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 lightdriven 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.

Efficiency and performance of light triggered molecular motors are crucial features that need to be mechanistically understood to improve the performance and enable conscious property tailoringf or specific applications. In this work, three differenth emithioindigo-based molecular motors are investigated and all four steps in their complete unidirectionalr otation are unraveled fully quantitatively. Transient absorption spectroscopy across twelveo rders of magnitude in time is used to probe the fs nuclear motions up to the ms thermal kinetics, covering the timeframe of the whole motor rotation. The newly knownf ull mechanisms allow simulation of the motor systems to scrutinize their performance at realistic illumination conditions. This highlights the importance of photoisomerization quantum yields for the rotationspeed. The substitution pattern in close proximity to the rotation axle influences the excited and ground state properties. Reduction of electron donation and concomitant increaseo fs teric hindrance leads to faster photoisomerization reactions with quasi-ballistic behavior,b ut also to as light decrease in the quantume fficiency. The expected decelerating effects of increased stericsa re primarily manifested in the ground state. Ap romising approach for nextgeneration hemithioindigom otors is to elevate electron donation at the rotor fragment followed by an increaseo f steric hindrance.

Intramolecular charge separation and enhanced spin orbit coupling explain the weak fluorescence of a T-shaped dyad comprising two strong fluorophores.

Nanoscale infrared (IR) resonators with sub-diffraction limited mode volumes and open geome- tries have emerged as new platforms for implementing cavity QED at room temperature. The use of infrared (IR) nano-antennas and tip nanoprobes to study strong light-matter coupling of molecular vibrations with the vacuum field can be exploited for IR quantum control with nanometer and femtosecond resolution. To accelerate the development of molecule-based quantum nano-photonic devices in the mid-IR, we propose a generally applicable semi-empirical methodology based on quantum optics to describe light-matter interaction in systems driven by femtosecond laser pulses. The theory is shown to reproduce recent experiments on the acceleration of the vibrational relaxation rate in infrared nanostructures, and also provide phys- ical insights for the implementation of coherent phase rotations of the near-field using broadband nanotips. We then apply the quantum framework to develop general tip-design rules for the exper- imental manipulation of vibrational strong coupling and Fano interference effects in open infrared resonators. We finally propose the possibility of transferring the natural anharmonicity of molecular vibrational levels to the resonator near-field in the weak coupling regime to implement intensity-dependent phase shifts of the coupled system response with strong pulses, and develop a vibrational chirping model to understand the effect. The semi-empirical quantum theory is equivalent to first- principles techniques based on Maxwell's equations, but its lower computational cost suggests its use a rapid design tool for the development of strongly-coupled infrared nanophotonic hardware for applications ranging from quantum control of materials to quantum information processing.

We investigate the photoinduced dynamics of perylene diimide dyads based on a donor-spacer-acceptor motif with polyyne spacers of varying length by pump-probe spectroscopy, time resolved fluorescence, chemical variation and quantum chemistry. While the dyads with pyridine based polyyne spacers undergo energy transfer with near-unity quantum efficiency, in the dyads with phenyl based polyyne spacers the energy transfer efficiency drops below 50%. This suggests the presence of a competing electron transfer process from the spacer to the energy donor as the excitation sink. Transient absorption spectra, however, reveal that the spacer actually mediates the energy transfer dynamics. The ground state bleach features of the polyyne spacers appear due to the electron transfer decay with the same time constant present in the rise of the ground state bleach and stimulated emission of the perylene energy acceptor. Although the electron transfer process initially quenches the fluorescence of the donor it does not inhibit energy transfer to the perylene energy acceptor. The transient signatures reveal that electron and energy transfer processes are sequential and indicate that the donor-spacer electron transfer state itself is responsible for the energy transfer. Through the introduction of a Dexter blocker unit into the spacer we can clearly exclude any through bond Dexter-type energy transfer. Ab initio calculations on the donor-spacer and the donor-spacer-acceptor systems reveal the existence of a bright charge transfer state that is close in energy to the locally excited state of the acceptor. Multipole-multipole interactions between the bright charge transfer state and the acceptor state enable the energy transfer. We term this mechanism coupled hole-transfer FRET. These dyads represent a first example that shows how electron transfer can be connected to energy transfer for use in novel photovoltaic and optoelectronic devices.

Weak-field coherent control of a two-photon transition in colloidal semiconductor nanocrystals (NCs) by tailored femtosecond laser pulses is demonstrated at room temperature. Ensembles of cadmium sulfide (CdS) and cadmium selenide (CdSe) NCs forming a colloidal suspension were irradiated by ultrashort infrared laser pulses being phase-modulated in frequency domain. The luminescence generated by electron−hole recombination after two-photon excitation (TPE) of the shaped pulses serves as a measure for NC excitation. In the experiment, we applied polynomial spectral phase functions of second-(GDD) and third-order (TOD), as well as phase jumps (θ-step), and studied the effect of the various shaped laser pulses on the excitation of both types of NCs. In view of potential applications in multilabel two-photon microscopy, both types of NCs are uniformely mixed in a single sample. We find that distinct pulse shapes enable selectivity among the excitation of both NC types in this mixture. Numerical simulations based on calculating the spectral overlap of the second order nonlinear optical spectrum and the TPE spectrum of the NCs are in good agreement with the measurement results. Analytic expressions derived for the second order power spectral density (PSD) because of combined GDD-TOD modulation and the θ-step rationalize this finding and show that GDD-TOD modulation provides a spectroscopic tool to investigate the TPE spectrum.

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.

The front cover artwork is provided by the groups of Henry Dube and Eberhard Riedle. The image shows an NMR tube at very low temperatures that is illuminated, leading to photoisomerization back and forth of a molecular motor. Quantum yields (Φ) for this process are measured in situ. Read the full text of the Research Article at 10.1002/cptc.202100232.