Thermochemical stabilities, electronic structures, and optical properties of C56X10 (X = H, F, and Cl) fullerene compounds

Stimulated by the recent isolation and characterization of C₅₆Cl₁₀ chlorofullerene (Tan et al., J Am Chem Soc 2008, 130, 15240), we perform a systematic study on the geometrical structures, thermochemistry, and electronic and optical properties of C₅₆X₁₀ (X = H, F, and Cl) on the basis of density functional theory (DFT). Compared with pristine C₅₆, the equatorial carbon atoms in C₅₆X₁₀ are saturated by X atoms and change to sp³ hybridization to release the large local strains. The addition reactions C₅₆ + 5X₂ --> C₅₆X₁₀ are highly exothermic, and the optimal temperature for synthesizing C₅₆X₁₀ should be ranged between 500 and 1000 K. By combining 10 X atoms at the abutting pentagon vertexes and active sites, C₅₆Cl₁₀ molecules exhibit large energy gaps between the highest occupied and lowest unoccupied molecular orbitals (from 2.84 to 3.00 eV), showing high chemical stabilities. The C₅₆F₁₀ and C₅₆Cl₁₀ could be excellent electron acceptors for potential photonic/photovoltaic applications in consequence of their large vertical electron affinities. The density of states is also calculated, which suggest that the frontier molecular orbitals of C₅₆X₁₀ are mainly from the carbon orbitals of two separate annulene subunits, and the contributions derived from X atoms are secondary. In addition, the ultraviolet-visible spectra and second-order hyperpolarizabilities of C₅₆X₁₀ are calculated by means of time-dependent DFT and finite field approach, respectively. Both the average static linear polarizability <α> and second-order hyperpolarizability <γ> of these compounds are larger than those of C₆₀ due to lower symmetric structures and high delocalization of π electron density on the two separate annulene subunits.