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Abstract:

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 "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.

Abstract:

The unique electronic structures of heteroatomic conjugated polymers (HCPs) offer an attractive platform to tune optoelectronic properties via a supramolecular coordination strategy. This study reports on an sp(2) nitrogen heteroatom containing fluorene-based copolymer namely poly(9,9-dioctylfluorene-co-9,9-dioctyldiazafluoren-2,7-yl) (PF8-co-DAF8), with ≈20% DAF8 units. Tuning the optoelectronic properties of PF8-co-DAF8 via supramolecular coordination with a Lewis acid (B(C6 F5 )3 or AlCl3 ) is explored. Formation of either the PF8-co-DAF8-B(C6 F5 )3 or PF8-co-DAF8-AlCl3 adducts reduces the optical gap and causes an attendant redshift of the photoluminescence spectra. Controlling the degree and strength of the coordination allows the emission color to be tuned from blue through to green and yellow. This strategy is successfully implemented for polymer light-emitting diodes, confirming the large degree of spectral tuning whilst maintaining good device performance. Maximum luminous efficiencies, η ≈ 1.55 cd A(-1) @ 2120 cd m(-2) , 1.32 cd A(-1) @ 1424 cd m(-2) , and 2.56 cd A(-1) @ 910 cd m(-2) are, respectively, recorded for the blue-emitting diodes with Commission Internationale de L'Eclairage (CIE) (x, y) coordinates = (0.16, 0.16), the white-emitting diodes with CIE (x, y) = (0.28, 0.38) and the green-emitting diodes with CIE (x, y) = (0.33, 0.52). The results highlight the versatility of the supramolecular coordination strategy in modifying the electronic structure of HCPs.

Abstract:

A novel supramolecular system comprising a complex of 9,9′-diphenyl-9H,9′H-2,2′-bifluorene-9,9′-diol (DPFOH) with poly(methyl methacrylate) (PMMA) is presented as an attractive system for optical gain in the ultraviolet. The analogue compound 9,9′-diphenyl-9H,9′H-2,2′-bifluorene (DPFO8) without an -OH substituent was synthesized alongside DPFOH to confirm the importance of its chemical structure to the thin-film microstructure. A hydrogen-bonding interaction allows the molecule such as DPFOH and a combination of DPFOH and PMMA to have an excellent solution-processed high quality coating film. In stark contrast to the DPFO8 system, we find that the addition of 1 wt % DPFOH to PMMA leads to spontaneous formation of a supramolecular complex via hydrogen bonding interactions, giving rise to a homogeneous film with relatively high photoluminescence quantum efficiency ∼38 (±5)%. The demonstration of ultraviolet laser action with peak wavelength emission at 385 nm provided further evidence of the high optical quality of the DPFOH/PMMA supramolecular complex films. The DPFOH/PMMA supramolecular complex has great potential for use in low-cost solution-processed optoelectronic devices.