Tuning the Ground and Excited State Dynamics of Hemithioindigo Molecular Motors by Changing Substituents

Excited State Dynamics in Unidirectional Photochemical Molecular Motors

Unidirectional photochemically driven molecular motors (PMMs) convert the energy of absorbed light into continuous rotational motion. As such they are key components in the design of molecular machines. The prototypical and most widely employed class of PMMs is the overcrowded alkenes, where rotational motion is driven by successive photoisomerization and thermal helix inversion steps. The efficiency of such PMMs depends upon the speed of rotation, determined by the rate of ground state thermal helix inversion, and the quantum yield of photoisomerization, which is dependent on the excited state energy landscape. The former has been optimized by synthetic modification across three generations of overcrowded alkene PMMs. These improvements have often been at the expense of photoisomerization yield, where there remains room for improvement. In this perspective we review the application of ultrafast spectroscopy to characterize the excited state dynamics in PMMs. These measurements lead to a general mechanism for all generations of PMMs, involving subpicosecond decay of a Franck-Condon excited state to populate a dark excited state which decays within picoseconds via conical intersections with the electronic ground state. The model is discussed in the context of excited state dynamics calculations. Studies of PMM photochemical dynamics as a function of solvent suggest exploitation of intramolecular charge transfer and solvent polarity as a route to controlling photoisomerization yield. A test of these ideas for a first generation motor reveals a high degree of solvent control over isomerization yield. These results suggest a pathway to fine control over the performance of future PMMs.

Proton-Coupled Photochromic Hemithioindigo: Toward Photoactivated Chemical Sensing and Imaging

We report photoswitchable fluorescent hemithioindigos (HTIs) where the metastable E isomers were stabilized by the proton-bridged intramolecular hydrogen bond. Titration experiments and computational analysis indicated that the E isomers were much more basic than the Z isomers, which enabled photoactivated colorimetric and fluorescent pH response in solvents and polypropylene films. The HTIs exhibited reversibly switchable fluorescence with the Z isomers being the most fluorescent. Moreover, the HTIs were lysosomotropic and the kinetic fluorescence evolution during photoswitching was able to differentiate subcellular compartments with different pH. The combination of photoenhanced basicity, switchable fluorescence, and proton-coupled photochromism lay the groundwork for a broad range of chemical and biological applications.

A photochemical method to evidence directional molecular motions

Light driven synthetic molecular motors represent crucial building blocks for advanced molecular machines and their applications. A standing challenge is the development of very fast molecular motors able to perform rotations with kHz, MHz or even faster frequencies. Central to this challenge is the direct experimental evidence of directionality because analytical methods able to follow very fast motions rarely deliver precise geometrical insights. Here, a general photochemical method for elucidation of directional motions is presented. In a macrocyclization approach the molecular motor rotations are restricted and forced to proceed in two separate ~180° rotation-photoequilibria. Therefore, all four possible photoinduced rotation steps (clockwise and counterclockwise directions) can be quantified. Comparison of the corresponding quantum yields to the unrestricted motor delivers direct evidence for unidirectionality. This method can be used for any ultrafast molecular motor even in cases where no high energy intermediates are present during the rotation cycle.

Active Mechanical Threading by a Molecular Motor

Molecular motors transform external energy input into directional motions and offer exquisite precision for nano‐scale manipulations. To make full use of molecular motor capacities, their directional motions need to be transmitted and used for powering downstream molecular events. Here we present a macrocyclic molecular motor structure able to perform repetitive molecular threading of a flexible tetraethylene glycol chain through the macrocycle. This mechanical threading event is actively powered by the motor and leads to a direct translation of the unidirectional motor rotation into unidirectional translation motion (chain versus ring). The mechanism of the active mechanical threading is elucidated and the actual threading step is identified as a combined helix inversion and threading event. The established molecular machine function resembles the crucial step of macroscopic weaving or sewing processes and therefore offers a first entry point to a “molecular knitting” counterpart.

Designing light-driven rotary molecular motors

The ability to induce and amplify motion at the molecular scale has seen tremendous progress ranging from simple molecular rotors to responsive materials. In the two decades since the discovery of light-driven rotary molecular motors, the development of these molecules has been extensive; moving from the realm of molecular chemistry to integration into dynamic molecular systems. They have been identified as actuators holding great potential to precisely control the dynamics of nanoscale devices, but integrating molecular motors effectively into evermore complex artificial molecular machinery is not trivial. Maximising efficiency without compromising function requires conscious and judicious selection of the structures used. In this perspective, we focus on the key aspects of motor design and discuss how to manipulate these properties without impeding motor integrity. Herein, we describe these principles in the context of molecular rotary motors featuring a central double bond axle and emphasise the strengths and weaknesses of each design, providing a comprehensive evaluation of all artificial light-driven rotary motor scaffolds currently present in the literature. Based on this discussion, we will explore the trajectory of research into the field of molecular motors in the coming years, including challenges to be addressed, potential applications, and future prospects.

All-Red-Light Photoswitching of Indirubin Controlled by Supramolecular Interactions

Red-light responsiveness of photoswitches is a highly desired property for many important application areas such as biology or material sciences. The main approach to elicit this property uses strategic substitution of long-known photoswitch motives such as azobenzenes or diarylethenes. Only very few photoswitches possess inherent red-light absorption of their core chromophore structures. Here, we present a strategy to convert the long-known purple indirubin dye into a prolific red-light-responsive photoswitch. In a supramolecular approach, its photochromism can be changed from a negative to a positive one, while at the same time, significantly higher yields of the metastable E-isomer are obtained upon irradiation. E- to Z-photoisomerization can then also be induced by red light of longer wavelengths. Indirubin therefore represents a unique example of reversible photoswitching using entirely red light for both switching directions.

Steric Effects on the Thermal Processes of Hemithioindigo Based Molecular Motor Rotation

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.

Effect of charge-transfer enhancement on the efficiency and rotary mechanism of an oxindole-based molecular motor

Harvesting energy and converting it into mechanical motion forms the basis for both natural and artificial molecular motors. Overcrowded alkene-based light-driven rotary motors are powered through sequential photochemical and thermal steps. The thermal helix inversion steps are well characterised and can be manipulated through adjustment of the chemical structure, however, the insights into the photochemical isomerisation steps still remain elusive. Here we report a novel oxindole-based molecular motor featuring pronounced electronic push-pull character and a four-fold increase of the photoisomerization quantum yield in comparison to previous motors of its class. A multidisciplinary approach including synthesis, steady-state and transient absorption spectroscopies, and electronic structure modelling was implemented to elucidate the excited state dynamics and rotary mechanism. We conclude that the charge-transfer character of the excited state diminishes the degree of pyramidalisation at the alkene bond during isomerisation, such that the rotational properties of this oxindole-based motor stand in between the precessional motion of fluorene-based molecular motors and the axial motion of biomimetic photoswitches.

Tuning the Ground and Excited State Dynamics of Hemithioindigo Molecular Motors by Changing Substituents

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 tailoring for specific applications. In this work, three different hemithioindigo-based molecular motors are investigated and all four steps in their complete unidirectional rotation are unraveled fully quantitatively. Transient absorption spectroscopy across twelve orders 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 known full 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 rotation speed. The substitution pattern in close proximity to the rotation axle influences the excited and ground state properties. Reduction of electron donation and concomitant increase of steric hindrance leads to faster photoisomerization reactions with quasi-ballistic behavior, but also to a slight decrease in the quantum efficiency. The expected decelerating effects of increased sterics are primarily manifested in the ground state. A promising approach for next-generation hemithioindigo motors is to elevate electron donation at the rotor fragment followed by an increase of steric hindrance.