Donor strength of pi-extended tetrathiafulvalenes: ionisation energies vs. oxidation potentials. A joint theoretical and experimental study

The pi-donor strength of extended tetrathiafulvalenes (TTFs) has been investigated as a function of the electronic character and length of the spacer between the two dithiole rings. First and second ionisation energies were calculated with B3LYP/6-311++G(2d, p)//B3LYP/6-31+G(d) and B3LYP/6-311++G(2d, p)//PM3 computational methods and correlated with the number of carbon atoms in the spacer. Theoretical IEs are compared to electrochemical oxidation potentials E(ox)s to elucidate the importance of solvation. There are remarkable differences between spacers comprised of sp(2) and sp carbon atoms. Thus, E-ox increases with IE for alkene spacers whereas there is no clear correlation for alkyne spacers, which is ascribed to a solvent effect. Calculations using a continuum solvent model support this interpretation. Moreover, we report the synthesis of a new alkyne-extended TTF that contains six triple bonds in the spacer. Its redox properties were investigated and discussed in context of the calculations. Finally, the donor strength of an alkyne-extended TTF, bis(1,3-dithiol-2-ylidene) hexa-2,4-diyne, was evaluated by its ability to form an inclusion complex with the cyclic acceptor cyclobis(paraquat-p-phenylene).