Advanced Porous Aromatic Frameworks: A Comprehensive Overview of Emerging Functional Strategies and Potential Applications

Porous aromatic frameworks (PAFs) are a fundamental group of porous materials characterized by their distinct structural features and large surface areas. These materials are synthesized from aromatic building units linked by strong carbon-carbon bonds, which confer exceptional rigidity and long-term stability. PAFs functionalities may arise directly from the intrinsic chemistry of their building units or through the postmodification of aromatic motifs using well-defined chemical processes. Compared to other traditional porous materials such as zeolites and metallic-organic frameworks, PAFs demonstrate superior stability under severe chemical treatments due to their robust carbon-carbon bonding. Even in challenging environments, the chemical stability and ease of functionalization of PAFs demonstrate their flexibility and specificity. Research on PAFs has significantly expanded and accelerated over the past decade, necessitating a comprehensive overview of key advancements in this field. This review provides an in-depth analysis of the recent advances in the synthesis, functionalization, and dimensionality of PAFs, along with their distinctive properties and wide-ranging applications. This review explores the innovative methodologies in PAFs synthesis, the strategies for functionalizing their structures, and the manipulation of their dimensionality to tailor their properties for specific potential applications. Similarly, the key application areas, including batteries, absorption, sensors, CO2 capture, photo-/electrocatalytic usages, supercapacitors, separation, and biomedical are discussed in detail, highlighting the versatility and potential of PAFs in addressing modern scientific and industrial challenges.

Synthesis, Characterization, and Roles of Vacancy Defects in Polymer and Graphitized Carbon Nitride Photocatalysts: A Comprehensive Review

Vacancy defect graphitic carbon nitride (g-C3N4) and conjugated polyimide (PI) polymer photocatalysts have become increasingly recognized as metal-free photocatalysts featuring an appropriate bandgap. The narrow absorption spectrum of visible light and the rapid recombination rate of the photoexcited charge carriers in PI polymers and g-C3N4 impede its photocatalytic performance. The presence of oxygen vacancies (OVs) in PI polymer photocatalysts, as well as nitrogen vacancies (NVs) and carbon vacancies (CVs) in g-C3N4, can significantly enhance the migration of photogenerated electrons. Adding vacancies to improve the electronic structure and band gap width can greatly enhance the photocatalytic efficiency of PI polymers and g-C3N4. Defect engineering is important for increasing the photocatalytic ability of PI-polymer and g-C3N4. There remains a notable absence of thorough review papers covering the synthesis, characterization, and applications of vacancy-rich PI-polymer and g-C3N4 in photocatalysis. This review paper examines the roles of OVs in PI-polymer, NVs, and CVs in g-C3N4 and thoroughly summarizes the preparation approaches employed before and after, as well as during polymerization. This review scrutinizes spectroscopic characterization techniques, such as EPR, XPS, PAS, XRD, FTIR, and NMR, for vacancy defect analysis. We also reviewed the role of vacancies, which include light absorption, photogenerated charge carrier separation, and transfer dynamics. This review could serve as a comprehensive understanding, a vacancy-engineered design framework, and a practical guide for synthesizing and characterizing.

Enhanced Photocatalytic Degradation of Environmental Pollutants Using a Triphenylamine-Based Polymer: Synthesis, Characterization, and Mechanistic Insights

Conjugated porous organic polymers have sparked growing research attention as photocatalysts owing to their high surface area, tunable pores, and capacity to collect and transfer light energy via their delocalized backbone. However, the synthesis methods for preparing these polymers require difficult experimental setups, such as high polymerization temperature, inert atmosphere, and use of transition metal catalysts. In the present work, a triphenylamine-based conjugated porous polymer (TPA-BPA) has been synthesized employing tris(4-aminophenyl)amine (TPA) and biphenyldicarboxaldehyde (BPA) as precursors via a one-pot Schiff base reaction in ambient conditions in the absence of the metal catalyst. The synthesized TPA-BPA polymer has been characterized using Fourier transform infrared spectroscopy, 13C cross-polarization magic angle spinning nuclear magnetic resonance, Brunauer-Emmett-Teller (BET), thermogravimetric analysis, field emission scanning electron microscopy, high-resolution transmission electron microscopy, valence band X-ray photoelectron spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, powder X-ray diffraction, electron paramagnetic resonance (EPR), and diffuse reflectance spectroscopy (DRS) techniques. The DRS analysis revealed that TPA-BPA has an optical band gap of 2.1 eV, demonstrating its semiconductive nature. The EPR study has shown that the synthesized polymer exhibits an intense radical signal at g = 2.00, confirming free radical generation upon photoexcitation and facilitating the breakdown of organic contaminants by photocatalysis. TPA-BPA possesses an exceptional porous structure with a surface area of 36 m2/g, as confirmed by BET studies, and high thermal stability up to 420 °C. It has been confirmed by photocatalytic studies that TPA-BPA shows effective degradation of methyl orange (92%), Congo red (91.5%), tobramycin (96%), and hydroquinone (85%) under visible light irradiation in 60 min. Owing to these observations, TPA-BPA can be an excellent candidate as a photocatalyst for environmental remediation. These findings pave the way for large-scale manufacturing of metal-free conjugated porous polymers as photocatalysts with variable photoelectrical characteristics.

Electrochemiluminescence enhanced by molecular engineering linear π-conjugated polymer: An ingenious ECL emitter for the construction of exosome sensing platform

Linear π-conjugated polymers (LCPs) with π-electron conjugation system have many remarkable optical characteristics such as fluorescence and electrochemiluminescence (ECL). However, the extremely strong interchain interaction and π-π stacking limit the luminescence efficiency. In this work, 1H-1,2,4-triazole-3,5-diamine was chosen as the polymer monomer and reacted with terephthalaldehyde via simple Schiff base condensation to synthesize LCPs. Subsequently, molecular engineering strategy was adopted to construct zirconium-based LCPs (MLCPs), which not only prevented π-π stacking but also ensured that extended π-coupling was maintained in the LCPs, thus effectively promoting charge transport and achieving strong luminescence. Second, the coreactant polyethyleneimine (PEI) was assembled onto the MLCPs (MLCPs@PEI) to further promote the emission of ECL. To further explore the potential of the obtained MLCPs@PEI as emerging ECL emitter, colorectal cancer exosome was chosen as model biomarker, and an innovative ECL ratiometric system based on MLCPs@PEI and luminol was designed to improve the validity and accuracy of the sensors. This research provides a fresh nanoplatform for exosome detection and broadens the application of LCPs in ECL immunoassay.

Large-Scale Continuous and In Situ Photosynthesis of Hydrogen Peroxide by Sulfur-Functionalized Polymer Catalyst for Water Treatment

Photocatalytic H2 O2 generation system based on polymer catalyst receives increasing attention in recent years; however, the insufficient charge separation efficiency and low oxygen adsorption/activation capacity severely limit their potential application. In this study, a sulfur (C=S) functionalized polymer catalyst is reported through a green water-mediated and catalyst-free multi-component reactions (MCRs) route. The sulfur functional group endows the polymer with a suitable energy band and facilitates the separation of photogenerated electron-hole pair. The reported polymer achieves a high H2 O2 production efficiency (3132 μmol g-1  h-1 ) in pure water without oxygen aeration. To demonstrate their potential in in situ wastewater treatment, a panel reactor system (20×20 cm) is constructed for large-scale production of H2 O2 , which realizes continuous degradation of emerging pollutants including antibiotics and bisphenol A under natural sunlight irradiation condition. The H2 O2 utilization efficiency of the photo-self-Fenton system using in situ generated H2 O2 is found 7.9 times higher than that of the traditional photo-Fenton system. This study offers new insights in green synthesis and design of functional polymer photocatalyst, and demonstrates the feasibility of panel reactor system for large-scale continuous H2 O2 photocatalytic production and water treatment.

Imine-Linked 2D Conjugated Porous Organic Polymer Films for Tunable Acid Vapor Sensing

Two-dimensional (2D) films of conjugated porous organic polymers (C-POPs) can translate the rich in-plane functionalities of conjugated frameworks into diverse optical and electronic applications while addressing the processability issues of their crystalline analogs for adaptable device architectures. However, the lack of facile single-step synthetic routes to obtain large-area high-quality films of 2D-C-POPs has limited their application possibilities so far. Here, we report the synthesis of four mechanically robust imine-linked 2D-C-POP free-standing films using a single-step fast condensation route that is scalable and tunable. The rigid covalently bonded 2D structures of the C-POP films offer high stability for volatile gas sensing in harsh environments while simultaneously enhancing site accessibility for gas molecules due to mesoporosity by structural design. Structurally, all films were composed of exfoliable layers of 2D polymeric nanosheets (NSs) that displayed anisotropy from disordered stacking, evinced by out-of-plane birefringent properties. The tunable in-plane conjugation, different nitrogen centers, and porous structures allow the films to act as ultraresponsive colorimetric sensors for acid sensing via reversible imine bond protonation. All the films could detect hydrogen chloride (HCl) gas down to 0.05 ppm, far exceeding the Occupational Safety and Health Administration's permissible exposure limit of 5 ppm with fast response time and good recyclability. Computational insights elucidated the effect of conjugation and tertiary nitrogen in the structures on the sensitivity and response time of the films. Furthermore, we exploited the exfoliated large 2D NSs and anisotropic optoelectronic properties of the films to adapt them into micro-optical and triboelectric devices to demonstrate their real-time sensing capabilities.

Cyano-Regulated Organic Polymers for Highly Efficient Photocatalytic H2 O2 Production in Various Actual Water Bodies

Photocatalytic production of H2 O2 has drawn significant attention in recent years, but the yield rate of current photocatalytic systems is still unsatisfactory. Moreover, the presence of various components in actual water bodies will consume the photogenerated charges and deactivate the catalyst, severely limiting the real applications of photocatalytic H2 O2 production. Herein, a cyano-modified polymer photocatalyst is synthesized by Knoevenagel condensation with subsequent thermal polymerization. The introduction of cyano group and sulfer (S), oxygen (O) elements modulates the microstructure and energy band of the polymer catalyst, and the cyano group sites can effectively adsorb and activate O2 , realizing the generation of H2 O2 in the two-step single-electron oxygen reduction process. The reported system achieves high H2 O2 generation rate up to 1119.2 µmol g-1 h-1 in various water bodies including tap water, river water, seawater, and secondary effluent. This simple and readily available catalyst demonstrates good anti-interference performance and pH adaptability in photocatalytic H2 O2 production in actual water bodies, and its photodegradation and sterilization applications are also demonstrated. This study offers new insights in developing polymer catalysts for efficient photocatalytic production of H2 O2 in various water bodies for practical application.

Azine- and imine-linked conjugated polyHIPEs through Schiff-base condensation reaction

A new alternative synthetic route to the transition-metal free π-conjugated polyHIPEs is reported, which combines the Schiff-base condensation reaction and an emulsion-templating technology.

Conjugated microporous organic polymer as fluorescent chemosensor for detection of Fe3+ and Fe2+ ions with high selectivity and sensitivity

A conjugated microporous organic polymer (TPA-Bp) comprised of triphenylamine (TPA) and 2,2'-bipyridine-5,5'-diformaldehyde (Bp) was prepared via the Schiff-base reaction under ambient conditions. TPA-Bp is an amorphous and microporous spherical nanoparticle with very high stability. TPA-Bp suspension in DMF displayed strong fluorescence emission and selective fluorescence quenching response towards Fe³⁺ and Fe²⁺ ions. The fluorescence intensity of TPA-Bp at 331 nm presents linear relationship with the concentrations of both Fe³⁺ and Fe²⁺ with low detection limits of 1.02 × 10^-5 M for Fe³⁺ and 5.37 × 10^-6 M for Fe²⁺. The results of X-ray photoelectron spectroscopy (XPS) and Fourier Transform infrared spectroscopy (FTIR) confirm the selective coordination of N atoms of pyridine unit with Fe ions. The fluorescence quenching of TPA-Bp upon the addition of Fe³⁺/Fe²⁺ ions can be attributed to the absorption competition quenching (ACQ) mechanism and the energy transfer between TPA-Bp and Fe³⁺/Fe²⁺ ions. This work demonstrates that the conjugated microporous polymers are promising candidates as luminescent sensor for detection of the special analytes in practical applications.

Pyrene-containing conjugated organic microporous polymers for photocatalytic hydrogen evolution from water

Pyrene based conjugated microporous polymers (CMPs) as photocatalysts with promising H₂ production efficiencies and very high stability.