LONG-RANGE ELECTRON TRANSFERS BETWEEN VERY SLOWLY DIFFUSING - TETRACYANOQUINODIMETHANE AND ITS RADICAL-ANION IN POLY(ETHER) SOLUTIONS

The homogeneous electron self-exchange rate between 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its radical anion (TCNQ-), dissolved in the poly(ethylene oxide) (PEO) polymer electrolyte solvents network PEO and MW 600 000 linear PEO, is measured by detecting the enhancement of the apparent diffusion rate of TCNQ- by the electron exchange reaction. We report results for the physical diffusion constant D(phys) (D(app,1/2)) of TCNQ- and the product k(delta)delta-2 of the TCNQ-/0 electron self-exchange rate constant k(delta) and the square of the center-to-center electron-transfer reaction distance-delta. D(phys) is made to vary from 3 X 10(-7) to 5 X 10(-12) cm2/s by choice of temperature and electrolyte concentration. Remarkably, k(delta)delta-2 changes in a nearly proportional manner over this > 10(5)-fold range of values. The explanation is not the trivial one of diffusion control of the rate of contact-collisions leading to reaction. The D(phys) vs k(delta)delta-2 correlation is interpreted in terms of a (D(phys)) diffusion rate-dependent reaction distance; electron transfers between very slowly diffusing donor and acceptor occur at distances (delta) larger than collision-contact distances (DELTA). Estimates of edge-to-edge electron-transfer distances yield values from contact up to ca. 16 angstrom and of self-exchange rate constants k(delta) that range from 4 x 10(9) to 1 x 10(3) M-1 s-1 for the maximum and minimum values observed for D(phys), respectively. The interpretation is supported by estimates of time constants-tau(et,delta) for the long range TCNQ-/0 electron transfer that give values within a factor of 2 of the sum of the time constants-tau(diff) and tau(et,DELTA) for, respectively, diffusion together of TCNQ- and TCNQ0 over the same long distance (delta - DELTA) followed by electron transfer at collision. That is, in mixed valent solutions around electrodes, when D(phys) is very small and the time constant-tau(diff) for physical diffusion correspondingly large, then even the slower electron transfers characteristic of long distance reactions can deliver charges on comparable timescales, tau(et,delta).