Linker Nitrogen Tunes Charge Polarity to Strengthen Built‐In Electric Fields in Covalent Organic Frameworks for Photocatalytic Oxidation

ABSTRACT

Covalent organic frameworks (COFs) are attractive platforms for heterogeneous photocatalysis, yet efficient exciton dissociation and charge separation remain intrinsically challenging in ordered organic frameworks. Here, we show that these limitations can be addressed by regulating the local electrostatic environment via linker nitrogen engineering. Within a common imine‐linked framework, systematic modulation of nitrogen content in the bridging linkers further tunes the overall electrostatic environment and strengthens the framework‐scale built‐in electric fields, as revealed by spatially resolved spectroscopic analyses and theoretical calculations. The strengthened built‐in electric fields lower the effective exciton binding energy, suppress recombination, promote directional charge separation, and improve charge utilization under illumination. As a result, the polarity‐engineered COFs exhibit excellent photocatalytic performance in two representative aerobic oxidation reactions under visible light and mild conditions, with TAPP‐Bpy‐COF affording >99% conversion and >99% selectivity within 1 h in both reactions. This work establishes linker nitrogen engineering as a chemically countable and general strategy for regulating exciton dynamics and charge utilization in COFs and provides a rational design principle for efficient organic photocatalysts.

Linker nitrogen engineering provides a chemically countable electrostatic handle in porphyrinic COFs, translating molecular polarity into framework‐scale built‐in electric fields. The strengthened fields lower exciton binding energies, promote charge separation, and enhance interfacial charge utilization, thereby enabling highly efficient metal‐free aerobic oxidations under visible light irradiation.

1. Introduction

Photocatalysis offers an environmentally benign route to translate solar photons into chemical reactivity for value‐added synthesis under mild conditions [1, 2, 3, 4, 5]. Efficient photocatalysis, however, critically depends on the coordination of photon absorption, excited‐state evolution, interfacial charge transfer, and surface reactions within a single platform. Covalent organic frameworks (COFs) combine modular π‐conjugated architectures with permanent porosity and structural programmability [6, 7, 8, 9, 10, 11], offering a versatile heterogeneous platform for light harvesting, substrate enrichment, and charge transport [12, 13, 14]. COF photocatalysts have therefore been explored for organic transformations [15, 16, 17, 18, 19, 20, 21, 22, 23], H₂ evolution [24, 25, 26, 27], CO₂ reduction [28, 29, 30, 31], and H₂O₂ production [32, 33, 34, 35, 36, 37]. Despite these advantages, the low dielectric constants of organic frameworks favor strong electron‐hole Coulomb attraction, resulting in high exciton binding energies and rapid recombination [38, 39]. Consequently, COF photocatalysis is often constrained by the disparity between efficient light harvesting and effective charge utilization at reactive interfaces.

To alleviate exciton confinement and charge recombination in COFs, diverse strategies have been explored, including conjugation extension [40, 41, 42, 43], donor–acceptor coupling [44, 45, 46, 47, 48], linkage polarity tuning [49, 50], heteroatom incorporation [51, 52], and interface engineering [53, 54]. While these approaches can improve charge separation [55], they often perturb multiple structural and electronic descriptors simultaneously, such as conjugation length, energy levels, polarity, and morphology, complicating direct structure–property correlations. The remaining challenge is therefore to identify a chemically simple, structurally programmable, and electrostatically quantifiable parameter that enables systematic regulation of exciton dynamics within a consistent COF platform.

Herein, we developed linker nitrogen engineering as a chemically well‐defined and countable polarity‐tuning strategy to regulate charge distribution and framework polarization in COFs (Scheme 1). Using porphyrinic units as photoactive nodes, we constructed a COF series in which the bridging linkers were systematically varied from all‐carbon aromatic to pyridinic and bipyridinic units. The number of nitrogen atoms in the linker thus served as an explicit structural parameter for stepwise electrostatic tuning while preserving the same reticular architecture. Increasing linker nitrogen content redistributed intraframework charge density and induced a framework‐scale potential response characteristic of strengthened built‐in electric fields, thereby lowering the effective exciton binding energy, suppressing recombination, and promoting directional charge separation, as corroborated by light‐assisted spectroscopies and theoretical calculations. This electrostatic modulation translated directly into enhanced performance in metal‐free, visible‐light aerobic oxidation, highlighting control through linker nitrogen engineering as a general route to connect molecular design with framework‐scale photocatalytic reactivity.

SCHEME 1.

Schematic illustration of the molecular to framework nitrogen‐modulation strategy.

2. Results and Discussion

2.1. Molecular Design and Structural Characterization of Nitrogen‐Engineered COFs

The successful synthesis of the COFs, TAPP‐BD‐COF, TAPP‐py‐COF, and TAPP‐Bpy‐COF was confirmed by comprehensive structural characterization. Figure 1a showed the structure of TAPP‐Bpy‐COF. Powder x‐ray diffraction (PXRD) patterns indicated that all three COFs exhibited discernible diffraction features consistent with ordered frameworks (Figure 1b). The experimental PXRD profile matched well with the simulated pattern of an AA‐stacked tetragonal model and showed characteristic reflections at 3.0° and 6.1°, assignable to the (100) and (200) planes. Pawley refinement afforded unit cell parameters of a = b = 30.1467 Å and c = 3.5035 Å (α = β = γ = 90°), and the reliability factors Rwp and Rp were below 10%, corroborating the proposed stacking model (Table S1). The PXRD patterns of TAPP‐BD‐COF and TAPP‐py‐COF were provided in the Supporting Information (Figure S3).

FIGURE 1.

Structural characterization of TAPP‐Bpy‐COF. (a) Schematic structure of TAPP‐Bpy‐COF. (b) PXRD pattern and simulated profile. (c) FT‐IR spectrum. (d) Solid‐state ¹³C NMR spectrum. (e) High‐resolution N 1s XPS spectrum. (f) HRTEM image.

Fourier transform infrared (FT‐IR) spectroscopy confirmed the formation of imine linkages in all three COFs, evidenced by pronounced C═N stretching bands at 1619 cm⁻¹ (TAPP‐BD‐COF), 1623 cm⁻¹ (TAPP‐py‐COF), and 1621 cm⁻¹ (TAPP‐Bpy‐COF) (Figures 1c and S4–S5). For TAPP‐Bpy‐COF, a new vibrational band appeared at 1497 cm⁻¹, which was attributed to the pyridyl ring and indicated the incorporation of the bipyridine unit. Solid‐state ¹³C NMR spectroscopy provided additional structural evidence for the successful formation of the COFs (Figures 1d and S6), with characteristic signals for the C═N carbon (154.67 and 151.28 ppm), phenyl and porphyrinic carbons (120–140 ppm). Comparable FT‐IR and solid‐state ¹³C NMR results were obtained for TAPP‐BD‐COF and TAPP‐py‐COF.

X‐ray photoelectron spectroscopy (XPS) further verified the chemical composition and bonding environments of the COFs. Taking TAPP‐Bpy‐COF as a representative example, the high‐resolution C 1s spectrum (Figure S7) showed two components at 284.5 and 286.1 eV, which were assigned to conjugated C═C and C─N/C═N environments, respectively. The N 1s spectrum (Figure 1e) displayed three signals at 397.9, 399.1, and 399.7 eV, corresponding to pyridinic nitrogen, C═N nitrogen, and pyrrolic nitrogen, respectively. Thermogravimetric analysis (TGA) indicated that activated TAPP‐Bpy‐COF remained thermally stable in air up to an onset decomposition temperature above 400°C (Figure S8), suggesting sufficient robustness for photocatalytic studies.

Scanning electron microscopy (SEM) images in Figure S9 showed that TAPP‐BD‐COF, TAPP‐py‐COF, and TAPP‐Bpy‐COF all adopt predominantly sheet‐like morphologies. The high‐resolution transmission electron microscopy (HRTEM) image of TAPP‐Bpy‐COF (Figure 1f) displayed well‐defined lattice fringes, indicative of structural order. The lattice‐fringe inset gave an interplanar spacing of approximately 2.71 nm, which was assigned to the (100) plane. This spacing was consistent with the PXRD results and the simulated structural model, supporting an ordered layered framework. Meanwhile, discernible lattice fringes were also observed in TAPP‐BD‐COF and TAPP‐py‐COF (Figure S10), suggesting that short‐range order was retained across the series. The porosity of the three COFs was evaluated by N₂ adsorption–desorption measurements at 77 K. As shown in Figure S11, the Brunauer–Emmett–Teller (BET) surface areas of TAPP‐BD‐COF, TAPP‐py‐COF, and TAPP‐Bpy‐COF were determined to be 524, 408, and 611 m² g⁻¹, respectively (Figures S11a–c). The pore size distributions derived from non‐local density functional theory (NLDFT) analysis (Figures S11d–f) showed main peaks centered at 2.51, 2.62, and 2.53 nm for TAPP‐BD‐COF, TAPP‐py‐COF, and TAPP‐Bpy‐COF, indicating that the three frameworks possess broadly similar pore structural features. To examine the influence of crystallinity on photocatalytic performance, control samples of TAPP‐Bpy‐COF with different degrees of crystallinity were also prepared (Figure S12). Despite discernible differences in PXRD intensity, their photocatalytic performances remained essentially unchanged (Figure S13).

2.2. Linker Nitrogen‐Induced Polarization Strengthens Built‐In Electric Fields

As shown in Figure 2, increasing linker nitrogen content further modulated the electrostatic environment within the porphyrinic COFs, providing a molecular basis for enhanced charge separation. Density functional theory (DFT) calculations based on representative imine‐linked framework fragments revealed that the dipole moment increased markedly from 1.83 D for TAPP‐BD‐COF to 4.03 D for TAPP‐Bpy‐COF (Figure 2a). These results indicate that, under a common imine‐linked framework background, increasing linker nitrogen content further enhances intraframework polarization and the electrostatic driving force for charge redistribution and directional charge separation in the COFs. The highest occupied molecular orbital (HOMO) was predominantly localized on the porphyrin rings across all three COFs (Figure S14). Meanwhile, the lowest unoccupied molecular orbital (LUMO) distribution varied among the COFs. For TAPP‐Bpy‐COF, the LUMO was primarily distributed over the bipyridine (Bpy) linker, with partial overlap with the HOMO. Further insights into the electron‐hole distribution in the excited state of TAPP‐Bpy‐COF were obtained using time‐dependent density functional theory (TD‐DFT) and electron‐hole excitation analysis (Figure S15). Notably, increasing nitrogen content reshaped the spatial electron‐hole distributions across the three COFs. In the charge density difference maps, the green regions indicated photoinduced electron accumulation, while the blue regions signified electron depletion. Furthermore, small values for the hole delocalization index (HDI) and electron delocalization index (EDI) suggested a high degree of hole and electron delocalization, respectively. The smallest HDI and EDI values observed (HDI = 4.46, EDI = 4.31) for TAPP‐Bpy‐COF implied significant levels of electron and hole delocalization. Zeta potential measurements revealed distinct surface charge responses among the COFs (Figure S16). TAPP‐BD‐COF (−11.6 mV), TAPP‐py‐COF (−21.8 mV), and TAPP‐Bpy‐COF (−36.2 mV) exhibited negative zeta potentials, consistent with strengthened interfacial electrostatic interactions.

FIGURE 2.

Charge polarization and surface photovoltage characterization. (a) Electrostatic potential (ESP) maps and calculated dipole moments of the COFs. (b) Spatially resolved surface photovoltage (SPV) spectra at the nanoscale under illumination. (c) Schematic illustration of the Kelvin probe force microscopy (KPFM) measurements of the contact potential difference (CPD), together with representative topography images and 3D CPD maps recorded in the dark and under 405 nm illumination. (d) Probability distributions of CPD in the dark and under illumination. (e) Differential surface potential response along the cross section.

To gain deeper insight into the charge‐transfer process, surface photovoltage (SPV) was measured by Kelvin probe force microscopy coupled with surface photovoltage spectroscopy (KPFM‐SPS). Figure 2b showed the SPV signals of the COFs, and TAPP‐Bpy‐COF exhibited the strongest response, suggesting more efficient photoinduced charge separation upon enhanced framework polarity. A light‐assisted Kelvin probe force microscopy (KPFM) was further performed to obtain spatially resolved surface potential maps (Figure 2c). A two‐pass lift‐mode protocol was used to acquire topography and CPD maps while minimizing topography‐potential crosstalk: the surface topography was first recorded in tapping mode, and the tip then retraced the stored profile at a fixed lift height to measure CPD. KPFM measured the contact potential difference (CPD) between a conductive tip and the sample surface, and the photoinduced CPD change was quantified as |ΔCPD| = |CPD _light − CPD _dark|. The comparable morphological features observed across samples minimized CPD variations arising from height differences, suggesting that the measured CPD contrast primarily reflected intrinsic electronic properties rather than topography‐induced offsets [56, 57, 58]. TAPP‐Bpy‐COF showed the largest ΔCPD (Figure 2d), and statistical analysis yielded a substantially larger photoinduced potential shift (ΔCPD = 66 mV) than TAPP‐py‐COF (35.5 mV), whereas TAPP‐BD‐COF exhibited only a weak shift with strong overlap between the dark and light distributions. Consistently, this enhancement stemmed from the strengthened built‐in electric fields (Figure 2e), which increased from 75 kV cm⁻¹ (TAPP‐BD‐COF) to 368 kV cm⁻¹ (TAPP‐Bpy‐COF).

2.3. Built‐In Electric Fields Lower Exciton Binding Energy and Promote Charge Separation

Larger photoinduced surface potential shifts signaled stronger built‐in electric fields and a greater electrostatic driving force at the framework level. These strengthened built‐in electric fields favored exciton dissociation, suppressed geminate recombination, and promoted more efficient charge separation at the early stage. Figure 3 offered further insight into the excited‐state kinetics governing exciton unbinding and charge separation. Notably, all COFs displayed distinct electron paramagnetic resonance (EPR) signals (g = 2.003) under illumination, evidencing the formation of photoinduced paramagnetic species (Figure S17). Among them, TAPP‐Bpy‐COF exhibited the strongest EPR signal, suggesting the highest steady‐state population of photoinduced paramagnetic species under identical conditions. Time‐resolved photoluminescence (TRPL) measurements (Figure 3a) revealed that TAPP‐Bpy‐COF exhibited a longer lifetime (7.92 ns) than TAPP‐BD‐COF (4.92 ns) and TAPP‐py‐COF (6.54 ns), suggesting slower decay dynamics.

FIGURE 3.

Excited‐state dynamics and temperature‐dependent photoluminescence of the COF series. (a) Time‐resolved photoluminescence (TRPL) decay traces. (b) 2D pseudo‐color femtosecond transient absorption (fs‐TAS) spectra of TAPP‐BD‐COF, TAPP‐py‐COF, and TAPP‐Bpy‐COF under 420 nm excitation. (c) Decay kinetics extracted at 600 nm for TAPP‐BD‐COF, TAPP‐py‐COF, and TAPP‐Bpy‐COF. (d–f) Temperature‐dependent steady‐state photoluminescence (PL) spectra recorded under 430 nm excitation (100–280 K), with Arrhenius fits used to determine the exciton binding energy (E _b).

Femtosecond transient absorption spectroscopy (fs‐TAS) was employed to probe the early‐time excited‐state dynamics (Figure 3b). Upon 420 nm excitation, all three COFs exhibited pronounced negative features at 480 and 600 nm, which were assigned to the ground‐state bleach (GSB) of the Soret and Q bands, respectively. A negative band at 670 nm overlapped with the steady‐state photoluminescence (PL, Figure S18) spectrum and was attributed to stimulated emission (SE) from the S₁–S₀ transition. The Q‐band GSB recovery kinetics extracted at 600 nm (0.5–5000 ps, Figure 3c) progressively slowed from TAPP‐BD‐COF (τ₁ = 1.71 ps, τ₂ = 22.87 ps) to TAPP‐py‐COF (τ₁ = 4.71 ps, τ₂ = 38.57 ps) and further to TAPP‐Bpy‐COF (τ₁ = 9.96 ps, τ₂ = 48.51 ps). This trend suggested more strongly suppressed excited‐state decay and longer‐lived photoinduced species in the bipyridine‐bridged framework, which facilitated subsequent charge transfer and catalysis.

To further examine the influence of the inserted linker structure on exciton dissociation, the exciton binding energy (E _b) was derived from temperature‐dependent PL spectra. E _b was used to describe the Coulomb attraction between photogenerated electrons and holes, making it a key parameter for determining the kinetics of exciton dissociation [59, 60]. The PL peak intensity of all three COFs increased as the temperature was lowered from 280 to 100 K (Figure 3d–f), mainly because thermally activated nonradiative decay channels were progressively suppressed at lower temperatures [61]. The apparent E _b was extracted by fitting the temperature‐dependent PL data to an Arrhenius‐type relation. Here, E _b reflected the effective barrier for thermally assisted exciton unbinding in these frameworks. By contrast, TAPP‐Bpy‐COF showed the lowest E _b (32.6 meV), markedly lower than that of TAPP‐BD‐COF (62.2 meV) and TAPP‐py‐COF (36.9 meV). The reduced E _b thus facilitated exciton dissociation, favoring charge separation and suppressing electron‐hole recombination. Taken together, these electrostatic and photophysical results establish a direct relationship between linker nitrogen variation, framework electrostatics, exciton dynamics, and photocatalytic performance in a series of structurally related COFs and provide the mechanistic basis for the superior photocatalytic performance of TAPP‐Bpy‐COF.

The optical and photoelectrochemical properties were investigated by UV–vis diffuse reflectance spectroscopy (DRS) and photoelectrochemical measurements. UV–vis diffuse reflectance spectra (Figure S19) showed a slight red shift for TAPP‐Bpy‐COF, suggesting only a modest change in visible‐light absorption among the three COFs. Therefore, the enhanced photocatalytic performance of TAPP‐Bpy‐COF is unlikely to be primarily governed by enhanced light harvesting, but is more closely associated with improved exciton dissociation and charge separation. The corresponding optical band gaps were estimated from Tauc plots (Figure S20), revealing that TAPP‐Bpy‐COF possessed the narrowest band gap. Mott–Schottky (M–S) measurements (Figure S21) revealed positive slopes in the linear regions, confirming the n‐type semiconductor nature of the COFs and indicating that electrons were the predominant charge carriers. As illustrated in Figure S22, the conduction band (CB) positions of these COFs are sufficiently negative to enable the reduction of O₂ to •O₂ ⁻ (−0.33 V vs NHE). The corresponding valence band (VB) positions are sufficiently positive to support oxidation of the organic substrates by photogenerated holes. These band edge alignments are therefore consistent with the proposed photocatalytic pathway. Electrochemical impedance spectroscopy (EIS, Figure S23) and transient photocurrent measurements (Figure S24) offered additional evidence for improved charge separation and more efficient charge extraction under illumination in TAPP‐Bpy‐COF. In the Nyquist plots, TAPP‐Bpy‐COF displayed the smallest semicircle, together with the highest photocurrent density, indicating the lowest charge‐transfer resistance and the most efficient photogenerated charge extraction under illumination.

2.4. Photocatalytic Dehydrogenative Aromatization and Oxidative Coupling Reactions

Quinoline derivatives are widely used in the fine chemical and pharmaceutical industries. Accordingly, the visible‐light‐driven oxidative dehydrogenation of 1,2,3,4‐tetrahydroquinoline (THQ) was selected as a model reaction to evaluate the impact of linker nitrogen modulation on photocatalytic reactivity. To elucidate the relationship between conversion and reaction time, aliquots of the reaction mixture were withdrawn at selected time intervals and analyzed by gas chromatography‐mass spectrometry (GC‐MS) (Figure 4a). The resulting time‐conversion profiles identified TAPP‐Bpy‐COF as the most active photocatalyst among the three COFs. Control experiments excluded background reactivity. Conversion remained below 1% without light irradiation, the catalyst, or O₂, and reached only 48% when O₂ was replaced by air (Figure 4b). The wavelength‐dependent activity closely overlapped with the absorption spectrum of TAPP‐Bpy‐COF, with the highest activity observed at 420 nm (Figure S25). TAPP‐Bpy‐COF displayed high durability, maintaining >90% of its initial activity over five consecutive runs (Figure 4c). Various substrates featuring diverse substituents were examined to expand the scope, and all were efficiently converted to the corresponding dehydrogenated products (Figures 4d and S26–S33). Furthermore, among reported photocatalysts for photooxidative THQ dehydrogenation, TAPP‐Bpy‐COF achieved an outstanding product yield and compared favorably with most reported photocatalysts (Table S2).

FIGURE 4.

Visible‐light‐driven photocatalytic oxidations across the three COFs. (a) Time‐dependent dehydrogenative aromatization of 1,2,3,4‐tetrahydroquinoline (THQ) catalyzed by TAPP‐BD‐COF, TAPP‐py‐COF, and TAPP‐Bpy‐COF, with error bars representing the standard deviation from three independent experiments. (b) Control experiments for THQ dehydrogenative aromatization using TAPP‐Bpy‐COF under varied reaction conditions. (c) Recycling performance of TAPP‐Bpy‐COF for THQ dehydrogenative aromatization. (d) Substrate scope for the dehydrogenative aromatization of THQ. (e) Oxidative coupling of benzylamine (BA). Typical conditions: substrate (0.1 mmol), catalyst (3 mg), MeCN (3 mL), O₂ atmosphere, 30 W blue LED (420 nm). Conversions and selectivities were determined by gas chromatography‐mass spectrometry (GC‐MS) (see the Supporting Information for details).

Encouraged by its superior activity in dehydrogenative aromatization, the photocatalytic performance of TAPP‐Bpy‐COF was further evaluated for the aerobic oxidation of benzylamine (BA, Figure 4e). Under identical conditions, TAPP‐Bpy‐COF delivered markedly higher substrate conversion than the reference TAPP‐BD‐COF and TAPP‐py‐COF at matched reaction times (Figure S34). After six consecutive cycles (Figure S35), TAPP‐Bpy‐COF still afforded comparable activity, evidencing excellent operational stability. Notably, benzylamine and all examined derivatives reached near‐quantitative conversion (>99%) to the corresponding coupled products within 1 h over TAPP‐Bpy‐COF (Figures S36–S43), regardless of whether the substrates bore electron‐donating substituents (─Me, ─OMe) or electron‐withdrawing substituents (─F, ─Cl, ─Br). Comparative analysis with reported metal‐based and metal‐free catalysts further highlighted the highly competitive performance of the nitrogen‐modulated porphyrinic COF system (Table S3).

2.5. Active Species Identification and Mechanistic Insights Into Nitrogen‐Induced Photocatalytic Oxidation

To probe the photocatalytic mechanism and clarify structure–function relationships, trapping experiments and electron paramagnetic resonance (EPR) measurements were performed, together with density functional theory (DFT) calculations. Under an O₂ atmosphere, TAPP‐Bpy‐COF delivered the best catalytic performance. This trend was consistent with the control experiments described above, which further confirmed that O₂ was indispensable for the reaction. Thus, a key question concerned the mechanistic role of O₂ in enabling the high conversion observed in photocatalytic oxidation. Scavenger tests were conducted using triethanolamine (TEOA, hole scavenger), benzoquinone (BQ, •O₂ ⁻ scavenger), isopropanol (IPA, •OH scavenger), and 1,4‐diazabicyclo[2.2.2]octane (DABCO, ¹O₂ scavenger) (Figure 5a,b). The results suggested a negligible contribution from •OH, whereas •O₂ ⁻ and h⁺ were identified as the primary reactive species in the photocatalytic oxidation reactions. In contrast, DABCO caused only a modest inhibitory effect, consistent with a limited contribution of ¹O₂ under these conditions. EPR spin‐trapping was used to assess the propensity of the COFs to generate reactive oxygen species and radical intermediates under illumination (Figures 5c and S44). 5,5‐dimethylpyrroline‐N‐oxide (DMPO) and 2,2,6,6‐tetramethylpiperidine (TEMP) were employed to trap •O₂ ⁻ and ¹O₂, respectively. In our system, TAPP‐Bpy‐COF showed a stronger propensity to activate O₂ toward •O₂ ⁻ formation, while ¹O₂ was detectable but remained a non‐dominant pathway, in agreement with the scavenger results.

FIGURE 5.

Active species identification and mechanistic insights across the three COFs. (a) Active species trapping experiments for oxidative dehydrogenation of THQ. (b) Active species trapping experiments for oxidative coupling of BA. (c) Spin‐trapping EPR spectra using DMPO as spin trap. (d) Time‐dependent generation of •O₂ ⁻. Experimental conditions: 25 µM NBT in CH₃OH (50 mL), 5 mg catalyst, stirred in the dark for 30 min, 30 W, 420 nm LED. (e) O₂ ⁻TPD profiles of TAPP‐BD‐COF, TAPP‐py‐COF, and TAPP‐Bpy‐COF. (f) Gibbs free energy diagram for oxygen reduction reaction (ORR) into H₂O₂. (g) Proposed photocatalytic reaction pathways for THQ dehydrogenation and BA oxidative coupling.

To gain qualitative insight into the relative O₂ interaction tendencies of the three COFs, we calculated the adsorption configurations and adsorption energies of O₂ at the biphenyl, pyridine, and bipyridine linker sites (Figure S45). DFT calculations showed that the O₂ adsorption energies were highly comparable across the three COFs, indicating only limited differences in O₂ binding strength. To further probe the O₂ activation behavior under photocatalytic conditions, nitroblue tetrazolium chloride (NBT) was employed to evaluate the relative generation of •O₂ ⁻ (Figure 5d) [62]. The results showed that the relative superoxide generation trend among the three COFs was consistent with the EPR results, thereby providing supplementary experimental evidence for their different relative O₂ activation tendencies. In addition, O₂ temperature‐programmed desorption (O₂‐TPD) experiments were conducted to examine the interaction between O₂ and the catalysts. As shown in Figure 5e, compared with TAPP‐BD‐COF and TAPP‐py‐COF, TAPP‐Bpy‐COF exhibited a stronger O₂ desorption response. Gibbs free energy analysis (Figure 5f) indicated that TAPP‐Bpy‐COF exhibited the most favorable thermodynamic pathway for the two‐electron reduction of O₂ to H₂O₂. In particular, the formation of the key *OOH intermediate on TAPP‐Bpy‐COF involved a lower Gibbs free energy change than on TAPP‐BD‐COF and TAPP‐py‐COF. Furthermore, the experimentally detected H₂O₂ formation trend was consistent with the Gibbs free energy analysis (Figure S46), providing additional support for the thermodynamic favorability of TAPP‐Bpy‐COF for the two‐electron O₂ reduction pathway. Taken together, these results suggest that •O₂ ⁻ and h⁺ are the primary reactive species responsible for substrate oxidation, whereas ¹O₂ contributes only to a minor extent; the detected H₂O₂ should be regarded mainly as a concomitant oxygen‐reduction product and a thermodynamic/experimental indicator of O₂ activation tendency, rather than the dominant oxidizing species in the present transformations.

As depicted in Figure 5g, a plausible mechanism for THQ dehydrogenation and BA oxidative coupling over TAPP‐Bpy‐COF was proposed based on the above results [23, 63, 64, 65]. The transformation proceeded predominantly via a charge‐transfer pathway that generated •O₂ ⁻. An energy‐transfer channel producing ¹O₂ could also have operated but likely contributed to a lesser extent. Upon light irradiation, electrons in TAPP‐Bpy‐COF were promoted from the valence band (VB) to the conduction band (CB), leaving photogenerated holes in the VB. The resulting charge separation enabled electron transfer to O₂ to generate •O₂ ⁻ and allowed the photogenerated holes to oxidize the substrate. In the THQ dehydrogenation reaction, photogenerated electrons reduced O₂ to the superoxide radical anion (•O₂ ⁻), while holes oxidized THQ to the corresponding radical cation. The resulting •O₂ ⁻ then facilitated sequential proton‐coupled electron‐transfer steps from this intermediate, thereby forming the C═N bond. Repetition of this sequence on the partially dehydrogenated intermediate ultimately completed aromatization to quinoline, with H₂O₂ concomitantly formed via O₂ reduction. In the benzylamine coupling reaction, benzylamine was oxidized by photogenerated holes to form the benzylamine radical cation (Ph‐CH₂NH₂ ^•+). The resulting •O₂ ⁻ then abstracted a proton from this radical cation to generate the imine intermediate (Ph─CH═NH) and H₂O₂. The imine intermediate then reacted with benzylamine to afford the coupled product.

3. Conclusion

In summary, this work demonstrated that linker nitrogen engineering provided a chemically countable electrostatic handle in porphyrinic COFs, enabling a direct and programmable link between molecular polarity and framework‐scale potential landscapes within a constant reticular platform. Beyond the intrinsic electrostatic contribution of the imine‐linked framework, linker nitrogen modulation further reshaped the internal electrostatic environment and strengthened the framework‐scale built‐in electric fields. These changes lowered the effective exciton binding energy, suppressed recombination, facilitated directional charge separation, and improved interfacial charge utilization. Importantly, these results established a clear correlation among structural modulation, internal electrostatics, exciton dynamics, and photocatalytic performance in ordered organic frameworks, and identified linker nitrogen engineering as an effective strategy for strengthening built‐in electric fields and promoting charge separation. This chemically countable design parameter thus offered a general design principle for the rational development of next generation COF photocatalysts and related organic framework catalysts.

Author Contributions

Junshan Li: conceptualization, formal analysis. Enwei Zhu: conceptualization, supervision. Chou‐Hung Hsueh: investigation. Hang Su: visualization. Jingyi Xu: writing – review and editing. Yujia Li: investigation, formal analysis, software. Meichi Chong: visualization, writing – review and editing. Qi Zhang: investigation, formal analysis, writing – review and editing. Siming Wang: investigation, formal analysis, writing – original draft, methodology. Jiaming Zhang: conceptualization, supervision, writing – review and editing. Yongfa Zhu: conceptualization, supervision, funding acquisition. Xiaolin Zhu: conceptualization, supervision, funding acquisition.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie72822‐sup‐0001‐SuppMat.pdf

Data Availability Statement

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