Structure, photophysics, and the order-disorder transition to theβphase in poly(9,9-(di-n,n-octyl)fluorene)

X-ray-diffraction, UV-vis absorption, and photoluminescence (PL) spectroscopies have been used to study the well-known order-disorder transition (ODT) to the $\beta$ phase in poly(9,9- (di-$n,n$-octyl)fluorene) (PF8) thin-film samples through a combination of time-dependent and temperature-dependent 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 $\beta$ phase. Rapid thermal quenching is employed for PF8 studies of pure $\alpha$-phase samples while extended low-temperature annealing is used for improved $\beta$-phase formation. Low temperature PL studies reveal sharp Franck-Condon type emission bands and, in the $\beta$ phase, two distinguishable vibronic subbands with energies of $\approx 199$ 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 subbands exhibit an anomalous temperature dependence. The PL studies also identify strongly correlated behavior between the relative $\beta$-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.