Facile Construction of a Supramolecular Organic Framework Using

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Article Cite This: ACS Omega 2019, 4, 8528−8538

http://pubs.acs.org/journal/acsodf

Facile Construction of a Supramolecular Organic Framework Using Naphthyl Viologen Guests and CB[8] Host via Charge-Transfer Complexation Kanagaraj Madasamy,†,‡ David Velayutham,†,‡ and Murugavel Kathiresan*,†,‡ †

Electroorganic Division and ‡Academy of Scientific and Innovative Research (AcSIR), CSIR−Central Electrochemical Research Institute, Karaikudi, 630003 TamilNadu, India

ACS Omega 2019.4:8528-8538. Downloaded from pubs.acs.org by 79.110.17.98 on 05/16/19. For personal use only.

S Supporting Information *

ABSTRACT: Herein, we report the synthesis of guest−host systems comprising naphthyl−viologen−naphthyl (Np−Vio−Np) and viologen−naphthyl−viologen (Vio−Np−Vio) guest molecules and their subsequent supramolecular polymerization in the presence of a CB[8] host in water. In addition, the guest complexation of ethyl-terminated trimeric viologen (ETV) with Np−Vio−Np and CB[8] was investigated. As a result of supramolecular interactions, 2D supramolecular organic frameworks with high internal periodicity were constructed. 1H NMR studies clearly demonstrated the formation of a host-stabilized chargetransfer complex via folding back (Np−Vio−Np and Vio−Np−Vio) in the presence of CB[8]. In the case of ETV + Np−Vio−Np + CB[8], a large polymeric network was formed as indicated by the NMR titrations. UV−vis and fluorescence studies clearly confirm the formation of an inter/intra molecular CT complex upon complexation with cucurbit[8]uril. The size obtained using the dynamic light scattering (DLS) method pinpoints the formation of larger supramolecular aggregates in the order of μm through host−guest assembly, which is further complemented by FESEM and TEM. SAXS measurements indicate the formation of a 2D supramolecular polymer/polymer aggregate with longrange order.

1. INTRODUCTION Construction of supramolecular polymers highly relies on the nature of the host/guest molecule used.1−3 Molecular interactions such as hydrogen bonding interactions, metalcoordination bonds, electrostatic interactions, donor−acceptor interactions, π−π interactions, ion−dipole interactions, and so forth are frequently employed for the construction of supramolecular polymers.3−7 In certain cases, even multiple driving forces are utilized that are identical to the biological systems.6 Supramolecular assemblies involving interlocked systems8 received wide attention owing to their key role in the construction of stimuli responsive smart materials including molecular switches, molecular displays, molecular wires, and intelligent artificial materials.9−11 Reversible and very directional secondary interactions12,13 such as hydrogen bonding, ion−dipole interactions, and charge−transfer interactions are used as versatile building tools to construct supramolecular polymers with specific functions as they offer controllability, reversibility, and stimuli responsive behavior.4,5,14,15 In particular, assembly of supramolecular polymers in aqueous solutions attracts wide attention as they mimic biological systems or find applications in the areas of optics, biology, or electronics.14,16,17 Several host and guest molecules such as cucurbit[n]urils, calixarenes, crown ethers, cyclodextrins, and so forth and viologens, pyridinium, substituted ammonium salts, and so forth were used to construct © 2019 American Chemical Society

supramolecular polymers based on ion−dipole, hydrophobic, or electrostatic interactions.6 N,N′-Dialkyl-4,4′-bipyridinium, commonly known as viologen (V2+), exists in different ionic states (V2+ ↔ V•+ ↔ V0), dicationic, monocation radical, and neutral states with respect to the applied electrochemical potential.18−20 The ease of dimerization (V •+ ↔ •+ V) of viologen cation radicals (V•+)21−23 and charge-transfer (CT) complex formation between the monocation radicals, the electron deficient viologen (dication) and electron rich aromatic entities [naphthalene, coumarin, tetrathiafulvalene, etc.]19,24−26 offer synthetic chemists to design new architectures to assemble different entities via supramolecular interactions resulting in the formation of stimuli-responsive supramolecular polymers.27 Cucurbit[8]uril commonly denoted as CB[8] is a macrocyclic cavity made up of glycoluril units that are interconnected by methylene bridges.28 It binds both neutral as well as cationic guest molecules via hydrophobic and cation−dipole interactions.29 Its specific binding properties and the cavity space allows for the incorporation of two appropriate aromatic guests, such as, viologen radical dimers22 or naphthyl and viologen units30 instantaneously. This binding behavior of CB[8] is widely investigated and utilized in the construction of Received: February 12, 2019 Accepted: May 6, 2019 Published: May 15, 2019 8528

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Figure 1. Chemical structures of interacting molecules; guest: linear axel viologen molecules (Np−Vio−Np, Vio−Np−Vio) and viologen trimer (ETV); host: cucurbit[8]uril.

Scheme 1. Synthesis of Naphthyl-Terminated Viologen Np−Vio−Np and Viologen-Terminated Naphthyl Molecules Vio−Np− Vio Molecules; Reaction Conditions; (i) CH3CN, 80 °C, 4 d, (ii) 3 M NH4PF6/H2O

Figure 2. 1H NMR spectra of (a) Np−Vio−Np (7.0 mM) molecule upon addition of 0.5, 1.0, 1.5, and 2.0 equiv CB[8] and CB[8] alone from bottom to top; (b) NOESY spectrum of Np−Vio−Np + 1.5 equiv CB[8] in D2O at 25 °C.

hydrophobic and CT interactions.5 As a result of these interactions, pseudorotaxane (supramolecular polymer) was formed. Very recently Chang et al. reported the linear supramolecular assembly through host−guest interactions using methyl viologen, coumarin guests, and CB[8] host.14 By

supramolecular polymers, where CB[8] is employed as a host as well as a bridging unit.31 Liu and co-workers reported the instantaneous molecular recognition between adamantine and β-CD with viologen and CB[8] guest−host pairs, which results in the formation of a quaternary complex guided by 8529

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Figure 3. 1H NMR spectra of (a) Vio−Np−Vio (5.20 mM) molecule upon addition of 0.5, 1.0, 1.5, and 2.0 equiv CB[8] and CB[8] alone, from bottom to top; (b) NOESY spectrum of Vio−Np−Vio + 1.5 equiv CB[8] in D2O at 25 °C.

units that are interconnected with flexible butyl linkers (Np− Vio−Np and Vio−Np−Vio) with CB[8]. 2.2.1. NMR Titration of Np−Vio−Np with CB[8]. Initially, we started our investigation on the formation of the CT complex (1:1) in Np−Vio−Np molecules within CB[8] cavity. CB[8] was added in small portions, and the corresponding 1H NMR spectra was recorded. Addition of 0.5 and 1.0 equiv CB[8] to the Np−Vio−Np sample resulted in the upfield shift of Np protons, that is, some of the Np protons appeared in the region of 6.67−6.43 and 6.03 ppm (upfield shift), which is indicative of the inclusion in the hydrophobic CB[8] cavity (Figure 2a).36,37 Encapsulation into the CB[8] cavity results in the conformational restriction of Np protons and as a result Np protons experience magnetically different environments resulting in peak splitting/peak shifts.36 In the free Np−Vio− Np molecule, the bipyridinium protons “a” and “b” appeared as a doublet at 8.8 and 7.6 ppm, respectively; however, in the presence of 0.5 and 1 equiv CB[8], the peak at 8.8 ppm has split into two peaks (i.e., a broad doublet and a broad quartet shifted to the upfield region indicating the interaction with Np moiety and encapsulation into the CB[8] cavity). Similarly, the peak at 7.6 ppm has split into three peaks (all appeared as broad doublets, of which two doublets appeared in the downfield region). All these results clearly indicate a face-toface π−π stacking with the naphthyl unit and an aromatic πelectron shielding of the bipyridinium units upon inclusion in the CB[8] cavity.38 Besides naphthalene and bipyridinium protons, the methylene protons too showed peakshifts, that is, the methylene protons c and f on linker were shifted to downfield (c = 4.73− 4.94 ppm, f = 4.01−4.49 ppm), indicating that both these −CH2 groups were not encapsulated into the CB[8] cavity. All these observations certainly indicate that one of the terminal Np units in the Np−Vio−Np molecule backfolds to

introducing an azobenzene monomer, the morphology of the formed polymer changes from linear to dendritic. Zhang et al. reported the construction of two dimensional (2D) supramolecular organic framework utilizing viologen cation radical and CB[8].32 Recently, we reported on the formation of reversible 2 dimensional supramolecular organic framework comprising viologen monocation radicals and CB[8], where dimeric, trimeric, and star-shaped viologen monocation radicals were used in the construction of supramolecular polymers within the CB[8] cavity.22 In this work, we report the synthesis of naphthyl-terminated viologen (Np−Vio−Np) and viologen-terminated naphthyl (Vio−Np−Vio) systems and their subsequent supramolecular polymer formation with ethylterminated trimeric viologen (ETV) in the CB[8] cavity (Figure 1).

2. RESULT AND DISCUSSION 2.1. Synthesis. The starting materials 1, 2, and 3 were synthesized using the reported procedures.33−35 All target compounds, Np−Vio−Np and Vio−Np−Vio, were obtained from nucleophilic substitution reactions between the corresponding alkyl halide with free or mono alkylated 4,4′bipyridine in CH3CN. The starting material 1 (10 fold excess) was reacted with 4,4′-bipyridine in CH3CN to afford Np− Vio−Np (80%). An identical reaction condition was followed for the synthesis of Vio−Np−Vio, where compound 2 was reacted with 3 (10 fold excess) to obtain Vio−Np−Vio (75%). Finally, the ETV molecule was synthesized using a procedure reported in our previous work.22 The as-synthesized molecules were then characterized by 1H, 13C, and DEPT NMR, ESI-MS, and elemental analyses (Scheme 1). 2.2. NMR Investigations. 1H NMR study was carried out to ascertain the binding process of viologen with naphthyl 8530

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Figure 4. (a) 1H NMR spectrum of ETV (4.30 mM) alone, ETV + 3.0 equiv CB[8], Np−Vio−Np alone, ETV + 3.0 equiv CB[8] + 1.0 equiv NP− Vio−NP, and ETV + 3.0 equiv CB[8] + 2.0 equiv NP−Vio−NP and CB[8] alone, from bottom to top; (b) NOESY spectrum of ETV + 3.0 equiv CB[8] + 1.0 equiv Np−Vio−Np in D2O at 25 °C.

chemical environment. Both the bipyridinium protons “c” and “d” (8.9 and 8.1 ppm) experienced downfield shifts (few set of peaks in the range of 9.37−8.07 ppm) upon addition of 0.5 equiv CB[8], indicating the face-to-face π−π stacking with the naphthyl unit and aromatic π-electron shielding of the bipyridinium units upon inclusion in the CB[8] cavity.38 In the presence of 1.0 equiv, some of bipyridinium peaks disappeared and broad peaks with low resolution were observed signifying that one of the bipyridinium unit is completely incorporated into the CB[8] cavity due to the hoststabilized charge−transfer interactions.39 When the CB[8] concentration is 1.5 equiv, the uncomplexed viologen unit also gets encapsulated indicated by slight upfield shift and peak broadening. Moreover, during the encapsulation, the protons o, p, q, and r on tether experience the shielded environments and suffer by steric hindrance as well. Consequently these protons appeared in the region of 3.19−1.59 ppm were shifted slightly upfield. Furthermore, 0.5 equiv addition of CB[8] (2.0 equiv in total) scarcely affected the peak intensity; however, minor upfield shifts on the bipyridinium and aliphatic protons were noted and almost all of the peaks appeared broadened, indicating the formation of larger aggregates/polymer via CT complexation. The NOESY spectrum for the [(Vio−Np−Vio) + 1.5 equiv CB[8]] complexed mixture gave additional information for the formation of ternary inclusion complex. The cross peaks between bipyridinium protons c and d and m, n, l, and i protons of Np units confirms the occurrence of both the units inside the CB[8] cavity as observed in the case of Np−Vio−Np (Figure 3b). 2.2.3. NMR Titration of ETV with CB[8] and Subsequent Addition of Np−Vio−Np. The self-assembly of ETV with CB[8] is well-known from our previous work where viologen monocation radicals were encapsulated into the CB[8] cavity

bipyridinium center upon addition of 1.0 equiv CB[8] via hoststabilized intramolecular CT complexation. Further 0.5 equiv addition (1.5 equiv in total) of CB[8] results in the complexation of another Np unit in Np−Vio−Np, that is, encapsulation results in the broadening of few Np signals, some of the peaks between 7.7 and 6.7 ppm were upfield shifted (the intensity of peaks between 6.7 and 6.4 ppm increased), indicating encapsulation into the second CB[8] cavity (due to the increased intensity of CB[8] signals, the aromatic signals of bipyridinium and naphthalene appeared very low in intensity). In nuclear Overhauser enhancement spectroscopy (NOESY) spectrum the cross peaks were observed between protons (a, j, and m), (b, k, and l) and (b, j, and m), when 1.5 equiv CB[8] is added, ascribed to the close packing of bipyridinium and Np moieties (one over the other) inside CB[8] cavity as shown in (Figure 2b). Finally, when 2.0 equiv of CB[8] is added, drastic changes in the NMR spectrum with low resolved signal was observed, which clearly demonstrates the formation of aggregates/polymer via CT complexation (repetitive orientation of Np and Vio units inside CB[8] cavity in water). 2.2.2. NMR Titration of Vio−Np−Vio with CB[8]. Similar self-assembly was observed in the case of Vio−Np−Vio molecule; here, viologen backfolded onto the naphthalene unit upon addition of first CB[8] unit, that is, both viologen and subsequent addition of CB[8] led to the complexation of free viologen. Addition of 0.5 equiv CB[8] resulted in the upfield shift of Np protons (7.4−6.7 ppm were shifted to 6.66−6.51, 6.01, 5.94 ppm), indicating the encapsulation of Np unit into the CB[8] cavity (Figure 3a). In contrast, bipyridinium protons showed a mixed response, that is, uncomplexed bipyridinium proton does not show any peak shift, whereas the complexed bipyridinium protons responded to their new 8531

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Figure 5. UV−vis spectrum of (a) Np−Vio−Np; (b) Vio−Np−Vio; and (c) ETV (0.28 mM) guest molecules. (d) fluorescence emission spectrum of Np−Vio−Np in different concentrations of CB[8] in H2O, rt.

to form supramolecular polymer.22 In this work, we were interested to complex dicationic ETV into the CB[8] cavity. The host−guest process was initiated by adding 1.5 and 3.0 equiv of CB[8] to the ETV solution in D2O, during which the bipyridinium protons displayed upfield and downfield shifts (two sets of broad peaks in the region of 9.18−8.91 and 8.72− 8.52 ppm) indicative of encapsulation into the hydrophobic cavity. When 3 equiv CB[8] is added to ETV, 1:1 stoichiometric complexation between CB[8] and ETV is expected. To this complexed solution, 1.0 equiv Np−Vio−Np molecule was added, so that the Np will undergo face-to-face π−π stacked intermolecular CT complex formation with complexed ETV inside the CB[8]. It is noteworthy that both the bipyridinium and naphthalene units experienced downfield and upfield shifts upon complexation as in the case of Np− Vio−Np, indicating the formation of intermolecular CT complex in the CB[8] cavity (Figure 4a). This observation is further confirmed by the cross peaks between bipyridinium and naphthalene protons in NOESY spectrum (Figure 4b). When adding 2.0 equiv Np−Vio−Np, the resultant spectrum did not show any significant changes because of poor solubility of the second guest in D2O. All these observations above certainly signify the formation of larger aggregates. In general, supramolecular systems undergo conformational changes (for example, folded form to an extended form) in response to an external chemical stimulus.39 Further, physical stimuli, such as, heat, ultrasound, are used to acquire better solubility of host and guest molecules and could also be used to assist the conformational changes. In this study, the guest molecules such as Np−Vio−Np, Vio−Np−Vio, and ETV are involved in larger aggregate/polymeric species formation in extended arrangements with CB[8] host. 2.3. UV−Vis and Fluorescence Study. UV−vis spectroscopy is a prominent tool which is frequently employed to probe the CT complexation process between electron-rich and electron-deficient species. During the NMR titration, color

change upon CB[8] addition was noticed, that is, the color of Np−Vio−Np changed from brown to pale orange (Supporting Information, Figure S1). The color changes could be attributed to the host-stabilized CT complexation.36 To gain more insight on the interaction and to investigate the formation of intramolecular CT complex, UV−vis titration was carried out by varying the concentration of CB[8], while the guest concentration was kept constant. Initial study was carried out between Np−Vio−Np and CB[8]. Upon addition of 1.0 equiv CB[8] to Np−Vio−Np (0.28 mmol) solution, visual color change from yellow to brown is noticed, signified by a new broad absorption band centered around 405 nm, corresponding to the formation of an intramolecular CT complex as indicated by 1H NMR titrations (Figure 5a). These results clearly indicate the backfolding of Np over the Vio moiety upon inclusion in the CB[8] cavity as this is the only possibility. Addition of further 1 equiv. CB[8] resulted in the increase in the intensity of CT band around 405 nm which is attributed to the extension of CT complex by self-exchange of CB[8] between free and CT-complexed entities within the same molecule leading to the formation of larger aggregates. In the case of Vio−Np−Vio, characteristic CT band was observed in the visible region centered around 415 and 560 nm in the presence of 1 and 2 equiv CB[8] (Figure 5b). In addition, the color of the solution turned from brown to purple, which is indicative of the formation of a CT complex between naphthalene and bipyridinium units inside the CB[8] cavity. Thus, the broad absorption bands between 400 and 600 nm clearly indicated the existence of intramolecular hostinduced CT interaction in the complexed species. 39 Furthermore, the CT interaction between the donor (Np) and the acceptor (Vio) units is strengthened by the space restrictions existing in the CB[8] cavity.36 Thus, the hostenhanced CT interaction allows for the formation of 1:1:1 ternary complex between Vio, Np, and CB[8] units, which 8532

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between Np and bipyridinium units as discussed in NMR and UV−vis study (Figure 5d).39,41 Further addition of CB[8] (2.0 equiv) led to the increase in the intensity of CT band, which is due to the extension of CT interaction between donor and acceptor species in the same Np−Vio−Np molecule as discussed in the UV−vis study. 2.4. Particle Size Measurements. DLS measurements is a key tool to state the existence of supramolecular polymeric species/larger aggregates in solution. The DLS measurements were carried out to get clear insights on the supramolecular polymer assemblies formed by different guest molecules (Np− Vio−Np, Vio−Np−Vio, and ETV) with CB[8] host in aqueous solution. The DH values for the as-formed selfassembled species were found to be 0.63 μm for Np−Vio−Np, 0.53 μm for Vio−Np−Vio, and 0.77 μm for ETV, respectively. These DH values clearly indicate the formation of larger aggregates (polymeric species) by the guest molecules in the presence of CB[8] host via self-assembly process (Figure 6). The formation of these larger aggregates could be attributed to the formation of 2D assemblies as reported in our previous study.14 The degree of polymerization is in the order, ETV − 3 (CB[8]−Np−Vio−Np) > Np−Vio−Np−2CB[8] > Vio−Np− Vio-2CB[8]. Herein, the degree of polymerization is more favored for the ETV − 3 (CB[8]−Np−Vio−Np) combination guided by intermolecular CT complexation, and this result is very much related to the results obtained in UV−vis studies. Zeta potential measurements were carried out to understand the stability of colloidal dispersion obtained from viologen guests (Np−Vio−Np, Vio−Np−Vio, and ETV) and CB[8] (Figure S3). The zeta potential values are +33.5, +34.6, and +38.0 mV for Np−Vio−Np−2CB[8], Vio−Np−Vio-2CB[8], and ETV + 3 (Np−Vio−Np + CB[8]), respectively. These values clearly indicate the formation of aggregates with good stability. 2.5. Diffusion-Ordered Spectroscopy. Diffusion-ordered spectroscopy (DOSY) experiment was carried out to examine the formation of aggregates/polymers as the diffusion coefficient (D) clearly distinguishes between the complexed and uncomplexed host−guest molecules, provided that there is a sufficient difference in their D values.42 The D values of guest molecules are considerably small compared to the complexed species resulting from the addition of 1.0 and 2.0 equiv CB[8] (CB[8] is added to the guest molecule in solution, Table 1).31 The D value of free Np−Vio−Np monomer was 2.88 × 10−10 m2/s, and the D value gradually decreased with the CB[8] addition, indicating the increase in molecular weight of the complex.10,31 When 2 equiv CB[8] is added, the host-stabilized CT complex of Np−Vio−Np showed a D value of 1.02 × 10−10 m2/s, which is nearly one-third of the D value of the

further extends as a supramolecular polymer or supramolecular aggregates.39,40 Similarly in the case of ETV molecules, the CT complex formation in the presence of CB[8] was monitored by incremental addition of CB[8] and second guest (Np−Vio− Np) in a different stoichiometric ratio (Figure 5c). UV−vis spectrum of ETV and ETV − 3CB[8] complexed species (blue and red line) shown in Figure 4c did not show any absorption peak in the range of 300−700 nm, indicating the absence of characteristic CT band in both ETV molecule and ETV − 3CB[8] complexed species. Upon addition of 1.0 equiv of Np−Vio−Np guest to the ETV − 3CB[8] complex, a broad absorption peak at 403 nm was observed. This is attributed to the encapsulation of the terminal Np group into the CB[8] cavity, which is already occupied with bipyridinium group of ETV molecule leading to the formation of intermolecular CT complex (color change from colorless to orange). Subsequent additions (2 and 3 equiv) of the second guest (Np−Vio−Np) molecule facilitated the CT complexation process as indicated by the enhancement of CT band and in addition blue shift was observed (from 403 to 398 nm). The intermolecular CT complexation in this case favors the formation of supramolecular polymers as indicated by the dynamic light scattering (DLS) measurements (Figure 6).

Figure 6. Particle size measurements of Np−Vio−Np (0.70 mM), Vio−Np−Vio (0.70 mM), and ETV (0.70 mM) molecules in the presence of CB[8] in different stoichiometric ratio in H2O, rt.

The fluorescence emission study was carried out for Np− Vio−Np in the presence of CB[8] to infer additional information on host-stabilized CT complexation. The fluorescence spectrum of Np−Vio−Np showed strong emission peak at 470 nm corresponding to naphthalene chromophore. Upon addition of 1.0 equiv CB[8], slight decrease in fluorescent intensity (30%) was observed with emission peak centered at 490 nm (red shift, Δλ = 20 nm) ascribed to improved CT complex formation inside the CB[8] cavity

Table 1. Diffusion Coefficients (D) of Guest Molecules in the Absence and Presence of CB[8] host/guest Np−Vio−Np Np−Vio−Np + 1 equiv CB[8] Np−Vio−Np + 2 equiv CB[8]

CB[8]

D (×10−10 m2/s) 2.88 1.99 1.02

host/guest

D (×10−10 m2/s)

Vio−Np−Vio Vio−Np−Vio + 1 equiv CB[8] Vio−Np−Vio + 2 equiv CB[8]

host/guest

D (×10−10 m2/s)

3.16 2.45

ETV ETV + 3 equiv CB[8]

5.88 5.49

0.93

ETV + 3 equiv CB[8] + 1 equiv Np−Vio−Np ETV + 3 equiv CB[8] + 2 equiv Np−Vio−Np ETV + 3 equiv CB[8] + 3 equiv Np−Vio−Np

5.13 4.78 3.38

2.75 8533

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Figure 7. FESEM images of (a) Np−Vio−Np (1.0 mM) + CB[8] (2.0 equiv); (b) Vio−Np−Vio (1.0 mM) + CB[8] (2.0 equiv); and (c) ETV (0.1 mM) + 3.0 equiv of CB[8] + Np−Vio−Np in H2O.

Figure 8. TEM images of supramolecular polymers of (a) Np−Vio−Np (0.50 mM) + 2 equiv CB[8], (b) Vio−Np−Vio (0.50 mM) + 2 equiv CB[8], and (c) ETV (0.50 mM) + 3 equiv Np−Vio−Np + CB[8] in H2O.

guest to the complexed species resulted in the gradual decrease of D values indicating the formation of larger species. 2.6. FESEM. Field emission scanning electron microscopy (FESEM) measurements were carried out to investigate the surface morphologies of the as-formed supramolecular polymers using Np−Vio−Np, Vio−Np−Vio, and ETV guests and CB[8] host through the host-stabilized CT complex process. These self-assembled polymers displayed different

corresponding monomer. In the case of Vio−Np−Vio molecule, the D value decreases from 3.16 × 10−10 to 9.33 × 10−11 m2/s upon addition of 2.0 equiv CB[8] to the guest solution. Thus, the significant decrease in D value confirms the formation of larger supramolecular aggregates/polymer in the presence of CB[8]. In the case of ETV and ETV − 3(CB[8]), the D values obtained were in the range of 5.88 and 5.49 × 10−10 m2/s, respectively. Incremental addition of Np−Vio−Np 8534

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morphologies based on the guest molecule used (aggregates, hexagonal, and rod-like morphologies) (Figure 7). The size of these particles ranged from 0.2 to 1.0 μm matching well with the DLS data and indicates the formation of larger aggregates or supramolecular polymers. 2.7. Transmission Electron Microscopy. The formation of supramolecular polymers was further investigated by transmission electron microscopy (TEM) (Figure 8). The existence of “antler” shape morphology in several nanometers to micrometer length in the case of Np−Vio−Np + 2 equiv CB[8] clearly indicated the formation of linear supramolecular polymer in the Np−Vio−Np molecule via self-assembly process with CB[8].14 In contrast, spherically distributed aggregate-type morphology was observed in the case of Vio− Np−Vio + 2 equiv CB[8], and dendritic type morphology with several micrometers was observed for the supramolecular polymers made-up of the ETV and Np−Vio−Np monomers via self-assembly with CB[8]. The higher dendritic size confirms the higher degree of polymerization and the sizes are in accordance with FESEM and particle size measurements. 2.8. SAXS Measurement. The as-assembled supramolecular polymers were analyzed by small angle X-ray scattering (SAXS) technique (Figure 9). D spacing values of

further confirmed by morphological measurements by SEM and TEM.47 2.9. Mechanism of Formation of Supramolecular Polymers. Based on the above results, we have proposed the following mechanism for supramolecular polymerization in these three cases as shown in Figure 10.

Figure 10. Mechanism showing the formation of Supramolecular polymerization in Np−Vio−Np, Vio−Np−Vio, and ETV + Np−Vio− Np in the presence of CB[8]. Figure 9. SAXS profile of Np−Vio−Np (7.0 mM), Vio−Np−Vio (5.20 mM), and ETV (4.30 mM) in the presence of CB[8] in different stoichiometric ratios in H2O.

3. CONCLUSIONS In summary, we have reported the construction of a 2D supramolecular organic framework using different viologen guest molecules (Np−Vio−Np, Vio−Np−Vio, and ETV) with CB[8]. The supramolecular interactions as well as the polymer formation were characterized by 1H NMR, UV−vis spectroscopy, CV, DOSY, DLS, SAXS, and microscopic techniques. 1H NMR investigations clearly demonstrated the formation of inclusion donor−acceptor complex in the hydrophobic CB[8] cavity. It is noteworthy that the 1 equiv addition of CB[8] resulted in the formation of an intramolecular complex where a naphthyl or viologen unit folds back onto its corresponding donor/acceptor moiety to form an inclusion complex with CB[8]. Further addition of CB[8] enhances the formation of the inclusion complex via exchange of a CB[8] molecule and results in the formation of large aggregates as indicated by the broad peaks or disappearance of corresponding peaks in the 1H NMR spectrum. UV−vis and fluorescence studies revealed the formation of inter/intra molecular CT complexation between viologen guests and CB[8] host. Particle size measurement studies revealed the formation of large self-assembled aggregates whose size was in the order of μm. Zeta potential measurements signified the formation of a colloid between the guest and host molecules with good stability. DOSY measurements showed variation in the diffusion coefficient values of uncomplexed and complexed molecules, indicating the

these polymers were calculated using the Bragg equation (nλ = 2d sinθ, where λ = 1.54 Å). The noticeable broad scattering peaks between 2θ = 0.5°−2.0° corresponds to Bragg reflection with {100} and {200} facet,43,44 indicating the formation of ordered mesoporous 2D hexagonal arrangements with p6mm symmetry.45,46 The broad peaks with d-spacing values around 5.0 and 9.0 nm further confirmed the ordered structural arrangements. This ordered internal periodicity is the main criteria for the existence of 2D structures.26 Hence, these observations clearly indicate that these self-assembled polymers form 2D architecture with long-range order. Although the DLS measurements indicate the formation of larger aggregates, SAXS shows the evidence for the formation of 2D framework with long-range order. All these results clearly indicate the following possibilities, (i) the supramolecular polymerization of the guest molecules with the CB[8] host leads to the formation of 2D framework with long-range order, or (ii) aggregates of these host and guest molecules leads to the formation of an ordered network, or (iii) a mixture of supramolecular polymers and their aggregates as indicated by DLS and SAXS, which is 8535

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slope of the straight line obtained when ln(I) is displayed against the gradient-pulse power’s square according to the following equation: ln(I) = −γ2G2Dδ2(Δ−δ/3), where I is the relative intensity of a chosen resonance, γ is the proton gyromagnetic ratio, Δ is the intergradient delay (60 ms), δ is the gradient pulse duration (varied between 1.5 and 5 ms), and G is the gradient intensity. 4.5. Zeta Potential and Particle Size Measurements. The size distribution of the viologen CT complexed species in the presence of CB[8] cyclic host molecule was investigated. For these measurements, viologen guest molecule concentration (7.0 mM) was kept constant.

formation of large molecular species. FESEM analysis revealed the formation of supramolecular polymers with different morphologies and their diameters were in the range of μm, supplementing the DLS results. Furthermore, SAXS analysis indicated the formation of 2D supramolecular polymer/ polymer aggregate with long-range order. Future studies will focus on the spectroelectrochemical investigations and energy storage applications of these systems.

4. EXPERIMENTAL SECTION 4.1. Materials. All starting materials and solvents were purchased from Sigma-Aldrich or Alfa Aesar and used without further purification. All reactions were performed under dry conditions. For host−guest interaction studies, synthesized cucurbit[8] uril was used.22 For measurements in organic media, the as-prepared viologen hexafluorophosphate salts were used, and for the measurements in aqueous media, the corresponding PF6− salts were anion exchanged to Br− salts by adding TBA·Br in CH3CN. 4.2. Characterization and Measurements. 1H NMR, 13 C NMR, and DEPT spectra were recorded on a Bruker 500 AVANCE spectrometer at 25 °C using CD3CN or DMSO-d6 as solvents and internal references. Elemental analyses were performed on Elementar Vario EL III Micro cube instrument. ESI-mass spectra were recorded on a Finnigan LCQ Advantage Max, Thermofisher Instrument. Zeta potential and DLS size distribution measurements were carried out in Nano ZS model ZEN 3600, Malvern Instrument and Microtrac-Nanotrac 250 instrument respectively. SAXS experiments were conducted on Rigaku (Rigaku Smart Lab) using Cu Kα radiation, the power of X-ray source was operated at 50 mA and 40 kV, and the dspacing values were calculated by the formula, d = 2π/q. For SAXs measurement, thin films of the respective polymers were prepared using dip coating method on glass plates. FESEM (Gemini Zeiss) and Transmission electron microscope (Tecnai 20 G2 (FEImake), Netherlands) were used to analyze the surface morphologies. 4.3. UV−Vis and Electrochemistry. Absorbance changes were measured with a Hewlett-Packard 8453 spectrophotometer. Cyclic voltammetry was carried out at room temperature in a standard three-electrode cell using H2O/0.1 M KCl as solvent/electrolyte. The working electrode was a glassy carbon disk (area = 0.07 cm2, Sinsil), and the counter electrode was a Pt wire. The reference electrode was a Ag/AgCl/KCl (3.0 M) electrode (Sinsil) separated by a salt bridge. Before each experiment, the working electrode was carefully polished with alumina powder and rinsed with distilled water. Potentiostatic control was provided by CH-Instrument and formal potentials (E0′) were calculated from cathodic and anodic peak potentials in CVs according to E0′ = (Epc + Epa)/2. The scan rate was 0.05 V/s. 4.4. 1H NMR Titrations. The 1H NMR titration studies were carried out on a Bruker 400 MHz spectrometer in D2O at 298 K. Aliquots of the CB[8] host was added to the guest solution. The concentrations of the guest molecules were Np− Vio−Np = 7.0 mM, Vio−Np−Vio = 5.20 mM, and ETV = 4.30 mM. For measurements in D2O, the corresponding PF6 salts were ion exchanged to Br salts by adding TBA.Br in CH3CN. Diffusion measurements were performed at different host concentrations using a 1H NMR pulsed gradient technique: the simulated spin-echo sequence which leads to the measurement of the diffusion coefficient D, where D is the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00392. Detailed synthetic procedures, cyclic voltammetry, zeta potential, FESEM, NMR, and MS data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Murugavel Kathiresan: 0000-0002-1208-5879 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M. Kanagaraj thanks DST-INSPIRE for his project assistantship; Dr. K.M. thanks DST-INSPIRE for the faculty award and DST-SERB Start-up Research grant for research funding. Director and support staff of CSIR−CECRI are gratefully acknowledged for their constant encouragement and support.

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REFERENCES

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