Polymerization of the rotor-stator compound ${\mathrm{C}}_{60}$-cubane under pressure

Cubane, ${\mathrm{C}}_8{\mathrm{H}}_8$, can be inserted into the octahedral voids of fullerene lattices to create a family of rotor-stator compounds. We have investigated the structural phase behavior of ${\mathrm{C}}_{60}\bullet {\mathrm{C}}_8{\mathrm{H}}_8$ by annealing a number of samples for up to $3\mathrm{h}$ at selected temperatures in the range $380–870\mathrm{K}$ under pressures up to $2\mathrm{GPa}$. The high-pressure treated materials were then investigated under ambient conditions using Raman spectroscopy and x-ray diffraction. ${\mathrm{C}}_{60}\bullet {\mathrm{C}}_8{\mathrm{H}}_8$ is found to have at least five different structural phases depending on treatment conditions. In addition to the known cubic and orthorhombic structures observed at atmospheric pressure, we find two polymeric states with pseudocubic and pseudoorthorhombic structures, respectively, based on the two original lattices and created by heating in different pressure ranges. These materials are believed to be copolymers of ${\mathrm{C}}_{60}$ and decomposition products of cubane. In contrast to the polymeric states of ${\mathrm{C}}_{60}$ the present polymer structures are determined by the topology of the original lattices rather than by the molecular structure. Above $700\mathrm{K}$ we find a carbon-rich amorphous state created when the cubane finally decomposes, releasing its hydrogen content in the form of hydrocarbons.

Figure 1

(Color online) Raman spectra of ${\mathrm{C}}_{60}\bullet {\mathrm{C}}_8{\mathrm{H}}_8$ obtained with (a) ${\mathrm{Ar}}^{+}$ ion laser, $514.5\mathrm{nm}$; (b) diode laser, $780\mathrm{nm}$; (c) He-Ne laser, $632.5\mathrm{nm}$. Inset in (a) shows high-energy range. The full vertical (black) lines mark the positions of the $H_g$ modes in ${\mathrm{C}}_{60}$, numbered as in the figure. The dotted vertical (blue) lines give the corresponding positions of the $A_g$ modes.

Figure 2

(Color online) X-ray diffraction results for the samples treated at high temperatures: ($a$, black) pristine ${\mathrm{C}}_{60}\bullet {\mathrm{C}}_8{\mathrm{H}}_8$ for reference; ($b$, red) $500\mathrm{K}$; ($c$, blue) $620\mathrm{K}$; ($d$, green) $870\mathrm{K}$. Diffraction indices are shown for reference.

Figure 3

(Color online) Raman spectra of differently treated samples obtained with an ${\mathrm{Ar}}^{+}$ ion laser, $514.5\mathrm{nm}$. ($a$, black) pristine ${\mathrm{C}}_{60}\bullet {\mathrm{C}}_8{\mathrm{H}}_8$ at ambient conditions; ($b$, red) treated at $0.5\mathrm{GPa}$ and $383\mathrm{K}$, unmodified structure; ($c$, green) treated at $620\mathrm{K}$ in vacuum, pseudocubic polymer; ($d$, blue) treated at $1\mathrm{GPa}$ and $524\mathrm{K}$, pseudocubic polymer; ($e$, orange) treated at $2\mathrm{GPa}$ and $499\mathrm{K}$, pseudo-orthorhombic polymer; ($f$, purple) treated at $870\mathrm{K}$ in vacuum, amorphous carbon.

Figure 4

(Color online) X-ray-diffraction data for pristine ${\mathrm{C}}_{60}\bullet {\mathrm{C}}_8{\mathrm{H}}_8$ ($a$, black) compared to data for samples treated at $0.5\mathrm{GPa}$ and $383\mathrm{K}$ ($b$, blue) and $460\mathrm{K}$ ($c$, green). Asterisk indicates a peak from talc.

Figure 5

(Color online) Raman spectra obtained with an ${\mathrm{Ar}}^{+}$ ion laser, $514.5\mathrm{nm}$, near the $A_g\left(2\right)$ mode of ${\mathrm{C}}_{60}$. ($a$, black) pristine ${\mathrm{C}}_{60}{\mathrm{C}}_8{\mathrm{H}}_8$; ($b$, red) treated at $0.5\mathrm{GPa}$ and $383\mathrm{K}$, unmodified structure; ($c$, blue) treated at $0.5\mathrm{GPa}$ and $460\mathrm{K}$, pseudocubic polymer; ($d$, green) treated at $1\mathrm{GPa}$ and $466\mathrm{K}$, pseudocubic polymer; ($e$, orange) treated at $2\mathrm{GPa}$ and $601\mathrm{K}$, pseudo-orthorhombic polymer.

Figure 6

(Color online) Raman spectra obtained with a diode laser, $783\mathrm{nm}$. ($a$, black) pristine ${\mathrm{C}}_{60}\bullet {\mathrm{C}}_8{\mathrm{H}}_8$; treated at $0.5\mathrm{GPa}$ and $383\mathrm{K}$ ($b$, blue); at $0.5\mathrm{GPa}$ and $410\mathrm{K}$ ($c$, green); and at $0.5\mathrm{GPa}$ and $460\mathrm{K}$ ($d$, red). The positions of ${\mathrm{C}}_8{\mathrm{H}}_8$ peaks are marked with vertical lines.

Figure 7

(Color online) (a) Raman spectra obtained with an ${\mathrm{Ar}}^{+}$ ion laser, $514.5\mathrm{nm}$; (b) x-ray-diffraction data for pristine ${\mathrm{C}}_{60}$ (bottom curves, black) and ${\mathrm{C}}_{60}$ dimers (top curves, red).

Figure 8

(Color online) X-ray-diffraction data for pristine ${\mathrm{C}}_{60}\bullet {\mathrm{C}}_8{\mathrm{H}}_8$ ($a$, black) as compared to the pseudo-orthorhombic polymer obtained at $2\mathrm{GPa}$ and $499\mathrm{K}$ ($b$, red), and the amorphous carbon phase obtained at $1\mathrm{GPa}$ and $700\mathrm{K}$ ($c$, purple). Vertical lines indicate the theoretical positions of peaks in the orthorhombic structure (see text).

Figure 9

(Color online) Raman spectra obtained with a He-Ne laser, $632.5\mathrm{nm}$, for the samples treated at $2\mathrm{GPa}$ and ($a$, blue) $426\mathrm{K}$, ($b$, orange) $601\mathrm{K}$. The asterisk indicates a peak from talc.

Figure 10

(Color online) High-pressure “reaction map” of ${\mathrm{C}}_{60}\bullet {\mathrm{C}}_8{\mathrm{H}}_8$ as deduced in this work. Different symbols denote different structures; full, almost horizontal lines denote the onset of irreversible polymerization reaction on heating; a dashed line indicates the approximate location of the cubic-orthorhombic phase boundary. A vertical line separates pseudo-cubic and pseudo-orthorhombic polymer states.