pi-dimer from bithiophene radical cations. Investigation of equilibrium constants as a function of substituent size and supporting electrolyte using fast conversion electrochemical cells

The procedure for the determination of second order equilibrium constants K , using spectroelectrochemistry has been re-examined. Incomplete conversion of the starting material led to non-homogeneous concentration profiles and to considerable errors in the determinations of K. Values for three dialky lthiobithiophene radical cations have been measured in which the size of th e thioalkyl group and of the counter anion of the supporting electrolyte ar e different. Large decreases observed in both cases may suggest that the as sociation takes the form of a sandwich of two radical cations with the coun ter-anion in the middle.