How Strain Controls Electronic Linewidth in Single β-Phase Polyfluorene Nanowires

Abstract

Low-temperature single-molecule fluorescence spectroscopy reveals pure, virtually defect-free chains of the one-dimensional crystalline beta-phase of polyfluorene. The likelihood of beta-phase formation is shown to correlate directly with the initial shape of the polymer chain, with extended chains preferentially forming this planarized phase. Planarized chains, characterized by a distinct spectroscopic signature can, however, exhibit substantial bending within the plane. This bending results in a strong increase in the elementary transition linewidth of the conjugated segment. The transition linewidth provides a lower limit to the electronic dephasing time of the excited state of >3 ps at 5 K. Remarkably, bending does not appear to disrupt the pi-electron conjugation so that the emission from a single bent beta-phase chromophore is not necessarily linearly polarized as is generally assumed.