Photoisomerization of hemithioindigo compounds: Combining solvent- and substituent- effects into an advanced reaction model

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.

Mechanistic Elucidation of the Hula-Twist Photoreaction in Hemithioindigo

The Hula-Twist (HT) photoreaction represents a fundamental photochemical pathway for bond isomerizations and is defined by the coupled motion of a double bond and an adjacent single bond. This photoreaction has been suggested as the defining motion for a plethora of light-responsive chromophores such as retinal within opsins, coumaric acid within photoactive yellow protein, or vitamin D precursors, and stilbenes in solution. However, due to the fleeting character of HT photoproducts a direct experimental observation of this coupled molecular motion was severely hampered until recently. To solve this dilemma, the Dube group has designed a molecular framework able to deliver unambiguous experimental evidence of the HT photoreaction. Using sterically crowded atropisomeric hemithioindigo (HTI) the HT photoproducts are rendered thermally stable and can be observed directly after their formation. However, following the ultrafast excited state process of the HT photoreaction itself has not been achieved so far and thus crucial information for an elementary understanding is still missing. In this work, we present the first ultrafast spectroscopy study of the HT photoreaction in HTI and probe the competition between different excited state processes. Together with extensive excited state calculations a detailed mechanistic picture is developed explaining the significant solvent effects on the HT photoreaction and revealing the intricate interplay between productive isomerizations and unproductive twisted intramolecular charge transfer (TICT) processes. With this study essential insights are thus gained into the mechanism of complex multibond rotations in the excited state, which will be of primary importance for further developments in this field.

Heterocyclic Hemithioindigos: Highly Advantageous Properties as Molecular Photoswitches

A survey of heterocyclic hemithioindigo photoswitches is presented identifying a number of structural motives with outstanding property profiles. The highly sought-after combination of pronounced color change, quantitative switching in both directions, exceptional high quantum yields, and tunable high thermal stability of metastable states can be realized with 4-imidazole, 2-pyrrole, and 3-indole-based derivatives. In the former, an unusual preorganization using isomer selective chalcogen- and hydrogen bonding allows to precisely control geometry changes and tautomerism upon switching. Heterocyclic hemithioindigos thus represent highly promising photoswitches with advanced capabilities that will be of great value to anyone interested in establishing defined and reversible control at the molecular level.

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

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.

Molecular structure, DNA binding mode, photophysical properties and recommendations for use of SYBR Gold

SYBR Gold is a commonly used and particularly bright fluorescent DNA stain, however, its chemical structure is unknown and its binding mode to DNA remains controversial. Here, we solve the structure of SYBR Gold by NMR and mass spectrometry to be [2-[N-(3-dimethylaminopropyl)-N-propylamino]-4-[2,3-dihydro-3-methyl-(benzo-1,3-thiazol-2-yl)-methylidene]-1-phenyl-quinolinium] and determine its extinction coefficient. We quantitate SYBR Gold binding to DNA using two complementary approaches. First, we use single-molecule magnetic tweezers (MT) to determine the effects of SYBR Gold binding on DNA length and twist. The MT assay reveals systematic lengthening and unwinding of DNA by 19.1° ± 0.7° per molecule upon binding, consistent with intercalation, similar to the related dye SYBR Green I. We complement the MT data with spectroscopic characterization of SYBR Gold. The data are well described by a global binding model for dye concentrations ≤2.5 μM, with parameters that quantitatively agree with the MT results. The fluorescence increases linearly with the number of intercalated SYBR Gold molecules up to dye concentrations of ∼2.5 μM, where quenching and inner filter effects become relevant. In summary, we provide a mechanistic understanding of DNA-SYBR Gold interactions and present practical guidelines for optimal DNA detection and quantitative DNA sensing applications using SYBR Gold.

Photopharmacology of Proteolysis-Targeting Chimeras: A New Frontier for Drug Discovery

Photopharmacology is an emerging field that uses light to precisely control drug activity. This strategy promises to improve drug specificity for reducing off-target effects. Proteolysis-targeting chimeras (PROTACs) are an advanced technology engineered to degrade pathogenic proteins through the ubiquitin-proteasome system for disease treatment. This approach has the potential to target the undruggable proteome via event-driven pharmacology. Recently, the combination strategy of photopharmacology and PROTACs has gained tremendous momentum for its use in the discovery and development of new therapies. This review systematically focuses on PROTAC-based photopharmacology. Herein, we provide an overview of the new and vibrant research on photoPROTACs, discuss the advantages and disadvantages of this approach as a biological tool, and outline the challenges it faces in a clinical setting.

Waveform control of molecular dynamics close to a conical intersection

Conical intersections are ubiquitous in chemical systems but, nevertheless, extraordinary points on the molecular potential energy landscape. They provide ultra-fast radiationless relaxation channels, their topography influences the product branching, and they equalize the timescales of the electron and nuclear dynamics. These properties reveal optical control possibilities in the few femtosecond regime. In this theoretical study, we aim to explore control options that rely on the carrier envelope phase of a few-cycle IR pulse. The laser interaction creates an electronic superposition just before the wave packet reaches the conical intersection. The imprinted phase information is varied by the carrier envelope phase to influence the branching ratio after the conical intersection. We test and analyze this scenario in detail for a model system and show to what extent it is possible to transfer this type of control to a realistic system like uracil.

Oxidized Hemithioindigo Photoswitches-Influence of Oxidation State on (Photo)physical and Photochemical Properties

The photophysical and photochemical properties of sulfoxide and sulfone derivatives of hemithioindigo photoswitches are scrutinized and compared to the unoxidized parent chromophores. Oxidation results in significantly blue-shifted absorptions and mostly reduction of photochromism while thermal stabilities of individual isomers remain largely unaltered. Effective photoswitching takes place at shorter wavelengths compared to parent hemithioindigos and high isomeric yields can be obtained reversibly in the respective photostationary states. Reversible solid-state photoswitching is observed for a twisted sulfone derivative accompanied by visible color changes. These results establish oxidized hemithioindigo photoswitches as promising and versatile tools for robust light-control of molecular behavior for a wide range of applications.

Green light powered molecular state motor enabling eight-shaped unidirectional rotation

Molecular motors convert external energy into directional motions at the nano-scales. To date unidirectional circular rotations and linear motions have been realized but more complex directional trajectories remain unexplored on the molecular level. In this work we present a molecular motor powered by green light allowing to produce an eight-shaped geometry change during its unidirectional rotation around the central molecular axis. Motor motion proceeds in four different steps, which alternate between light powered double bond isomerizations and thermal hula-twist isomerizations. The result is a fixed sequence of populating four different isomers in a fully unidirectional trajectory possessing one crossing point. This motor system opens up unexplored avenues for the construction and mechanisms of molecular machines and will therefore not only significantly expand the toolbox of responsive molecular devices but also enable very different applications in the field of miniaturized technology than currently possible.

Molecular motors have predominantly been limited to circular or linear directional motions. Here, the authors design a molecular motor that moves unidirectionally along a more complex figure-eight trajectory by a distinct four-step mechanism that alternates between photochemical and thermal reaction steps.