Temperature-controlled charge transfer mechanism in a polymer film incorporating a redox molecule as studied by potential-step chronocouloabsorptometry

Charge transport (CT) in a Nafion film incorporating [Ru(bpz)(3)](2+) (bpz = 2,2'-bipyrazine) was investigated using a potential-step chronocouloabsorptometry technique. The reductive CT by [Ru(bpz)(3)](2+/+) was found to take place by a combination mechanism of physical displacement of the complex and charge hopping between the molecules of the complex. This is a different mechanism from that of the oxidative CT (occurring by a charge hopping mechanism) in an earlier reported Nafion/[Ru(bpy)(3)](2+/3+) (bpy = 2,2'-bipyridine) system. The combination mechanism in CT was also reported in an earlier Nafion/[(NH3)(5)Ru-O-Ru(NH3)(4)-O- Ru(NH3)(5)](6+) (Ru-red) system. In the present [Ru(bpz)(3)](2+) system, the activation energy (E-a = 78 kJ.mol(-1)) of physical displacement is higher than that (40 kJ-mol(-1)) of charge hopping. In the Ru-red system, on the contrary, the Ea (13 kJ.mol(-1)) of physical displacement is lower than that (55 kJ-mol(-1)) of charge hopping. As a result, the fraction (Phi(c)) of the contribution of charge hopping to the initial CT rate decreased with increasing temperature for the [Ru(bpz)(3)](2+) System, whereas it increased with increasing temperature for the Ru-red system. The physical displacement was enthalpy-controlled in the [Ru(bpZ)(3)](2+) system at 25 degreesC. By contrast, it was entropy-controlled in the Ru-red system. The featured factors influencing physical displacement and charge hopping were discussed.