Unraveling the Morphology and Macroscopic Alignment of Poly(9,9-di-n-octylfluorenyl-2,7-diyl) for Enhanced Polarized Emission

In this paper, we introduce simple procedures not only to enhance the polarized photoluminescence (PL) of poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) films but also to vary the relative concentrations of PFO α- and β-phases. Enhanced molecular ordering was obtained by creating surface grooves on quartz substrates onto which PFO films were deposited and treated at temperatures within the PFO nematic phase. More specifically, the molecular order parameter increased from ca. 0.10 for films on quartz substrates to 0.57 when PFO was deposited on grooves generated by sliding a heated polytetrafluoroethylene (PTFE) bar on the substrate, and the films were treated at 180, 230, and 250 °C. Furthermore, higher ordering was reached with PFO adsorbed via self-assembly than by dip coating. Upon combining polarized absorption, PL, and emission ellipsometry, we could confirm the macroscopic alignment and the change from α- to β-phase in PFO films. The approach presented here serves to control polarized emission, which is relevant for applications such as optical displays, flat panels, optical storage, and optoelectronic devices, and to identify the structural properties of photoluminescent polymers.