPO. Moreover, the robustness of TpBTD-COF is preserved by TEMPO. Remarkably, TpBTD-COF could endure multiple cycles of sulfoxidation, even with higher conversions than the fresh one. TpBTD-COF photocatalysis with TEMPO implements diverse aerobic sulfoxidation by an electron transfer pathway. This work highlights that benzothiadiazole COFs are an avenue for tailor-made photocatalytic transformations.

Red light photocatalysis of conjugated microporous polymers based on fused thiophenes for selective oxidation of amines.

By virtue of tailorable building blocks, the band gaps and electronic structures of conjugated microporous polymers (CMPs) can be readily adjusted at the molecular level. Generally, the building blocks possessing extended π-conjugations result in exceptional photocatalytic performances. In this work, the direct CH arylation of fused thiophenes, thieno[3,2-b]thiophene (TT) and dithieno[3,2-b:2',3'-d]thiophene (DTT), with 1,3,6,8-tetrabromopyrene affords two CMPs, namely TT-Py-CMP and DTT-Py-CMP. The expansion of π-conjugations of the fused thiophenes from TT to DTT gives rise to a bathochromic shift about 30 nm from TT-Py-CMP to DTT-Py-CMP. Besides, systematic characterizations suggest the optoelectronic properties of DTT-Py-CMP are better than that of TT-Py-CMP. Furthermore, DTT-Py-CMP drives better red light photocatalysis than TT-Py-CMP for the selective oxidation of amines with molecular oxygen. The selective oxidation of benzyl amines by red light photocatalysis of DTT-Py-CMP progresses via an electron transfer pathway with high selectivities for imines. This work provides new insights that fused thiophenes could be the stepping stone in designing CMPs for expansive visible light photocatalysis.

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, 2 mol%) illuminated by blue LEDs. Phenyl methyl sulfide could be almost entirely converted into phenyl methyl sulfoxide with 99% of selectivity within 18 min. 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.

Selective oxidation of amines powered with green light and oxygen over an anthraquinone covalent organic framework.

The exploration of emerging photocatalysts like covalent organic frameworks (COFs) is an essential but challenging endeavor to find sustainable solutions for selective organic transformations. Anthraquinones are envisaged to construct COFs for visible light photocatalysis because their derivatives are employed industrially as oxidation catalysts or organic dyes. Herein, an anthraquinone COF, TpAQ-COF, is successfully constructed with 1,3,5-triformylphloroglucinol (Tp) and 2,6-diaminoanthraquinone (AQ). Then, the selective oxidation of amines over TpAQ-COF is implemented. Amines can be effectively converted into corresponding imines over TpAQ-COF powered with green light and oxygen, during which superoxide radical anion is discerned as the pivotal reactive oxygen species. This work suggests that COFs could inherit the advantages of molecular building blocks for selective reactions powered with broad visible light.

Visible-light-induced selective photocatalytic aerobic oxidation of amines into imines on TiO2.

Imines are important intermediates for the synthesis of fine chemicals, pharmaceuticals, and agricultural chemicals. Selective oxidation of amines into their corresponding imines with dioxygen is one of the most-fundamental chemical transformations. Herein, we report the oxidation of a series of benzylic amines into their corresponding imines with atmospheric dioxygen as the oxidant on a surface of anatase TiO(2) under visible-light irradiation (λ>420 nm). The visible-light response of this system was caused by the formation of a surface complex through the adsorption of a benzylic amine onto the surface of TiO(2). From the analysis of products of specially designed benzylic amines, we demonstrated that a highly selective oxygenation reaction proceeds via an oxygen-transfer mechanism to afford the corresponding carbonyl compound, whose further condensation with an amine would generate the final imine product. We found that when primary benzylic amines (13 examples), were chosen as the substrates, moderate to excellent selectivities for the imine products were achieved (ca. 38-94%) in moderate to excellent conversion rates (ca. 44-95%). When secondary benzylic amines (15 examples) were chosen as the substrates, both the corresponding imines and aldehydes were detected as the main products with moderate to high conversion rates (ca. 18-100%) and lower selectivities for the imine products (ca. 14-69%). When tribenzylamine was chosen as the substrate, imine (27%), dibenzylamine (24%), and benzaldehyde products (39%) were obtained in a conversion of 50%. This report can be viewed as a prototypical system for the activation of C-H bonds adjacent to heteroatoms such as N, O, and S atoms, and oxofuctionalization with air or dioxygen as the terminal oxidant under visible-light irradiation using TiO(2) as the photocatalyst.