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

Abstract

Ultrafast broadband UV−vis transient circular dichroism (TrCD) and transient absorption (TA) spectroscopy are employed to study the dynamics of chiral polymer thin films. Helical cholesteric arrangements are formed by mixing chiral binaphthalene derivatives with the intrinsically achiral polyfluorene-based copolymer F8BT. The induced circular dichroism (ICD) response of F8BT shows a mirrorimage-like shape when the chirality of the additive is changed. Peaks in the CD spectra reach large absolute values of about 800 mdeg. Photoexcitation by ultrashort laser pulses results in TrCD spectra of opposite sign for cholesteric polymer films of different helicity, demonstrating transient chiral discrimination on the femtosecond time scale over the wavelength range 260−700 nm. Differences between the excited-state and ground-state CD spectra considerably depend on the electronic transition. The excited-state processes in these systems are evaluated by a detailed kinetic analysis providing rate constants for the relaxation of the different singlet exciton states as well as singlet−singlet annihilation processes of F8BT involving diffusive bimolecular recombination and Forster resonance energy transfer.

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

“…As explained in more detail below, the latter effect is again consistent with the formation of a charge-pair state from S 1 by singlet−singlet annihilation (SSA). This process becomes faster at high pump fluence, and thus high initial exciton number densities in the polymer 25 , 39 , as described by our kinetic modelling below. Therefore, under such high-fluence conditions, the S 1 population is already considerably depleted at 3 ps.…”

“…4b ) and is due to circular selective scattering. At high pump fluence, CPSE at 3 ps will be largely absent because of fast depletion of the S 1 state due to singlet−singlet annihilation, leading to an apparent blue-shift of the band in the TrCD spectrum 25 . Note that the noise characteristics of the TA and TrCD signals (and thus also the CPSE contribution) are governed by laser noise, i.e.…”

“…20 μJ pulse −1 ) was converted into left-circular polarisation by means of a high-precision quarter-wave plate (B. Halle Nachfl.). The circularly polarised beam then traversed a DKDP Pockels cell (Eksma MP1), replacing the BBO-based Pockels cell used in an older design 24 , 25 . Switching from left-circular to right-circular polarisation was achieved by applying a pulsed half-wave voltage of 2500 V. The circularly polarised 400 nm seed pulses were focused into a translating CaF 2 plate (thickness 3 mm), generating a broadband multifilament UV−Vis supercontinuum (260−700 nm), which was then separated into a signal and a reference beam by means of a broadband beam splitter.…”

“…In this work, we present an advanced setup for TrCD detection using a Pockels-cell-based polarisation-switching design employing a deuterated KDP crystal (replacing the previously used BBO 24 , 25 ). Consequently, the time resolution is improved to 100 fs, with the two decisive benefits of a substantial increase in the intensity of the circularly polarised multifilament supercontinuum over the spectral range covered (260−700 nm) and an accompanying reduction of its intensity fluctuations.…”

Pentacene–Bridge Interactions in an Axially Chiral Binaphthyl Pentacene Dimer

“…While CD is responsive to molecular arrangements in dimers at the ground state, excited-state CD is highly sensitive to coupling between monomers and monomer–bridge interactions in the excited state. Recent work on excited-state CD in axially chiral dimers and cholesteric phases of conjugated polymers , has revealed crucial insights into the role of molecular structure and its rearrangement on the excited-state relaxation dynamics. However, little is known about the impact of chirality on iSF dynamics.…”