The Influence of Alkyl‐Chain Length on Beta‐Phase Formation in Polyfluorenes

Abstract

Di‐n‐alkyl substituted polyfluorenes with alkyl chain lengths of 6, 7, 8, 9, and 10 carbon atoms (PF6, PF7, PF8, PF9, and PF10) are studied in dilute solution in MCH using optical spectroscopy. Beta‐phase is formed upon cooling in solutions (∼ 7 µg mL−1) of PF7, PF8, and PF9 only, which is observed as an equilibrium absorption peak at ∼ 437 nm and strong changes in the emission spectra. Beta‐phase is formed upon thermal cycling to low temperature in solutions (∼7 µg mL−1) of PF7, PF8, and PF9, which is observed as an equilibrium absorption peak at ∼ 437 nm and strong changes in the emission spectra. Beta phase is found to occur more favorably in PF8 than in PF7 or PF9, which is attributed to a balance between two factors. The first is the dimer/aggregate formation efficiency, which is poorer for longer (more disordered) alkyl chain lengths, and the second is the Van der Waals bond energy available to overcome the steric repulsion and planarize the conjugated backbone, which is insufficient in the PF6 with a shorter alkyl chain. Beta phase formation is shown to be a result of aggregation, not a precursor to it. A tentative value of the energy required to planarize the fluorene backbone of (15.6 ± 2.5) kJ mol−1 monomer is suggested. Excitation spectra of PF6, PF7, PF8, and PF9 in extremely dilute (∼ 10 ng mL−1) solution show that beta phase can form reversibly in dilute solutions of PF7, PF8 and PF9, which is believed to be a result of chain collapse or well dispersed aggregates being present in solution from dilution of more concentrated solutions. PF7, PF8, and PF9 also form beta phase in thermally cycled solid films spin‐cast from MCH. However, in the films the PF7 formed a larger fraction of beta phase than the PF9, in contrast to the case in solutions, because it is less likely that the close‐packed chains in the solid state will allow the formation of planarized chains with the longer PF9 side chains.

Network structure of polyfluorene sheets as a function of alkyl side chain length

“…Figure 6 shows the optical absorption spectra of PF N -MCH at 10 mg/mL with the modeled fits to the spectra including the 0-0 peak, three vibronic peaks with several multiplicities, as well as a distinctive asymmetric peak at 2.9 eV. The data are qualitatively similar to those reported earlier [32,34] but the modeling allows us to estimate the relative fractions of polymer assemblies and locally free chains. The vibronic replicas used correspond to modes with energies of 85 meV (v1), 156 meV (v2), and 199 meV (v3) which were selected from fits to the β-phase PF8 photoluminescence spectrum and which agree well with Raman spectroscopy of PF8 [44].…”

“…The v2 peak increases at the expense of the v3 peak with increasing N, thus pointing to the increasing fraction of membranes and decreasing fraction of ordered sheets. The intrinsic stiffness of polymer backbones is similar for all N, as indicated by similar room temperature spectra before annealing for dilute solutions [34]. The asymmetric peak at 2.85 eV is indicative of the amount of β phase present.…”

“…If the assemblies are assumed to be quasi-two-dimensional membranes, the isotropic phase transition temperature (T * mem ) for the membrane generally increases with decreasing N [33] and is unattainable for PF6. At room temperature, the optical characteristics matching those of the PF8 β phase are found for PF7 and PF9 in MCH both at high (∼1 vol %) [32] and low (∼0.002 vol %) [34] concentrations, while the same is seen for PF10 in thin films [35], with its thermal dependency following the lower T * mem [33]. The presence of sheetlike assemblies [26] or membranes [33] in PFs mixed with MCH (or PF N -MCH) is well established and discussed in terms of their internal structure [32] and macrophase separation [30], thermal behavior [30,32], and the existence of β phase [25,34,35].…”

Spectroscopic studies of different poly3hexylthiophene chain environments in a polyfluorene matrix

“…[12] Several studies have been reported, including mixing of polyfluorenes with different emissive materials and applying them in single or multiple layers. [13,14] We also know that many polyfluorenes form there own distinct phases in films, commonly known as the beta phase, [15,16,17] thus it is a ideal component of a blend to study. Polythiophenes and their regioregular forms are another class of well-known semiconductor polymers known to show complex morphology and morphological controlled pho-2 tophysics, where side chain structure and quality of solvent/environment also plays a significant role in their photophysical properties.…”

Side-Chain Influence on the Mass Density and Refractive Index of Polyfluorenes and Star-Shaped Oligofluorene Truxenes

“…Their widespread use has benefitted from their tunable their photonic and electronic properties, which can be tailored by various methods including addition of substituents to the main core of the molecule 12,13,14 , molecular engineering of the band gaps 15 , control of morphology to extend conjugation length 16,17 , and control of film thickness 18 . The resulting materials properties in the solid-state depend not only on the electronic properties of individual molecules but also on the density of the film, i.e.…”