Dube Research Group - Faculty for Chemistry and Pharmacy

Welcome to the Research Group of Henry Dube!

Functional Supramolecular Systems

Simultaneous Complementary Photoswitching of Hemithioindigo Tweezers for Dynamic Guest Relocalization

Remote control of complex molecular behavior and function is one key problem in modern chemistry. Using light signaling for this purpose has many advantages, however the integration of different photo processes into a wholesome yet complex system is highly challenging. Here we report an alternative approach to increase complexity of light control-simultaneous complementary photoswitching-in which spectral overlap is used as an advantage to drastically reduce the signaling needed for controlling multipart supramolecular assemblies. Two photoswitchable molecular tweezers respond to the same light signals with opposite changes in their binding affinities. In this way the configuration of two host tweezers and ultimately the dynamic relocation of a guest molecule can be trigged by only one signal reversibly in the same solution. This approach should provide a powerful tool for the construction of sophisticated, integrated, and multi-responsive smart molecular systems in any application driven field of chemistry.

published with open access in Nature Communications.

Photocontrol of Polar Aromatic Interactions by a bis-Hemithioindigo Based Helical Receptor

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.

Molecular Machines

Complete Mechanism of Hemithioindigo Motor Rotation

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 light-driven 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.

published in J. Am. Chem. Soc.

Direct Observation of Hemithioindigo-Motor Unidirectionality

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.

Sunlight Powered kHz Rotation of a Hemithioindigo Based Molecular Motor

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.

published with open access in Nature Communications.

Photochemistry and Photophysics

Hemiindigo - Highly Bistable Photoswitching at the Biooptical Window

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.

Ingredients to TICT Formation in Donor Substituted Hemithioindigo

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.

Bistable Photoswitching of Hemithioindigo with Green and Red Light - Entry Point to Advanced Molecular Digital Information Processing

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

Twisted Hemithioindigo Photoswitches: Solvent Polarity Determines the Type of Light-Induced Rotations

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.

Making Fast Photoswitches Faster - Using Hammett Analysis to Understand the Limit of Donor-Acceptor Approaches for Faster Hemithioindigo Photoswitches

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.

more coming soon...

 
 

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