Coordination polymers based on bis(bipyridyl)alkane ligands: Film preparation and charge-transport characteristics

Electroactive coordination polymers have been prepared by reacting Fe2+ and Co2+ With bis(bipyridyl)alkane (bba) ligand films that have been spin coat ed onto glassy carbon electrodes. The resulting films exhibit redox charact eristics similar to those of electropolymerized vinyl(bipyridyl) metal comp lexes. The stability of the electrode-bound films increases as the length o f the linking alkyl chain decreases. Better stability is also obtained with Fe2+ films than Co2+ films and with the alkyl-linking group attached at th e 4-position rather than the 5-position. Charge-transport measurements usin g chronoamperometry have been conducted on poly-Fe-II/bba films in which th e alkyl linker consists of 2, 4, and 6 methylene groups. Values of (DC)-C-1 /2 (D = charge-transfer diffusion coefficient; C = concentration of redox s ites) range from 9.5 x 10(-8) to 2.3 x 10(-8) mol/cm(2) s(1/2) at 298 K and follow a trend of decreasing (DC)-C-1/2 With increasing alkyl chain length for the 4-position-linked Fe-II/bba polymers. No such trend is observed fo r the analogous 5-position-linked systems. Activation parameters E-a and De lta H double dagger range from 22 to 31 kJ/mol, whereas Delta S double dagg er ranges from -30 to -75 J/mol K. The results are consistent with a mechan ism in which the increase in methylene chain length causes a decrease in th e rate of charge transport due to the lower frequency of electron-transfer collisions between neighboring redox sites. However, polymer segmental moti on is also believed to play a secondary role in this mechanism.