We have examined the microstructure, solution processing, thermal transitions, and mechanical properties of regioregular poly(nonylbithiazole) (PNBT). PNBT can adopt three distinct colors in the solid stateyellow, red, and metallic greendepending on processing conditions such as polymer concentration, solvent evaporation rate, temperature, and pressure. Microstructural variations were observed between samples of different colors, including the intensification of optical absorption with the degree of crystalline order and/or crystal size. Yellow samples lacked well-developed three-dimensional order, whereas red and green samples were semicrystalline. The development of a lyotropic mesophase was observed at intermediate concentrations in a “diffusion couple” geometry. A crystal model with two planar, π-stacked chains per unit cell was shown to be consistent with the experimental diffraction data. The simulated orthorhombic unit cell parameters are a = 2.38 nm, b = 0.72 nm, and c = 0.79 nm.

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The synthesis of poly(4,4′-dinonyl-2,2′-bisoxazole-5,5′-diyl) (PNBO) was performed through a nickel-mediated coupling of 5,5′-dibromo-4,4′-dinonyl-2,2′-bisoxazole. The solid-state morphology of PNBO was determined by powder X-ray diffraction (XRD) which showed strong peaks due to lamellar spacing and three-dimensional ordering. The polymer has absorbance maxima of 446 and 464 nm in solution, and 475 nm in the film. A low wavelength shoulder near 510 nm is ascribed to the absorption of aggregated molecules. The emission spectra are concentration dependent and consistent with emission from excimers. Essentially all the emission in the solid state is due to excimers. The physical and chemical properties of PNBO are compared to the sulfur analogues, poly(4,4′-dinonyl-2,2′-bithiazole-5,5′-diyl) (PNBTz) and poly(3-alkylthiophene)s.

The ∼2.4 nm (100) lattice planes of electrooptically active poly(nonylbithiazole) (PNBT) and poly(nonylbisoxazole) (PNBO) crystallites have been imaged by means of low-dose high-resolution transmission electron microscopy (HRTEM). The average dimensions of the observed crystallites were (14 ± 9) × (13 ± 6) nm (width vs length) and (80 ± 36) × (22 ± 9) nm for PNBT and PNBO, respectively. Atomization of dilute polymer solutions led to the formation of discontinuous thin films containing numerous holes. The influence of these surfaces on the crystallization process was evident in the preferential parallel orientation of the (100) fringes with respect to the film edges. Evidence of considerable lattice bending about the [010] (π−π stacking) direction was observed in both compounds. Two-dimensional maps of the local lattice curvature revealed substantial differences in the modes of deformation of the two polymers:  PNBT crystallites curved in a continuous fashion, whereas PNBO crystallites localized the distortions. Possible factors that could influence the observed deformation mechanisms are discussed.

Regioregular, alkyl-substituted poly(heteroaromatics) tend to crystallize in π-stacked structures as opposed to the herringbone pattern characteristic of non-substituted, planar aromatic molecules, e.g. sexithiophene. π-Stacking, favored by the crystallization of the sidechains, induces planarity in the conjugated backbone. The increased planarity causes a large (50-100 nm) shift in λmax (abs.) compared to the solution phase in which the backbone is twisted out of coplanarity, π-Stacking is a form of ground state aggregation that causes “fine structure” on the absorption peak. Excitation into the aggregate states rapidly leads to the formation of excimers as shown by the excitation spectra. In -stacked polymers, photoluminescence is almost entirely from the excimer states.