Coherent acoustic phonon dynamics in chiral copolymers

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.

Mapping the broadband circular dichroism of copolymer films with supramolecular chirality in time and space

“…The oscillation in the absorption signal arises from a coherent acoustic phonon, which is induced by the pump laser pulse and propagates back and forth between the polymer−glass and polymer−nitrogen interface. The period of this oscillation is directly proportional to the thickness of the film ( d = τ a · c L /4), where τ a is the measured oscillation period of (100 ± 2), (171 ± 3) and (388 ± 3) ps for the films investigated, and c L is the longitudinal sound velocity of c-PFBT, which we previously determined as (2490 ± 150) m s −1 29 . The inset of Fig.…”

“…The film thickness was determined in an all-optical, non-invasive fashion using laser-based picosecond ultrasonics 27 – 29 from the TA traces of our ultrafast measurements. The TA kinetics of the polymer films exhibited pronounced damped oscillations at the edges of the ground-state absorption bands (see Fig.…”

“…The films were then annealed in a nitrogen atmosphere at 150 °C for 15 min. The thickness of the polymer films was determined in a contactless fashion by picosecond ultrasonics 27 , 28 as d = 0.25· τ a · c L , where τ a is the measured coherent acoustic phonon oscillation period (obtained from the ultrafast transient absorption kinetics averaged over the wavelength range 470−510 nm) and c L is the known longitudinal sound velocity of c-PFBT (2490 m s −1 ) 29 .…”

Ultrafast UV–Vis Transient Absorption and Circular Dichroism Spectroscopy of a Polyfluorene Copolymer Showing Large Chiral Induction

“…The thickness of the thin films (typically in the range 125−160 nm) was determined in a contactless fashion by using coherent acoustic phonon spectroscopy, as described in our previous publication. 15 An example for such a measurement is provided in Figure S2.1.…”

“…12−14 In a follow-up study, we were able to detect the first coherent oscillations ever observed in an ultrafast TrCD experiment, which arise from the propagation of a coherent acoustic phonon inside the chiral PFPh film. 15 In this contribution, we expand our TrCD investigations toward cholesteric copolymer arrangements with large induced chirality. Chiral induction is an intriguing phenomenon and has been known for a long time.…”