Dual‐Quenching Charge Transfer Unlocks Record Nitrate‐to‐Ammonia Photocatalytic Conversion in Redox‐Active Eosin Y Polymers

ABSTRACT

Efficient photocatalytic nitrate reduction reaction (NO₃RR) is vital for mitigating nitrogen pollution and producing green ammonia (NH₃). Although organic polymer photocatalysts show great potential for NO₃RR, they frequently suffer from low charge separation efficiency. This limitation largely comes from the lack of suitable redox‐active moieties incorporated in the polymer photocatalysts toward NO₃RR. Herein, we embed the redox‐active Eosin Y (EY) into a conjugated polymer backbone to synthesize a series of EY–X polymers (where X = benzene, biphenyl, or fluorene), in which the extended π‐conjugation can promote dual dynamic and static quenching for directional electron transfer. Upon visible‐light excitation, EY forms a long‐lived radical anion (EY^•−) that stores and relays electrons to nitrate, while ground‐state complexation between polymer and nitrate preorganizes the substrate for photoinduced directional electron transfer. This dual‐pathway mechanism extends charge‐separated lifetimes, inhibits recombination, and enhances electron delivery. Consequently, under cocatalyst‐free conditions, the EY–BE polymer achieves a record high NH₃ production of 215 µmol g⁻¹ h⁻¹. Experimental and computational investigations support the reversible EY/EY^•− cycle and the nitrate‐binding ground‐state complex as the origin of activity and selectivity. This work demonstrates a rational strategy leveraging reversible redox‐active chromophores to integrate dual quenching for designing high‐performance NO₃RR photocatalysts.

The integration of the redox‐active Eosin Y units into conjugated polymers facilitates directional charge transfer and effectively reduces the energy barrier for nitrate‐to‐ammonia conversion through a dual‐pathway (dynamic and static) quenching mechanism. This strategy achieves a record ammonia production rate of 215 µmol g⁻¹ h⁻¹.

1. Introduction

Ammonia (NH₃) plays a pivotal role as both an essential industrial chemical and a potential clean‑energy carrier, yet its large‐scale synthesis still relies on the energy‐intensive Haber–Bosch process with substantial greenhouse‐gas emissions [1, 2]. Meanwhile, nitrate (NO₃ ⁻) pollutants are pervasive in aquatic environments and pose serious ecological risks [3]. Photocatalytic nitrate reduction reaction (NO₃RR) offers a sustainable strategy to simultaneously purify wastewater and upcycle “waste nitrogen” into high‐value‐added NH₃, thereby integrating environmental remediation with chemical production [4]. Although inorganic catalysts dominate this reaction, they often suffer from metal‐ion leaching and limited tunability [5, 6]. In contrast, conjugated microporous polymers (CMPs) have emerged as promising alternatives owing to their structural flexibility and adjustable photophysical properties. Recent efforts have focused on precise donor–acceptor engineering [7, 8, 9], side‐chain functionalization [9], heteroatom integration [10], and heterostructure design [11] to finely tune electronic structures and enhance catalytic efficiency. Although recent advances have been made in polymer photocatalysts for NO₃RR, challenges such as low charge‐separation efficiency, limited electron transfer to the substrate, and competing side reactions like hydrogen evolution continue to limit their efficiency and selectivity in ammonia production. Thus, it is desirable to develop new CMPs with efficient charge separation and transport capabilities as well as appropriate redox potential toward NO₃RR.

In response to these challenges, ground‐state complexation can preorganize the catalyst‐substrate interface, allowing static quenching to stabilize these interactions for efficient electron transfer [12], while dynamic quenching facilitates photoinduced charge separation [10]. Such complementary pathways effectively enhance charge separation and suppress recombination, addressing the limitations of low charge‐transfer efficiency. Redox‐active Eosin Y (EY, 2‐(2,4,5,7‐tetrabromo‐6‐hydroxy‐3‐oxo‐3H‐xanthen‐9‐yl)benzoic acid) units, which have facilitated their widespread application in photocatalytic redox reactions, especially in nitro compound reduction (Scheme 1a) [13, 14], can be incorporated into the polymer backbone to enable directional electron transfer.

SCHEME 1.

Schematic of (a) homogeneous EY molecular catalyst system for nitro reduction, and (b) heterogeneous EY‐based redox‐active polymer (RAP) photocatalyst system for efficient NO₃ ⁻‐to‐NH₃ conversion.

To implement this concept, we construct the dual‐quenching charge transfer in redox‐active EY‐based CMPs for the visible light‐driven NO₃ ⁻‐to‐NH₃ conversion (Scheme 1b), in which a series of EY‐based conjugated redox‐active polymers (EY–BE, EY–BP, and EY–FL) were designed and synthesized. Among them, EY–BE achieved a record high ammonia production rate of 215 µmol g⁻¹ h⁻¹ with an ammonia synthesis selectivity of 93.6%, surpassing the previously reported benchmark of 149.6 µmol g⁻¹ h⁻¹ for pure organic polymer photocatalysts [7]. Mechanistic studies — including transient absorption spectroscopy (TAS), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) calculations — unveil a dual‐pathway mechanism driven by both static and dynamic quenching. Specifically, the formation of a ground‐state complex [EY–BE, NaNO₃] initiates static quenching, enabling direct photoinduced electron transfer (PET). Concurrently, upon light irradiation, the EY unit undergoes charge separation to generate a long‐lived radical anion (EY^•−). This radical serves as a key intermediate to capture and transfer photogenerated electrons to NO₃ ⁻, thereby regenerating EY in the process and enabling a reversible EY/EY^•− redox cycle. Together, these complementary pathways facilitate efficient, directional electron delivery, suppress charge recombination, and significantly lower the energy barrier for NO₃RR. Ultimately, this work demonstrates the rational integration of a redox‐active EY unit into a polymer matrix, enabling efficient photocatalytic NO₃ ⁻ reduction via a dual‐intermediate redox mechanism, and provides a clear design strategy for developing polymer photocatalysts for NO₃RR at the molecular level.

2. Results and Discussion

In this system, the EY–X polymers (where X = BE, BP, and FL) were synthesized via Sonogashira cross‐coupling reactions of EY with 1,4‐diethynylbenzene (BE), 4,4’‐diethynylbiphenyl (BP), and 2,7‐diethynyl‐9H‐fluorene (FL), respectively. The detailed synthetic procedures and control experiments are provided in the (Figure 1a and Figure S1).

FIGURE 1.

(a) Synthetic routes of EY–X (X = BE, BP, and FL) RAPs. (b) FT‐IR spectra of EY–X (X = BE, BP, and FL) and their corresponding precursors. (c) C 1s XPS of EY–X (X = BE, BP, and FL). (d) Solid state ¹³C NMR spectra of EY–X (X = BE, BP, and FL).

The chemical structure of each polymer was comprehensively characterized by Fourier‑transform infrared spectroscopy (FTIR), X‑ray photoelectron spectroscopy (XPS), solid‑state ¹³C nuclear magnetic resonance (¹³C ssNMR), and powder X‑ray diffraction (PXRD). FTIR spectroscopy (Figure 1b) provides preliminary confirmation of polymer formation: the terminal alkyne ≡C─H) stretch at 3250 cm⁻¹ and the aryl‑bromide (Ar–Br) band at 730 cm⁻¹ disappear in the polymer spectra, indicating successful Sonogashira coupling [15, 16]. High‐resolution XPS detects only a trace Br signal, with no detectable residual metal signal, indicating that the metal content is below the XPS detection limit (∼0.1 atomic percent) [17]. Furthermore, the C 1s and O 1s spectra (Figure 1c and Figures S4 and S5) confirm the expected chemical environments, including C─C, C═C, C≡C, C─O, and C═O [18, 19, 20, 21, 22]. ¹³C ssNMR spectra (Figure 1d) further validate the structural backbone; aromatic carbons adjacent to alkynyl linkages appear around 100 ppm [23], while alkynyl carbons resonate near 80 ppm [24]. Notably, EY–FL displays an additional distinct fluorene methylene signal at ∼30 ppm [25]. Finally, PXRD shows only broad features, revealing the largely amorphous nature of the polymers (Figure S8) [26].

Collectively, these characterization results provide consistent evidence supporting the successful synthesis and target structures of all three EY‐based conjugated polymers. Scanning electron microscope (SEM) images show that the materials assemble into a randomly arranged, nanoparticle‐like morphology (Figure S10). Furthermore, Energy Dispersive X‐ray Spectroscopy (EDS) mapping also reveals a homogeneous distribution of carbon and oxygen across the polymer surfaces, confirming the successful coupling of the corresponding precursors (Figures S11 and S12) [27]. To assess porosity, N₂ adsorption–desorption isotherms were recorded at 77 K. The Brunauer–Emmett–Teller (BET) surface areas of EY–BE, EY–BP, and EY–FL are 498.40, 19.82, and 32.15 m² g⁻¹, respectively (Figures S13 and S14). The significantly higher surface area of EY–BE is expected to facilitate substrate accessibility, thereby benefiting its photocatalytic performance [28]. In contrast, the relatively low surface areas of EY–BP and EY–FL are likely associated with more densely packed frameworks or partial pore collapse during polymer formation, which may be related to their lower synthetic yields. Notably, all three EY‐based polymers exhibit pronounced hysteresis loops in their isotherms, supporting the presence of mesoporous architectures [29].

The thermal stability and processability of photocatalysts are critical for their practical engineering applications [30, 31]. Thermogravimetric analysis (TGA) reveals that all three EY‐based polymers retain over 88% of their initial mass up to 400°C. The minor weight loss at low temperatures is attributed to the desorption of adsorbed species, whereas the major mass loss at elevated temperatures corresponds to decomposition of the polymer backbones. Importantly, because the boiling and decomposition temperatures of triethanolamine (TEOA) were far below 400°C, the TGA results further confirm its effective removal from the system. This finding perfectly aligns with the absence of nitrogen signals in FT‐IR, XPS, and EDS analyses, conclusively ruling out the possibility that the detected NH₃ during photocatalysis originates from the decomposition or transformation of residual TEOA. Furthermore, EY–BE exhibits a higher peak degradation temperature (477.5°C) than EY–BP and EY–FL, indicating superior thermal stability (Figure S15) [32].

Regarding particle morphology, static light scattering (SLS) measurements were employed to evaluate particle size and distribution. EY–BE shows a smaller median diameter (d(0.5) = 6.35 µm) and smaller surface‐area‐weighted mean diameter (D[3,2] = 5.84 µm), alongside a reduced fraction of large agglomerates (d(0.9) = 18.22 µm). These characteristics facilitate more uniform dispersion and improved accessibility of the catalytically active sites. Conversely, EY–BP exhibits a larger particle size (d(0.5) = 14.57 µm), and EY–FL shows severe aggregation (d(0.9) = 76.56 µm), both of which restrict effective dispersion and interfacial interactions. Consequently, these results indicate that EY–BE exhibits relatively better dispersion stability, which may be beneficial for photocatalytic applications (Figures S16 and S17 and Table S1) [33].

The photocatalytic performance of a material is governed not only by its chemical structure and morphology but also by the alignment of its energy levels [34]. Molecular‐level structural modifications enable precise tuning of energy band positions [35]. All three EY‐based photocatalysts exhibit strong absorption in the visible‐light region (Figure 2a), indicating good potential for photocatalytic applications [36]. Tauc plot analysis reveals band gap energies (E _g) of 1.92, 1.88, and 1.86 eV for EY–BE, EY–BP, and EY–FL, respectively (Figure S18). Mott–Schottky analysis further indicates that their flat‐band potentials are −0.97, −0.84, and −0.74 V (vs. SCE) (Figure S20). Correspondingly, the conduction band edge potentials (E _CB) are determined to be −0.93, −0.80, and −0.70 V (vs. NHE) for EY–BE, EY–BP, and EY–FL, respectively (Figure 2b) [37]. These values are all more negative than the thermodynamic threshold for NO₃ ⁻ reduction to NH₃ (−0.58 V vs. NHE, pH = 7) [38, 39], confirming their energetic feasibility for this reaction. Notably, EY–BE has the most negative E _CB, indicating a greater driving force for photocatalytic NO₃ ⁻ reduction [34].

FIGURE 2.

(a) UV–vis diffuse reflectance spectra (DRS) of EY–X (X = BE, BP, and FL). (b) Band diagram of EY–X (X = BE, BP, and FL). (c) Performance comparison of EY–X (X = BE, BP, and FL) for NO₃ ⁻‐to‐NH₃ production rate rate. (d) Wavelength‐dependent AQY and UV–vis DRS of EY–BE. (e) ¹H NMR spectroscopic characterization of NH₄ ⁺ isotopic variants. (f) The production rate, selectivity, and conversion efficiency of photocatalytic products. (g) Photocatalytic efficiency of NH₃ synthesis by EY–X (X = BE, BP, and FL) dispersed in a 10 ppm NaNO₂ deionized water solution under vacuum conditions. (h) Comparison of NO₃ ⁻‐to‐NH₃ production rate between our photocatalysts and other reported organic‐polymer‐based photocatalysts.

To maximize the NH₃ production rate, we optimized the sodium nitrate (NaNO₃) concentration in the reaction solution using Nessler's reagent colorimetry (Figures S22 and S23) [22]. These evaluations revealed 10 ppm as the optimal NaNO₃ concentration, at which EY–BE achieves an NH₃ production rate of 215 µmol g⁻¹ h⁻¹ without any hole scavenger. This rate surpasses those of EY–BP (166 µmol g⁻¹ h⁻¹) and EY–FL (154 µmol g⁻¹ h⁻¹) (Figure 2c), as well as outperforms benchmarked polymer photocatalysts reported to date (Figure 2h and Table S2). Additionally, EY–BE achieves an apparent quantum yield (AQY) of 0.26% at 420 nm. The wavelength‐dependent AQY profile closely matches its optical absorption spectrum (Figure 2d), confirming that the reaction is genuinely light‐driven (the light intensity was calibrated via a five‐point sampling method). All three EY‐based photocatalysts demonstrated excellent operational durability, maintaining stable NH₃ production across five consecutive cycles (Figure S24). Additionally, Post‐reaction FTIR and PXRD analyses showed no discernible structural changes in EY–BE (Figures S25 and S26), further confirming its stability [20]. To further evaluate the kinetic behavior of the system, time‐dependent photocatalytic experiments over 6 h were conducted for EY–BE with hourly sampling. As shown in Figure S27, the NH₃ production gradually accumulates with increasing irradiation time without any observable deactivation, indicating sustained catalytic activity and steady‐state operation over extended reaction periods.

Finally, to rigorously address the potential catalytic contribution of metallic impurities introduced during synthesis, inductively coupled plasma optical emission spectroscopy (ICP‐OES) was performed. As detailed in Table S3, the residual Pd contents are present in the polymers; however, its content exhibited no clear correlation with the respective photocatalytic performance of the samples. Moreover, control experiments reveal that the deliberate addition of exogenous metals (Pd or Cu) to the reaction system has no significant effect on the catalytic activity (Figure S28). Together, these findings suggest that while trace metal residues are present, they make a negligible contribution to the overall catalytic performance, which is instead intrinsically driven by the organic polymer frameworks.

To elucidate the mechanism of photocatalytic NO₃ ⁻ reduction to NH₃, a series of quenching experiments were conducted (Figure S29). First, negligible NH₃ generation is detected in the absence of light, photocatalyst, or NaNO₃, indicating that light irradiation, photocatalyst, and substrate are all essential for NH₃ synthesis. In addition, the introduction of the electron scavenger NaIO₃ resulted in a significant decrease in NH₃ yield, while the addition of the hole scavenger ethylene glycol (EG) led to a marked increase in NH₃ production. These observations confirm that the NH₃ formation proceeds through a reduction process involving PET. Moreover, in the presence of AgNO₃ as an electron scavenger, all three EY‐based photocatalysts exhibited oxygen production rates exceeding 180 µmol g⁻¹ h⁻¹ (Figure S30), thereby confirming the active involvement of photogenerated holes in the four‐electron water oxidation reaction (4 e⁻ WOR, 2 H₂O → O₂ + 4 H⁺ + 4 e⁻) [40]. Under the standard nitrate reduction conditions without AgNO₃, EY−BE produced O₂ and NH₃ at rates of 45.7 and 215 µmol g⁻¹ h⁻¹, respectively, supporting that water oxidation acts as the oxidative half‐reaction during nitrate reduction. It should be noted that the detected gas‐phase O₂ rate is lower than the theoretical stoichiometric requirement for the observed NH₃ production. This apparent discrepancy suggests that part of the photogenerated holes or oxygen‐related species may be consumed, trapped, or retained through pathways that do not immediately release gas‐phase O₂. To probe the fate of these hole polarons and oxygen‐related species, in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) and O₂ temperature‐programmed desorption (O₂‐TPD) measurements were performed. The in situ DRIFTS spectra exhibited clear changes in the 1788, 1743, and 1645 cm⁻¹ bands upon O₂ exposure (Figure S31), indicating that O₂ adsorption affects the local surface environment of EY−BE, as evidenced by the bands assigned to carbonyl‐related vibrations and the electronic‐state modulation of the conjugated C═C framework. Moreover, O₂‐TPD revealed a high oxygen desorption capacity of 383 µmol g⁻¹ (Figure S32). These results suggest that the apparent discrepancy in oxidative equivalents can be reasonably attributed to the strong surface retention of O₂‐related species and mild surface passivation‐like processes within the polymer matrix, which altogether restrict the immediate evolution of gaseous O₂ while preserving the overall catalytic charge balance. To confirm the sole nitrogen source for NH₃ production, ¹⁵N isotope labeling experiments were conducted using Na¹⁵NO₃ as the exclusive nitrogen source [41]. The detection of only ¹⁵NH₄ ⁺ and the absence of ¹⁴NH₄ ⁺ confirmed that no other nitrogen contaminants interfered with the results (Figure 2e).

During the multistep photocatalytic reduction of NO₃ ⁻‐to‐NH₃, potential side reactions such as nitrite (NO₂ ⁻) accumulation and hydrogen gas (H₂) evolution should be considered when evaluating NH₃ selectivity [42]. To assess the extent of these competitive side reactions during photocatalysis, quantitative analyses of NO₂ ⁻ in the liquid phase and H₂ in the gas phase were performed (Figure S33) [43, 44]. The results show that among the three EY‐based polymers, EY–BE exhibits the highest selectivity for NH₃ formation (93.6%) and the most efficient NO₃ ⁻ conversion (18.3%), outperforming both EY–BP and EY–FL (Figure 2f). Notably, no gas‐phase H₂ evolution was detected for any of the samples. Furthermore, under full‐spectrum Xe lamp irradiation, all three EY‐based photocatalysts demonstrated substantial improvements in both NH₃ production rate and nitrate conversion efficiency (Figure S34). The practical applicability of the photocatalytic system was further evaluated by testing EY–BE under different pH conditions (pH = 8–12), which are representative of neutral‐to‐mildly alkaline wastewater environments. EY–BE exhibited only minor activity fluctuations across the investigated pH range, indicating relatively stable activity under these conditions (Figure S35). Additionally, EY–BE demonstrates high photocatalytic efficiency in directly reducing the NO₂ ⁻ intermediate to NH₃, achieving an NH₃ production rates of 282 µmol g⁻¹ h⁻¹ (Figure 2g). This suggests that EY–BE effectively interacts with reaction intermediates, thereby boosting NH₃ production.

Investigating the charge transfer properties of photocatalysts is essential for understanding their charge separation and transport behavior [21]. The charge transfer characteristics of the photocatalysts were analyzed using transient photocurrent measurements, electrochemical impedance spectroscopy (EIS), steady‐state photoluminescence (PL), and time‐resolved photoluminescence (TRPL) experiments. Among the three EY‐based polymers, EY–BE exhibited the highest transient photocurrent (Figure 3a), the lowest electrochemical impedance (Figure S36a), reduced fluorescence intensity (Figure S36b), and the longest fluorescence lifetime (Figure 3b), indicating superior charge separation efficiency and stability [45, 46]. The built‐in electric field of a semiconductor drives charge‐carrier separation, enhancing their migration to the surface for photocatalytic reactions. The strength of this field directly correlates with carrier separation efficiency [47]. In open‐circuit voltage (OCV) and zeta potential measurements, EY–BE showed a stronger built‐in electric field than EY–BP and EY–FL (Figure S37), suggesting a higher space charge density. This result further supports EY–BE's superior carrier separation efficiency observed in the photoelectrochemical tests and its enhanced dispersion stability. Additionally, temperature‐dependent PL spectra analyzed using the Arrhenius equation (Figure S38) revealed the exciton binding energies (E _b). EY–BE exhibits the lowest E _b (35.51 meV), indicating lower energy loss during exciton recombination, which reduces recombination tendencies and facilitates charge transfer [48].

FIGURE 3.

(a) I–t curves of EY–X (X = BE, BP, and FL). (b) Time‐resolved photoluminescence (TRPL) decay profiles and corresponding fitted lifetime of EY–X (X = BE, BP, and FL). (c) Absorption difference spectra of EY–BE in dimethyl sulfoxide (DMSO) upon titration with NaNO₃ from 0 to 50 mm (inset: magnified view of the 500–600 nm region). (d) Stern–Volmer plots of steady‐state (I ₀/I vs. [NaNO₃]) and time‐resolved (τ₀/τ vs. [NaNO₃]) TRPL quenching monitored at 550 nm. (e) Nanosecond transient absorption spectra (ns‐TAS) of EY–BE at selected delay times after pulsed laser excitation (λ_ex = 532 nm, argon atmosphere) in the absence of NaNO₃. (f) The ns‐TAS kinetic traces of EY–BE were probed at 550 nm in the absence and the presence of high‐concentration NaNO₃. Femtosecond transient absorption spectra (fs‐TAS) of EY–BE in DMSO (λ_ex = 350 nm, argon atmosphere) in (g) the absence, and (h) the presence of high‐concentration NaNO₃. The top panels show the population dynamics obtained through global analysis.

Density functional theory (DFT) calculations on the three EY‐based photocatalysts provide further insights. As shown in Figure S39, the frontier molecular orbitals exhibit spatial separation: the highest occupied molecular orbitals (HOMOs) delocalize over both the EY unit and the aromatic bridging moieties (X), while the lowest unoccupied molecular orbitals (LUMOs) localize predominantly on the EY segment. This separation favors photoinduced electron localization on the EY fragment, consistent with the experimentally observed EY^•− radical under illumination. Dipole moment analysis reveals that EY–BE has the largest dipole along the y‐axis (7.056 D), exceeding EY–BP (6.478 D) and EY–FL (6.842 D) (Figure S40), indicating a stronger intrinsic charge asymmetry that likely enhances local electric fields and directional charge separation. All three EY‐based photocatalysts possess a π‐conjugated coplanar backbone, promoting strong intramolecular charge separation and ordered π–π stacking, which facilitates efficient in‐plane charge transport, interchain coupling, and overall photocatalytic activity.

In order to further elucidate the photophysical and electrochemical behavior of the EY‐based polymer (EY–BE), a combination of steady‐state UV–vis titration, FT‐IR spectroscopy, infrared thermal imaging, cyclic voltammetry (CV), spectroelectrochemical analysis, and transient absorption spectroscopy (ns‐TAS and fs‐TAS) was carried out.

Steady‐state UV–vis titration experiments in dimethyl sulfoxide (DMSO) reveal that both small‐molecule EY and the polymer EY–BE show gradual increases in absorption intensity near 540 nm upon titration with NaNO₃ (Figure 3c and Figure S41). This observation confirms the emergence of an electronic interaction between the EY unit and NO₃ ⁻, indicating new electronic interactions and the formation of [EY–BE, NaNO₃] in both systems [12]. The existence of this ground‐state complex is supported by FT‐IR spectroscopy and infrared thermal imaging (Figures S42 and S43). Upon the addition of NaNO₃, characteristic NO₃ ⁻ vibrational bands emerge, the aromatic C═C stretching vibration redshifts, and an exothermic response is detected. These observations provide initial evidence for the presence of the [EY–BE, NaNO₃] complex [49]. Stern–Volmer analysis of both PL and TRPL experiments reveals distinct quenching behaviors (Figure 3d and Figure S44). When steady‐state emission intensity is used, the Stern–Volmer plot displays an upward curvature, indicative of simultaneous static and dynamic quenching processes [12]. Fitting yields a dynamic quenching constant (K _D, representing diffusion‐controlled collisional quenching) of 0.286 M⁻¹ and a static quenching constant (K _S, corresponding to ground‐state complex formation) of 113 M⁻¹.

Spectroelectrochemical measurements reveal that applying potentials from 0 to −1.0 V (vs. NHE, determined from CV [50], Figure S45) results in a gradual decrease of the ground‐state absorption band at ∼540 nm and the simultaneous appearance of a new absorption near 365 nm, confirming the electrochemical generation of the EY radical anion (EY^•−) (Figure S46). Under 532 nm excitation, the ns‐TAS spectra of EY–BE (Figure 3e and Figures S47 and S48) exhibit a pronounced ground‐state bleach (GSB) at ∼550 nm and a positive transient at ∼450 nm, characteristic of a long‐lived charge‐separated state (BE^•+–EY^•−) [8, 51]. The kinetic traces at 550 nm (Figure 3f) demonstrate that the GBS lifetime of EY–BE increases from 23.7 µs (without NaNO₃) to 32.7 µs (with NaNO₃), accompanied by an initial amplitude decrease significantly (Figures S49 and S50). The lifetime increase indicates a longer charge‐separated state compared to the excited state of EY–BE while the initial amplitude drop suggests the formation of a ground‐state complex between EY–BE and NaNO₃ ([EY–BE, NaNO₃]).

With the aim of directly monitoring the ultrafast electron transfer dynamics underlying both static and dynamic quenching processes, which are beyond the temporal resolution of ns‐TAS, femtosecond visible–near‐infrared transient absorption spectroscopy (fs‐TAS) was further employed (Figure 3g,h and Figures S51 and S52). Under 350 nm pulsed excitation in argon‐saturated DMSO, fs‐TAS reveals a pronounced excited‐state absorption band at 550 nm for EY–BE. Upon addition of NaNO₃, the 550 nm signal shows a significant decrease in amplitude, indicating that the excited‐state population is primarily reduced through a static ground‐state complexation pathway. Global kinetic analysis further reveals three characteristic time constants (τ₁ = 4.32 → 4.38 ps, τ₂ = 147 → 141 ps, and τ₃ = 5.72 → 5.37 ns). The nearly unchanged τ₁ (hot‐S₁ → relaxed‐S₁) suggests that the ultrafast energy relaxation and solvation process is unaffected by NaNO₃ [52]. In contrast, the slight shortening of τ₂ reflects the enhanced deactivation of a transient precursor (charge‐transfer state) [53] via dynamic quenching or electron transfer in the presence of high‐concentration NaNO₃, while the significant shortening of τ₃ indicates that the long‐lived S₁ component is highly sensitive to electron‐transfer‐induced deactivation, accelerating both radiative and nonradiative relaxation. The combination of the marked decrease in fs‐TAS amplitude and the modest lifetime shortening is in line with steady‐state fluorescence titration and time‐resolved photoluminescence results, suggesting the coexistence of static ground‐state complexation and dynamic excited‐state quenching processes during photocatalysis [51].

Furthermore, to evaluate the advantages of heterogeneous photocatalysts and formation of the [EY–BE, NaNO₃] complex in NO₃RR, we introduced control samples comprising small‐molecule EY and two polymer photocatalysts, BE–BE and DPP–BE (the latter incorporating the classical diketopyrrolopyrrole dye unit DPP) (Figure S2). Successful synthesis of BE–BE and DPP–BE was confirmed by FT‐IR, XPS, ¹³C ssNMR, PXRD, and N₂ sorption measurements, while their energy‐level alignment was analyzed to assess electronic properties (Figures S3, S6, S7, S9, S14, S19, and S21). Among them, BE–BE and DPP–BE cannot form ground‐state complexes with NaNO₃ (Figure S53). The photocatalytic performance of EY–BE was found to be markedly superior to that of BE–BE, DPP–BE, and small‐molecule EY, with NH₃ production rates of 215, 131, 130, and 94 µmol g⁻¹ h⁻¹, respectively (Figure S54). This result further confirms that the formation of the ground‐state complex formed between EY–BE and NaNO₃ plays a crucial role in enhancing the efficiency of photocatalytic NO₃ ⁻‐to‐NH₃ conversion.

Overall, static quenching arises from ground‐state complex formation, which decreases the concentration of free EY–BE and extends the charge‐separated‐state lifetime by suppressing back‐electron transfer. Meanwhile, dynamic quenching originates from the generation of the long‐lived EY^•− species, which acts as an electron trap to enable sequential electron accumulation and efficient transfer to NO₃ ⁻. The synergistic interplay between these two quenching pathways promotes directional charge transfer and stabilizes reactive intermediates, ultimately enhancing the photocatalytic NO₃ ⁻‐to‐NH₃ conversion activity of EY–BE.

The redox‐mediated charge transfer process of the EY‐based polymers was further validated by electron paramagnetic resonance (EPR) and in situ DRIFTS measurements. Under illumination, all three EY‐based photocatalysts exhibited a characteristic sextet signal corresponding to the DMPO–EY^•− adduct, confirming the generation of photoinduced radical intermediates (Figure 4b and Figure S55) [45, 54, 55]. Moreover, in situ DRIFTS results show that, under argon‐purged conditions and light irradiation, the spectrum of EY–BE displays a decrease in the characteristic absorption band of quinone carbonyl groups (Ar = O, 1700 cm⁻¹) and a concurrent increase in the signal corresponding to phenolic hydroxyl groups (Ar–OH, 2950 cm⁻¹) (Figure 4c) [23]. These changes indicate that, in the absence of the NaNO₃ substrate, the EY moiety undergoes a reversible structural transformation during the PET process, leading to the formation of electron‐rich EY^•− species. Furthermore, upon introduction of the NaNO₃ substrate, a significant decrease in the intensity of the NO₃ ⁻ bands (1620 and 1384 cm⁻¹) and a simultaneous increase in the NH₄ ⁺ bands (3000, 1850, and1420 cm⁻¹) were detected under illumination (Figure 4d), demonstrating that the surface NO₃ ⁻ was effectively converted to NH₄ ⁺ under light irradiation [4, 56] (Figure 4a).

FIGURE 4.

(a) Structural changes in the redox reversible cycle of EY active units. (b) EPR spectrum of EY–BE in DMSO under argon atmosphere with illumination. In situ DRIFTS difference spectra of EY–BE during Xe lamp irradiation (λ > 420 nm, argon atmosphere), (c) without NaNO₃ aqueous solution, and (d) with NaNO₃ aqueous solution. (e) Gibbs free energy (ΔG) diagrams of NO₃RR at the EY^•− center of EY–X (X = BE, BP, and FL). (f) Comparison of the overall ΔG for NO₃ ⁻ adsorption and the initial hydrogenation step in EY–X (X = BE, BP, and FL) at EY and EY^•− centers. (g) Molecular dynamics (MD) simulation snapshots of EY–BE in aqueous solution. (h) Radial distribution functions (RDFs) of water around the EY moieties in EY–X (X = BE, BP, and FL). (i) The rotational energy barriers (ΔE) of three systems by scanning the dihedral angles between the EY unit and the adjacent electron‐donating X moieties from 0° to 180°.

To gain mechanistic insights into the superior NO₃ ⁻‐to‐NH₃ conversion activity, DFT calculations were performed to investigate both the reaction pathway and the excited‐state electronic structure of the three EY‐based photocatalysts. These calculations reveal that, upon photoexcitation, the photogenerated electron preferentially localizes on the EY moiety (Figure S56), leading to the formation of the EY^•− radical. In EY–BE, this radical species exhibits a more electron‐rich nature, as evidenced by a lower Bader charge of 0.194 |e|. The resulting negatively charged, coplanar π‐conjugated backbone facilitates strong interaction with adsorbed NO₃ ⁻ ions, thereby promoting efficient interfacial electron transfer and subsequent photoredox activity. Consistency between this theoretical results and the experimental data on NO₂ ⁻ reduction further confirms the superior activity and selectivity of EY–BE. These findings suggest that EY–BE not only stabilizes key intermediates but also actively engages in their further transformation, enabling efficient NH₃ production through multiple pathways.

Building on these electronic structure insights, we further simulated the full NO₃ ⁻‐to‐NH₃ reaction pathway on the three EY‐based photocatalysts (Figure 4e and Figures S57–S60). The overall process involves two main stages: deoxygenation and hydrogenation [7, 57], with the initial NO₃ ⁻ adsorption and subsequent hydrogenation to *NO₂OH identified as the rate‐limiting steps due to their high Gibbs free energy (ΔG) barriers. Notably, NO₃ ⁻ preferentially adsorbs onto the carbonyl‐bearing aromatic ring of the EY unit, and EY–BE exhibits the weakest adsorption energy among the three, which favors the occurrence of static quenching. According to first‐principles calculations, taking EY–BE as an example, ΔG of the key intermediate *NO₂OH significantly decreases from 1.82 to 1.29 eV after the reversible structural transformation from EY to EY^•−, representing a nearly 30% reduction compared to the untransformed state. This result supports the conclusion that the EY^•− radical species generated via dynamic quenching plays a crucial role in lowering the energy barrier of this intermediate. The lowest overall energy barrier observed for EY–BE aligns well with its enhanced NH₃ production rate and strongest EPR signals for reactive intermediates.

In addition to electronic structural effects, the local solvent microenvironment around the active sites plays a pivotal role in determining photocatalytic efficiency, especially during the hydrogenation steps [58, 59]. This is because proton supply and water molecule adsorption behavior directly influence the hydrogenation process in nitrate reduction [60]. Molecular dynamics (MD) simulations of the radial distribution functions (RDFs) of water molecules around the EY core reveal that all three EY‐based photocatalysts are strongly hydrated (Figure 4g,h), consistent with their good aqueous dispersibility. Notably, EY–BE shows the highest local water density at its active sites, reflecting enhanced hydrophilicity and a denser hydrogen‐bonding network that promotes efficient proton transfer via the Grotthuss mechanism. MD simulation data of solvation free energy (Figure S61) further confirm that EY–BE has the strongest solvation among the three EY‐based polymers, supporting its superior hydrophilicity and accelerated reaction kinetics in aqueous media. Beyond solvation, the structural flexibility of the polymer backbone also affects the accessibility of catalytic sites. The rotational energy barriers (ΔE) were evaluated by scanning the dihedral angles between the EY unit and adjacent electron‐donating X moieties over a range of 0°–180°, revealing that EY–BE exhibits the lowest ΔE, indicative of its superior conformational flexibility that promotes better active‐site exposure and interfacial interactions. Together with its reduced energy barriers in key reaction steps, the cooperative effects of solvation and conformational dynamics underscore the crucial role of donor‐unit‐regulated microenvironments in achieving the outstanding photocatalytic NO₃ ⁻‐to‐NH₃ performance of EY–BE.

Based on experimental results and theoretical calculations, this study reveals the molecular mechanism of efficient photocatalytic NO₃ ⁻‐to‐NH₃ conversion over EY‐based polymer photocatalysts (EY–X). Upon light irradiation, EY–X undergoes effective charge separation, forming the highly reducing radical anion EY^•−, which enables dynamic quenching and drives multi‐electron transfer to NO₃ ⁻. Simultaneously, static quenching arises from ground‐state complex formation between EY–X and NO₃ ⁻, promoting substrate pre‐organization and directional electron transfer. The EY unit imparts strong redox activity and high electron conductivity, while the donor unit tunes the local environment to facilitate NO₃ ⁻ adsorption via hydrogen bonding, enhances proton‐coupled electron transfer, and stabilizes key intermediates. The synergy of static and dynamic quenching pathways accelerates charge transport, activates NO₃ ⁻ efficiently, and enables selective NH₃ production through a robust photocatalytic cycle (Figure 5).

FIGURE 5.

Illustration of the dual‐pathway quenching mechanism in EY–X photocatalysts for NO₃ ⁻‐to‐NH₃ conversion.

3. Conclusion

In this work, we have developed a fully organic polymer photocatalyst by incorporating a redox‐active EY unit, achieving efficient NO₃ ⁻‐to‐NH₃ conversion with an ammonia production rate of 215 µmol g⁻¹ h⁻¹, surpassing all previously reported polymer photocatalysts. Mechanistic investigations reveal a synergistic dual‐pathway quenching mechanism. In this process, ground‐state complexation (static quenching) facilitates substrate preorganization and directional electron transfer, while photoinduced charge separation (dynamic quenching) generates long‐lived EY^•− radicals for multielectron NO₃ ⁻ reduction. The EY unit provides strong redox activity and high electron conductivity, while the tailored donor segment optimizes the catalytic microenvironment to promote proton‐coupled electron transfer and stabilize key intermediates. These insights offer both a solid mechanistic foundation and a versatile design blueprint for developing next‐generation organic polymer photocatalysts.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: adma73686‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.