Abstract
Coherent phonon oscillations in the UV-Vis transient absorption and circular dichroism response of two chiral polyfluorene-based copolymer thin films are investigated. A slow oscillation in the hundred picosecond regime indicates the propagation of a longitudinal acoustic phonon with a frequency in the gigahertz range through cholesteric films of PFPh and PFBT, which allow for the optical determination of the longitudinal sound velocity in these polymers, with values of (2550 ± 140) and (2490 ± 150) m s−1, respectively. The oscillation is induced by a strain wave, resulting in a pressure-induced periodic shift of the electronic absorption bands, as extracted from a Fourier analysis of the transient spectra. The acoustic phonon oscillation is also clearly detected in the transient circular dichroism (TrCD) response of PFPh, indicating a transient pressure-induced shift of the CD spectrum and possibly also phonon-induced chirality changes via pitch length modulation of the cholesteric helical polymer stack.
Highlights
Picosecond ultrasonics1 has become a powerful tool for the laserbased, contactless investigation of thin films and nanostructures covering the frequency range from a few to several hundred gigahertz.2–10In a system of interest, e.g., a metal, semiconductor, or polymer thin film, acoustic phonons are generated either directly by an ultrashort laser pulse11–13 or by exciting an attached transducer layer,14–17 which launches an acoustic pulse in the adjacent film structure
We investigate the viscoelastic response of thin nanolayers of two previously unstudied chiral polyfluorene copolymers denoted as PFPh and PFBT using ultrafast coherent acoustic phonon spectroscopy in transmission mode
Violet and blue regions indicate ground state bleach (GSB) or stimulated emission (SE), whereas yellow to red colors are due to excited state absorption (ESA)
Summary
Picosecond ultrasonics has become a powerful tool for the laserbased, contactless investigation of thin films and nanostructures covering the frequency range from a few to several hundred gigahertz.2–10In a system of interest, e.g., a metal, semiconductor, or polymer thin film, acoustic phonons are generated either directly by an ultrashort laser pulse or by exciting an attached transducer layer, which launches an acoustic pulse in the adjacent film structure. Previous optical investigations largely employed detection at a single or a few probe wavelengths in order to extract information regarding longitudinal and transversal (shear) acoustic waves in various thin film materials, and many of these measurements have been performed in reflective geometry.. Previous optical investigations largely employed detection at a single or a few probe wavelengths in order to extract information regarding longitudinal and transversal (shear) acoustic waves in various thin film materials, and many of these measurements have been performed in reflective geometry.4,6,11,16,17,20 In this contribution, we investigate the viscoelastic response of thin nanolayers of two previously unstudied chiral polyfluorene copolymers denoted as PFPh and PFBT (see top of Fig. 1 for their chemical structures) using ultrafast coherent acoustic phonon spectroscopy in transmission mode. Based on the evaluation of the complete spectrotemporal datasets from broadband transient absorption, we aim at determining accurate values for the previously unknown longitudinal speed of sound inside these polymer compounds and obtaining a clear picture of the physical processes responsible for the oscillatory optical response
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