Facile separation, spectroscopic identification, and electrochemical properties of higher trifluoromethylated derivatives of the [70]fullerene.

We survey the structure and electronic properties of the family of higher trifluoromethylated C-70(CF3)(n) molecules with n=14, 16, 18, and 20. Twenty-two available compounds, of which thirteen are newly obtained and characterized, demonstrate the broad diversity of p-system topologies, which enabled us to study the interplay between the CF3 addition pattern and the electronic properties. UV/Vis spectroscopic and cyclic voltammetric studies demonstrate the importance of the exact addition pattern rather than the plain number of addends. Of particular interest is the skew pentagonal pyramid (SPP) addition pattern, which enables formation of closed-shell cyclopentadienyl anions C-70(CF3)(n-1)(-) through CF3 detachment upon electron transfer. A detailed study of the process is presented for a SPP-C-70(CF3)(16) where potentiostatic electrolysis at the second reduction potential gives C-70(CF3)(15)(-) oxidizable to a persistent C-70(CF3)(15)(center dot) radical. Together with the literature data for the lower C-70(CF3)(n) compounds with n=2-12, the present results show good correlation between the experimental boundary level positions and the DFT predictions. The compounds turn out to be electron acceptor molecular semiconductors with experimental LUMO energies and HOMO-LUMO gaps within the ranges of -4.3 to -3.7 eV and 1.6 to 3.3 eV, respectively, depending on the shape of the conjugated fragments. The HOMO levels fall within the range of -5.6 to -6.9 eV and show linear correlation with the number of addends.