Preparations of Core H2O-Bound 5, 10, 15, 20-Tetrakis-4-chlorophenyl Porphyrin, P 1 , and O-Methylation of Phenol and Its P-Substituted Analogues.

The treatment of a dichloromethane solution of 5, 10, 15, 20-tetrakis-4-chlorophenyl porphyrin, P 1 , with methanolic solutions of each of phenol, p-amino phenol, and p-nitro phenol for just 1 h results in the formation of water-molecule-bound amorphous solids of P 1 . In addition to the straightforward access to the H2O-molecule-coordinated species of P 1 thus produced, the another chief advantage of this synthetic strategy is the successful recoveries of anisole, p-amino anisole, and p-nitro anisole at the end of the reactions. The present work therefore further reports the use of P 1 as an efficient catalyst for the selective O-methylation of phenols using methanol as an environmentally friendly methylating agent. The H2O-bound amorphous solids of P 1 exhibit higher-intensity absorption as well as photoluminescence emission bands in dichloromethane compared with the parent crystalline form. Further, the measurement of the solid-state emission properties of both the crystalline and amorphous forms indicates quenching of fluorescence bands corresponding to amorphous solids in comparison with that of parent crystalline form. The crystalline form of P 1 and the H2O-bound amorphous solids were further studied by scanning electron microscope/transmission electron microscope (SEM/TEM), powder X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), and thermal analysis techniques. The results of these studies indicate change in morphological and structural features, surface areas, porosities, and thermal robustness upon core coordination of water molecules with the macrocyclic rings of P 1 .

Introduction

Porphyrins are derived from natural pigments such as chlorophyll, bacteriochlorophyll, and heme, which are composed of modified pyrrole subunits interconnected at their α carbon atoms and constitute a robust 22π electron system.[1] Highly conjugated porphyrin macrocycles exhibit a remarkable tendency to aggregate due to strong intermolecular Van der Waals-like attractive forces between the neighboring molecules.[2] The noncovalently bound molecular arrays of porphyrins are drawing greater attention due to their unique role in molecular devices, as models of natural light-harvesting pigments like chlorophylls and for supramolecular syntheses.[3] The electronic properties of aggregates deviate significantly from those of parent monomers, leading to a marked change in their exciton dynamics.[4] Porphyrins also serve as an excellent ligand to almost all of the metal ions of the periodic table due to their unique conformational flexibilities.[5] In nature, metalloporphyrins play a vibrant role in vital biological processes. For example, heme (iron porphyrin) plays an active role in oxygen transport and storage, electron transport for enzymatic redox reactions, signal transduction, ligand binding, and control of gene expression.[6] Chlorophylls (magnesium porphyrin) are photoactive pigments of photosynthesis in green plants.[7] In nature, chlorophyll molecules are arranged in the form of supramolecular self-assemblies to form polymeric porphyrin wires for long energy and electron transport (light harvesting) essential for the process of photosynthesis.[8] Inspired by the remarkably unique activities manifested by the natural metalloporphyrins, detailed studies on large numbers of synthetic porphyrins, metalloporphyrins, and peripheral substituted porphyrins have been actively carried out by the researchers.[9] However, the chemistry of central N–H units (amine moieties) in the core of free-base porphyrins, which have been often described as the precursor for metal insertion as well as central imine nitrogens, has not received adequate attention as compared to their metallo derivatives. The nonplanar tetrapyrrole cores of porphyrin can be involved in the formation of H-bond with suitable acceptor molecules. The nonplanar central cores of porphyrins allow incorporation and binding of guest molecules by means of electrostatic interactions (H-bond).[10]

In this article, we present the use of methanolic solutions of each of phenol, p-amino phenol, and p-nitro phenol as weak acids to generate the core water-molecule-bound amorphous solids of P. Successful recovery of anisoles at the end of each reaction is the remarkable feature of this method. Further, the present methodology also helped us in determining the morphological and structural features of the H2O-bound amorphous solids thus produced. The H2O-bound solids of P induced by methanolic solutions of phenol, p-amino phenol, and p-nitro phenol have been designated as P, P, and P, respectively.

Results and Discussion

The cyclocondensation reaction between pyrrole and p-chloro benzaldehyde in refluxing propionic acid followed by column chromatography afforded us the required free-base porphyrin P in decent yield.[11] In the present study, we carried out the reactions between dichloromethane solution of P and methanolic solutions of phenolic moieties at the refluxing temperature of 60 °C for 1 h. After the completion of the reactions, we ended up getting a high yield of purplish residues corresponding to the water-molecule-bound amorphous solids of P, i.e., P, P, and P, respectively (Scheme ). To isolate the various O-methylated compounds, i.e., anisole, p-amino anisole, and p-nitro anisole, which are present in the filtrates of the end products, we first allowed each of the filtrate to evaporate at room temperature and then loaded it on basic alumina columns. Elutions were first carried with dichloromethane to remove any porphyrin impurity. When the faint purple liquid corresponding to free-base porphyrins stopped eluting from the respective columns, the columns were then eluted with pure methanol. During these processes, we were able to collect clear straw, dark brown, and amber liquids corresponding to anisole, p-amino anisole, and p-nitro anisole that were eluted later from the columns with a polar solvent, i.e., methanol (Figure ). Direct methylation of phenol with methanol is complicated in that O-methylation of phenol is difficult to achieve compared to C-methylation.[12] However, in the present methodology, the selective O-methylation of phenols clearly suggests that porphyrins acted as efficient catalysts and specifically catalyzed O-methylation of phenols.

Scheme 1

O-Methylation of Phenols and Core Binding of H2O Molecules with Molecules of P

Figure 1

Fractions of the methanolic solutions of (a) anisole (b) p-amino anisole, and (c) p-nitro anisole isolated during the mid of elutions from the columns.

Plausible Mechanisms

On the basis of results given above, the plausible reaction mechanisms for the generation of water-molecule-bound amorphous solids of P–P and O-methylated phenols have been deduced, which are shown in Figures and 3, respectively. The acidic hydrogens of phenol and p-substituted phenols, i.e., p-amino phenol and p-nitro phenol, form hydrogen bonds with imino nitrogen atoms of porphyrin cores, resulting in the protonation of imino nitrogens and generation of phenoxide ions. After this, the reaction can proceed in two different possible pathways. The first possible pathway is that the resonance-stabilized phenoxide ions attack the methyl group (−CH3) of methanol to produce anisole and p-substituted anisoles, i.e., p-amino anisole and p-nitro anisole, and the resulting hydroxyl ions thus generated being strong nucleophiles attack the imino protons of porphyrin cores to generate water molecules (Figure ). The second possible pathway is that the protonation of methanol takes place, leading to the expulsion of water molecules and generation of methyl carbocations (−CH3+). The methyl carbocations are then attacked by phenoxide ions to form anisoles (Figure ). The second pathway is however more feasible than the first one as hydroxide ions are considered as a bad leaving group. On the other hand, water is a good leaving group and, therefore, the reaction occurs more possibly via the second pathway. It is also worth noting that since the stability of the methyl carbocation (−CH3+) is less, the whole reaction is occurring in a concerted manner. The porphyrin rings play an important role of accepting the protons from phenolic moieties and then donating the accepted protons to the hydroxyl group (−OH). Hence, the macrocycles provide a solid platform for the reaction to occur in a concerted manner. The water molecules thus liberated are then simultaneously trapped within the porphyrin cores by a number of intermolecular hydrogen bonds, leading to the formation of stacking frameworks of the macrocycles (Scheme ). The hydrogen bonds may be N (porphyrin)···H (water) or O (water)···H (porphyrin) as shown in Scheme . The nonplanarity of tetrapyrrole cores (amine and imine units) of P(13) allows the binding of guest molecules (H2O) with porphyrin cores, and the key driving force for the generation of this kind of stable systems is the attractive electrostatic interaction (N–H···O bond) between two polar groups, i.e., H2O molecules and tetrapyrrole cores.[10] It should be noted here that dichloromethane just played a role in mixing P with methanolic solutions of phenols, and by the time O-methylation of phenols and coordination of liberated water molecules with porphyrin core occur at 60 °C, it vaporizes through the open condenser due to its low boiling point.

Figure 2

Catalytic mechanism (I) for O-methylation of phenols with methanol over porphyrin and trapping of water molecules within the tetrapyrrole core.

Figure 3

Catalytic mechanism (II) for O-methylation of phenols with methanol over porphyrin and trapping of water molecules within the tetrapyrrole core.

To further understand the reactivity of phenol and its p-substituted derivatives in the formation of anisoles, a Hammett sigma plot was measured. A Hammett sigma plot is the plot of the logarithm of rate constant against the corresponding σ values of the substituents.

The dissociation constant pKa is generally expressed in terms of Log Kai.e.,orThe Hammett equation is given aswhere ρ is the Hammett reaction constant and σ is the Hammett sigma parameter and is defined by the equationwhere x = H, p-NO2, and p-NH2.

The pKa values of phenol, p-nitro phenol, and p-amino phenol in methanol are −14.3, −11.30, and −7.4, respectively.[14]

When Log Ka was plotted against the Hammett constant σ, a linear free energy relationship with a positive slope (ρ = 1) was obtained as shown in Figure . Therefore, O-methylation of phenols to yield the corresponding anisoles showed sensitivity to para substitution of phenol. Further, the yield of the isolated anisoles significantly correlated with the Hammet sigma parameters of the substituents.

Figure 4

Hammett plot for O-methylation of phenols in methanol.

The spectroscopic techniques such as 1H NMR spectroscopy, Fourier transform infrared spectroscopy (FT-IR) spectroscopy, and electron spray ionization (ESI) mass spectrometry enabled the full characterization of mononuclear free-base porphyrin P and its water-molecule-bound solid analogues, i.e., P–P. The formations of anisoles were also confirmed successfully by 1H NMR spectroscopy and electron spray ionization (ESI) mass spectrometry.

1H NMR Spectroscopy

The comparative and selected portions of the 1H NMR spectra of P–P are shown in Figure . The full spectra are provided in the Supporting Information as Figures S1–S4. The β pyrrole protons (Ha) of P appear as singlet at δ 8.86 ppm, whereas the ortho phenyl protons (Hc) and meta phenyl protons (Hb) of P resonate as doublets at δ 8.15/8.17 and 7.76/7.78 ppm, respectively. Due to the strong ring current effect, the inner imino protons (NH) of the porphyrin core resonate in the upfield region at δ −2.83 ppm. After treating P with a methanolic solution of each of phenol, p-amino phenol, and p-nitro phenol, a slight change in the 1H NMR spectra of P was observed (Figure ). In the spectra of P–P, the signals corresponding to Ha, Hc, and Hb are slightly downfield shifted by δ 0.01 ppm compared with the resonance signals of P. Similarly, the signal due to imino protons, NH, is also slightly downfield shifted by δ 0.01 ppm except for P, which resonates at the same δ value as that of P (Table S1). The line widths (Table S1) of resonance signals decrease in the order P > P > P > P (NH), P > P > P > P (Ha), and P > P > P > P (Hc/Hb). Hence, the downfield shift and broadening of resonance signals of P–P relative to P may be attributed to the existence of hydrogen bond between porphyrin and bound water molecules.[15] The downfield shifts of resonance signals further suggest that porphyrins are stacked in a J-type manner rather than an H-type manner, which induces an upfield shift of resonance signals.[16] The appearance of a slight downfield shift of resonance signals in amorphous forms, P–P, leads to the inference that some of the hydrogen bonds between water molecules and porphyrin cores are able to remain intact in CDCl3 solutions. The CDCl3 molecules may hinder the hydrogen bonds between water molecules and porphyrin cores, and as a result of this hindrance by the NMR solvent, the stacking frameworks of porphyrins may disrupt in CDCl3 solutions. Also as a side note, it can be seen that the intensities of the majority of the resonance signals in the case of compounds treated with phenolic moieties are higher than the parent compound, P.

Figure 5

Selected portions of the 1H NMR spectra (400 MHz pulse FT) of P (cyan), P (pink), P (yellow), and P (blue), ∼5 × 10–5 M in CDCl3.

The 1H NMR spectra of isolated arylic ethers, i.e., anisole, p-amino anisole, and p-nitro anisole, were also measured in dimethyl sulfoxide (DMSO) (Figures S5–S7). The proton of NMR spectrum of anisole shows a doublet (δ 6.74/6.76 ppm) and a triplet (δ 7.14/7.16/7.18 ppm) signal in the aromatic region corresponding to two arylic protons (Ha and Hb) (Figure S5). The second triplet signal of anisole is partially obscured by its doublet signal. Similarly, the NMR spectrum of p-amino anisole reflects a pair of doublets due to arylic protons Ha (δ 6.40/6.42 ppm) and Hb (6.46/6.48 ppm) followed by a singlet signal (δ 4.15 ppm) due to p-aminoproton (Hd) resonance (Figure S6). The NMR spectrum of p-nitro anisole also consists of two sets of doublets at δ 6.42/6.44 and 7.90/7.93 ppm corresponding to arylic protons (Ha and Hb) (Figure S7). The signals corresponding to methoxy proton resonances in all of the three molecules are, however, partially merged with the signal due to undeuterated DMSO molecules at δ ∼ 3.4 ppm.

FT-IR Spectroscopy

Figure shows the various stretching frequencies of the crystalline form, P and the amorphous solids, P–P. The number of infrared-active vibration modes of P–P are summarized in Table S2. The N–H stretching and bending frequencies for P appear at 3321.00 and 963.27 cm–1, respectively. However, for the compounds treated with phenolic moieties, there occurs a slight shift in the N–H vibrations. The N–H stretching for the water-bound porphyrins appears at 3311.87 (P), 3312.52 (P), and 3313.08 cm–1 (P). Similarly, the N–H bending occurs at 963.88 (P), 963.61 (P), and 963.78 cm–1 (P) [Figure (right)]. Further, the γ15, γ5, and υ24 modes of pyrrole fold vibration for P are centered at 784.03, 797.32, and 829.94 cm–1, respectively. In the compounds treated with phenolic moieties, notably the peak corresponding to the γ15 mode disappears, whereas peaks due to γ5/υ24 modes are observed at 790.48/824.94 (P), 790.70/825.50 (P), and 792.38/827.63 cm–1 (P). Also, the band due to the Cβ–H vibration of P at 1217.14 cm–1 is split into two (marked with stars) in the compounds treated with phenolic moieties and appears at 1211.21/1220.44 (P), 1211.57/1220.67 (P) and 1211.46/1220.30 cm–1 (P). The FT-IR spectra of compounds treated with phenolic moieties further show a band due to O–H stretching vibrations (marked with circles) located at 3355.48 (P), 3355.16 (P), and 3354.49 cm–1 (P). The presence of O–H frequencies clearly indicates the presence of bound water molecules within the three-dimensional frameworks of the macrocyclic rings, P–P. For P, a band due to in-plane pyrrole bending is found at 844.78 cm–1;[17] however, in the compounds treated with phenolic moieties, this band is split into two (marked with stars) and appears at 840.61/853.18 (P), 840.96/853.30 (P), and 841.97/853.13 cm–1 (P). A peak at ∼670 cm–1, which is obscured in P, can be seen in the FT-IR spectra of compounds treated with phenolic moieties. This band can be assigned to the N–H out-of-plane bending deformation. The comparatively broader band due to in-plane CH deformation, ∼1088 cm–1, sharpens up in the compounds treated with phenolic moieties.[17] These kinds of observed changes in the peak shape (splitting of single peaks or disappearance of split peak) and peak positions of P–P relative to P indicate the presence of intermolecular H-bonding induced by the water molecules in the case of P–P.

Figure 6

FT-IR spectra of P–P.

Mass Spectroscopy

To determine the fragmentation patterns of P and those of the compound treated with phenolic moieties, P, ESI mass spectra of P and P have been further recorded. As can be observed from Figure S8, P exhibits a molecular ion peak at m/z 753.02 (calculated value 752.64). On the other hand, the molecular ion peak is not observed in the mass spectrum of P (Figure S9). In fact, P shows a number of unique peaks attributed to the presence of bound water molecules with the macrocyclic rings. For example, the peak at m/z 1566.39 (calculated value 1565.32) is assigned to the three water molecule-coordinated [C44H26Cl4N4] dimer that is further bound to another water molecule that is being shared with three other [C44H26Cl4N4] molecules. Similarly, the peak at m/z 2320.60 (calculated value 2317.96) may be due to three water molecule-coordinated [C44H26Cl4N4] trimer that is further bound to another water molecule that is being shared with three other [C44H26Cl4N4] molecules. The peak at m/z 437.30 (calculated value 435.89) corresponds to a single water molecule-coordinated fragment molecule, [C26H14ClN4]. Another peak at m/z 546.34 (calculated value 547. 48) may be due to a water-molecule-coordinated [C32H18Cl2N4] fragment. Likewise, the peak at m/z 793.52 (calculated value 793.17) may correspond to two water molecule-coordinated [C44H26Cl4N4] species, which is also bound with another H2O molecule that is being shared with four other [C44H26Cl4N4] molecules.

The ESI mass spectrum of anisole (Figure S10) reflects the parent ion peak at m/z 107.04 (calculated value 108.14). Similarly, the negative ion ESI mass spectrum of p-amino anisole generates the parent ion peak at m/z 122.06 (calculated value 123.15) (Figure S11). A strong signal at m/z 108.08 (calculated value 108.14) corresponding to the fragmentation of the p-amino group from the ionized parent molecule was also observed in the negative ion ESI mass spectrum of p-amino anisole as shown in Figure S11. For p-nitro anisole, the parent ion peak was not observed; however, two significant peaks were found at m/z 108.04 and 138.05 (calculated values 108.14 and 138.09) in the negative ion ESI mass spectrum (Figure S12), indicating the fragmentation of −NO2 and −CH3 groups from the parent compound.

Absorption Spectroscopy

Figures and 8 represent the UV–visible absorption spectra of P and those of P–P in dichloromethane at ∼10–5 and ∼10–4 M, respectively. The core coordination of water molecules with P induces marked changes in the electronic structure as can be tracked by UV–visible spectroscopy. P shows one strong Soret or B band in the near-UV region, ∼418 nm, and four lower-intensity Q bands in the visible region, ∼515, 549, 591, and 646 nm. The B and Q bands arise due to π–π* electronic transitions of the porphyrin ring and correspond to the S0–S2 and S0–S1 electronic transitions, respectively.[18] On treating dichloromethane solution of P with methanolic solutions of each of phenol, p-amino phenol, and p-nitro phenol, amorphous solids, P–P, are formed, which induces a hyperchromic effect on the absorption bands. The increase in the intensities of the absorption bands in P–P relative to P may be ascribed to the presence of intermolecular H2O-bound stacks of macrocyclic rings, which disrupt in dichloromethane. The disruption of P–P in dichloromethane seems to be favored due to dipole–dipole interactions between dichloromethane and porphyrin as well as hydrogen bonding between dichloromethane and water molecules, which may increase the original hydrogen bonding between water molecules and porphyrin molecules. Notably, a small hump (marked with stars) appears for each of the compounds treated with phenolic moieties, P–P (Figures and 8), giving partially split B bands. The appearance of these small humps fairly suggests that not all of the intermolecular H2O-bound stacks of porphyrins are disrupted in dichloromethane and that some of the stacking architectures of porphyrins are able to remain intact in dichloromethane. Further, the appearance of these small humps at a higher wavelength (λmax ∼ 423 nm) in comparison with the main intense B band (λmax = 418 nm) indicates that the stackings of porphyrins induced by core-bound water molecules occur in a J-type supramolecular manner.[19] It can be seen from Figures and 8 that the intensities of absorption bands for P–P increase at a higher solute concentration. The FWHM of B absorption bands (Table S3) follows the trends P > P > P > P (∼10–5 M) and P > P > P > P (∼10–4 M). The intensities of absorption bands corresponding to P–P at both the concentrations (∼10–5 and ∼10–4 M) decrease in the following manner: P > P > P.

Figure 7

[Left] B and Q absorption bands of P–P; [right] magnified view of B bands of P–P in dichloromethane (∼10–5 M).

Figure 8

[Left] B and Q absorption bands of P–P; [right] magnified view of B bands of P–P in dichloromethane (∼10–4 M).

The electronic absorption spectra of anisole, p-amino anisole, and p-nitro anisole were further recorded in methanol at 298 K (Figures S13–S15). Anisole shows a band due to the π–π* (S0–S1) electronic transition of benzene ring at 289 nm (Figure S13). In the case of p-amino anisole, this band appears at a longer wavelength, 294 nm (Figure S14). The existence of an aniline-type B band in p-amino anisole indicates that the spectrum resembles the monosubstituted compound, which involves substituents that preferentially interact with the benzene ring.[20] On the other hand, the presence of the electron-accepting nitro group at the para position of methoxy benzene causes a bathochromic shift of the π–π* (S0–S1) absorption band relative to simple anisole, that is, the B absorption band in p-nitro anisole appears at 314 nm (Figure S15). The p-nitro anisole also shows a characteristic band due to the n–π* transition at 389 nm.

Emission Spectroscopy

Figure shows the photoluminescence emission spectra of P–P (λexc = 418 nm) in dichloromethane. Both P and P–P exhibit two characteristic S1 emissions, i.e., Q(0,0) and Q(0,1) at 656 and 720 nm followed by a less common S2 emission, i.e., B(0,0) at 421 nm. The appearance of the B(0,0) band in addition to Q(0,0) and Q(0,1) bands signifies an appreciable mirror-image relationship between the corresponding absorption and photoluminescence emission bands of P–P.[21] The smaller Stokes shift between the S0–S2 excitation and S0–S2 emission (∼3 nm) in P–P further suggests that the molecules do not undergo much change in their geometries upon exciting from ground states to the electronically excited states.[22] It is clear from Figure that the emission intensities corresponding to P–P are higher relative to P. The higher fluorescent emission intensities of P–P compared with P may be due to the presence of H2O-bound layers of porphyrins in the former that disrupt in dichloromethane to give more fluorescent monomers.

Figure 9

Emission spectra of P–P (c = 5.8 × 10–6 M) in dichloromethane.

Figure illustrates the solid-state photoluminescence emission spectra of P–P (λexc = 418 nm) under ultraviolet irradiation. In the solid state, the highly crystalline free-base porphyrin, P, and the highly amorphous solids, P–P, exhibit the higher-intensity B(0,0) band ∼420 nm; however, there is complete quenching of weaker-intensity Q(0,0) and Q(0,1) bands in all of the compounds, i.e., P–P. The disappearance of the weaker-intensity Q(0,0) and Q(0,1) bands is due to concentration quenching (CQ) of emission bands in the solid state.[23] The intensity order of the B(0,0) band in P–P follows the trend P > P > P > P. The higher intensity of P relative to P–P is attributed to the crystallization-induced emission enhancement (CIEE) effect in P.[24] In the amorphous solids, the presence of water-molecule-bound stacks of porphyrins induces quenching of emission bands. The quenching occurs due to possible energy migration through the stacks of porphyrin rings.[25]

Figure 10

Emission spectra of P–P in the solid state at 418 nm excitation under ultraviolet irradiation.

The photoluminescence emission spectra of anisole, p-amino anisole, and p-nitro anisole have been further recorded in methanol at 298 K. Anisole reflects one sharp S1 emission at 328 nm (λexc = 298 nm) (Figure S16). On the other hand, the emission spectra of p-substituted anisoles at the same excitation wavelength, i.e., 298 nm, reflect two characteristic peaks at 374/455 nm (p-amino anisole) (Figure S17) and 382/450 nm (p-nitro anisole) (Figure S18).

Powder XRD Analyses

The internal structures of P–P were investigated by powder XRD analyses. The XRD pattern of P–P (Figure ) exhibits some additional diffraction peaks (marked with stars) in comparison with the diffraction pattern of P. The rest of the diffraction peaks in P–P undergo a change in positions (Tables S4–S7) as well as relative intensities in comparison with the diffraction peaks of P. For example, the higher-intensity peak at 2θ 6.19° in P appears at 2θ 6.05, 6.11, and 6.05° in P, P, and P, respectively. Similarly, the peaks at 2θ 15.17 and 15.99° in P are shifted toward lower 2θ regions in P–P. Also, the higher-intensity peaks at 2θ 17.25 and 20.32° in P appear as medium-intensity peaks at slightly different 2θ regions in the case of P–P. The average crystallite sizes of the as-prepared samples were calculated according to Scherrer’s formula (eq ) by considering the three most intense diffraction peaks.where D is the crystallite size of the particles, λ is the wavelength of X-ray, β is the full width at half-maximum (FWHM) of the diffraction peak, and θ is the diffraction angle. The average crystallite size for P calculated using highly intense peaks at 2θ 6.19, 17.25, and 20.32° was found to be 58.79 nm. Similarly, the average crystallite sizes for P–P calculated using three most intense peaks at 2θ 6.05, 19.50, and 23.44 (P); 6.11, 22.03, and 23.47 (P); and 6.05, 20.42, and 23.50° (P) were found to be 40.24, 47.93, and 49.59 nm, respectively. The calculated crystallite sizes indicate that P–P are of smaller crystallite size as compared to P.

Figure 11

Powder XRD patterns for P (blue), P (green), P (magenta) and P (purple).

Scanning Electron Microscope/Transmission Electron Microscope (SEM/TEM) Analyses

To determine the change in surface morphology of P upon the formation of P–P, SEM/TEM analyses of P and those of P have been carried out. The SEM micrograph reflects that the surface morphology of P is rough and the individual particles are of spherical shape [Figure (left)]. The rough surface morphology and appearance of approximately distinct spheres of particles reflect poor connectivity among the individual molecules. On the other hand, relatively smooth surface morphology with multilayered arrangement of constituent particles is clearly visible for P [Figure (right)]. The appearance of a smooth surface for P indicates strong connectivity among the constituent particles within the layered structures of P.

Figure 12

SEM micrographs of [left] P and [right] P.

The TEM micrographs of P exhibit a number of irregular-shaped layered arrangements of particles (Figure ). On the other hand, the particles of P form relatively smooth nanosheet-like structures with one sheet lying over another in a slipped manner (Figure ). An individual nanosheet has a square-like geometry with edge length ranging from 0.8 to 2.0 μm. This kind of nanosheet-like arrangement was earlier also observed for SnIV5-(4-pyridyl)-10, 15, 20-triphenyl porphyrin [SnPyTriPP] with the edge length of individual nanosheets ranging from 0.3 to 1.0 μm.[26] On the basis of the crystal structure of SnT(4-Py)P[27] that reflects hydrogen bonding of porphyrins with water molecules in between the layers, it was suggested that the nanosheet crystals of [SnPyTriPP] might involve similar structural motifs. Notably, in the present case also, the appearance of similar nanosheet-like architectures for amorphous solid, P, might be attributed to the presence of hydrogen bonding of type (porphyrin) ···H (water) or O (water) ···H (porphyrin) via a bridging water molecule, resulting in the formation of layered nanosheet-like structures.

Figure 13

TEM micrographs of P.

Figure 14

TEM micrographs of P.

Nitrogen Gas Adsorption/Desorption Analyses

Figure shows the BET isotherms of P and P. The N2 sorption surface analysis parameters are listed in Table . The N2 gas adsorption study at 77 K showed that the amount of N2 adsorbed by P [Figure (left)] first increases with the increasing pressure and reaches a maximum value of ∼0.02 P/P0. After this, adsorption decreases with increasing pressure. At ∼0.05 P/P0, the adsorption becomes constant. However, at ∼0.10 P/P0, the adsorption again increases with the increasing pressure with maximum at ∼0.15 P/P0. At ∼0.05 P/P0, the adsorption again becomes constant till the pressure reaches ∼0.22 P/P0. This type of change in the adsorption isotherm of P reflects the variation in pore sizes of the sample. At 0.22 P/P0, a negative value of adsorption was found, indicating that almost all of the available pores have been filled with N2 and no further adsorption is possible with a further increase in pressure. At higher pressure, i.e., 0.9 P/P0, a small amount N2 has been squeezed into some of the unfilled pores of the sample. On the other hand, P maintained a type IV isotherm and showed uniform pore size distribution in the mesoporous region [Figure (right)]. P has a much higher surface area, pore volume, and pore diameter as compared to P. Hence, the coordination of water molecules within the porphyrinic cores of P induces remarkable changes in the surface area, pore volume, and pore diameter of P.

Figure 15

N2 adsorption–desorption isotherms of [left] P and [right] P.

Table 1

Results of BET Analysis for P and P

sample	specific surface area (m2 g–1)	average pore volume (cm3 g–1)	average pore diameter (nm)
P1	0.8557	0.00044055	2.0594
P2	1.9086	0.0054951	11.517

Thermal Analysis

To further determine the change in thermal stability of P upon the core coordination of water molecules, thermogravimetric analysis (TGA) analyses of P and P were carried out (Figure ). The initial stage of thermal decomposition for P [Figure (olive green)] involves evaporation of physically adsorbed water molecules at ∼110 °C. The sudden fall in the graph at this temperature indicates weak interaction of adsorbed water molecules with the porphyrin ring. After dehydration of lattice water molecules takes place, the mass remains constant between 250 and 430 °C followed by an abrupt weight loss at ∼470 °C due to collapse of porphyrin scaffolds. For P [Figure (magenta)], a continuous mass loss (though slowly) with the increasing temperature was observed in the initial stage of thermal decomposition, indicating first the loss of lattice water molecules between 100 and 150 °C followed by a loss of coordinated water molecules between 150 and 240 °C.[28] At ∼490 °C, the mass loss then increases with an increase in temperature, indicating the gradual collapse of entire porphyrin scaffolds. The residual mass percentages for P and P at 955 and 910 °C are found to be 54.39 and 52.63%, respectively.

Figure 16

TGA curves for P (olive green) and P (magenta).

Conclusions

In summary, we report a very simple as well as efficient synthetic methodology for the preparation of tetrapyrrole H2O-bound amorphous solids of P. The amorphous solids were prepared by simply carrying out the reactions between dichloromethane solutions of P and methanolic solutions of phenols. The core coordination of water molecules with P induces a notable change in the 1H NMR, electronic absorption, and photoluminescence emission spectra of P. SEM/TEM and XRD analyses indicate a change in surface morphology and structure of P upon tetrapyrrolic core incorporation of water as guest molecules. The N2 gas adsorption–desorption studies reflect that the treatment of P with methanolic solutions of phenolic moieties dramatically increases the surface area, pore volume, and pore diameter of the macrocyclic rings. The thermal analytical data clearly suggest that amorphous solids contain water molecules in the bound state. Successful isolation of phenol, p-amino anisole, and p-nitro anisole undoubtedly demonstrates the role of P as a highly efficient catalyst for the selective O-methylation of phenols by methanol, which is an eco-friendly methylating agent.

Materials and Methods

The analytical reagent (AR)-grade chemicals were used for the preparation P–P. The AR-grade chloroform, dichloromethane, methanol, phenol, p-amino phenol, p-nitro phenol, pyrrole, and propionic acid were all purchased from Sigma-Aldrich and were used as received without any further purification. Basic alumina for column chromatography was also procured from Sigma-Aldrich. FT-IR spectra was recorded on PerkinElmer Spectrum as KBr pellets. 1H NMR spectra were recorded on a Bruker spectrometer, model AV 400 N (400 MHZ), using CDCl3 as a solvent and TMS as an internal reference. UV–visible spectra were obtained on a PG spectrophotometer, model T-90, using CH2Cl2. Photoluminescence emission spectroscopic studies were recorded on F-4700 FL Spectrophotometer using CH2Cl2 as solvent at room temperature. The solid state emission spectra were also recorded on F-4700 FL Spectrophotometer using Xenon lamp as excitation source. The ESI mass spectra were acquired on Bruker Compass Data Analysis 4.1. Powder X-ray diffraction (PXRD) analysis was carried on a D8 X-ray diffractometer (Bruker) at a scanning rate of 12° min–1 in the 2θ range from 0 to 50°, with Cu Kα radiation (λ = 0.15405 nm). Scanning electron microscopy (SEM) micrographs of the samples were recorded on FEI Nova Nano SEM 450. Transmission electron microscopy (TEM) was carried out on a Tecnai G2 20 S-TWIN transmission electron microscope with a field emission gun operating at 200 kV. The samples for TEM measurements were prepared by evaporating a drop of the colloid onto a carbon-coated copper grid. TGA of the samples was carried out on a PerkinElmer instrument under a nitrogen atmosphere at a scanning rate of 20 °C per minute. BET methods were employed to perform the specific surface area analysis, using a Micromeritics ACAP 2020 analyzer. All of the samples were outgassed at 400 °C for 5 h before the measurements. The yield percentages of the amorphous solids, P–P, were calculated simply by dividing the mass of P–P obtained by the mass of P used initially.

Synthesis of 5, 10, 15, 20-Tetrakis-4-chlorophenyl Porphyrin, P

P was synthesized according to the modified Adler’s method.[12] Pyrrole (20 mmol) and 4-chloro benzaldehyde (20 mmol) were added to a 250 mL round-bottomed flask containing 50 mL of propionic acid solution. The reaction mixture was then allowed to reflux at 60 °C for half an hour. After the completion of the reaction, the mixture was cooled at room temperature and then stored in a refrigerator for 2 h. The shiny purple crude product was then filtered, and the residues were air-dried completely. The crude product was then subjected to purification on a basic alumina column using pure chloroform as the eluting solvent. The purified crystalline solid was finally recovered by evaporation of chloroform.

P was obtained as a purple solid with 70% yield. FT-IR (KBr, νmax, cm–1): 3321.00 ν(N–H)pyrrole, 1346.69 ν(C–N), 1487.61 ν(C=N), 3028.33 ν(C–H)ph, 3063.27 ν(C–H)py. 1H NMR (CDCl3, 400 MHz, δ, ppm): −2.84 (s, 2H, NH), 7.76–7.78 (d, 8H, ArHm), 8.15–8.17 (d, 8H, ArHo), 8.86 (s, 8H, βH). UV–vis (λmax, nm, CH2Cl2): 418 (B band), 515, 550, 590, 647 (Q bands).

Preparations of H2O-Molecule-Coordinated Porphyrins, P–P

To each 250 mL round-bottomed flask containing 15 mL of methanolic solution of each of phenol, p-amino phenol, and p-nitro phenol (3.086 × 10–2 mol) was added 6.602 × 10–4 mol of P dissolved in 10 mL of dichloromethane. The mixtures were then refluxed at 60 °C for 1 h. After allowing the mixture to cool at room temperature, the mixtures were filtered over Whatman chromatographic filter paper. The purplish amorphous solid residues were washed repeatedly with pure methanol and then air-dried completely. To ensure complete drying, a warm stream of air was given to the residues for 2–3 min.

P was obtained as a purple solid with 75% yield. FT-IR (KBr, νmax, cm–1): 3311.87 ν(N–H)pyrrole, 1346.24 ν(C–N), 1487.90 ν(C=N), 3029.05 ν(C–H)ph, 3063.53 ν(C–H)py. 1H NMR (CDCl3, 400 MHz, δ, ppm): −2.84 (s, 2H, NH), 7.76–7.79 (d., 8H, ArHm), 8.15–8.17 (d., 8H, ArHo), 8.87 (s, 8H, βH). UV–vis (λmax, nm, CH2Cl2): 418 (B band), 515, 549, 591, 647 (Q bands).

P was obtained as a purple solid with 70% yield. FT-IR (KBr, νmax, cm–1): 3312.52 ν(N–H)pyrrole, 1346.84 ν(C–N), 1486.95 ν(C=N), 3029.31 ν(C–H)ph, 3062.70 ν(C–H)py. 1H NMR (CDCl3, 400 MHz, δ, ppm): −2.83 (s, 2H, NH), 7.76–7.78 (d., 8H, ArHm), 8.15–8.17 (d., 8H, ArHo), 8.87 (s, 8H, βH). UV–vis (λmax, nm, CH2Cl2): 418 (B band), 515, 550, 590, 647 (Q bands).

P was obtained as a purple solid with 73% yield. FT-IR (KBr, νmax, cm–1): 3313.08 ν(N–H)pyrrole, 1346.80 ν(C–N), 1486.72 ν(C=N), 3028.98 ν(C–H)ph, 3063.03 ν(C–H)py. 1H NMR (CDCl3, 400 MHz, δ, ppm): −2.83 (s, 2H, NH), 7.76–7.78 (d., 8H, ArHm), 8.15–8.17 (d., 8H, ArHo), 8.87 (s, 8H, βH). UV–vis (λmax, nm, CH2Cl2): 418 (B band), 515, 550, 591, 647 (Q bands).

Recoveries of Anisoles

During the filtration of the end product of the reactions between phenolic moieties and free-base macrocycle in methanol/dichloromethane mixtures, anisoles were successfully collected after the evaporation of the filtrates. The collected anisoles were then purified on basic alumina columns using pure organic solvents of dichloromethane and methanol as eluting agents. Elution with dichloromethane first removed the faint purple liquid corresponding to a small amount of free-base impurities. Anisoles eluted later from the columns with the polar solvent, i.e., methanol.

Anisole was obtained as a clear straw liquid with 40% yield. 1H NMR (CDCl3, 400 MHz, δ, ppm): 3.44 (s, 3H, OCH3), 6.74–6.76 (d., 2H, ArHo), 7.14–7.16–7.18 (t., 3H, ArHm,p). HR-MS (ESI) (+ve mode): m/z 108.04 calcd 108.14 for C6H5O molecular ion.

P-amino anisole was obtained as a dark brown liquid with 55% yield. 1H NMR (CDCl3, 400 MHz, δ, ppm): 3.40 (s, 3H, OCH3), 6.46–6.48 (d., 2H, ArHo), 6.40–6.42 (d., 2H, ArHm), 4.14 (s, 2H, NH2). HR-MS (ESI) (−ve mode): m/z 108.08 calcd 108.14 for C6H5O ion and 122.06 calcd 123.15 for C7H9NO molecular ion.

P-nitro anisole was obtained as an amber liquid with 52% yield. 1H NMR (CDCl3, 400 MHz, δ, ppm): 3.48 (s, 3H, OCH3), 6.42–6.44 (d., 2H, ArHo), 7.90–7.93 (d., 2H, ArHm). HR-MS (ESI) (−ve mode): m/z 108.04 calcd 108.14 for C6H5O ion and 138.05 calcd 138.12 for C6H7NO ion.