Synthesis of alkyl sulfonated fullerenes without catalyst: improved water solubility by the sulfonate groups

Two new alkyl sulfonated fullerene (C60) derivatives C60(O)14(OH)4(OCH2CH2CH2SO3Na)5 (2), C60(O)15(OSO3Na)5(OCH2CH2CH2SO3Na)6 (4) with high water-solubility were synthesized by different routes. One derivative (4) was facilely synthesized from a sulfonated C60 derivative C60(O)11(OH)4(OSO3Na)8 (3) in aqueous phase without the addition of any catalyst and has the highest water solubility (118 mg/mL) and smallest aggregation size (49.2 nm) in neutral aqueous solution (0.1 wt%). Derivative 2 was synthesized from fullerenol C60(O)8(OH)13 (1) directly in DMF affording enhanced water solubility compared to 1. The estimated average molecular formulae of these new alkyl sulfonated C60 derivatives were determined by XPS spectroscopy and confirmed by FTIR and 1H NMR spectroscopies. The measured water solubility of the sulfonated C60 derivatives follows the order of 3 < 2 < 4, suggesting that the larger number of functional groups is directly correlated to the higher water solubility, and the sulfonate groups can improve the water solubility greatly. The aggregation size of 2-4 measured by dynamic light scattering (DLS) is all smaller than that of fullerenol 1, indicating that more functional groups result in the improvement of the dispersion of fullerenol in water. ESR study indicates that 2-4 have the ability to produce stable radicals, and the intensity of the ESR signal decreases evidently with the increase of the number of functional groups from 3 to 2 to 4.