Chain conformation and the photophysics of polyfluorenes

Abstract

Conjugated polymers may combine the optical and electronic properties of semiconductors with the desirable mechanical and processing capabilities that characterise polymeric “plastic” materials. The first demonstration of electroluminescence from conjugated-polymer-based devices 25 years ago created the field of Plastic Electronics which has since witnessed spectacular scientific and technological progress, yielding novel devices for applications in light-emitting diodes, photovoltaics, electronics, photonics, sensing, and many others. The molecular nature of conjugated polymers results in weak, typically van der Waals-type, interchain bonding which enables straightforward and cost-effective processing of these materials from the melt or solutions. However, the trade-off for this desirable characteristic is that the solid-state microstructure of conjugated polymers can be extremely sensitive to processing conditions, leading to a wide variety of structural polymorphs that limit reproducibility of the optoelectronic properties of devices and, hence, prevent their commercialisation. For this reason small organic molecules are often used in preference to conjugated polymers, since the former’s microstructure can be more precisely controlled via chemical synthesis and deposition protocols. Nevertheless, there are examples of conjugated polymers that also exhibit well-defined structures. This has been demonstrated, among others, for poly(9,9-dioctylfluorene) (PFO) – a widely-studied blue-emitting conjugated polymer – which can adopt a precisely-defined planar-zigzag chain conformation termed the “β-phase”. Introducing a fraction of β-phase chain segments leads to dramatic – and often advantageous – changes to the optoelectronic properties of PFO. In order to harness the β-phase chain conformation of PFO with the aim of optimising its solid-state optoelectronic properties, three distinct directions need to be investigated, which motivated the research presented in this thesis. First, the mechanism responsible for the formation of the β-phase of PFO is still little-understood; specifically, the role of inter-chain interactions in the formation of intra-chain (conformational) order remains ambiguous. To this end, the formation of the β-phase in PFO solutions with common organic solvents was systematically investigated. Unexpectedly, it is found that solution-crystallisation of PFO proceeds via polymer-solvent compound formation, enabled by adoption of the β-phase conformation which features along-chain cavities that allow for simultaneous inclusion/intercalation of solvent molecules of matching volume. Remarkably, it is shown that the physically-bound intercalated solvent stabilises the β-phase conformation in the absence of specific inter-chain interactions. This makes the β-phase conformation distinct from other conformational isomers and crystalline forms of PFO. An additional consequence of β-phase formation by complexation with the solvent is that the β-phase chain conformation is inherently well-defined and crystallisation-condition-invariant in terms of its intermonomer torsion angle. Second, while selected aspects of the influence of the β-phase on the optoelectronic properties of PFO have been reported, many of them deserve further study. For instance, the effect of the relative fraction of β-phase chain segments on absorption and photoluminescence spectra, as well as photoluminescence quantum efficiency (PLQE), is examined. It is demonstrated that, in comparison to the in-plane isotropic, “glassy” films, PLQE is enhanced by up to 15% for a narrow range (7–9%) of β-phase fractions, introduced by film immersion in solvent-nonsolvent mixtures. Furthermore, analysis of dilute (≤ 1 wt % PFO) oriented blends of PFO with polyethylene shows that the β-phase conformation is metastable in the absence of solvent and, therefore, cannot be oriented by tensile drawing. Consequently, retaining a fraction of β-phase chain segments in tensile-drawn PFO-polyethylene blends leads to strong photoluminescence (PL) depolarisation which limits the resulting optical anisotropy. Third, the ability to locally generate the β-phase conformation on sub-micrometre scale in glassy PFO thin films, which is of interest for fabricating a range of photonic structures, has so far presented a challenge from both the structuring and the imaging perspective. A novel form of dip-pen nanolithography is shown herein to provide an effective means of spatial patterning of the β-phase conformation on length scales ≤ 500 nm while retaining the planar format of the film. The accompanying increase in refractive index associated with the adoption of the β-phase conformation can enable a variety of visible-wavelength, conjugated polymer photonic elements, two of which are modelled using complex photonic dispersion calculations. With regard to imaging of the β-phase patterns, spectroscopic Raman mapping is shown to provide a useful alternative to confocal PL microscopy, with an additional advantage of being able to estimate the relative fraction of β-phase chain segments in the patterns. The research presented in this thesis explore the chain conformation of PFO, specifically its β-phase conformational isomer, as an additional parameter space within which its optoelectronic properties can be tailored for optimal device performance. Further improvements to the resolution of dip-pen nanolithography-based spatial patterning of the β-phase may offer the prospect of a versatile approach to visible-wavelength “conformational” metamaterials. The reported molecular PFO compounds can more generally be formed with the organic solvent replaced by a small molecule of desired optoelectronic properties, thereby allowing the fabrication of ultra-regular molecular-level blends comprising a PFO host and a small-molecular guest. A judicious choice of guest molecule might therefore enable new functionalities in PFO-based devices.