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Article

Sn(IV) Porphyrin-Based Ionic Self-Assembled Nanostructures and Their Application in Visible Light Photo-Degradation of Malachite Green

by
Nirmal Kumar Shee
and
Hee-Joon Kim
*
Department of Chemistry and Bioscience, Kumoh National Institute of Technology, Gumi 39177, Korea
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(7), 799; https://doi.org/10.3390/catal12070799
Submission received: 1 July 2022 / Revised: 16 July 2022 / Accepted: 19 July 2022 / Published: 20 July 2022
(This article belongs to the Special Issue Nanostructured Materials for Solar and Visible Light Driven Photocatalysis)
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Abstract

:
A series of porphyrin-based ionic complexes were prepared through the reaction of two porphyrin precursors, 5,10,15,20-tetrakis(4-(2-pyridyl)phenyl)porphyrin H2TPhPyP (1) and trans-dihydroxo [5,10,15,20-tetrakis(4-(2-pyridyl)phenyl)porphyrinato]tin(IV) Sn(OH)2TPhPyP (2), with various acids (HCl, HNO3, CF3COOH, H2SO4, H2CO3, and H3PO4). The complexes were characterized via elemental analysis, 1H nuclear magnetic resonance spectroscopy, electrospray ionization mass spectrometry, Fourier transform infrared (FT-IR) spectroscopy, ultraviolet (UV)-visible spectroscopy, fluorescence spectroscopy, and field-emission scanning electron microscopy (FE-SEM). Each compound exhibited different results for UV-visible, fluorescence, FT-IR, and FE-SEM studies depending on the counter anions. The complexes possessed different self-assembled nanostructures based on electronic interactions between the cations of compounds 1 and 2 with different counter anions. These aggregated species are stabilized by electrostatic forces and the π-π stacking interactions between the two porphyrin rings, in which the counter anions play an important bridging role. The counter anions also play an important role in controlling the morphology and photocatalytic properties of the as-developed materials. The complexes were then used for the photocatalytic degradation of the malachite green (MG) dye in aqueous media under visible light irradiation for up to 70 min. A morphology-dependent photocatalytic degradation of the MG dye was observed for all the ionic complexes, with efficiencies ranging from 50% to 95%.

1. Introduction

Agricultural and industrial activities generate significant amounts of toxic compounds that are discharged into water ecosystems, causing severe environmental issues. Large quantities of organic dyes, herbicides, phenols, pesticides, plasticizers, biphenyls, amino, and nitro compounds are discharged into water bodies, endangering aquatic life and water potability [1,2]. Several methods to remove hazardous pollutants from wastewater have been reported, including those based on physicochemical techniques, such as adsorption [3], membrane filtration [4], ion-exchange [5], chemical precipitation [6], electrolysis [7], and advanced oxidation processes (AOPs) [8]. Among them, AOPs are the most commonly used methods owing to their simple operation, high degradation rate without the generation of secondary pollutants, and degradation of the entire pollutant to less-toxic compounds (CO2 and H2O). These methods are based on the degradation of pollutant molecules by reactive oxygen species (ROS) generated in situ using a suitable photocatalyst. Photocatalysts absorb light from sunlight and generate ROS, which facilitate the degradation of organic pollutants in water [9,10,11]. TiO2 has been used as a promising photocatalyst owing to its high stability, low toxicity, and high photocatalytic efficiency for the degradation of pollutants in water. However, a large band gap (~3.2 eV) restricts the use of TiO2 to only ultraviolet light (λ < 370 nm). In addition, the high recombination rate of photo-induced electron-hole pairs lower the quantum efficiency of photocatalytic processes [12,13].
Porphyrinoids (free-base porphyrins or metalloporphyrins) can be used as alternatives to TiO2 as they have a high absorption efficiency through a wider wavelength range (in the UV-vis region). However, the use of porphyrinoids in homogeneous catalysis has drawbacks, which include the aggregation of porphyrin in solution, low reusability, and poor recovery from the reaction mixture. To overcome these challenges, porphyrin-based nano-or microstructured materials have been proposed and have received significant attention from the basic and applied chemistry research community over the last two decades [14,15,16,17]. These nanomaterials display unique features, including large surface areas and high physicochemical stability, compared to their parent compounds. Highly ordered and versatile geometric features render these materials suitable for applications in other fields, including chemical sensing [18], molecular recognition [19], catalysis [20], solar energy conversion and storage [21], and biomedical purposes [22]. Porphyrin molecules aggregate in solution and self-assemble to form larger stable molecules. Numerous intermolecular noncovalent interactions (e.g., hydrogen bonding interactions, π-π stacking interactions, ligand coordination, electrostatic interactions, van der Waals interactions, and hydrophobic and hydrophilic effects) facilitate the self-assembly of porphyrin molecules [23,24,25,26,27]. Therefore, the fabrication of well-defined prominent nanostructures from porphyrin compounds is a challenge. Various methods, including ionic assembly [28], metal-ligand coordination [29], re-precipitation [30], surfactant-assisted synthesis [31], and sonication [32], have been used for the construction of porphyrin-based self-assembled nanostructures. Ionic assemblies are extensively used for the construction of porphyrinoid-based nanosheets [33], nanotubes [28], nanospheres [34], nanofibers [35], nanorods [32], and nanocomposites [36] from various metalloporphyrins. Sn(IV) porphyrin complexes are suitable choices for the construction of functional porphyrin nanoaggregates because Sn(IV) centers are oxophilic and can readily form stable six-coordinate complexes with two trans-axial oxyanion ligands, such as alkoxides and carboxylates. In addition, these complexes have unique optical properties and are diamagnetic; therefore, structural information can be readily obtained using nuclear magnetic resonance (NMR) spectroscopy. They can also be used in the production of hydrogen gas [37,38,39,40,41,42,43].
Previously, we demonstrated the supramolecular ionic self-assemblies of highly-charged Sn(IV) porphyrin cations stabilized by counter anions [44,45]. In the present paper, the formation of Sn(IV) porphyrin-based ionic self-aggregated nanostructures has been described using Sn(IV) porphyrin complex 2 as the cation source (Chart 1). Precursor porphyrin 1 has also been used in competitive studies. The counter-anion source used was 1% aqueous solution of monoprotic acids (HCl, HNO3, and CF3COOH), diprotic acids (H2SO4 and H2CO3), and a triprotic acid (H3PO4). In addition, the photocatalytic efficiency of these nanostructures was examined by monitoring the degradation of malachite green (MG) in an aqueous solution. MG is used as a natural green dye in textile industries, such as leather, wool, and silk [46]. MG, which possesses a cationic complex structure with a triphenylmethane core, is a carcinogenic, non-biodegradable, and toxic compound. Therefore, this dye can be used as a model pollutant to test the photocatalytic efficiency of the prepared catalysts in the laboratory [47,48].

2. Results and Discussions

2.1. Syntheses and Characterization

Scheme 1 illustrates the preparation of compounds 1 and 2. First, H2TphPyP (1) was prepared from 4-(2-pyridyl)benzaldehyde and freshly distilled pyrrole in the presence of propionic acid under reflux conditions. Subsequently, compound 1 was treated with tin(II) chloride in the presence of pyridine, followed by hydrolysis, to form Sn(OH)2TphPyP (2). Compounds 1 and 2 were fully characterized using various spectroscopic techniques, including elemental analysis, 1H-NMR spectroscopy, ESI-MS, UV–vis spectroscopy, fluorescence spectroscopy, and Fourier transform infrared (FT-IR) spectroscopy.
1H-NMR spectra of compounds 1 and 2 were recorded in CDCl3 and are shown in Figures S1 and S2. For compound 1, the characteristic signal at 2.73 ppm corresponds to the NH protons of the pyrrole ring, while the protons in the pyridyl ring appear at 8.84, 8.04, 7.90, and 7.35 ppm. The phenyl protons of compound 1 are at 8.35 ppm, and the β-pyrrole protons appear at 8.94 ppm. Compound 2 exhibits a peak at 7.43 ppm corresponding to the axial Sn-OH protons. The pyridyl ring protons appear at 8.86, 8.07, 7.92, and 7.38 ppm. The β-pyrrole and phenyl protons of compound 2 appear at 9.23 and 8.45 ppm, respectively. ESI-MS spectra of compounds 1 and 2 are shown in Figures S3 and S4, respectively. Compound 1 has a molecular peak at 927.27 [1 + H]+, while 2 exhibits a peak at 1075.19 [2 + H]+. Furthermore, compound 2 has a low-intensity peak at 538.08, indicating the ion [2 + 2H]2+.
The UV-vis absorption spectra of compound 1 and its ionic complexes from various acids are shown in Figure 1. A sharp Soret band (419 nm) and four Q-bands (515, 551, 590, and 645 nm) were observed. After treatment with various acids to form the ionic complexes, the Soret band red-shifted (419→422 nm for HNO3, CF3COOH, and HCl; 419→442 nm for H2SO4), and an additional band at 443 nm in the case of HCl and CF3COOH was observed. In the case of H2CO3, the Soret band did not shift, but it was slightly broad compared with that of compound 1. Conversely, Q-bands broadened with increasing intensity owing to the formation of ionic complexes. The Soret band of the ionic complexes of compound 1 and H3PO4 showed a broad shift (half-width changed from 396 to 443 nm), and the Q bands red-shifted (515→524, 551→560, 590→596, and 645→654 nm). Similar trends were observed in the UV-vis absorption spectra of compound 2 and its ionic complexes with different acids (Figure 2). Compound 2 exhibited a typical sharp Soret band at 427 nm and Q-bands at 520, 560, and 601 nm. Blue shifts were observed in the Soret bands of all the ionic complexes of the acids: 427→420 nm for HCl, CF3COOH, HNO3, and H2SO4; 427→416 nm for H2CO3; 427→418 nm for H3PO4. The corresponding Q-bands also blue-shifted (551 and 592 nm). However, the Q bands also blue-shifted compared to those of compound 2 (566 and 609 nm).
The fluorescence spectra of compound 1 and its ionic complexes with various acids are shown in Figure 3. Compound 1 has a typical two-band emission spectrum (654 and 705 nm). After treatment with various acids to form ionic complexes, the intensities of these two bands decreased without shifting. The fluorescence spectra of compound 2 and its ionic complexes are shown in Figure 4. Compound 2 has two emission bands (601 and 652 nm). Red shifts were observed for HCl and CF3COOH ionic complexes (601→606 nm and 651→655 nm, respectively), while the emission intensity decreased considerably. In the case of H2SO4, HNO3, H2CO3, and H3PO4 ionic complexes, these two bands did not shift, but their intensity decreased compared to that of compound 2. The formation of the ionic complexes was further examined via FT-IR spectroscopy, as shown in Figures S5 and S6. Characteristic peaks were observed for NO3− (1351 cm−1), SO42− (1110 cm−1), CF3COO− (1671 cm−1), CO32− (1410 cm−1), and PO43− (1010 cm−1).

2.2. Supramolecular Ionic Self-Assembly to Nanostructures

The self-assembly of compounds 1 and 2 and their ionic complexes were analyzed via field-emission scanning electron microscopy (FE-SEM). For sample preparation, each complex was suspended in water at a fixed concentration (c = 1 mM) and centrifuged at 13,500 rpm for 10 s. Thereafter, the solution was drop-casted onto the surface of a copper tape for deposition and dried in air, followed by Pt coating prior to the FE-SEM studies. The morphologies of the self-assembled nanostructures are shown in Figure 5 and Figure 6. The average length of the spherical particles of compound 1 was from 110 to 200 nm (Figure 5a). In contrast, the average particle size of the truncated polyhedron of compound 2 was from 200 to 900 nm with a width of 100 nm (Figure 5b).
After the formation of ionic complexes of compound 1 with various acids, its morphology changed significantly. In the case of ionic complexes derived from H2CO3, the average particle size was from 250 to 350 nm, with a width of 50 nm (Figure 6a). The average particle size of the ionic complexes of compound 1 and HNO3 was smaller (70–20 nm) (Figure 6b). In contrast, the particle size of the ionic complexes of compound 1 and H2SO4 was larger than that of compound 1, ranging from 250 to 600 nm (Figure 6c). Fused nanorods were observed in the ionic complexes of compound 1 and HCl, with the average length ranging from 160 to 40 nm (Figure 6d). The morphology of the ionic complexes of compound 1 and CF3COOH exhibited rectangular-shaped structures (210 × 170 × 80 nm3) with several small particles (Figure 6e). Flake-shaped particles were observed in ionic complexes of compound 1 and H3PO4 (Figure 6f), with average particle size ranging from 200 to 260 nm in length and a width of 50 nm. From Figure 6, porphyrin ionic complexes of compound 1 and various acids self-assembled into nanostructures, and the morphology of these particles depends on the counter anions.
The morphologies of the self-assembled nanostructures obtained upon the formation of ionic complexes derived from compound 2 with various acids are shown in Figure 7. In the case of ionic complexes of H2CO3, the average particle size of the sphere ranged from 70 to 140 nm (Figure 7a). Fused nanoflakes were observed in the ionic complex of compound 2 with HNO3 (Figure 7b). A square-shaped morphology was observed in the complex of compound 2 with H2SO4, with the average length of the fused square particles being 290 nm (Figure 7c). Fused rectangular-shaped structures (240 × 160 × 55 nm3) were observed for the ionic complexes of compound 2 and HCl (Figure 7d). The morphology of the ionic complexes of compound 2 and CF3COOH was considerably different from that of compound 2 (Figure 7e) and demonstrated rectangular-shaped structures (220 × 90 × 40 nm3) along with fused spheres (average diameter of ~120 nm). The morphologies of the ionic complexes of compound 2 and H3PO4 consisted of stone-like particles with lengths ranging from 90 to 300 nm and a width of 20 nm (Figure 7f). From Figure 7, it can be concluded that the counter anions affect the morphology of the ionic complexes derived from Sn(IV) porphyrin 2 with different acids.

2.3. Photocatalytic Performance for the Degradation of MG Dye

We have examined the photocatalytic activity of the photocatalysts developed in this study for the degradation of MG dye in aqueous solutions under visible-light irradiation. A negligible decay of the MG dye was observed in the absence of either a photocatalyst or visible light (Figure S7). ESI-MS spectra were recorded after 30 min in each photodegradation experiment. New small molecules from the degradation of the MG dye were formed, which was confirmed by the appearance of new peaks in the mass spectra. The time-dependent absorption spectra of MG in the presence of photocatalysts generated from compound 2 with H3PO4 under visible light irradiation are shown in Figure S8. From Figure 8 and Figure 9, all the photocatalysts (generated from compounds 1 and 2 with various acids) were effective for the photodegradation of MG, and their photocatalytic performance depended on the morphology of the nanoaggregates. The degradation rates of MG dye in the presence of a photocatalyst can be determined from its degradation efficiency, (C0 − C)/C0, where C0 is the initial concentration of MG, and C is the concentration at time t. The observed degradation rates of MG were 50% for compound 1 and 53, 60, 66, 73, 86, and 90% for the photocatalysts derived from compound 1 with H2CO3, HNO3, H2SO4, HCl, CF3COOH, and H3PO4, respectively (Figure 8).
In contrast, the observed degradation rate of MG was 65% for compound 2 and 67, 70, 75, 80, 91, and 95% for the photocatalysts derived from compound 2 with HNO3, H2SO4, HCl, H2CO3, CF3COOH, and H3PO4, respectively (Figure 9). From the above observation, it was clear that the photocatalysts derived from 1 and 2 with H3PO4 showed the optimum performance towards the decay of MG dye. To further elucidate the reaction kinetics for the degradation of the MG dye, the pseudo-first-order rate equation was used for the photocatalytic degradation experiments. The equation was expressed as ln(C0/C) = kt, where C0 is the initial concentration of dye, C represents dye concentration at irradiation time t, and k denotes the pseudo-first-order rate constant. Using the data displayed in Figure 8 and Figure 9, the reaction kinetics of the MG dye degradation are presented in Figures S9 and S10. The first-order rate constant for the degradation of MG by compound 1 was 0.009 min−1, and the other rate constants were 0.011, 0.013, 0.015, 0.017, 0.028, and 0.032 min−1 for the photocatalysts derived from compound 1 with H2CO3, HNO3, H2SO4, HCl, CF3COOH, and H3PO4, respectively (Figure S9).
Similarly, the first-order rate constant for the degradation of MG by compound 2 was 0.014 min−1 and the other rate constants were 0.015, 0.016, 0.018, 0.022, 0.032, and 0.039 min−1 for the photocatalysts derived from compound 2 with HNO3, H2SO4, HCl, H2CO3, CF3COOH, and H3PO4, respectively (Figure S10). Previously, various investigations concerning the MG dye degradation have been reported in the literature. Our observed rate constants were comparable to or even better than those reported for other photocatalysts such as TiO2 (0.0064 to 0.140 min−1) [47], Zn(II)porphyrin with H2O2 (0.0042 to 0.0054 min−1) [48], Au-Ag/TiO2 (0.00987 to 0.2595 min−1) [49], and CuFe2O4 (0.0142 min−1) [50]. In addition, the recovery of the photocatalysts from the reaction vessel was easily handled by a continuous filtration-wash-dry procedure. The efficiency of the photocatalyst remained unchanged after recovery, and it showed excellent recyclability for the degradation of MG dye (Figure S11). It should be noted that the efficiency of photocatalyst (derived from 2 with H3PO4) remained almost intact even after 10 consecutive cycles. As depicted in Figure S11, 95% MG was degraded within 70 min for the first run, and the photocatalytic performance slightly decreased, marked by 91% MG degradation, after 10 consecutive cycles. The morphology of the catalysts remained intact after their use in the degradation experiment, as confirmed by the FE-SEM image of the photocatalyst derived from compound 2 with H3PO4 (Figure S12).
To examine the effect of dye/catalyst ratio on the performance of the photocatalysts, different concentrations of the MG dye aqueous solution (5, 10, 15, 20, 30, and 40 mg L−1) with a fixed amount of catalyst (5 mg each time) were used for each time. From Figure S13, it was revealed that the degradation rate gradually decreased as the concentration of MG dye increased. The effect of the temperature and the pH on the degradation of MG dye has been also examined. From the Figure S14, it was observed that degradation efficiency increased with increasing temperature. As shown in Figure S15, the pH of the MG dye solution affected the degradation rate. The rate of degradation increased from pH 2 to pH 7, and then decreased up to pH 11.
Several possible mechanisms for the photodegradation of pollutant dyes in aqueous solutions using porphyrin-based photocatalysts have been reported [11,15,16,17]. The mechanism consists of five steps for porphyrin-based nanomaterials (P). When the photocatalyst absorbs visible-light irradiation, electrons in the valence band (VB) are excited to the conduction band (CB). The electron–hole pairs (e−/h+) are formed through this process at the surface of the photocatalyst (step 1). Intermolecular π–π interactions stabilized the electronic delocalization over the nanomaterials and minimize the recombination energy of the excited electrons. Next, the photogenerated holes (h+) react with H2O to produce reactive species of hydroxyl radicals (•OH). On the other hand, the reaction of the electrons with the dissolved O2 generate superoxide radical anions (O2−•) (steps 2 and 3). Then these reactive species (superoxide radical anions and hydroxyl radicals) react with the MG dye and mineralized it into low molecular compounds (steps 4 and 5).
P + hν →P∗ (e− + h+)
H2O + h+→•OH + H+
O2 + 2e−→O2−•
•OH + MG→Degraded products
O2−• + MG→Degraded products.
Reactive species involved in the photocatalytic degradation of the MG dye by the photocatalyst generated from compound 2 and H3PO4 were identified via radical trapping analysis [51]. To investigate the effect of different reactive species on the photodegradation of the MG dye, tert-butyl alcohol (tBuOH) and para-benzoquinone (BQ) were used to capture •OH and O2−•, respectively (Figure S16). From Figure S16, the degradation rate was affected by the presence of tBuOH as well as BQ, suggesting that hydroxyl radicals and superoxide radicals are the major reactive species that contribute to the catalytic oxidation of the MG dye. For further details on the detection of all intermediate species involved in the dye degradation process (all oxygen species, such as 1O2 and 3O2, •OH, or O2−•), readers can refer to our previous reports [11,15,16,17].
In order to examine the adsorption properties of the photocatalysts towards the MG dye, the Brunauer–Emmett–Teller (BET) surface area of the photocatalysts (derived from 1 and 2 with H3PO4) was measured. Figure S17 presents the N2 adsorption-desorption isotherms of the photocatalysts (derived from 1 and 2 with H3PO4). Each photocatalyst exhibited a low surface area, for example the BET surface area of the best performing photocatalysts (derived from 1 and 2 with H3PO4) were estimated to be 4.5 m2/g and 12.7 m2/g, respectively. As shown in Figure S18, it took about 30 min to reach the absorption-desorption equilibrium, 2% and 5% of MG dye were adsorbed by the photocatalysts derived from 1 and 2 with H3PO4, respectively. Lower adsorption of dye on the surface of photocatalysts have some advantages as the bare active sites are readily available for photodegradation. However, the absorption of dye on the surface of photocatalyst not only depends on the surface area of the catalyst but also the interaction of dye with active sites of the photocatalysts and various interaction at the junction of solid-liquid interface.
The reaction mixture was analyzed after 30 min irradiation of visible light using ESI-MS (Figure S19) to study the details of the degradation pathway of the MG dye. New peaks appeared in the mass spectra imply that the MG dye was degraded into new small molecules [52,53]. By analyzing mass spectral data in Figure S19, possible intermediates for the degradation of the MG dye are given in Figure 10. The degradation of the MG dye can occur in two different ways. First, the reaction of the hydroxyl radical with the MG cation forms a carbinol intermediate with m/z 347.2. This intermediate is further fragmented into smaller intermediates with m/z 269.1, 226.1, and 199.0 and hydroquinone (m/z 111.0). Second, the N-demethylation of the MG cation forms an intermediate (m/z 273.1), followed by the formation of a hydroxyl radical species (m/z 291.1) and finally fragmentation into a molecule with m/z 213.1. Further successive breakdown leads to the formation of hydroquinone. Hydroquinone is further fragmented into benzoquinone and then to maleic acid. Finally, all intermediate molecules were further fragmented and mineralized into CO2 and H2O. Moreover, the total organic carbon (TOC) value was measured to evaluate the removal of the MG dye by photocatalysts [54]. The TOC removal percentage achieved using the photocatalyst generated from compound 2 and H3PO4 was only 75%.

3. Materials and Methods

All chemicals were purchased from Tokyo Chemical Industry Co. LTD. (Tokyo, Japan) and used without further purification. Elemental analysis was conducted using an EA 1110 Fisons analyzer (Used Lab Machines Limited, London, UK). 1H-NMR spectra were recorded on a Bruker BIOSPIN/AVANCE III 400 spectrometer at 293 K (Bruker BioSpin GmbH, Silberstreifen, Rheinstetten, Germany). ESI-MS spectra were recorded using a Thermo Finnigan linear ion trap quadrupole mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). Steady-state UV-vis and fluorescence spectra were recorded on a Shimadzu UV-3600 spectrophotometer and a Shimadzu RF-5301PC fluorescence spectrophotometer, respectively. FT-IR spectra were measured using a Shimadzu FTIR-8400S spectrophotometer (Shimadzu, Tokyo, Japan). FE-SEM images were obtained using a MAIA III system (TESCAN, Brno, Czech Republic). The Brunauer–Emmett–Teller (BET) surface area using N2 adsorption isotherms at 77 K was measured with an analyzer (BELSORP-mini volumetric adsorption equipment). The total organic carbon (TOC) values of the degradation of MG dye were examined with a JEOL JEM-3010 total organic carbon analyzer to calculate the extent of mineralization.

3.1. Synthesis of 5,10,15,20-Tetrakis [4-(2-Pyridyl)phenyl]porphyrin H2TPhPyP (1)

Freshly distilled pyrrole (0.38 mL, 5.5 mmol) was added dropwise to a solution of 4-(2-pyridyl)benzaldehyde (1.00 g, 5.5 mmol) in propionic acid (200 mL) under reflux. After 3 h, the solvent was evaporated to dryness under vacuum to obtain an oily residue. Thereafter, the oily residue was neutralized with trimethylamine and dissolved in a minimum amount of N,N-dimethylformamide. The reaction mixture was stored in a refrigerator overnight. The resulting solid was filtered, washed with hot water, and dried. The crude product was purified using column chromatography (SiO2, eluent: CH2Cl2/MeOH = 90:10) to afford 5,10,15,20-tetrakis [4-(2-pyridyl)phenyl]porphyrin 1. The crude product was recrystallized from CH2Cl2/n-hexane to obtain a violet-purple crystalline powder. Yield: 0.127 g (10%). Anal calculated for C64H42N8: C, 83.27; H, 4.59; N, 12.14. Found: C, 83.05; H, 4.67; N, 12.28. 1H-NMR (400 MHz, CDCl3, ppm): δ −2.73 (s, 2H, NH), 7.35 (m, 4H, H4-Py), 7.90 (m, 4H, H3-Py), 8.04 (d, J = 7.8 Hz, 4H, H5-Py), 8.35 (dd, 16H, Ho,m-Ph), 8.84 (d, J = 4.8 Hz, 4H, H2-Py), 8.94 (s, 8H, β-pyrrole). UV-visible (CHCl3): λnm (log ε), 419 (5.78), 515 (4.47), 551 (4.33), 590 (4.10), 645 (4.04). Emission (CHCl3, λnm): 654 and 705.

3.2. Synthesis of Trans-dihydroxo-[5,10,15,20-tetrakis(4-(2-pyridyl)phenyl)porphyrinato]tin(IV) Sn(OH)2TPhPyP (2)

H2TPhPyP (0.092 g, 0.1 mmol) was dissolved in pyridine (50 mL), and SnCl2∙2H2O (0.22 g, 0.85 mmol) was added to the solution and refluxed for 15 h. Subsequently, pyridine was removed under vacuum, and the residue was dissolved in CHCl3 and filtered through a celite pad to remove excess SnCl2. The filtrate was evaporated, and the crude was re-dissolved in a solvent mixture (50 mL) consisting of tetrahydrofuran (THF; 40 mL) and H2O (10 mL). K2CO3 (0.33 g, 1.45 mmol) was added to the reaction mixture, and it was refluxed for 10 h. THF was removed under reduced pressure conditions, and the mixture was cooled to 5 °C to precipitate the product. The solid compound was then filtered and dried in an oven. Recrystallization (CHCl3/CH3CN) afforded a reddish compound 2. Yield 90 mg (84%). Anal calculated for C64H42N8O2Sn: C, 71.59; H, 3.94; N, 10.44; R, 14.03. Found: C, 71.42; H, 4.11; N, 10.37; R, 14.10. 1H-NMR (400 MHz, CDCl3, ppm): δ −7.38 (m, 4H, H4-Py), 7.43 (s, 2H, Sn-OH), 7.92 (m, 4H, H3-Py), 8.07 (d, J = 7.7 Hz, 4H, H5-Py), 8.45 (m, 16H, Ho,m-Ph), 8.86 (d, J = 4.7 Hz, 4H, H2-Py), 9.23 (m, 8H, β-pyrrole). UV-visible (CHCl3): λnm (log ε), 427 (5.50), 520 (3.70), 560 (4.35), 601 (4.25). Emission (CHCl3, λnm): 601 and 652.

3.3. Synthesis of the Ionic Complexes of H2TPhPyP (1) and Sn(OH)2TPhPyP (2) Using Various Acids

Ten milligrams of each of the porphyrin compounds (1 and 2) were dissolved in a 1% aqueous solution (5 mL) of each of the following acids–HCl, HNO3, CF3COOH, H2SO4, H2CO3, and H3PO4, and acetone (20 mL) was layered over for slow diffusion for 7 d. Subsequently, the crystalline materials were filtered, washed with water, and air-dried.

3.4. Photocatalytic Investigation

The photocatalytic efficiency of the self-assembled ionic nanostructures was studied by monitoring the degradation of MG dye in an aqueous solution by the catalysts, using malachite oxalate green salt as the source of MG. In a typical procedure, 5 mg of the photocatalyst was added to 150 mL of aqueous MG solution (20 mg L−1, distilled water at pH 7.0) with stirring. The mixture was then placed in the dark for 30 min to attain an adsorption-desorption equilibrium. The irradiation process was conducted using a 150 W xenon arc lamp with a UV cut-off filter (ABET technologies, Old gate lane Milford, Milford, CT, USA) at room temperature. At specified intervals, 3 mL of the suspension was centrifuged to remove the photocatalyst. Subsequently, the solution was analyzed by recording absorption variations in the UV-Vis spectra at 617 nm to determine the concentration of MG.

4. Conclusions

In this study, various porphyrin-based ionic complexes were prepared through the reaction of two porphyrin precursors, compounds 1 and 2, with various acids (HCl, HNO3, CF3COOH, H2SO4, H2CO3, and H3PO4). Depending on the counter anion [55,56], these complexes readily assembled into different nanostructures, based on the electronic interaction of cations of compounds 1 or 2 with different counter anions. These nanostructures can be further stabilized through π-π stacking interactions between the phenyl-extended porphyrin rings. Furthermore, the morphology-dependent photocatalytic degradation of MG dye in aqueous solution under visible light irradiation for 70 min was observed for all the ionic complexes, with efficiencies ranging from 50% to 95%. Therefore, counter anions can affect the morphology as well as the optical properties of porphyrin-based species. In addition, the aggregates are good photocatalysts for the treatment of wastewater.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal12070799/s1, 1H-NMR spectra, mass spectra, and results of the photocatalytic degradation of MG are shown in Figures S1–S19. Figure S1: 1H-NMR spectrum of compound 1; Figure S2: 1H-NMR spectrum of compound 2; Figure S3: ESI-MS spectrum of compound 1; Figure S4: ESI-MS spectrum of compound 2; Figure S5: FT-IR spectra of compound 1 and its ionic complexes; Figure S6: FT-IR spectra of compound 2 and its ionic complexes; Figure S7: UV-vis spectroscopic observation of the photocatalytic degradation of MG dye in aqueous solution under visible light irradiation by photocatalyst 1 and 2; Figure S8: Absorption spectral change of MG dye for the photocatalytic degradation by photocatalyst generated from 2 and H3PO4 in aqueous solution under visible light irradiation; Figure S9: Kinetics for the photocatalytic degradation of MG under visible light irradiation by photocatalyst generated from 1 and various acids; Figure S10: Kinetics for the photocatalytic degradation of MG under visible light irradiation by photocatalyst generated from 2 and various acids; Figure S11: Recyclability of the photocatalyst generated from 2 and H3PO4 towards the degradation of MG dye; Figure S12: FE-SEM images of photocatalyst generated from 2 and H3PO4 (after and before the degradation of MG dye); Figure S13: Effect of initial concentration of MG on the degradation by the photocatalyst (5 mg) generated from 2 and H3PO4; Figure S14: Effect of temperature on the MG degradation by the photocatalyst generated from 2 and H3PO4; Figure S15: Effect of pH of the solution on the MG degradation by the photocatalyst generated from 2 and H3PO4; Figure S16: Photocatalytic degradation of MG dye in aqueous solution by the photocatalyst generated from 2 and H3PO4 with the addition of different scavengers under visible light irradiation; Figure S17: N2 adsorption-desorption isotherms at 77 K for the photocatalysts derived from 1 and 2 with H3PO4; Figure S18: Adsorption of MG dye for the photocatalysts derived from 1 and 2 with H3PO4; Figure S19: ESI-MS spectrum (positive ion mode) of the reaction mixture of MG with the photocatalyst generated from 2 and H3PO4 after 30 min of visible light irradiation.

Author Contributions

Investigation, methodology, data curation, visualization, formal analysis, validation, software, and writing, N.K.S.; conceptualization, review, editing, supervision, project administration, and funding acquisition, H.-J.K. All authors read and agreed to the published version of the manuscript.

Funding

National Research Foundation of Korea (NRF) grant (Grant No. 2022R1F1A1074420).

Data Availability Statement

Data are available in the article and Supplementary Materials.

Acknowledgments

This study was supported by a National Research Foundation of Korea (NRF) grant (Grant No. 2022R1F1A1074420) funded by the Korean Government (MSIT).

Conflicts of Interest

The authors declare no conflict of interest.

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Chart 1. Chemical structures of compounds 1 and 2 used in this study.
Chart 1. Chemical structures of compounds 1 and 2 used in this study.
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Scheme 1. Synthesis of compounds 1 and 2. Reaction conditions: (a) propionic acid, reflux, (b) SnCl2∙2H2O/Pyridine, and (c) THF + Water/K2CO3.
Scheme 1. Synthesis of compounds 1 and 2. Reaction conditions: (a) propionic acid, reflux, (b) SnCl2∙2H2O/Pyridine, and (c) THF + Water/K2CO3.
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Figure 1. UV-visible spectra of compound 1 and its different ionic complexes in CHCl3.
Figure 1. UV-visible spectra of compound 1 and its different ionic complexes in CHCl3.
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Figure 2. UV-visible spectra of compound 2 and its different ionic complexes in CHCl3.
Figure 2. UV-visible spectra of compound 2 and its different ionic complexes in CHCl3.
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Figure 3. Fluorescence spectra of compound 1 and its different ionic complexes in CHCl3. Optical density (OD) of the sample solution was fixed at 0.20.
Figure 3. Fluorescence spectra of compound 1 and its different ionic complexes in CHCl3. Optical density (OD) of the sample solution was fixed at 0.20.
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Figure 4. Fluorescence spectra of compound 2 and its different ionic complexes in CHCl3. OD of the sample solution was fixed at 0.20.
Figure 4. Fluorescence spectra of compound 2 and its different ionic complexes in CHCl3. OD of the sample solution was fixed at 0.20.
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Figure 5. FE-SEM images for the morphology of compounds 1 (a) and 2 (b).
Figure 5. FE-SEM images for the morphology of compounds 1 (a) and 2 (b).
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Figure 6. FE-SEM images depicting the morphology of ionic complexes derived from compound 1. (a) 1 + H2CO3, (b) 1 + HNO3, (c) 1 + H2SO4, (d) 1 + HCl, (e) 1 + CF3COOH, and (f) 1 + H3PO4.
Figure 6. FE-SEM images depicting the morphology of ionic complexes derived from compound 1. (a) 1 + H2CO3, (b) 1 + HNO3, (c) 1 + H2SO4, (d) 1 + HCl, (e) 1 + CF3COOH, and (f) 1 + H3PO4.
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Figure 7. FE-SEM images illustrating the morphology of ionic complexes derived from compound 2. (a) 2 + H2CO3, (b) 2 + HNO3, (c) 2 + H2SO4, (d) 2 + HCl, (e) 2 + CF3COOH, and (f) 2 + H3PO4.
Figure 7. FE-SEM images illustrating the morphology of ionic complexes derived from compound 2. (a) 2 + H2CO3, (b) 2 + HNO3, (c) 2 + H2SO4, (d) 2 + HCl, (e) 2 + CF3COOH, and (f) 2 + H3PO4.
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Figure 8. Photocatalytic degradation of MG dye in aqueous solution (pH 7, temperature 298 K) under visible light irradiation by photocatalysts generated from compound 1 with various acids.
Figure 8. Photocatalytic degradation of MG dye in aqueous solution (pH 7, temperature 298 K) under visible light irradiation by photocatalysts generated from compound 1 with various acids.
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Figure 9. Photocatalytic degradation of the MG dye in aqueous solution (pH 7, temperature 298 K) under visible light irradiation by photocatalysts generated from compound 2 with various acids.
Figure 9. Photocatalytic degradation of the MG dye in aqueous solution (pH 7, temperature 298 K) under visible light irradiation by photocatalysts generated from compound 2 with various acids.
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Figure 10. Possible intermediates of the reaction between MG and the photocatalyst under visible light irradiation in aqueous solution.
Figure 10. Possible intermediates of the reaction between MG and the photocatalyst under visible light irradiation in aqueous solution.
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Shee, N.K.; Kim, H.-J. Sn(IV) Porphyrin-Based Ionic Self-Assembled Nanostructures and Their Application in Visible Light Photo-Degradation of Malachite Green. Catalysts 2022, 12, 799. https://doi.org/10.3390/catal12070799

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Shee NK, Kim H-J. Sn(IV) Porphyrin-Based Ionic Self-Assembled Nanostructures and Their Application in Visible Light Photo-Degradation of Malachite Green. Catalysts. 2022; 12(7):799. https://doi.org/10.3390/catal12070799

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Shee, Nirmal Kumar, and Hee-Joon Kim. 2022. "Sn(IV) Porphyrin-Based Ionic Self-Assembled Nanostructures and Their Application in Visible Light Photo-Degradation of Malachite Green" Catalysts 12, no. 7: 799. https://doi.org/10.3390/catal12070799

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