Structural and optical properties of highly hydroxylated fullerenes: Stability of molecular domains on the $C60$ surface

The excitation spectra and the structural properties of highly hydroxylated $C60(OH)x$ fullerenes (so-called fullerenols) are analyzed by comparing optical absorption experiments on dilute fullerenol-water solutions with semiempirical and density functional theory electronic structure calculations. The optical spectrum of fullerenol molecules with 24–28 OH attached to the carbon surface is characterized by the existence of broad bands with reduced intensities near the ultraviolet region (below $∼500nm$⁠) together with a complete absence of optical transitions in the visible part of the spectra, contrasting with the intense absorption observed in $C60$ solutions. Our theoretical calculations of the absorption spectra, performed within the framework of the semiempirical Zerner intermediate neglect of diatomic differential overlap method [Reviews in Computational Chemistry II, edited by K. B. Lipkowitz and D. B. Boyd (VCH, Weinheim, 1991), Chap. 8, pp. 313–316] for various gas-phase-like $C60(OH)26$ isomers, reveal that the excitation spectra of fullerenol molecules strongly depend on the degree of surface functionalization, the precise distribution of the OH groups on the carbon structure, and the presence of impurities in the samples. Interestingly, we have surprisingly found that low energy atomic configurations are obtained when the OH groups segregate on the $C60$ surface forming molecular domains of different sizes. This patchy behavior for the hydroxyl molecules on the carbon surface leads in general to the formation of fullerene compounds with closed electronic shells, large highest occupied molecular orbital-lowest unoccupied molecular orbital energy gaps, and existence of an excitation spectrum that accounts for the main qualitative features observed in the experimental data.