Electrochemical, spectroelectrochemical and EPR properties of poly(pyrrole-viologens)

Spectroelectrochemical and EPR studies of three viologens, N-methyl-N′-(2-pyrrol-1-yl-ethyl)-4,4′-bipyridinium ditetrafluoroborate (MPEB²⁺), N-methyl-N′-(2-pyrrol-1-yl-propyl)-4,4′-bipyridinium ditetrafluoroborate (MPPB²⁺) and N,N′-bis-(2-pyrrol-1-yl-propyl)-4,4′-bipyridinium ditetrafluoroborate (BPPB²⁺), are presented. All three viologens undergo a two-step reduction leading to significant changes in their electronic spectra and EPR responses. Oxidative polymerization of the above compounds results in the formation of good quality polymer films. Modified electrodes obtained by the polymerization of viologen-substituted pyrroles have been studied by electrochemical, spectroelectrochemical and EPR methods. It has been shown that N-substituted pyrroles can be electrochemically polymerized to give films in which grafted viologen units retain their spectroelectrochemical and magnetic properties. Like viologens not bound to the polymer matrix, they undergo a two-step reversible reduction resulting in clearly different electronic spectra for each reduction state and an EPR response showing the formation and anihilation of radical cations. Analysis of the electronic spectra obtained for the first reduction product strongly indicates that the radical cations formed undergo dimerization to a degree which is dependent on the mobility of the viologen molecule in the polymer matrix. The following sequence of this association ability has been established: (MPPB²⁺)₂ > (MPEB²⁺)₂ > (BPPB²⁺)₂. EPR spectra originating from the reduction of viologens covalently entrapped in the polymer matrix are broad and do not reveal any hyperfine structure. This observation strongly indicates that spin sites are immobilized and that the rate of electron transfer is high. Poly(pyrrole-viologen)-modified electrodes are stable in non-aqueous solutions and can be reversibly switched between the two reduction states. In aqueous solutions only the first viologen redox system is reversible. However, the system is unstable with respect to the second redox couple.