Tuning intermolecular interactions in di-octyl substituted polyfluorene via hydrostatic pressure

TL;DR

This study investigates how hydrostatic pressure influences the optical and vibrational properties of dioctyl-substituted polyfluorene, revealing phase transitions, increased electron-phonon interactions, and the effects of side chain variations.

Contribution

It provides new insights into pressure-induced structural and electronic changes in polyfluorenes with different side chains, highlighting phase transitions and electron-phonon coupling effects.

Findings

PL vibronics red-shift at 26 meV/GPa in as-is sample

At least two phase transitions at 1.1 GPa and 4.2 GPa in thermally annealed sample

Raman C-C stretch frequency increases with pressure at 7.2 cm$^{-1}$/GPa

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 dioctyl-substituted 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.