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.

Hemithioindigo molecular motors undergo very fast unidirectional rotation upon irradiation with visible light, which has prevented a complete analysis of their working mechanism. In this work, we have considerably slowed down their motion by using a new synthesis for sterically hindered motor derivatives. This method allowed the first observation of all four intermediate states populated during rotation. The exact order in which each isomeric state is formed under irradiation conditions was elucidated using low temperature H NMR spectroscopy in conjunction with other analytical methods. At the same time, complete unidirectionality could also be directly shown. Access to slowly rotating hemithioindigo motors opens up a plethora of new applications for visible-light-induced unidirectional motions, especially in areas such as catalysis, smart materials, and supramolecular chemistry.

Substituted indigo derivatives undergo photoisomerization of the central double bond if both nitrogen atoms are functionalized. Indigo itself however does not photoisomerize because of a competing and highly efficient excited‐state proton transfer. In this work, we show that also mono‐arylated indigo undergoes photoisomerization despite still possessing one nitrogen‐bound proton and the likely presence of a competing intramolecular excited‐state proton transfer. The two different isomers exhibit strongly different absorptions and therefore can be distinguished by the naked eye. Different to diaryl‐substituted indigo, thermal decay of the metastable cis isomers of mono‐arylated derivatives can be greatly accelerated by small amounts of water (by more than a factor of 300 for measured half‐lives). Such tunability is of high interest for applications that require quick and autonomous switching‐off of photoresponsive trigger units.

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.

Tuning the thermal behavior of light driven molecular motors is fundamentally important for their future rational design. In many molecular motors thermal ratcheting steps are comprised of helicity inversions, energetically stabilizing the initial photoproducts. In this work we investigated a series of five hemithioindigo (HTI) based molecular motors to reveal the influence of steric hindrance in close proximity to the rotation axle on this process. Applying a high yielding synthetic procedure, we synthesized constitutional isomeric derivatives to distinguish between substitution effects at the aromatic and aliphatic position on the rotor fragment. The kinetics of thermal helix inversions were elucidated using low temperature 1 H NMR spectroscopy and an in situ irradiation technique. In combination with a detailed theoretical description, a comparative analysis of substituent effects on the thermal helix inversions of the rotation cycle is now possible. Such deeper understanding of the rotational cycle of HTI molecular motors is essential for speed regulation and future applications of visible light triggered nanomachines.

Hemithioindigo molecular motors undergo very fast unidirectional rotation upon irradiation with visible light, which has prevented ac omplete analysis of their working mechanism. In this work, we have considerably slowed down their motion by using an ew synthesis for sterically hindered motor derivatives.This method allowed the first observation of all four intermediate states populated during rotation. The exact order in whiche ach isomeric state is formed under irradiation conditions was elucidated using low temperature 1 HNMR spectroscopyi nc onjunction with other analytical methods.A tt he same time,c omplete unidirectionality could also be directly shown. Access to slowly rotating hemithioindigo motors opens up ap lethora of new applications for visible-light-induced unidirectional motions,especially in areas such as catalysis,smart materials,and supramolecular chemistry.

The front cover artwork is provided by Thomas Bartelmann and Henry Dube of the Department of Chemistry, Ludwig‐Maximilians‐Universität Munich (Germany). The image shows photoisomerization of mono‐arylated indigo upon irradiation with red light. Read the full text of the Article at https://doi.org/10.1002/cptc.201700228.

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.