A Hydrogen Bonded Organic Framework Constructed from Mixed Valence Fe Clusters for Efficient H₂O₂ Photosynthesis

Abstract

A critical bottleneck in photocatalytic H₂O₂ production lies not only in the severe non-radiative energy losses incurred from enhancing light absorption, but also in a fundamental proton-electron kinetic imbalance. Here we introduce a light–heat–proton coupling strategy that harnesses the dissipated photothermal energy to activate proton dissociation from carboxylic acids, thereby creating a productive driving force for redox catalysis. A hydrogen-bonded organic framework (HOF-FJU-200) incorporating mixed-valence Fe²⁺/Fe³⁺ clusters is constructed via a metalloligand approach. The intervalence charge transfer transitions within these clusters generate strong photothermal conversion while extending light absorption to the NIR-II region ( ~ 2500 nm). The resulting thermal energy activates proton dissociation from unpaired carboxylic acid groups, synchronizing proton release with photoinduced electron transfer. This cooperative mechanism effectively channels non-radiative heat into catalytic function, achieving an H₂O₂ production rate of 10657 μmol·g⁻¹·h⁻¹ without sacrificial agents. By directly coupling light, heat, and proton dynamics within a single framework, this work establishes a general paradigm for utilizing non-radiative energy to regulate proton-driven photocatalytic reactions.

Photocatalytic H₂O₂ production is limited not only by severe non-radiative energy losses but also by a fundamental imbalance between proton and electron transfer kinetics. Here, the authors develop a light–heat–proton coupling strategy that exploits dissipated photothermal energy to promote proton dissociation from carboxylic acids, creating a driving force for efficient redox catalysis and H₂O₂ generation.

Introduction

Hydrogen-bonded organic frameworks (HOFs) have recently emerged as promising multifunctional materials, although their development still lags behind that of metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs)^1–9. The uniqueness of HOFs, particularly the easy regeneration through simple recrystallization, solution processability, intrinsic self-healing capability, and more preferential flexible-robust nature, has enabled scientists to develop a series of HOFs for their applications on gas storage, gas separation, molecular recognition, proton conduction, and sensing^2,10–13. It is envisioned that HOFs can be developed as efficient photocatalysts as well, although this potential has been rarely fulfilled yet¹⁴.

Photocatalytic hydrogen peroxide (H₂O₂) synthesis proceeds without sacrificial agents is an appealing route toward sustainable oxidant production, avoiding the energy-intensive anthraquinone process. To achieve efficient photocatalytic performance, materials must absorb a broad spectrum of light, particularly extending into the visible and near-infrared regions, to maximize the utilization of solar energy¹⁵. Traditional strategies to extend optical absorption, including band-gap narrowing in inorganic semiconductors and hybrid frameworks, often come at the cost of weakened redox potentials and heightened non-radiative recombination¹⁶. As a result, although heat can in some cases enhance reaction rates, a significant portion of the absorbed energy is lost through thermal dissipation pathways rather than being harnessed for chemical conversion.

Furthermore, the central challenge extends beyond light absorption to the precise control of the intricate pathways that govern energy utilization and dissipation. The photocatalytic synthesis of H₂O₂ proceeds primarily via two half-reactions: the oxygen reduction reaction (ORR) and the water oxidation reaction (WOR). The desired 2-electron ORR pathway (O₂ + 2H⁺ + 2e⁻ → H₂O₂) features intrinsically fast kinetics, which allows for the efficient utilization of photogenerated electrons. In contrast, the complementary WOR (via the 2-electron: 2H₂O + 2 h⁺ → H₂O₂ + 2H⁺ or 4-electron: 2H₂O + 4 h⁺ → O₂ + 4H⁺) is intrinsically kinetically sluggish¹⁷. Therefore, a critical bottleneck arises from the common kinetic mismatch between proton generation and electron transfer, particularly under illumination (Fig. 1a). The rate of proton generation often lags behind electron transfer kinetics, creating a local proton-deficient microenvironment at the catalytic sites¹⁸. This imbalance severely throttles the ORR pathway and limits the overall light-to-chemical efficiency.

Fig. 1. Schematic comparison.

a Conventional photocatalytic routes. b Our proposed system for H₂O₂ photosynthesis from water and oxygen, which proceeds without any sacrificial agents.

HOFs featuring diverse hydrogen-bonding motifs and abundant π–π interactions, may provide an intrinsic platform to overcome this kinetic limitation. The dynamic and reversible nature of hydrogen bonds allows efficient proton dissociation and transfer, while π–π stacking among aromatic units promotes charge migration and suppresses charge recombination^19–22. We hypothesize that in a specially designed HOF with strong near-infrared (NIR) absorption, the thermal energy generated from non-radiative relaxation can be harnessed to accelerate proton dissociation, thereby synchronizing proton and electron transfer during photocatalysis. Such a light–heat–proton coupled mechanism enables photogenerated carriers and thermally activated protons to act cooperatively, substantially boosting the photocatalytic efficiency for H₂O₂ production (Fig. 1b). While photothermal-enhanced H₂O₂ production has been demonstrated in polymeric and single-atom catalytic systems, for example by Ye et al. and Wang et al.^23,24, and NIR-responsive HOFs for photothermal conversion have been reported by Su et al.²⁵, and proton-promoted H₂O₂ synthesis has been previously described by Han et al.²⁶, the “light–heat–proton coupling” mechanism has not been reported before and represents a novel approach to harnessing non-radiative energy for catalytic reactions. The realization of this concept, however, requires a material that simultaneously possesses efficient photothermal centers and readily accessible proton sources, a combination that remains elusive²⁷.

Here we introduce a hydrogen-bonded organic framework, HOF-FJU-200, constructed through a metalloligand strategy involving mixed-valence Fe²⁺/Fe³⁺ clusters. The incorporation of these clusters endows the framework with exceptionally broad light absorption, extending from the visible to the near-infrared region, reaching up to 2500 nm through intervalence charge-transfer transitions. The π–π stacking interactions between adjacent ligands facilitate efficient intermolecular electron transfer, while the unpaired carboxyl groups act as proton sources that respond to the local photothermal effect. The combination of these features enables a synergistic interaction between photoexcitation and thermally induced proton dynamics. As a result, HOF-FJU-200 achieves an outstanding H₂O₂ production rate of 10657 μmol·g⁻¹·h⁻¹ without the use of sacrificial agents, demonstrating an effective strategy for integrating photothermal and proton-coupled processes within a single soft framework material.

Results

Crystal structures of HOF-FJU-200

Dinuclear clathrochelate complexes bearing carboxylic acid functional groups (denoted as CLC-COOH) were successfully synthesized via metal-templated condensation reactions (Fig. 2a). These reactions involved the combination of 4-cyanophenylboronic acid and 4-hydroxy-3,5-((hydroxyimide)methyl) benzoic acid, in the presence of both Fe³⁺ and Fe²⁺ salts. Subsequently, high-quality black octahedral single crystals of HOF-FJU-200 were obtained by slowly diffusing methanol into a solution of CLC-COOH in N,N-Dimethylformamide (DMF) (Supplementary Fig. 1a). Notably, rapid pouring of methanol into the CLC-COOH-containing DMF solution yields uniformly sized HOF-FJU-200 nanoparticles (Supplementary Fig. 1b and Supplementary Fig. 1c). Single-crystal X-ray diffraction (SCXRD) analysis confirms that the framework of HOF-FJU-200 assembled from CLC-COOH molecules as the fundamental building blocks (Fig. 2b). X-ray photoelectron spectroscopy (XPS) indicates that the iron species in HOF-FJU-200 retain a mixed-valence state (Fe²⁺/Fe³⁺) (Supplementary Fig. 2), which is also commonly observed in stable inorganic compounds such as Magnetite (Fe₃O₄) and Prussian blue. Electrostatic Potential Surface (ESP) calculations reveal that cyano groups exhibit regions of pronounced negative electrostatic potential due to the electron-withdrawing effect, while carboxylic acid-substituted benzene displays positive potential areas (Fig. 2c). This complementary charge distribution may significantly enhance the interactions between the two systems through π–π stacking. The weak hydrogen bonds formed between the oxygen atoms of boric acid and the CH groups on adjacent molecules further stabilize this assembly (Fig.1d, Supplementary Fig. 3).

Fig. 2. Crystal structures of HOF-FJU-200.

a Schematic of the synthesis of CLC-COOH monomer. b The structure of CLC-COOH monomer. c The calculated electrostatic potential surface of CLC-COOH monomer. d Illustration of CH···OB hydrogen bonds and π-π interaction between two adjacent CLC-COOH molecules in HOF-FJU-200. e The structure of the tetramer in HOF-FJU-200 formed from four CLC-COOH monomers and its polyhedral model. f The total polyhedral model, highlighting a dia topological structure. g The total structure of HOF-FJU-200 and (h) highlighting unpaired R-COOH groups with the channel. (Gray, carbon, blue, nitrogen, pink, oxygen, yellow, boron, and purple, Fe). All H atoms, except those on the carboxylic acid group, are omitted for clarity.

In fact, each of the four CLC-COOH molecules contributes only one benzonitrile group for the formation of the tetramer, leaving one remaining benzonitrile group on each molecule available to participate in the construction of adjacent tetramers (Fig. 2e). Consequently, each triangular-pyramidal tetramer assembles with four neighboring triangular-pyramidal tetramers in the four corner-facing directions by sharing CLC-COOH molecules (Fig. 2e). This connection pattern continues, resulting in the formation a dia topological structure (Fig. 2f). Thus, HOF-FJU-200 can be regarded as π-interaction-driven hierarchical three-dimensional (3D) superstructures, which is quite challenging to achieve due to limited directional control and the weakness of intermolecular interactions²². Remarkably, unlike the conventional propensity of carboxylic-acid-based supramolecules to form hydrogen bonds, this unique stacking configuration does not depend on hydrogen-bonding interactions between carboxylic acids (Fig. 2e, h). This distinctive assembly mode endows HOF-FJU-200 with the rare characteristic of being a hydrogen-bonded organic framework featuring unpaired-carboxylate channels (Fig. 2f, g).

Unpaired-carboxylate channels have already been successfully engineered in MOF and COF families due to the directional assembly attributes^28,29. Research has shown that carboxyl groups can facilitate proton conduction^30,31. This function enables the conversion of superoxide (·O₂⁻), a key intermediate in H₂O₂ photocatalysis, into H₂O₂. However, in HOFs, these functional groups often act as either strong hydrogen-bond donors or acceptors^32,33. Consequently, they actively engage in intermolecular hydrogen bonding, contributing to framework formation and effectively immobilizing protons. This makes the preservation of unpaired carboxyl moieties exceedingly difficult in HOFs. Gao et al. reported a rare example of a HOF featuring unpaired carboxyl groups by utilizing paired residual carboxylic acids³⁴. The assembly of a carboxylic acid-based HOF without employing any hydrogen bonding from the carboxylic acids, thereby keeping all carboxyl groups unpaired, had never been achieved prior to HOF-FJU-200.

Photoelectric properties and band edge potentials

Therefore, this distinctive property renders HOF-FJU-200 a promising candidate as a potential photocatalyst for H₂O₂ production. Prior to photocatalyst study, esterified analogues of both the clathrochelate complex (CLC-COOCH₃) and its corresponding HOF framework (HOF-FJU-201) were synthesized (Supplementary Fig. 4a and Supplementary Fig. 4b). SCXRD analysis reveals that in HOF-FJU-201, CLC-COOCH₃ molecules form dimers stabilized primarily by π–π stacking between cyanophenyl groups and methyl-ester-carboxylate phenyl rings (Supplementary Fig. 4c). These dimers further assemble via C–H···O hydrogen bonding into 1D porous channels (Supplementary Fig. 4d–g). Solid-state UV-Vis-NIR diffuse reflectance spectroscopy reveals dual-range light harvesting capabilities in these HOFs, demonstrating both strong visible absorption and pronounced NIR-II response (1000-2500 nm region, Fig. 3a). This near-infrared absorption primarily originates from the mixed-valence Fe²⁺/Fe³⁺ dinuclear sites that enable efficient NIR absorption via intervalence charge transfer (IVCT), a well-established phenomenon in Fe²⁺/Fe³⁺ systems such as Magnetite (Fe₃O₄) and Prussian blue analogs³⁵. Tauc plot analyses reveal optical band gaps of 1.54 eV (HOF-FJU-200) and 1.43 eV (HOF-FJU-201) (Supplementary Fig. 5), which are substantially narrower than those of conventional HOFs ( > 2.0 eV). This narrowing enables HOF-COOR systems (R = H, CH₃) to efficiently harness low-energy solar photons, significantly advancing their potential for photon-functional applications.

Fig. 3. Photoelectric properties and band edge potentials.

a UV/Vis-NIR DRS spectra of HOF-FJU-200 and HOF-FJU-201. b Illustration of electronic band structures of HOF-FJU-200 and HOF-FJU-201. c EIS Nyquist plots of HOF-FJU-200 and HOF-FJU-201. d Transient photocurrent density of HOF-FJU-200 and HOF-FJU-201.

Mott-Schottky measurements confirm n-type semiconductivity of the materials (positive slopes, Supplementary Fig. 6), with flat-band potentials at –0.78 V (HOF-FJU-200) and –0.65 V vs. Ag/AgCl (HOF-FJU-201). After conversion to the normal hydrogen electrode (NHE) using the factor +0.198 V, these values become –0.58 V for HOF-FJU-200 and –0.45 V vs. NHE for HOF-FJU-201. For n-type semiconductors, the conduction band is typically at least 0.10 eV more negative than the flat-band potential. Consequently, the conduction band (CB) positions were calculated to be –0.68 V for HOF-FJU-200 and –0.55 V vs. NHE for HOF-FJU-201. The favorable CB positions of HOF-FJU-200 and HOF-FJU-201 indicate their potential for H₂O₂ production via the O₂ reduction reaction (ORR) (Fig. 3b). The corresponding valence band (VB) positions at pH 7 (vs. NHE) are +0.86 V for HOF-FJU-200 and +0.88 V for HOF-FJU-201 (Fig. 3b). Electrochemical impedance spectroscopy (EIS) revealed a smaller semicircular Nyquist radius for HOF-FJU-200 compared to HOF-FJU-201 (Fig. 3c), indicating lower charge transfer resistance in HOF-FJU-200. Under photostimulation, HOF-FJU-200 exhibited a stronger transient photocurrent response than HOF-FJU-201, signifying superior efficiency in electron-hole separation and charge transfer within the HOF-FJU-200 framework (Fig. 3d). These photoelectrochemical differences likely account for the significant disparity in photocatalytic H₂O₂ production performance.

H₂O₂ production and photothermal performance

HOF-FJU-200 is stable in methanol, ethanol, isopropanol, n-propanol, toluene, diethyl ether, and even boiling water (Supplementary Figs. 7–11). It was treated with pure water for 10 exchange cycles to completely remove the guest solvent molecules within its pores (Supplementary Fig. 12), thereby eliminating their interference with the photocatalytic reaction. To evaluate their photocatalytic H₂O₂ production capabilities, reactions were conducted under visible light irradiation (λ > 420 nm; 300 W Xe lamp) in O₂-saturated water contained in 30 mL vials, without addition of any sacrificial agents. The concentration of generated H₂O₂ was quantified by titration with Ce(SO₄)₂. Strikingly, HOF-FJU-200 showcased a distinctive catalytic behavior that deviated from the established patterns commonly seen in prior photocatalytic materials. In most cases, the catalytic yield of these traditional materials usually follows either a linear progression with time or a gradual decrease. In contrast, during the initial half-hour, HOF-FJU-200 exhibited a relatively modest catalytic activity, registering a production rate of 4706 μmol·g⁻¹·h⁻¹. However, beyond this phase, its activity abruptly surged to 10657 μmol·g⁻¹·h⁻¹, transitioning into a sustained linear production regime (Fig. 4a). HOF-FJU-200 cannot directly utilize low-energy infrared photons (λ > 810 nm) to drive H₂O₂ production due to the band-gap limitation, whereas the 720–810 nm NIR light is effective (Supplementary Fig. 13). The apparent quantum yield (AQY) of HOF-FJU-200 was 11.58% at 420 nm, and at near-infrared wavelengths it reached 2.56% at 720 nm and 2.35% at 810 nm, demonstrating a broad spectral response and efficient use of NIR light for photocatalysis (Table S2). The solar-chemical conversion efficiency (SCC) of HOF-FJU-200 reached 0.162%. Furthermore, such self-accelerating performance exceeds all previously reported results for discrete molecule-based photocatalysts, including HOFs (Fig. 4c, Supplementary Tables 3–4). More significantly, it outperforms the best MOFs and rivals the top-performing COFs (Supplementary Tables 5–6). Single crystals of HOF-FJU-200 can be retained in an aqueous H₂O₂ solution at a concentration of 0.1 M, far exceeding the concentration generated during photocatalysis (0.0005 M), which rules out H₂O₂-induced structural degradation (See CIF file: HOF-FJU-200-H₂O₂, Supplementary Fig. 14). Post-reaction PXRD, FT-IR, SCXRD and XPS analysis confirmed unchanged crystallinity (Supplementary Figs. 15–19) and five consecutive photocatalytic cycles demonstrated >95% retention of activity with simple catalyst recovery, collectively attesting to the material’s robust operational stability (Supplementary Fig. 20). Turn-on/turn-off cycling experiments confirm that HOF-FJU-200 exhibits excellent anti-photobleaching property with no significant loss of activity over repeated cycles (Supplementary Fig. 21).

Fig. 4. H₂O₂ production and photothermal performance of photocatalysts.

a Temporal H₂O₂ photoproduction curves over HOF-FJU-200 and HOF-FJU-201 in O₂ and pure water under visible light. b The H₂O₂ evolution rates under photocatalysis by HOF-FJU-200, HOF-FJU-201, along with their corresponding monomers, CLC-COOH and CLC-COOCH₃. c Comparison of artificial H₂O₂ photosynthesis by HOF photocatalysts with HOF-FJU-200 (activation rate). d The H₂O₂ evolution rates of HOF-FJU-200 were measured under both light and dark conditions at varying temperatures. e Temperature change of HOF-FJU-200 solutions under visible light irradiation (λ > 420 nm; 300 W Xe lamp) over time. f Photothermal conversion plot of HOF-FJU-200 in solid state under 808 nm laser irradiation with power intensities of 0.1, 0.3 and 0.5 W cm⁻². g IR thermal images of HOF-FJU-200 solutions under visible light irradiation (λ > 420 nm; 300 W Xe lamp). h IR thermal images of HOF-FJU-200 in solid state under 808-nm laser irradiation (0.5 W cm⁻²) for 300 s, followed with the laser turned off. The error bars in (a, b, and d) represent the standard deviation over 3 individual experiments. Source data are provided as a Source Data file.

The production rate of HOF-FJU-200 (10657 μmol·g⁻¹·h⁻¹) vastly surpassed that of HOF-FJU-201 (718 μmol·g⁻¹·h⁻¹) (Fig. 4a, b). To decouple packing structural contributions, we assessed their monomeric molecules, CLC-COOH and CLC-COOCH₃. Powder X‑ray diffraction (PXRD) of the monomers shows that they are almost entirely amorphous at room temperature, indicating no long‑range periodic stacking (Supplementary Fig. 22). The CLC-COOH monomer exhibits substantially higher H₂O₂ production than the CLC-COOCH₃ monomer (3951 vs 562 μmol·g⁻¹·h⁻¹, respectively) (Fig. 4b). Notably, both HOFs outperform their monomers (HOF-FJU-200: 10657 vs 3951; HOF-FJU-201: 718 vs 562 μmol·g⁻¹·h⁻¹), demonstrating that it is the accessible pore channels within the HOF frameworks, rather than only the external surface molecules, that facilitate mass transport and contribute significantly to the enhanced catalytic generation of H₂O₂ (Fig. 4b).

Experimental mechanistic investigation

To elucidate the origin of this activity surge, we systematically evaluated HOF-FJU-200’s photocatalytic performance under controlled thermal conditions. In the absence of light, H₂O₂ production was negligible, demonstrating that light, rather than heat, is the indispensable component for catalysis (Fig. 4d). Under illumination, the activity exhibited Arrhenius-type enhancement with increasing temperature ( < 80 °C), confirming thermal acceleration of the catalytic kinetics (Fig. 4d). Significantly, the measured activity at 80 °C ( ~ 10000 μmol·g⁻¹·h⁻¹) precisely matched the post-surge rate observed in the kinetic profile. Under isothermal conditions at 20 °C to minimize the thermal effects, HOF-FJU-200 (with free –COOH) yielded 2406 μmol·g⁻¹·h⁻¹, whereas the esterified analogue HOF-FJU-201 (with –COOCH₃) yielded only 370 μmol·g⁻¹·h⁻¹. This trend persisted in the amorphous monomer controls: CLC-COOH gave 1385 μmol·g⁻¹·h⁻¹ while CLC-COOCH₃ gave 255 μmol·g⁻¹·h⁻¹. The consistent superiority of the carboxylic acid-containing species across both crystalline and amorphous states confirms that internal proton availability is critical for enhanced catalysis, independent of morphology or packing effects (Supplementary Fig. 23). Beyond this threshold, activity declined markedly due to thermally-driven H₂O₂ decomposition (2H₂O₂ → 2H₂O + O₂). Subsequent H₂O₂ decomposition tests confirmed this deduction: almost no decomposition occurred at 80 °C (Supplementary Fig. 24-25), while significant decomposition was observed at 100 °C (Supplementary Fig. 26). This temperature-dependent behavior, particularly the congruence at 80 °C, establishes that the abrupt activity surge originates from in-situ thermal elevation within the catalytic system during photocatalysis.

Under 808-nm laser irradiation (0.5 W/cm²), solid HOF-FJU-200 crystals rapidly heated to 166 °C within 10 seconds (heating rate >12 °C/s) (Fig. 4f–h), while the corresponding aqueous photocatalytic system reached 78 °C in 30 min (Fig. 4e–g). HOF-FJU-201 exhibits comparable photothermal conversion efficiency with HOF-FJU-200 (Supplementary Fig. 27). Fluorescence spectroscopy revealed negligible emission intensity from HOF-FJU-200 and HOF-FJU-201, indicating severe non-radiative decay under illumination (Supplementary Fig. 28). This complete quenching of emission is attributed to an intervalence charge transfer (IVCT) effect, which provides an efficient non-radiative decay pathway that suppresses radiative recombination³⁶. Consequently, the mixed-valence Fe²⁺/Fe³⁺ dinuclear sites in HOF-FJU-200 and HOF-FJU-201 enable ultra-efficient photothermal conversion through synergistic mechanisms: intervalence charge transfer (IVCT) broadens near-infrared (NIR) absorption, and polaron formation accelerates thermal relaxation³⁷. This synergistic interplay indicates a novel light–heat–proton coupling pathway in HOF-FJU-200, where light absorption, heat generation, and carboxylate-facilitated proton transport collectively drive the enhanced photocatalytic H₂O₂ production. In contrast, HOF-FJU-201 is constructed from ester-based monomers. Although it exhibits comparable photothermal efficiency, it lacks the ability to dissociate protons internally. Without an intrinsic proton source, the reaction must rely on inert water molecules to supply protons, a kinetically sluggish process that renders its self-accelerating catalytic behavior far less pronounced than that of HOF-FJU-200.

To further elucidate the mechanisms of H₂O₂ generation by HOF-FJU-200, a series of experiments was conducted to identify the key species. The proton conduction behaviors of HOF-FJU-200 and HOF-FJU-201 were analyzed using alternating current (AC) impedance spectroscopy on pelletized samples under controlled humidity and temperature (Supplementary Fig. 29). At 90% relative humidity (RH), the proton conductivity of both materials exhibited a significant increase with rising temperature (Fig. 5a, Supplementary Table 8). Neglecting the thermal decomposition of H₂O₂, these dissociated protons facilitate the photocatalytic reaction with superoxide radicals (·O₂⁻) to produce H₂O₂ (·O₂⁻ + 2H⁺+e⁻→H₂O₂). Notably, the proton conductivity of HOF-FJU-200 was significantly higher than that of HOF-FJU-201, with corresponding activation energy (Eₐ) values of 0.304 eV and 0.405 eV, respectively (Figs. 5a, b and Supplementary Table 8). This indicates that the presence of carboxylic acid groups facilitates proton dissociation, thereby promoting H₂O₂ synthesis, which is consistent with the substantially higher catalytic activity observed for HOF-FJU-200 compared to HOF-FJU-201.

Fig. 5. Experimental mechanistic investigation.

a Compare proton conductivity of HOF-FJU-200 and HOF-FJU-201 under 90% RH with different temperatures. b Arrhenius plots of HOF-FJU-200 and HOF-FJU-201 between 30 and 90 °C at 90% RH. Least-squares fitting are shown as a solid line. c H₂O₂ evolution rates at different conditions photocatalyzed by HOF-FJU-200. d EPR spectra of HOF-FJU-200 and HOF-FJU-201 in the dark and under visible light irradiation, using DMPO as the spin-trapping reagent. e, f In situ DRIFTS spectra acquired over different illumination times for the HOF-FJU-200 and HOF-FJU-201 photocatalytic systems, respectively. The error bars in c represent the standard deviation over 3 individual experiments. Source data are provided as a Source Data file.

H₂O₂ formation was negligible in the dark and significantly reduced under light illumination in a N₂ atmosphere (Fig. 5c), demonstrating that both O₂ and light are essential for photocatalytic H₂O₂ production. In the presence of sodium iodate (NaIO₃) and lanthanum oxide (La₂O₃), HOF-FJU-200 can photocatalyse H₂O to O₂ confirmed by online gas chromatography (GC) analysis (Supplementary Table 7). Scavenger studies with AgNO₃ (e⁻ acceptor), tert-butanol (TBA, ∙OH quencher), and p-benzoquinone (pBQ, ·O₂⁻ scavenger) demonstrated: TBA induced no significant activity suppression, ruling out ∙OH involvement; pBQ reduced H₂O₂ production to near-background levels, confirming ·O₂⁻ as indispensable; AgNO₃ caused substantial activity loss, verifying photogenerated electron participation (Fig. 5c). Parallel control experiments on HOF-FJU-201 showed similar mechanistic dependencies. EPR spectroscopy with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin-trapping revealed no detectable ·O₂⁻ signals in darkness but distinct DMPO-·O₂⁻ adducts upon illumination, with HOF-FJU-200 exhibiting greater signal intensity than HOF-FJU-201, which correlates positively with its superior H₂O₂ performance (Fig. 5d). This integrated evidence establishes a sequential two-electron oxygen reduction reaction (ORR) pathway: O₂ + e⁻→·O₂⁻ followed by ·O₂⁻ + 2H⁺+e⁻→H₂O₂, where photogenerated electrons and superoxide radicals constitute fundamental active species.

In situ Fourier Transform Infrared (FTIR) spectroscopy was employed to dynamically track intermediates formed during photocatalytic H₂O₂ generation over HOF-FJU-200 and HOF-FJU-201. The spectra were collected at 10-minute intervals under Xe lamp irradiation with continuous O₂ flow, using samples pre-wetted with water. Progressive spectral evolution revealed three key features: the O-O stretching vibration at ~945 cm⁻¹ intensified with irradiation time (Fig. 5e, f), while emerging peaks at ~1177 cm⁻¹ (assigned to ·O₂⁻) and ~1237 cm⁻¹ (OOH*) confirmed superoxide radical and hydroperoxyl intermediate formation. In addition, The vibrational band observed in the 1000–1030 cm⁻¹ range, assigned to the C–O–O stretching mode, confirms the adsorption of O₂ on aromatic carbon sites³⁸. The progressive enhancement of the peroxide (·O₂⁻) intermediate-derived C–O ( ~ 1280–1290 cm⁻¹) and O–O ( ~ 1050–1060 cm⁻¹) stretching bands, is accompanied by a cumulative enhancement of H₂O₂-associated O–O bending vibration ( ~ 1320–1340 cm⁻¹)³⁹. Furthermore, the vibrational feature observed in the 1095–1105 cm⁻¹ region are ascribed to intermediates of water oxidation⁴⁰. Notably, HOF-FJU-200 exhibits a more pronounced intensity of ·O₂⁻ signal than HOF-FJU-201. This enhancement can be attributed to the deprotonated carboxylic acid groups of HOF-FJU-200, which promotes efficient and sustained electron transfer to O₂, thereby favoring the continuous formation and accumulation of ·O₂⁻ species.

Significantly, new vibrational peaks associated with –COO⁻ groups appeared near 1430 cm⁻¹ in HOF-FJU-200 under illumination (Fig. 5f). This indicates that a fraction of the carboxyl groups in HOF-FJU-200 underwent proton dissociation during the photocatalytic reaction, undergoing reversible interconversion between –COOH and –COO⁻, which aligns with our earlier hypothesis. This phenomenon was not observed for HOF-FJU-201. Collectively, these time-resolved vibrational signatures unambiguously corroborate the proposed two-electron ORR pathway (O₂ → ·O₂⁻ → OOH* → H₂O₂). Rotating ring-disk electrode (RRDE) experiments were performed to investigate the selectivity toward the oxygen reduction reaction (ORR) pathway. Linear sweep voltammetry (LSV) curves of two HOFs were recorded in an O₂-saturated 0.1 M phosphate buffer solution at different rotating speeds (Supplementary Fig. 30a, b). The calculated n value for HOF-FJU-200 was 2.01, which is closer to 2 than that of HOF-FJU-201 (2.34), indicating that HOF-FJU-200 exhibits higher O₂-to-H₂O₂ selectivity under the same conditions (Supplementary Fig. 30 c).

Density functional theory calculations and proposed reaction pathway

Density functional theory (DFT) calculations comparing the reaction pathways of HOF-FJU-200 and HOF-FJU-201 revealed critical mechanistic insights. While HOF-FJU-200 frameworks feature potential catalytic sites (–COOH, –CN, –CH groups), the carboxylic acid moiety demonstrates energetically favorable catalysis compared to alternative sites (Fig. 6a and Supplementary Fig. 31), establishing –COOH as the dominant active center. In HOF-FJU-201, the carbonyl group also represents the most suitable catalytic site (Fig. 6a and Supplementary Fig. 32). It is well established that the *OOH intermediate is a crucial step in H₂O₂ synthesis. Crucially, during the conversion from *O₂ to *OOH, HOF-FJU-200 exhibits a significantly lower energy barrier compared to HOF-FJU-201 (Fig. 6b and Supplementary Figs. 3 and 34). Additional calculations demonstrate that the free carboxyl groups within the framework, once deprotonated, can significantly lower the energy barriers (ΔG = 0.07420 eV) of the critical reaction steps compared to their non-deprotonated counterparts (ΔG = 0.31250 eV). Although the energy barrier of the deprotonated HOF-FJU-200 increases slightly with temperature (Supplementary Fig. 35, Supplementary Table 9), the values remain much smaller than those of the non-deprotonated system. This confirms that the decisive factor underlying the superior performance of HOF-FJU-200 is the temperature-promoted deprotonation of carboxylic acid groups, which supplies protons on demand to the catalytic sites. Therefore, based on experimental and computational results, a possible mechanism is proposed (Fig. 6c): Initially, O₂ is adsorbed onto the -COOH sites. Upon light irradiation, the temperature of the HOF framework increases, promoting proton generation through dissociation of carboxylic acid groups and H₂O. Simultaneously, the organic segment generates photogenerated electrons, and then adsorbed O₂ obtains the electron and proton to generate *OOH intermediate. Subsequently, *OOH acquires a second proton and electron to produce H₂O₂. This proton transfer process can also facilitate water dissociation, boosting O₂ evolution.

Fig. 6. Density functional theory calculations and proposed reaction pathway.

a The calculated adsorption energies of O₂ at different active sites of HOF-FJU-200 and HOF-FJU-201 b Energy diagrams depicting the reduction pathway of adsorbed O₂ to H₂O₂ on the carbonyl sites of HOF-FJU-200 and HOF-FJU-201. c Proposed mechanism for the photosynthesis of H₂O₂ using HOF-FJU-200 as the catalyst.

In summary, this work establishes a light–heat–proton coupling strategy within a hydrogen-bonded organic framework to address the challenges in photocatalytic hydrogen peroxide (H₂O₂) synthesis. By incorporating mixed-valence Fe²⁺/Fe³⁺ clusters, this framework achieves broad optical absorption extending into the NIR-II region ( ~ 2500 nm) via intervalence charge transfer. This enhancement enables exceptional photothermal conversion efficiency, driving proton dissociation from carboxylic acid groups and promoting proton-coupled electron transfer during photocatalysis. This cooperative coupling enables a highly efficient and self-accelerating photocatalytic process, delivering a record H₂O₂ production rate of 10657 μmol·g⁻¹·h⁻¹ without sacrificial agents. By directly coupling light absorption, heat generation, and proton dynamics within a hydrogen-bonded organic framework, this study introduces a new paradigm that transforms non-radiative thermal loss into catalytic functionality, thereby offering a general blueprint for a wide range of proton-coupled electron transfer reactions beyond H₂O₂ synthesis.

Methods

Synthesis of HOF-FJU-200

Black polyhedral crystals of HOF-FJU-200 were obtained by dissolving 5 mg of CLC-COOH in 1 mL of DMF via sonication; the solution was then placed in a 20 mL vial containing 3 mL of methanol for a 5-day diffusion, after which the crystals were collected by filtration and washed with methanol.

Synthesis of HOF-FJU-201

Black strip crystals of HOF-FJU-201 were obtained by dissolving 5 mg of CLC-COOCH₃ in 1 mL of DMA via sonication; the solution was then placed in a 20 mL vial containing 3 mL of methanol for a 5-day diffusion, after which the crystals were collected by filtration and washed with methanol.

Synthesize nanoparticles of HOF-FJU-200

Black nanoparticles of HOF-FJU-200 were synthesized by first dissolving 50 mg of CLC-COOH in 10 mL of DMF with sonication. Subsequently, 50 mL of methanol was added immediately, and the mixture was stirred for 2 h. After the resulting suspension was allowed to stand for 24 h, the black nanoparticles were collected via centrifugation (three cycles) and washed with methanol.

Synthesize nanoparticles of HOF-FJU-201

Black nanoparticles of HOF-FJU-201 were synthesized by first dissolving 50 mg of CLC-COOCH₃ in 10 mL of DMA with sonication. Subsequently, 50 mL of methanol was added immediately, and the mixture was stirred for 2 hours. After the resulting suspension was allowed to stand for 24 h, the black nanoparticles were collected via centrifugation (three cycles) and washed with methanol.

Supplementary information

Supplementary Information ^{(3MB, pdf)}

Transparent Peer Review file ^{(2.1MB, pdf)}

Source data

Source Data ^{(7.4MB, xlsx)}

Author contributions

X.F. and R.Z. contributed equally to this work. X.F., B.C., and Z.Z. conceived the project and designed the experiments. R.Z., F.Y., and S. C.carried out the preparation, characterization, and electrochemical measurements of the HOF catalysts. X.F., R.Z., and S.X. prepared the figures. X.F., B.C., and Z.Z. analyzed the data and wrote the manuscript with input from all authors. All authors discussed the results and commented on the manuscript.

Peer review

Peer review information

Nature Communications thanks the anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Funding

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (22575048, 22101048, 22271046, and 22373015), the National Science Fund for Distinguished Young Scholars of China (22425102), the Natural Science Foundation of Fujian Province (2025J01628), and the Open Project of the National Key Laboratory of Structural Chemistry (20250002).

Data availability

Data are available from the corresponding authors upon request. The entries of CCDC 2476017, 2547150, and 2476018 contain the supplementary crystallographic data for HOF-FJU-200, HOF-FJU-200-H2O2, and HOF-FJU-201. These data can be obtained free of charge at http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12, UnionRoad, Cambridge CB2 1EZ, U.K. Fax: (Internet)+44-1223/336-033. E-mail: depos-it@ccdc.cam.ac.uk. All other data supporting the findings of this study are available in the Article and Supplementary information/Source data file. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at https://doi.org/10.1038/s41467-026-76362-1.