Integration of individual molecular components such as molecular motors or switches into larger meta-functional systems represents a current challenge at the forefront of molecular machine research. Here we present a modular supramolecular approach to relay the photoinduced geometry changes of a hemithioindigo based molecular motor into catalytic efficiency of a chemical reaction. Using the intrinsic chemical nature of the motor for recognition of different hydrogenbonding organocatalysts a greater than 10-fold modulation in binding affinity is achieved upon photoisomerization. This change in affinity is then translated effectively into control of catalytic competence of the organocatalysts without direct interference by the motor. As an example the organocatalysed Michael addition reaction between nitrostyrene and 3-methoxy-dimethyl aniline was modulated in situ by visible light irradiation. Thus, dynamic and reversible remote control of catalytic processes by the switching capacity of a hemithioindigo molecular motor is established in a multicomponent chemical system. The high intrinsic modularity of this approach presents further advantages, e.g., for easy tailoring of conditions or facile exchange of catalysts and reactions. These results represent a first stepping stone into integrated chemical networks regulated by molecular machines in a fully dynamic way.
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.
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