Direct Observation of Hemithioindigo‐Motor Unidirectionality

Abstract

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.

Recent Implementations of Molecular Photoswitches into Smart Materials and Biological Systems

“…The HTI scaffold has also been applied as a visible‐light‐driven molecular motor, in which four subsequent light‐ and heat‐induced isomerization steps result in a full turn around the central double bond (Figure ), similarly to that of prototypical Feringa motors . The initially observed fast (with a frequency of around 1 kHz at 20 °C) unidirectional rotation can be slowed down 50‐fold by increasing its steric hindrance; thus allowing for detailed investigations into the rotation mechanism . Recently, the same group reported a HTI‐based molecular motor fully devoid of thermal ratcheting in the ground state.…”

“… A HTI‐based visible‐light‐driven molecular motor with a frequency of up to 1000 Hz at ambient temperature . Exchange of the methoxy substituents for an isopropyl group slows the rotation down to 20 Hz, which enables direct 1 H NMR spectroscopic observation of all four states below −100 °C . Adapted with permission from ref.…”

Photoefficient 2nd generation molecular motors responsive to visible light

“…1C). [29][30][31] A related photon-only molecular motor was disclosed in 2018. 32 However, in view of future applications it is evident that there is a major need for distinct rotary motors which are able to perform full 360 rotations using low-energy light.…”

2‐(4′‐Pyridyl‐N‐oxide)‐Substituted Hemithioindigos as Photoresponsive Guests for a Super Aryl‐Extended Calix[4]pyrrole Receptor

“…Thes witching between the two states shows high efficiency and proceeds with little photofatigue. In recent years, HTI derivativesh ave been exploited as peptide modulators [13] and as prototypes of molecular logic gates, [14] machines, [15,16] and supramolecular receptors, [17] and their switching properties have been studied [18] and modulated [19] for possible applicationsa tt he bio-optical window (l = 650-900nm). The tetra-a isomers of aryl-and super aryl-extended calix [4]pyrroles are known to form thermodynamically and kinetically stable 1:1i nclusionc omplexes with pyridyl-N-oxide derivatives.T he association constant values determined for these inclusion complexes are in the range of 10 4 to 10 7 m À1 .…”