Promotion of the Polyfluorene Beta-Phase: A First Principles Study*

Two configurations of polyfluorenes - potentially important for their blue emission properties in organic devices - have been identified in single molecule spectroscopy experiments: a glassy phase with random torsional angles between fluorene units, and a planar beta-phase. The twisted conformtaion is known to be the lowest energy structure, and the factors which promote the stabilization of the beta-phase are unclear. We present a density functional theory study of ways in which polyfluorene molecules could be manipulated to favor the formation of the photophysically more stable beta-phase. Extension along the molecular axis, which increases the stability of the planar conformation relative to the glassy phase, and the role of side groups are examined in fluorene oligomers and a polyfluorene infinite in the molecular axis. Implications for excited state properties, including fluorescence, will be discussed in the context of these results.