Metalloporphyrin-Based Covalent Organic Frameworks: Design, Construction, and Photocatalytic Applications

Abstract

Metalloporphyrin-based covalent organic frameworks (MPor-COFs) are emerging porous crystalline materials that combine the optoelectronic properties of metalloporphyrins with the highly ordered structure of COFs. Such a combination not only extends the light absorption spectrum of COFs by incorporating porphyrins but also improves the separation and transport capabilities of photo-generated electrons and holes by leveraging the structural advantages of organic frameworks. At the same time, the metal ions embedded in the porphyrin ring provide abundant active sites and optimize charge transfer channels, showing particular advantages in photocatalysis. The molecular design, construction, and photocatalytic application of MPor-COFs were reviewed in this paper. The intrinsic relationship among the structure, optoelectronic properties, and specific photocatalytic application received special attention. First, the role of the metal center in regulating the electronic structure and photophysical property of porphyrin monomers was introduced, as well as the impact of bond type on framework stability and charge transport efficiency. Then, the synthesis strategies for MPor-COFs were summarized. Finally, the applications of these materials in photocatalysis were critically reviewed, and their prospects and challenges in energy conversion and environmental remediation were also discussed.

1. Introduction

The widespread use of fossil resources, such as coal, oil, and natural gas, become the foundation of modern industrial civilization. However, their non-renewable nature and over-exploitation lead to increasingly serious problems, including resource depletion and environmental pollution [1,2]. Solar energy is considered a cornerstone for building a renewable energy system owing to its advantages of large reserves, cleanliness, and environmental friendliness, which represent the strategic direction for resolving the current energy crisis [3,4]. Inspired by the directional transmission of photo-generated charges and precise regulation of redox reactions in natural photosynthesis, photocatalysis converting solar energy into storable chemical energy has become a green technological paradigm that integrates energy conversion and environmental remediation [5,6]. The photocatalyst is essential to photocatalysis, and thus, developing efficient photocatalysts is critical for improving solar energy conversion efficiency [7,8]. Photocatalysts with high photoelectric conversion efficiency require certain crucial characteristics, including wide spectral response, high carrier mobility, and an appropriate redox potential gradient [9].

Since the pioneering work of TiO₂ in photocatalytic water splitting by Fujishima and Honda in 1972 [10], a diverse system of photocatalytic materials has been gradually established over fifty years, including inorganic semiconductors, organic polymers, and metal complexes [11,12]. Among them, inorganic photocatalysts such as TiO₂ are noted for their high chemical stability and low preparation cost. In response to their inherent wide bandgap and the tendency for photogenerated charge carriers to recombine, researchers have developed various modification strategies. These approaches, including element doping, heterojunction construction, and morphology control, have effectively expanded the visible-light response and improved charge separation efficiency [13,14,15]. These efforts have significantly advanced the development and application exploration of TiO₂-based materials in the field of photocatalysis.

In contrast, organic photocatalytic systems with tailorable molecular structure and tunable light absorption spectrum provide a new paradigm in overcoming the above shortcomings of inorganic photocatalysts [16,17]. In natural photosystems, chlorophyll captures light across a wide spectrum through a large π-conjugated framework, and the central metal of chlorophyll cooperates with enzyme systems to achieve efficient separation and directional transfer of charge carriers. Such a biological mechanism provides important inspiration for the design of artificial photocatalysts [17,18]. Porphyrin molecules exhibit excellent photoresponse properties, benefiting from their unique conjugated ring with 18 π electrons. The flexible modification of their peripheral substituents and central metals even aggregating states can precisely regulate the frontier orbital energy levels and redox potentials, making them highly promising for photocatalytic hydrogen production, CO₂ reduction, and pollutant degradation [17,18,19,20,21,22].

Covalent organic frameworks (COFs), emerging crystalline porous materials, provide an ideal platform for photocatalyst design due to their high specific surface area [23], adjustable pore size [24], ordered pore structure [25], excellent stability [26], and tunable functionality [27]. The modular design of COFs allows for precise regulation of optical and electronic properties, thereby optimizing photocatalytic performance [28,29,30]. When porphyrin units are incorporated into the COFs, both the light-capturing ability and the charge separation and transfer efficiency will be improved, laying the foundation for efficient photocatalysis [31,32,33,34,35,36,37,38,39,40,41,42].

MPor-COFs facilitate photocatalytic performance by introducing metal ion coordination sites at the center of the porphyrin ring. The coordination of metal ions can alter the electronic structure of porphyrin, enhance the efficiency of photo-generated charge separation and provide abundant catalytically active sites [43,44,45,46,47]. The incorporation of metalloporphyrins enables COFs to have a wider absorption spectrum [48,49,50,51,52] and higher carrier mobility [53,54,55,56,57,58]. Through rational molecular design, metalloporphyrin-based COFs have also demonstrated wide applications in gas storage and separation [59,60], chemical sensing [61,62,63], energy storage devices [64,65,66,67], drug delivery [68,69] and photodynamic therapy [70,71], showcasing their great potential in solar energy conversion and utilization.

2. Strategies for the Design of MPor-COFs

Metalloporphyrin-based covalent organic frameworks (MPor-COFs), emerging porous crystalline materials, exhibit enormous potential in photocatalysis, energy conversion, and environmental remediation by integrating the optoelectronic properties of metal porphyrins with the highly ordered structure of COFs [72,73,74,75]. To achieve efficient photo-generated charge separation and transfer, and powerful photocatalytic activity, the key to the molecular design lies in the multi-scale precise regulation from the molecular level to the macroscopic frameworks [76,77,78,79,80]. Therefore, the design of MPor-COFs should focus on two aspects: the selection of the central metals in porphyrin and the organic linkers.

2.1. Selection of the Metal Center

The design of metalloporphyrin monomers is essential for the construction of MPor-COFs. The selection of the metal center directly affects the optical properties, electronic structure, and catalytic activity of the porphyrin monomers [81]. Central metal ions influence the electronic density and energy levels of the porphyrin ring, and subsequently affect their light absorption, charge separation efficiency, and reaction pathways. Porphyrin with main group metal ions, such as Mg²⁺ and Zn²⁺, is typically an excellent photosensitizer. In contrast, porphyrin embedded with transition metal ions, such as Cu²⁺, Fe³⁺, Co²⁺, and Ni²⁺, is more likely to serve as catalytically active centers [82,83], as shown in Figure 1a–c.

The central metal ions are crucial in regulating the electronic structure of metalloporphyrins. The electronegativity, d-electron configuration, and ionic radius of the central metal ions directly decide the electronic cloud distribution of the porphyrin ring, altering the energy level of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), thereby affecting the light absorption properties and redox potentials [84]. For example, the LUMO energy levels of Mg-porphyrin and Zn-porphyrin are obtained as −4.0 eV and −3.9 eV through electrochemical and UV-Vis spectral analysis, respectively, which are highly matched with the LUMO energy level of Cu-porphyrin (−4.1 eV), as shown in Figure 1d. This alignment of energy levels promotes the transfer of electrons from the photosensitizer (Mg-porphyrin and Zn-porphyrin) to the catalyst (Cu-porphyrin) [85], as shown in Figure 1e.

The metal center of porphyrins directly influences their light-capturing and energy-transfer efficiency. For example, metal ions such as Zn²⁺ and Mg²⁺, which have filled or vacant d-orbitals, prevent non-radiative energy decay caused by low-energy d-d transitions in the corresponding porphyrin complexes. Thus, they exhibit longer singlet and triplet lifetimes, a feature that enables them to act as efficient photosensitizers, allowing for prolonged excited-state persistence and efficient energy transfer [82]. The metal center also has a decisive impact on the absorption spectrum of porphyrins, which is directly related to the efficiency of light capture and energy transfer processes. Metalloporphyrins with central Mg²⁺ or Zn²⁺ ions showed strong absorption peaks in the Soret band (around 425 nm) and the Q band (around 550–600 nm), covering a large portion of the visible light spectrum. This makes them ideal for broad-spectrum light-capturing applications. In contrast, metalloporphyrins with central copper (Cu) or cobalt (Co) ions exhibit redshift or spectral broadening due to electronic transitions (d-d transitions) in the metal’s d-orbitals, which extends their absorption spectrum toward the red region. Additionally, they are often accompanied by fluorescence quenching [86].

The selection of the central metal ions should consider their photostability. Under photocatalytic conditions, Mg²⁺ and Zn²⁺ centers exhibit higher stability, with less attenuation of the Soret band absorption after prolonged irradiation. However, Cu²⁺ and Co²⁺ in the center of porphyrin may undergo valence change during the reduction process, which can affect their long-term application [87,88]. Therefore, it is essential to balance the light-capturing ability, charge separation efficiency, catalytic activity, and photostability when selecting the central metal for the MPor-COFs.

Figure 1. (a) Graphical representation of the formation of phycobilisome by the co-assembly of protein scaffolds and bilins; (b) Chemical structures of the anionic metalloporphyrins used as the photosensitizer (Ps-M, green) and catalysts (Cat-M, red) as well as PDDA used as a counterion polyelectrolyte; (c) Graphical representation of the co-assembly of the metalloporphyrins and PDDA into hierarchical supramolecular nanostructures for photocatalytic H₂ production in water; (d) HOMO and LUMO energy levels of the metalloporphyrins as photosensitizers and catalysts in water (M = Mg²⁺, Zn²⁺, Mn³⁺, Fe³⁺, Ni²⁺, Cu²⁺, Co²⁺, [M] = 2 mM); (e) Graphical representation for the dissipation mechanism of the donor excitation energy. Reproduced with permission [85].

Figure 1. (a) Graphical representation of the formation of phycobilisome by the co-assembly of protein scaffolds and bilins; (b) Chemical structures of the anionic metalloporphyrins used as the photosensitizer (Ps-M, green) and catalysts (Cat-M, red) as well as PDDA used as a counterion polyelectrolyte; (c) Graphical representation of the co-assembly of the metalloporphyrins and PDDA into hierarchical supramolecular nanostructures for photocatalytic H₂ production in water; (d) HOMO and LUMO energy levels of the metalloporphyrins as photosensitizers and catalysts in water (M = Mg²⁺, Zn²⁺, Mn³⁺, Fe³⁺, Ni²⁺, Cu²⁺, Co²⁺, [M] = 2 mM); (e) Graphical representation for the dissipation mechanism of the donor excitation energy. Reproduced with permission [85].

2.2. Design of the Linkage

Porphyrin molecules serve as ideal building blocks for constructing covalent organic frameworks (COFs) owing to their highly symmetrical, rigid, and planar structure, as well as their extended macrocyclic conjugated system [89]. Such a molecular structure not only imparts inherent stability to the COFs but also provides numerous modifiable sites for further functionalization. This facilitates the introduction of diverse functional groups, thereby enabling precise customization of the functionality [90]. In the construction of COFs, the synergistic design of the building units and linking bonds is important in regulating the overall performance of the material [91,92,93,94,95]. Almost all mature linkages in COFs have been successfully combined with porphyrin units to construct porphyrin-based COFs with distinct structures and varied properties. The choice of the linkers directly affects the stability, crystallinity, and pore properties of the frameworks. Some representative porphyrin building units, along with corresponding linkers, were collected and shown in Figure 2. As the variety of the structural units continues to expand, researchers have conducted extensive studies on the structural characteristics of porphyrin-based COFs and explored their applications in different fields.

Porphyrins with peripheral amino or aldehyde groups are usually adopted to construct Por-COFs, although there are diversified porphyrin units. Borate ester bonds had been used to construct the first porphyrin-based covalent organic framework (COF-66), which was synthesized using borate bonds. However, the limited availability of monomers containing borate groups and the poor stability of the COFs restrict their practical application [96]. By comparison, imine bonds, formed through Schiff base condensation reactions [96,97], not only promote the formation of high-crystallinity COFs due to their excellent reversibility and dynamic equilibrium characteristics but also support the construction of complex topological structures. This provides great potential for subsequent functional modifications, including the introduction of donor–acceptor structures [98,99] and metal active sites [100,101], and has become a mainstream choice in the design of COFs. In addition, hydrazone linkages derived from imine bonds [73,102] have also been used to construct COFs and applied in photocatalytic hydrogen production [103] and other fields.

The development and upgrade of irreversible linking bonds significantly improve the structural stability and functional diversity of COFs. Por-COFs with sp²-carbon linkages, formed by Knoevenagel condensation, have an extended π-conjugated structure throughout the entire 2D lattice. This will effectively enhance the material’s acid-base resistance, reduce the electronic polarization caused by imine bonds, and improve the overall electronic transport capabilities [104]. The first sp²-carbon-linked COFs were prepared by Jiang’s group in 2017 [105], while the first sp²-carbon-linked Porphyrin-based COFs (Por-sp²c-COFs) were reported in 2019 [106], which exhibited high activity and cycling stability in photocatalytic oxidation reactions.

The introduction of imidazole linkages can extend the heterocyclic structure, allowing COFs with enhanced stability and biocompatibility. Zhao et al. [107] reported the synthesis of M-PyPor-COFs with imidazole linkages through the Debus-Radziszewski reaction in 2022. This material efficiently generates singlet oxygen under visible light irradiation. Meanwhile, Dong et al. [108] synthesized NiPc-CoPor-imi-COF by forming imidazole linkages between aldehyde and o-phenylenediamine units. The material exhibits bimetallic active sites and efficiently catalyzes the reduction of CO₂. Recent studies have found that the nitrogen atom on the imidazole ring, with a lone pair of electrons, acts as a highly selective metal coordination site in manipulating COFs to capture metal ions with high efficiency and selectivity [109].

Polyimide bonds, classic irreversible linkages formed through the condensation of carboxyl and amine groups, are strong covalent bonds widely used in the electrocatalytic reduction of CO₂. NiPc-2HPorCOF, prepared by Zhang et al. [110], is an efficient electrocatalyst for CO₂ reduction. Polyimide bonds serve as important structural units for the green synthesis of COFs with high crystallinity and excellent stability. In addition, triazine linkages, diyne linkages, and other connecting bonds have also been used in the construction of porphyrin-based COFs, further enriching the material’s structure and properties. TA-Por-sp²-COF was synthesized by the reaction of cyanide’s cyclotrimerization under the catalysis of trifluoromethanesulfonic acid [111]. In this porphyrin-based COF, the triazine ring acts as a strong electron acceptor, while the porphyrin unit serves as an electron donor, and the two parts are covalently connected to form an over-conjugated system with enhanced photocatalytic performance. Similarly, a COF (COF-JLU10) with diyne linkages was formed [112], in which a fully π-conjugated framework was generated. An extended, delocalized 2D π-conjugated network was formed by the diyne-linked porphyrin macrocyclic conjugated system. These diverse linking strategies provide flexible pathways for the customized design of COFs (Figure 3).

3. Construction of MPor-COFs

The precise introduction of metal ions into the center of the porphyrin ring becomes a key step to unlock their high photocatalytic performance when MPor-COFs are synthesized. This metallization process can affect the electronic structure of the porphyrin units, the efficiency of photo-generated charge separation, and the photocatalytic activity of the materials [113,114,115,116,117,118,119,120,121,122,123]. Up to now, three methods are often used to introduce metal ions into the Por-COFs, which are post-synthesis metallization, pre-metallization followed by synthesis, and one-pot synthesis.

3.1. Post-Synthesis Metallization

Post-synthesis Metallization necessitates the prior synthesis of a high-quality porphyrin-based COF precursor. Fabrication of these precursors relies on methods that provide precise control over nucleation and growth dynamics [124], primarily including solvothermal, microwave-assisted, and ionothermal synthesis. The solvothermal method utilizes high-temperature and high-pressure conditions in sealed vessels to facilitate reversible covalent bonding formation, which promotes framework self-repair and high crystallinity [125,126]. However, it is characterized by long reaction times, sensitive to solvent conditions, and typically yields microcrystalline powders [127,128]. In contrast, microwave-assisted synthesis employs rapid microwave irradiation for uniform heating, significantly shortening the reaction times and often improving both crystallinity and yield, though it is strongly dependent on polar solvents [129,130]. Alternatively, the ionothermal method uses low-melting-point ionic liquids as combined solvents and catalysts, avoiding high-pressure risks. Its limitations include harsh conditions that may restrict monomer compatibility and result in poor crystallinity due to restricted bond reversibility [131].

Subsequently, using the synthesized porphyrin-based COF as the precursor, the metalation is carried out via a coordination reaction with target metal salts in an appropriate solvent. During this process, the nitrogen atoms within the porphyrin core coordinate with the metal ions, leading to the formation of stable metal-nitrogen bonds and the incorporation of metal centers into the framework. This strategy avoids possible dissociation or valence state changes of the metal center under the harsh conditions of COF synthesis [132]. In 2017, Xu et al. [132] successfully synthesized MPor-COFs (COF-366-Zn and COF-366-Co) using COF-366 as the precursor and zinc acetate or cobalt acetate as the target metal ions under solvent reflux conditions. X-ray photoelectron spectroscopy (XPS) analysis revealed that Zn in COF-366-Zn exists in the form of Zn²⁺. The metal loading before and after catalytic cycling was close to the theoretical stoichiometric value, confirming the efficient anchoring and uniform distribution of metal sites. The results prove that near-quantitative metallization of the porphyrin units can be obtained through careful selection of metal precursors and reaction conditions.

3.2. Pre-Metallization Followed by Synthesis

Pre-metallization followed by synthesis is one of the most versatile strategies for constructing MPor-COF materials [133,134]. The core concept of this method is to obtain metalloporphyrin first and then use it as a structural unit for MPor-COF formation. The success of this strategy depends on whether the metallization condition is compatible with the subsequent COF construction process or not. It is critical to maintain the structural stability and coordination integrity of the metalloporphyrin monomers during the condensation reaction [99]. Typical metallization processes are usually based on liquid coordination chemistry principles, where free porphyrins and corresponding metal salts are heated under reflux in an appropriate organic solvent and an inert atmosphere. This allows the efficient and selective embedding of the target metal ions into the porphyrin macrocycle to form well-defined metalloporphyrin monomers [133,134]. This pre-metallization procedure not only provides stoichiometrically controllable building units for subsequent COF formation but also avoids some issues, such as framework distortion or unsaturated coordination, that usually occur in post-synthesis metallization [132]. This provides inspiration for the rational design of structurally ordered and tunable MPor-COFs.

3.3. One-Pot Metallization Method

The one-pot metallization method refers to placing all reactants, including monomers, metal salts, and morphology control agents, in the same reaction system to simultaneously achieve framework construction, metal ion coordination, and in situ protonation in one step. This method omits intermediate separation and, through precise reaction condition design, allows multiple chemical processes (such as imine condensation, metal coordination, and acid protonation) to occur simultaneously. In this case, complex nanomaterials with different functionalities were prepared. One-pot metallization method was used to prepare MPor-COFs by the reaction of 5,10,15,20-tetra(4-aminophenyl)porphyrin, terephthalaldehyde and FeCl₃ in a mixed solvent of acetonitrile and acetic acid at room temperature [38]. Imine condensation, Fe³⁺ coordination, and protonation were achieved simultaneously, simplifying the traditional multi-step process. However, the addition of FeCl₃ requires balancing the protonation and metallization ratio to prevent the instability of COFs. A donor–acceptor structured Por-COF-Zn was also synthesized by one-pot reaction of 5,10,15,20-tetra(4-aminophenyl)porphyrin, thiophene[3,2-b]thiophene-2,5-dialdehyde, and zinc acetate [135]. The reaction temperature needs multiple optimizations to avoid the collapse of the pore structure.

Nevertheless, traditional synthesis methods (pre-metallization followed by synthesis and post-synthesis metallization) often face the dilemma of insufficient crystallinity and irregular porosity in preparing high-quality MPor-COFs [136,137]. The underlying cause lies in the autoaggregation of porphyrin monomers, leading to disordered stacking [138,139,140]. Inspired by the concept of “porphyrin polymer ladders” [141,142], Bourda and coauthors [143] proposed an innovative synthesis strategy in which pyridine molecules were used as “assembly agents” in a one-pot method (Figure 4). Assembling agents were used to coordinate with metal ions in an octahedral coordination geometry. Linear coordination polymers were formed and used as templates to guide the ordered stacking of COF layers. The controllable construction of MPor-COFs with high crystallinity and high surface area can be achieved by this approach. The strategy not only effectively suppresses disordered aggregation but also provides a novel synthesis pathway for the design and preparation of structurally precise and functionally integrated MPor-COFs.

Overall, compared to traditional step-by-step synthesis strategies, the one-pot metallization method significantly simplifies the synthetic process by integrating multiple reaction steps into one single operation. This avoids repeated separation and purification steps and reduces energy consumption, and at the same time minimizes the risk of side reactions [144,145]. By the control of reaction parameters such as solvent polarity [146], temperature [127], and additives [147], the one-pot method can finely regulate the frameworks, including the microstructure, pore structure, and photophysical properties [148,149]. This provides a more efficient and controllable synthesis pathway for high-performance functional materials.

4. Photocatalytic Applications

The material’s energy band structure, photo-generated charge separation efficiency and active site distribution can be regulated by careful selection of the metal center and elaborate design of linking bonds. Meanwhile, highly crystalline and stable frameworks can be constructed through diverse synthetic methods. These structural characteristics established in earlier stages lead to outstanding performance in photocatalytic reactions. This chapter will focus on the specific applications of MPor-COFs in various cutting-edge fields.

4.1. Photocatalytic Hydrogen Production

Hydrogen energy is considered an efficient and environmentally friendly energy due to its high energy density and clean characteristics. Traditional hydrogen production processes rely on the water-gas shift reaction, which consumes a lot of energy and generates substantial CO₂ emissions. In contrast, photocatalytic hydrogen production directly uses solar energy and water to produce hydrogen, becoming an environmentally friendly and sustainable hydrogen production pathway because there are no carbon emissions during the entire process [150,151]. Among various photocatalysts, MPor-COFs have attracted special attention due to their excellent visible light-responsive properties. The energy band structure and photo-generated carrier migration ability of these materials can be precisely controlled at a molecular level, providing an ideal platform for building high-performance photocatalytic systems, in which metal ion coordination plays a key role in the hydrogen production process.

Metal ions in MPor-COFs can regulate charge separation and migration efficiency by influencing the LMCT (ligand-to-metal charge transfer) process, which directly influences charge-carrier dynamics. MPor-DETH-COFs (M = H₂, Co, Ni, Zn) were synthesized by the condensation of aldehyde porphyrin precursor (p-MPor-CHO) and 2,5-diethoxyterephthalonitrile (DETH) [99] (Figure 5a). As photocatalysts, hydrogen production is sustained using H₂PtCl₆ as the co-catalyst and triethanolamine as the sacrificial agent under visible light irradiation. The hydrogen production rates are ranged as follows: CoPor-DETH-COF < H₂Por-DETH-COF < NiPor-DETH-COF < ZnPor-DETH-COF. The photocatalyst exhibits excellent stability, and the average hydrogen evolution rate over ZnPor-DETH-COF reaches up to 413 μmol g⁻¹ h⁻¹. Notably, it maintains its photocatalytic hydrogen production activity without significant degradation after 120 h of continuous operation. The d-electron configuration of metal ions determines the strength of the LMCT process. As shown in Figure 5b, for Co²⁺ (3d⁷), a prominent LMCT process occurs, effectively trapping photogenerated holes at the metal center and severely suppressing their migration through the macrocycle-on-macrocycle pathway. This leads to rapid charge recombination, accounting for its lowest activity. In contrast, for Zn²⁺ (3d¹⁰), the LMCT process is forbidden. This allows for efficient hole migration via the porphyrin ring and electron transfer through a Zn···Zn chain. This prolongs the charge separation lifetime and improves hydrogen production efficiency. The fluorescence lifetime of ZnPor-DETH-COF is the longest (nanosecond scale), while CoPor-DETH-COF has almost no emission (picosecond scale), consistent with the electronic structure of the metal ions.

The simultaneous coordination of metal ions and axial ligands can also achieve ultrathin MPor-COFs, which can shorten the charge transport distance. Different metal ions have a distinct impact on the exfoliation effect (Figure 5c). Compared to Mg²⁺, Cu²⁺ produces a more stable metal porphyrin structure, resulting in more complete exfoliation and ultimately thinner nanoplatelets. The metal coordination alters the electronic structure of the porphyrin material. Solid-state diffuse reflectance spectra show that e-CON (Cu, epy) has an extended absorption band, close to the near-infrared region. Photoelectrochemical tests confirm that it generates photocurrents under both visible light and near-infrared light, predicting the potential in photocatalytic hydrogen evolution across a wide spectral range [152]. The photocatalytic hydrogen evolution activity of DhaMTph (M = Cu, Ni) 136 (P8, P10+L12) improved 3.7 times compared to DhaTph. The photocatalytic activity was further enhanced 2.2 times when they were made into a nanosheet structure [153].

Figure 5. (a) Schematic representation of the synthesis of MPor-DETH-COFs; (b) LMCT mechanism indicated by DFT calculations. The isosurface of the electron orbitals of blue VBM and magenta CBM (upper panel) and schematic illustrations of the hole-electron transport processes (lower panel) in MPor-DETH-COFs: H2Por-DETH-COF, CoPor-DETH-COF, NiPor-DETH-COF, ZnPor-DETH-COF. The balls in different colours represent different atoms: H, white; C, grey; N, blue; Co, olive green; Ni, light blue; Zn, orange. Reproduced with permission [99]; (c) Scheme for the synthesis of exfoliated DhaTph. The addition of metals and axial ligands induces exfoliation of the covalent organic frameworks composed of porphyrin units. Reproduced with permission [153].

Figure 5. (a) Schematic representation of the synthesis of MPor-DETH-COFs; (b) LMCT mechanism indicated by DFT calculations. The isosurface of the electron orbitals of blue VBM and magenta CBM (upper panel) and schematic illustrations of the hole-electron transport processes (lower panel) in MPor-DETH-COFs: H2Por-DETH-COF, CoPor-DETH-COF, NiPor-DETH-COF, ZnPor-DETH-COF. The balls in different colours represent different atoms: H, white; C, grey; N, blue; Co, olive green; Ni, light blue; Zn, orange. Reproduced with permission [99]; (c) Scheme for the synthesis of exfoliated DhaTph. The addition of metals and axial ligands induces exfoliation of the covalent organic frameworks composed of porphyrin units. Reproduced with permission [153].

4.2. Photocatalytic Carbon Dioxide Reduction

Photocatalytic reduction of carbon dioxide to high-value-added chemicals is considered a key strategy for achieving carbon neutrality with both environmental benefits and energy value [154,155]. COFs with tunable optoelectronic properties, high specific surface area, and ordered pore structure provide an ideal platform for photocatalytic CO₂ reduction. Specifically, MPor-COFs offer various performance optimization strategies to enhance the catalytic efficiency.

Firstly, the catalytic performance of materials can be improved by increasing the exposure of active sites. COF-367-Co, ultra-thin two-dimensional COF nanosheets, were prepared and successfully applied in the photocatalytic reduction of CO₂ (Figure 6a) [156]. A production rate of 1062 μmol g⁻¹ h⁻¹ and a selectivity of 78% for CO were obtained using a ruthenium complex as the photosensitizer and ascorbic acid as the electron donor. In comparison, the COF nanosheets without cobalt or physically mixing with free Co²⁺ ions showed significantly lower photocatalytic activity. This proves that the Co-N₄ center in the cobalt porphyrin is the actual catalytically active site, rather than metal ions physically adsorbed on the surface. Moreover, COF-367-Co nanosheets exhibited significantly improved CO₂ adsorption capacity compared to the metal-free COF-367 nanosheets. This indicates that the presence of the cobalt center enhances the capturing ability for CO₂, which is the precondition for efficient catalysis.

Periodic and ordered donor–acceptor (D-A) structures can be introduced into the COF framework to promote the separation of photo-generated charge carriers by utilizing their strong electron-donating and electron-withdrawing effects, and thus enhance the photocatalytic CO₂ reduction performance of the material. For example, MPor-COFs with D-A structures (PD-COF-23-Ni) were prepared by the condensation of NiTAPP and DPP-CHO, and showed astonishing selectivity of 99% for CO and a higher reaction rate of 40.0 μmol g⁻¹ h⁻¹, 2-fold of that over the metal-free COFs [157]. Moreover, such photocatalytic reduction of CO₂ didn’t require additional photosensitizers and noble metal cocatalysts. DFT calculations showed that the HOMO is mainly distributed in the porphyrin units, while the LUMO is located in the DPP units. The Ni(II) coordination optimized the energy level structure, accelerated the transfer of photo-generated electrons from porphyrins to DPP, and suppressed the recombination of electrons and holes. NiTAPP serves as a photosensitizer to absorb visible light and as a catalytic site. During the light irradiation, NiTAPP⁺ is reduced by TEOA to generate NiTAPP⁻, then electrons in NiTAPP⁻ transfer to CO₂, and thus a continuous catalytic cycle is formed, as shown in Figure 6c. Recently, cobalt-5,10,15,20-tetra(4-aminophenyl)porphyrin (Co-Por) was used as an electron acceptor unit, and three COFs (Co-Por-TA, Co-Por-TT, Co-Por-BDT) were synthesized via Schiff base condensation with three different conjugated donor units (phenyl TA, thienothiophene TT, and benzo[1,2-b:4,5-b’] dithiophene BDT) [158], as shown in Figure 6d. These COFs can catalyze the reduction of CO₂ to CO using Re(bpy)(CO)₃Cl as the photosensitizer and TEOA as the sacrificial agent under visible light irradiation. The activity sequence was Co-Por-BDT (1424 μmol g⁻¹ h⁻¹) > Co-Por-TT (1182 μmol g⁻¹ h⁻¹) > Co-Por-TA (642 μmol g⁻¹ h⁻¹). This trend in performance was linked to the charge-carrier densities of donor units and π-conjugation lengths, which influenced the light absorption and excitonic effects. Optical studies showed that Co-Por-BDT with the most extended conjugation had the narrowest band gap and highest absorbance. Theoretical simulations indicated that stronger D-A interactions in Co-Por-BDT reduced exciton binding energy, enhancing charge-carrier mobility and separation. Meanwhile, Co²⁺ ions directly participate in the CO₂ reduction process in these reactions. They acted as electron transfer centers to promote the adsorption and activation of CO₂ molecules. Besides, Co coordination optimized the electronic structure of the porphyrins, in which the HOMO mainly focused on the donor unit and the LUMO located in the Co-Por acceptor unit. In this way, a clear donor–acceptor structure was formed, and the recombination of electrons and holes was suppressed.

The adsorption energy of the active site on the reaction intermediates can also be adjusted by changing the metal ion or spin state of the porphyrin center. In this way, the activity and selectivity of the materials can be optimized. COF-367-Co³⁺ (S = 0) and COF-367-Co²⁺ (S = 1/2) were prepared and used in the reduction of CO₂ to generate HCOOH [159]. They achieved the first spin state of metal ions in a COF, and then altered the spin state of the cobalt center without changing the framework structure by oxidizing Co²⁺ to Co³⁺ in air. This provided a new paradigm for regulating the electronic structure of COFs (Figure 6e). The production rate of HCOOH for COF-367-Co³⁺ (S = 0) reached 93.0 μmol g⁻¹ h⁻¹, significantly higher than that of COF-367-Co²⁺ (S = 1/2), while the production of CO and CH₄ was suppressed. More importantly, its HCOOH production activity showed no significant decrease over five consecutive cycles, demonstrating excellent reusability. DFT calculations indicated that Co³⁺ (S = 0) lowered the energy barrier of HCOOH production but increased the energy barrier for further conversion of HCOOH, thereby improving the selectivity. The porphyrin ring provides a rigid coordination environment in which the Co²⁺/Co³⁺ ions were stabilized. The framework structure remained unchanging during the oxidation state change, and thus, single-variable regulation was achieved.

Figure 6. (a) Schematic illustration of the synthesis of the COF-367 NSs. Reproduced with permission [160]; (b) Frontier orbitals from DFT calculations; (c) the proposed photocatalytic mechanism of PD-COF-23-Ni. Reproduced with permission [157]; (d) Schematic illustration of the building units and synthesis process of Co-Por-COFs. Photocatalytic CO₂ production and photo and electrochemical analyses of the Co-Por-COFs. Reproduced with permission [158]; (e) Rational Fabrication of COF-367-Co Featuring Different Spin States of Co Ions toward Photocatalytic CO₂ Reduction. Reproduced with permission [159].

Figure 6. (a) Schematic illustration of the synthesis of the COF-367 NSs. Reproduced with permission [160]; (b) Frontier orbitals from DFT calculations; (c) the proposed photocatalytic mechanism of PD-COF-23-Ni. Reproduced with permission [157]; (d) Schematic illustration of the building units and synthesis process of Co-Por-COFs. Photocatalytic CO₂ production and photo and electrochemical analyses of the Co-Por-COFs. Reproduced with permission [158]; (e) Rational Fabrication of COF-367-Co Featuring Different Spin States of Co Ions toward Photocatalytic CO₂ Reduction. Reproduced with permission [159].

4.3. Photocatalytic Synthesis of Organics

Compared to photocatalytic hydrogen production and carbon dioxide reduction, the application of metal porphyrin-based COFs in the photocatalytic synthesis of organics is still in its infancy. Till now, only a few types of reactions have been reported. Therefore, expanding the photocatalytic systems to achieve more complex and diverse organic synthesis holds great potential.

Quinolines and quinazolinones are a class of drug molecules with excellent physiological and pharmacological activities. They are widely used not only in the pharmaceutical field but also in the pesticide industry [160,161]. However, traditional synthetic methods for 4-quinazolinone often involve dehydrogenative cross-coupling reactions, suffering from harsh reaction conditions, low yields, and difficulties in recovering homogeneous catalysts [162]. Two photosensitive and stable COFs (TAPP-An and TAPP-Cu-An) were designed and synthesized (Figure 7a,b) [163]. These COFs were used as efficient, heterogeneous tandem photocatalysts for the rapid synthesis of 4-quinazolinone compounds at room temperature under light irradiation. This is the first time that crystalline COF tandem catalysts have been successfully applied in the synthesis of important drug molecules. This opens up a new pathway toward organic synthesis, particularly in drug synthesis. The metal site plays a crucial role in the synthesis of 4-quinazolinone (the intramolecular oxidative cross-coupling reaction). The Cu site in TAPP-Cu-An works with the An unit synergistically in the one-pot reaction. The An units ensure high selectivity of the first dehydrogenation oxidation step, while the Cu sites guarantee rapid and efficient conversion of the intermediates into the final products. This synergistic effect makes TAPP-Cu-An outperform TAPP-An (which contains only the An unit) and homogeneous TAPP-Cu molecules in the one-step reaction. The non-metallic TAPP-An catalyst exhibited lower selectivity (62%), whereas TAPP-Cu-An with Cu²⁺ ions achieves nearly 100% selectivity. Moreover, TAPP-Cu-An demonstrated excellent recyclability, showing no significant loss in substrate conversion or product selectivity over seven consecutive reaction cycles.

The sunlight-driven nitrating pyridine ring addition reaction provides a promising pathway for CO₂ utilization and offers a green alternative to traditional thermocatalytic fixation methods that rely on high temperature or high pressure. Metal-free m-DBPA-COF and nickel-metallized m-NiDBPA-COF were prepared based on the trans-A₂B₂ type porphyrin D-A COFs [33] (Figure 8a). The materials can efficiently catalyze the CO₂ and nitrating pyridine ring addition reaction under visible light irradiation. The introduction of nickel ions is crucial. Firstly, they synergize with the D-A electronic push-pull effect through ligand-to-metal charge transfer (LMCT) to significantly enhance the separation and transport efficiency of photo-generated charges. Secondly, they provide the Lewis acid sites required for substrate activation, as shown in Figure 8b. Thanks to these advantages, m-NiDBPA-COF exhibited excellent photocatalytic performance.

4.4. Photocatalytic Degradation of Pollutants

Efficient treatment of organic pollutants becomes crucial for environmental governance and human health [164]. Among various treatments, photocatalytic degradation of organic pollutants is an effective and energy-saving method. Semiconductor photocatalysts were originally designed for the removal of pollutants [165]. At present, more and more photocatalysts, including metal porphyrin-based COFs, have shown great potential in the degradation of pollutants.

Despite the unique advantages of photocatalytic technology, it has a series of technical challenges in practical application. Traditional Fenton technology relies on Fe²⁺ to activate H₂O₂ to generate reactive oxygen species (ROS) for pollutant degradation. However, it is facing problems such as safety and cost arising from the external addition of H₂O₂, stringent requirements for acidic conditions, and iron sludge deposition. Although photo-Fenton technology can achieve in situ generation of H₂O₂ with the help of photocatalysts, reducing dependence on external reagents, it still requires the introduction of Fe²⁺, and the efficiency of H₂O₂ synthesis is limited by the low separation efficiency of photo-generated carriers and poor proton adsorption capacity [166,167].

Figure 7. (a) Synthesis of 4-quinazolinone drug molecules through an efficient photocatalytic dehydrogenative cross-coupling reaction by photosensitive and stable porphyrin-based COF; (b) Schematic of the synthesis of TAPP-An and TAPP-M-An. Reproduced with permission [167].

Figure 7. (a) Synthesis of 4-quinazolinone drug molecules through an efficient photocatalytic dehydrogenative cross-coupling reaction by photosensitive and stable porphyrin-based COF; (b) Schematic of the synthesis of TAPP-An and TAPP-M-An. Reproduced with permission [167].

Figure 8. (a) Design and synthesis of m-DBPA-COF and m-NiDBPA-COF; (b) Proposed photocatalytic CO₂ cycloaddition reaction mechanism. Reproduced with permission [33].

Figure 8. (a) Design and synthesis of m-DBPA-COF and m-NiDBPA-COF; (b) Proposed photocatalytic CO₂ cycloaddition reaction mechanism. Reproduced with permission [33].

To overcome these problems, researchers have designed complex materials with dual functions: in situ generation and activation of H₂O₂. For example, Liu [168] and his colleagues synthesized COF-HMTBD by condensing tetrahydroporphyrin (TAPP) and metal porphyrins (M-TAPP, M = Fe/Co/Cu) with benzothiadiazole (BD), in which TAPP and M-TAPP were used as an electron donor while BD was an electron acceptor. This material showed outstanding performance in Rhodamine B (RhB) degradation and excellent cycle stability, in which RhB was completely degraded within 3 h (Figure 9a). Notably, the Fe³⁺/Fe²⁺ in the material can catalyze the decomposition of H₂O₂ into ·OH and ·O₂⁻ via a Fenton-like reaction, which results in approximately 60% of RhB degradation even in the dark conditions. The more detailed photocatalyst performance is shown in

Table 1. Comparative Analysis of Photocatalytic Performance for Porphyrin-Based Covalent Organic Frameworks in Organic Pollutant Degradation.

Material	Pollutant	Degradation Efficiency (%)	Time (min)	Specific Surface Area (m2/g)	Charge Carrier Lifetime (ns)
COF-HTBD [168]	RhB	Negligible	180	17.72	0.53
COF-HFeTBD [168]	RhB	100	180	32	2.76
COF-HCuTBD [169]	RhB	55	180	44	1.05
COF-HCoTBD [169]	RhB	47	180	776.98	-
CuTB-COF [169]	RhB	66.4	60	800	4.13
CuTP-COF [169]	RhB	98.7	60	1726	4.21

.

The design of catalysts has been developed towards “self-sustaining” to overcome the inherent limitations of traditional Fenton processes, in which exogenous addition of Fe²⁺ and H₂O₂ is required, iron sludge is generated, and the reaction usually operates at strict acidic conditions (pH ≤ 3). Two donor–acceptor COFs (CuTB-COF and CuTP-COF) were designed and synthesized through imine condensation reactions using CuTAPP as the electron donor [169]. In these systems, the CuTAPP unit acts as a dual-function active site, enabling simultaneous photocatalytic generation and activation of H₂O₂. Under visible light irradiation, the ordered D-A structure promotes charge separation efficiently. The photogenerated electrons reduce O₂ to H₂O₂ via a two-electron oxygen reduction pathway; meanwhile, the Cu²⁺/Cu⁺ redox cycle activates H₂O₂ into hydroxyl radicals (·OH) through a Fenton-like mechanism. The higher performance of CuTP-COF is attributed to its enhanced charge separation efficiency, as evidenced by its stronger photocurrent response, longer lifetime of charge carriers, and narrower bandgap compared to that of CuTB-COF. This optimized electronic structure endows the materials with exceptional pollutant adsorption capacity and photocatalytic degradation performance. Notably, CuTP-COF achieved a remarkable RhB removal rate of 98.7% within 60 min without exogenous Fe²⁺ or H₂O₂. The crystallinity and activity of the material were preserved over five cycles. Compared to traditional Fenton processes, these “self-sustaining” COF catalysts offer significant advantages: they operate effectively under neutral conditions, eliminate iron sludge accumulation, avoid the safety risks associated with H₂O₂ storage, and enable a closed-loop degradation cycle through in situ H₂O₂ production and activation. Moreover, the integration of adsorption capability with photocatalytic function creates a synergistic system where pollutant concentration and degradation are optimized within the same material framework.

Metalloporphyrin-based COFs, with designable pore structures, ordered electron donor–acceptor units, and metal active sites, integrating the photocatalytic H₂O₂ production with the activation together, significantly improved the efficiency and economy of organic pollutant degradation via Fenton-like reactions. Compared to early Fenton-like systems that required external reagents such as H₂O₂ and Fe²⁺, the present self-cycling catalysts capable of in situ H₂O₂ generation and activation presented a clear development direction, offering new ideas for the design of efficient, low-energy wastewater treatment photocatalysts.

5. Conclusions and Outlook

In summary, MPor-COFs have shown unique advantages in photocatalysis by combining the optoelectronic properties of metalloporphyrins with the highly ordered porous structure of COFs. Metalloporphyrin units feature broad spectral absorption, long-lived excited states, and tunable electronic structures, while the COF framework endows them with high specific surface area, tunable pore sizes, excellent stability, and modular design capabilities. The introduction of metal ions further optimizes the electronic structure of the porphyrin rings, enhances the efficiency of photo-generated charge separation and provides more active sites for the catalytic reactions. On the one hand, conjugated chemical bonds and π-π stacking interactions construct efficient electron transport pathways, significantly enhancing carrier mobility and excited-state lifetimes. On the other hand, the tunability and precise structural control of COFs enable researchers to deeply understand the structure–activity relationship, facilitating the rational design of high-efficiency photocatalysts.

Although significant progress has been made in photocatalytic hydrogen production, CO₂ reduction, organic synthesis, and pollutant degradation over MPor-COFs, several challenges have to be dealt with in practical applications. Problems, including complex preparation processes, difficulty in controlling crystallinity, insufficient structural stability, and low charge transport efficiency, limit their further application. Critically, the industrial scalability of these materials remains a bottleneck, as achieving large-scale production with consistent quality and performance poses considerable challenges. Future research should focus on precise design and synthesis at the molecular and even atomic level. Optimizing the metal center selection and linkage strategies can be used to enhance electron–hole separation. Additionally, synthesis methods need to enhance crystallinity, porosity, and morphological control. Furthermore, it is essential to achieve efficient light energy conversion and targeted applications, expanding the material’s light absorption range and regulating structural features. Metalloporphyrin-based COFs are expected to play a more significant role in energy conversion and environmental remediation through interdisciplinary innovation.