A study by spectroelectrochemical FTIR and density functional theory calculations of the reversible complexing ability of an electroactive tetrathiafulvalene crown

We report on the study of the electrochemically targeted complexation/expulsion of a metal cation (Ba2+) by a crown ether tetra(thiomethyl)tetrathiafulvalene derivative (crown-TTM-TTF). Real time, in situ FTIR spectroelectrochemistry was used to obtain spectroscopic evidence of this electrochemically triggered phenomenon. Density functional theory calculations allowed the spectral information collected to be assigned. Both experimental and theoretical results clearly show that neutral crown-TTM-TTF complexes well Ba2+. Complexation is evidenced by a significant downshift of the frequency corresponding to the asymmetric stretching of the C-O-C ether groups. Concerning the cation crown-TTM-TTF, the spectroscopic signal of the complex form was difficult to identify, first because of the rather low value of the complexation constant and second because the vibration modes involving the oxygen atoms (which are the most affected by the complexation) were found by calculation to occur in the lower spectral region (<1000 cm(-1)), which is not accessible in our experimental conditions. In the case of the dication crown-TTM-TTF, it is now clear that the complex form does not exist, which means that the electrochemical formation of the dication necessarily involves the expulsion of the barium ion.