Design of a nitrogen-rich perylene-triazine porous organic polymer for iodine and CO₂ adsorption

Abstract

Porous organic polymers (POPs) have emerged as promising materials for environmental remediation owing to their high surface area, structural tunability, and chemical stability. In this work, we present a nitrogen-rich POP named PYTP, synthesized via a solvent-assisted polycondensation process from triazine, perylene, and pyridine-based polymeric building blocks. The resulting polymer features a nitrogen-rich conjugated framework and was obtained in high yield using a simple, scalable method. Structural characterization through FT-IR, solid-state ¹³C CP-MAS NMR, and FE-SEM confirmed the successful formation and stability of the PYTP framework. BET surface area analysis revealed a specific surface area of 135 m² g⁻¹ and an average pore size of 1.2 nm, indicating a microporous nature favourable for gas uptake. The PYTP polymer exhibited outstanding performance in iodine capture, with a vapor adsorption capacity of 355 wt% and 97% removal efficiency from hexane solution. Elemental mapping confirmed the presence of iodine, and recyclability tests demonstrated excellent reusability over multiple cycles in both vapor and solution phases. Significantly, PYTP also achieved high removal efficiency of crude iodine under real-time conditions, highlighting its applicability for practical environmental remediation scenarios. Furthermore, CO₂ adsorption studies showed a maximum uptake of ∼50 cc g⁻¹ (9.81 mg g⁻¹), indicating good CO₂ uptake capacity with no additional adsorption beyond P/P₀ ≈ 0.9. This behaviour reflects efficient pore filling and strong CO₂–framework interactions. These findings underscore PYTP as a highly effective, reusable, and dual-functional adsorbent with strong potential for real-time radioactive iodine removal, carbon capture, and broader nuclear waste management applications.

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1. Introduction

Growing concerns over climate change, environmental degradation, and the accelerating rise in greenhouse gas emissions have intensified the global urgency to transition from fossil fuels to more sustainable and environmentally friendly energy sources. Among these alternatives, nuclear power stands out as a reliable, low-emission energy option capable of meeting large-scale energy demands with a minimal carbon footprint.^1–5 Nevertheless, despite its advantages, the large-scale deployment of nuclear technology faces critical environmental and safety challenges, among them, the long-term management and safe disposal of radioactive waste.^6–8 A particularly problematic class of nuclear waste includes volatile radioactive iodine isotopes, such as iodine species (¹²⁹I and ¹³¹I), which possess long half-lives and high environmental mobility, enabling them to migrate into the atmosphere or aquatic systems and eventually accumulate in the food chain.^9–12 Once ingested, these isotopes concentrate in the thyroid gland and disrupt biological processes, leading to severe long-term health effects, including cancer. At the same time, the continued rise in carbon dioxide (CO₂) emissions from fossil fuel combustion remains the primary driver of global warming, ocean acidification, and climate instability.^13–15 These dual challenges highlight the urgent need for advanced functional materials capable of not only capturing and immobilizing elemental iodine (I₂) but also efficiently adsorbing CO₂. Such dual-function materials would offer significant promise for addressing both nuclear waste management and greenhouse gas mitigation, thereby contributing to environmental safety and sustainable energy development.^16–19

In recent times, adsorption-based technologies have been recognized as among the most effective strategies to mitigate environmental pollutants. These approaches offer effective solutions for both CO₂ sequestration and iodine capture because of their versatility, scalability, operational simplicity, and cost-effectiveness.^20–22 Among the various adsorbent materials investigated, porous organic polymers (POPs) have gained significant attention due to their intrinsic features, including high surface areas, tunable pore structures, and excellent thermal and chemical stability. These properties make POPs ideal candidates for environmental remediation, particularly for the uptake of greenhouse gases such as CO₂ and volatile contaminants like iodine. The adsorption performance of POPs can be further improved by increasing the number of active sites within their framework, which facilitates stronger host–guest interactions and enhances overall uptake efficiency.^23–27 One effective strategy is the incorporation of heteroatoms (e.g., N, S, P, O and B) into the porous network. The lone pair electrons of these heteroatoms generate Lewis basic sites that strengthen iodine binding through Lewis acid–base interactions, while also promoting CO₂ capture via dipole–quadrupole interactions.^28–31 Beyond heteroatom functionalization, perylene units have also been incorporated as structural motifs to enhance adsorption properties. The extended π-conjugation and inherent stability of perylene enable strong π–π stacking and charge-transfer interactions, which are essential for capturing and stabilizing iodine species within the framework.^32–35 In addition, the highly conjugated perylene domains improve CO₂ uptake by increasing framework polarizability and providing favorable quadrupole interaction sites.^36–38

This study presents the synthesis and characterization of a novel porous polymer, PYTP, obtained via a solvent-assisted polycondensation method. The design employs a multi-monomer strategy in which pyridine, a nitrogen-rich electron-donating heterocycle, is incorporated into a conjugated backbone constructed from triazine and perylene moieties. By adopting this design, the incorporation of pyridine not only increases the overall nitrogen content but also provides additional electron density and coordination sites, making the polymer more interactive toward iodine species. This structural modification yields a π-conjugated, nitrogen-enriched conjugated framework exhibits high stability, excellent chemical and thermal resistance, and robust mechanical properties, making it suitable for deployment in radioactive environments. Importantly, the PYTP framework establishes an Acceptor–Acceptor–Donor (A–A–D) configuration, in which electron-deficient triazine and perylene units are coupled with electron-rich pyridine. This configuration enhances electron mobility within the framework, promotes strong host–guest interactions, and significantly improves iodine binding efficiency. In addition to iodine uptake, PYTP demonstrated efficient CO₂ adsorption, attributed to its microporous structure and nitrogen functionalities that facilitate dipole–quadrupole interactions with CO₂ molecules. The adsorption performance of PYTP was further validated under practical conditions, where iodine-spiked real water samples were efficiently purified, confirming its strong potential for real-world applications. To verify these structural and functional features, a comprehensive physicochemical characterization was carried out, including FT-IR, solid-state ¹³C CP-MAS NMR, HRMS, FE-SEM, TGA, XPS, and BET surface area analysis. All of these analyses confirmed the formation of a nitrogen-rich, stable, and structurally robust framework. In summary, these attributes establish PYTP as a multifunctional platform with strong potential for the adsorption of both radioactive iodine and CO₂, particularly in realistic environmental scenarios.

2. Experimental section

2.1. Reagents and solvents

Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) was obtained from TCI Chemicals, while 2-pyridinecarboxaldehyde, 2,4-diamino-6-methyl-1,3,5-triazine, and imidazole were purchased from Spectrochem and Sigma Aldrich. In addition, analytical grade solvents including ethanol, methanol, hexane, and ethyl acetate were procured from Avra Chemicals and used directly for the experimental work. All reagents were of commercial grade and employed without further purification throughout the study.

2.2. Characterization methods

A variety of analytical techniques were employed to characterize the synthesized materials. ¹H NMR spectra were obtained on a 400 MHz Bruker spectrometer, while solid-state ¹³C cross-polarization magic angle spinning (CP-MAS) NMR spectra were recorded at 500 MHz and 100 MHz using Bruker instruments. Fourier-transform infrared (FT-IR) spectra were measured in ATR mode with a JASCO-4100 spectrometer, and high-resolution mass spectrometry (HRMS) was carried out using a Waters Xevo G2XS-QT system. UV-visible absorption spectra were acquired on an Agilent 8453 diode array spectrophotometer with hexane as the solvent. The surface morphology and microstructural features were analyzed by field emission scanning electron microscopy (FE-SEM) using a Quanta 250 FEG instrument (Thermo Fisher Scientific). Thermogravimetric analysis (TGA) was performed on a TA Instruments SDT Q600 thermal analyzer under a nitrogen atmosphere to assess thermal stability. Powder X-ray diffraction (PXRD) analysis was carried out using an X’Pert powder diffractometer from Malvern Panalytical. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a ULVAC-PHI VersaProbe 4 spectrometer equipped for elemental analysis. Gas sorption measurements, including BET surface area analysis and CO₂ adsorption isotherms, were performed using a Quantachrome Nova Station B gas sorption analyzer.

2.3. Synthetic procedure

Synthesis of (E)-6-(2-(pyridin-2-yl)vinyl)-1,3,5-triazine-2,4-diamine (PY-TRI). For the synthesis, 2-pyridinecarboxaldehyde (0.19 g, 1.75 mmol) was first dissolved in 25 mL of methanol, and the resulting solution was added dropwise to another methanolic solution (25 mL) containing 6-methyl-1,3,5-triazine-2,4-diamine (310 mg, 2.20 mmol) and 20% KOH. The reaction mixture was stirred continuously at 60 °C for approximately 5 days, and its progress was monitored by thin-layer chromatography (TLC). After completion, the mixture was cooled to room temperature. The resulting precipitate was collected by filtration and air-dried to give a yellow crystalline solid.

The spectroscopic characterization of compound PY-TRI (Yield: 88%) revealed distinct FT-IR absorptions at 3483 cm⁻¹ (–NH₂), 1506 cm⁻¹ (C=CH), 3211.45 cm⁻¹ (C–H), and 1536.51 cm⁻¹ (C=C). The solid-state ¹³C CP-MAS NMR spectrum displayed resonances at 133 ppm (C=C), 124 and 126 ppm (aromatic carbons), 163 ppm (triazine carbon), and 142 ppm (C=CH), as shown in Fig. S2. High-resolution mass spectrometry (HRMS) confirmed the molecular ion peak at m/z 215.1051 (Fig. S3), consistent with the molecular formula C₁₀H₁₀N₆.

Synthesis of PYTP polymer. Compound PY-TRI (0.45 g, 1.37 mmol) was combined with perylene-3,4,9,10-tetracarboxylic dianhydride (1.05 g, 2.68 mmol) in 55 g of imidazole. Zinc acetate (0.38 g, 1.72 mmol) served as the catalyst, and the reaction mixture was maintained at 150 °C under continuous stirring for 48 h. Following cooling to room temperature, the mixture was poured into methanol and stirred briefly to precipitate the polymer. The solid was repeatedly washed with methanol to remove excess imidazole and unreacted monomers. Finally, the product was collected by filtration, dried thoroughly, and obtained as a deep maroon polymeric material, hereafter referred to as PYTP.

The spectroscopic analysis of PYTP (Yield: 90%) revealed characteristic FT-IR absorptions at 1344 cm⁻¹, corresponding to the imide group, and at 1763.54 cm⁻¹, assigned to the carboxylic dianhydride. In the solid-state ¹³C CP-MAS NMR spectrum, signals were observed at 161 ppm, attributed to triazine carbons, while resonances at 121 and 122 ppm were assigned to C=N triazine groups. Additional peaks at 178 ppm were imide group, and further resonances at 140 and 143 ppm corresponded to CH=CH carbons.

2.4. Iodine vapor adsorption

In this study, iodine vapor adsorption was evaluated by placing approximately 10 mg of PYTP polymer in an open vial, which was subsequently placed inside a tightly sealed glass container with excess crystalline iodine placed at the base. The container was maintained at 75 °C under ambient pressure to create a saturated iodine vapor atmosphere. At predetermined time intervals, the vial was removed, allowed to cool to room temperature, and weighed to determine the iodine uptake. The adsorption capacity was expressed as a weight percentage and calculated using the following equation:

a = (m₂ − m₁)/m₁ × 100 wt%

where m₁ (g) is the initial polymer mass and m₂ (g) is the mass after iodine adsorption.

2.5. Adsorption of Iodine from hexane solution

The kinetics of iodine adsorption in organic solution were studied using an iodine/n-hexane system. A solution was prepared by dissolving 10 mg of iodine in 10 mL of n-hexane and sonicating it for 10 min to ensure complete dissolution and homogeneity. Approximately 10 mg of the PYTP polymer was then introduced into the iodine/n-hexane solution, and the adsorption process was monitored at defined time intervals. The residual iodine concentration in the supernatant was determined using UV-Vis spectroscopy, and the values were calculated using the Lambert–Beer law. The adsorption capacity (q_t, mg g⁻¹) was calculated according to the following equation:

q_t = (C₀ − C_t)/W × M

where C₀ and C_t (mg mL⁻¹) are the initial and time-dependent iodine concentrations, V is the solution volume (10 mL), and W is the mass of the PYTP polymer (10 mg). In this study, n-hexane was used as a nonpolar solvent that does not react with either iodine or the polymer, thereby preventing side reactions and enabling direct comparison with previous literature reports. For recyclability evaluation, the PYTP–I₂ powder was immersed in ethanol for 24 h to remove the adsorbed iodine. The material was then thoroughly dried and reused for subsequent adsorption–desorption cycles to assess its reusability.

2.6. CO₂ adsorption measurements

The CO₂ adsorption–desorption isotherms were measured using a Quantachrome Nova Station B gas sorption analyzer at 273 K. Before analysis, the sample was activated by degassing under vacuum at 200 °C for 4 h to remove physisorbed species. High-purity CO₂ was used as the adsorbate, and the isotherm was recorded over a relative pressure (P/P₀) range from approximately 0 to 1. All adsorption points were collected under equilibrium conditions using the standard equilibration settings of the instrument. The CO₂ uptake values were obtained directly from the recorded isotherm, and the specific surface area was calculated from the low-pressure region using the multipoint BET method. Data processing and analysis were performed using the NovaWin software.

3. Results and discussion

The synthetic pathway for PYTP is shown in Scheme 1. The molecular architecture of PYTP combines three structural motifs: pyridine, triazine, and perylene. Among these, perylene and triazine act as electron-accepting components, while pyridine functions as a donor unit and introduces additional nitrogen coordination sites into the framework. This arrangement establishes an Acceptor–Acceptor–Donor (A–A–D) type electronic configuration, incorporating extended π-conjugation, electron-withdrawing acceptor sites, and nitrogen donor sites within a single structure. Such a design is expected to promote strong host–guest interactions with iodine through Lewis acid–base and charge-transfer mechanisms, while simultaneously enhancing CO₂ adsorption via dipole–quadrupole interactions. The polymer was obtained as a black colour solid and exhibited high stability, making it suitable for subsequent adsorption studies.

Scheme 1 Synthetic route of PYTP polymer.

Solid-state ¹³C CP-MAS NMR spectroscopy serves as a powerful technique for structural characterization, especially useful for materials with limited solubility in common organic solvents. Using this method, both the intermediate (PY-TRI) and the final polymer were analyzed. In the ¹³C CP-MAS spectrum of PY-TRI (Fig. S1), a resonance at 133 ppm is attributed to the vinylene linkage, confirming the successful formation of the intermediate. Additional confirmation was obtained from FT-IR spectroscopy, which displayed a characteristic absorption band at 1506 cm⁻¹ corresponding to the vinylene C=C stretching vibration as depicted in Fig. 1. Further confirmation was provided by HRMS-ESI (Fig. S2), where a molecular ion peak at m/z 215.1051 was consistent with the calculated value. The validated PY-TRI intermediate was subsequently subjected to polycondensation with perylene, affording the target PYTP polymer. The formation of PYTP was confirmed by FT-IR and solid-state ¹³C CP-MAS NMR spectroscopy. In the FT-IR spectrum of the monomer PYR-CHO, a strong C=O stretching band was observed at ∼1690 cm⁻¹, along with characteristic aldehyde C–H stretching vibrations in the region of 2920–2850 cm⁻¹ shown in the Fig. 1. For the intermediate PY-TRI, the aldehyde C=O band disappeared, while new absorptions appeared at ∼1590 cm⁻¹, assigned to C=N stretching, and at ∼1506 cm⁻¹, attributed to the CH=CH linkage.³⁹ A broad absorption at 3483 cm⁻¹ corresponded to the –NH₂ stretching vibrations of the triazine unit. In the final polymer PYTP, the disappearance of the –NH₂ stretching band at 3483 cm⁻¹, together with the emergence of a new absorption at 1344 cm⁻¹ characteristic of the imide functionality, confirmed the conversion of the intermediate into the polymer.⁴⁰ A minor band at 1763 cm⁻¹ is also observed, which reflects the presence of a small fraction of dianhydride carbonyl groups naturally embedded within the rigid porous PYTP network. Such residual C=O features are well documented in porous organic polymers, where steric hindrance and restricted diffusion inherently limit the accessibility of internal reactive sites during framework formation.⁴¹ This behaviour is characteristic of robust POP architectures and is fully consistent with the successful formation of the imide-linked PYTP polymer. Additional bands corresponding to C=O and C–N stretching further supported the incorporation of imide and pyridine units into the framework.

Fig. 1 FT-IR spectra of compound PYR-CHO, PY-TRI and PYTP polymer.

The ¹³C CP-MAS NMR spectrum of PYTP Fig. S3 provided further structural validation. Resonances at 161 and 162 ppm were attributed to carbon atoms in the triazine core, consistent with values reported for triazine-based polymers. A distinct resonance at 178 ppm was assigned to the imide carbonyl carbon, comparable to values observed in imide-linked structures. Signals in the region around 140 ppm were attributed to aromatic CH carbons of the perylene units, while a resonance at 122 ppm was assigned to the vinylene carbons bridging the triazine and pyridine groups. Minor variations in chemical shifts are likely due to structural features unique to PYTP.

3.1. N₂ adsorption isotherm

The porosity of PYTP was assessed by nitrogen adsorption–desorption analysis at 77 K. The BET surface area was found to be 135 m² g⁻¹, exceeding that of previously reported perylene–triazine frameworks and indicating superior textural properties. The resulting isotherm displayed a Type I profile Fig. 2(a), characterized by a sharp uptake in the low-pressure region (P/P₀ < 0.1) followed by a plateau, confirming the predominantly microporous nature of the framework.⁴¹ The pore size distribution Fig. 2(b) further supported this observation, showing a dominant pore width of 1.2 nm. The presence of uniform micropores is particularly advantageous, as they provide abundant binding sites for guest molecules and thereby facilitate efficient adsorption of both iodine and CO₂.

Fig. 2 (a) Nitrogen adsorption–desorption isotherm of PYTP and (b) Pore size distribution curve of PYTP.

3.2. FE-SEM and mapping

The surface morphology of PYTP was characterized using field-emission scanning electron microscopy (FESEM). As shown in Fig. 3(a), PYTP exhibits a flake-like morphology consisting of compactly arranged fragments with rough surfaces. This morphology is expected to provide abundant contact sites, thereby enhancing surface interactions and improving adsorption capacity. After iodine uptake, PYTP, now denoted as PYTP–I₂, exhibited distinct morphological changes, as shown in Fig. 3(b), with iodine deposits clearly visible on the surface as well as within the pores of the polymer framework. These observations were further supported by elemental mapping analysis. For PYTP, the mapping images revealed the presence of carbon (C), nitrogen (N), and oxygen (O), in agreement with the expected polymer composition. In contrast, PYTP–I₂ displayed an additional iodine (I) signal along with C, N, and O, confirming the successful capture of iodine. The uniform distribution of iodine throughout the structure demonstrates its effective integration into the porous framework. These results highlight the strong affinity of PYTP toward iodine species and reinforce its potential as a highly efficient adsorbent material. The EDX spectrum in the image shows distinct peaks confirming the presence of C, N, O, and I in the sample as shown in Fig. S1(a) and Fig. S4 (b). The strong iodine peaks around 4 keV clearly indicate its significant contribution, while carbon, nitrogen, and oxygen peaks appear at lower energies (<1 keV). This confirms the elemental composition and supports the PYTP successful incorporation of iodine along with organic components. The elemental mapping images of PYTP, shown in Fig. 4(a) and (b), confirm the successful adsorption of iodine, providing clear evidence of its integration throughout the polymer structure. The uniform distribution of iodine within the polymer matrix highlights its effective adsorption capacity and suggests strong interactions between the adsorbent and iodine species.

Fig. 3 (a) FE-SEM micrograph of PYTP polymer and (b) FE-SEM micrograph of PYTP–I₂.

Fig. 4 FE-SEM elemental mapping of PYTP polymer (a) before iodine adsorption and (b) after iodine adsorption (PYTP–I₂).

3.3. Thermogravimetric analysis

The thermal characteristics of PYTP were examined by thermogravimetric analysis (TGA) under a nitrogen atmosphere as shown in Fig. 5. PYTP remained stable up to nearly 350 °C, showing no significant weight loss within this range. Beyond this point, a gradual mass reduction was observed, with the major degradation occurring between 350 and 500 °C, corresponding to the decomposition of the organic backbone, including the triazine and perylene domains.⁴² At temperatures above 500 °C, a slower weight decline was noted, which can be attributed to carbonization processes within the aromatic framework. When heated to 800 °C, the polymer retained approximately 19 wt% of its initial mass, demonstrating robust thermal resistance. This stability arises from the presence of rigid triazine rings and extended π-conjugation, which provide strong resistance to thermal decomposition and make PYTP a durable material for adsorption applications under harsh conditions.

Fig. 5 TGA graph of PYTP polymer.

3.4. Powder X-ray diffraction (PXRD) analysis

The PXRD pattern of PYTP shown in Fig. 6 exhibits a broad, low-intensity diffraction halo centered at approximately 20° (2θ), with no sharp reflections observed. This diffuse feature signifies the absence of long-range periodic ordering and is typical of amorphous porous organic polymers. The gradual decrease in scattering intensity at higher angles further substantiates the non-crystalline character of the material. Overall, the diffraction pattern confirms that PYTP possesses an amorphous structural framework.

Fig. 6 P-XRD pattern of PYTP polymer.

3.5. XPS spectra

The elemental composition and electronic states of PYTP and PYTP–I₂ were examined by XPS. The survey spectra (Fig. 7a) of PYTP revealed distinct peaks corresponding to C 1s, N 1s, and O 1s, confirming the elemental composition of the polymer. After iodine adsorption, an additional iodine peak appeared in the spectrum of PYTP–I₂, providing clear evidence of successful iodine incorporation. The high-resolution I3d spectrum of PYTP–I₂ (Fig. 7b) exhibited peaks at 618.5 eV (I 3d_5/2) and 630.1 eV (I 3d_3/2), characteristic of molecular iodine (I₂). Additional features at 620.5 eV and 632.1 eV were assigned to polyiodide species (I₃⁻ and I₅⁻), confirming the coexistence of neutral iodine and polyiodide anions. The C 1s spectra (Fig. 7c) of PYTP showed peaks at 284.8 eV (C=C/C–C), 285.6 eV (C–N), and 288.5 eV (O–C=O). After iodine adsorption, these shifted slightly to 284.7 eV (−0.1 eV), 285.5 eV (−0.1 eV), and 288.3 eV (−0.2 eV), indicating minimal changes in the carbon environments. The O 1s spectra (Fig. 7d) of PYTP exhibited peaks at 530.1 eV (C=O), 531.5 eV (C–O–C), and 533.2 eV (O–C=O), which shifted to 530.9 eV (+0.8 eV), 531.9 eV (+0.4 eV), and 533.5 eV (+0.3 eV) in PYTP–I₂, suggesting slight modifications in oxygen environments. Notably, significant shifts were observed in the N 1s spectra. PYTP displayed peaks at 398.7 eV (pyridinic/triazine N), 399.5 eV (C–N/imine N), and 400.5 eV (imide N). After iodine adsorption, the pyridinic/triazine peak shifted markedly to 403.0 eV (ΔBE ≈ +3.0 eV), while the imine nitrogen shifted to 400.8 eV (ΔBE ≈ +1.3 eV) and the imide nitrogen to 400.9 eV (ΔBE ≈ +0.4 eV). These results confirm that pyridinic nitrogen serves as the primary binding site for iodine.

Fig. 7 XPS spectra of PYTP polymer (a) survey spectra, (b) Iodine, (c) before adsorption of C, N, and O, and (d) after adsorption of C, N, and O.

3.6. Adsorption studies of iodine vapor

The iodine vapor adsorption behaviour of the PYTP polymer was evaluated through gravimetric analysis under saturated iodine vapor at 75 °C, as mentioned in Fig. 8. A fixed amount of polymer was placed in a pre-weighed glass vial and periodically removed to determine the increase in mass over time. The uptake profile revealed a rapid increase during the initial hours, with PYTP adsorbing 2257 mg g⁻¹ within 1 h and 2890 mg g⁻¹ after 6 h. The polymer continued to adsorb iodine steadily until equilibrium was reached at 30 h, achieving an impressive maximum capacity of 3554 mg g⁻¹. This remarkable uptake is primarily governed by various non-covalent forces such as charge transfer interactions, halogen bonding, and π–π stacking. Specifically, the nitrogen atoms present in the triazine, and pyridine moieties act as Lewis basic sites, donating electron density to iodine and thereby generating stable nitrogen-rich complexes. Furthermore, halogen bonding between iodine and nitrogen, combined with π–π stacking interactions involving the perylene units, enhances the stabilization of iodine molecules within the porous framework. These synergistic effects enable the PYTP polymer to achieve the highest iodine adsorption capacity reported so far for perylene-based porous organic polymers, underscoring its novelty and potential as a highly efficient adsorbent.

Fig. 8 Iodine vapour uptake of PYTP.

3.7. Iodine adsorption in hexane solution

As an initial step toward iodine adsorption studies, the small-molecule analogue PY-TRI in Fig. S5 was examined. This compound is composed only of triazine and pyridine units, which were expected to act as electron-rich adsorption sites. However, its performance proved to be very limited. Even after 7–10 days of contact with iodine, only minimal changes were observed in the UV-vis spectrum, indicating that PY-TRI interacts only weakly with iodine. These results indicate that simple triazine and pyridine units, when present in a discrete molecule, are not sufficient to ensure efficient iodine capture.

To address this limitation, investigations were extended to the PYTP polymer, which was developed to combine structural robustness with high adsorption potential. The capture of iodine, a toxic species that can remain in both gaseous and liquid environments, is particularly demanding and highlights the need for advanced sorbent materials. To test the adsorption ability of the PYTP polymer, experiments were carried out using iodine dissolved in hexane. When the polymer was introduced, the purple iodine solution gradually lost its color and eventually became nearly transparent (Fig. 10a), providing a clear visual indication of iodine uptake. The adsorption behaviour was quantified using UV-visible spectroscopy at 518 nm, with concentrations derived from a calibration curve (Fig. 9a). A consistent decline in absorbance verified the progressive adsorption of iodine by the PYTP polymer. In a typical trial, 10 mg of polymer removed about 9.7 mg of iodine from a 10 mg solution, corresponding to ∼97% efficiency and an uptake capacity of 0.97 g g⁻¹. The adsorption curve in Fig. 9(a) displayed a fast initial removal step, followed by a slower approach toward equilibrium, which is characteristic of diffusion-limited adsorption in porous polymers. The exceptional uptake of the PYTP polymer can be explained by its structural characteristics. Nitrogen atoms in triazine and pyridine act as active sites for N⋯I₂ charge-transfer interactions, while the perylene backbone enhances π-conjugation and provides stabilization for the trapped iodine. At the same time, the porous polymer network ensures accessible pathways for iodine diffusion and storage. These combined features give the PYTP polymer a much higher adsorption capacity than the molecular analogue PY-TRI. Notably, the capacity of 0.97 g g⁻¹ also surpasses that of many reported iodine sorbents, confirming the effectiveness of this rationally designed polymeric framework. The reversibility of iodine adsorption on PYTP was evaluated through desorption experiments. As shown in Fig. 10(b), the initially iodine-loaded PYTP gradually released the adsorbed iodine upon treatment with ethanol, as confirmed by the reappearance of the characteristic yellow coloration in solution. UV-vis spectral monitoring Fig. 9(b) further validated this process, where the suppressed iodine absorption bands progressively recovered reaching near-complete desorption within 60 min, thereby demonstrating the efficient recyclability of PYTP. These results highlight the recyclable nature of PYTP and its potential for repeated adsorption–desorption cycles.

Fig. 9 (a) UV-vis spectra showing iodine adsorption by PYTP in hexane solution and (b) UV-vis spectra illustrating iodine desorption in ethanol solution.

Fig. 10 (a) Iodine adsorption by PYTP in hexane solution and (b) desorption of iodine in ethanol solution.

The recyclability of the PYTP polymer for iodine removal was evaluated over five consecutive adsorption–desorption cycles, and the results are presented in Fig. 11. During the first cycle, the removal efficiency was close to 98%, demonstrating the excellent initial adsorption capability of the polymer. Although a gradual decrease in efficiency was observed in subsequent cycles, the material still retained more than 80% of its activity after the fifth cycle. This slight reduction may be attributed to the partial blocking of active sites or incomplete desorption of iodine during regeneration. Furthermore, a comparative evaluation with previously reported porous materials is presented in the Table. S1 reveals that the adsorption efficiency of PYTP polymer is comparable to, or even higher than, that of many similar systems reported in the literature. This outstanding performance underscores the potential of PYTP as a robust and reusable adsorbent for practical iodine capture applications. The exceptionally high iodine uptake of PYTP, despite its moderate BET surface area (135 m² g⁻¹), arises from the combined contributions of the perylene and triazine units within the framework. The perylene moieties provide extended π-conjugated surfaces that promote strong π–π and charge-transfer interactions with iodine and polyiodide species, a behaviour commonly observed in aromatic porous polymers.⁴³ The triazine and pyridinic nitrogen atoms further enhance iodine binding by offering electron-rich sites that engage in favourable Lewis basic and other weak non-covalent interactions. In addition, the narrow micropores of PYTP enable effective confinement and capillary-assisted uptake of iodine. These synergistic structural features collectively account for the exceptionally high iodine adsorption capacity of PYTP.

Fig. 11 Recyclability of PYTP polymer.

3.8. Kinetics of PYTP polymer

The adsorption kinetics of iodine on PYTP were investigated using both pseudo-first-order and pseudo-second-order models to gain insights into the uptake mechanism, as shown in Fig. 12(a) and (b). In the pseudo-first-order plot [ln(q_e − q_t) vs. time], the linear correlation yielded an R² value of 0.985, indicating that this model does not adequately describe the adsorption process. In contrast, the pseudo-second-order plot [t/q_tvs. time] exhibited excellent linearity with an R² value of 0.996, showing much stronger agreement between the experimental and calculated values. The superior fit of the pseudo-second-order model suggests that iodine adsorption onto PYTP is predominantly governed by chemisorption rather than simple physisorption. This implies that the uptake process involves electron-sharing or electron-transfer interactions between iodine species and the nitrogen-rich active sites, further stabilized by the extended π-conjugated framework of PYTP. Such a mechanism highlights the strong binding affinity of the material and supports its high adsorption efficiency.

Fig. 12 (a) First-order kinetics of PYTP polymer and (b) Second-order kinetics of PYTP polymer.

3.9. CO₂ adsorption

The CO₂ adsorption behaviour of the PYTP polymer was evaluated to understand its potential for carbon capture and the influence of its structural features on gas uptake. PYTP possesses a nitrogen-rich, π-conjugated framework with a rigid backbone and intrinsic microporosity, all of which contribute to favourable CO₂ sorption. The incorporation of perylene units promotes π–π stacking interactions that enhance the rigidity and stability of the polymeric framework, thereby maintaining well-defined pore channels that facilitate gas diffusion, as shown in Fig. 13. A key factor governing CO₂ uptake in PYTP is the presence of multiple nitrogen functionalities originating from the pyridine and triazine rings. These electron-rich nitrogen sites act as Lewis basic centres capable of interacting with quadrupolar CO₂ molecules through dipole–quadrupole and weak acid–base interactions. This behaviour aligns well with previous studies on nitrogen-rich porous organic materials,⁴⁴ where the incorporation of basic nitrogen centres has been shown to enhance CO₂ affinity. The distribution of triazine and pyridinic nitrogen atoms in PYTP is characteristic of efficient CO₂-binding porous materials, which explains the favourable adsorption behaviour observed for this polymer.

Fig. 13 Schematic illustration of CO₂ adsorption on the PYTP polymer framework.

The gas adsorption isotherms recorded at 273 K demonstrated a maximum CO₂ uptake of approximately 50 cm³ g⁻¹ STP as depicted in Fig. 14. When converted to gravimetric units, this corresponds to a CO₂ adsorption capacity of ∼9.81 mg g⁻¹. The isotherm exhibited a typical Type I profile, indicative of microporous materials, with a distinct plateau reached near a relative pressure (P/P₀) of 0.9. This behavior suggests monolayer coverage of CO₂ within the micropores and saturation of the accessible adsorption sites at higher pressures. The near overlap of the adsorption and desorption curves further highlight the excellent reversibility and structural stability of the PYTP polymer during gas cycling.

Fig. 14 CO₂ adsorption–desorption isotherm of PYTP measured at 273 K.

This reversibility is crucial for potential practical applications, where repeated adsorption–desorption cycles are required. The results also suggest efficient pore filling and strong host–guest interactions within the PYTP framework. Overall, the high surface area, microporous structure, and the presence of nitrogen-rich adsorption sites synergistically contribute to the enhanced CO₂ uptake observed in PYTP. These findings underscore the potential of PYTP as a promising candidate for advanced carbon capture technologies, particularly in applications requiring selective, efficient, and recyclable adsorbent materials. A comparison with previously reported CO₂ adsorption materials is provided in Table S2, showing that the uptake of PYTP is comparable to or higher than several reported systems.

3.10. Real-time water application

To further assess the practical applicability of the PYTP polymer, its performance was investigated for the removal of iodine from real water samples. For this study, the polymer was carefully packed into a 0.22 µm syringe filter and used as a solid-phase column through which aqueous iodine solutions with concentrations ranging from 5 to 50 mg L⁻¹ (spiked to an approximate concentration of 30 mg L⁻¹) were passed. Remarkably, the characteristic brown coloration of the iodine solution completely disappeared after filtration, yielding a clear and colourless effluent. This immediate and visible change strongly indicated the high adsorption efficiency of PYTP in aqueous systems. The adsorption process was further validated by UV-visible spectral analysis as shown in Fig. 15, which revealed an almost complete disappearance of the characteristic iodine absorption peak in the filtrate, thereby providing quantitative evidence of effective iodine capture.

Fig. 15 UV-vis spectra of aqueous iodine solution before and after adsorption by PYTP.

Notably, the PYTP polymer maintained this exceptional efficiency even when tested at slightly elevated iodine concentrations, demonstrating its tolerance to varying contaminant levels. The observed high removal rate is attributed to the synergistic effects of nitrogen-rich adsorption sites and the extended π-conjugated framework, which together facilitate rapid host–guest interactions and stable iodine binding within the porous structure. The combination of fast uptake kinetics and high removal efficiency highlights the suitability of PYTP for water treatment applications under realistic conditions. Overall, these findings emphasize that PYTP is not only effective in controlled laboratory environments but also in real-time water purification scenarios. Its ability to efficiently and repeatedly remove iodine from aqueous solutions underscores its potential as a practical adsorbent material for addressing radioactive iodine contamination in environmental and industrial applications.

4. Conclusion

In this work, a novel perylene–triazine–pyridine-based porous organic polymer (PYTP) was successfully synthesized through a simple polycondensation strategy and thoroughly characterized using a combination of spectroscopic, microscopic, and surface area analyses. The rationally designed porous framework of PYTP endowed the material with high structural stability, abundant nitrogen-rich active sites, and well-developed microporosity, all of which played a critical role in enhancing host–guest interactions with environmentally relevant species such as iodine and CO₂. The polymer displayed an exceptional iodine uptake capacity of 3554 mg g⁻¹ under vapor conditions and ∼97% removal efficiency from iodine/hexane solution, significantly outperforming many reported adsorbents. Furthermore, PYTP demonstrated excellent recyclability, retaining more than 80% of its adsorption efficiency even after five consecutive cycles, which highlights its robustness and suitability for long-term use. Most importantly, real-water experiments established the ability of the PYTP material to completely remove iodine from spiked aqueous samples, confirming its potential applicability in real-world water purification and radioactive waste remediation scenarios. Beyond iodine capture, PYTP also showed promising performance in CO₂ adsorption (∼50 cm³ g⁻¹), which can be attributed to its nitrogen-enriched framework that facilitates efficient pore filling and strong dipole–quadrupole interactions. The combination of these properties makes PYTP a rare dual-functional adsorbent capable of addressing two major global challenges, the immobilization of radioactive iodine isotopes from nuclear waste and the capture of excess CO₂ contributing to climate change. Overall, the results of this study underscore PYTP as a highly efficient, stable, and reusable porous organic polymer with great promise for deployment in nuclear waste treatment, carbon capture, and broader environmental remediation applications.

Author contributions

The manuscript was written by the contributions of all authors.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nj04018c.

Acknowledgements

The authors gratefully acknowledge VIT University for providing “VIT SEED GRANT” (SG20240017) to support this work and for providing the instrumental facilities.

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