Electron Delocalization and Dimerization in Solid ${\mathrm{C}}_{59}\mathrm{N}$ Doped ${\mathrm{C}}_{60}$ Fullerene

Electron spin resonance and ab initio electronic structure calculations show an intricate relation between molecular rotation and chemical bonding in the dilute solid solution. The unpaired electron of ${\mathrm{C}}_{59}\mathrm{N}$ is delocalized over several ${\mathrm{C}}_{60}$ molecules above 700 K, while at lower temperatures it remains localized within short range. The data suggest that below 350 K rigid ${\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{−}}{\mathrm{C}}_{60}$ heterodimers are formed in thermodynamic equilibrium with dissociated rotating molecules. The structural fluctuations between heterodimers and dissociated molecules are accompanied by simultaneous electron spin transfer between ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{59}\mathrm{N}$ molecules. The calculation confirms that in the ${\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{−}}{\mathrm{C}}_{60}$ heterodimer the spin density resides mostly on the ${\mathrm{C}}_{60}$ moiety, while it is almost entirely on ${\mathrm{C}}_{59}\mathrm{N}$ in the dissociated case.

Figure 1

ESR spectra in dilute ${\mathrm{C}}_{59}\mathrm{N}:{\mathrm{C}}_{60}$ solid solution. (a) Spectra at 150 GHz near the $T_c=261\mathrm{K}$ phase transition of solid ${\mathrm{C}}_{60}$. The two ESR lines are assigned to nonrotating ${\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{−}}{\mathrm{C}}_{60}$ heterodimers and neutral, rotating ${\mathrm{C}}_{59}\mathrm{N}$. We propose that structural fluctuations lead to a simultaneous observation of the two species. The ${\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{−}}{\mathrm{C}}_{60}$ signals are also shown magnified. (b) 9.4 GHz ESR spectra at high temperatures. Above 600 K the ${\mathrm{C}}_{59}\mathrm{N}$ ESR is gradually replaced by the ESR of $\mathrm{C}_{60}{}^{-}$ anions. This is attributed to delocalization of electrons over many ${\mathrm{C}}_{60}$ sites.

Figure 2

Temperature dependence of the ESR intensity ratio, $R(T)$, of the species attributed to ${\mathrm{C}}_{59}\mathrm{N}\mathrm{\text{−}}{\mathrm{C}}_{60}$ heterodimers and ${\mathrm{C}}_{59}\mathrm{N}$ at 9 (squares), 75 (dots), and 225 (triangles) GHz. $R(T)$ is approximately proportional to the heterodimer concentration. Solid lines: calculated curves; see text.

Figure 3

Proposed energy level diagram of ${\mathrm{C}}_{59}\mathrm{N}:{\mathrm{C}}_{60}$. $E_{0>}(E_{0<})$: heterodimer $T>T_c$, ($T<T_c$); $E_{>}(E_{<})$: dissociated neutral ${\mathrm{C}}_{59}\mathrm{N}$ free radical $T>T_c$, ($T<T_c$); $E_i$: pair of ${\mathrm{C}}_{59}{\mathrm{N}}_{+}$ and first neighbor $\mathrm{C}_{60}{}^{-}$ ions (electron delocalized to a cluster of ${\mathrm{C}}_{59}\mathrm{N}$ and 12 ${\mathrm{C}}_{60}$ molecules); $E_{\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{e}}$: electron delocalized over large distances.

Figure 4

Ab initio calculation of molecular structures and the spin densities, $\rho (z)$, along the molecular axis $z$ integrated in the plane perpendicular to $z$. (a) The ground state heterodimer and (b) the dissociated pair with a fixed intermolecular C-C separation $d=0.30\mathrm{n}\mathrm{m}$. Note the almost complete transfer of an electron spin from ${\mathrm{C}}_{60}$ to ${\mathrm{C}}_{59}\mathrm{N}$ with the dissociation of the heterodimer.