Tuning Intermolecular Interactions in Dioctyl-Substituted Polyfluorene via Hydrostatic Pressure

Abstract

Polyfluorenes (PFs) represent a unique class of poly para-phenylene based blue-emitting polymers with intriguing structure-property relationships. Slight variations in the choice of functionalizing side chains result in dramatic differences in the inter-and intra-chain structures in PFs. We present photoluminescence (PL) and Raman scattering studies of bulk samples and thin films of dioctylsubstituted PF (PF8) under hydrostatic pressure. The bulk sample was further thermally annealed at 1.9 GPa. The PL vibronics of the as-is sample red-shift at an average rate of 26 meV/GPa.The thermally annealed sample is characterized by at least two phase transitions at 1.1 GPa and 4.2 GPa, each of which has a different pressure coefficient for PL vibronics. The Huang-Rhys factor, a measure of the electron-phonon interaction, is found to increase with increasing pressures signaling a higher geometric relaxation of the electronic states. The Raman peaks harden with increasing pressures; the intra-ring C-C stretch frequency at 1600 cm −1 has a pressure coefficient of 7.2 cm −1 /GPa and exhibits asymmetric line shapes at higher pressures, characteristic of a strong electron-phonon interaction. The optical properties of PF8 under high pressure are further contrasted with those of a branched side chain substituted PF.

Persistence of nematic liquid crystalline phase in a polyfluorene‐based organic semiconductor: A high pressure study

“…The increase in line width with pressure is symptomatic of an enhanced inter-chain interaction, and is observable in most amorphous conjugated polymers. 13 As shown in Figure 6(b), the FWHM of the 0-0 peak increases smoothly with pressure, without any discontinuities. The changes in the PL energies and line widths with pressure are consistent with enhanced intra-and interchain interactions but are not indicative of any changes in the overall ordering of 3/8-PF2/6.…”

“…11 Hydrostatic pressure provides another tuning parameter for altering intra-and interchain interactions, without changing the chemical makeup, for ascertaining structure-property relationships in p-conjugated polymers. 12 There are previous high pressure works, for example of PF8, 13 polyphenylene vinylene (PPV), 14 poly(p-phenylene), 15 F8BT, 16,17 poly(3-octylthiophene), 18 poly(3,3'-dioctyl-2,2'-bi-thiophene), 19 and P3HT blended with a fullerene derivative. 20 These examples shed light on the changes in optical excitations to conformational and other structural changes.…”

Structural study of helical polyfluorene under high quasihydrostatic pressure

“…The additional redshift, due to planarization of the backbone, typically shows up as a nonlinear shift in the PL energies as observed, for example, for PF8 [12] and parahexaphenyl [37].…”

“…The PL linewidth under pressure is an indicator of enhanced interchain interactions [12]. The discontinuity in the 0-0 PL FWHM at ∼2.7 GPa could reflect a transition to the weakly ordered state with a concomitant planarization of the backbone.…”

“…Morandi et al [10] studied PPV up to 5 GPa and reported a bathochromic shift in π -π * transition explained by the enhanced intermolecular interactions. Paudel et al [11,12] studied a variety of polyfluorenes, including PF8, and observed a redshift in the PL spectrum also explained by the higher conjugation and supposed planarization of the main chain.…”