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

Aleksandr Perevedentsev,Ji-Seon Kim,Nathan Chander,Donal D C Bradley

doi:10.1002/polb.24106

Aleksandr Perevedentsev, Ji-Seon Kim + Show 2 more

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https://doi.org/10.1002/polb.24106

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Abstract

ABSTRACTPoly(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 “β‐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 β‐phase chain segments. Results show that the photoluminescence quantum efficiency (PLQE) of PFO films is highly sensitive to both the β‐phase fraction and the method by which it was induced. Notably, a PLQE of ∼69% is measured for PFO films possessing a ∼6% β‐phase fraction induced by immersion in solvent/nonsolvent mixtures; this value is substantially higher than the average PLQE of ∼55% recorded for other β‐phase films. Furthermore, a linear relationship is observed between the intensity ratios of selected Raman peaks and the β‐phase fraction determined by commonly used absorption calibrations, suggesting that Raman spectroscopy can be used as an alternative means to quantify the β‐phase fraction. As a specific example, spatial Raman mapping is used to image a mm‐scale β‐phase stripe patterned in a glassy PFO film, with the extracted β‐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

Highlights

The physical geometry, or conformation, of aflexible macromolecule represents an additional parameter space within which its properties and functionalities can be modified
The absorption spectra of b-phase PFO films are understood to comprise a linear superposition of the contributions from (i) disordered glassy chain segments yielding the broad peak centered at 380 nm and (ii) b-phase segments, the extended geometry of which results in a red-shifted absorption peak at 435 nm.[16]
The results clearly demonstrate the higher sensitivity of the PL data to b-phase fraction that, motivated our subsequent investigation of Raman spectroscopy as an alternative means to quantify the b-phase fraction in PFO films

Summary

Introduction

The physical geometry, or conformation, of a (semi-)flexible macromolecule represents an additional parameter space within which its properties and functionalities can be modified. Poly(9,9-dioctylfluorene) (PFO) is a widely studied conjugated polymer that exhibits a range of attractive properties, such as efficient blue photoluminescence and electroluminescence, high optical gain, and charge-carrier mobility, as well as good thermal stability, resulting in numerous proposed applications in, among others, light-emitting diodes (LEDs) and lasers,[8,9,10,11] sensing,[12,13] and photonics.[14,15] In addition to this, the remarkably diverse polymorphism of PFO is the focus of extensive research effort—both fundamental and application- The example of cellulose and starch— two stereoisomers of glucose—is well-known, while conformational changes are of fundamental importance to bimolecular interactions of proteins as well as the crystallization behavior of polymers.[1,2] In the case of conjugated polymers, which typically feature electronic wavefunctions primarily localized onto individual chains,[3] the chain conformation, as determined by the torsion angles between the constituent molecular subunits, affects the electronic coupling along the chain and, therewith, a wide range of the resulting optoelectronic properties.[4,5,6] While lack of control over the formation of conformational isomers and the associated structural polymorphs generally presents an obstacle for achieving consistent solid-state properties,[7] there are notable exceptions of conjugated polymers for which well-defined and reproducible chain conformations are known to exist.

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