Magnetic Fullerenes inside Single-Wall Carbon Nanotubes

${\mathrm{C}}_{59}\mathrm{N}$ magnetic fullerenes were formed inside single-wall carbon nanotubes by vacuum annealing functionalized ${\mathrm{C}}_{59}\mathrm{N}$ molecules encapsulated inside the tubes. A hindered, anisotropic rotation of ${\mathrm{C}}_{59}\mathrm{N}$ was deduced from the temperature dependence of the electron spin resonance spectra near room temperature. Shortening of the spin-lattice relaxation time $T_1$ of ${\mathrm{C}}_{59}\mathrm{N}$ indicates a reversible charge transfer toward the host nanotubes above $\sim 350\mathrm{K}$. Bound ${\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{−}}{\mathrm{C}}_{60}$ heterodimers are formed at lower temperatures when ${\mathrm{C}}_{60}$ is coencapsulated with the functionalized ${\mathrm{C}}_{59}\mathrm{N}$. In the 10–300 K range, $T_1$ of the heterodimer shows a relaxation dominated by the conduction electrons on the nanotubes.

Figure 1

ESR spectra of (a) ${\mathrm{C}}_{59}\mathrm{N}@\mathrm{SWCNT}$, (b) ${\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{:}}{\mathrm{C}}_{60}@\mathrm{SWCNT}$, and (c) crystalline ${\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{:}}{\mathrm{C}}_{60}$ at 300 K. 1, 2, and 3 denote the ${}^{14}\mathrm{N}$ hyperfine triplet lines with a nuclear state of $I=1$, 0, and $-1$, respectively. The asterisk in (a) shows a weak impurity signal. The arrows in (b) and (c) indicate the ${\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{−}}{\mathrm{C}}_{60}$ heterodimer. The fit in (b) shows the deconvolution of the ESR signal into the ${}^{14}\mathrm{N}$ triplet of monomer ${\mathrm{C}}_{59}\mathrm{N}$ and the ${\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{−}}{\mathrm{C}}_{60}$ heterodimer components.

Figure 2

Linewidths of the ${}^{14}\mathrm{N}$ triplet components in crystalline ${\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{:}}{\mathrm{C}}_{60}$ (open symbols) and $({\mathrm{C}}_{59}\mathrm{N},{\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{:}}{\mathrm{C}}_{60})@\mathrm{SWCNT}$ (solid symbols) samples (a) below 350 K and (b) above 290 K. The stars show the data for the temperature range where the linewidths of the three components are equal. The solid lines are guides to the eye.

Figure 3

Concentration of ${\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{−}}{\mathrm{C}}_{60}$ bound heterodimers in ${\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{:}}{\mathrm{C}}_{60}@\mathrm{SWCNT}$. The solid curve is a fit with parameters explained in the text. The dashed curve shows the same quantity for crystalline ${\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{:}}{\mathrm{C}}_{60}$ above the 261 K phase transition. Note the much higher heterodimer concentration for the peapod material. The inset shows the temperature evolution of the spectra.

Figure 4

(a) Temperature dependence of the ESR linewidth for the heterodimer signal. (b) The homogeneous contribution to the linewidth with a linear fit (solid line). The corresponding $T_1$ values are shown on the right axis.