Polythiophene hybrids of transition-metal bis(salicylidenimine)s: Correlation between structure and electronic properties

The synthesis, electrochemistry, and spectroscopic behavior of tetradentate bis(salicylidenimine) transition metal complexes 5-9 are reported. Appending these complexes with 3,4-ethylenedioxythiophene (EDOT) moieties allows for electrochemical polymerization at much lower potentials than the parent salen complexes. The resulting polymers display well-defined organic-based electrochemistry at potentials <0.5 V vs Fc/Fc(+). The EDOT-modified N,N'-ethylene bis(salicylidene), N,N'-o-phenylene bis(salicylidene), and N,N'-trans-cyclohexylene bis(salicylidene) complexes 5a-b, 6a-b, and 8a-b display cyclic voltammograms with four organic-based redox waves. Increasing the interchain separation through the use of nonplanar bis(salicylidene) ligands results in only two redox waves. The conductivity of the copper-based polymers decreases with increasing interchain spacing, with the maximum conductivity being 92 S cm(-1) for poly(5a) and 16 S cm(-1) for poly(7a). The nickel complexes were less sensitive to increased interchain separation and showed conductivities greater than 48 S cm(-1) regardless of the interchain spacing and near 100 S cm(-1) in the case of poly(6b). In situ spectroelectrochemistry was consistent with the segmented electronic nature of these polymers. Cyclic voltammetry of an analogous uranyl complex. 5c, revealed that two electrons per repeat unit were removed during oxidation. Consideration of our collective investigations, which also included in situ EPR spectroscopic studies, led to a postulation that pi-aggregation processes are occurring in those polymers which are allowed to have close interchain spacing.