Encyclopedia
Please note this is a comparison between Version 1 by Huacheng Zhang and Version 3 by Jason Zhu.
The molecular structure of pillar[n]arene could serve different roles in the fabrication of attractive carbon nanotube-based materials. Firstly, pillar[n]arene has the ability to provide the structural basis for enlarging the cylindrical pillar-like architecture by forming one-dimensional, rigid, tubular, oligomeric/polymeric structures with aromatic moieties as the linker, or forming spatially “closed”, channel-like, flexible structures by perfunctionalizing with peptides and with intramolecular hydrogen bonding. Interestingly, such pillar[n]arene-based carbon nanotube-resembling structures were used as porous materials for the adsorption and separation of gas and toxic pollutants, as well as for artificial water channels and membranes. In addition to the art of organic synthesis, self-assembly based on pillar[n]arene, such as self-assembled amphiphilic molecules, is also used to promote and control the dispersion behavior of carbon nanotubes in solution. Furthermore, functionalized pillar[n]arene derivatives integrated carbon nanotubes to prepare advanced hybrid materials through supramolecular interactions, which could also incorporate various compositions such as Ag and Au nanoparticles for catalysis and sensing.
- pillar[n]arene
- carbon nanotube
- synthesis
- hybrid materials
- composites
1. Mimicking the Structure and Characteristics of Carbon Nanotubes by Functionalized Pillar[n]arene
1.1. Preparing Linear Pillar[n]arene-Based Oligomer/Polymer via Rigid Aromatic Bridges
Due to the possession of rigid pillar-like molecular structures and an electron-rich cavity [1][48], the skeleton of functionalized pillar[n]arene was employed as previous pieces for the construction of linear oligomeric and polymeric architectures, such as P1–P7 (Scheme 1), by introducing rigid aromatic bridging subunits to mimic carbon nanotubes [2][3][4][5][6][42,49,50,51,52]. Several classic organic syntheses such as heterocyclization and Pd-catalyzed coupling reactions [7][8][9][10][53,54,55,56] have been thoroughly employed. Particularly, those carbon nanotube-resembling linear pillar[n]arene-based porous materials have been used in diverse applications such as gas absorption [11][12][57,58], as well as the absorption and separation of toxic pollutants in water [13][59]. For example, P2 exhibits a recognition towards solvent molecules such as dichloromethane [4][50], whereas P3 has the ability to capture toxic pollutants such as adiponitrile [5][51]. In addition, P5 can selectively absorb CO2 rather than N2 or methane [6][52], whereas P6 and P7 were used for separating propane gas from the simulated gas mixture of methane and propane [2][42].
Scheme 1. Chemical structures of pillar[5]arene-based polymers P1–P7.
1.2. Preparing Peptide-Appended Pillar[n]arene Processing Intramolecular Hydrogen Bonds
Except for introducing rigid aromatic moieties as the bridging linker for the construction of pillar[n]arene-based polymeric architectures, diverse designs and synthetic strategies were carried out for mimicking carbon nanotubes; for example [14][60], the peptide-appended pillar[n]arene P8 (Scheme 2) was produced to introduce intramolecular interactions such as hydrogen bonding [15][61] to form “closed”, tubular, molecular architectures [16][62], as well as to resemble the performance of carbon nanotubes as artificial water channels [17][18][63,64] and permeable membranes [19][20][21][65,66,67]. It was revealed that the average single-channel osmotic water permeability [19][65] and the ion rejection of P8 were greatly analogous to those of carbon nanotubes [21][67]. Furthermore, the pore density [22][68] of P8-based channel arrays was much higher than that of carbon nanotube-based ones [19][65]. It was also found that the flexible conformation of the peptide-appended pillar[n]arene was available for water permeability [20][23][24][66,69,70].
2. Dispersion of Carbon Nanotube by Using Functionalized Pillar[n]arene
Water-soluble pillar[n]arene could include hydrophobic guest molecules [25][26][71,72], producing pillar[n]arene-based self-assembled amphiphiles (PSAs) [27][28][73,74] and resembling the performance of general surfactant-dispersing carbon nanotubes [29][75]. For example [30][76], water-soluble carboxylate-perfunctionalized [31][77] pillar[6]arene P9 (Scheme 3) has the ability to recognize the hydrophobic pyrene [32][78] derivative G1 (Scheme 3) in the stoichiometry of 1/1, and aggregate into vesicular architectures [33][79] in an aqueous solution as confirmed by transmission electron microscopy (TEM) [34][80]. Since the water solubility of P9 changes with the change of pH value, the morphology of P9 ⸧ G1-based self-assemblies could be also controlled. In addition [30][76], P9 could include another pyrene derivative G2 (Scheme 3) with the association constant (Ka) of (8.04 ± 0.68) 104 M−1. Additionally, the obtained amphiphilic inclusion could further disperse multi-walled carbon nanotubes well by sonication in aqueous solutions as confirmed by TEM and scanning electron microscopy (SEM). The π–π stacking interactions [35][81] between pyrene subunits and carbon nanotubes played a significant role in this process as confirmed by fluorescence spectroscopy [36][82].
Similarly [37][83], another pyrene derivative G3 (Scheme 3 and Table 1) which is responsive to UV irradiation and degrades into F1 and F2 (Scheme 3) was further employed as the hydrophobic guest to be included by P9. Thus, the amphiphilic inclusion P9 ⸧ G3 was obtained and exhibited the critical aggregation concentration (CAC) [38][84] of 1.0 × 106 mol L−1, which has the capacity of dispersing multi-walled carbon nanotubes in aqueous solutions as confirmed by TEM. Interestingly, the dispersion of carbon nanotubes could be controlled upon UV irradiation according to the formation/deformation of G3.
3. Decoration of Carbon Nanotube by Using Functional Pillar[n]arene
The mono-hydroxyl functionalized pillar[5]arene P10 (Scheme 4) coupled with the alkyl bromide-modified polyfluorene F3 (Scheme 4) via a classic nucleophilic substitution reaction led to the formation of pillar[5]arene-functionalized polymer P11 (Scheme 4 and Table 1) [39][85]. Due to the possession of the pillar[5]arene cavity on the structural skeleton, P11 could form a complex with a neutral poly(ethylene glycol) (PEG) derivative with two ending subunits—alkylnitrile G4 (Scheme 4) via donor–acceptor interactions. Particularly, this obtained host–guest inclusion P11 ⸧ G4 not only has the capacity for properly exfoliating and dispersing single-walled carbon nanotubes in organic solvents (600 μg mL−1)as confirmed by 1H NMR, Raman, UV–Vis–near-infrared (NIR) spectra, and thermogravimetric analysis (TGA), but also affords the preparation of pillar[5]arene-based polymer-carbon nanotube composite organogels [40][41][42][88,89,90] in 1,2-dichlorobenzene (40 wt%) via non-covalent interactions [39][85].
Besides the organic and polymeric composition, diverse inorganic materials were also employed for fabricating different functional, carbon nanotube-based, hybrid materials. For example [43][86], the water-soluble phosphate-perfunctionalized pillar[6]arene P12 (Scheme 5 and Table 1) could decorate the surface of a single-walled carbon nanotube at room temperature via π–π stacking interactions between the carbon nanotube and benzene moieties of P12 by sonicating in aqueous solutions, as confirmed by zeta potentials and Fourier transform infrared (FTIR) spectroscopy. Particularly, Ag nanoparticles could be further well dispersed on the surface of the carbon nanotube due to the coordination environment provided by the cavity of P12. Thus, the obtained hybrid materials containing Ag nanoparticles, carbon nanotubes and P12 exhibited strong catalytic activity towards a series of guest molecules such as 4-nitrophenol (G5, Scheme 5), methylene (G6, Scheme 5) and paraquat (G7, Scheme 5), paving the way for efficient electrochemical sensing of highly toxic herbicides.
Except for loading Ag nanoparticles, Au nanoparticles [45][46][47][91,92,93] could also be introduced into carbon nanotube-based hybrid materials via the assistance of pillar[n]arene. For example, water-soluble hydroxyl pillar[5]arene P13 (Scheme 5) was also used for dispersing single-walled carbon nanotubes in aqueous solutions via non-covalent interactions, and further assisted in promoting the formation of Au nanoparticles on the surface of carbon nanotubes, leading to the formation of hybrid materials Au@(P13 interacting with carbon nanotubes) [44][87]. It has been revealed that such hybrid materials had reasonable performances in catalyzing an ethanol oxidation reaction (EOR), as well as sensing p-dinitrobenzene (G8, Scheme 5) because of pillar[5]arene cavities.
