Hemithioindigo molecular motors undergo very fast unidirectional rotation upon irradiation with visible light, which prevented a complete analysis of their working mechanism. In this work we have considerably slowed down their motion via a new synthesis for sterically hindered motor derivatives. This method allowed for the first time 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 1H NMR spectroscopy in conjunction with other analytic methods. At the same time complete unidirectionality could also directly be shown. Access to slow rotating hemithioindigo motors opens up a plethora of new applications for visible light induced unidirectional motions, especially in areas such as catalysis, smart materials, or supramolecular chemistry.
published in Angew. Chem. Int. Ed.
Damit sich Moleküle wie Motoren verhalten, dürfen sie sich nur in eine Richtung bewegen. Ob das funktioniert, hängt davon ab, ob in den Molekülen Asymmetrie vorliegt. Diese molekularen Motoren bewegen sich mit Frequenzen im Kilo- und Megahertzbereich; Licht gibt ihnen die Energie dafür.
published in Nachrichten aus der Chemie.
Photodriven molecular motors are able to convert light energy into directional motion and hold great promise as miniaturized powering units for future nanomachines. In the current state of the art, considerable efforts have still to be made to increase the efficiency of energy transduction and devise systems that allow operation in ambient and non-damaging conditions with high rates of directional motions. Especially the need for ultraviolet light to induce the motion of virtually all available light-driven motors hampers the broad applicability of these systems. We describe here a hemithioindigo-based molecular motor, which is powered exclusively by nondestructive visible light (up to 500 nm) and rotates completely directionally with kHz frequency at 20 C. This is the fastest directional motion of a synthetic system driven by visible light to date permitting materials and biocompatible irradiation conditions to establish similarly high speeds as natural molecular motors.
open access in Nature Communications.
Hemiindigo is a long known chromophore that absorbs in the blue part of the spectrum but has almost completely been ignored as potential photoswitch. Herein we show how the absorption of hemiindigo is shifted to the red part of the visible spectrum and how nearly perfect photoswitching can be achieved using blue or green and red light. Five derivatives were investigated giving very high isomeric yields in both switching directions, i.e. >90% E isomer after irradiation with 470 to 530 nm light and 99% Z isomer with 590 up to 680 nm light. At the same time the thermal bistability is extraordinarily high leading to half-lives of the pure isomeric states of up to 83 years at 25 °C. The herein developed photoswitches show photochromism in the visible enabling the two isomeric states to be distinguished by the naked eye. Substituted hemiindigos therefore constitute extremely promising new photoswitches with excellent properties for applications in biology, chemistry, or material sciences.
published in J. Am. Chem. Soc.
TICT formation in hemithioindigo photoswitches has recently been reported and constitutes a second deexcitation pathway complementary to photoisomerization. Typically this behavior is not found for this type of photoswitches and it takes special geometric and electronic conditions to realize it. Here we present a systematic study that identifies the molecular preconditions leading to TICT formation in donor substituted hemithioindigo, which can thus serve as a frame of reference for other photoswitching systems. By varying the substitution pattern and providing an in-depth physical characterization including time resolved and quantum yield measurements we found that neither a ground state pre-twisting along the rotatable single bond nor the introduction of strong push-pull character across the photoisomerizable double bond alone leads to formation of TICT states. Only the combination of both ingredients produces light induced TICT behavior in polar solvents.
published in J. Phys. Chem. Lett.
Photoswitches reacting to visible light instead of harmful UV irradiation are of very high interest due to the mild and broadly compatible conditions of their operation. Shifting the absorption into the red region of the electromagnetic spectrum usually comes at the cost of losing thermal stability of the metastable state - the switch switches off by itself. Only recently have photoswitches become available that combine visible light responsiveness with high bistability. However, shifting the wavelengths for bistable photoswitching beyond 600 nm is still a great challenge without involving secondary processes such as two-photon absorption or sensitization. We present a simple hemithioindigo photoswitch, that can efficiently be photoisomerized using green and red light while maintaining a high thermal barrier of the metastable state. This highly sought after properties allow for selective switching in a mixture of hemithioindigo dyes. In addition protonation can be used as second independent input altering the light response of the switch and allows construction of advanced molecular digital information processing devices. This is demonstrated by realizing a broad variety of logical operations covering combinational and sequential logic behavior. By making use of the protonation-induced loss of thermal bistability a high security keypad lock can be realized, which distinguishes the sequences of three different inputs and additionally erases its unlock state after a short time.
published in Chemistry - A European Journal
Controlling the internal motions of molecules by outside stimuli is a decisive task for the generation of responsive and complex molecular behavior and functionality. Light induced structural changes of photoswitches are of special high interest due to the ease of signal application and high repeatability. Typically photoswitches use one reaction coordinate in their switching process and change between two more or less defined states. Here we report on new twisted hemithioindigo photoswitches enabling two different reaction coordinates to be used for the switching process. Depending on the polarity of the solvent either complete single bond (in DMSO) or double bond (in cyclohexane) rotation can be induced by visible light. This mutual independent switching establishes an unprecedented two-dimensional control of intramolecular rotations in this class of photoswitches. The mechanistic explanation involves formation of highly polar twisted intramolecular charge-transfer species (TICT) in the excited state and is based on a large body of experimental quantifications, most notably ultrafast spectroscopy and quantum yield measurements in solvents of different polarity. The concept of pre-twisting in the ground state to open new independent reaction coordinates in the excited state should be transferable to other photoswitching systems.
published in J. Am. Chem. Soc.
Hemithioindigo (HTI) is an emerging photoswitch with many advantageous properties compared to the commonly used photoswitches like azobenzenes, spiropyranes, or dithienylethenes. In this DIGEST the syntheses, physical and photophysical properties of HTI photoswitches and mechanistic explanations for the latter are reviewed. Emphasis will be placed on those distinct properties that render HTIs into unique phototools. Additionally, a broad variety of applications ranging from supramolecular to biological chemistry is presented to highlight the great potential of HTIs as upcoming, alternative photoswitches.
published in Tetrahedron Letters.
Hemithioindigo (HTI) photoswitches have a tremendous potential for biological and supramolecular applications due to their absorptions in the visible in conjunction with ultrafast photoisomerization and high thermal bistability. Being able to rationally tailor their photophysical properties for a specific application is the key to exploit the full potential of HTIs as photoswitching tools. In this work we used time-resolved absorption spectroscopy and Hammett analysis to discover an unexpected principal limit to the photoisomerization rate for donor substituted HTIs. Using stationary absorption and fluorescence measurements in combination with theoretical investigations, we offer a detailed mechanistic explanation for the observed rate limit. An alternative way of approaching and possibly even exceeding the maximum rate by multiple donor substitution is demonstrated, which gave access to the fastest HTI photoswitch reported to date.
published in Chemistry - A European Journal.
The first example of a bis-Hemithioindigo (bis-HTI) based molecular receptor was realized. Its folding and selective binding affinity for aromatic guest molecules can precisely be controlled by visible light and heat. The thermodynamically stable state of the bis-HTI is the s-shaped planar Z,Z-configuration. After irradiation with 420 nm light only the E,Z-configuration is formed in a highly selective photoisomerization. The E,Z-isomer adopts a helical conformation because of the conscious implementation of repulsive sterical interactions. The E,Z-configured helix is able to recognize electron poor aromatic guests exclusively via polar aromatic interactions and also distinguishes between regioisomers. After heating the Z,Z-configuration is completely restored and the aromatic guest molecule is efficiently released.
published in Chemistry - A European Journal.
more coming soon...