Relation between charge transfer and solvent polarity in fullerene derivatives: NMR studiesElectronic supplementary information (ESI) available: Table S1: chemical shifts (ppm) in C6D6 (ETN = 0.111), CDCl3 (ETN = 0.259), and CD2Cl2 (ETN = 0.309). See http://www.rsc.org/suppdata/jm/b2/b203112b/