X-ray diffraction studies of oriented poly(3-n-alkylthiophene) (P3AT's) solution-cast films show that these samples are characterized by well-organized lamellar structures whereby stacks of planar thiophene main chains are uniformly spaced by the alkyl side chains. In spite of considerable hydrocarbon chemical bonding and conformational disorder, a high degree of structural regularity is observed with appreciable three-dimensional ordering of the polymer chains. Analysis of the resultant scattering data in combination with model structure factor calculations leads to suggestive models in which an open packing of the alkyl chains is maintained with considerable side-chain disorder and side-chain mobility. There is evidence that this flexibility allows for an additional ordering process to occur at the interface formed by the alkyl side-chain end groups and that the liquid crystalline behavior is intimately related to the appearance of this structure.

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The diverse steady-state spectroscopic properties of poly(di-n-octylfluorene) are addressed from a molecular-level perspective. Modeling of representative oligomers support the experimental observation of at least three distinguishable classes of conformational isomers with differing chain torsion angles. One class appears to be populated by a single compact structural isomer, and this conforms to the so-called beta phase. A rigorous Franck-Condon analysis of the photoluminescence in conjunction with Frenkel-type exciton band structure calculations is performed. These results accurately reproduce all major spectral features of the photoabsorption and those of singlet exciton emission.

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X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to study the well-known order-disorder transition (ODT) to the β phase in poly(9,9-(di-n,noctyl)fluorene)) (PF8) thin film samples through combination of time-dependent and temperaturedependent measurements. The ODT is well described by a simple Avrami picture of one-dimensional nucleation and growth but crystallization, on cooling, proceeds only after molecular-level conformational relaxation to the so called β phase. Rapid thermal quenching is employed for PF8 studies of pure α phase samples while extended low-temperature annealing is used for improved β phase formation. Low temperature PL studies reveal sharp Franck-Condon type emission bands and, in the β phase, two distinguishable vibronic sub-bands with energies of approximately 199 and 158 meV at 25 K. This improved molecular level structural order leads to a more complete analysis of the higher-order vibronic bands. A net Huang-Rhys coupling parameter of just under 0.7 is typically observed but the relative contributions by the two distinguishable vibronic sub-bands exhibit an anomalous temperature dependence. The PL studies also identify strongly correlated behavior between the relative β phase 0-0 PL peak position and peak width. This relationship is modeled under the assumption that emission represents excitons in thermodynamic equilibrium from states at the bottom of a quasi-one-dimensional exciton band. The crystalline phase, as observed in annealed thin-film samples, has scattering peaks which are incompatible with a simple hexagonal packing of the PF8 chains.