Recent advances in stimuli-responsive covalent organic frameworks: from mechanisms to applications

Abstract

Stimuli-responsive covalent organic frameworks (COFs) represent an emerging class of adaptive materials that synergize dynamic behavior with inherent porosity. This integration facilitates multi-stimuli responsiveness and confers substantial application potential across diverse domains. Herein, we systematically review stimuli-responsive behaviors and related mechanisms of smart COFs under physical, chemical, and combinatorial stimuli. Contemporary advances in material design are critically evaluated to correlate structural motifs with application performance in next-generation devices. Current research challenges and future development perspectives are analyzed to advance molecular engineering strategies. This work aims to establish foundational design principles for extending stimuli-responsive COF functionality to unexplored application scenarios.

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Shen Xu

Dr Shen Xu received his PhD degree in 2019 from the Nanjing University of Posts & Telecommunications (NJUPT) under the co-supervision of Prof. Wei Huang and Prof. Runfeng Chen. Then he worked as a postdoctoral fellow in the City University of Hong Kong supervised by Prof. Qichun Zhang. Currently, Dr Xu is working as a specially-appointed professor in NJUPT. His research focuses on organic optoelectronic materials, especially covalent organic frameworks, and their applications.

Shengqiang Xue

Shengqiang Xue received his BEng degree from Yantai University in 2023. He is currently a graduate student at the Nanjing University of Posts and Telecommunications. His research focuses on the design, synthesis, and applications of smart covalent organic frameworks with resonant structures.

Le Yu

Le Yu obtained a Bachelor's degree in Polymer Materials from Bengbu University in 2024. Currently, he is pursuing his graduate studies at the Nanjing University of Posts and Telecommunications, where his research interests are centered on the design and applications of smart luminescent covalent organic frameworks.

Qichun Zhang

Qichun Zhang holds the position of full professor at the City University of Hong Kong (CityU, Hong Kong, China). Prior to joining CityU, he served as a tenured associate professor at the Nanyang Technological University of Singapore (NTU, Singapore). Currently, he is recognized as one of the Clarivate Analytics’ top 1% highly cited researchers in the cross-field from 2018 to 2024. His research focuses on carbon-rich conjugated materials and their potential applications. To date, he has published over 600 papers with citations more than 47 000 and an h-index of 119.

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1. Introduction

The advent of the intelligence era imposes heightened demands on information exchange.^1–3 Consequently, smart materials have garnered significant attention in materials science owing to their adaptive behavior under external stimuli.^4–6 Molecular reorganization or condensed-phase transitions in these materials induce corresponding alterations in physicochemical properties, thereby creating opportunities for deployment across diverse fields including sensing,⁷ optoelectronics,⁸ and biomedicine.⁹ Nevertheless, conventional stimuli-responsive materials—particularly organic variants—often exhibit densely packed architectures that constrain structural tunability.^10–12 Furthermore, the fundamental responsive performances impede the integration of multifunctional systems for practical applications.^13–15

Porous organic polymers, characterized by tunable chemical functionality and high specific surface area derived from inherent porosity, exhibit significant potential for integration with multifunctional components, enabling deployment in diverse complex scenarios.^16–19 Among porous organic polymers, covalent organic frameworks (COFs) have garnered substantial research interest since Yaghi's seminal report in 2005.^20–24 Compared with other porous counterparts, COFs are distinguished by four cardinal advantages: (i) crystalline ordering facilitating rapid mass transport and charge carrier mobility through defined pore channels; (ii) structural programmability permitting precise functional group incorporation for multifunctional design; (iii) covalent-bonded frameworks ensuring exceptional chemical/mechanical stability; and (iv) exclusive light-element composition eliminating heavy metals to confer cost-effectiveness and biocompatibility. These attributes establish COFs as versatile candidate materials for catalysis,^25–29 gas separation,^30–32 optoelectronics,^33–35 energy storage devices,^36–39 and biomedical applications.^40–42

Recent advances have incorporated stimuli-responsive moieties into COFs through covalent integration within skeletons or as pendant groups. This strategy not only enhances COF adaptability but also enables synergistic coupling with multifunctional components.^43,44 The inherent porosity and structural modularity of COFs, on the one hand, facilitate multi-functionalization via concurrent integration of diverse stimuli-responsive units; on the other hand, the stimulus accessibility through pore-mediated diffusion pathways is enhanced, thereby improving response sensitivity. This covalent integration paradigm creates new avenues for designing advanced responsive materials and elucidating structure–property correlations. Since 2016, publication outputs on stimuli-responsive COFs have exhibited exponential growth, reflecting intensified research focus (Scheme 1a). This burgeoning field necessitates a systematic assessment of design methodologies and application landscapes. While existing reviews address related themes,^45–47 most of them precede 2023. Given accelerated progress—particularly the threefold surge in publications over the past triennium—a critical evaluation of recent breakthroughs is imperative.

Scheme 1 (a) Publication numbers by year in the field of stimuli-responsive COFs since 2009. The data were obtained by searching “responsive covalent organic framework*” on Web of Science. (b) Schematic illustration of involved stimuli, applications, and prospects in this review.

This review systematically categorizes stimuli-responsive mechanisms in COFs into physical stimuli (photo, electrical, thermal, magnetic, and mechanical) and chemical stimuli (pH, ion, solvent, explosives, and bio-chemicals) according to stimulus modality, with emphasis on molecular design principles and structure–property correlations. Multi-stimuli-responsive systems are subsequently analyzed. Applications leveraging these dynamic COFs are then critically evaluated, with performance benchmarks contextualized against operational requirements. A critical assessment of field advancements and persistent challenges follows, highlighting seminal contributions and recent breakthroughs (Scheme 1b). Collectively, this work aims to inspire innovative molecular design concepts and functional integration of stimuli-responsive COFs. We believe this contribution could elucidate fundamental structure–dynamics relationships and establish design guidelines for practical implementation in real-world scenarios.

2. Physical stimuli-responsive COFs

In recent years, the responsive behaviors of COFs to physical stimuli have garnered significant attention. Capitalizing on their structural adaptability, these materials can dynamically tune their properties—such as porosity, conductivity, or luminescence—in response to external physical stimuli, including light, electric fields, temperature, magnetic fields, and mechanical force. Such stimulus-responsive characteristics thereby unlock new avenues for deploying COFs as advanced functional materials. This section provides a comprehensive overview of the mechanisms and behaviors of COFs under various physical stimuli, discussing recent advancements and outlining promising future research trajectories.

2.1. Photo-responsive COFs

Photo-responsive COFs are mainly constructed by introducing photo-responsive groups. Exposure to specific light can trigger the structural change of COFs, thus leading to various changes involving material color, luminescence, pore structure, etc. Photo-responsive COFs can be applied in a wide range of fields such as gas sensing, controlled drug release, and optical switching.

2.1.1. Photochromism. Introducing photochromic units such as azobenzene,^48,49 spiropyran,^50,51 and diarylethene^52,53 into the COF frameworks is an effective way to endow them with unique optical properties. Light irradiation of specific wavelength can trigger reversible configuration isomerization of active groups.

The trans configuration is usually the steady state of azobenzene groups owing to the lower energy, which can transfer to cis under UV irradiation (Scheme 2).⁵⁴ The difference in configuration usually causes the change in light absorption, thereby exhibiting photochromism. This isomerization can be reversed by visible light irradiation or heating. Liang et al. introduced azobenzene groups as side chains via a post-modification method to synthesize COF-Azo.⁵⁵ Under UV light, the COF-Azo transforms into the cis isomer along with absorption intensity decrease at 330 nm and increase at 460 nm, illustrating photochromic behavior (Fig. 1a–c). Interestingly, this isomerization also induces a selectivity switch on hydrated ions. The sulfonic group-encapsulated COF-Azo (COF-AzoSO₃H) membrane shows high mono/monovalent ion selectivity (6.1) in trans form and high mono/multivalent ion selectivity (20.7) in cis form, due to the hydrophilicity change during isomerization.

Scheme 2 Mechanisms of representative chromic behaviors.

Fig. 1 Photo-responsive COFs. (a) Photoisomerization of COF-AzoSO₃H membranes under UV or visible light irradiation. (b) UV-vis spectra of the COF-Azo dispersion under UV light. (c) Photoisomerization cycle of the COF-Azo dispersion with alternative UV and visible light irradiation. Reproduced with permission.⁵⁵ Copyright 2024, Elsevier. (d) Comparison between conventional azobenzene isomerization and a new strategy for tetrafluoroazobenzene isomerization. Reproduced with permission.⁵⁶ Copyright 2024, Wiley-VCH GmbH. (e) Schematic of the light-induced structural change of the PAH and photos of the reversible color change of the PAH in DI water. Reproduced with permission.⁵⁸ Copyright 2023, American Chemical Society. (f) Solid-state absorption spectrum diversification of the COF-O film upon irradiation at 365 nm and the COF-C film upon irradiation at >550 nm (inset: time-dependent profile). Reproduced with permission.⁵⁹ Copyright 2019, Wiley-VCH GmbH. (g) Proposed mechanism for controlling ¹O₂ generation via competitive ET pathways based on DFT calculations. Reproduced with permission.⁶⁰ Copyright 2022, American Chemical Society. (h) Structural diagram of the COFilm/R-BBNA, R-chirCOFilm, and R-chirCOFilm-H samples with circular dichroism (CD) and CPL changes. Reproduced with permission.⁶² Copyright 2025, American Chemical Society. (i) UV/vis reflectance spectra of COF-D1 after green light irradiation (520 nm, 20 mW cm², 20 min) and heat (60 °C, 20 min); inset shows the corresponding photographic images. Reproduced with permission.⁶⁵ Copyright 2023 Chinese Journal of Chemistry. (j) Charge/discharge curves under light/dark intermittent conditions. Reproduced with permission.⁶⁹ Copyright 2024, Wiley-VCH GmbH.

To improve the thermal stability and realize the photo-only control of azobenzene isomerization, Jiang et al. employed fluorine-substituted azobenzene as the side chain (Fig. 1d).⁵⁶ Differently, the trans to cis transformation is activated by 560 nm visible light irradiation. The isomerization not only changes the absorption spectra leading to intensity increase at 296 nm and decrease at 410 nm, but also changes the framework structure and pore size which directly affect the gas adsorption and release performance of COFs. Reverse isomerization from cis to trans can be achieved by the illumination of 420 nm light.

Photo-induced cyclization reactions can also be utilized to achieve the photo-response of COFs. Intramolecular cyclization (dithienylethene and spiropyran units) and intermolecular cycloaddition reactions (unsaturated bonds) were both investigated to realize photoswitching. Different from cis–trans isomers which are usually introduced as side-chains owing to their lower steric hindrance compared with the skeleton, photocyclization units can act as both side chains and backbones.

Spiropyran is a type of well-studied photoswitch material.⁵⁷ Liang et al. dispersed a sulfonic group-encapsulated spiropyran into an imine COF (RT-COF-1).⁵⁸ The existence of the sulfonic group makes spiropyran in the ring-open form (MC, Scheme 2) under ambient conditions. Upon 450 nm light irradiation, the dispersion of the composite in water quickly changes from orange to colorless because of transforming into the ring-closed form (SP). An automatic reverse occurs after turning off the blue light for several seconds (Fig. 1e). The SP form is more conducive to the diffusion of H₂O₂ and exhibits great potential in controllable chemical removal.

Dithienylethene is another kind of classic photo-responsive molecule which undergoes cyclic reaction under photoirradiation. Zhang and co-workers introduced 1,2-bis(5-formyl-2-methylthien-3-yl)cyclopentene (DAE-O, Scheme 3) as the linker of COF-O.⁵⁹ 365 nm UV irradiation could activate the ring-closure reaction to form COF-C and lead to an obvious color change. The increment of the absorption band intensity at 640 nm proves this transformation. This ring-closure reaction is reversible by irradiation with visible light of wavelength greater than 550 nm (Fig. 1f).

Scheme 3 Monomers utilized to construct stimuli-responsive COFs with two (top), three (middle), and four (bottom) functional groups.

Jiang and co-workers also inserted dithienylethene into the backbone of a COF to build an o-COF.⁶⁰ UV illumination triggers the ring-closure reaction to form a c-COF and induces obvious changes in absorption and emission. The color of the material changes from dark red to dark blue, while the emission peak of the o-COF at 673 nm is suppressed and blue-shifted, which can be ascribed to the reduced triplet energy level of dithienylethene, thereby enabling the energy transfer from the porphyrin unit (Fig. 1g).

Most 2D COFs form dense packing with the interlayer distance shorter than 4 Å, which meets the Schmidt's ring addition rule (<4.2 Å).⁶¹ Therefore, two close-packed vinyl groups are favorable for cycloaddition by photo-induction. Very recently, Gu et al. introduced chiral bis(allyloxy)-binaphthalene into an achiral olefin-linked COF.⁶² The vinyl linkages undergo [2+2] cycloaddition with adjacent vinyl groups and allyloxy groups under UV irradiation and induce circularly polarized luminescence (CPL, Fig. 1h). These reactions interrupt the conjugation, thus blue-shifting the absorption maximum from ∼450 to ∼340 nm and fluorescence peaks from 520 to 407 nm. Upon heating, part of the interlayer vinyl four-membered rings breaks, which further blue-shifts the emission peak to 383 nm, but the CPL property is retained due to stable chiral crosslinked structures.

The anthracene molecules can undergo similar [4+4] cycloaddition at the 9- and 10-positions to form concavo-convex cycloaddition dimers.⁶³ Jiang et al. utilized anthracene as the linker to construct a Ph-An-COF, in which anthracene units form cycloaddition dimers under UV irradiation achieved by close packing with an interlayer distance of 3.4 Å.⁶⁴ Meanwhile, the absorption peaks at 278 and 373 nm decreased and the peak at 303 nm increased. Besides, the emission of the Ph-An-COF was significantly quenched due to the non-luminescent nature of cycloaddition dimers. When heated at 100 °C in the dark, the cycloaddition dimers can be converted back into anthracene units along with a color reverse change.

Most photocyclization reactions are triggered by UV light, which may cause photo-bleaching and restriction in bio-applications. Wang and co-workers introduced donor–acceptor Stenhouse adducts (DASAs), a new class of photochromic materials, as the side chains of conventional COFs.⁶⁵ Under green light irradiation, DASA side chains undergo a linear to cyclic isomerization which interrupts the linear conjugation, thus causing an apparent color change from brown to light yellow (Fig. 1i). Upon heating or when kept in the dark, the isomerization can be reverted.

2.1.2. Electrical conductivity change. When COFs are combined with photoactive molecules, light stimulation often induces molecular structural or electron density changes, thereby influencing the conductivity of COFs. Recently, Dong et al. reported another type of photochromic COF based on photoinduced electron transfer (PET).⁶⁶ With the coexistence of light and oxygen, the DBTB-DETH-COF synthesized through an ultrasonic method transfers electrons to oxygen atoms, which generates superoxide radicals and causes a color change from yellow-to-olive and a conductivity change evidenced by doubled photocurrent intensity. These changes can be reversed via heating at 100 °C for 5 min.

The cycloaddition reaction of anthracene not only changes the absorption and color, but also manipulates the conductivity of the COF. The original COF-DaTp film developed by Xuan et al. exhibits an average current of 80 nA when a bias voltage of 0.1 V is applied.⁶⁷ The structural transformation triggered by 365 nm illumination disrupts the original conjugated system, which reduces electron delocalization and dramatically decreases the average current to 9 nA. The conductivity can be recovered by irradiation with 254 nm light. This photo-responsive conductivity change enables an optoelectronic synaptic device with 32 conductance states.

Sun et al. encapsulated hydroxynaphthol blue (HB) in the pores of a COF, thus achieving a light-controlled bifunctional nanofluidic device.⁶⁸ Under illumination, the PET process occurs in HB molecules which is accompanied by proton transfer, driving HB molecules into a charge-separated state. Subsequently, the resistance decreased from 1260 to 435 kΩ, along with a 1.7-fold increase of the maximum output power density. This light-dependent behavior was proven by a controlled experiment in which the conductivity change is negligible after light illumination in the absence of HB.

The NT-COF developed by Cai and colleagues possesses a donor–acceptor structure that promotes efficient generation and separation of photogenerated electron–hole pairs under illumination.⁶⁹ These carriers effectively participate in charging/discharging processes to facilitate internal electrochemical reactions and reduce charging voltage and internal overpotential, hence reducing battery energy consumption (Fig. 1j).

2.1.3. Photothermal conversion. Porphyrin is known to be an efficient photothermal unit.⁷⁰ Tian et al. synthesized a porphyrin-based COF and modified it to form nanoparticles (NPs).⁷¹ The photothermal conversion efficiency (PTCE) of COF NPs under a 671 nm laser is approximately 13.7%, and the photothermal performance remains stable after 5 cycles, indicating that the ordered structure effectively maintains the durability (Fig. 2a).

Fig. 2 Photothermal COFs. (a) Temperature curves of COF NPs under five cycles of photothermal heating by a 671 nm laser at 1.5 W cm⁻². Reproduced with permission.⁷¹ Copyright 2024, Elsevier. (b) Temperature changes of ICG solution, ICG@COF NPs and COF-OH NPs during 600 s of 808 nm laser irradiation at 1.0 W cm². Reproduced with permission.⁷² Copyright 2024, Chinese Chemical Society (CCS), Institute of Chemistry of Chinese Academy of Sciences (IC), and Royal Society of Chemistry. (c) Temperature changes of tumors in the only laser group and HCOF@Au + laser group subjected to 808 nm laser irradiation (1 W cm⁻²) for 5 min. Reproduced with permission.⁷³ Copyright 2024, American Chemical Society. (d) UV absorption spectra of the TPAT COF with different sizes dispersed in tetrahydrofuran. (e) Single cycle of the heating and cooling curves of the TPAT COF. Reproduced with permission.⁷⁵ Copyright 2022, American Chemical Society. (f) Photothermal conversion graphs for both COFs on the quartz glass when illuminated with an 880 nm laser at 1.5 W cm⁻². Reproduced with permission.⁷⁶ Copyright 2022, Elsevier.

Wu et al. combined a porphyrin-based COF (COF-OH) with indocyanine green (ICG), another famous photothermal agent, to additionally improve the performance.⁷² Compared with the free ICG (Fig. 2b), the composite showed a much higher temperature increase, reaching a PTCE of 56.7% under irradiation with an 808 nm laser (1.0 W cm⁻²). Such high increment can be attributed to the synergistic effect of the two compounds with an extended absorption range.

Badiei and co-workers reported another kind of photothermal COF (denoted as HCOF) based on tri-phenoxy-triazine nodes and phenyl linkages.⁷³ The π-conjugated framework and close packing endow the HCOF with good photothermal performance. This can be further enhanced to achieve a PTCE of 32.9% by forming a composite with Au particles utilizing the surface plasmon resonance effect (Fig. 2c).

Tuning the absorption to the NIR region is the most effective way to construct photothermal COFs.⁷⁴ Consequently, Xie et al. utilized thienoisoindigo (TII, acceptor) and triphenylamine (TPA, donor) groups to construct a D–A type COF named a TPAT COF.⁷⁵ The D–A structure fundamentally enhances the intramolecular charge transfer, thus resulting in a significant red-shift to the NIR II region (Fig. 2d). On this basis, the TPAT COF achieves a PTCE of 48.2%, demonstrating excellent photothermal performance (Fig. 2e).

Zhao and colleagues employed naphthalene diimide (NDI) as the acceptor to build a D–A COF, while TPA and 1,3,5-triphenylbenzene act as donors.⁷⁶ The stronger electron-donating ability of TPA should be responsible for the higher PTCE of NDI-N-COF, which reaches 77.9% under 880 nm laser irradiation (1.5 W cm⁻², Fig. 2f). This result further validates the effectivity of the D–A structure to construct photothermal COFs.

Incorporating photo-active groups into COFs is still the most effective way to achieve photo-responsive behaviors and realize applications based on photoswitching. It is worth mentioning that the easiness of COF modification enables the utilization of multiple groups involving intramolecular and interlayer photo-reactions, which offers great potential for multi-wavelength responsive behaviors in complicated applications.

2.2. Electro-responsive COFs

The electrical properties of COFs were rarely investigated in the early stage due to their poor conductivity.⁷⁷ Thanks to the revolutionary development of reticular chemistry,⁷⁸ highly conductive COFs have been applied in optoelectronic devices such as transistors and memory devices.^79–81 The electrical-responsive COFs have also been developed which exhibit optical or electrical changes. Due to the recently published reviews^33,82 on the electronic properties of COFs, only electrochromism (EC) behaviors will be discussed in this section. Nearly half of EC COF examples were published in the last year, illustrating the focused interest and rapid development in this field.

EC mainly refers to the materials that undergo internal redox reactions under an applied potential, causing changes in the molecular structure or electron distribution.⁸³ These changes alter the absorption spectrum and ultimately lead to reversible color changes. COF_3PA-TT synthesized by Wang and co-workers exhibits notable EC during redox processes.⁸⁴ In 0.1 M LiClO₄/propylene carbonate electrolyte, the color of COF_3PA-TT changes from dark red to dark brown as the potential increases from 0 V to 1.4 V, and reverts to dark red at 0 V. The brown color is caused by the absorption increases in the 400–600 nm visible region and ∼1200 nm near-infrared (NIR) region, which can be attributed to the energy transfer from triarylamine to thienothiophene-diimine and generation of triarylamine cation radicals (Fig. 3a). The response time is calculated to be 20 and 18.5 s for coloring and bleaching, respectively.

Fig. 3 Electro-responsive COFs. (a) Absorption spectra and pictures (inset) of COF_3PA-TT EC electrodes at 0 and 1.4 V. Reproduced with permission.⁸⁴ Copyright 2019, American Chemical Society. Cyclic voltammetry curves (vs. SCE) of (b) the COF_TTB electrode and (c) the A-COF_TTB electrode measured in 0.1 M NaClO₄/CAN solution at a scan rate of 100 mV s⁻¹, and the electrode images of different states (insets). (d) Presumptive order of oxidation reactions in the A-COF_TTB. Reproduced with permission.⁸⁷ Copyright 2025, Science China Press. (e) Visuals of four-state multielectrochromism recorded from the film electrode surface and (f) the redox behavior of the TAPA-PDA ^ecCOF film in a three-electrode system. Reproduced with permission.⁸⁸ Copyright 2024, Wiley-VCH GmbH. (g) Cyclic voltammetry scans of a Py-ttTII film on ITO/glass recorded at a scan rate of 20 mV s⁻¹ in a three-electrode setup. Insets: photographs of the COF film in the neutral (−700 mV) and oxidized (+500 mV) states. (h) Switching speed at 550 nm extracted from ten oxidation/reduction cycles (symbols: data points, red line: average). Reproduced with permission.⁸⁹ Copyright 2021, The Authors, American Chemical Society Limited (Creative Commons CC BY). (i) Color change of electrochromic behavior of the TPDA-DHBD film and schematic representation of the successive redox processes in the electroactive fragment of the TPDA-DHBD film governing the electrochromic behavior. Reproduced with permission.⁹⁰ Copyright 2024, Wiley-VCH GmbH. (j) Changes in the optical spectrum of the EC-COF-1 film as a function of applied potential. Reproduced with permission.⁹¹ Copyright 2020, The Authors, Springer Nature Limited (Creative Commons CC BY).

Chen and colleagues investigated the difference in EC performance between isomeric COFs.⁸⁵ Two D–A COFs denoted as Py-C-TP-COF and Py-N-TP-COF were constructed using pyrene and tetraphenyl-1,4-benzenediamine, only with the difference in imine orientation. When increasing the applied potential to +1.1 V, the absorption peak of the Py-C-TP-COF at 437 nm decreased. Meanwhile, two new peaks appeared at 562 and 1050 nm due to the charge transfer between D and A as well as the formation of phenylamine polarons and then bipolarons, respectively. The whole process led to the color change from orange-yellow to light green. In contrast, the Py-N-TP-COF only showed a slight color change from yellow to light yellow. Systematic investigation reveals that the isomerism of imine bonds induces a much smaller resistance of the Py-C-TP-COF than that of the Py-N-TP-COF, which results in faster charge transfer, lower onset potential, and shorter EC response time of the Py-C-TP-COF.

Multi-state EC provides great potential for various applications such as high-density logic circuits. Wang et al. synthesized COF_TPDA-PDA based on TPDA (Scheme 3).⁸⁶ In 0.1 M NaClO₄/CH₃CN electrolyte, COF_TPDA-PDA expresses a color transition from plum to gray when a potential of +0.75 V is applied. This can be ascribed to the oxidation of TPA to generate cation radicals, which further form mixed-valence systems that enable the delocalization of electrons over two TPA centers. Along with the increase of potential to +0.90 V, more TPA cations form and the mixed-valence systems are disassembled, resulting in the color change from gray to light blue. In 2025, they studied the impacts of different linkages on EC performances.⁸⁷ The COF_TTB with imine linkage was reduced to A-COF_TTB with amine linkage. Similar to previous work where an EC COF was constructed using TPA and imine linkage, COF_TTB shows two oxidation peaks in the range of 0–1.2 V (Fig. 3b and c). In contrast, A-COF_TTB exhibits 4 distinct peaks, corresponding to 4 oxidation states. The new peaks lower than 0.5 V should be attributed to the oxidation of the amine linkages (Fig. 3d).

Recently, Ho et al. realized four-state EC in the TAPA-PDA ^ecCOF, which was constructed using TPA and benzene.⁸⁸ The electrochemical analysis demonstrated that the TAPA-PDA ^ecCOF has 3 oxidation peaks at 0.42, 0.61, and 0.98 V. Correspondingly, obvious color changes (orange → pear → green → cyan) can be captured during the cyclic voltammetry scan, illustrating four-state EC (Fig. 3e and f). The three oxidation processes should be ascribed to the respective formation of radical nitrogen cations at the linkage and TPA center.

In 2021, Bein and colleagues constructed an EC COF (Py-ttTII COF) with a quite fast response using a D–A–D linker based on the thienoisoindigo moiety.⁸⁹ The color change of the Py-ttTII COF from dark green to black originates from the two-electron electrochemical oxidation to the ttTII⁺ and ttTII²⁺ species and can be fully reversed during reduction (Fig. 3g). The response times can reach 0.38 s and 0.2 s for the oxidation and reduction, respectively (Fig. 3h).

In 2024, Qiu and colleagues synthesized the TPDA-DHBD film via an electrochemical interface polymerization method which shows excellent crystallinity and uniform morphology.⁹⁰ The existence of TPDA induces two oxidation peaks at ∼1.0 and ∼1.5 V corresponding to oxidation of two nitrogen atoms (Fig. 3i). There also exists a reduction peak at ∼−1.2 V owing to the reduction of the imine linkage. Therefore, the TPDA-DHBD film has four states in total with obvious color differences.

Most EC COFs exhibit a color change; however, rare examples can realize the change between the colored state and transparent state. This is attributed to the difficulty in tuning absorption from the visible region to the non-visible region. Zhang and colleagues developed a D–A COF utilizing TPDA as the donor and 2,1,3-benzothiadiazole as the acceptor.⁹¹ Based on LiClO₄/propylene carbonate-based gel electrolyte, the absorption at 370 and 574 nm rapidly suppressed upon applying a potential of 2 V due to the formation of polarons and then bipolarons (Fig. 3j). Therefore, the film color changed from blue-purple to transparent.

Benefitting from the abundant redox potentials of COFs, multiple EC states and fast response have been achieved. However, most EC performances were restricted in nitrogen-containing moieties. Besides, though the intrinsic insolubility of COFs provides excellent stability, it also brings challenges in processibility. Therefore, more efforts are needed before practical use.

2.3. Thermo-responsive COFs

The configuration change or chemical reactions of organic molecules may occur when absorbing enough energy from the environment to overcome the activation energy. Similarly, some COFs show reversible or irreversible temperature-dependent changes.

In 2021, Deng and colleagues developed a thiol-based COF showing an irreversible color change as the temperature rises.⁹² The thiol groups dominate the thermochromic behavior via the formation of interlayer disulfide bonds at high temperature. At 55 °C, the absorption at ∼575 nm significantly rises within 10 h, illustrating the great potential for application as a thermal history indicator (Fig. 4a).

Fig. 4 Thermo-responsive COFs. (a) Corresponding visible color at different temperatures over 96 h (0.4 mg mL⁻¹). Reproduced with permission.⁹² Copyright 2021 Wiley-VCH GmbH. (b) Temperature-switched aggregation and redispersion of h-COF-g-PNIPAM in water. Reproduced with permission.⁹⁵ Copyright 2022, American Chemical Society. (c) Plots of the proton conductance vs temperature for pristine COF-TAHA and IL₈₀₀@COF with different IL mass contents at 98% RH. Reproduced with permission.⁹⁶ Copyright 2023, American Chemical Society. (d) Schematic illustration of the topological polymerization routes of Py-BDA. Copyright 2025, Science China Press. (e) UV-vis DRS of ITO/Py-BDA and ITO/Py-BDA@350 °C thin films. Reproduced with permission.⁹⁷ Copyright 2025, Science China Press. UV-vis absorption spectra of (f) CityU-4, (g) CityU-5, and (h) CityU-6 with/without heat treatment, as well as corresponding (i)–(k) solid ESR spectra and after one year of storage under ambient conditions. Reproduced with permission.⁹⁸ Copyright 2023, American Chemical Society.

Poly(N-isopropyl acrylamide) (PNIPAM) is well-known as a thermal-sensitive polymer.^93,94 Its molecular chain contains both hydrophilic amide groups and hydrophobic isopropyl groups, endowing thermochromism owing to the conformation transition from coil to globule when exceeding the lower critical solution temperature (LCST, ∼32 °C). In 2022, Liu et al. grafted poly(NIPAM-DMEAm-Ald) on hollow COF NPs to construct thermochromic h-COF-g-PNIPAM.⁹⁵ When the temperature of its dispersion was above the LCST, the transmittance at 450 and 500 nm dramatically dropped from ∼100% to ∼10%, resulting in an obvious color change (Fig. 4b).

The LCST-type thermo-response can also be achieved by an ionic liquid (IL)-combined COF. In 2023, Wang and co-workers introduced a series of PEG-functionalized ILs into the skeleton of COF-TAHA.⁹⁶ These ILs bound on the COF backbone through hydrogen bonding manifest reversible phase separation at 98% RH (relative humidity). For instance, the proton conductivity of IL800@COF-20% gradually increases along with the increase of temperature but below the LCST. Once exceeding the LCST, the proton conductivity dramatically drops an order of magnitude. This change can be well reversed by decreasing the temperature below LCST (Fig. 4c).

Very recently, Chen et al. developed an imine COF (Py-BDA) with diacetylene groups.⁹⁷ Compared to the counterparts with monoalkynyl or without acetylene, Py-BDA can undergo an interlayer solid-state polymerization reaction to form enyne chains when treated at 350 °C (Fig. 4d). After polymerization, the absorption band of the Py-BDA@350 °C film broadens, thus changing the color of the film (Fig. 4e).

In 2023, Zhang and co-workers synthesized a series of phenylenediacetonitrile-based COFs (denoted as CityU-4 to -6).⁹⁸ Upon heat treatment (250 °C for CityU-4 and CityU-5, 220 °C for CityU-6) for 10 min in air, the colors of three COFs change from orange to dark red (Fig. 4f–h). Systematic investigations reveal that the color change originated from the formation of open-shell radicals. Owing to their lower energy, the COF radicals can be stored for more than one year without any intensity decrease of electron spin resonance (ESR) signals under ambient conditions (Fig. 4i–k). The open-shell form can be reversed to close-shell COFs via ethanol treatment to quench the radicals.

Phase transition and structural isomerism are widely utilized to obtain thermochromism in organic small molecules, but usually suppressed by the robust skeleton of COFs, which led to rare reports of the thermo-responsive COFs. The employment of flexible structures or formation of stable radicals like CityU-4 should be helpful to overcome these restrictions.

2.4. Magneto-responsive COFs

Materials with ferromagnetism, paramagnetism, or diamagnetism can exhibit responsive behaviors in a certain magnetic field.⁹⁹ COFs that are capable of responding to a magnetic field find their unique applications in recycled materials detection.¹⁰⁰ As the most well-known magnetic material, Fe₃O₄ NPs are often utilized to construct magnetic materials, including COF composites. To date, most magnetic-responsive COFs rely on the combination with Fe₃O₄.^101–103 Core–shell magnetic materials based on Fe₃O₄ nuclear have long been investigated,¹⁰⁴ but Guo et al. employed a COF as the shell with high crystallinity for the first time, using an in situ disorder-to-order reformation.¹⁰⁵ 50% of the saturation magnetization of Fe₃O₄ can be retained after forming the composite (32 emu g⁻¹, Fig. 5a). To date, hundreds of magnetic COFs utilizing this most effective method have been reported and applied in various scenarios, mainly on the basis of their magnetic-responsive behavior to improve the efficiency of separation and recycling.^106–108

Fig. 5 (a) Magnetic hysteresis loops of Fe₃O₄ and Fe₃O₄@v-COF. Reproduced with permission.¹⁰⁵ Copyright 2023, American Chemical Society. (b) Temperature dependence of the spin susceptibility, determined using the superconducting quantum interference device magnetometer for the iodine-doped-sp²c-COF; emu, electromagnetic units. (c) Magnetic (M)–applied field (H) profiles at different temperatures (red, 2 K; blue, 5 K; purple, 10 K; brown, 20 K; green, 100 K; black, 300 K). The nonlinearity of the curves denotes the ferromagnetic phase transition. Reproduced with permission.¹¹¹ Copyright 2017, The American Association for the Advancement of Science. (d) Solid-state ESR spectrum of the χ_MT–T plot at 293 K. Reproduced with permission.¹¹⁴ Copyright 2018, Wiley-VCH GmbH. (e) Temperature dependence of magnetization obtained with an applied field of B = 500 Oe. Plot of ^χ−1 versus T. Reproduced with permission.¹¹⁵ Copyright 2019, American Chemical Society. (f) Moment vs. field measurements collected at 5 K on CORN-COF-1-TCNE (blue) and CORN-COF-1 (red). Reproduced with permission.¹¹⁶ Copyright 2024, Wiley-VCH GmbH.

Except for Fe₃O₄, other ferromagnetic materials can also be employed to construct magnetic COF composites. Very recently, Sitti and colleagues developed a kind of magnetic Janus nanoparticle for the magnetically monitored visible drug delivery.¹⁰⁹ After coating the TAPB-TPA COF onto three-layered upconversion nanoparticles (UCNP) to construct a UCNP-COF, Ni and Au were sputtered to form magnetic Janus particles. Interestingly, these particles can be monitored by a neodymium (NdFeB) magnet without aggregation. Combined with the luminescence properties of UCNPs and high specific surface area of the composite, the Janus particles are able to be applied as the controlled visible drug delivering agents.

The intrinsic magnetic-responsive behaviors of purely organic COFs are highly related to the radicals. The limited reports involving COF radicals should be ascribed to their instability and the difficulty of constructing a fully conjugated skeleton. Jiang et al. reported the first radical COF which was constructed by introducing 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) into the COF as the side chains, but the magnetic properties were not investigated.¹¹⁰ Later, they synthesized a crystalline sp² carbon-conjugated COF via Knoevenagel condensation reaction,¹¹¹ just after the first olefin-linked COF.¹¹² After iodine doping and oxidation, radicals are generated in the sp²c-COF which enormously increases its conductivity from 6.1 × 10⁻¹⁴ S m⁻¹ to 7.1 × 10⁻² S m⁻¹. ESR results reveal that electrons are localized at pyrene knots, rather than forming bipolarons. Interestingly, the doped sp²c-COF shows diamagnetism at room temperature and gradually transfers to paramagnetism along with the temperature decrease. When the temperature further decreases, ferromagnetic phase transition occurs in the doped sp²c-COF with the Curie temperature of 8.1 K (Fig. 5b and c). Notably, the highly ordered crystalline structure of the sp²c-COF plays a significant role in the conductive and magnetic properties. Cui and colleagues predicted that this sp²c-COF and its imine-linked analogue (replacing cyanoethylene with imine) are the first two examples possessing organic-ligand-based Lieb lattices.¹¹³ The ferromagnetism transition was also proven by the calculation.

In 2018, Wu et al. constructed a radical COF (PTM-CORF) based on polychlorotriphenylmethyl, a well-studied stable organic radical, via the Eglinton coupling reaction.¹¹⁴ This radical COF exhibits anti-ferromagnetism with an exchange interaction of −375 cm⁻¹ according to the positive linear relationship between the product of temperature and molar magnetic susceptibility and the temperature (Fig. 5d). When PTM was combined with 1,3,5-tris(4-formylphenyl) (Tz) by Ruoff and co-workers to form PTMR-CTF, it became paramagnetism (Fig. 5e).¹¹⁵

Recently, Milner et al. developed a new kind of fully-conjugated COF with tetraazafulvalene linkages based on the dimerization of N-heterocyclic carbene (NHC).¹¹⁶ After being oxidized by tetracyanoethylene (TCNE), the generated radical anion TCNE˙⁻ can be encapsulated in its pores. These radical anions lead to the Curie–Weiss paramagnetism, which is far different from closed-shell COFs (Fig. 5f).

Only a few examples of paramagnetic COFs have been reported, and ferromagnetic COFs are even rarer. As mentioned above, the radical in COFs should be pivotal, and the involved electronic process in multi-dimensional conjugated systems remains uncovered.

2.5. Mechano-responsive COFs

Mechanical-responsive COFs are also rarely reported. To date, only piezofluorochromic COFs that exhibit alterations in molecular conformation under ultrahigh pressure and pressure-induced conductivity change have been developed.

In 2024, Fang et al. synthesized a series of bicarbazole-based COFs with different topologies, rigidities, and conjugation systems.¹¹⁷ As the pressure increases, two 3D COFs show a fluorescent intensity increase and then a decrease with a continuous bathochromic shift (Fig. 6a and b). The fluorescence enhancement at relatively low pressure (<6 GPa) is closely related to the reduced interlayer distance and enhanced interactions. In contrast, the heightened energy dissipation plays a dominant role, thereby inducing fluorescence quenching. For 2D COFs with close packing, the high pressure only induces enhanced π–π interactions and energy dissipation presented as luminescence quenching. Besides, three 2D imine COFs with low rigidity show rare blue-shifted fluorescence during the onset of pressure. The blue-shifted emission is possibly attributed to the enhanced vibration of imine bonds that suppressed the energy transfer.

Fig. 6 Mechano-responsive COFs. Fluorescence spectra and intensity-wavelength plots with corresponding chromaticity coordinates and photographs under UV irradiation (λ_ex = 355 nm) of (a) JUC-620 and (b) JUC-621 at various pressures from 101 kPa to 13.5 GPa and 16 GPa, respectively. Reproduced with permission.¹¹⁷ Copyright 2024, Wiley-VCH GmbH. (c) Schematic of torsional deformation in the benzene ring bridging cyanide (C=N) groups. (d) Pressure-dependent PL peak positions (left axis) and intensities (right axis) for methanol@CTFs. Reproduced with permission.¹¹⁸ Copyright 2025, Wiley-VCH GmbH. Schematic illustration of ion accumulation and transport (e) by the electric field and (f) coupled electric field and mechanical pressure. (g) Membrane conductance as a function of external pressure in KCl solutions of different concentrations. (h)–(j) Poisson–Nernst–Planck (PNP) modeling of the ion transport through the H2TPP-COF monolayer driven by the coupled electric field and mechanical forcing and the time-dependent distribution of the net charge carriers. Reproduced with permission.¹²² Copyright 2023, American Chemical Society.

Very recently, Yao and colleagues observed another rare example of blue-shifted piezofluorochromism.¹¹⁸ The emission intensity gradually increases along with the increase of the pressure and reaches the maximum at 1.22 GPa. Meanwhile, the emission peak blue-shifts from 507 nm and reaches 450 nm at 3.51 GPa. According to the theoretical simulation, this abnormal blue-shift originated from the significant increase of dihedral angles between phenyl groups and adjacent Tz rings, from 8.9° and 4.8° to 27.7° and 24.1°, respectively (Fig. 6c). The introduction of methanol induces the enhanced hydrogen-bonding with phenyl groups, thus resulting in structural perturbations and suppressing π–π stacking. Further applied pressure of a diamond anvil cell can overcome this distortion and strengthen the interlayer stacking which causes the emission red-shift and quenching (Fig. 6d).

In low-permeability systems (e.g., nanotubes, nanoslits, and single nanopores), external pressure usually enhances ionic conductivity.^119–121 However, for the highly permeable H₂TPP-COF monolayer membrane investigated by Li et al., external mechanical force causes anomalous decreases in ionic conductivity (Fig. 6e and f).¹²² The negative surface potential of the membrane allows the selective cation transport, which induces the opposite streaming current of the applied electric field (Fig. 6g–j).

In 2024, Zhang and co-workers introduced fluorinated alkyl as side chains to construct piezoelectric COFs (denoted as CityU-13 and CityU-14).¹²³ This molecular design successfully constructs highly non-centrosymmetric structures with strong dipole moments (theoretical value: 27.84 Debye) and long-range dipole alignment. The strategy effectively induces spontaneous polarization, enabling the piezoelectric nanogenerators to demonstrate record-high open-circuit voltages (CityU-13: 60 V; CityU-14: 50 V), an exceptional piezoelectric coefficient (d₃₃: 20.9 pC N⁻¹), an instantaneous power output of 3.3 μW, and stable performance exceeding 14 days. This work provides new insights for piezoelectric COFs in self-powered systems.

To date, only a few examples of piezo-responsive COFs have been proposed and no stretch-responsive COFs have been established yet. Digging the potential of packing and interlayer interactions should be promising for novel mechano-responsive COFs.

3. Chemical-responsive COFs

COFs exhibiting responsiveness to chemical stimuli, such as acids and bases, solvents, ions, and bio-chemicals, have garnered significant research interest. Through precise design of the COF structure and a fundamental comprehension of the underlying responsive mechanisms, these frameworks can be engineered to deliver targeted functionality and enhanced performance, thereby addressing the practical demands of detection, extraction, and complicated bio-applications.

3.1. pH-responsive COFs

The widely existing pH gradients in external environments and living organisms provide natural “environmental switches” for pH-responsive COFs, enabling them to achieve functions such as targeted delivery, spatiotemporal controlled release, and on-demand therapy. The core design strategy of pH-responsive COFs relies on introducing proton-sensitive chemical groups or dynamic bonding structures, which regulate the physical and chemical properties of the materials through dynamic bond breaking, protonation/deprotonation, or intermolecular force changes, therefore responding to specific pH signals.

Hydrolysis of dynamic covalent bonds under acidic conditions is a common mechanism for pH-responsive COFs, mainly involving their cleavage which leads to disintegration of the COF structure or expansion of pores to achieve various applications.

Boron-containing COFs, constructed using boroxine or boronate ester bonds, are the first class of porous, crystalline, pure organic frameworks.¹²⁴ Their high crystallinity is enabled by the high reversibility of the self-condensation reaction of aryl boronic acid (boroxine) or the condensation reaction between aryl boronic acid and o-diphenol (boronate ester). Therefore, hydrolysis is widely observed in boron-containing COFs.¹²⁵ Particularly, the existence of acid can catalyze this hydrolytic process, thus activating acid-responsive properties and can be applied in biomedical fields, especially in targeted drug delivery.¹²⁶ In 2023, Zhang and coworkers utilized the very first boroxine-linked COF (COF-1) as the drug loading and delivering material.¹²⁷ Its acid responsiveness primarily originates from the protonation of B–O bonds, which makes it susceptible to nucleophilic attack by oxygen atoms in water molecules. COF-1 was combined with an albumin hydrogel to improve the stability under neutral conditions. But a slightly acidic environment (pH = 5.5, tumor microenvironment) can highly promote the dissociation of the albumin hydrogel and hydrolysis of the COF-1 structure, thereby releasing the loaded drug molecules at the tumor site.

Imine-linked COFs are synthesized via Schiff base reaction of amine and aldehyde under the catalysis of acid.¹²⁸ The addition of acid increases the reversibility of Schiff base reaction that can improve the crystallinity, but also weakens the stability of imine bonds. Recently, Trabolsi synthesized alkyn-nCOF-cRGD to load doxorubicin (Dox) and exploited the acidity to control the speed of hydrolysis of imine bonds.¹²⁹ A slight acidic environment (pH = 6.4) induced controlled release of Dox, while more acidic conditions (pH = 5.4) led to the rapid and complete release, demonstrating a sensitive acid-response (Fig. 7a). The Au@RCOF@PDA composite was developed by Badiei et al. based on an imine-linked TFPTA–PDA COF.¹³⁰ Similarly, this COF is relatively stable with a slow hydrolytic rate under neutral conditions (pH = 7.4), but undergoes rapid hydrolysis to release drug molecules in acidic tumor microenvironments (pH 5.0).

Fig. 7 pH-responsive COFs. (a) Schematic illustration of constructing Alkyn-nCOF-cRGD@Dox and its pH response. Reproduced with permission.¹²⁹ Copyright 2024, The Authors, American Chemical Society Limited (Creative Commons CC BY). (b) Protonation and deprotonation occur at the imine sites and phenolic hydroxyl moiety, respectively. Reproduced with permission.¹³² Copyright 2023, American Chemical Society. (c) Overview of the acid-exfoliation and film-casting procedures. Reproduced with permission.¹³³ Copyright 2020, Wiley-VCH GmbH. (d) Absorption spectra of JUC-556-[HZ]_0.50 (E) upon protonation in TFA solution at low concentrations. (e) Absorption spectra of JUC-556-[HZ]_0.50 (Z) upon deprotonation in Et₃N solution at low concentrations. (f) Absorption spectra of JUC 556-[HZ]_0.50 (E) upon protonation in HCl solution with increasing concentrations. Reproduced with permission.¹³⁸ Copyright 2021, Wiley-VCH GmbH. (g) Naked eye photographs of TA-COF powders upon exposure to HCl and NH₃ vapors. Reproduced with permission.¹³⁹ Copyright 2023, American Chemical Society. (h) Solid-state UV-vis spectra and colors of the TG-DMPZ modified test paper at different pH values. Reproduced with permission.¹⁴⁰ Copyright 2024, Wiley-VCH GmbH. (i) Lateral size (left) and zeta potential (right) of a COF_BTC solution at different pH values. Inset: photos of COF_BTC at different pH values. (j) Exfoliation and dissolution route of COF_BTC in alkaline media and photo of the stable solution (at pH 13). Reproduced with permission.¹⁴² Copyright 2019, American Chemical Society.

Hydrolysis-caused acid-response is irreversible due to cracking of the COF structure. In contrast, the protonation-induced pH-response with no structure collapse is reversible, enabling a wider application range. The surface charge or conformation of COFs can be regulated through changes in the protonation states of functional groups.

The protonation of imine bonds refers to the process in which the nitrogen atom (N) of the imine accepts a proton (H⁺) to form a positively charged protonated intermediate.¹³¹ This process is a core step in acid–base reactions and plays a key role in the diverse changes in the properties and applications of imine-based COFs. In 2023, Liu et al. constructed a free-standing membrane by liquid–liquid interfacial condensation reaction between DTA and TAPA.¹³² Treatment with acidic solution (pH = 1) immediately changes its color from bright red to black, which can be reversed via exposure to an alkaline solution (pH = 13) for several minutes (Fig. 7b).

When an excess amount of trifluoroacetic acid (TFA) is added to the 2D imine-linked BND-TFB COF powder synthesized by Dichtel et al., the color of the powder changes from bright orange to red and can be dispersed in organic solvents to form a polydisperse suspension (Fig. 7c).¹³³ At the same time, their crystallinity and porosity apparently decrease. This can be attributed to the protonation of the imine linkages by TFA, hence generating electrostatic repulsion between adjacent layers. This repulsion overcomes the interlayer interactions, inducing the rapid exfoliation of COF powders into nanosheets to achieve dispersion.

Hydrazone bonds are also pH-responsive functional groups whose protonation states vary with the pH value.¹³⁴ When pH reaches 3, both imine and imide bonds of hydrazone linkage were protonated, which became a specific recognition site of Au(III) ions and caused fluorescence quenching. In order to control the pH-sensitive release process, some protecting groups can be introduced into the system. For instance, Yang et al. attached soybean phospholipid as the encapsulating group to improve the stability and hemocompatibility of a hydrazone-modified COF (NCOFs-NN-DOX@SP).¹³⁵

Amine groups become protonated and positively charged at pH < pK_a (∼9.0). Taking LZU-1@BTA developed by Liu et al. as an example, in acidic environments, secondary amines in LZU-1 and 1-H-benzotriazole (BTA) are protonated, generating positive charges that drive BTA release via electrostatic repulsion.¹³⁶ Deprotonation caused by alkaline environments leads to both negative charges and similar repulsion. This pH responsiveness is critical for corrosion protection.

Azo groups exhibit a similar ζ potential change to imine and amine groups. ATOMe synthesized by Zheng and co-workers via condensation reaction of 4,4′-azodianiline (Azo) and 2,4,6-trimethoxy-1,3,5-benzenetricarbaldehyde (TpOMe) shows excellent stability in a harsh environment.¹³⁷ Its ζ potential is negative (−2.7 mV) under neutral conditions, which reverses to positive under acidic conditions. Based on this dramatic change, ATOMe is capable of loading drugs with a positive charge and releasing them in acidic microenvironments.

Fang et al. synthesized 3D hydrazone-functionalized covalent organic frameworks (JUC-556-[HZ]_x), which exhibit pH-triggered switching properties.¹³⁸ Upon adding TFA, the UV-vis absorption peak intensity showed a decrease at 283 nm and an increase at 249 nm along with a color change from yellow to red, indicating the protonation of pyridine and transformation of hydrazone groups from the E configuration to the Z configuration (Fig. 7d–f). When Et₃N was added, the spectra reverted to the initial state, confirming the reversible isomerization process.

Abbaspourrad and colleagues synthesized two COFs with tetrazine linkage (TA-COF-1 and TA-COF-2).¹³⁹ Exposure to HCl solution or vapor results in a rapid color change of TA-COF-1 from orange to dark brown, while that of TA-COF-2 becomes orange. The original colors fully recover upon removal of HCl and exposure to NH₃ vapor (Fig. 7g). For TA-COF-1 suspended in 1,4-dioxane, HCl treatment causes a red-shift in the fluorescence spectra originated from protonation of the tetrazine ring, which reduces the energy barrier for π–π* transitions. The basic conditions can reverse the protonation to its neutral state.

Gao and co-workers synthesized guanidine-based COFs.¹⁴⁰ Under acidic conditions, the nitrogen atoms in the guanidine group accept protons to form positively charged guanidinium ions, leading to changes in intramolecular charge distribution and the electronic structure of the conjugate system, thereby causing the color change from red to orange. Under basic conditions, the NH groups in the guanidine lose protons to form negatively charged guanidinate anions. At this time, the guanidine group exists in a resonance-stabilized conjugate structure, leading to a red-shift in the absorption spectrum and deepening the color to dark brown or black (Fig. 7h).

In contrast to nitrogen-containing moieties that become positively charged after protonation under acidic conditions, carboxyl groups (–COOH) can be deprotonated and possess a negative charge when pH > pK_a (weakly acidic to basic). Therefore, this pH-response enables the loading and release of negatively charged drugs such as in the colon. In 2019, Yang Li et al. synthesized carboxyl-functionalized COF-TzDBd via a one-pot method through the direct condensation of Tz and 4,4′-diamino-[1,1′-biphenyl]-2,2′-dicarboxylic acid (DBd).¹⁴¹ The surface properties of COF-TzDBd and the ionization states of dyes change with the variation of pH values. Crystal violet and brilliant green are cationic dyes, which are positively charged in solution. At high pH, the carboxyl groups are deprotonated, thus possessing negative charges to enhance the capability of dye adsorption.

The coulombic attraction and repulsion between charged COFs and alkali/acid to manipulate the COF stacking was reported by Xiang et al.¹⁴² The COF_BTC with quasi-phthalocyanine iron centers shows a positive charge owing to the existence of ferric iron. Under basic conditions (pH ≥ 10), alkali molecules or hydroxyl groups can insert into the COF interlayer because of the coulombic attraction, resulting in the COF exfoliation and dissolution to form a stable solution. The exfoliated COF monolayers carry negative charges on the surface (ζ = −21.6) which prevents aggregation through the electrostatic repulsion effect (Fig. 7i). However, under acidic conditions (pH ≤ 7), the surface charge of COF_BTC is neutralized (ζ = +6.27), causing rapid aggregation and precipitation. The precipitate can be redissolved by tuning the pH to alkaline, demonstrating a reversible pH-responsive behavior (Fig. 7j).

The pH change can induce diverse responses of COFs including color, ζ potential, and packing mode. These features can be integrated with other responsive behaviors and in favor of various applications.

3.2. Ion-responsive COFs

The exacerbation of excessive sewage discharge, heavy metal pollution, and radionuclide leakage brings severe challenges to wastewater detection and purification. Stimuli-responsive COFs can achieve highly sensitive detection, high-capacity adsorption, and dynamic regeneration of target ions through precise molecular recognition and synergistic multi-mechanisms, providing innovative solutions for green chemistry and sustainable development.

Heteroatoms with lone-pair electrons (N, O, F, etc.) in the COF skeleton can form coordination bonds with metal ions which causes a fluorescence change, while some anions can reverse this process via electrostatic competition.¹⁴³ Zhao et al. synthesized a fluorescent COF namely TFPB-PI(F)-COF containing phenanthro[9,10-d]imidazole moieties.¹⁴⁴ The suspension of TFPB-PI(F)-COF exhibits blue fluorescence upon excitation with 320 nm light. When nitrogen atoms of imidazole and imine bonds as well as fluorine atoms within the framework selectively coordinate with Cu²⁺/Al³⁺ ions, the fluorescence is highly quenched through the ligand–metal charge transfer and PET mechanisms. But the fluorescence can be recovered by introducing PO₄³⁻ to capture metal ions through competitive strong electrostatic attraction originating from its high-density negative charge (Fig. 8a).

Fig. 8 Ion and solvent responsive COFs. (a) Proposed coordination and recognition process of TFPB-PI(F)-COF toward Cu²⁺/Al³⁺followed by PO₄³⁻. Reproduced with permission.¹⁴⁴ Copyright 2023, Elsevier. (b) Electron transfer process during copper ion sensing with sp²-TPE-COF NSs. Reproduced with permission.¹⁴⁵ Copyright 2023, Wiley-VCH GmbH. (c) Detection and removal of Hg²⁺ using COF-LZU8 based on fluorescence quenching and photographs of COF-LZU8 under a UV lamp (λ_ex = 365 nm) before (left) and after (right) the adsorption of Hg²⁺. Reproduced with permission.¹⁴⁷ Copyright 2016, American Chemical Society. (d) Schematic diagram of TFPT-BTAN-AO regeneration detection and extraction of UO₂²⁺. Reproduced with permission.¹⁴⁸ Copyright 2020, The Authors, Springer Nature Limited (Creative Commons CC BY). (e) Calculation of interlayer distances and interaction energies before and after the Fe(III)-assisted aqueous-phase exfoliation of the COF into the corresponding soluble CON. Inset: a photograph of the dispersion of CONs in aqueous solution. Reproduced with permission.¹⁵² Copyright 2018, The Authors, Springer Nature Limited (Creative Commons CC BY). (f) Structural transformation illustration and PXRD patterns of PEG-COF-4 under different humidity conditions. Reproduced with permission.¹⁵³ Copyright 2022, Wiley-VCH GmbH. (g) Space-filling mode illustration of the pores in water and immersed in acetone. Reproduced with permission.¹⁵⁵ Copyright 2023, American Chemical Society. (h) Energy landscape of COF Tp-TAPT computed using DFT. Reproduced with permission.¹⁵⁷ Copyright 2024, The Authors, Science Limited (Creative Commons CC BY-NC).

Yao and co-workers synthesized a tetraphenylethylene (TPE)-based fully conjugated COF denoted as (sp²)-TPE-COF.¹⁴⁵ The (sp²)-TPE-COF nanosheets exhibit a highly selective fluorescence quenching effect towards Cu²⁺ in aqueous solution. The cyano groups in the COF skeleton serve as recognition sites, and the lone pair electrons of nitrogen atoms chelate with Cu²⁺ to form a coordination complex. The energy level of the lowest unoccupied molecular orbital (LUMO) of (sp²)-TPE-COF is higher than the redox potential of Cu²⁺/Cu⁺, allowing excited-state electrons to transfer from the COF to Cu²⁺, leading to fluorescence quenching (Fig. 8b).

Apart from unsaturated nitrogen and halogen atoms, thioether groups are also capable of coordinating with metal ions, especially heavy metal ions and noble metal ions.¹⁴⁶ Wang and coworkers synthesized COF-LZU8 with densely distributed thioether groups, which can effectively detect mercury ions (Hg²⁺).¹⁴⁷ The coordination with Hg²⁺ leads to electron transfer from the π-conjugated framework to the unoccupied orbitals of mercury thus quenching the fluorescence of COF (Fig. 8c). Na₂S solution treatment can effectively exchange Hg²⁺ out to recover the fluorescence.

In 2020, Qiu et al. synthesized TFPT-BTAN-AO with amino and hydroxyl groups, which can coordinate with UO₂²⁺ and result in a significant decrease of the fluorescence intensity and lifetime of TFPT-BTAN-AO.¹⁴⁸ This quenching effect is due to the electron transfer from COF to UO₂²⁺ (Fig. 8d).

Other types of coordination including bipyrine unit,¹⁴⁹ O,N,O′-chelation,¹⁵⁰ and Se,N coordination¹⁵¹ have also been reported to construct ion-sensitive COFs.

The coordination can also lead to a packing mode change. TpBD was prepared by Guo et al. with β-keto-enamine linkage.¹⁵² When FeCl₃ was added to the COF suspension in water and sonicated, the bulk COF powder gradually turned to nanosheets to form a clear solution. Theoretical calculations reveal that Fe³⁺ coordinates with the tri(N-salicylideneaniline) units in the COF, while the Fe atoms and corresponding ions are located outside the plane, which thickens the single atomic layer of the COF and increases the interlayer distance (Fig. 8e). More importantly, the interlayer attraction energy is greatly reduced and overcome by the electrostatic repulsion thereby causing self-exfoliation.

3.3. Solvent-responsive COFs

In applications such as chemical separation, flexible sensors, and adaptive devices, the solvent-responsive behaviors of COFs exhibit unique advantages. These materials can achieve real-time sensing of and functional feedback to the external solvent environment through precise molecular-level design. Compared with traditional porous counterparts, COFs provide innovative solutions for intelligent separation and flexible actuation through the synergistic effects of weak interactions and dynamic bonding mechanisms.

In 2022 Zhang et al. synthesized PEG-COF-x membranes via interfacial polymerization.¹⁵³ When humidity increases, the membrane bends from the dense upper surface to the rough lower surface; it recovers to its original shape as humidity decreases. In a natural humidity gradient environment above water, the membrane undergoes continuous periodic bending and recovery due to hygroscopic-dehumidification cycles. The PEG segments, containing numerous ether groups, capture water vapor molecules through hydrogen bonding and lead to segmental swelling. Upon moisture absorption, PEG chains transition from a folded to an extended state, increasing inter-chain spacing and driving expansion of the COF layered structure (Fig. 8f). During dehumidification, the segments contract, restoring the ordered structure.

In 2024, the TG-DFP COF film was synthesized by Trabolsi and co-workers through the copolymerization of pyridine-2,6-dicarbaldehyde (DFP) and triaminoguanidine hydrochloride salt (TG_Cl).¹⁵⁴ The TG-DFP COF film exhibits a rapid and reversible response to humidity which causes dramatic mechanical motion. The adsorbed water molecules can form hydrogen bonds with the COF, leading to the formation of a thin hydrated surface layer. Therefore, the originated electrostatic repulsion and ion–dipole interactions result in rapid bending and flipping of the film.

Different solvents can also change the pore structure of COFs. In 2023, Liu et al. synthesized a COF membrane through the reaction of a rigid trialdehyde monomer (TFP) and a flexible triamine monomer (TAEA).¹⁵⁵ The COF membrane has a flexible structure and a microporous structure in water with a pore size of 10.2 Å. However, a larger pore size of 12.1 Å was observed in acetone (Fig. 8g). The strong hydrogen bonds formed between water molecules and the COF framework, which causes the fold of flexible TAEA segments and reduces the interlayer spacing, should be responsible for the smaller pore size in water.

Ma et al. synthesized an ionic PyVg-COF based on viologen. The PyVg-COF can be self-exfoliated into single/multi-layer nanosheets and dissolved in polar organic solvents (e.g., NMP, DMSO, and DEF) to form stable solutions.¹⁵⁶ The strong electrostatic repulsion of the viologen backbone leads to low interlayer interaction energy (2.22 eV), hence suppressing the π–π stacking and forming AB stacking. But the interaction energies between polar solvents (e.g., DEF and DMF) and the COF backbone are significantly higher (5.12 eV and 4.87 eV) than interlayer interaction energy, allowing solvent molecules to intercalate between layers, disrupt stacking, and achieve dissolution of COFs.

In 2024, Zhao et al. prepared the COF Tp-TAPT via interfacial synthesis.¹⁵⁷ In polar organic solvents, the interlayer stacking mode of the COF Tp-TAPT membrane transitions from quasi-AB stacking to AB stacking, accompanied by a reduction in the pore size (Fig. 8h). Upon solvent removal, the membrane structure reverts to the initial quasi-AB stacking state, demonstrating reversible structural changes. In the 2D layered structure of the COF, intralayer connections are maintained by covalent bonds, while interlayer interactions are held by π–π stacking and dispersion forces. Polar organic solvent molecules form hydrogen bonds or dipole–dipole interactions with polar groups (e.g., imine bonds) between COF layers, thereby weakening interlayer noncovalent interactions and prompting relative interlayer displacement to alter the stacking mode.

These interesting solvent-responses in the COF structure provide unique potential in molecular sieving and separation.

3.4. Explosive-responsive COFs

The tunable structure of COFs allows the introduction of functional groups for specific interactions with explosive molecules through various mechanisms. Additionally, the pores of COFs provide abundant adsorption sites and molecular diffusion channels, significantly improving detection efficiency. Their stimuli-responsive characteristics enable visual and quantitative detection of explosives, meeting the needs of velocity and accuracy in practical detecting applications.

In 2023, Bai et al. synthesized two COFs (Pythz-COF and Pyaph-COF) with different numbers of hydrazone bonds.¹⁵⁸ Both COFs showed obvious fluorescence quenching phenomena towards nitrophenol explosives including 2,4,6-trinitrophenol (TNP), dinitrophenol (DNP), and p-nitrophenol (PNP), but the Pythz-COF with more hydrazone linkages delivers better detection performance. This demonstrates the significant role of hydrazone in fluorescence quenching. A systematic study uncovers that the static PET from hydrazone to nitrophenol groups and competitive absorption are the main mechanisms of quenching.

A similar phenomenon can also be observed in imine-linked COFs. Bhattacharya and colleagues synthesized two imine-linked COFs with pyrene and TPE groups, namely TPEPy and TPEB.¹⁵⁹ Both COFs exhibit fluorescence quenching towards various nitroaromatic compounds (NACs), with the fluorescence intensity gradually decreasing as the concentration of NACs increases. The quenching efficiency is positively correlated with the number of nitro groups and the presence of phenolic hydroxyl groups, as the PET quenching mechanism is also dominant. Interestingly, a new fluorescence peak of TPEPy arises along with the further addition of nitroaromatic explosives, exhibiting a firstly turn-off then turn-on emission (Fig. 9a). This can be ascribed to the protonation of imine bonds caused by a decreased pH value. The protonated imine bonds interrupt the conjugation of the aromatic framework, thus leading to the intrinsic emission of pyrene and TPE groups.

Fig. 9 Explosive- and bio-chemical-responsive COFs. (a) Fluorometric titration of the TPEPy with different analytes. Reproduced with permission.¹⁵⁹ Copyright 2024, American Chemical Society. (b) UV-vis DRS (black) and PL (blue) spectra of pristine (dashed line) and acidified (solid line) ETTA-CHDA COFs in the solid state. Reproduced with permission.¹⁶⁰ Copyright 2023, The Authors, American Chemical Society Limited (Creative Commons CC BY). (c) LUMO and HOMO orbital energies of DMA, DCN, MB²⁻ and TfaTta⁺. (d) The PET process from the HOMO of DMA and DCN to the HOMO of TfaTta⁺. Reproduced with permission.¹⁶⁵ Copyright 2024, American Chemical Society.

In 2023, Müllen and colleagues synthesized a series of COFs with cyclohexane linkers.¹⁶⁰ Compared with the counterparts with the benzene linker, cyclohexane groups significantly increase the interlayer distance owing to the steric hindrance and interruption of the conjugation, hence improving the luminescence efficiency. The stronger basicity of diaminocyclohexane linkers also promotes the sensitivity to explosives such as TNP. It is worth mentioning that although the acids may cause the swift protonation of imine bonds, it only leads to the slight bathochromic shift of absorption and emission spectra rather than fluorescence intensity decrease. For instance, 45 and 23 nm of red-shifts are observed for the ETTA-CHDA COF in absorption and emission spectra, respectively (Fig. 9b). The quenching effect on TNP should be ascribed to the PET from the TPE moiety to TNP. The synergy of excellent luminescence performance, large Brunauer–Emmett–Teller (BET) specific surface area, and the large one-dimensional channels enhances the detection performance. Apart from TNP, the ETTA-CHDA COF also shows excellent detection of phenyl glyoxylic acid. In contrast, the formation of nonluminescent complexes should be responsible for the fluorescence quenching.

Two β-ketoenamine-linked COFs denoted as 3BD and 3′PD exhibit fluorescence quenching responses to triacetone triperoxide (TATP).¹⁶¹ Studies have shown that TATP may oxidize the enamine moieties in the COFs, leading to a gradual decrease in the emission intensity accompanied by a blue shift.

3.5. Bio-chemical-responsive COFs

Various kinds of enzymes exist in living organisms, with differential expression levels across tissues and cells. Enzyme-responsive COFs utilize enzymes overexpressed in tumor tissues or specific cells to achieve precise regulation of drug release or other functions. In 2022, Zhou and colleagues synthesized a thiol-targeting nanoinhibitor (Ag-TA-CON@EBS@PEG) based on an enzyme-responsive COF loaded with silver nanoparticles (AgNPs) and ebselen (EBS) for the treatment of bacterial infections.¹⁶² The TA-COF in Ag-TA-CON@EBS@PEG contains azo bonds, which can be specifically recognized and disrupted by azoreductase. Once the azo bonds are broken, the original structure of TA-COF collapses and releases AgNPs and EBS from the nano-inhibitor, achieving specific responsiveness to enzymes in bacterial infection microenvironments.

Redox reactions are the most significant reactions to support vital activity. Due to the existence of thiol groups, glutathione (GSH) is an effective reductive agent. The concentration of GSH in tumor cells is 100–1000 times higher than that in normal cells, indicating its capability as a tumor indicator. The DSPP-COF with disulfide bonds synthesized by Dong et al. is stable under physiological conditions.¹⁶³ However, the high concentration of GSH can reduce and break the disulfide bonds, which causes the collapse of the COF structure. On this basis, the DSPP-COF is able to act as a GSH-responsive drug delivery system.

Recently, Yin and colleagues synthesized a template-free self-assembled hollow microtubular COF, MT-COF-18 Å.¹⁶⁴ The boronic acid structure in MT-COF-18 Å can specifically interact with glucose to form boronate ester reversibly. This binding changes the micro-structure of MT-COF-18 Å, making it easier to release insulin.

In 2024, Yan et al. modified the neutral COF TfaTta into a multifunctional ionic COF (TfaTta–MB) via the Menshutkin reaction and ion-exchange.¹⁶⁵ This material shows regular fluorescence quenching on two organochlorine pesticides, dicloran (DCN) and dicamba (DMA). TfaTta–MB has an electron-deficient backbone, while DMA and DCN are electron-rich molecules. The HOMO energy levels of DMA and DCN are located between the LUMO and HOMO energy levels of TfaTta⁺ (Fig. 9c). After light excitation, the electrons on the HOMO of DMA or DCN molecules autonomously transfer to the HOMO of TfaTta⁺, preventing the electrons on the LUMO of TfaTta⁺ from returning to its HOMO, thus resulting in fluorescence quenching (Fig. 9d).

In 2024, Wang and colleagues synthesized two COFs, Se-COF and Se-BCOF, based on TPE and diselenide-bridged monomers.¹⁶⁶ The diselenide bonds in the COF are sensitive to reactive oxygen species (ROS), which causes bond cleavage. Therefore, effective drug delivery to the target site can be achieved.

H₂O₂ has a similar impact on azo bonds. COF-TpAzo with azo bonds was synthesized by Tian and colleagues.¹⁶⁷ The β-keto-enamine linkage provides the coordination sites to Fe ions, which can undergo Fenton reaction to generate hydroxyl radicals (˙OH). The high oxidizability of ˙OH can cleave the azo bonds in the COF structure, enabling H₂O₂ responsive drug release.

4. Multi-stimuli-responsive COFs

Multi-stimuli-responsive COFs demonstrate the capacity to exhibit either synergistic or orthogonal responses to distinct external stimuli. This enables sophisticated functions through the reversible and dynamic reconfiguration of their structural or physicochemical properties, revealing substantial potential for advanced applications.

Anthracene molecules can undergo not only [4+4] cycloaddition reaction to form a dimer, but also [4+2] cycloaddition with singlet oxygen (¹O₂) under illumination.¹⁶⁸ Zhang et al. constructed a 3D COF with anthracene units denoted as NKCOF-14.¹⁶⁹ The loose packing of the 3D framework prevents the formation of dimers and allows the high accessibility of anthracene units. Along with the prolongation of the 660 nm light irradiation, the fluorescence intensity gradually decreases due to the capture of ¹O₂ to form NKCOF-14-O (Fig. 10a). In addition, the cycloaddition can be reversed by heating at 100 °C for 1 h (Fig. 10b).

Fig. 10 Multi-stimuli-responsive COFs. (a) Ball-and-stick structural representation of part of the reversible conversion of NKCOF-14 to NKCOF-14-O after binding of oxygen and release (C: grey, O: pink, and N: blue). (b) The fluorescence of NKCOF-14 and NKCOF-14-O dispersed solutions under the 365 nm irradiation (solvent: ethanol) and the schematic illustration of the methods of the invisible ink and the anti-fake label. Reproduced with permission.¹⁶⁸ Copyright 2021, Royal Society of Chemistry. (c) Absorption spectra of COF_TAPA-TFPB film electrodes at −0.2 and 0.8 V. The inset is a photograph of the COF_TAPA-TFPB films at 0 and 0.80 V. (d) Response time of the COF_TAPA-TFPB film electrodes at 532 nm with a switching time of 60 s by applying voltage between −0.5 and 0.75 V. Reproduced with permission.¹⁷⁰ Copyright 2024, American Chemical Society. (e) Photographs of TG-DFP COF powder at temperatures of 25 and 100 °C (under different humidity conditions). (f) Optical images of the reversible color change in day-light (top) and under UV light (bottom panel) of a cotton fabric coated with TG-DFP COF powder between humid and dry air. (g) Humidity-dependent emission spectra of the TG-DFP COF after exposure to different RHs. Reproduced with permission.¹⁷¹ Copyright 2024, Wiley-VCH GmbH. (h) Zeta potentials of RDAP in pH 5.0, pH 6.0, pH 6.8 or pH 7.4 buffer. Statistical data are presented as mean ± SD (n = 3). Reproduced with permission.¹³⁷ Copyright 2024, Wiley-VCH GmbH. (i) Release capability of ABTS from ABTS@Fe-DhaTph in PBS with various pH values. Reproduced with permission.¹⁷³ Copyright 2024, Science China Materials. (j) Possible soluble mechanism explanation. (k) Different concentration solutions of TPT-TAB-COF in methyl alcohol, from left to right, 0.1 0.2, 0.5, 1, and 2 mg mL⁻¹. (l) Fluorescence titration of the TPT-TAB-COF in methyl alcohol upon gradual addition of iodine aqueous solution (inset: fluorescence change images before (left) and after (right) I₂ addition). Reproduced with permission.¹⁷⁴ Copyright 2023, American Chemical Society.

A TPA-based COF_TAPA-TFPB film was prepared by Wan et al. to realize electrchromism.¹⁷⁰ In a 0.1 M NaClO₄ and CH₃CN electrolyte system, the TPA moieties can be oxidized via an applied voltage of 0.75 V to form a cation radical, thereby tuning the color from yellow to deep red (Fig. 10c). The coloring and bleaching times can reach 18.6 and 0.7 s, respectively (Fig. 10d). Besides, the fluorescence of the COF film can be fully quenched upon the applied voltage. The oxidation and reduction nature of the chromism indicates that the oxidation process can also be reversed by reductants. Indeed, dopamine, a reducing agent with affinity toward the TPA groups, can effectively reduce the oxidized COF_TAPA-TFPB film to the neutral state, significantly accelerating the recovery of absorption and fluorescence.

Recently, Trabolsi et al. developed a thermo- and hydrachromic COF (TG-DFP COF) via the combination of TG_Cl and pyridine.¹⁷¹ Although the monomers are non-luminescent, the TG-DFP COF shows bright yellow fluorescence at low temperature owing to the charge transfer from pyridine to TG⁺. As the temperature rises from −15 °C to 105 °C, the fluorescence intensity dramatically decreases and then the peak bathochromically shifts to 600 nm. The thermochromism in COFs is primarily related to hydrogen-bonding with atmospheric water molecules (Fig. 10e and f). The increasing temperature breaks these bonds and then changes the charge transfer path. Due to the hydrophilicity feature and the vital role of H-bonds, the TG-DFP COF also exhibits hydro-responsive behavior (Fig. 10g). Turn-on fluorescence and a red-to-yellow color change can be observed when the RH value increases from 0% to 20%, illustrating the dual-response characteristics.

Zheng et al. developed a pH and hypoxia dual-responsive system based on ATOMe.¹³⁷ The covalently attached poly-L-lysine (PLL) reverses the ζ potential from negative to positive thus enabling the further attachment of Dox and PD-L1 siRNA. Besides, the ζ potential of the composite increases in response to the decreased pH due to the protonation of PLL, which results in the escape from lysosomes (Fig. 10h). Meanwhile, the highly expressed azoreductase in the hypoxic tumor can reduce the azo bonds to release the drugs in pores.

Fe-DhaTph developed by Dong et al. can catalyze the decomposition of H₂O₂ and show peroxidase (POD)-like activity. After further loading with 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), a composite namely ABTS@Fe-DhaTph was constructed.¹⁷² The ABTS can be oxidized to oxABTS in the presence of H₂O₂, which leads to the coloring from colorless to green. Moreover, the acidic conditions can promote the catalysis upon H₂O₂ conversion, illustrating pH-responsive behavior. Besides, the composite shows a high photothermal efficiency of 39.8% at high H₂O₂ concentration.

Zhang and colleagues also built a synergistic therapeutic system based on COF-1.¹⁷³ The hydrolysis of boroxine bonds in COF-1 can be highly promoted by acids, hence exhibiting pH response and enabling drug release (Fig. 10i). The polydopamine linked on the COF can not only scavenge ROS, but also act as a NIR light-triggered photothermal unit.

In 2023, Ma et al. synthesized the TPT-TAB-COF via the condensation reaction of 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TPT) and p-phenylenediamine (PPDA).¹⁷⁴ The TPT-TAB-COF remains in solid powder form in aprotic solvents but becomes soluble in protic solvents after exchanging imine bonds with imidazole linkages (Fig. 10j). This obvious solubility change is ascribed to the formation of hydrogen bonds between the imidazole acceptor and protic solvent donor, which enhances the interaction between the COF framework and the solvents, thus achieving protic solvent detection (Fig. 10k). Besides, the TPT-TAB-COF also shows the capability of iodine detection. It originates from the iodine binding on nitrogen atoms which induces energy transfer caused fluorescence quenching (Fig. 10l).

Lately, Wang and colleagues designed an imine COF by combining porphyrin with alkyl chains and then dispersed them in a liquid crystal polymer (LCP).¹⁷⁵ The porphyrin units endow the composite with excellent photothermal performance, while the LCP enables the capability of stimuli-responsive actuation of the composite film. In detail, the temperature of this actuator can increase sharply for 53.6 °C within 5 seconds when irradiated with 808 nm light at 625 mW cm⁻² (Fig. 11a). Meanwhile, the film bends with an angle of 53.4°, which can be attributed to the opposite thermal harmomegathus of the homotropically-aligned surface and planar-aligned surface. Moreover, the actuator based on the splayed-oriented LCP film exhibits differential expansion rates on opposing sides, which also leads to the bending. Upon exposure to DCM vapor, this actuator begins to bend within 1 s and finishes curling within 4 s (Fig. 11b).

Fig. 11 Multi-stimuli-responsive COFs. (a) Photographic and infrared thermal imaging analysis of the POR-BHA COF LCP actuator. (b) Sequential photographs illustrating the reversible deformation of POR-BHA COF-LCP in response to DCM vapor exposure. Reproduced with permission.¹⁷⁵ Copyright 2024, Wiley-VCH GmbH. (c) Schematic fabrication process of the COC/PVDF bilayer membrane. (d) Temperature change of COC membranes under NIR light irradiation (808 nm) with different light intensities (0.2, 0.4, and 0.6 W cm⁻²). (e) Optical photos of the actuator under NIR light (0.6 W cm⁻²) irradiation. (f) Corresponding infrared thermal images of the actuator under NIR light (0.6 W cm⁻²) irradiation. (g) Schematic of the COC/PVDF energy converter system. Reproduced with permission.¹⁷⁶ Copyright 2024, Wiley-VCH GmbH.

Most recently, Wang et al. developed a similar multi-stimuli-responsive actuator but based on a different mechanism.¹⁷⁶ COF-TASA with porphyrin units serves as the photothermal generator, while cellulose nanofiber (CNF) and PVDF act as the hygroscopic agent and thermal expansion layer, respectively (Fig. 11c). The driving force of the bending is generated from the differential thermal expansion of two layers. The COF-TASA exhibits an excellent photothermal effect with a record-high PTCE of 79.9% under 808 nm excitation, which is attributed to the zwitterionic resonance structure (Fig. 11d). The hydrophilicity and hygroscopicity of CNF enable the humidity-responsive feature which can be improved by the excellent mass transport of COF-TASA and abundant hydrogen-bonds between them. Therefore, this composite demonstrates photo- and humidity-induced bending (Fig. 11e). Moreover, the mechanical force of this actuator can be converted to the electrical energy due to the piezoelectric feature of PVDF (Fig. 11f).

These examples have shown the unlimited possibilities of multi-stimuli-responsive COFs in structural combination and functional integration. The multiple-in one strategy that enables solving complicated practical issues using one material has attracted much attention in various fields.

5. Applications

Stimulus-responsive COFs have emerged as a focal point of research in smart materials, due to their unique integration of dynamic, reversible (or irreversible) responsiveness to external environmental changes and well-defined porous architectures that facilitate efficient mass transport and adsorption. These programmable behaviors enable precise control over material properties and support the integration of multiple functions, providing innovative solutions to address complex practical issues. This section surveys the diverse application landscape of stimulus-responsive COFs and emphasizes their distinctive advantages across various domains.

5.1. Sensing and detection

Benefitting from the combination of porous structure and easiness of modification, stimuli-responsive COFs have shown great potential in detection and sensing. The functions including selectivity and sensitivity can be customized against specific needs.

5.1.1. Temperature indicator. The thiol-functionalized COF synthesized by Wang and co-workers shows a gradual color change from bright yellow to purple.⁹² The increased temperature can remarkably promote the speed of this color change and result in color difference after a certain time at different temperatures. Besides, this change is irreversible thus enabling the application as a time–temperature indicator to track the thermal history of foods during storage and transportation.

5.1.2. Chirality. Luminescent chiral COFs can be used to achieve chirality detection.¹⁷⁷ The chirCOFilm constructed by Gu et al. exhibits excellent CPL response characteristics suitable for highly sensitive optical sensors.⁶² The initial doped composites of COFilm/R-BBNA and COFilm/S-BBNA show no obvious CPL signal. In contrast, the chirCOFilm formed by photoinduced cyclization exhibits an obvious CPL signal at 407 nm, with the luminescence dissymmetry factor reaching 1.2 × 10⁻³. Hence, the highly sensitive CPL detection of (R)-(+)-1-phenylethanol (R-PE) and (S)-(−)-1-phenylethanol (S-PE) enantiomers can be realized owing to the CPL signal difference (Fig. 12a).

Fig. 12 Sensing and detection. (a) Photo images of S-chirCOFilm probing of (0%, 0.5%, and 5%) S- (left)/R-PE (right) excitation at 365 nm under the left circularly polarized filter and right circularly polarized filter. Reproduced with permission.⁶² Copyright 2023, American Chemical Society. (b) Luminescence spectra of COF-JLU3 in THF suspensions containing different concentrations of Cu²⁺ (0 to 2 × 10⁻⁴ M), λ_ex = 400 nm. Inset: the corresponding images of COF-JLU3 under UV irradiation at 365 nm before and after titration with Cu²⁺ ions. Reproduced with permission.¹⁷⁸ Copyright 2016, Royal Society of Chemistry. (c) In situ fluorescence spectra of TFPB-PI(F)–COF@Cu²⁺ (0.003 mg L⁻¹) in the presence of the different anions (3 μmol L⁻¹). Reproduced with permission.¹⁴⁴ Copyright 2024, Elsevier. Ion separation of COF membranes in the binary system under (d)  UV and (e) visible light irradiation. Reproduced with permission.⁵⁵ Copyright 2024, Elsevier. (f) Fluorescence titration of COF-LZU8 dispersed in acetonitrile upon the gradual addition of Hg²⁺ (λ_ex = 390 nm). Inset: photographs of the fluorescence emission change (under a UV lamp with λ_ex = 365 nm) upon the addition of Hg²⁺. Reproduced with permission.¹⁴⁷ Copyright 2016, American Chemical Society. (g) Hg²⁺ sorption kinetics of COF-S-SH with a Hg²⁺ initial concentration of 5 ppm at a V/m ratio of 46 500 mL g⁻¹. Reproduced with permission.¹⁷⁹ Copyright 2016, American Chemical Society. (h) Fluorescence emission spectra of TFPT-BTAN-AO upon gradual addition of UO₂²⁺ (λ_ex = 277 nm). Inset photos show the fluorescence emission change (under a 365 nm UV lamp) of TFPT-BTAN-AO after addition of UO₂²⁺. (i) The selective adsorption of the test ions. Error bars represent S.D. n = 3 independent experiments. Reproduced with permission.¹⁴⁸ Copyright 2020, The Authors, Springer Nature Limited (Creative Commons CC BY). (j) Relative fluorescence intensity of CON at different pH values. (k) Relative fluorescence intensity of CONN at different pH values. (l) Relative fluorescence intensity of CONNCO at different pH values. Reproduced with permission.¹⁸⁰ Copyright 2021, American Chemical Society. (m) Naked eye color changes of MOH-Tf1₂Tf2₁ in water solutions with different pH values (1 to 14). (n) The linear relationship between fluorescence emission intensity and pH values (1 to 6). (o) The reversible pH response of MOH-Tf1₂Tf2₁ for five acid–base cycles. Reproduced with permission.¹⁸¹ Copyright 2024, Elsevier. (p) Color (left) and fluorescence (right) changes of the test paper upon alternate exposure to dry HCl and NH₃ gases and upon sequential exposure to the air atmosphere saturated with water at 80 °C, that above concentrated HCl solution (room temperature), and that above concentrated NH₃ solution (room temperature). Reproduced with permission.¹⁸² Copyright 2021, American Chemical Society.

5.1.3. Ion detection. A hydrogen-bond-assisted azine-linked COF (COF-JLU3) was employed as a fluorescent probe by Liu et al. to detect trace toxic heavy metal ions.¹⁷⁸ COF-JLU3 achieves rapid Cu²⁺ identification through ion enrichment via its porous structure and luminescence response, exhibiting a detection limit (LOD) of 0.31 μM and a linear range of 0–0.4 μM (Fig. 12b). It withstands a pH of 5–11 and can be regenerated via EDTA elution with a fluorescence recovery rate >90%.

In 2024, the TFPB-PI(F)-COF was synthesized by Zhang et al., which also exhibits a fluorescence quenching response to Cu²⁺/Al³⁺ with high selectivity.¹⁴⁴ The LODs for Cu²⁺ and Al³⁺ are 2.3 and 4.2 nM, while the quenching efficiencies are 95% and 90%, respectively. Interestingly, the fluorescence quenching can be recovered by adding PO₄³⁻ to achieve the relayed detection of PO₄³⁻ with an LOD of 10 nM (Fig. 12c). This recovery is also highly selective, as no obvious response to other common anions (F⁻, Cl⁻, Br⁻, etc.) is exhibited. The achieved “on–off–on” fluorescence change can be well repeated for 5 cycles, indicating the excellent reversibility.

The isomerism of azo groups enables the selective separation of ions.⁵⁵ In the cis form, COF-AzoSO₃H exhibits a high mono/divalent selectivity of 22.5 for the Li⁺/Mg²⁺ system and nearly no mono/monovalent selectivity (1.1) for the K⁺/Li⁺ system (Fig. 12d). Differently, after changing to trans form upon UV irradiation, the K⁺/Li⁺ selectivity reaches 6.9, while Li⁺/Mg²⁺ selectivity further enhances to 36.6 (Fig. 12e). This dynamic multi-selectivity can be applied to realize Li⁺ recycling from Li batteries.

Zhang and colleagues synthesized two hydrazone-linked COFs containing O,N,O′-chelating sites, namely Bth-Dha and Bth-Dma.¹⁵⁰ The Bth-Dma COF exhibits significantly higher fluorescence quenching efficiency towards Fe³⁺ than other metal ions (such as Na⁺, K⁺, Mg²⁺, Cu²⁺, etc.) with the LOD reaching 0.17 μM. The loading capacity for Fe³⁺ is approximately 11.9 wt%. Meanwhile, common anions (Cl⁻, Br⁻, NO₃⁻, etc.) show no significant impact on the fluorescence quenching efficiency.

COF-LZU8, a thioether-functionalized fluorescent COF, was synthesized via a bottom-up method, focusing on efficient detection and removal of Hg²⁺.¹⁴⁷ It achieves real-time fluorescence quenching within minutes with a LOD of 25 ppb, and the quenching rate for Hg²⁺ is significantly higher than that for other metal ions (Fig. 12f). When COF-LZU8 was treated with a 10 ppm Hg²⁺ solution, the residual concentration could reach lower than 0.2 ppm, with a removal rate >98%. After Hg²⁺ adsorption, COF-LZU8 can be regenerated by treating with 10 equiv. of Na₂S solution to release Hg²⁺ from thioether sites.

Ma and colleagues prepared COF-S-SH by grafting thiol derivatives onto a vinyl-functionalized mesoporous COF-V through post-synthetic modification.¹⁷⁹ This material demonstrates outstanding performance in Hg removal, with adsorption capacities of 1350 mg g⁻¹ for Hg²⁺ and 863 mg g⁻¹ for Hg⁰. The distribution coefficient reaches as high as 2.3 × 10⁹ mL g⁻¹, enabling the rapid decrease of the Hg²⁺ concentration from 5 ppm to below 0.1 ppb (Fig. 12g).

Qiu et al. synthesized an sp² carbon-conjugated fluorescent COF namely TFPT-BTAN-AO.¹⁴⁸ Upon treating with UO₂²⁺, TFPT-BTAN-AO exhibits rapid fluorescence quenching within 2 seconds, realizing real-time detection. The adsorption capacity of the equilibrium is as high as 98% within 45 minutes, demonstrating ultrafast adsorption kinetics (Fig. 12h and i). Moreover, the saturated adsorption capacity reaches 128 mg g⁻¹ in 3.0 M nitric acid, which illustrates strong acid resistance.

5.1.4. pH sensing. Mandal and colleagues synthesized three COFs (CON, CONN, and CONNCO), which exhibit distinct fluorescence intensity changes at different pH values.¹⁸⁰ CON shows reduced fluorescence intensity along with the decrease of the pH value (Fig. 12j). In contrast, CONN exhibits quasi-linear increased fluorescence under the same conditions (Fig. 12k). As for CONNCO, it shows a slightly increased fluorescence intensity as pH decreases from 7 to 4, but the intensity sharply drops when the pH value exceeds this range due to the break of hydrogen bonds (Fig. 12l). Notably, CON achieves a detection limit of 0.1 pH units and rapid response to protons in aqueous solutions within 10 s.

A hydrazone-linked COF denoted as MOH-Tf1₂Tf2₁ was synthesized by Su et al. to serve as an efficient pH sensor.¹⁸¹ In the pH range of 1–6, fluorescence intensity exhibits a good linear relationship with pH, allowing precise pH monitoring via fluorescence intensity changes with high sensitivity. In the case of pH from 7 to 14, the relationship between fluorescence intensity and the pH value is nonlinear, but the color change from orange to crimson is quite obvious, indicating its potential as a colorimetric pH sensor (Fig. 12m and n). After multiple pH switching between 1 and 14, the fluorescence intensity of MOH-Tf1₂Tf2₁ can be nearly recovered (Fig. 12o).

Some COFs exhibit gas responses correlated with pH, expanding their application in gas sensing. For instance, VCOF-PyrBpy shows rapid and reversible responses to HCl and NH₃.¹⁸² When HCl-modified test strips are exposed to dry HCl gas, their color instantly changes from yellow to dark red with fluorescence quenching. The color and fluorescence can be restored via exposure to gaseous NH₃ (Fig. 12p). This pH-responsive behavior provides a new method for detecting specific gases, featuring fast response and good reversibility with potential applications in environmental monitoring and industrial production.

5.1.5. Wastewater treatment. TPT-TAB-COF and TPT-PPDA-COF synthesized by Ma et al. emit blue fluorescence in methanol solution which can be quenched upon exposure to I₂.¹⁷⁴ TPT-TAB-COF shows better performance as an I₂ detector, with a LOD as low as 75 nM and a Stern–Volmer constant (K_sv) of 2 × 10⁴ M⁻¹ (Fig. 13a). This high detection sensitivity enables the rapid identification of low-concentration iodine in aqueous solutions.

Fig. 13 (a) Fluorescence titration of the TPT-TAB COF in methyl alcohol upon gradual addition of iodine aqueous solution. Inset: fluorescence change images before (left) and after (right) I₂ addition. Reproduced with permission.¹⁷⁴ Copyright 2023, American Chemical Society. Charge-driven molecular sieving effects of COF films towards (b) methylene blue, (c) rhodamine B, and (d) methyl orange. Reproduced with permission.¹³² Copyright 2023, American Chemical Society. (e) Rejection of solvent-responsive COF membranes in different solvents (water and acetone). Reproduced with permission.¹⁵⁵ Copyright 2023, American Chemical Society. (f) Dye rejection performance of the COF Tp-TAPT membrane using the feed with different polarity gradients. Reproduced with permission.¹⁵⁷ Copyright 2024, The Authors, Science Limited (Creative Commons CC BY-NC). The Stern–Volmer plots of (g) the Pythz-COF and (h) the Pyaph-COF about (I₀/I − 1) values versus the concentrations of TNP (inset: the enlarged Stern–Volmer plots at low concentrations of TNP). Reproduced with permission.¹⁵⁸ Copyright 2022, Elsevier. The cyclability of recovery efficiency of benzimidazoles based on (i) Fe₃O₄@COF-IL and (j) Fe₃O₄@COF/IL on different adsorbents. The error bar depicts the standard deviation obtained from three replicated measurements. Reproduced with permission.¹⁸⁵ Copyright 2025, Elsevier. (k) Schematic illustration of Fe₃O₄@COF@Os based LFA for PSA detection. Reproduced with permission.¹⁰⁶ Copyright 2024, Wiley-VCH GmbH.

In 2023, Liu et al. prepared a 160 nm-thick free-standing COF membrane via liquid–liquid interfacial reaction.¹³² The COF membrane exhibits tiny changes after immersion in solutions with pH 1 to 11 for 130 days and heating to 317 °C in a nitrogen atmosphere, demonstrating excellent chemical and thermal stability. Through the charge-driven molecular sieving effect induced by acid–base treatment, the membrane can screen organic dyes with sizes much smaller than the membrane pores and persistent organic pollutants with opposite charge and similar size. The protonation induces the rejection rate increase of methylene blue and rhodamine B (RhB) from 42.38% and 25.38% to 94.00% and 83.18%, respectively (Fig. 13b and c). Besides, the deprotonation treatment leads to the rejection rate increase of indigo carmine and methyl orange from 40.24% and 19.52% to 98.44% and 41.05%, respectively (Fig. 13d). Therefore, this pH-responsive membrane enables the controlled purification of water and extraction of various dyes.

They also realized the adjustment of dye filtration via a solvent-responsive COF membrane through pore size manipulation.¹⁵⁵ In water, the membranes efficiently reject various dyes, with rejection rates of ∼99% for neutral red, rose bengal, brilliant blue R, etc. In acetone, the pore size expands, thus allowing dyes permeance and differentiated separation effects in different solvents (Fig. 13e). By adjusting the mixing ratio of water and acetone, gradient rejection of different organic dyes can be achieved by a single membrane, avoiding the need for traditional multi-stage membrane separation processes.

Zhao and colleagues prepared the COF Tp-TAPT membrane possessing similar solvent-responsive sieving capability, but shows an opposite solvent effect.¹⁵⁷ Specifically, it exhibits moderate rejection rates towards several dye aqueous solutions. In contrast, the rejection rates dramatically increase to >95% when water is replaced by polar organic solvents which causes the sharply shrunk pore size (Fig. 13f). The rejection rate of acid fuchsin in ethanol is 95% with a permeance of 18 kg m⁻² h⁻¹ bar⁻¹, while in n-hexane, the rejection rate is 53% with a permeance of 33 kg m⁻² h⁻¹ bar⁻¹. For methylene blue, the rejection rate in polar solvents exceeds 99% and the permeance is over 15 kg m⁻² h⁻¹ bar⁻¹. Additionally, it is applicable for curcumin recovery, where the rejection rate of curcumin in polar solvents increases from 37% to 92% with a permeance exceeding 20 kg m⁻² h⁻¹ bar⁻¹.

CAT-PAH@RT-COF-1-based nanomotors constructed by Liang et al. are able to remove water pollutants like RhB.⁵⁸ In practical applications, light-controlled nanomotor movement enhances contact/adsorption with pollutants for efficient water remediation. Under static conditions, the removal rates of RhB by the blank COF and nanomotor are 31.3% and 25.1% respectively, which increase to 63.1% and 65.2% after mechanical stirring. In the RhB solution containing 50 mM H₂O₂, the removal rate of RhB by the nanomotor under blue light irradiation within 30 minutes can reach 79.8%, while those under red light and in the dark are only 18.0% and 17.3% respectively.

5.1.6. Explosive detection. Based on the fluorescence quenching or adsorption response properties of COFs, various portable detection devices can be fabricated for rapid on-site detection of explosives. The ETTA-CHDA COF developed by Müllen and colleagues can be used to detect the explosive TNP.¹⁶⁰ In THF solution, as the concentration of TNP increases, the PL intensity of ETTA-CHDA COF decreases significantly, with a quenching degree of up to 98%. This indicates that the ETTA-CHDA COF has a significant response to TNP. Even at low TNP concentrations, it can cause a significant change in the PL intensity, with a low LOD of 188 ppb. The good responsive reversibility of the ETTA-CHDA COF allows the reuse to reduce the detection cost.

Bai et al. used hydrazone COFs for the visible recognition of nitrophenol explosives, such as TNP, DNP, and PNP, based on the fluorescence quenching effect.¹⁵⁸ For the Pythz-COF, the K_sv for TNP reaches 7.9 × 10⁴ M⁻¹, with a LOD of 0.76 μM and a response time of only 3 minutes (Fig. 13g). The K_sv values for DNP and PNP are 3.9 × 10⁴ M⁻¹ and 4.4 × 10⁴ M⁻¹, with LODs of 2.4 μM and 24.9 μM, respectively. As for the Pyaph-COF whose half of the hydrazone linkages are replaced by imine bonds, it exhibits a slightly lower K_sv for TNP (3.9 × 10⁴ M⁻¹) and higher LOD (1.9 μM), demonstrating a better detecting ability of hydrazone bonds (Fig. 13h).

Wang and Zhang employed the DHB-TFP COF as a real-time monitor to detect PNP and TNP in environmental water samples with high sensitivity and selectivity.¹⁸³ The DHB-TFP COF shows LODs of 0.40 and 11.15 μM for PNP and TNP, respectively, with a linear fluorescence quenching relationship.

Recently, Bhattacharya and colleagues synthesized TPEPy and TPEB COFs to detect various nitroaromatic compounds.¹⁵⁹ Notably, TPEPy exhibits a unique “turn-off/on” fluorescence switching behavior toward highly acidic TNP, with the emission peak blue-shifting from 530 nm to 420 nm. The materials show significant selectivity for aromatic nitro compounds, with no obvious response to aliphatic nitro compounds. The detecting performance remains stable after 10 cycles of use and enables real-time fluorescence monitoring via paper strips. TPEPy and TPEB have initial emission peaks at 535 nm and 530 nm, respectively, with quenching efficiencies of 65–76% and K_sv of 0.36–1.25 × 10⁴ M⁻¹ toward several NACs.

5.1.7. Magnetic extraction. The combination of smart COFs and magnetic materials can enhance the efficiency of recycling. A boric acid-functionalized magnetic COF nanocomposite was reported by Li et al. for the magnetic solid-phase extraction of trace endocrine disrupting chemicals in meat samples.¹⁸⁴ The LODs can reach 0.08–0.72 μg kg⁻¹ with limits of quantification (LOQs) of 0.29–1.99 μg kg⁻¹, illustrating high sensitivity.

Recently, Zhang et al. also used a magnetic COF for magnetic solid-phase extraction of aflatoxins.¹⁰⁸ Fe₃O₄@COF (ETTBA-ND) exhibits excellent adsorption performance with capacity lying from 46.7 to 52.3 mg g⁻¹, while recovery rates range from 80.34% to 105.60%. Notably, the LODs are extremely low, between 1.6 and 9.9 ng kg⁻¹ with a wide linear range of 0.01–100 μg L⁻¹, enabling the detection of trace aflatoxins in foods.

COFs possess high specific surface areas and regular pore structures, providing abundant sites for the loading of ionic liquids (ILs) and facilitating the diffusion and adsorption of target molecules (benzimidazoles).¹⁸⁵ Fe₃O₄@COF-IL and Fe₃O₄@COF/IL, which combined magnetic COF with ILs, were constructed by Liu et al. to extract benzimidazoles from human plasma. These composites show excellent selectivity, high recovery rates (>95%), low LODs (20–100 ng L⁻¹) and a wide linearity range (1–1000 μg L⁻¹). The covalently linked composite (Fe₃O₄@COF-IL) possesses much better cycling stability, retaining more than 90% of the initial recovery rate after 10 cycles (Fig. 13i). In contrast, only 30% to 60% efficiency can be held by Fe₃O₄@COF/IL under the same conditions (Fig. 13j).

Chen and co-workers developed Fe₃O₄@COF@Os as the POD-mimic nanozyme based on the superparamagnetism of the magnetic COF and high POD-mimic activity endowed by Os to separate targets.¹⁰⁶ The high saturation magnetization values of the composite enable the quick separation of target biomarkers and effective avoidance of background interference. In order to diagnose serum cancer, the composite was utilized as an enrichment and catalytically active colorimetric tag in a lateral flow assay. An ultralow LOD of 3.83 pg mL⁻¹ can be achieved, which is 3 orders of magnitude lower than that of conventional Au NPs (Fig. 13k).

The structural advantages of COFs bring outstanding sensing and detection performance toward various objects. Going beyond fundamental sensing, the achievements of molecular sieving, controlled separation, and pollutant extraction reveal the considerable potential of smart COFs for practical implementation.

5.2. Optoelectronic devices

The direct utilization of stimuli-response features of COFs is to fabricate sensors. On the basis of that, optoelectronic devices such as logic gates and photoelectric conversion can be achieved by further signal exchange or exploiting the multi-responsiveness.

The COF_TPDA-PDA film developed by Wang et al. exhibits three-state EC with quick response and long duration, which is applicable in logic gate devices.⁸⁶ Applied potentials of +0.75 V and +0.90 V represent input 1 (erase) and input 2 (write), while the absorbance at 740 and 1150 nm indicate output 1 and output 2, respectively (Fig. 14a and b). As a result, a flip-flop memory system with two NOR gates could be established (Fig. 14c).

Fig. 14 Optoelectronic devices. Switching of the (a) input potentials (vs. Ag/AgCl) at 0.75 and 0.90 V and (b) output absorbance at 740 and 1150 nm of the COF_TPDA-PDA film. (c) Logic circuit and truth table of the set/reset flip-flop logic. Reproduced with permission.⁸⁶ Copyright 2021, Wiley-VCH GmbH. (d) Scheme and truth table of the logic gate. Reproduced with permission.¹⁷⁰ Copyright 2024, American Chemical Society. (e) Visualization of the on/off state with an LED switched by temperature regulation. Reproduced with permission.⁹⁶ Copyright 2023, American Chemical Society. (f) The photoswitch device of the DBTB-DETH-COF. Reproduced with permission.⁶⁷ Copyright 2024, The Authors, Springer Nature Limited (Creative Commons CC BY). (g) Schematic diagram of the optoelectronic synaptic device sensing and computing integration. (h) The original license plate image (left), noisy license plate image (middle), denoised license plate image (right), and their corresponding grayscale value distributions. Reproduced with permission.⁶⁵ Copyright 2024, Wiley-VCH GmbH.

Wan et al. constructed another logic gate device using a multi-responsive fluorescence-switchable COF.¹⁷⁰ The fluorescence of the COF_TAPA-TFPB film can be quenched upon an applied potential of +0.75 V and recovered by adding dopamine solution. Defining an applied potential of +0.75 V as In1, addition of dopamine solution as In2, and “NOT” of In2 as . Meanwhile, the existence or absence of fluorescence represents 1 or 0 of Out, respectively (Fig. 14d). Therefore, an “” logic gate can be demonstrated as consisting of In2, and Out.

Integrating photochromic molecules into 2D COFs enables dynamic regulation of COF electronic properties. For instance, the conductivity of SP-integrated COF increases by 39% compared to the original COF after 30 minutes of UV irradiation.¹⁸⁶ This property can be harnessed to develop multifunctional and electroactive 2D materials with potential applications in electronic devices such as sensors and transistors.

The thermochromic COF based on an LCST-like mechanism also exhibits a significant conductivity change due to the phase separation at high temperature.⁹⁶ On this basis, Wang and colleagues constructed a smart device which series-connected IL₈₀₀@COF-20% with an LED light. At 55 °C, the proton conductivity reaches 4.71 × 10⁻³ S cm⁻¹, which enables the turn-on state of LEDs. When the temperature rises to 80 °C, the conductivity significantly decreases to 4.91 × 10⁻⁴ S cm⁻¹ and leads to the turn-off of the light (Fig. 14e). The on–off switch can undergo at least 20 cycles without significant attenuation in conductivity, illustrating good cycling stability.

Dong et al. developed a similar smart device based on a photochromic COF.⁶⁷ After doping with graphene, the DBTB-DETH-COF is able to sustain the turn-on state of the LED light in the presence of light and air due to the generation of radicals. When the radicals were quenched by heating, the low conductivity leads to a turn-off state (Fig. 14f).

The cycloaddition reaction of anthracene not only features the photochromism of COF-DaTp, but also features the conductivity change.⁶⁵ Consequently, Zhang and co-workers used COF-DaTp as the active layer to fabricate an optoelectronic synapse device (Fig. 14g). 32 conductive states realized by this device enable optical recognition and noise reduction of handwritten data. More than 90% of the noise with gray values in the range of 50–200 can be removed, and thus the gray value range can be adjusted to 0–50 and 200–255 and the image contrast can be enhanced (Fig. 14h).

COFs are widely utilized as electrodes or electrolytes in secondary batteries.^187–189 The smart COFs such as NT-COFs can further reduce the charging voltage and internal overpotential via the photo-generated charge carriers.⁶⁹ Therefore, when served as the cathode in an aqueous zinc-organic battery with a metallic zinc anode, the discharge capacity apparently increased from 308 mAh g⁻¹ on overcast days to 430 mAh g⁻¹ on sunny days (Fig. 15a–c). This change is reversible, which makes NT-COF based batteries a potential candidate with smart modulation to combine with solar cells (Fig. 15d).

Fig. 15 (a) Schematic diagram of the photodetector structure (FTO/NT-COF/Ag) and (b) its photocurrent response at 0 V bias voltage under both dark and light conditions (at an intensity of 1 sun, 100 mW cm⁻²). (c) GCD curves under different daytime conditions (sunny, cloudy, and overcast) within a specific time period. (d) Cycling performance evaluation of photo-responsive Zn//NT-COF cells under ambient weather conditions. Reproduced with permission.⁶⁹ Copyright 2024, Wiley-VCH GmbH. (e) Schematic of the experimental setup for measuring the trans membrane ionic transport. (f) I–V plots recorded for symmetrical 1 M KCl aqueous solutions under various conditions. (g) Output power density with 0.01 and 0.5M NaCl aqueous solutions. Reproduced with permission.⁶⁸ Copyright 2023, The Authors, Springer Nature Limited (Creative Commons CC BY).

Sun and colleagues constructed a composite including COF-301 with a hydrogen-bond network and photosensitizer HB.⁶⁸ HB converts light into charge carriers, which decreases the resistance and promotes the ionic conductivity (Fig. 15e and f). Under the simulated estuarine salinity gradients (0.5 M NaCl vs. 0.01 M NaCl) and light irradiation, the nanofluidic device exhibits excellent energy conversion performance with a peak power density of 129 W m⁻² (Fig. 15g).

Nonetheless, the potential of smart COFs in optoelectronic devices remains underexplored. The stimuli-responsive features could facilitate the realization of environmentally adaptive and self-healing functionalities within the device.

5.3. Bio-applications

Stimuli-responsive COFs have demonstrated unique application potential in the field of biomedicine. By utilizing specific responses to various stimuli such as light, pH, temperature, or redox chemicals, applications including controlled drug release, disease treatment, and bioimaging can be realized. Considering the existing review article,⁴⁵ only representative advances in bio-applications will be discussed in this section.

5.3.1. Drug delivery. The porous crystalline framework and designable pore structure of COFs enable the precisely regulated drug loading. By modulating the surface potential and matching the size of pores with drug molecules, high drug loading capacity and selective loading can be achieved. In addition, intelligent drug release can be realized by introducing pH-sensitive groups, cleavable covalent bonds (such as disulfide bonds), or specific recognition units.

Trabolsi and co-workers developed a gastric-resistant nCOF nanoparticle for oral insulin delivery.¹⁹⁰ Insulin is loaded between the nanosheet layers, protecting insulin from degradation in in vitro digestive fluids and exhibiting glucose-responsive release properties. The hydroxyl groups of glucose interact with nitrogen atoms in the COF framework through hydrogen bonding, prompting structural changes in the COF nanosheets to release insulin from interlayers. Compared with other sugars, serum, or amino acids, glucose-triggered insulin release is highly specific. Additionally, the drug stability reaches <5% amount of release within 24 hours in simulated gastric juice (pH 2.0), ensuring the drug would not prematurely release in the stomach (Fig. 16a). In a hyperglycemic environment, insulin is almost completely released within 7.5 hours, and the release rate is significantly faster than that under normal blood glucose or glucose-free conditions.

Fig. 16 Drug loading and therapy. (a) In vitro accumulated insulin-release from the TTA-DFP-nCOF/insulin-FITC at 37 °C in PBS (10 mM) and pH 2.0 (black), or pH 7.4 at several glucose concentrations. Reproduced with permission.¹⁹⁰ Copyright 2021, Royal Society of Chemistry. (b) Schematic illustration of the multifunctional approach of using engineered nCOFs for targeted chemotherapy within a tumor environment. (c) In vitro Dox release from Alkyn-nCOF cRGD at 37 °C in 10 mM PBS solution at a pH of 7.4 or 5.4. Reproduced with permission.¹²⁹ Copyright 2024, The Authors, American Chemical Society Limited (Creative Commons CC BY). (d) 5-Fu release behavior triggered by GSH with different concentrations. Reproduced with permission.¹⁶³ Copyright 2023, Royal Society of Chemistry. (e) DC_AC50 release from DC_AC50@nano TKPP-COF under NIR irradiation at different power densities. Reproduced with permission.¹⁹¹ Copyright 2023, Royal Society of Chemistry. (f) Schematic illustration of the preparation and application of cationic COF NPs. (g) Tumor growth curves of mice treated with various groups. (h) Images and (i) average tumor weight of dissected tumor tissues on day 10 of mice treated with various groups. (j) Establishment of subcutaneous CT26 tumors and the therapeutic schedule in vivo. Reproduced with permission.⁷¹ Copyright 2024, Elsevier. (k) Schematic illustration of the synthesis of the COF_Zn@HPTA hydrogel and its mechanism for co-incubation diagnosis and repair of diabetic wounds. Reproduced with permission.¹⁹⁴ Copyright 2024, Wiley-VCH GmbH. (l) Linear time data of UAPi. (m) Schematic diagram of the mechanism regarding CAT-like activity and the mechanism of enhancing enzymatic activity by NIR-II. Reproduced with permission.¹⁹⁵ Copyright 2024, Wiley-VCH GmbH.

Another approach to manipulate drug release of COFs is to change their structures in response to stimulus sources. For instance, Trabolsi and colleagues developed an alkynyl-functionalized nanoscale COF (Alkyn-nCOF) and constructed a nanoplatform (Alkyn-nCOF-cRGD) through click chemistry modification with a cyclic Arginyl-glycyl-aspartic acid (cRGD) peptide.¹²⁹ By loading the chemotherapeutic drug Dox into Alkyn-nCOF-cRGD, precise drug release at tumor sites can be achieved in response to acidic environments (Fig. 16b). In vivo experiments demonstrated that the drug loading efficiency of Alkyn-nCOF for Dox was 61 ± 4 wt% and that of Alkyn-nCOF-cRGD for Dox was 42 ± 3 wt%. At a physiological pH of 7.4, the COF can stably encapsulate drugs and avoid premature release. In contrast, Dox is rapidly and completely released at tumor sites with a pH of 5.4, owing to the structural damage and fragmentation of the COF (Fig. 16c).

Dong et al. synthesized a biodegradable disulfide-linked porphyrin COF which was loaded with 5-fluorouracil (5-Fu) to form a multifunctional nanomedicine.¹⁶³ The disulfide bonds in the COF can be reductively cleaved by the high concentration of GSH in tumor cells, leading to the dissociation of the COF framework and the release of the loaded drug. 96.9% of the 5-Fu was released within 24 hours under the condition of 10 mM GSH, while only 11.3% was released in the absence of GSH, achieving selective drug release at the tumor site (Fig. 16d).

5.3.2. Therapy. Photodynamic therapy (PDT), characterized by precision, minimal invasiveness, and low drug resistance, has become a key approach for treating tumors and infectious diseases. With the development of new photosensitizers, delivery technologies, and combined therapies, its application scenarios continue to be expanded. Especially in combination with nanomaterials such as COFs, it is expected to overcome the limitations of traditional PDT, promote its evolution toward high efficiency and intelligence, and bring new clinical transformations.

Dong and colleagues synthesized a ROS-responsive COF composite (namely DC_AC50@nano TKPP-COF) composed of porphyrin photosensitizers, ROS-responsive dithioacetal moieties, and loaded drug DC_AC50 for synergistic chemotherapy and PDT of cancer.¹⁹¹ The porphyrin photosensitizer generates ¹O₂ under 660 nm red light irradiation to directly kill tumor cells. The loading capacity of DC_AC50 is 1.209 μmol mg⁻¹. Only 15.8% of DC_AC50 is released within 48 hours in the dark (Fig. 16e). However, under 660 nm light with low power density (200 mW cm⁻²), the release rate reaches 95.2% within 36 hours, while the release rate can reach 97.7% within 12 hours using high power density 660 nm light (600 mW cm⁻²).

Wang and colleagues synthesized a pillararene-embedded COF (PCOF) for treating periodontitis.¹⁹² The activation of PCOF by visible light (440 nm blue light) generates reactive oxygen species, which synergizes with ROS-responsive drug release to enhance the antibacterial effect. Under illumination, it mainly produces ¹O₂ and superoxide anions (O₂˙⁻), without ˙OH (verified by ESR spectroscopy). DPBF probe detection shows that the generation of ¹O₂ reduces the absorbance of DPBF by 90% after 20 min of illumination, confirming the efficient production of singlet oxygen.

Stimulus-responsive COFs significantly improve the precision and safety of photothermal therapy through three key characteristics: environmental-sensitive release, targeted enrichment, and microenvironment regulation. Their applications in tumor ablation and infection clearance not only overcome the targeting limitations of traditional photothermal agents but also expand therapeutic boundaries through multifunctional integration. Tian and co-workers constructed a novel cationic multifunctional nanoscale COF nucleic acid delivery system based on photosensitizers and cationic monomers for photodynamic and photothermal combined therapy (Fig. 16f).⁷¹ COF nanoparticles (COF NPs) generated various ROS species including ¹O₂, ˙OH, and O₂˙⁻ under 671 nm laser irradiation, confirmed by indicator characterization and ESR tests. In terms of photothermal performance, the temperature of the 200 μg mL⁻¹ COF NP solution increased from room temperature to 77 °C within 300 s under 671 nm laser irradiation with an intensity of 2.0 W cm⁻², achieving a PTCE of approximately 13.7%. The triple therapy combining PTT, PDT, and drug release shows the best treatment effect, which achieved the smallest tumor size and the lightest weight (Fig. 16g–j). Notably, the PTT of COF NPs played a more important role than PDT.

COF_FePc nanosheets exhibit strong NIR absorption and acid-insensitive photothermal effects.¹⁹³ Under 808 nm laser (2 W cm⁻²) irradiation, the temperature of 200 μg mL⁻¹ COF_FePc solution increases from 25 °C to 53.2 °C with a PTCE of 38.7%. The acidic environment (pH 3.5–7.4) has no significant influence on the photothermal effect, ensuring stability in the lysosomal environment. In the 4T1 tumor model, the tumor volume of the COF_FePc + 808 nm group decreases by ∼80% compared with the PBS group, and the tumor weight is reduced to 20% of that in the PBS group after 14 days.

Zhang and colleagues synthesized a new material (COF_Zn@HPTA) which can intelligently regulate local blood glucose levels and promote wound healing, thereby expressing potential for treatment of diabetic wounds.¹⁹⁴ The COF_Zn shows glucose oxidase-like behavior due to its redox-active sites to reduce the blood glucose level (Fig. 16k). Besides, photothermal conversion of the COF_Zn@HPTA hydrogel assists the antibacterial effect against Escherichia coli and Staphylococcus aureus. When the COF concentration was 400 μg mL⁻¹, the killing rate of both bacteria exceeded 95%, effectively inhibiting bacterial infections in diabetic wounds.

Shui and co-workers designed a hypoxia-responsive COF nanoplatform namely UAPiGCH.¹⁹⁵ The COF contains azo ligands, which can be cleaved by bioreductases overexpressed in the hypoxic tumor microenvironment. This leads to the collapse of the COF skeleton and release of encapsulated drugs, in which, UAPi exhibits good photothermal conversion performance in the NIR-II region. Under the irradiation of a 1064 nm laser with a power density of 1.0 W cm⁻² for 5 min, 100 μg mL⁻¹ of UAPi reached a temperature of 50.8 °C, with a PTCE of 41.9% (Fig. 16l and m). Cyclic “heating–cooling” tests confirmed the stability of its photothermal conversion.

5.3.3. Imaging. The stimuli-responsive luminescence of COFs enables the enrichment and signal change at the desired sites in the biological system. Moreover, the capability of combining drug loading and therapy provides COFs with unique advantages in multi-function integrated applications.

Zhang et al. synthesized a multi-responsive fluorescent TFPB-PI (F)-COF.¹⁴⁴ It achieves highly selective detection of Cu²⁺ and Al³⁺ through coordination interactions and chelation-enhanced quenching effects and can be applied to visualization of Cu²⁺ and PO₄³⁻ in living cells. When the concentration of TFPB-PI (F)-COF reached 100 μg L⁻¹, the survival rate of HeLa cells after 24 hours of incubation still exceeded 85%, indicating its good biocompatibility and applicability for cell imaging. Observation under a confocal fluorescence microscope with 418 nm laser excitation showed intact cell morphology under a bright field. In the blue fluorescence channel, COF-treated cells exhibited bright fluorescence, which significantly weakened after adding Cu²⁺ and recovered after further addition of PO₄³⁻ (Fig. 17a). The uniform distribution of intracellular fluorescence signals indicated that the COF can effectively enter cells and respond to ion changes.

Fig. 17 Imaging. (a) Confocal fluorescence images of TFPB-PI(F)-COF-loaded HeLa cells in PBS (pH 7.4) incubated with Cu²⁺ for 30 min and subsequently PO₄³⁻ for an extra 30 min. Reproduced with permission.¹⁴⁴ Copyright 2023, Elsevier. (b) Luminescence image of the TPE-ss COF material at different excitation wavelengths after addition of the sample to the medium. Ex vivo fluorescence images of (c) mice heart tissue and (d) a mouse model after intravenous injection of the TPE-ss COF, and (e) the corresponding cardiac tissue average fluorescence intensities. Reproduced with permission.¹⁹⁶ Copyright 2022, Wiley-VCH GmbH.

Gao and colleagues synthesized a COF nanocarrier, TPE-ss COF, based on aggregation-induced emission luminogens and redox-responsive disulfide bonds.¹⁹⁶ The TPE-ss COF emits blue light under 405 nm excitation with a photoluminescence quantum yield of 23%. The detectable fluorescence signal enables cellular or in vivo imaging to achieve therapy monitoring (Fig. 17b). The disulfide bonds in the framework can respond to high concentration GSH in myocardial ischemia/reperfusion (MI/R) tissues, triggering COF degradation and drug release. Simultaneously, the GSH response process can be real-time monitored through changes in fluorescence signals, achieving “imaging-therapy” integration (Fig. 17c–e).

The bio-applications of smart COFs demonstrate the excellent examples of utilizing multi-stimuli-responses.

5.4. Other applications

5.4.1. Controllable gas adsorption. Apart from the selective separation of ions, the isomerism of azo groups also enables the controlled gas adsorption. [4F-Azo]X-TPB-DMTP-COFs exhibit good CO₂ adsorption and storage capability, which is tunable under different configurations achieved by a photo-sensitive azo unit (Fig. 18a and b).⁵⁶ Specifically, the azo groups in the cis form occupy less space than that in the trans form, thereby exhibiting larger specific surface areas. The transformation of two configurations can be triggered by irradiation at different wavelengths, achieving photodynamically controlled adsorption. The BET surface area of [trans-4F-Azo]_0.50-TPB-DMTP-COF is 1176 m² g⁻¹, which increases to 1439 m² g⁻¹, after transforming to the cis form. Meanwhile, the pore sizes increase from 2.31 and 2.81 nm to 2.59 and 2.95 nm, respectively (Fig. 18c and d).

Fig. 18 Other applications. N₂ sorption isotherms of (a) [trans-4F-Azo]X-TPB-DMTP-COFs and (b) [cis-4F-Azo]X-TPB-DMTP-COFs (X = 0.17, red curve; X = 0.34, blue curve; X = 0.50, sky blue curve; X = 0.67, green curve; X = 1.0, black curve). Pore size (black dots) and distribution (blue dots) profiles of (c) [cis-4F-Azo]0.50-TPB-DMTP-COF and (d) [trans-4F-Azo]0.50-TPB-DMTP-COF. Reproduced with permission.⁵⁶ Copyright 2024, Wiley-VCH GmbH. (e) ζ potential curves of CAP@CS and TpPa-1 at different pH values (4–8.5). Reproduced with permission.¹⁹⁷ Copyright 2023, American Chemical Society. (f) Tafel polarization curves and (g) Nyquist plots of the Mg alloy, PES coating, TpBD coating, 2-MB coating, MB-Ag-COF-1 coating, MB-Ag-COF-2 coating, and MB-Ag-COF-3 coating in the 3.5 wt.% NaCl solution. (h) 3D current density maps recorded using the scanning vibrating electrode technique (SVET) and Nyquist plots over the artificial crack on respective PES and MB-Ag-COF-2 self-repairing coatings. Reproduced with permission.¹⁹⁸ Copyright 2024, Wiley-VCH GmbH. (i) Conversion versus time plot with the COF as the photocatalyst by alternating irradiation. Reproduced with permission.⁶⁰ Copyright 2022, American Chemical Society.

Similarly, the photo-responsive COF Ph-An-COF undergoes a structural transformation under light irradiation to form dimers.⁶⁴ Its BET surface areas can be tuned from 1864 m² g⁻¹ to 1456 m² g⁻¹. Meanwhile, the fluorescence intensity reduces significantly, demonstrating the controllable gas storage with optical monitoring.

5.4.2. Corrosion protection and self-repair. The electrostatic interactions of TpPa-1 can be modulated by pH to achieve controlled chemical release. Chen et al. loaded the capsaicin@chitosan (CAP@CS) microcapsules onto TpPa-1 through physical adsorption to construct the CAP@CS/TpPa-1 composite, which could be dispersed in fluorocarbon resin to form an intelligent coating.¹⁹⁷ At a pH of 5–7, both CAP@CS and TpPa-1 carry positive charges, with an adsorption capacity of ∼105 mg g⁻¹ and a maximum CAP release amount of 49.37 ppb after 5 hours. When pH exceeds 7, CAP@CS remained positively charged, while TpPa-1 became negatively charged, and the ζ potential difference reached 35 mV at a pH of 8.5, thus generating maximum electrostatic attraction (Fig. 18e). This indicates the excellent tunability of CAP release via pH control. TpPa-1 improves the dispersibility of CAP@CS and acts as a physical barrier to extend the propagation path of corrosive media, endowing the coating with excellent antibacterial and anticorrosive properties.

2D imine COFs (TpPA, TpBD, and TpTD) with precisely regulated pore sizes of 1.5–2.9 nm were developed by Wang and colleagues.¹⁹⁸ After loaded with Ag⁺ and the corrosion inhibitor (2-mercaptobenzimidazole, 2-MB), they form composite coatings with polyethersulfone (PES) to protect low-density magnesium alloys. When the coating contacts environmentally existing corrosive anions (Cl⁻, Br⁻, SO₄²⁻, etc.), Ag ions in the COF pores can react with corrosive ions to form precipitates to reduce the contact between corrosive media and the magnesium alloy substrate. Nanofluidic channels enable rapid release of 2-MB with a release rate as high as 141.2 mg g⁻¹ min⁻¹, which is 200% faster than that of traditional carriers like carbon nanotubes. When the coating is damaged, released 2-MB forms a passivation film with magnesium ions (Mg²⁺) at the crack, blocking corrosion paths and achieving autonomous coating repair. In a 3.5 wt% NaCl solution, the corrosion current density of the MB-Ag-COF-2 coating is as low as 3.73 × 10⁻¹⁰ A cm⁻², which is 3 orders of magnitude lower than that of a pure PES coating (Fig. 18f). Besides, the impedance modulus of the coating at a low frequency of 0.1 Hz reaches 6.3 × 10³ kΩ cm⁻², also much better than that of the pure PES coating (Fig. 18g). After damage by an ∼80 μm-diameter crack, MB-Ag-COF-2 begins releasing 2-MB within 5 minutes, accompanied by the drop of corrosion current density from 3.8 × 10⁻⁶ A cm⁻² to 5.3 × 10⁻⁷ A cm⁻² within 7 hours (Fig. 18h). The high repair efficiencies of nearly 100% demonstrates the broad adaptability of this composite to severe corrosive environments.

5.4.3. Photo-switchable catalysis. ¹O₂ is essential in some oxidation reactions, such as conversion of benzylamine to N-benzylidene.^199,200 The photo-responsive COF developed by Jiang et al. contains dithienylethene moieties.⁶⁰ When dithienylethene is in open form (o-COF), the higher energy level than porphyrin induces the energy transfer from porphyrin to oxygen to generate ¹O₂. Therefore, the o-COF can catalyze the conversion of benzylamine to N-benzylidene under blue light irradiation with ultrahigh yield exceeding 99% (Fig. 18i). In contrast, the nitrogen atmosphere dramatically decreases the yield to 7.5%, illustrating the significance of photogenerated ¹O₂ from o-COF. The counterpart in closed form has a low energy level thereby capturing the energy from porphyrin and leading to limited ¹O₂ generation, which results in much lower conversion yield.

Conclusions and challenges

The past two decades have witnessed significant advances in COFs, particularly as smart materials with broad applicability. This review has systematically summarized the response mechanisms, design strategies, and application developments of stimuli-responsive COFs. Their customizable molecular structures, high specific surface area, and well-defined porous channels endow smart COFs with abundant and controllable responsive behaviors. Responsiveness can be achieved through not only the introduction of functional groups, but also intrinsic skeletal design via different mechanisms. Moreover, the rapid development of reticular chemistry and high functionalization flexibility of COFs have facilitated the realization of multi-stimuli-responsive systems. Beyond traditional application fields such as sensing and detection, stimuli-responsive COFs have demonstrated exceptional performances in biomedicine and intelligent devices. The integration of sophisticated functions through multi-stimuli-responsive COFs offers tailored solutions to meet the demanding requirements of practical applications, exhibiting particular promise in disease diagnosis and therapy.

Though huge advances have been achieved in stimuli-responsive COFs, there still exists some deficiencies that restrict their applications.

(1) The dynamic linkages of COFs with moderate stability provide opportunities for the controllable drug release. In contrast, the operational durability of optoelectronic devices necessitates robust material stability to ensure the long-term performance. While several highly stable linkages including vinyl, dioxane, and all-aryl have been developed, their further functionalization and integration with active groups remain challenging. Therefore, designing new skeletons that simultaneously achieve high stability and facile modification is an urgent need.

(2) Currently, the multi-stimuli-responsive COFs remain scarce. One key limitation lies in the predominant reliance on molecular-level modifications to achieve stimulus responsiveness, rather than leveraging condensed matter levels such as packing arrangements or interlayer interactions. As a result, stimuli-responsive behaviors that depend on bulk material properties are underdeveloped,²⁰¹ e.g., mechano-responsive COFs. Only a few examples of piezofluorochromic COFs have been proposed and their performances were measured using a diamond anvil cell, and stretch-responsive COFs have yet to be realized. Hence, digging the potential of packing and intermolecular interactions would provide more opportunities in multi-stimuli-responses of COFs.

On the other hand, the integration of stimuli-responsive COFs with other smart materials, such as metal–organic frameworks,^202–204 hydrogen-bonded organic frameworks,^205–207 and 2D Mxene,^208–212 offers a promising alternative pathway. Such composite materials can circumvent the synthetic challenges associated with post-modification of individual components while enabling synergistic multi-responsive behavior. This approach facilitates the creation of sophisticated material systems that combine diverse stimulus-actuated functionalities without requiring intricate chemical functionalization of a single framework.

(3) The controllable synthesis of highly crystalline COFs and fabrication of large-area oriented COF films remain significant challenges. Currently, most COF syntheses rely on solvothermal methods, which are often limited in scalability and yield materials with non-uniform crystallinity. Furthermore, the introduction of functional groups via post-modification frequently compromises the crystallinity; for instance, the porosity of aminated COF decreases by more than 30% after protonation, affecting responsive performance. Despite these obstacles, several pioneering studies have demonstrated routes toward large-scale synthesis. Besides, the cost-effectiveness of COFs has been significantly improved. For instance, the production costs of NKCOF-41²¹³ and NKCOF-62,²¹⁴ developed by Zhang et al. at the kilogram-scale are estimated at approximately $50 per kg and $69 per kg, respectively. These values are significantly lower than those of conventionally lab-synthesized COFs, which typically range from $500 to $1000 per kg. Very recently, Zhang et al. summarized the advances in industrial production of COFs.²¹⁵ The development of scalable synthetic protocols that concurrently allow efficient and structurally benign functionalization could effectively overcome these limitations and pave the way for advanced applications.

Nevertheless, recent advances in stimuli-responsive COFs remain highly promising. Progress in synthetic methods, coupled with the development of novel structural scaffolds and functional motifs, is enhancing the adaptive and intelligent capabilities of these materials. It is anticipated that the prosperity of smart COFs and their unique advantages will unlock significant opportunities across a broad spectrum of applications.

Author contributions

All authors contributed to the writing and revising of this review article.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Acknowledgements

S. X. thanks the financial support from the Natural Science Foundation of Jiangsu Province (BK20240640), the Jiangsu Specially-Appointed Professor Plan, the Natural Science Research Start-up Foundation of Recruiting Talents of Nanjing University of Posts and Telecommunications (NY224003), and the Hua Li Talents Program of Nanjing University of Posts and Telecommunications. Q. Z. acknowledges the financial support from the City University of Hong Kong (7020148; 9239116; 9240189; 9380117; 9678403; 9680375; R-IND26401 and R-IND26402) and the Hong Kong Branch of National Precious Metals Material Engineering Research Center (NPMM).

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