DFT study of the interaction free energy of pi-pi complexes of fullerenes with buckybowls and viologen dimers

We present a theoretical investigation, by means of DFT protocols, of the complexation thermodynamics of (i) complexes of C-70 and C-60 fullerenes with bowl-shaped hexabenzocoronene derivatives and (ii) complexes of C-60 with viologen dimers. The recent functionals of the M06 family, accounting for pi-pi interactions to a good level of approximation, have been used to calculate the interaction free energies. For the former complexes, the good agreement between the calculated results and the experimental data confirms the reliability of the protocol used. On these grounds, we then checked the stability of a series of complexes of C-60 with some viologen dimers, 1BPnBP1 (n = 6-9), where two N-methylated bipyridinium units are linked by an alkyl chain of variable length, acting as molecular tweezers. Both forms of the viologen cores, that is the cation (each core doubly charged) and the reduced neutral form, were considered. For a suitable chain length a free energy gain upon complexation is predicted for the neutral form while the complexation of C-60 by the cationic form is disfavoured mainly for entropic reasons.