Structural and Spectroscopic Investigations of Bulk Poly[bis(2-ethyl)hexylfluorene]

X-ray diffraction, optical spectroscopy (photoluminescence, photoluminescence excitation, Raman scattering), and molecular modeling are used in a broad study of structure and structural phase behavior within bulk poly(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl) (PF2/6). The nascent polymer initially appears in a disordered state, but annealing at modest temperatures, just above the glass transition temperature (T g) of 332 K, initiates structural evolution toward the well-known hexagonal-type phase. Cooling from the high-temperature liquid crystal mesophase yields the same basic phase structure but with much improved long-range order. Despite these large-scale changes in the PF2/6 interchain ordering, optical spectroscopy resolves only subtle variations in the spectroscopic features. Thermal cycling above 330 K (near T g) yields red shifts in the Franck−Condon (FC) emission. These shifts are, in part, irreversible, but temperatures above 250 K retain a persistent red shift of the FC emission on heating. Temperatures above 330 K (near T g) show little frequency shift of the Raman-active modes until cycling to temperatures above that of the liquid crystalline phase transition (at 430 K). Thereafter, monotonic shifts are observed. Excitation measurements clearly resolve two distinct defect emission peaks and specify a mobility edge for singlet exciton migration at reduced temperatures. Structure factor refinements are consistent with a 5/2 polyfluorene helix incorporating a multitude of structurally distinct conformational isomers. The alkyl side chains include conformational disorder as well and are oriented, on average, perpendicular to the helices. Combinatorial modeling of over 4000 structural isomers (i.e., gas-phase decamers at zero temperature) identifies a relatively large number of energetically favorable chain conformations. Models approximating 5/1 and 5/2 helices yield stable, low-energy structures. Preference for a 5/2 helix cannot be rigorously established at present.