Tracking intermediates of photocatalytic hydrogen peroxide generation from a heptazine-based covalent organic polymer in water

Abstract

Metal-free organic photocatalysts with tailorable molecular structures that possess light-harvesting capability, tunable band gaps, accessible active sites and preorganized pathways for charge carrier migration are ideal candidates for generating H₂O₂ from O₂. To design such photocatalysts, it is imperative to formulate a structure–activity relationship through a detailed mechanistic understanding of photoexcited charge carrier generation, reactivity and dynamics. Inspired by the polymeric carbon nitride (PCN) backbone which is well known to stabilize anion radicals, here we have synthesized a new covalent organic polymer, RedCN, for photocatalytic H₂O₂ generation by integrating analogous heptazine-hydrazine moieties with 2,4,6-triformylphloroglucinol (Tp) via a β-keto-enamine linkage. RedCN exhibited remarkable activity for H₂O₂ generation in an aqueous medium with numbers reaching up to 1460 μmol per g per hour while maintaining high activity even in seawater. Broadband femtosecond transient absorption spectroscopy provides the first optical signature of the heptazine-Tp anion radical that decays reactively within ∼70 ps in the presence of O₂. Electron paramagnetic resonance (EPR) spectroscopy revealed the presence of both photogenerated radicals delocalized on the RedCN framework as well as the O₂ reduced superoxide intermediate. We subsequently used the H₂O₂ photoreaction scheme to carry out controlled oxidation of organic sulfides to sulfoxides. Our work therefore paves the way for rational design of heptazine-based photocatalysts for effective H₂O₂ production from O₂ while demonstrating promise for coupled photoredox catalysis in an aqueous medium under ambient conditions.

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Introduction

Hydrogen peroxide is an environmentally friendly oxidizing agent with diverse applications, including wastewater treatment,^1–3 chemical synthesis,^4–6 as a disinfectant, and as a potential energy carrier in fuel cells.^7–10 Its energy density is comparable to that of compressed hydrogen (H₂), positioning H₂O₂ as a promising clean energy alternative to H₂ due to its superior ease of storage and transportation.¹¹ Currently, H₂O₂ is primarily synthesized through the anthraquinone reduction–oxidation process,^12–14 a method that demands significant energy input, relies on precious metal catalysts such as palladium and nickel, and generates substantial waste and toxic byproducts.¹⁵ Consequently, the development of green and sustainable methods for H₂O₂ production is imperative.

Photocatalytic synthesis of H₂O₂ from water and oxygen under solar irradiation has emerged as a highly promising approach.¹⁶ This method not only aligns with principles of sustainability by utilizing abundant solar energy but also facilitates on-site and on-demand H₂O₂ production, thereby eliminating the need for its storage and transportation. Metal-free polymers have emerged as highly promising catalysts for solar-driven H₂O₂ production, attributed to their tunable molecular structures, superior selectivity for two-electron ORR, and minimal H₂O₂ decomposition activity compared to metal-based^17–19 inorganic semiconductors.^20–24 Among these, PCN, a polymer composed of heptazine motifs, stands out as a prominent metal-free organic semiconductor.^25,26 The heptazine moieties, characterized by their high nitrogen content, exhibit notable electron affinity and a conjugated π-electron system that governs the optical and electronic properties of PCN. Functionalized PCN frameworks are known in the literature to stabilized and store the excited state electrons in the form of their anion radicals.^27,28

Hirai and co-workers demonstrated that PCN is capable of producing H₂O₂ with good selectivity via the 2e⁻ ORR photocatalytic pathway.²⁹ However, its photocatalytic performance remains suboptimal due to inherent limitations such as low surface area, limited visible-light absorption (<450 nm), and rapid charge carrier recombination.³⁰ To enhance the photocatalytic efficiency of PCN, various modifications have been explored, including heterostructure formation,³¹ heteroatom doping,³² donor–acceptor engineering,³³ and defect introduction,^34,35 all aimed at improving its photocatalytic activity. Interestingly heptazine building blocks have shown immense promise in this photocatalytic reaction as highlighted by recent reports of hundreds of micromolar levels of H₂O₂ production per gram per hour.^36–38 However, despite these advancements, a comprehensive and systematic understanding of the heptazine core structure and the intrinsic factors governing its photocatalytic behaviour is missing. Mechanistic studies conducted thus far primarily focused on elucidating the reasons for improved activity by comparing the material's properties and performance before and after modification. Building on the unique structural features of PCN and addressing its intrinsic drawbacks, we propose a novel strategy that involves the rational integration of active heptazine motifs with synergistic functional units while unravelling the mechanism using spectroscopic techniques that track excited state catalytic intermediates and products. This approach seeks to unlock new potential in the design of high-performance photocatalysts for sustainable H₂O₂ production.

Herein, we synthesized a covalent organic polymer (COP) by integrating heptazine-hydrazine moieties with 2,4,6-triformylphloroglucinol (Tp) through a β-keto-enamine linkage, denoted as RedCN (Scheme 1). The resulting material absorbs the whole visible range and demonstrates enhanced photocatalytic H₂O₂ production via a two-step ORR pathway and a two-electron WOR pathway. Additionally, RedCN was found to maintain robust photocatalytic H₂O₂ generation in water over a wide pH range (3–11), with a good performance observed even in seawater. To elucidate the mechanistic aspects of the photocatalytic process, we employed electron paramagnetic resonance (EPR) and broadband femtosecond transient absorption spectroscopy, providing valuable insights into the reaction pathways and charge carrier dynamics. TA data showed the absorption signature of the heptazine-Tp anion radical for the first time in such a material, which decays faster under excess O₂ conditions. Moreover, we demonstrated the in situ catalytic oxidation of aromatic sulfides to sulfoxides in a water–isopropanol mixture, using the H₂O₂ generated within the reaction system. This work highlights the potential of RedCN as a versatile photocatalyst for sustainable H₂O₂ production and oxidative transformations and also provides mechanistic insights into H₂O₂ production.

Scheme 1 Schematic representation of the synthesis route used to obtain the photocatalyst, RedCN. The basic building block is the melem unit which forms the basis for all heptazine-based extended materials. Here, pTSA is p-toluenesulfonic acid.

Results and discussion

Synthesis and characterization of the photocatalyst (RedCN)

To synthesize the COP, the heptazine hydrazine molecule (melem-NH) was selected as the monomeric unit. Due to the inherent electron deficient nature of the heptazine rings, the nucleophilicity of the terminal amine groups is significantly reduced, thereby limiting their reactivity. To overcome this limitation, the terminal amines were chemically modified to hydrazine functionalities to enhance their reactivity. These modifications were done by performing a reaction between melem and aqueous hydrazine solution by the hydrothermal process mentioned in the ESI.† After synthesis, melem-NH is characterised by FTIR, ¹³C NMR and Raman spectroscopy (as shown in Fig. S1–S3†). RedCN was then synthesized through a condensation reaction between the monomer melem-NH and 1,3,5-triformylphloroglucinol (Tp) by a mechanochemical process. Fourier transform infrared (FTIR) spectroscopy was used to elucidate the structural information of RedCN. FTIR analysis revealed a significant decrease in the intensity of the N–H stretching peaks at 3220 and 3300 cm⁻¹, along with the emergence of a new C=C peak at 1582 cm⁻¹ and a C–N peak at 1279 cm⁻¹, not present in FTIR of melem-NH (as shown in Fig. 1(a)).^39,40 These observations confirm the successful polymerization of melem-NH with Tp. The presence of a C=O peak at 1632 cm⁻¹ further validated the formation of β-keto-enamine linkages. Additionally, a characteristic peak at 782 cm⁻¹, attributed to the heptazine breathing modes, and a peak at 1180 cm⁻¹, corresponding to the N–N bond, confirmed the incorporation of heptazine rings and hydrazine functionality in RedCN (as shown in Fig. 1(a)).^41,42 The XPS survey of RedCN shows peaks corresponding to O 1s, N 1s, and C 1s, confirming the presence of C, N and O elements (ESI Fig. S4(a)†). Upon deconvoluting the C 1s spectra, three distinct peaks are observed at binding energies of 284.5, 286.5, and 288.1 eV (as shown in Fig. 1(b)). The peak at 284.5 eV represents a mixture of C–C and C=C (C adventitious peak), which serves as the internal calibration peak for all elements in the XPS analysis. The peak at 286.5 eV corresponds to C=O, confirming that keto tautomerism occurred after imine bond formation in the material. The peak at 288.1 eV represents N–C=N (sp² carbon of heptazine), indicating the presence of heptazine moieties.^36,43 The N 1s spectrum exhibits three peaks: the peak at 404.6 eV corresponds to the C=N–C bond, the second peak at 400.2 eV is attributed to N–N, confirming the presence of hydrazine functionality, and the third peak, at a binding energy of 397.8 eV, is assigned to the N–H bond, consistent with literature reports (as shown in Fig. 1(c)).^44,45 The O 1s XPS data show two peaks at 530.1 eV and 528.7 eV (as shown in ESI Fig. S4(b)†). The peak at 530.1 eV corresponds to O=C,⁴⁶ similar to what is observed in the C 1s spectra, while the peak at 528.7 eV is likely due to non-stoichiometry in the system.⁴⁷ The XPS results provide further evidence, in agreement with FTIR analysis, confirming the formation of RedCN. To further confirm the keto–enol tautomerism and the chemical structure of RedCN, ¹³C CP/MAS solid state NMR was used. The peaks in the spectra, as shown in ESI Fig. S5,† at 180 ppm correspond to carbonyl carbon formed via keto–enol tautomerism. The peak at 151 ppm is attributed to carbon in the C–N bond and peaks at 103 and 100 ppm are due to the C=C bond formed in the structure. This further confirms the successful reaction between melem-NH and Tp. The peaks at 162 and 154 ppm correspond to the carbons in the heptazine units. A small peak at 191 ppm suggests that some free aldehyde groups from partially reacted Tp are also present. These observations from FTIR, XPS and ¹³C NMR collectively confirm the formation of the polymeric framework through the reaction between melem-NH and Tp.

Fig. 1 (a) FTIR of melem-NH (blue trace), Tp (orange trace) and RedCN (red trace); XPS of RedCN. (b) Deconvoluted C-1s XPS spectra of the sample; (c) deconvoluted N-1s spectra of the sample.

The powder X-ray diffraction (PXRD) pattern, as shown in the ESI (Fig. S6(a)),† demonstrated the amorphous nature of RedCN, characterized by broad diffraction peaks. Two prominent peaks were observed at 2θ values of 11.6° and 27.2°, which were assigned to the in-plane porosity and the π–π stacking of the 2D layers of RedCN, respectively. The interlayer spacing between the 2D layers was calculated to be 3.2 Å, further confirming the structural arrangement of the material. The PXRD of melem-NH (as shown in Fig. S6(b)†) demonstrated sharp peaks confirming that melem-NH is crystalline in nature. This indicates a reduction in the crystallinity of melem-NH upon polymer formation.

To assess the surface area and pore size distribution of RedCN and melem-NH, the N₂ adsorption–desorption experiment was conducted at 77 K. The analysis shows that RedCN and melem-NH exhibit a typical Type III isotherm. This type of isotherm is generally for mesoporous materials with a hysteresis loop at high partial pressure (Fig. S7†). The calculated Brunauer–Emmett–Teller (BET) surface area of RedCN is 208 m² g⁻¹ which is much higher than that of melem-NH (7 m² g⁻¹) (shown in Fig. S8†). The BJH pore size analysis suggests that RedCN exhibited a broad pore size distribution with maximum pore sizes of 3 nm to 8 nm, which shows compromised crystallinity (ESI Fig. S7(a)†). Scanning electron microscopy (SEM) images of RedCN show an extended porous network with flake like morphology whereas melem-NH has 2D flake like morphology (Fig. S9†). The surface of RedCN displays pores of various sizes, indicating a heterogeneous distribution (Fig. S9(a) and (b)†). The results from SEM, XRD and BET surface analysis suggest that RedCN possesses a disordered architecture useful for applications requiring extensive surface interaction.

To evaluate the potential photocatalytic activity of the prepared RedCN, UV-Vis diffuse reflectance spectroscopy (DRS), EPR and photocurrent measurements were done. The absorption edge of melem-NH is up to 450 nm whereas RedCN exhibits robust and wide range absorption covering the whole UV to visible range, suggesting its enhanced ability to harvest light (Fig. 2(a)). Using the Kubelka–Munk function, the band gap of melem-NH and RedCN was measured to be 3.52 eV and 2.10 eV respectively (Fig. 2(b)). The reduced band gap in RedCN is probably due to its bonding with Tp. This indicates that RedCN definitely has more delocalized excitons due to the polymeric nature of the material.

Fig. 2 (a) UV-Vis DRS spectra and (b) band gap plots of melem-NH (blue) and RedCN (red). (c) EPR spectra of RedCN in the dark (black) and under light illumination with a white light LED (with a UV filter, ≥400 nm) (red), and (d) photocurrent measurement of RedCN (red) and melem-NH in the 1.5 AM solar spectrum (blue).

The electron paramagnetic resonance (EPR) spectrum of RedCN was recorded at 110 K to investigate the formation of any radical species upon light illumination. The EPR spectra showed a paramagnetic absorption signal with a g-value of 2.0065, when RedCN was exposed to white light. The experimental results indicate that light irradiation induces the generation of radicals within the material which are absent in the dark, as shown in Fig. 2(c). Furthermore, on increasing the duration of light illumination, an increase in the EPR signal was observed, suggesting an increase in the concentration of radical species (Fig. S10†). These findings highlight the photochemical activity of RedCN and its ability to generate light-induced radical species, which could play a significant role in its photocatalytic applications.

To further investigate the photogenerated electron transfer kinetics, the photocurrent responses of RedCN and melem-NH were analyzed. As shown in Fig. 2(d), melem-NH does not show any response to the current upon light on/off switching indicating the rapid recombination of photogenerated electron–hole pairs. RedCN displayed a clear and rapid photocurrent response within the 1-minute light on-off cycling, suggesting more effective charge separation and facile electron transfer from RedCN to the counter electrode via charges and their mobility within RedCN upon light illumination, which are critical for the photocatalyst performance.

Photocatalytic H₂O₂ production performance of RedCN

The photocatalytic H₂O₂ production was investigated using RedCN and melem-NH as catalysts in an oxygen saturated water–IPA (9 : 1) mixture, pure water and simulated sea water under visible light illumination. Fig. 3(a) depicts the kinetic plot for RedCN dispersed in the water–IPA mixture, which shows a gradual increase in H₂O₂ production with prolonged visible light exposure. RedCN exhibits a remarkable initial photocatalytic H₂O₂ production rate of 1460 μmol g⁻¹ h⁻¹ during the first hour of illumination. The amount of H₂O₂ generated is determined by the colorimetric method using an Fe²⁺ xylenol orange complex as an indicator (Fig. S11†). Even in the absence of IPA (sacrificial agent), RedCN displays an impressive photocatalytic H₂O₂ production rate of 1231 μmol g⁻¹ h⁻¹. However, with an increase in the time of illumination the difference in the H₂O₂ production between conditions with and without IPA increased. This behaviour is attributed to the enhanced recombination of photogenerated electron–hole pairs in the absence of IPA, leading to a reduction in the free charge carriers. After 3 h of irradiation, the photocatalytic production of H₂O₂ reached 2832 μmol g⁻¹ and 1833 μmol g⁻¹ with and without IPA, respectively.

Fig. 3 Photocatalytic performance of RedCN: (a) kinetic plot of H₂O₂ production in the presence (green) and absence (orange) of IPA (sacrificial agent); photocatalytic H₂O₂ production in (b) RedCN and melem-NH dispersed in IPA–water (1 : 10) and (c) DI water and simulated sea water without IPA. All these experiments are done under O₂ saturated conditions (d) O₂ and N₂ purging and (e) with benzoquinone (O₂^−. quencher); the light used in all these experiments is a xenon lamp (AM 1.5 G like spectrum with an intensity of approximately 93 mW cm⁻²) for 1 h (f) EPR spectra of DMPO–O₂⁻˙ for RedCN, illuminated with a white light LED with a UV filter.

In comparison, under identical conditions, the monomer melem-NH produced only 42.3 μmol per g per h of H₂O₂ (Fig. 3(b)). The polymerization of melem-NH with Tp increased the H₂O₂ production by 34-fold. This increase in H₂O₂ production is attributed to better charge separation and porosity in RedCN. Porosity facilitates efficient mass adsorption and diffusion, provides better exposure to active sites and shortens the charge carrier migration distance from the bulk to the surface, which increases the photocatalytic efficiency.⁴⁸ Polymerization of melem-NH with Tp gives better charge separation due to the difference in their redox potentials. Similarly, under identical conditions PCN produces 438 μmol g⁻¹ h⁻¹, which is lower than that of RedCN. This highlights the enhancement achieved through structural modifications that come with heptazine linkage with Tp (Fig. S12†).

The wavelength-dependent H₂O₂ production of RedCN yielded an apparent quantum yield (AQY) of 6.77% at 400 nm (Fig. S13†). To ensure the reusability of the catalyst, the H₂O₂ production was evaluated throughout four repeating cycles, each cycle of 3 h. H₂O₂ production by RedCN decreased minimally (<20%) during the cycling experiments (Fig. S14†). To assess the chemical stability post-photocatalysis, FTIR analysis of RedCN was performed. All peaks and peak positions remained the same before and after photocatalysis, suggesting that the chemical composition remained unchanged and demonstrating good photochemical stability (as shown in Fig. S15†). To gain further insight into the structural stability of the material after photocatalysis, we conducted PXRD and SEM analyses following 3 hours of photocatalytic reaction. As shown in the SEM images (Fig. S16†), the flake-like morphology undergoes some changes; however, the porous structure remains largely preserved. The PXRD pattern reveals a decrease in the intensity of the peak at 11.6° (2θ), indicating slight structural modifications after photocatalysis (shown in Fig. S17†). Despite these changes, the H₂O₂ production remains largely unaffected, as demonstrated in ESI Fig. S14.†

The photocatalytic decomposition of H₂O₂ on the RedCN surface was investigated under light irradiation. As shown in the data in Fig. S18,† there was no significant change in the H₂O₂ concentration during the first 30 minutes. Even after 3 hours, only a ∼10% decrease in the H₂O₂ concentration was observed, indicating minimal decomposition of H₂O₂ on the RedCN surface. Considering the limited availability of fresh water,^49,50 the photocatalytic performance of RedCN in stimulated sea water was also investigated (Fig. 3(c)). Remarkably, it shows 850 μmol g⁻¹ h⁻¹ of H₂O₂ production in the absence of any sacrificial agents, highlighting the RedCN's potential for practical applications and its photocatalytic properties even at high pH and in the presence of many ions such as chlorides, carbonates and sulphates. Photocatalytic H₂O₂ production can proceed via both ORR and the WOR pathways in the absence of IPA. To elucidate the role of oxygen in the ORR process, a series of controlled experiments were conducted. When oxygen purging was discontinued, and the reaction was conducted under ambient air conditions, the H₂O₂ production rate decreased significantly from 1460 μmol g⁻¹ h⁻¹ to 950 μmol g⁻¹ h⁻¹ (Fig. S19†). This observation highlights the critical role of sufficient oxygen availability in facilitating efficient photocatalytic H₂O₂ generation. Interestingly, RedCN demonstrated a substantial H₂O₂ production rate of 350 μmol g⁻¹ h⁻¹ even under nitrogen purging conditions, where oxygen was absent (as shown in Fig. 3(d)). Upon completely removing the O₂ mixture by freeze–pump–thaw, the reaction mixture still showed some H₂O₂ production upon light illumination (Fig. S19†). This observation suggests the involvement of the alternative reaction pathways contributing to H₂O₂ formation, potentially involving the WOR mechanism.

Mechanistic study

To identify the active oxygen species involved in H₂O₂ production, quenching experiments and spin-trapping EPR spectroscopy were performed. When benzoquinone, a known superoxide scavenger, was introduced into the system, H₂O₂ production was completely suppressed, indicating the pivotal role of superoxide radicals in the reaction (Fig. 3(e)). To further validate the involvement of superoxides as active oxygen species, 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was employed as a spin-trapping agent. As depicted in Fig. 3(f), EPR spectra exhibited distinct DMPO–O₂⁻˙ adduct peaks⁵¹ under light irradiation, whereas no such peaks were observed in the dark. This result unequivocally confirms that, upon shining light, excited-state electrons are generated, which are taken up by the oxygen, leading to the generation of superoxide radicals (O₂⁻˙). This suggests that the sequential two-electron, two-step ORR pathway significantly contributes to H₂O₂ production (shown in Scheme 2).

Scheme 2 Schematic illustration of different reaction pathways towards H₂O₂ production using RedCN.

To further explore the role of oxygen in H₂O₂ production, spectroscopic techniques were employed. Photoluminescence (PL) measurements of RedCN were conducted under inert, ambient, and oxygen-saturated conditions. Upon excitation at 450 nm, RedCN displayed a PL emission with a peak maximum at 587 nm under all three conditions. However, the PL signal intensity decreased progressively with an increasing oxygen concentration in the reaction medium (Fig. S20†). This observation suggests that the ORR plays a significant role in H₂O₂ production. The increased oxygen availability facilitates the capture of photoexcited electrons by oxygen molecules, thereby reducing the recombination of electron–hole pairs. Consequently, the reduced recombination leads to a decrease in PL intensity, further confirming the critical involvement of oxygen in the photocatalytic process. PL spectra of melem-NH were also recorded with excitations at 350 nm and 450 nm (shown in Fig. S21†). On excitation at both wavelengths similar PL spectra are obtained. The emission spectra have the peak maximum at 445 nm which is blue shifted as compared to RedCN (as shown in Fig. S21†). PL intensity of RedCN is significantly lower than that of melem-NH, indicating low recombination of holes and electrons in RedCN compared to melem-NH. This reduced recombination contributes to significantly increased H₂O₂ production in RedCN as compared to melem-NH.

To gain deeper mechanistic insights, the excited state dynamics of covalent polymers was tracked by femtosecond transient absorption (TA) spectroscopy measurements.^26,52 TA spectra of RedCN dispersed in the water–IPA mixture (9 : 1) were recorded under inert, ambient, and oxygen-rich conditions with pump-probe delays ranging from −1 ps to 2 nanoseconds following 480 nm excitation. The TA spectra for all three conditions exhibited a negative signal in the visible range from 450 to 800 nm, attributed to ground-state bleach (GSB) and stimulated emission, as illustrated in Fig. S22.† In the near-infrared (NIR) region (Fig. 4(a), (b) and S23†), a broad excited-state absorption (ESA) signal was observed from 1000 nm to 1300 nm, persisting for several hundred picoseconds. Using a model compound in which heptazine hydrazine is linked to aminated Tp (as shown in Fig. 5(a)), TD-DFT calculations were performed to calculate the excitation energies of its anion radical. The TD-DFT results showed a broad absorption feature for the anion radical, with a peak at 325 nm, along with a broad feature spanning the wavelength greater than 2400 nm (as shown in ESI Fig. S24†). As shown in Fig. 5(b), the broad anion radical absorption feature of the model compound is overlaid with the experimental data obtained from TA measurements. In the visible region, the excited-state absorption overlaps with the ground-state bleach (GSB) and simulated emission features, making it difficult to resolve. However, in the near-infrared (NIR) region, the broad excited-state absorption closely resembles the anion radical feature of the model compound. The decay time of this ESA feature is influenced by the presence and absence of oxygen in the system. This confirms that the oxygen is playing an important role in modulating the excited state dynamics.

Fig. 4 Femtosecond transient absorption (TA) spectra after 480 nm excitation at various time delays in the NIR region of RedCN under (a) inert and (b) O₂ saturated conditions. SVD analysis of the transient absorption of RedCN in the NIR region using a three-state model under (c) inert and (d) O₂ saturated conditions.

Fig. 5 (a) Model monomeric compound used for TD-DFT calculations. (b) Calculated absorption spectra of anion radicals (red); the stick shows the direct transitions, comparing the computed and experimental anion radical spectra (black) obtained from TA spectroscopy upon 480 nm excitation.

Under inert (Fig. 4(a)) and ambient conditions in the NIR region (Fig. S23†), a distinct GSB signal was detected between 850 and 950 nm at early time points and this increased over time due to the decay of the ESA state. This GSB signal persisted up to 2 nanoseconds under inert conditions, indicating that the molecules remained in the excited state for extended durations. As the excited state (anion radical) is decaying, the GSB is increasing under the inert conditions. In contrast, under excess oxygen conditions (Fig. 4(b)), the GSB signal was absent and only the ESA feature is visible, which decays in a few hundred picoseconds, suggesting efficient quenching of the excited state by oxygen. To further analyse the TA spectral features, singular value decomposition (SVD) was performed using a three-state sequential kinetic model, providing the corresponding lifetimes of the associated excited states of RedCN under various conditions, as shown in Fig. 4(c), (d) and S25.† Under inert conditions, the initial component (black trace) exhibited a lifetime of 0.46 ps, which increased to 1.8 ps under ambient conditions and 1.5 ps in the oxygen-rich environment. The second component (red trace) had a lifetime of 145 ps under both inert and ambient conditions but decreased significantly to 70 ps under oxygen-rich conditions. These observations suggest that the excited state is being quenched by molecular oxygen, leading to a rapid decay of the corresponding lifetime in 70 ps in the presence of excess oxygen which persists for a longer duration of 145 ps under inert conditions. We assign these three components first to an anion radical of RedCN, followed by its relaxation which is 145 ps under inert and 70 ps under excess O₂ conditions. The third component (green trace) was long-lived across all three conditions; it is only the GSB signal that was detected under inert conditions due to the unquenched excited states. TA data further highlight the influence of oxygen in modulating the excited-state dynamics of RedCN. TA measurements of melem-NH were also conducted with excitations at 480 nm and 400 nm (power: 0.4 mW, 0.26 mW and 0.34 mW, respectively). However, no significant excited-state signals were observed under these conditions, further indicating limited charge carrier dynamics in melem-NH (shown in Fig. S26†).

The complementary oxidation half-reaction was also thoroughly investigated. When isopropanol (IPA) was employed as a sacrificial agent, the second half-reaction involved the oxidation of IPA to acetone, as confirmed by ¹H NMR analysis. After 3 hours of photoreaction, in NMR we obtained one singlet at 2.1 ppm of acetone along with peaks of IPA (shown in ESI, Fig. S27†). In the absence of a sacrificial agent, water oxidation occurs, enabling RedCN to produce 350 μmol g⁻¹ h⁻¹ of H₂O₂ even under nitrogen-rich conditions. Water oxidation can proceed either directly to oxygen via a four-electron transfer mechanism or to H₂O₂via a two-electron transfer process (shown in Scheme 2).⁵³ To confirm the production of oxygen, gas chromatography (GC) analyses were performed at every 15-minute interval during photocatalysis. No trace of O₂ was detected, effectively ruling out the possibility of water oxidation to O₂ within the system. To further investigate the water oxidation reaction (WOR) pathway, a quenching experiment was conducted using t-butyl alcohol, a known hydroxyl radical scavenger. The H₂O₂ production rate decreased to 520 μmol g⁻¹ h⁻¹ from 1460 μmol g⁻¹ h⁻¹ (Fig. S28†), indicating the involvement of a two-step WOR mechanism in the process. Under higher acidic conditions also, the H₂O₂ production decreases, which further confirms the involvement of the hydroxyl radical. As shown in Fig. S29,† at pH 3, the H₂O₂ production decreased to 1019.5 μmol g⁻¹ h⁻¹ and upon increasing the pH from 3 to 5 and 7 the H₂O₂ production also increased. To further prove the involvement of the OH radical, an OH quenching dye hydroxyphenyl fluorescein (HPF) is used. HPF on reacting with the OH radical gives a fluorescence signal at 515 nm upon excitation at 488 nm. As shown in Fig. S30,† on increasing the time of light illumination the fluorescence intensity increases, which also confirms the formation of the OH radical.

In situ oxidation of sulfides

Since we generate both oxidizing and reducing equivalents in our COP material, we tried to couple it to carry out a photocatalytic organic transformation in tandem with H₂O₂. Although it is well known that hydrogen peroxide (H₂O₂) is capable of facilitating the oxidation of sulfides to sulfoxides, overoxidation of sulfides to sulfones is a major problem. To overcome this issue, in this study, photocatalytically generated H₂O₂ was employed for the in situ oxidation of sulfides to sulfoxides under visible light illumination (as shown in Fig. S31†). After 5 hours of irradiation, thioanisole exhibited a 97% conversion to sulfoxide, as confirmed by ¹H NMR (refer to ESI Fig. S32†). To systematically investigate the influence of substituents on sulfides, substrates with electron-donating (–OMe) and electron-withdrawing (–F) functional groups were utilized. The NMR results revealed that, after 5 hours of illumination, 4-fluorothioanisole was completely consumed and achieved a 95% conversion to 4-fluorophenyl methyl sulfoxide (Fig. S33†). However, in the case of 4-methoxythioanisole, complete consumption was not observed after 5 hours of light illumination. NMR analysis of the reaction mixture showed a 33% conversion to 4-methoxyphenyl methyl sulfoxide (Fig. S34†). Electron withdrawing groups attached to the sulfides decrease the electron density on sulfur facilitating faster oxidation by H₂O₂ whereas electron donating groups increase the electron density on sulfur reducing its reactivity towards H₂O₂. These findings underscore the significant role of electronic effects in the efficiency of photocatalytic organic sulfide oxidation reactions.

Conclusions

In summary, we have successfully developed a heptazine hydrazine-based covalent organic polymer capable of efficient photocatalytic H₂O₂ production under visible light irradiation. The integration of heptazine hydrazine with 1,3,5-triformylphloroglucinol (Tp) enables a novel two-electron, two-step water oxidation pathway (WOR) for H₂O₂ generation in addition to the two-step oxygen reduction reaction (ORR) pathway. Chemical quenching reactions along with direct EPR detection confirm that superoxide and hydroxide radicals are the intermediate species in the reaction. The broadband femtosecond TA in conjunction with electronic absorption calculations provided the first direct detection of the anion radical absorption spectra. TA and PL measurements emphasize the role of O₂ in controlling the lifetime of the reactive radical delocalized on the RedCN framework. This work highlights the potential of heptazine-based polymeric systems in facilitating dual reaction pathways, enhancing overall photocatalytic efficiency. The findings from this study provide valuable insights into the molecular-level intermediates of heptazine-hydrazine based metal-free polymeric photocatalysts. These materials hold immense promise for applications in artificial photosynthesis, sustainable energy production, and environmental remediation.

Data availability

The data supporting this article has been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to thank Ms Sushma Kundu and Prof. Vivek Polshettiwar for assistance in BET measurements. We thank Mrs Mamta Joshi for NMR, Mr Dnyaneshwar Avhad for EPR, Ms Payal Allawadhi for GC measurement, and Mr Debdulal Bag for photoelectrochemical measurements. Mr Vilas J. Mhatre is acknowledged for XRD measurements. We thank Mr Jayesh B Parmar. Mr Manish is acknowledged for SEM data. We thank Ms Subhra Ranjita Pattanayak and Dr T. N. Narayanan for BET analysis of melem-NH and Mr Tushar Ranjan Panda and Dr Malay Patra for HPF dye. We acknowledge the NMR Facility at TIFR Hyderabad for ¹³C (CP/MAS) solid state NMR. The authors also acknowledge discussions with Mr Dipin Tomar, Dr Debojyoti Roy, Mr Kishan Yadav, Ms Aisworika Mohanty, and Ms Shashi (TIFR Mumbai). K. T., D. K. and J. D. acknowledge support from the Department of Atomic Energy (DAE), Government of India, under Project No. 12-R&D-TFR-5.10-0100.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of the methodology, additional experimental data figures and details of computations. See DOI: https://doi.org/10.1039/d5ta02777b

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