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. 2026 Sep 25;16(45):57394–57404. doi: 10.1039/d6ra07039f

Pyrylium and thio-pyrylium integrated olefin-linked cationic conjugated porous polymers for the adsorption of organic dyes in aqueous medium

Maruti Vibhuti Ravikumar a, Balagopalan Gayathri a, Vellanki Lakshmi a,✉
PMCID: PMC13613152  PMID: 42798934

Abstract

Conjugated porous polymers (CPPs) have attracted significant interest for environmental remediation, particularly in removing organic dyes from water, due to their permanent porosity, chemical robustness, and extended π-conjugated frameworks. In this work, we demonstrate that cationic CPPs can be highly effective adsorbents for a wide range of organic dyes in aqueous solution. The pyrylium-based CPPs (TMPA-O and TMPB-O) and thio-pyrylium-based CPPs (TMPA-S and TMPB-S) were synthesized and systematically evaluated for the adsorption of both cationic dyes, malachite green (MG), methylene blue (MB), and crystal violet (CV), and an anionic dye, methyl red (MR). Detailed kinetic and adsorption isotherm analyses revealed fast uptake behaviour and substantial adsorption capacities across the dye set. The adsorption of cationic dyes was screened by polarizable counterions (BF4− and CF3SO3−), which formed counterion-mediated bridges with the polymer and dye, thereby reducing electrostatic repulsion between the cationic dye and the polymer. The highest adsorption capacity was observed for MG with TMPB-S, reaching 667 mg g−1, and TMPB-S also showed notable uptake of MB (92 mg g−1) and MR (485 mg g−1). The superior performance of TMPB-S is attributed to the combination of its relatively higher surface area and pore volume among the investigated materials, alongside the counterion mediation in cationic dye adsorption. These findings bypass the limitations of charge-based adsorption and highlight cationic CPPs with polarizable counterions as effective and versatile options for removing dyes from water.

TMP-based cationic CPPs were synthesized and used for adsorbing dyes from water through polarizable BF4−/CF3SO3− counterions, which facilitate bridging between the dye and the polymer, as well as π–π and pore interactions.graphic file with name d6ra07039f-ga.webp

1. Introduction

The increasing demand for clean water with rapid industrial development has intensified the need for efficient technologies to remove organic contaminants from aqueous environments.1,2 Synthetic dyes are particularly a difficult class of pollutants due to their widespread use in the textile, printing, pharmaceutical, and chemical sectors, as well as their great durability against chemical and biological degradation.3 A large volume of dye-containing effluents is generated during industrial processing, and a significant fraction of these colored compounds is discharged into water bodies without complete treatment.4 Many commercial dyes exhibit considerable resistance to photolysis, oxidation, and microbial breakdown due to their highly conjugated aromatic structures and ionic substituents, which enable them to persist in natural fluids for extended periods. Dye removal is a significant environmental concern because many dyes can limit light penetration, interfere with photosynthesis, and have detrimental or mutagenic effects on aquatic organisms, even at low levels.5 Various physical and chemical methods, such as coagulation, membrane filtration, photocatalytic degradation, and advanced oxidation processes, have been studied for wastewater treatment.6,7 Even though these techniques can effectively remove pollutants, they usually entail high costs, complex operations, and sometimes produce secondary waste during treatment. Because of these limitations, adsorption has become one of the most widely used methods for removing dyes from water.8 The main advantages of adsorption are its simple procedure, high efficiency, and suitability for treating various types of dye pollutants.9–11

The adsorption efficiency mainly depends on the adsorbent material's characteristics. The interaction between the adsorbents and the dyes is significantly influenced by factors such as surface characteristics, pore structure, and the presence of functional groups. Consequently, widely used adsorbents, including metal oxides,12,13 activated carbon,14,15 and natural clays16 have been utilized extensively. However, achieving strong interactions with dyes that vary in molecular size, charge, or functional groups is challenging, as these conventional adsorbents often lack sufficient control over pore size and surface characteristics. Thus, introducing charged sites onto frameworks and incorporating counterions within these ionic frameworks further enhances the adsorption performance by providing ion-exchange pathways and stabilizing the dye-framework via electrostatic interactions.4,5,17,18 Therefore, in recent years, more attention has been given to the development of new adsorbent materials with tunable porosity and surface-active sites to improve dye-removal performance.

Porous materials have emerged as an important class of adsorbents owing to their permanent porosity, high stability and flexible molecular design. In contrast to conventional inorganic adsorbents such as clays, metal oxides, zeolites, and mesoporous silica, porous frameworks (MOFs, COFs, PAFs) offer greater control over pore size, surface properties, and chemical composition by rational incorporation of functional groups within their structures.19–29 Among these, conjugated porous organic polymers (POPs and CPPs) have received much attention in recent years due to their π-conjugated structure, which allows strong interaction with aromatic dye molecules, mainly through π–π stacking between the polymer backbone and the dye rings.20 Furthermore, the presence of heteroatoms such as nitrogen, sulfur, or oxygen, and ionic active sites in the polymer improves the adsorption behaviour of the materials by increasing hydrogen bonding, dipole–dipole, and coordination interactions. Due to the synergistic effects of these structural and chemical properties, CPPs often exhibit high dye uptake and relatively rapid adsorption towards a wide range of organic dyes in aqueous solutions.23 Most of the ionic porous polymers act as strong adsorbents via an ion-exchange mechanism and electrostatic attraction.4,5,17 However, the influence of counterion-mediated dye adsorption is relatively unexplored, which could overcome the limitations imposed by conventional electrostatic repulsions in the adsorption of cationic dye by cationic polymers and vice versa.

Based on these considerations, designing conjugated porous polymers with built-in ionic functionalities can be an effective approach for removing dyes with diverse structures and charges from aqueous media. Pyrylium- and its derivative-based porous materials have gained considerable interest lately due to their flexibility in utilising them as different counterion-based monomeric units. Only two pyrylium-based reports are known and used for the construction of covalent organic frameworks (COFs) from triazine-functionalized aldehydes for photoredox catalysis and CO2 adsorption.30,31 Taking the advantage of flexibility in achieving the different counterions pyrylium and thio-pyrylium based monomers were choosen to design ionic based porous polymers for efficient adsorption of dyes. In the present work, four cationic conjugated porous polymers were prepared, consisting of polarizable counterions from pyrylium and thio-pyrylium based monomers, yielding highly conjugated networks with permanent porosity. Among the synthesized materials, TMPB-S, having cationic thio-pyrylium units and triflate counterions in the framework, showed higher adsorption capacities for both cationic (MG, MB) and anionic (MR) dyes. Detailed kinetic and adsorption analysis, along with a mechanistic explanation, revealed fast uptake and substantial adsorption capacities for all polymers across the dyes. Overall results indicate that cationic CPPs with polarizable counterions can serve as efficient adsorbents and be useful in wastewater purification.

2. Experimental section

2.1. Materials

1,3,5-Tribromo benzene, potassium carbonate (K2CO3), benzaldehyde, benzoic acid, benzoic anhydride, and trifluoromethanesulfonic acid (triflic acid) were purchased from Spectrochem Pvt. Ltd, Mumbai (India). 2,4,6-Trimethylpyrylium tetrafluoroborate (TMP) was purchased from Thermo Fisher Scientific India Private Ltd, and triphenylamine was purchased from Sigma-Aldrich. The tris(4-formylphenyl)amine (TFPA),32 1,3,5-tris(4-formylphenyl)benzene (TFPB)33 and model molecule (TMP-Ph)30 were synthesised accordingly based on the reported procedures.

2.2. Synthesis of TMPA-O and TMPB-O polymers

2,4,6-Trimethylpyrylium tetrafluoroborate (TMP) (100 mg, 0.48 mmol), tris(4-formylphenyl)amine (TFPA) (158 mg, 0.48 mmol)/tris(4-formylphenyl)benzene (TFPB) (187.4 mg, 0.48 mmol), benzoic acid (58 mg, 0.48 mmol), and benzoic anhydride (163 mg, 0.72 mmol) were slightly pre-ground collectively using mortar and pestle then subsequently loaded to an oven-dried pressure tube, and heated at 120 °C for 3 days. After cooling to room temperature, the precipitate was collected and washed with methanol, dichloromethane, and acetone three times (30 mL × 3), followed by Soxhlet extraction with tetrahydrofuran for 2 days, and then dried under vacuum at 100 °C for 24 h. The polymers TMPA-O and TMPB-O of dark black colour were obtained with yields of about ∼80%. (TMPA-O: FT-IR (ATR, cm−1): 1638, 1400–1585, 1270, 965. Solid-state (ss) 13C NMR (ppm): 151, 146, 144, 129; TMPB-O: FT-IR (ATR, cm−1): 1635, 1420–1590, 965. Solid-State (ss) 13C NMR (ppm): 142, 140, 136, 128).

2.3. Synthesis of TMPA-S and TMPB-S polymers

Thio-pyrylium-based CPPs were synthesized by following the reported procedure with slight modifications.31TMP (42 mg, 0.2 mmol) was dissolved in a 10 mL solvent mixture of DMF/o-DCB (7 : 3 v/v) in an oven-dried pressure tube and stirred for 10 min, and Na2S (93.6 mg, 1.2 mmol) was added, and the reaction was allowed to proceed at 25 °C for 20 min. After 20 minutes, CF3SO3H (1 mL) was added, and the mixture was stirred for an additional 30 minutes at the same temperature. The intermediate prepared in this way was directly used to condense with aldehyde units. TFPA (66 mg, 0.2 mmol)/TFPB (78 mg, 0.2 mmol) were added to the tube, along with CF3SO3H (1 mL). The mixture was then sonicated for 10 min to fully disperse the reactants and subsequently heated in an oil bath at 120 °C for 3 days in the dark. After the mixture cooled to room temperature, the precipitate that had formed was collected, washed with methanol, dichloromethane and acetone (30 mL × 3), followed by Soxhlet washing for 2 days using tetrahydrofuran and then dried under vacuum at 100 °C for 24 h. to afford thio-pyrylium CPPs TMPA-S and TMPB-S in 82 and 92% yields, respectively. TMPA-S: FT-IR (ATR, cm−1): 1638, 1420–1585, 1270, 965. Solid-state (ss) 13C NMR (ppm): 143, 141, 136, 129; TMPB-S: FT-IR (ATR, cm−1): 1635, 1425–1580, 965. Solid-state (ss) 13C NMR (ppm): 146, 141, 138, 129.

3. Structural analysis of CPPs

Pyrylium-based cationic conjugated porous polymers TMPA-O and TMPB-O were synthesized by solid-state aldol condensation between trimethyl pyrylium tetrafluoroborate (TMP) and triformyl phenyl amine (TFPA) or triformyl phenyl benzene (TFPB) (Scheme 1a). Meanwhile, a three-component aldol condensation was used to synthesize thio-pyrylium-based TMPA-S and TMPB-S from TMP and TFPA or TFPB, respectively, using Na2S as the third component (Scheme 1b).

Scheme 1. Synthesis of TMP-based CPPs, (a) TMPA-O and TMPB-O, (b) TMPA-S, and TMPB-S.

Scheme 1

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Fourier transform infrared (FTIR) spectroscopy was used to analyze and conclude the successful formation of CPPs. The obtained results were compared with the structurally similar reference compound TMP-Ph and their corresponding monomers (Fig. S1 and S2). The presence of a binding vibration peak of the trans-vinylene bond at ∼965 cm−1 in polymers confirms the formation of an olefin link, which is absent in monomers. The stretching band of C Created by potrace 1.16, written by Peter Selinger 2001-2019 O+ of monomer TMP has been shifted to 1638, 1635, and 1640 cm−1 in TMPA-O, TMPB-O, and model compound TMP-Ph, respectively. The corresponding peak was absent in thio-pyrylium polymers, which signifies that the complete conversion of pyrylium unit from monomer (TMP) to thio-pyrylium unit in TMPA-S and TMPB-S, and the same was detected by the peak corresponding to C Created by potrace 1.16, written by Peter Selinger 2001-2019 S+ at 1614 and 1605 cm−1. The strong stretching signal at ∼1270 cm−1 in TFPA and its constituent polymers (TMPA-O and TMPA-S) is due to the vibration of the amine N–R3 (R = phenyl) bond, which is absent in TFPB polymers (Fig. 1a). The solid-state 13C NMR of the materials indicated the presence of C Created by potrace 1.16, written by Peter Selinger 2001-2019 O+ and C Created by potrace 1.16, written by Peter Selinger 2001-2019 S+ functionalities in pyrulium and thiopyrulium-based CPPs, respectively (Figs. S5–S8). The peaks at 151 and 142 ppm correspond to C Created by potrace 1.16, written by Peter Selinger 2001-2019 O+, in TMPA-O and TMPB-O, respectively. Similarly, peaks at 143 and 146 ppm correspond to C Created by potrace 1.16, written by Peter Selinger 2001-2019 S+ of the thio-pyrylium-based polymers (TMPA-S and TMPB-S) (Fig. 1b).

Fig. 1. (a) FTIR Spectrum and (b) solid state 13C NMR of the polymers TMPA-O, TMPB-O, TMPA-S, TMPB-S and TMP-Ph.

Fig. 1

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The porous characteristics, including porosity, surface area, and pore volumes, of the CPPs were calculated using N2 sorption analysis at 77 K and 1 bar pressure, employing the Brunauer–Emmett–Teller (BET) method. The obtained BET profiles were found to be a combination of type-II and type-IV nitrogen sorption adsorption isotherms (Fig. S9 and S10). The obtained surface areas of CPPs were 20.76, 27.87, 42.56, and 64.82 m2 g−1, along with pore volumes of 0.031, 0.027, 0.058, and 0.092 cc g−1, for TMPA-O, TMPB-O, TMPA-S, and TMPB-S, respectively. The Barrett–Joyner–Halenda (BJH) pore distribution curves illustrated the mesoporous nature of the CPPs, with an average pore diameter of 3.4–3.8 nm. The morphological analysis of polymers was captured using Scanning Electron Microscopy (SEM) (Fig. S11). The SEM analysis revealed an agglomerated, collapsed surface with a porous morphology for TMPA-O. Similarly, TMPB-O demonstrated hierarchical pore morphology with surface macropores and crater-like cavities. The granular surface morphology was observed in TMPA-S and TMPB-S.

On the other hand, the PXRD of the polymers reflected the amorphous nature for both pyrylium and thio-pyrylium CPPs (Fig. S12) with broad peaks at ∼20° and ∼17°, respectively. The thermal stability of the polymers was examined by TGA. The materials remained stable up to 350 °C, with a loss of nearly 20% (wt%) (Fig. S13). Further, the chemical stability of CPPs was examined by exposing them to extreme pH conditions for 3 days and analysing the samples by FTIR and by monitoring the colour changes of the polymers (Fig. S14 and S15). The polymers exhibited high stability in 12 M HCl. But the polymers were unstable in 12 M NaOH due to hydrolysis of pyrylium by nucleophilic ring-opening reaction at high pH.30

Structural elucidation of formed CPPs was also demonstrated using X-ray photoelectron spectroscopy (XPS). The high-resolution scan of C 1s revealed peaks corresponding to C Created by potrace 1.16, written by Peter Selinger 2001-2019 O+ at 286.6 and 285.5 eV for TMPA-O and TMPB-O, respectively. Similarly, C Created by potrace 1.16, written by Peter Selinger 2001-2019 S+, C(CF3SO3−) peaks were observed at 285.5, 290.9 eV and 285.6, 290.9 for TMPA-S and TMPB-S, respectively. In O 1s scan showed peaks at 532.4 and 531.8 corresponding to C Created by potrace 1.16, written by Peter Selinger 2001-2019 O+ (Fig. 2a), also the C Created by potrace 1.16, written by Peter Selinger 2001-2019 S+ group peaks were observed at 163.63 and 163.70 eV in S 2p high scan for TMPA-S and TMPB-S, respectively (Fig. 2b). Similarly, all other peaks corresponding to carbon, oxygen, boron, fluorine, and sulphur were also observed and confirmed the formation of pyrylium and thio-pyrylium CPPs (Fig. S16).

Fig. 2. (a) Comparison of O 1s XPS spectra of TMPA-O and TMPB-O. (b) Comparison of S 2p XPS spectra of TMPA-S and TMPB-S.

Fig. 2

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The optical absorption behaviour and electronic conjugation of the CPPs have been evaluated using Diffuse Reflectance spectroscopy (DRS) (Fig. 3a). TFPA polymers (TMPA-O and TMPA-S) showed an extended threshold red edge in the NIR region ∼1200 nm. The less electron-donating monomer-based polymers (TFPB) showed red edges at 820 and 1004 nm for TMPB-S and TMPB-O, respectively. Thus, it indicates that the TFPA unit will donate electron density to the TMP and TMP-S units in extended conjugation. The Kubelka–Munk (K–M) plots were used for calculating indirect optical band gaps of the polymers (Fig. 3b). The polymers were found to show narrow band gaps in the range of 1.0 to 1.5 eV.

Fig. 3. (a) DRS spectra of TMPA-O, TMPB-O, TMPA-S, TMPB-S, and model compound TMP-Ph. (b) Kubelka–Munk plots of the CPPs.

Fig. 3

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Further, to gain insight into the electronic structure, redox behaviour, and electron-accepting characteristics of the synthesized CPPs, Cyclic Voltammetry (CV) was performed. In cyclic voltammetric plots, the oxidation peaks were not prominent due to the extended conjugation, making it difficult for the polymers to lose electrons. But there was a clear difference observed in the reduction peaks of pyrylium and thio-pyrylium CPPs. All the polymers exhibited a reversible reduction in the range of −1.18 to −1.59 V vs. Fc/Fc+. The pyrylium polymers (TMPA-O and TMPB-O) showed reduction peaks at −1.18 V vs. Fc/Fc+ and −1.21 V vs. Fc/Fc+, respectively. On the otherside, thio-pyrylium CPPs TMPA-S and TMPB-S exhibited reduction peaks at significantly higher potentials, of −1.47 V and −1.59 V, respectively (Fig. S17). Indicating that the pyrylium CPPs are much easier to reduce than thio-pyrylium CPPs due to the higher electron-accepting nature of the pyrylium unit than its counterpart.

4. Dye adsorption performance for polymer materials

To evaluate the dye adsorption capacities of polymers, we investigated the dye removal efficiency of the polymers (TMPA-O, TMPB-O, TMPA-S, and TMPB-S) against a set of commonly environment-polluting dyes in an aqueous medium. The polymer material of 3 mg was taken in a 15 mL glass vial containing 10 mL of dye solutions such as Malachite Green (MG, 50 mg L−1), Crystal Violet (CV, 50 mg L−1), Methyl Red (MR, 50 mg L−1), Cango Red (CR, 50 mg L−1), Reactive Red 120 (RR120, 50 mg L−1), Reactive Red 16 (RR16, 50 mg L−1), Sunset Yellow (SY, 50 mg L−1), Methyl Orange (MO, 50 mg L−1), Methylene Blue (MB, 12.5 mg L−1), Toluidine Blue (TB, 12.5 mg L−1), Rhodamine B (RhB, 12.5 mg L−1). Here, the concentration of dyes deliberately used in much higher than the natural environment to ensure the detectability of the adsorption behaviour of polymer materials using UV-visible spectroscopy. The dye and polymer mixtures in the vials were stirred at 600 rpm at room temperature for 3 hours. The removal efficiencies (%R) were calculated using the following eqn (1).

4. 1

where C0 is the initial concentration, Ct is the concentration of dye after t hour.

Among all the dyes, those with higher removal efficiencies were analyzed further using kinetics, adsorption isotherms, and pH dependency. All four CPPs exhibited good adsorption behaviour towards Malachite Green (MG) with removal efficiencies of 74.2%, 78.4%, 88.8%, and 99.2%, respectively (Fig. 4). Additionally, TMPA-S polymer adsorbed Crystal Violet (CV) with a removal efficiency of 97%. Alongside the TPB counterpart TMPB-S, adsorbed Methyl Red (MR) and Methylene Blue (MB) with removal efficiencies of 91% and 97.4%, respectively. The adsorption rate and potential rate-controlling steps were evaluated by fitting the data to linear pseudo-first-order (eqn (2)), pseudo-second-order (eqn (3)), and intraparticle diffusion models (eqn (4)) at different time intervals. In fact, the initial concentration of dyes for kinetic studies was chosen based on the differences in the absorption behaviour and molar absorptivity of the dyes (MG = 200 mg L−1, CV = 50 mg L−1, MB = 12.5 mg L−1, and MR = 50 mg L−1). Langmuir and Freundlich adsorption isotherms (eqn (5) and (6), respectively) were used to estimate the maximum adsorption (Qm) of dyes by the CPPs; both results and parameters are listed in Tables S1 and S2, and the concentrations were calculated at λmax of each dye.

4. 2
4. 3
Qt = Kdiff√t + C 4
4. 5
4. 6

where Qt and Qe (mg g−1) are the amount of dye adsorbed at time ‘t’ and at equilibrium, respectively, and Qm (mg g−1) is the maximum adsorption capacity of the adsorbent. Ce is the equilibrium concentration of dye. K1 (min−1) and K2 (g mg−1 min−1) are the rate constants of pseudo-first and second-order kinetics, respectively. KL and KF (L g−1) are the Langmuir and the Freundlich adsorption isotherm constants, respectively.

Fig. 4. The removal efficiencies of (a) TMPA-O, (b) TMPB-O, (c) TMPA-S, and (d) TMPB-S for different dyes.

Fig. 4

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From the kinetic curves of MG adsorption on CPPs, we observed that the adsorption process follows pseudo-first-order kinetics, with R2 values of 0.959, 0.888, 0.941, and 0.855 for TMPA-O, TMPB-O, TMPA-S, and TMPB-S, respectively (Fig. S18–S21). Alongside the Weber–Morris model, which yielded higher R2 values (0.827–0.897), indicating the major role played by intraparticle diffusion within the pores in adsorbing the dye. The pseudo-first-order kinetics were also used to find the equilibrium adsorption capacities (Qe), and were found to be 531 mg g−1, 278 mg g−1, 578 mg g−1, and 326 mg g−1. Whereas the pseudo-second-order model with a very low value of R2 (<0.258), signifies the absence of on-site chemisorption, the overall MG adsorption by CPPs is consistent with diffusion-driven physisorption on the polymer surfaces. Along with MG, TMPA-S also adsorbed Crystal Violet (CV), which follows the pseudo-second-order model (R2 = 0.974) and the pseudo-first-order model (R2 = 0.873), with a contribution from the intra-particle diffusion model (R2 = 0.889) (Fig. S22). The TMPB-S polymer also adsorbed Methylene Blue (MB) and Methyl Red (MR), in addition to MG. In the case of MB adsorption on TMPB-S, the pseudo-first-order model yielded a physically meaningful equilibrium capacity (Qe = 42.05 mg g−1) and rate constant (K1 = 7.55 × 10−3 min−1) with the highest correlation (R2 = 0.941), ruling out pseudo-second-order and chemisorption (R2 = 0.018) (Fig. S23). Notably, the anionic azo dye Methyl Red (MR) exhibits a mixed kinetic regime, with both the first-order and second-order kinetic models (R2 = 0.999 and 0.998, respectively) providing a good fit when absorbance intensity was considered at 525 nm in UV-vis (Fig. S24a). Additionally, it proposes good intraparticle diffusion into the pores, with a reasonable fit (R2 = 0.944), suggesting a transition from diffusion-controlled uptake to interaction-controlled adsorption near equilibrium. However, the λmax at 525 nm of the MR + polymer solution shifted towards ∼425 nm as the adsorption contact time increased (Fig. S24). The shift in the absorption can be attributed to changes in the electronic environment of MR due to pH changes when the polymer is added, and to electrostatic attraction between the dye and cationic polymer matrix (Scheme S4).34 These results suggest that TMPB-S not only adsorbs MR but also alters its microenvironment, leading to the observed hypsochromic spectral transition.

The maximum adsorption capacities towards MG were determined using adsorption isotherms and were found to be 617 mg g−1, 267 mg g−1, 529 mg g−1, and 667 mg g−1 for TMPA-O, TMPB-O, TMPA-S, and TMPB-S, respectively (Fig. S25). Among all synthesized polymers, TMPB-S exhibited the maximum adsorption capacity towards malachite green (MG), reaching up to 667 mg g−1. In TMPA-S, CV adsorption was accompanied by a better Langmuir isotherm fit (R2 = 0.902) and a maximum adsorption of CV of 149 mg g−1, indicating that interaction-controlled adsorption on relatively uniform surface sites (Fig. S26a and b). TMPB-S indicated that both the Langmuir and the Freundlich isotherms exhibit poor and yet comparable correlation (R2 ≈ 0.6) for MB, indicating neither model solely describes adsorption equilibrium, suggesting that the adsorption mechanism is more complex than ideal monolayer, multilayer physisorption, and is likely influenced by multiple interactions, framework–guest interactions, electrostatic effects, including pore accessibility and diffusion into pores (Fig. S26c and d). The maximum amount of MB adsorbed was 92 mg g−1. For MR, both the Langmuir and Freundlich isotherms (R2 ≈ 0.95) adequately described the equilibrium data, indicating favourable adsorption on heterogeneous sites. The maximum adsorption was calculated to be 485 mg g−1 (Fig. S26e and f).

The adsorption kinetics are best described by the pseudo-first-order model in the adsorption of cationic dyes, indicating that the overall adsorption proceeds predominantly through occupation of available surface adsorption sites, although multiple framework–guest interactions may contribute to the overall adsorption behaviour. Weber–Morris analysis reveals a significant boundary-layer effect, followed by intraparticle diffusion, confirming a multi-step adsorption mechanism. The dye-adsorbed polymers were analyzed using FT-IR spectroscopy. The peaks correspond to C Created by potrace 1.16, written by Peter Selinger 2001-2019 O+, C Created by potrace 1.16, written by Peter Selinger 2001-2019 S+ of polymers, and the peaks corresponding to counterions have been retained with a slight change in peak position and sharpness of the peaks. The existence of counterion peaks after adsorption indicates that those were not replaced during the adsorption (Fig. S27 and S28). Apparently, the appearance and slight shift in bands correspond to dyes, indicating the interaction with the cationic framework and counterions present in polymers.

The dye adsorption behaviour of the cationic CPPs investigated can be rationalised by a synergistic, multi-interaction mechanism rather than by electrostatic interactions or by counterion replacement within the polymer matrix alone. Despite the intrinsic positive charge of the polymer frameworks, efficient uptake of both cationic and anionic dyes is achieved through the combined effects of hydrogen bonding, extended π–π interactions between the aromatic dye molecules and the π-conjugated polymer backbone and pore confinement within the permanently porous network, along with dye–counterion–polymer framework bridge formation. The superior performance of thio-pyrylium-based TMPB-S is attributed to its higher accessible surface area and pore volume, which promote stronger interactions and more efficient pore filling. And also, the polymer containing triflate counterions exhibits higher cationic dye adsorption compared to the BF4− analogue due to the higher coordinating nature35 and polarizability36 of triflate counterion, which facilitates ion-pair rearrangement and enhances stabilization of the cationic dye within the porous framework. Kinetic and isotherm analyses further confirm that dye uptake proceeds via favourable surface interactions and multilayer adsorption under ambient aqueous conditions.

5. Adsorption behaviour of materials at various pH ranges

In general, the adsorption of dyes also depends on the solution's pH. The effect of the solution's pH was investigated through pH-controlled dye adsorption studies. Since the CPPs were not stable at extreme high pH, i.e., in an extremely alkaline solution due to the hydrolysis of pyrylium/thiophyrylium unit in polymers,31 the adsorptions were recorded in a pH range of 1 to 10. It was observed that the high amount of MG was adsorbed by the polymers at lower pH, suggesting the introduction of more positive charge to MG that helps in stabilizing the counter-ion bridges between the dye, counterion, and the polymer framework (Fig. 5). At pH 10, adsorption was decreased due to the competition between counterions and hydroxyl ions. In contrast, thio-pyrylium-containing polymers, particularly TMPB-S, exhibit maximum adsorption at neutral pH, attributed to enhanced framework polarizability and reduced dependence on ionic interactions. These results demonstrate that rational backbone rigidity and heteroatom selection enable efficient cationic dye adsorption despite electrostatic repulsion.

Fig. 5. Removal efficiency of MG by TMPA-O, TMPB-O, TMPA-S, and TMPB-S after the constant contact time of 3 h at different pH values (Co = 50 mg L−1).

Fig. 5

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Similarly, CV adsorption by TMPA-S and MB adsorption by TMPB-S were observed over a broad pH range, confirming that the adsorption is mostly governed by counterion mediation, π–π stacking and dispersion interactions within the thio-pyrylium CPPs. In contrast, MR shows strong pH sensitivity, with maximal adsorption under acidic and neutral conditions and significantly reduced uptake at pH > 7, due to its anionic form and increased solvation. These results demonstrate that TMPB-S enables efficient cationic dye adsorption through electrostatic interactions and counterion-mediation while retaining charge selectivity for pH-responsive dyes (Fig. 6).

Fig. 6. Removal efficiency of CV (Co = 50 mg L−1) by TMPA-S (a), MB (Co = 12.5 mg L−1), and MR (Co = 50 mg L−1) by TMPB-S (b & c) after the constant contact time, after 3 h at different pH values.

Fig. 6

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Alongside adsorption in different pH solutions, the influence of electrostatic interactions between adsorbing dyes and polymers were examined by conducting adsorption experiments using dyes prepared in 0.1 M NaCl (Fig. S29). The removal efficiencies of the dyes (MG, CV, MB, and MR) in high-ionic-strength solution were observed, with little deviation from the adsorption efficiencies in water at room temperature and ambient pressure conditions. If adsorption is based on electrostatic interactions, the removal efficiency should decrease with NaCl treatment, as the salt ions would hinder electrostatic interactions between the dyes and the polymer matrices, which was observed with MR adsorption by TMPB-S (>10%). In other cases, the removal efficiency was not greatly affected, indicating the absence of electrostatic interactions in adsorbing those dyes, which is consistent with our observations.

Further, the selectivity of different charged dyes was investigated by conducting adsorption analysis using a 1 : 1 mixture of oppositely charged dyes. For cationic dyes (MG, CV, MB), MO was used as the counteranionic dye, and CV was used for MR. As shown in Fig. 7 and 8, the UV-vis spectral analysis before and after the adsorption of dye mixture by polymers showed high selectivity towards their respective dyes by adsorbing them after 3 hours and leaving other counter dye in the solution itself. The adsorption capacities of all dyes are comparable to those of previously reported adsorbents (Tables S3–S6). The maximum adsorptions of MG, CV, and MB by polymers are moderate to low, whereas that of MR by TMPB-S is the highest among reported materials. Methyl red adsorption by porous organic polymers has not been reported much due to its electronic environmental changes in aqueous solution.

Fig. 7. UV-vis absorption spectra for the MG/MO aqueous solution in the presence of respective CPPs (a–d) before (0 min) and after (180 min) the adsorption.

Fig. 7

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Fig. 8. UV-vis absorption spectra of CV/MO by TMPA-S (a), MB/MO, and MR/CV by TMPB-S (b & c) before (0 min) and after (180 min) the adsorption.

Fig. 8

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The adsorbed polymers were recovered by washing the dye-loaded polymers with excess acetone, MeOH, 0.1 M HCl, and 0.1 M NaOH until no colour was released from the polymers. The recovered, dried polymers were analyzed using FT-IR (Fig. S30) and showed negligible differences in IR peak positions, indicating the polymer's recovery capacity. However, to evaluate long-term recyclability, systematic adsorption–desorption cycling experiments will be investigated in future work. Thus, the polymers integrated with polarizable counterions have proven to be materials that, despite being cationic in nature, can adsorb both kinds of dyes by overcoming electrostatic repulsion, with recoverability and reusability.

6. Conclusions

In summary, we have synthesized four cationic CPPs based on pyrylium and thio-pyrylium units. Pyrylium-based polymers TMPA-O and TMPB-O were synthesized using solid-state polymerization. While the thio-pyrylium-based CPPs, TMPA-S and TMPB-S, were synthesized using a multicomponent reaction between TMP and sodium sulphide before reacting with aldehydic units TFPA and TFPB, respectively. CPPs exhibited exceptional adsorbing behaviour for dyes of both kinds (cationic and anionic). TMPB-S exhibits the highest adsorption capacity for MG, reaching 667 mg g−1, while also showing notable uptake of MB (92 mg g−1) and MR (485 mg g−1). And its TPA analogue, TMPA-S, displays the highest affinity for CV, with an adsorption capacity of 149 mg g−1. The superior cationic dye adsorption performance of TMPB-S is attributed to its higher surface area, higher pore volume among the investigated polymers and counterion mediation between dye and polymer, which promote strong interactions with aromatic dye molecules. The adsorption of MR was driven by electrostatic attraction between MR and the polymer matrix, as evidenced by the decrease in removal efficiency in 0.1 M NaCl. These findings overcome the conventional hypothesis about charge-based adsorption limitations and establish cationic CPPs with polarizable counterions as promising, versatile platforms for the removal of dyes from water. Overall, these findings demonstrate that thoughtfully combining framework structure and counterion nature enables cationic CPPs to function as flexible, charge-independent adsorbents for organic dyes in aqueous medium.

Author contributions

M. V. Ravikumar: writing – original draft, data curation, methodology, conceptualisation, B. Gayathri: data curation, Dr. Vellanki Lakshmi: writing – editing, revision and correction of the manuscript, resources and supervision of the work.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

RA-016-D6RA07039F-s001

Acknowledgments

Lakshmi V. thanks SERB, India, for partially supporting this research through a Start-up research grant (SRG/2021/002251). Ravikumar M. V. and Gayathri B., thank NITK for the research fellowship. In addition, the authors are grateful to Prof. B.B. Das from NITK for the TGA Facility, JNCASR for XPS, the sophisticated central instrumentation facility (SCIF), IIT Dharwad, and the central research facility (CRF), NITK, and all its staff members for letting us use the facilities and assisting us with the material characterizations.

Data availability

The authors confirm that the data supporting this article are included in the supplementary information (SI). Supplementary information: synthetic information of monomers, model compound and polymers, experimental analysis including FTIR, NMR, BET, SEM, PXRD, TGA, XPS and CV along with dye adsorption kinetics and isotherms. See DOI: https://doi.org/10.1039/d6ra07039f.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-016-D6RA07039F-s001
RA-016-D6RA07039F-s001.pdf (4.9MB, pdf)

Data Availability Statement

The authors confirm that the data supporting this article are included in the supplementary information (SI). Supplementary information: synthetic information of monomers, model compound and polymers, experimental analysis including FTIR, NMR, BET, SEM, PXRD, TGA, XPS and CV along with dye adsorption kinetics and isotherms. See DOI: https://doi.org/10.1039/d6ra07039f.

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