Advances in Conjugated Microporous Polymers

Conjugated microporous polymers (CMPs) are a unique class of materials that combine extended π-conjugation with a permanently microporous skeleton. Since their discovery in 2007, CMPs have become established as an important subclass of porous materials. A wide range of synthetic building blocks and network-forming reactions offers an enormous variety of CMPs with different properties and structures. This has allowed CMPs to be developed for gas adsorption and separations, chemical adsorption and encapsulation, heterogeneous catalysis, photoredox catalysis, light emittance, sensing, energy storage, biological applications, and solar fuels production. Here we review the progress of CMP research since its beginnings and offer an outlook for where these materials might be headed in the future. We also compare the prospect for CMPs against the growing range of conjugated crystalline covalent organic frameworks (COFs).

Metal-free photocatalysts for hydrogen evolution

This article provides a comprehensive review of the latest progress, challenges and recommended future research related to metal-free photocatalysts for hydrogen productionviawater-splitting.

Conjugated porous polymers: incredibly versatile materials with far-reaching applications

This review discusses conjugated porous polymers and focuses on relating design principles and synthetic methods to key properties and applications such as (photo)catalysis, gas storage, chemical sensing, energy storage and environmental remediation.

Rapid metal-free synthesis of pyridyl-functionalized conjugated microporous polymers for visible-light-driven water splitting

A rapid metal-free synthetic approach was applied to synthesize pyridyl-functionalized conjugated microporous polymers (PCMPs) via an aminative cyclization between aryl aldehydes and ketones without any metal catalysts.

Structurally Diverse Covalent Triazine-Based Framework Materials for Photocatalytic Hydrogen Evolution from Water

A structurally diverse family of 39 covalent triazine-based framework materials (CTFs) are synthesized by Suzuki-Miyaura polycondensation and tested as hydrogen evolution photocatalysts using a high-throughput workflow. The two best-performing CTFs are based on benzonitrile and dibenzo[b,d]thiophene sulfone linkers, respectively, with catalytic activities that are among the highest for this material class. The activities of the different CTFs are rationalized in terms of four variables: the predicted electron affinity, the predicted ionization potential, the optical gap, and the dispersibility of the CTFs particles in solution, as measured by optical transmittance. The electron affinity and dispersibility in solution are found to be the best predictors of photocatalytic hydrogen evolution activity.

High performance photoanodic catalyst prepared from an active organic photovoltaic cell - high potential gain from visible light

Photoelectrochemical oxidation of thiols was enhanced with a threshold potential of -0.35 V vs. Ag/AgCl by the use of a ZnPc/PCBM:P3HT/ZnO electode, which was prepared by removing the PEDOT:PSS/Au electrode of an inverted OPV device and coating it with ZnPc. A co-photocatalysis property of ZnPc was observed in the photoelectrochemistry and scanning Kelvin probe microscopy.

A π-Conjugated, Covalent Phosphinine Framework

Structural modularity of polymer frameworks is a key advantage of covalent organic polymers, however, only C, N, O, Si, and S have found their way into their building blocks so far. Here, the toolbox available to polymer and materials chemists is expanded by one additional nonmetal, phosphorus. Starting with a building block that contains a λ5 -phosphinine (C5 P) moiety, a number of polymerization protocols are evaluated, finally obtaining a π-conjugated, covalent phosphinine-based framework (CPF-1) through Suzuki-Miyaura coupling. CPF-1 is a weakly porous polymer glass (72.4 m2  g-1 BET at 77 K) with green fluorescence (λmax =546 nm) and extremely high thermal stability. The polymer catalyzes hydrogen evolution from water under UV and visible light irradiation without the need for additional co-catalyst at a rate of 33.3 μmol h-1  g-1 . These results demonstrate for the first time the incorporation of the phosphinine motif into a complex polymer framework. Phosphinine-based frameworks show promising electronic and optical properties, which might spark future interest in their applications in light-emitting devices and heterogeneous catalysis.

Photocatalytically active ladder polymers

Conjugated ladder polymers (cLaPs) are introduced as organic semiconductors for photocatalytic hydrogen evolution from water under sacrificial conditions. Starting from a linear conjugated polymer (cLiP1), two ladder polymers are synthesized via post-polymerization annulation and oxidation techniques to generate rigidified, planarized materials bearing dibenzo[b,d]thiophene (cLaP1) and dibenzo[b,d]thiophene sulfone subunits (cLaP2). The high photocatalytic activity of cLaP1 (1307 μmol h-1 g-1) in comparison to that of cLaP2 (18 μmol h-1 g-1) under broadband illumination (λ > 295 nm) in the presence of a hole-scavenger is attributed to a higher yield of long-lived charges (μs to ms timescale), as evidenced by transient absorption spectroscopy. Additionally, cLaP1 has a larger overpotential for proton reduction and thus an increased driving force for the evolution of hydrogen under sacrificial conditions.

Two-dimensional semiconducting covalent organic frameworks via condensation at arylmethyl carbon atoms

Construction of organic semiconducting materials with in-plane π-conjugated structures and robustness through carbon-carbon bond linkages, alternatively as organic graphene analogs, is extremely desired for powerfully optoelectrical conversion. However, the poor reversibility for sp² carbon bond forming reactions makes them unavailable for building high crystalline well-defined organic structures through a self-healing process, such as covalent organic frameworks (COFs). Here we report a scalable solution-processing approach to synthesize a family of two-dimensional (2D) COFs with trans-disubstituted C = C linkages via condensation reaction at arylmethyl carbon atoms on the basis of 3,5-dicyano-2,4,6-trimethylpyridine and linear/trigonal aldehyde (i.e., 4,4″-diformyl-p-terphenyl, 4,4'-diformyl-1,1'-biphenyl, or 1,3,5-tris(4-formylphenyl)benzene) monomers. Such sp² carbon-jointed-pyridinyl frameworks, featuring crystalline honeycomb-like structures with high surface areas, enable driving two half-reactions of water splitting separately under visible light irradiation, comparable to graphitic carbon nitride (g-C₃N₄) derivatives.

Novel Diketopyrrolopyrrole-Based π-Conjugated Molecules Synthesized Via One-Pot Direct Arylation Reaction

Diketopyrrolopyrrole (DPP) is an important type of π-conjugated building block for high-performance organic electronic materials. DPP-based conjugated materials are usually synthesized via Suzuki, Stille, or Negishi cross-coupling reactions, which require organometallic precursors. In this paper, a series of novel phenyl-cored DPP molecules, including five meta-phenyl-cored molecules and four para-phenyl-cored molecules, have been synthesized in moderate to good yields, in a facile manner, through the Pd-catalyzed direct arylation of C–H bonds, and their optoelectrical properties have been investigated in detail. All new molecules have been fully characterized by NMR, MALDI-TOF MS, elemental analysis, UV–visible spectroscopy, and cyclic voltammetry. This synthetic strategy has evident advantages of atom- and step-economy and low cost, compared with traditional cross-coupling reactions.

Layered Thiazolo[5,4- d] Thiazole-Linked Conjugated Microporous Polymers with Heteroatom Adoption for Efficient Photocatalysis Application

Conjugated microporous polymers (CMPs) with high surface areas, tunable building blocks, and fully conjugated structures have found important applications in optoelectronics. Here, we report a new series of CMPs with tunable band gaps by introducing thiazolo[5,4- d] thiazole as the linkage. Because they are synthetic polymers, the geometries and structures could be rationally designed. Their intrinsic wide visible-light absorption properties and layered architectures endow them with a promising photocatalytic performance. The role of geometries, surface areas, and morphologies of the CMPs in photocatalysis abilities is examined and discussed. The results indicate that geometries have a direct impact on the surface areas and morphologies of the CMPs and thus exert great influence on photocatalysis.

Porous networks based on iron(ii) clathrochelate complexes

Microporous networks based on boronate ester-capped iron(ii) clathrochelate complexes are described. The networks were obtained by covalent cross-linking of tetrabrominated clathrochelate complexes via Suzuki-Miyaura polycross-coupling reactions with diboronic acids, or by Sonogashira-Hagihara polycross-coupling of clathrochelate complexes with terminal alkyne functions and 1,3,5-tribromobenzene. The networks display permanent porosity with apparent Brunauer-Emmett-Teller surface areas of up to SABET = 593 m2 g-1. A clathrochelate complex based on an enantiopure dioximato ligand was used to prepare chiral networks. One of these networks was shown to preferentially absorb d-tryptophan over l-tryptophan.

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Fluorine-Phenanthroimidazole Porous Organic Polymer: Efficient Microwave Synthesis and Photocatalytic Activity

A porous polymer containing a fluorophenylphenanthroimidazole core was easily prepared via one-pot Suzuki-Miyaura cross-coupling reactions under microwave heating. These new metal-free polymers have demonstrated heterogeneous photocatalytic activity toward aza-Henry reaction with reasonable recyclability. Their preparation require a minimal workup to build porous networks with control over the apparent surface area and pore volume from suitable molecular building blocks containing 2-(1 H-phenanthro[9,10- d]imidazol-2-yl)-3,5-difluorophenol (PhIm-2F), as rigid and multitopic node, which afforded a conjugated porous polymer (CPP-PhIm-2F). A series of fluorinated ligands have shown their capability in the preparation of soluble and supported cationic Ru(bpy)₂(F-phenanthroimidazole) complexes by reaction with Ru(bpy)₂Cl₂ and demonstrating a beneficial effect of two fluorine atoms on the photocatalytic effect.

Computational high-throughput screening of polymeric photocatalysts: exploring the effect of composition, sequence isomerism and conformational degrees of freedom

We discuss a low-cost computational workflow for the high throughput screening of polymeric photocatalysts.

Understanding structure-activity relationships in linear polymer photocatalysts for hydrogen evolution

Conjugated polymers have sparked much interest as photocatalysts for hydrogen production. However, beyond basic considerations such as spectral absorption, the factors that dictate their photocatalytic activity are poorly understood. Here we investigate a series of linear conjugated polymers with external quantum efficiencies for hydrogen production between 0.4 and 11.6%. We monitor the generation of the photoactive species from femtoseconds to seconds after light absorption using transient spectroscopy and correlate their yield with the measured photocatalytic activity. Experiments coupled with modeling suggest that the localization of water around the polymer chain due to the incorporation of sulfone groups into an otherwise hydrophobic backbone is crucial for charge generation. Calculations of solution redox potentials and charge transfer free energies demonstrate that electron transfer from the sacrificial donor becomes thermodynamically favored as a result of the more polar local environment, leading to the production of long-lived electrons in these amphiphilic polymers.

While inorganic semiconductors are well-studied for their solar-to-fuel energy conversion abilities, organic materials receive far less attention. Here, authors prepare linear conjugated polymers as H₂ evolution photocatalysts and rationalize photocatalytic activities with fundamental properties.

Sulfone-containing covalent organic frameworks for photocatalytic hydrogen evolution from water

Nature uses organic molecules for light harvesting and photosynthesis, but most man-made water splitting catalysts are inorganic semiconductors. Organic photocatalysts, while attractive because of their synthetic tunability, tend to have low quantum efficiencies for water splitting. Here we present a crystalline covalent organic framework (COF) based on a benzo-bis(benzothiophene sulfone) moiety that shows a much higher activity for photochemical hydrogen evolution than its amorphous or semicrystalline counterparts. The COF is stable under long-term visible irradiation and shows steady photochemical hydrogen evolution with a sacrificial electron donor for at least 50 hours. We attribute the high quantum efficiency of fused-sulfone-COF to its crystallinity, its strong visible light absorption, and its wettable, hydrophilic 3.2 nm mesopores. These pores allow the framework to be dye-sensitized, leading to a further 61% enhancement in the hydrogen evolution rate up to 16.3 mmol g^-1 h^-1. The COF also retained its photocatalytic activity when cast as a thin film onto a support.

Supramolecular Organic Photocatalyst Containing a Cubanelike Water Cluster and Donor-Acceptor Stacks: Hydrogen Evolution and Dye Degradation under Visible Light

Supramolecular organic photocatalysts are scarcely explored for the generation of sustainable energy as well as for environmental remediation purposes. An organic photocatalyst, containing a cubanelike water cluster and donor-acceptor stacks, was efficiently developed through a supramolecular approach. The material exhibited remarkable photocatalytic hydrogen generation, in the absence of any cocatalyst, with excellent stability and recyclability. The photoactivity was further assessed through time-resolved photoluminescence and electrochemical impedance spectroscopy. The material also exhibited highly efficient sunlight-driven photocatalytic activity through the degradation of harmful organic dye methylene blue.

Dibenzothiophene-S,S-Dioxide-Based Conjugated Polymers: Highly Efficient Photocatalyts for Hydrogen Production from Water under Visible Light

Three dibenzothiophene-S,S-dioxide-based alternating copolymers were synthesized by facile Suzuki polymerization for visible light-responsive hydrogen production from water (> 420 nm). Without addition of any cocatalyst, FluPh2-SO showed a photocatalytic efficiency of 3.48 mmol h^-1 g-¹ , while a larger hydrogen evolution rate (HER) of 4.74 mmol h^-1 g^-1 was achieved for Py-SO, which was ascribed to the improved coplanarity of the polymer that facilitated both intermolecular packing and charge transport. To minimize the possible steric hindrance of FluPh2-SO by replacing 9,9'-diphenylfluorene with fluorene, Flu-SO exhibited a more red-shifted absorption than FluPh2-SO and yielded the highest HER of 5.04 mmol h^-1 g^-1 . This work highlights the potential of dibenzothiophene-S,S-dioxide as a versatile building block and the rational design strategy for achieving high photocatalytic efficiency.

Fluorescent Sulphur- and Nitrogen-Containing Porous Polymers with Tuneable Donor-Acceptor Domains for Light-Driven Hydrogen Evolution

Light-driven water splitting is a potential source of abundant, clean energy, yet efficient charge-separation and size and position of the bandgap in heterogeneous photocatalysts are challenging to predict and design. Synthetic attempts to tune the bandgap of polymer photocatalysts classically rely on variations of the sizes of their π-conjugated domains. However, only donor-acceptor dyads hold the key to prevent undesired electron-hole recombination within the catalyst via efficient charge separation. Building on our previous success in incorporating electron-donating, sulphur-containing linkers and electron-withdrawing, triazine (C₃ N₃ ) units into porous polymers, we report the synthesis of six visible-light-active, triazine-based polymers with a high heteroatom-content of S and N that photocatalytically generate H₂ from water: up to 915 μmol h^-1  g^-1 with Pt co-catalyst, and-as one of the highest to-date reported values -200 μmol h^-1  g^-1 without. The highly modular Sonogashira-Hagihara cross-coupling reaction we employ, enables a systematic study of mixed (S, N, C) and (N, C)-only polymer systems. Our results highlight that photocatalytic water-splitting does not only require an ideal optical bandgap of ≈2.2 eV, but that the choice of donor-acceptor motifs profoundly impacts charge-transfer and catalytic activity.

Embedding Carbon Nitride into a Covalent Organic Framework with Enhanced Photocatalysis Performance

We report a new strategy to construct porous carbon nitride (PCN) by embedding a heptazine unit-the primary building block of carbon nitride-into the backbone of a covalent organic framework (COF). The strategy results in a new type of PCN which bears a fibrous morphology, high surface area and wide visible absorption. The photocatalytic performance was evaluated by photodegradation of an organic dye. We found that the introduction of the heptazine unit has a prominent effect on the catalytic activity, which demonstrates an effective strategy to prepare carbon nitride materials. This work opens up a new way for the preparation of carbon nitride for photocatalysis applications.

Fast tuning of covalent triazine frameworks for photocatalytic hydrogen evolution

A fast and facile route for the optimization of covalent triazine frameworks (CTFs) for photocatalytic hydrogen production is presented. Within 10 minutes a CTF with low photocatalytic activity can be converted into a highly active photocatalyst. Optimized CTF catalysts show an average hydrogen evolution rate of 1072 μmol h^-1 g^-1 under visible light (>420 nm).

The effect of molecular structure and fluorination on the properties of pyrene-benzothiadiazole-based conjugated polymers for visible-light-driven hydrogen evolution

The linear non-fluorinated polymer L-PyBT exhibited an impressive hydrogen evolution rate up to 83.7 μmol h⁻¹ under visible light irradiation.

Photocatalytic Oxygen Evolution from Functional Triazine-Based Polymers with Tunable Band Structures

Conjugated polymers (CPs) are emerging and appealing light harvesters for photocatalytic water splitting owing to their adjustable band gap and facile processing. Herein, we report an advanced mild synthesis of three conjugated triazine-based polymers (CTPs) with different chain lengths by increasing the quantity of electron-donating benzyl units in the backbone. Varying the chain length of the CTPs modulates their electronic, optical, and redox properties, resulting in an enhanced performance for photocatalytic oxygen evolution, which is the more challenging half-reaction of water splitting owing to the sluggish reaction kinetics. Our results could stimulate interest in these functional polymers where a molecular engineering strategy enables the production of suitable semiconductor redox energetics for oxygenic photosynthesis.

Covalent Triazine Frameworks via a Low-Temperature Polycondensation Approach

Covalent triazine frameworks (CTFs) are normally synthesized by ionothermal methods. The harsh synthetic conditions and associated limited structural diversity do not benefit for further development and practical large-scale synthesis of CTFs. Herein we report a new strategy to construct CTFs (CTF-HUSTs) via a polycondensation approach, which allows the synthesis of CTFs under mild conditions from a wide array of building blocks. Interestingly, these CTFs display a layered structure. The CTFs synthesized were also readily scaled up to gram quantities. The CTFs are potential candidates for separations, photocatalysis and for energy storage applications. In particular, CTF-HUSTs are found to be promising photocatalysts for sacrificial photocatalytic hydrogen evolution with a maximum rate of 2647 μmol h-1  g-1 under visible light. We also applied a pyrolyzed form of CTF-HUST-4 as an anode material in a sodium-ion battery achieving an excellent discharge capacity of 467 mAh g-1 .

Conjugated Microporous Polymer Nanosheets for Overall Water Splitting Using Visible Light

Direct water splitting into H₂ and O₂ using photocatalysts by harnessing sunlight is very appealing to produce storable chemical fuels. Conjugated polymers, which have tunable molecular structures and optoelectronic properties, are promising alternatives to inorganic semiconductors for water splitting. Unfortunately, conjugated polymers that are able to efficiently split pure water under visible light (400 nm) via a four-electron pathway have not been previously reported. This study demonstrates that 1,3-diyne-linked conjugated microporous polymer nanosheets (CMPNs) prepared by oxidative coupling of terminal alkynes such as 1,3,5-tris-(4-ethynylphenyl)-benzene (TEPB) and 1,3,5-triethynylbenzene (TEB) can act as highly efficient photocatalysts for splitting pure water (pH ≈ 7) into stoichiometric amounts of H₂ and O₂ under visible light. The apparent quantum efficiencies at 420 nm are 10.3% and 7.6% for CMPNs synthesized from TEPB and TEB, respectively; the measured solar-to-hydrogen conversion efficiency using the full solar spectrum can reach 0.6%, surpassing photosynthetic plants in converting solar energy to biomass (globally average ≈0.10%). First-principles calculations reveal that photocatalytic H₂ and O₂ evolution reactions are energetically feasible for CMPNs under visible light irradiation. The findings suggest that organic polymers hold great potential for stable and scalable solar-fuel generation.

Tailored Band Gaps in Sulfur- and Nitrogen-Containing Porous Donor-Acceptor Polymers

Donor-acceptor dyads hold the key to tuning of electrochemical properties and enhanced mobility of charge carriers, yet their incorporation into a heterogeneous polymer network proves difficulty owing to the fundamentally different chemistry of the donor and acceptor subunits. A family of sulfur- and nitrogen-containing porous polymers (SNPs) are obtained via Sonogashira-Hagihara cross-coupling and combine electron-withdrawing triazine (C₃ N₃ ) and electron-donating, sulfur-containing linkers. Choice of building blocks and synthetic conditions determines the optical band gap (from 1.67 to 2.58 eV) and nanoscale ordering of these microporous materials with BET surface areas of up to 545 m²  g^-1 and CO₂ capacities up to 1.56 mmol g^-1 . Our results highlight the advantages of the modular design of SNPs, and one of the highest photocatalytic hydrogen evolution rates for a cross-linked polymer without Pt co-catalyst is attained (194 μmol h^-1  g^-1 ).

Photocatalytic oxygen evolution from low-bandgap conjugated microporous polymer nanosheets: a combined first-principles calculation and experimental study

Nanostructured semiconducting polymers have emerged as a very promising class of metal-free photocatalytic materials for solar water splitting. However, they generally exhibit low efficiency and lack the ability to utilize long-wavelength photons in a photocatalytic oxygen evolution reaction (OER). Here, based on first-principles calculations, we reveal that the two-dimensional (2D) aza-fused conjugated microporous polymer (aza-CMP) with a honeycomb network is a semiconductor with novel layer-dependent electronic properties. The bandgap of the as-synthesized aza-CMP nanosheets is measured to be 1.22 eV, suggesting that they can effectively boost light absorption in the visible and near infrared (NIR) region. More importantly, aza-CMP also possesses a valence band margin suitable for a photocatalytic OER. Taking advantage of the 2D layered nanostructure, we further show that the exfoliated ultrathin aza-CMP nanosheets can exhibit a three-fold enhancement in the photocatalytic OER. After deposition of a Co(OH)₂ cocatalyst, the hybrid Co(OH)₂/aza-CMP photocatalyst exhibits a markedly improved performance for photocatalytic O₂ evolution. Furthermore, first-principles calculations reveal that the photocatalytic O₂ evolution reaction is energetically feasible for aza-CMP nanosheets under visible light irradiation. Our findings reveal that nanostructured polymers hold great potential for photocatalytic applications with efficient solar energy utilization.

A Hyper-cross-linked Polynaphthalene Semiconductor with Excellent Visible-Light Photocatalytic Performance in the Degradation of Organic Dyes

Hyper-cross-linked polynaphthalene nanoparticles (PNNs) capable of catalyzing the degradation of organic pollutants upon exposure to visible light have been developed. The nascent and metal-free PNNs with a porous structure, high specific surface area, and narrow bandgap are chemically and thermally stable in the catalytic system, which make it promising as a kind of excellent photocatalytic material compared to conventional photocatalysts. The photocatalytic activity of the as-obtained PNNs exhibits remarkable photocatalytic performance for the degradation of rhodamine B (RhB) and methyl blue (MB) under the irradiation of visible light. The easy preparation, high catalytic activity, and recyclability of the PNNs open new opportunities in the visible-light-promoted degradation of organic pollutants.

Trends and challenges for microporous polymers

Recent trends and challenges for the emerging materials class of microporous polymers are reviewed. See the main article for graphical abstract image credits.