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

Abstract

The π-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_oxs to elucidate the importance of solvation. There are remarkable differences between spacers comprised of sp² 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).