Fullerene-based one-dimensional crystalline nanopolymer formed through topochemical transformation of the parent nanowire

Large-scale practical applications of fullerene $\left({\text{C}}_{60}\right)$ in nanodevices could be significantly facilitated if the commercially available micrometer-scale raw ${\text{C}}_{60}$ powder were further processed into a one-dimensional nanowire-related polymer displaying covalent bonding as molecular interlinks and resembling traditional important conjugated polymers. However, there has been little study thus far in this area despite the abundant literature on fullerene. Here we report the preparation and characterization of such a ${\text{C}}_{60}$-based polymer nanowire, ${\left({-\text{C}}_{60}\text{TMB-}\right)}_n$, where $\text{TMB}=1,2,4$-trimethylbenzene, which displays a well-defined crystalline nanostructure, exceptionally large length-to-width ratio and excellent thermal stability. The material is prepared by first growing the corresponding nanowire through a solution phase of ${\text{C}}_{60}$ followed by a topochemical polymerization reaction in the solid state. Gas chromatography, mass spectrometry and ${}^{13}\text{C}$ nuclear magnetic resonance evidence is provided for the nature of the covalent bonding mode adopted by the polymeric chains. Theoretical analysis based on detailed calculations of the reaction energetics and structural analysis provides an in-depth understanding of the polymerization pathway. The nanopolymer promises important applications in biological fields and in the development of optical, electrical, and magnetic nanodevices.

Figure 1

(Color online) GC analysis of the ${\text{C}}_{60}\text{TMB}$ nanowires. (a) GC analysis of the as-prepared sample. (b) GC profile of the aged sample. The GC peaks at 1.34 min in (a) and 1.35 min in (b) are attributable to the remnant of ${\text{CCl}}_4$ solvent, and the peaks at 2.93 or 2.95 min in the spectra are due to an impurity present in the system.

Figure 2

(Color online) MS analysis of ${\text{C}}_{60}\text{TMB}$ nanowires. The profiles (a) and (b) correspond to the GC peaks at 2.80 and 3.10 min (elution times) as shown in Fig. 1a.

Figure 3

(Color online) (a) Direct polarization MAS ${}^{13}\text{C}$ solid state spectrum of pure ${\text{C}}_{60}$. (b) Solution-state (in deuterochloroform) ${}^{13}\text{C}$ NMR spectrum of pure 1,2,4-trimethylbenzene. (c) Solid-state CP-MAS ${}^{13}\text{C}$ NMR spectrum of the as-prepared ${\text{C}}_{60}$-1,2,4-trimethylbenzene adduct. (d) Solid-state CP-MAS ${}^{13}\text{C}$ NMR spectrum of the aged ${\text{C}}_{60}$-1,2,4-trimethylbenzene adduct. (e) The spectra calculated using the ab initio B3LYP/6-31G(d) method. Solid line (blue fill (in the online version)), dashed line (green fill (in the online version)), and dotted line (red fill (in the online version)) show the corresponding spectra for the ${\text{C}}_{60}{\text{-TMB-C}}_{60}$ complex as shown in (f), pure 1,2,4-trimethylbenzene and pristine ${\text{C}}_{60}$, respectively. The numbers labeled in (e) show the chemical shifts of the corresponding carbon atoms marked in (f).

Figure 4

A schematic shows the polymerization reaction pathway. The label X in the dimer indicates a residue derived from the TMB molecule during the reaction. Hydrogen atoms are omitted in this diagram for clarity.

Figure 5

(Color online) Laser micro-Raman spectroscopic analysis shows the profiles of the as-made (1) and the aged (2) ${\text{C}}_{60}\text{TMB}$ nanowire sample.

Figure 6

(a) SEM image of the ${\left({-\text{C}}_{60}\text{TMB-}\right)}_n$ nanopolymer. (b) SAED pattern shows the crystalline nature of the polymer. The inset in (b) is a typical TEM image of a nanopolymer associated with the SAED measurement.

Figure 7

(Color online) Computed structures of the reaction products (a) ${\text{C}}_{60}{\text{-TMB}}^{-2\text{H}}{-\text{C}}_{60}$ and (b) ${\text{HC}}_{60}{\text{-TMB}}^{-2\text{H}}{-\text{C}}_{60}\text{H}$, corresponding to reactions (7, 8) in the text. The distances between the fullerenes are indicated and they are the values calculated using the B3LYP/6-21G method. The inset shows the $s{p}^3$ hybridization of the fullerene carbon atoms resulting from the formation of the covalent bonds with ${\text{TMB}}^{-2\text{H}}$.

Figure 8

(Color online) Enthalpy of reaction (8) calculated using the semiempirical AM1 method (Refs. 55, 56) plotted as a function of the reaction coordinate, which characterizes the separation of the 1,2,4-TMB molecule from the fullerenes in the ${\text{HC}}_{60}{\text{-TMB}}^{-2\text{H}}{-\text{C}}_{60}\text{H}$ complex (see text). The indices in the plot mark the different stages of the reaction that are illustrated in the lower part of the figure.

Figure 9

(Color online) Calculated structures of two low-energy isomers of the $3{\text{C}}_{60}\text{TMB}$ complex as the product of reaction (9) in the text. The distances between fullerene centers correspond to the geometries optimized at the B3LYP/STO-3G level of calculation.

Figure 10

(Color online) Possible polymerization scenarios of the ${\text{C}}_{60}\text{TMB}$ 1D nanocrystals. In (a) we show the geometry of the unit cell (the TMB molecules embedded in the unit cell are not shown). The distances between the centers of the fullerenes are indicated. The geometry of a ${\text{C}}_{60}{\text{-TMB-C}}_{60}$ molecule is shown in (b). The distance between the two fullerenes in this case is $14.4\text{Å}$, very close to the dimensions of the unit cell. Plots (c) and (d) demonstrate the cross section of a nanowire, viewed along the $c$ axis, and illustrate two possible linking schemes between the fullerenes. The bars between the fullerenes indicate the TMB molecules. The polymerization can occur along either the $a$ (plot c) or the $b$ axis (plot d) of a nanowire (Refs. 37, 38).

Figure 11

(Color online) Distance between the centers of two fullerenes in a ${\text{C}}_{60}{\text{-TMB-C}}_{60}$ complex as a function of angle $\phi$, which is defined as the angle between the planes formed by the atoms (1)-(2)-(3) and (2)-(3)-(4), as shown in the inset.

Figure 12

(Color online) Possible fitting sites of (a) the $2{\text{C}}_{60}\text{TMB}$ complex and (b) the $3{\text{C}}_{60}\text{TMB}$ complex, in the unit cell of the parent nanowire. Distances are given in Ångstroms and correspond to experimental data (Ref. 37).

Figure 13

(Color online) ${}^{13}\text{C}$-NMR spectra calculated for three isomeric states of the TMB-TMB dimer using the ab initio B3LYP/6-31G(d) method. The geometries of the molecules used in the calculation are shown in the inset. Labels indicate the chemical shifts of different types of carbon atoms in the system. Note that the chemical shifts in (c) are much more degenerate than (a) and (b) due to the enhanced symmetry of the studied isomer.