Spectroscopic properties of poly(9,9-dioctylfluorene) thin films possessing varied fractions of [beta]-phase chain segments: enhanced photoluminescence efficiency via conformation structuring

Poly(9,9-dioctylfluorene) (PFO) is a widely studied blue-emitting conjugated polymer, the optoelectronic properties of which are strongly affected by the presence of a well-defined chain-extended "[beta]-phase" conformational isomer. In this study, optical and Raman spectroscopy are used to systematically investigate the properties of PFO thin films featuring a varied fraction of [beta]-phase chain segments. Results show that the photoluminescence quantum efficiency (PLQE) of PFO films is highly sensitive to both the [beta]-phase fraction and the method by which it was induced. Notably, a PLQE of 69% is measured for PFO films possessing a 6% [beta]-phase fraction induced by immersion in solvent/nonsolvent mixtures; this value is substantially higher than the average PLQE of 55% recorded for other [beta]-phase films. Furthermore, a linear relationship is observed between the intensity ratios of selected Raman peaks and the [beta]-phase fraction determined by commonly used absorption calibrations, suggesting that Raman spectroscopy can be used as an alternative means to quantify the [beta]-phase fraction. As a specific example, spatial Raman mapping is used to image a mm-scale [beta]-phase stripe patterned in a glassy PFO film, with the extracted [beta]-phase fraction showing excellent agreement with the results of optical spectroscopy. © 2016 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1995-2006