Photonic Switching Porous Organic Polymers toward Reversible Control of Heterogeneous Photocatalysis

Recent Advances on Diarylethene-Based Photoswitching Materials: Applications in Bioimaging, Controlled Singlet Oxygen Generation for Photodynamic Therapy and Catalysis

Photoswitching materials have emerged as a promising class of compounds that possess manifold interesting properties rendering their widespread use from photoswitches, regulators to optoelectronic devices, security technologies and biochemical assays. Diarylethenes (DAE) constitute one such category of photoswitchable compounds, where the key features of stability, photoisomerization wavelengths, quantum yield and variability in the photoisomers significantly depend on their derivatization. The last decade has witnessed a surge in the engagement of DAEs in different areas of chemical and biological sciences, like biomarkers, controlled generation of singlet oxygen, photo-dynamic therapy, chemosensing, catalysis, etc. In all the cases, the photoswitchability of DAE is the principal regulating factor along with its emission properties according to the appended groups. Previous reviews on applications of DAE-based systems did not predominantly cover all the aspects of biological and industrial implementations. They have covered only one field of application either in the biological science or in the synthetic aspect or photochromic aspects only. This review is a coalition of all those aspects in last six years. Here the variation of properties of the DAE systems with respect to structural diversifications have been discussed in detail along with their potential applications in bioimaging of cells, regulating singlet oxygen generation for photodynamic therapy and catalysis of organic reactions, and their future prospects. A tabular presentation of the photophysical properties of DAE derivatives adds to the basic understanding of this subject at a glance. We hope that this cumulative collection of contemporary research on DAE, as presented in this review, will enhance the knowledge of the readers about synthetic design anticipating their properties well in advance, and will certainly motivate researchers to generate new DAE architectures with superior chemical and biological properties in future.

The Application of Porous Organic Polymers as Metal Free Photocatalysts in Organic Synthesis

Concerns about increasing greenhouse gas emissions and their effect on our environment highlight the urgent need for new sustainable technologies. Visible light photocatalysis allows the clean and selective generation of reactive intermediates under mild conditions. The more widespread adoption of the current generation of photocatalysts, particularly those using precious metals, is hampered by drawbacks such as their cost, toxicity, difficult separation, and limited recyclability. This is driving the search for alternatives, such as porous organic polymers (POPs). This new class of materials is made entirely from organic building blocks, can possess high surface area and stability, and has a controllable composition and functionality. This review focuses on the application of POPs as photocatalysts in organic synthesis. For each reaction type, a representative material is discussed, with special attention to the mechanism of the reaction. Additionally, an overview is given, comparing POPs with other classes of photocatalysts, and critical conclusions and future perspectives are provided on this important field.

Nitrogen-Rich Conjugated Microporous Polymers with Improved Cobalt(II) Density for Highly Efficient Electrocatalytic Oxygen Evolution

Developing efficient oxygen evolution catalysts (OECs) made from earth-abundant elements is extremely important since the oxygen evolution reaction (OER) with sluggish kinetics hinders the development of many energy-related electrochemical devices. Herein, an efficient strategy is developed to prepare conjugated microporous polymers (CMPs) with abundant and uniform coordination sites by coupling the N-rich organic monomer 2,4,6-tris(5-bromopyrimidin-2-yl)-1,3,5-triazine (TBPT) with Co(II) porphyrin. The resulting CMP-Py(Co) is further metallized with Co2+ ions to obtain CMP-Py(Co)@Co. Structural characterization results reveal that CMP-Py(Co)@Co has higher Co2+ content (12.20 wt %) and affinity toward water compared with CMP-Py(Co). Moreover, CMP-Py(Co)@Co exhibits an excellent OER activity with a low overpotential of 285 mV vs RHE at 10 mA cm-2 and a Tafel slope of 80.1 mV dec-1, which are significantly lower than those of CMP-Py(Co) (335 mV vs RHE and 96.8 mV dec-1). More interestingly, CMP-Py(Co)@Co outperforms most reported porous organic polymer-based OECs and the benchmark RuO2 catalyst (320 mV vs RHE and 87.6 mV dec-1). Additionally, Co2+-free CMP-Py(2H) has negligible OER activity. Thereby, the enhanced OER activity of CMP-Py(Co)@Co is attributed to the incorporation of Co2+ ions leading to rich active sites and enlarged electrochemical surface areas. Density functional theory (DFT) calculations reveal that Co2+-TBPT sites have higher activity than Co2+-porphyrin sites for the OER. These results indicate that the introduction of rich active metal sites in stable and conductive CMPs could provide novel guidance for designing efficient OECs.

Engineering of Phenylpyridine- and Bipyridine-Based Covalent Organic Frameworks for Photocatalytic Tandem Aerobic Oxidation/Povarov Cyclization

Covalent organic frameworks (COFs) are emerging as a new class of photoactive organic semiconductors, which possess crystalline ordered structures and high surface areas. COFs can be tailor-made toward specific (photocatalytic) applications, and the size and position of their band gaps can be tuned by the choice of building blocks and linkages. However, many types of building blocks are still unexplored as photocatalytic moieties and the scope of reactions photocatalyzed by COFs remains quite limited. In this work, we report the synthesis and application of two bipyridine- or phenylpyridine-based COFs: TpBpyCOF and TpPpyCOF. Due to their good photocatalytic properties, both materials were applied as metal-free photocatalysts for the tandem aerobic oxidation/Povarov cyclization and α-oxidation of N-aryl glycine derivatives, with the bipyridine-based TpBpyCOF exhibiting the highest activity. By expanding the range of reactions that can be photocatalyzed by COFs, this work paves the way toward the more widespread application of COFs as metal-free heterogeneous photocatalysts as a convenient alternative for commonly used homogeneous (metal-based) photocatalysts.

Unveiling the Synthetic Potential of 1,3,5-Tri(10H-phenothiazin-10-yl)benzene-Based Optoelectronic Material: A Metal-Free and Recyclable Photocatalyst for Sequential Functionalization of C(sp2)-H Bonds

1,3,5-Tri(10H-phenothiazin-10-yl)benzene (3PTZ) is endowed with unique redox and photoresponsive characteristics and has been utilized as a p-type redox center for organic battery cathode material and a room-temperature phosphorescence (RTP) material, respectively. Conversely, its exploration in other research fields, particularly organic synthesis, remains unknown. Here, we demonstrate that 3PTZ-POP synthesized via cross-linking of 3PTZ is capable of harvesting visible-light photons and selectively converting solar energy to chemical energy. Specifically, 3PTZ-POP functions as a metal-free and recyclable photocatalyst to promote the sequential C(sp²)-H functionalizations of N-arylacrylamides with readily available trifluoromethylsulfonyl chloride as the radical precursor. An array of 3,3-disubstituted 2-oxindoles bearing a pharmaceutically important CF₃ moiety are delivered in moderate to excellent yields under mild and sustainable conditions.

Design and preparation of functional azo linked polymers for the adsorptive removal of bisphenol A from water: Performance and analysis of the mechanism

In order to effectively remove refractory bisphenol A (BPA) from water, a novel nitrogen doped organic porous functional azo linked polymer (ALP-p) was designed and prepared according to the physicochemical characteristics of propane linked to two phenol hydroxyl groups. This ALP-p was synthesized with 98.5% yield, from pararosaniline and phloroglucinol, via the diazo coupling reaction to produce multiple adsorption functional groups of benzene ring, hydroxyl group and azo group. This functional material showed high adsorption capacity of 357.8 mg/g for 50 mg/L BPA, at 20 °C. The adsorption kinetics and isotherms were described by the pseudo-second-order and Langmuir model, respectively. The major adsorption mechanisms were attributed to the high specific surface area (259.8 m²/g) and pore volume (0.56 cm³/g) related surface adsorption and pore diffusion through porous stereoscopic stacking cavity anchorage. The functional group from the three-dimensional skeleton structures of ALP-p for BPA anchoring endowed chemisorption via π-π interaction between benzene rings and hydrogen-bonding (O-H⋯O, C-H⋯N, C-H⋯O and C-H⋯C) with the hydrogen atom of benzene ring, -OH from BPA and -OH, NN from ALP-p, respectively. The coexisting organic pollutants and alkali environment posed a negative effect on adsorption, while salinity had no significant effect on the process. The adsorption capacity and recovery of ALP-p were >93.5% and 81.6% after five cycles of operation.

Porous organic polymers for light-driven organic transformations

As a new generation of porous materials, porous organic polymers (POPs), have recently emerged as a powerful platform of heterogeneous photocatalysis. POPs are constructed using extensive organic synthesis methodologies, with various functional organic units being connected via high-energy covalent bonds. This review systematically presents the recent advances in POPs for visible-light driven organic transformations. Herein, we firstly summarize the common construction strategies for POP-based photocatalysts based on two major approaches: pre-design and post-modification; secondly, we categorize and summarize the synthesis methods and organic reaction types for constructing various types of POPs. We then classify and introduce the specific reactions of current light-driven POP-mediated organic transformations. Finally, we outline the current state of development and the problems faced in light-driven organic transformations by POPs, and we present some perspectives to motivate the reader to explore solutions to these problems and confront the present challenges in the development process.

Hydrazone-linked 2D porphyrinic covalent organic framework photocatalysis for visible light-driven aerobic oxidation of amines to imines

Covalent organic frameworks (COFs) have recently gained rising consideration for visible light photocatalysis. Their property could be accurately established with specific reactions in which the most investigated one turns out to be the aerobic oxidation of amines. In this contribution, a hydrazone-linked 2D (two-dimensional) porphyrinic COF, Por-DETH-COF, was assembled from 5,10,15,20-tetrakis(4-benzaldehyde)porphyrin (p-Por-CHO) and 2,5-diethoxyterephthalohydrazide (DETH) and its photocatalytic activity was duly appraised with the aerobic oxidation of amines. Thereby, the red light-driven selective oxidation of benzyl amines to imines was obtained in very high conversions and selectivities with ambient air as the oxidant. Importantly, the photocatalytic system exhibited remarkable compatibility of functional groups and extensive scope of benzyl amines. Notably, the Por-DETH-COF photocatalyst displayed outstanding recyclability after five successive cycles. This work suggests that 2D COFs could contribute a unique juncture for selective organic transformations by photocatalysis.

Template-Induced {Mn2}-Organic Framework with Lewis Acid-Base Canals as a Highly Efficient Heterogeneous Catalyst for Chemical Fixation of CO2 and Knoevenagel Condensation

The target for the self-assembly of functional microporous metal-organic frameworks (MOFs) could be realized by employing ligand-directed and/or template-induced strategies, which prompted us to explore the synthetic technique of d¹⁰ secondary-building-unit-based nanoporous frameworks. Here, the exquisite combination of a paddle-wheel [Mn₂(CO₂)₆(OH₂)₂] cluster and a TDP^6- ligand contributes one robust honeycomb framework of {(Me₂NH₂)₂[Mn₂(TDP)(H₂O)₂]·3H₂O·3DMF}_n (NUC-31; DMF = N,N-dimethylformamide), whose activated state with the removal of associated aqueous molecules characterizes the outstanding physicochemical properties of nanochannels, penta- and tetracoordinated Mn²⁺ serving as highly open metal sites, rich Lewis base sites (rows of C═O groups and N_pyridine atoms), and excellent thermal stability. Moreover, it is worth mentioning that Lewis acid-base sites on the inner surface of the channels in activated NUC-31 successfully form one unprecedented canal-shaped acid-base confined space with evenly distributed open metal sites of Mn²⁺ and N_pyridine atoms as the canal bottom as well as two rows of C═O groups serving as dyke dams. Catalytic experiments displayed that activated NUC-31 could serve as an efficient heterogeneous catalyst for the chemical fixation of CO₂ with epoxides into cyclic carbonates under mild conditions. Furthermore, NUC-31 could effectively catalyze the reaction Knoevenagel condensation, which should be ascribed to the synergistic polarization effect aroused from its plentiful Lewis base sites in the confined channel space. Hence, these results demonstrate that the employment of ligand-directed and template-dependent strategies could overcome the self-assembled barriers of functional microporous MOFs and achieve unexpected frameworks.

V═O Functionalized {Tm2}-Organic Framework Designed by Postsynthesis Modification for Catalytic Chemical Fixation of CO2 and Oxidation of Mustard Gas

In terms of recently documented references, the introduction of V═O units into porous MOF/COF frameworks can greatly improve their original performance and expand their application prospects due to a change in their electronegativity. In this work, by a cation-exchange strategy, a consummate combination of separate 4f [Tm₂(CO₂)₈] SBUs and 3d [V^IVO(H₂O)₂] units generated the functionalized porous metal-organic framework {(Me₂NH₂)₂[VO(H₂O)][Tm₂(BDCP)₂]·3DMF·3H₂O}_n (NUC-11), in which [Tm₂(CO₂)₈] SBUs constitute the fundamental 3D host framework of {[Tm₂](BDCP)₂}_n along with [V^IVO(H₂O)₂] units being further docked on the inner wall of channels by covalent bonds. Significantly, NUC-11 represents the first example of V═O modified porous MOFs, in which uncoordinated carboxylic groups (-CO₂H) further grasp the functional [V^IVO(H₂O)₂] units on the initial basic skeleton along with the formation of covalent bonds as fixed ropes. Furthermore, activated samples of NUC-11 displayed a good catalytic performance for the chemical synthesis of carbonates from related epoxides and CO₂ with high conversion rate. Moreover, by employing NUC-11 as a catalyst, a simulator of mustard gas, 2-chloroethyl ethyl sulfide, could be quickly and efficiently oxidized into low-toxicity products of oxidized sulfoxide (CEESO). Thus, this study offers a brand new view for the design and synthesis of functional-units-modified porous MOFs, which could be potentially applied as an excellent candidate in the growing field of efficient catalysis.