Fast dimerisation of the triparaquat radical dication

The one-electron reduction of 2,4,6-tris(N-methylpyridinium-4-yl)pyridine t rication, triparaquat, TPQ(3+), synthesized in the form of the perchlorate salt, was studied by cyclic voltammetry on a platinum disc electrode. It wa s found that the reduction peak is centered at -880 mV (vs. SCE) at 500 mV s(-1) scanning rate. The subsequent oxidation takes place at +75 mV, indica ting an irreversible process, but no changes in the solution content were f ound after the oxidation. The blue colored reduction product reacts anomali stically slowly with oxygen with a second-order rate constant of (0.65 +/- 0.01) mol(-1) dm(3) s(-1). EPR experiments showed that the reduction produc t in ethanol solution does not possess an unpaired electron. A low temperat ure gamma radiolysis experiment revealed that the spectral peak maximum of the reduction product of TPQ(3+) solubilized in PVA film is shifted similar to 200 nm with respect to that obtained in aqueous solution at room temper ature. Pulse radiolysis experiments proved that this seemingly inconsistent set of data was a consequence of the very fast dimerisation process 2TPQ(2 +).reversible arrow (TPQ)(2)(4+) (k(dim) = 1.4 x 10(9) mol(-1) dm(3) s(-1)) . Cyclic voltammetry experiments on a mercury electrode revealed that the s econd reduction step corresponds to the dimer reduction. The third and four th reduction steps were irreversible and initiated further chemical reactio ns of triparaquat, which was consistent with the data obtained by the prolo nged steady state gamma radiolysis.