Coherent acoustic phonon dynamics in chiral copolymers

Mirko Scholz; Marius Morgenroth; Min Ju Cho; Dong Hoon Choi; Thomas Lenzer; Kawon Oum

doi:10.1063/1.5124438

Coherent acoustic phonon dynamics in chiral copolymers

Struct Dyn. 2019 Dec 23;6(6):064502. doi: 10.1063/1.5124438. eCollection 2019 Nov.

Authors

Mirko Scholz¹, Marius Morgenroth¹, Min Ju Cho², Dong Hoon Choi², Thomas Lenzer¹, Kawon Oum¹

Affiliations

¹ Physical Chemistry, University of Siegen, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany.
² Department of Chemistry, Research Institute for Natural Sciences, Korea University, 5 Anam-dong, Sungbuk-gu, Seoul 136-701, South Korea.

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.