The Front Cover depicts an NMR tube at very low temperatures that is illuminated, leading to photoisomerisation back and forth of a hemithioindigo molecular motor in solution within the tube. It is possible to measure quantum yields (Φ) for this process in situ. More information can be found in the Research Article by Henry Dube, Eberhard Riedle and co-workers.

Modern synthetic methods can be highly complex and proceed via multiple sequential processes. The intricate concepts can rarely be checked in all necessary detail due to the lack of suitable analysis methods. For photoinitiated reactions the starting point can be chosen to any desired precision by short or ultrafast light pulses. However, the optical analysis is blind to many aspects. Pairing the illumination of the reaction mixture with other online detection schemes proves extremely valuable. Herein we present a combination of variable temperature NMR spectroscopy with in-situ illumination and photon-counting to monitor the short-lived intermediates and gain a comprehensive and quantitative picture of the working of a hemithioindigo-based molecular motor. Application of the scheme is not limited to the field of molecular machines, we rather think that it will be of great use for any detailed identification and characterization of chemical intermediates in photochemistry, photoredox catalysis, photoswitching, or photoresponsive materials.

We describe a setup for time-resolved photoemission electron microscopy with aberration correction enabling 3 nm spatial resolution and sub-20 fs temporal resolution. The latter is realized by our development of a widely tunable (215-970 nm) noncollinear optical parametric amplifier (NOPA) at 1 MHz repetition rate. We discuss several exemplary applications. Efficient photoemission from plasmonic Au nanoresonators is investigated with phase-coherent pulse pairs from an actively stabilized interferometer. More complex excitation fields are created with a liquid-crystal-based pulse shaper enabling amplitude and phase shaping of NOPA pulses with spectral components from 600 to 800 nm. With this system we demonstrate spectroscopy within a single plasmonic nanoslit resonator by spectral amplitude shaping and investigate the local field dynamics with coherent two-dimensional (2D) spectroscopy at the nanometer length scale ("2D nanoscopy"). We show that the local response varies across a distance as small as 33 nm in our sample. Further, we report two-color pump-probe experiments using two independent NOPA beamlines. We extract local variations of the excited-state dynamics of a monolayered 2D material (WSe2) that we correlate with low-energy electron microscopy (LEEM) and reflectivity measurements. Finally, we demonstrate the in situ sample preparation capabilities for organic thin films and their characterization via spatially resolved electron diffraction and dark-field LEEM.

We discriminate different field dynamics across distances as small as 33 nm within a plasmonic slit resonator using aberration-corrected photoemission electron microscopy and a tunable broadband optical parametric amplifier at 1 MHz repetition rate.

We developed a high repetition rate optical parametric chirped-pulse amplification (OPCPA) laser system based on fiber-laser-seeded Innoslab to generate few-cycle pulses around 2 µm with passively stable carrier-envelope phase (CEP) by difference frequency generation (DFG). Incorporating a piezo mirror before the DFG stage permits rapid CEP control. The OPCPA system is seeded by a stable supercontinuum generated in bulk material with the picosecond Innoslab pulses. Few-cycle pulses with durations of 17 fs and energies of over 100 μJ were produced in a single OPCPA stage. Three different nonlinear crystals: BBO, BiBO, and LNB were tested in the final parametric amplifier, and their average power related limitations are addressed.

Exciton dynamics in a HJ-aggregate of cyanine dye TTBC are investigated by transient absorption with a time resolution of <60 fs and power-dependent emission spectroscopies. Both measurements are consistent with an exciton delocalization length of ~28 monomers. A model assuming diffusive exciton motion reveals that the exciton mobility is at least bimodal and restricted to one spatial dimension. J-band diffusion rates of 2.69 and 2.79e-3 cm2s-1 are found, leading to maximal diffusion lengths of 449 and 14.5 nm. The findings indicate that exciton-exciton annihilation is the origin of effective uphill transfer. This mechanism, if present under solar radiation, maybe useful for organic photovoltaic systems.

Two-dimensional electronic spectroscopy (2DES) is a powerful spectroscopic technique for the study of dynamics in coupled multi-chromophore systems, which has been successfully applied in the visible and infrared ranges. Its extension to the UV range (2DUV) is extremely promising for the study of biomolecules but poses several technical challenges: (i) the requirement of phase-locked pulse pairs; (ii) generation of extremely broadband UV pulses; (iii) dispersion management for the UV range. Here we present a setup able to generate ultrabroad phase-locked UV pump pulses with bandwidth from 260 to 310 nm. The UV pump pulse pairs are generated by Achromatic Second Harmonic Generation (ASHG) of a phase-locked, visible pulse pair (Fig. 1(a)). A non-collinear optical parametric amplifier (NOPA) produces 10-μJ, <; 6-fs-pulses in the 500-640 nm range. Two delayed, phase-locked replicas of the visible pulses are then generated by TWINS [1], a compact ultrastable interferometer consisting of a sequence of α-BBO birefringent wedges that allows varying with extreme accuracy and up to 1 ps the delay between the two replicas. The broadband visible replicas are subsequently chirped in a 45-mm block of SF57 and frequency doubled to the UV range by ASHG [2]. In this scheme, the visible pulse is dispersed by a fused silica (FS) prism and the angular dispersion is converted to a lateral dispersion by a second prism. The resulting collimated beam is focused into a 300μm β-BBO crystal for SHG. By suitably choosing the prism separation (152 cm) and the focal length (35 mm) we match the propagation direction of each component with its SHG phase-matching angle.

We study the excitation transfer in various geometric arrangements of rylene dimers using absorption, fluorescence and transient absorption spectra. Polarization and detection frequency dependencies of transient absorption track the interplay of transfer and vibrational relaxation within the dyads. We have resolved microscopic parametrization of intermolecular coupling between rylenes and reproduced transport data. Dynamical sampling of molecular geometries captures thermal fluctuations for Quantum Chemical estimate of couplings for orthogonally arranged dyad, where static estimates vanish and normal mode analysis of fluctuations underestimates them by an order of magnitude. Nonperturbative accounts for the modulation of transport by strongly coupled anharmonic vibrational modes is provided by a vibronic dimer model. Vibronic dynamics is demonstrated to cover both the Förster transport regime of orthogonally arranged dyads and the strong coupling regime of parallel chromophores and allows us to model signal variations along the detection frequency.

Herein we report the first photochromic polyoxometalate (POM)-based diarylethene (DAE) coordination complex, prepared by ligation of two cobalt(III)-incorporated borotungstates [BIIIWVI11O39CoIII]6- with the ditopic pyridyl-containing diarylethene (C25H16N2F6S2). The solution-state composition, structure, and stability of the assembly were probed using 1H and 19F nuclear magnetic resonance spectroscopy (NMR), electrospray ionization quadrupolar time-of-flight mass spectrometry (ESI-QTOF-MS), ultraviolet-visible spectroscopy (UV-vis), and small-angle X-ray scattering (SAXS), revealing that the complex self-organizes to adopt a molecular dumbbell structure due to electrostatic and steric considerations. This conformation is a prerequisite for the photocyclization reaction. The assembly was found to be switchable between two states using visible light due to the perturbation of the DAE electronic structure on coordination to the POM. We present photophysical data, including the reaction quantum efficiency of the molecular switch in both directions measured using a custom-built quantum yield determination setup in addition to fatigue resistance on prolonged irradiation.

The vibrational dynamics of the OH stretching mode in Ba(ClO4)2 trihydrate are investigated by means of femtosecond infrared spectroscopy. The sample offers plane cyclic water trimers in the solid phase that feature virtually no hydrogen bond interaction between the water molecules. Selective excitation of the symmetric and asymmetric stretching leads to fast population redistribution, while simultaneous excitation yields quantum beats, which are monitored via a combination tone that dominates the overtone spectrum. The combination of steady-state and time-resolved spectroscopy with quantum chemical simulations and general theoretical considerations gives indication of various aspects of symmetry breakage. The system shows a joint population lifetime of 8 ps and a long-lived coherence between symmetric and asymmetric stretching, which decays with a time constant of 0.6 ps.

The excited-state dynamics of a T-shaped bichromophoric molecule, consisting of two strong fluorophores, diphenyloxazole and diphenylpyrazoline, directly linked in an orthogonal geometry, was investigated. Despite the weak coupling ensured by this geometry and confirmed by the electronic absorption spectra, this dyad exhibits only weak fluorescence in both apolar and polar solvents, with fluorescence lifetimes ranging from 200 ps in CHX to 10 ps in ACN. Ultrafast spectroscopic measurements reveal that the fluorescence quenching in polar solvents is due to the population of a charge-separated state. In non-polar solvents, this process is energetically not feasible, and a quenching due to an efficient intersystem crossing (ISC) to the triplet manifold is proposed, based on quantum-chemical calculations. This process occurs via the spin-orbit charge-transfer (SOCT) ISC mechanism, which is enabled by the charge-transfer character acquired by the S1 state of the dyad upon structural relaxation and by the orthogonal arrangement of the molecular orbitals involved in the transition. The same mechanism is proposed to explain why the recombination of the charge-separated state is faster in medium than in highly polar solvents, as well as to account for the fast decay of the lowest triplet state to the ground state.

Hemithioindigo-based molecular motors are powered by nondamaging visible light and provide very fast directional rotations at ambient conditions. Their ground state energy profile has been probed in detail, but the crucial excited state processes are completely unknown so far. In addition, very fast processes in the ground state are also still elusive to date and thus knowledge of the whole operational mechanism remains to a large extent in the dark. In this work we elucidate the complete light-driven rotation mechanism by a combination of multiscale broadband transient absorption measurements covering a time scale from fs to ms in conjunction with a high level theoretical description of the excited state. In addition to a full description of the excited state dynamics in the various time regimes, we also provide the first experimental evidence for the elusive fourth intermediate ground state of the original HTI motor. The fate of this intermediate also is followed directly proving complete unidirectionality for both 180° rotation steps. At the same time, we uncover the hitherto unknown involvement of an unproductive triplet state pathway, which slightly diminishes the quantum yield of the E to Z photoisomerization. A rate model analysis shows that increasing the speed of motor rotation is most effectively done by increasing the photoisomerization quantum yields instead of barrier reduction for the thermal ratcheting steps. Our findings are of crucial importance for improved future designs of any light-driven molecular motor in general to yield better efficiencies and applicability.

Intense laser fields with controlled waveform are an important tool in strong-field and attosecond physics. Mid-inirared laser systems based on difference frequency generation (DFG) provide passively carrier-envelope phase (CEP) stable femtosecond pulses [1]. As Mid-IR generation is inherently inefficient, systems based on optical parametric amplification (OPA) and DFG are either limited by the peak power [2] or repetition rate [3].

We demonstrate a compact source of energetic and phase-locked multi-terahertz pulses at a repetition rate of 190 kHz. Difference frequency mixing of the fundamental output of an Yb:KGW amplifier with the idler of an optical parametric amplifier in GaSe and LiGaS2 crystals yields a passively phase-locked train of waveforms tunable between 12 and 42 THz. The shortest multi-terahertz pulses contain 1.8 oscillation cycles within the intensity full width at half-maximum. Pulse energies of up to 0.16 μJ and peak electric fields of 13 MV/cm are achieved. Electro-optic sampling reveals a phase stability better than 0.1 π over multiple hours, combined with free carrier-envelope phase tunability. The scalable scheme opens the door to strong-field terahertz optics at unprecedented repetition rates.

Light induced bond cleavage is an ubiquitous process in large molecules, yet its quantum nature is not fully understood. We present a comprehensive description of the ultrafast light induced CCl bond cleavage in diarylmethyl chlorides combining femtosecond transient absorption measurements with ab initio calculations. We observe a delayed appearance of radicals (80fs) and cations (125fs). The excited state wavepacket moves initially toward two conical intersections and the passing through these intersections determines the partitioning into the differing product channels. Different locations of the conical intersections explain the observed delay times.

Overcoming limitations of interferometric techniques in the UV, we present an alternate scheme to directly gauge the group delay with fs precision utilizing the coherent artifact in pump-probe measurements. Femtosecond-UV and NIR mirrors are characterized.

Multiscale transient absorption and streak camera measurements yield a clear picture of pyrene excimer and triplet dynamics. Linking pyrenes can accelerate relaxation dynamics by three orders of magnitude. Oligomers with tailored properties are now feasible.

A simple and easy to implement technique for femtosecond pulse characterization is proposed and experimentally verified. It is based on the introduction of a known amount of dispersion (by controlling the number of passes through dispersive material) and subsequent recording of the spectral positions of second harmonic peaks obtained in a non-linear crystal. Such dependence allows for direct retrieval of the pulse spectral phase. The presented pulse characterization method is beneficial especially for broadband pulses, where the second harmonic spectrum exceeds the detection bandwidth of a single spectrometer.

Diffraction and microscopy with ultrashort electron pulses can reveal atomic-scale motion during matter transformations. However, the spatiotemporal resolution is significantly limited by the achievable quality of the electron source. Here we report on the emission of femtosecond single/few-electron pulses from a flat metal surface via two-photon photoemission at 50–100 kHz. As pump we use wavelength-tunable visible 40 fs pulses from a noncollinear optical parametric amplifier pumped by a picosecond thin-disk laser. We demonstrate the beneficial influence of photon energies close to the photocathode’s work function for the coherence and duration of the electron pulses. The source’s stability approaches the shot noise limit after removing second-order correlation with the driving laser power. Two-photon photoemission offers genuine advantages in minimizing emission duration and effective source size directly at the location of photoemission. It produces an unprecedented combination of coherent, ultrashort and ultrastable single/few-electron wave packets for time-resolving structural dynamics.

A quantitative investigation of the relaxation dynamics of higher-lying vibrational states is afforded by a novel method of infrared pump-repump-probe spectroscopy. The technique is used to study the dynamics of OH stretching overtones in NaClO4·HDO monohydrate. We observe a continuous decrease of the energy separation for the first four states, i.e. v01 = 3575 cm(-1), v12 = 3370 cm(-1), and v23 = 3170 cm(-1), respectively. The population lifetime of the first excited state is 7.2 ps, while the one of the second excited state is largely reduced to 1.4 ps. The relaxation of the v = 2 state proceeds nearly quantitatively to the v = 1 state. The new information on the OH stretching overtones demands improved theoretical potentials and modeling of the H bond interactions. This work shows the potential of the new technique for the precise study of complex vibrational relaxation pathways.