Structure-property relationships in bithiophene and bithiazole polymers.

A series of polymers containing 4,4'-dinonyl-2,2 '-bithiophene and 4,4'-dinonyl-2,2 '-bithiazole was synthesized in order to probe structure-property relationships in conjugated polymers. Each polymer possesses a 4,4 '-dinonyl-2,2'-bithiazole or 4,4 '-dinonyl-2,2'-bithiophene central ring with pendent nonylthiazole or nonylthiophene heterocycles connected to yield either HH-TT or TH-TT-HT-HH (H = head and T = tail) coupling patterns. By UV-vis spectroscopy, polymers with HT couplings have more planar structures, both in solution and solid state, than corresponding polymers with HH couplings. By x-ray diffraction (XRD), films of polymers with HT couplings have greater degrees of crystallinity than polymers with HH couplings. Though polymers with HT couplings have greater degrees of crystallinity, polymers with HH couplings have higher melting points, which suggests higher enthalpies for the crystalline regions of HH polymers. The electronic properties of the seven polymers were investigated by cyclic voltammetry (CV) and spectroelectrochemical experiments. As determined by CV, polymers containing strictly thiophene heterocycles are only able to be oxidized, polymers containing strictly thiazole heterocycles are only capable of undergoing reduction and polymers containing both thiophene and thiazole heterocycles can be both oxidized and reduced within the potential range, +2.3 V. to -2.5 V. Polymers with HT couplings have lower redox potentials than polymers with HH-containing polymers. The UV-vis-NIR spectra of the polymers were taken upon application of negative or positive potentials and the results were interpreted using the pi-stacking, spin-paired electron model proposed by Miller et al. This model was expanded to consider the n-doped state of the polymers, and we suggest the formation of anion pi-stacks when the LUMO is singly occupied. The thermochromic properties of the seven polymers were investigated via variable-temperature UV-vis spectroscopy and variable-temperature XRD. Polymers with HT couplings show greater changes in morphology upon heating than do polymers with HH couplings. By variable-temperature XRD, polymers with HH couplings can be thermally-annealed to more crystalline morphologies than obtained by solvent-casting, whereas polymers with HT couplings are thermally-annealed to morphologies similar to those obtained by solvent-casting. The optical, structural, and electronic properties of bithiophene and bithiazole polymers can be vastly altered by controlling the composition and connectivity of the monomers.Ph.D.Polymer chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/127311/2/3029309.pd