Measurements of the electronic circular dichroism (CD) are highly sensitive to the absolute configuration and conformation of chiral molecules and supramolecular assemblies and have therefore found widespread application in the chemical and biological sciences. Here, we demonstrate an approach to simultaneously follow changes in the CD and absorption response of photoexcited systems over the ultraviolet−visible spectral range with 100 fs time resolution. We apply the concept to chiral polyfluorene copolymer thin films and track their electronic relaxation in detail. The transient CD signal stems from the supramolecular response of the system and provides information regarding the recovery of the electronic ground state. This allows for a quantification of singlet−singlet annihilation and charge-pair formation processes. Spatial mapping of chiral domains on femtosecond time scales with a resolution of 50 μm and diffraction-limited steady-state imaging of the circular dichroism and the circularly polarised luminescence (CPL) of the films is demonstrated.

The ultrafast charge carrier dynamics of the metaldeficient lead-free perovskite derivative methylammonium antimony iodide ((CH 3 NH 3 ) 3 Sb 2 I 9 , MASbI) is studied using femtosecond UV−vis−NIR transient absorption spectroscopy. The transient and steady-state absorption spectra show strong excitonic features. This is confirmed by an analysis of the near-band-gap absorption using Elliott's theory and its reformulation by Tanguy with subsequent modeling of the free-carrier and exciton contributions using the Saha equation. Time scales of the different carrier scattering processes are identified. The kinetics in the UV−vis range is largely independent of pump laser fluence suggesting that the dynamics involves localized excitons, with recombination time constants of 190 ps and >5 ns. Damped oscillations in the kinetics indicate coherent phonon dynamics and strong electron-phonon coupling. Fourier transformation of the time-domain data provides the steady-state Raman spectrum of MASbI which is dominated by vibrations of the [Sb 2 I 9 ] 3− anion. Exciton dissociation at the interfaces with mesoporous TiO 2 and a triarylamine-based hole transport material is found to be negligible. Our experiments suggest that it will be challenging to fabricate efficient photovoltaic devices from such lead-free antimony-and also bismuth-based zero-dimensional perovskite derivatives, because nonradiative exciton recombination largely governs their carrier dynamics.

Low-dimensional copper halides, such as CsCu 2 I 3 , have emerged as promising LED materials featuring strongly Stokes-shifted photoluminescence with high quantum yield. Previous calculations suggest an exciton self-trapping mechanism; however, direct experimental evidence for this process is still lacking. Here, we present femtosecond UV−vis transient absorption experiments of CsCu 2 I 3 thin films. The films were analyzed by SEM, XRD, and 133 Cs/ 63 Cu NMR for crystallinity and defects. Unique spectral dynamics is observed. The band gap absorption exhibits a characteristic double-peak structure arising from the 130 meV spin− orbit splitting of the copper d electrons. Emission at the direct band gap disappears because of the formation of the lowest-energy self-trapped exciton state. We determined the time constant of 12 ps for the trapping process of thermally relaxed free excitons, with an energy barrier of at least 60 meV. The data are successfully modeled by global kinetic analysis, providing also accurate time constants for charge carrier cooling processes.

We observed efficient induction of chirality in polyfluorene copolymer thin films by mixing with helicene-type chiral additives based on the dibenzo-[c,h]acridine motif. Images obtained from circular dichroism (CD) and circularly polarized luminescence (CPL) microscopy provide information about the chiral arrangements in the thin films with diffraction-limited resolution. The CD signal shows a characteristic dependence on the film thickness, which supports a supramolecular origin of the strong chiral response of the copolymer. In particular, we demonstrate the discrimination between films of opposite chirality based on their ultrafast transient chiral response through the use of femtosecond broadband CD spectroscopy and a newly developed setup for transient CPL spectroscopy with 28 ps time resolution. A systematic variation of the enantiomeric excess of the chiral additive shows that the "Sergeants and Soldiers" concept and "Majority Rules" are not obeyed.

We present a study of the photoinduced dynamics of the chiral polyfluorene-phenylene copolymer PFPh in THF and in cholesteric thin films. After photoexcitation at 370 nm in THF, ultraviolet–visible–near-infrared (UV–vis–NIR) transient absorption spectra show fast subpicosecond to picosecond intrachain migration of singlet excitons, solvation dynamics, and an exciton lifetime of 410 ps. The PFPh thin film features also interchain singlet exciton migration and exhibits shorter (2.1 and 240 ps) and longer lifetime components (2800 ps, interchain recombination). Furthermore, a setup for ultrafast UV–vis broadband transient circular dichroism (TrCD) spectroscopy has been developed. Fast supramolecular relaxation processes are observed, which are linked to changes in the anisotropic polarizability and pitch length of the cholesteric film. Such combined ultrafast transient CD and absorption experiments hold promise to reveal not only details of relaxation processes in supramolecular arrangements but also structural rearrangements of chiral molecular systems featuring CD signals in the UV–vis region.

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.

Hybrid organic−inorganic halogenidocuprates based on copper(I) represent materials with rich structural diversity and high photoluminescence (PL) quantum yield, yet the mechanism responsible for their efficient, strongly Stokes-shifted emission is still unclear.Here we report the successful preparation of (CH 3 NH 3 ) 4 Cu 2 Br 6 thin films with a zerodimensional molecular salt structure featuring "isolated" [Cu 2 Br 6 ] 4− ions. Time-resolved broadband PL measurements provide an excited-state lifetime of 114 μs at 298 K. Results from femto-to microsecond UV−vis−NIR transient absorption experiments combined with DFT/ TDDFT calculations suggest the formation of a long-lived structurally relaxed triplet species through intersystem crossing (61 ps), which almost exclusively decays by phosphorescence. In addition, time scales for structural relaxation and cooling processes are extracted from a global kinetic analysis of the transient spectra. Calculations for the isolated [Cu 2 Br 6 ] 4− anion and the (CH 3 NH 3 ) 4 Cu 2 Br 6 crystal suggest a strong impact of the crystal environment on the structure of the anion.

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.