Circular Dichroism Photoswitching with a Twist: Axially Chiral Hemiindigo

Hemiphosphoindigos as a platform for chiroptical or water soluble photoswitching

Photoswitches are important molecular tools to precisely control the behavior of matter by using light irradiation. They have found application in virtually all applied chemical fields from chemical biology to material sciences. However, great challenges remain in advanced property design including tailored chiroptical responses or water solubility. Here, hemiphosphoindigo (HPI) photoswitches are presented as capable phosphorus-based photoswitches and a distinct addition to the established indigoid chromophore family. Phosphinate is embedded in the core indigoid chromophore and the resulting optimized photoswitches display high thermal stabilities, excellent fatigue resistance and high isomer enrichment. A series of planar, twisted and heterocyclic HPIs are investigated to probe design strategies for advantageous photophysical properties. The phosphinate provides a platform for easily accessible, water-soluble photoswitches, especially interesting for biological applications. Its chiral nature further allows light-induced modulation of chiroptical properties. HPIs therefore open up a distinct structural space for photoswitch generation and advanced light-responsive applications.

Silanediol-Bay-Bridge Rigidified Axially Chiral Perylene Bisimide

Chiral organic molecules with a complementing π-structure are highly desired to obtain materials with good semiconducting properties and pronounced chirality effects in the visible region. Herein, we introduce a novel design strategy to achieve an axially chiral and rigid perylene bisimide (PBI) dye by attaching the chirality-inducing 2,2'-biphenoxy moiety at one side of the bay area and the rigidity-inducing di-tert-butylsilanediol bridge on the other side. This yielded a new bay-functionalized PBI derivative carrying the combination of a highly rigid and, simultaneously, an axially chiral perylene core. As a result, the derivative exhibits well-resolved absorption and emission spectra in the visible region, with a fluorescence quantum yield close to unity. Furthermore, the M- and P-enantiomers were found to be stable with a racemization barrier of 102 kJ mol-1 and, hence, could be successfully separated by chiral chromatography and studied by circular dichroism (CD) spectroscopy. This rigidified chiral-PBI could also be crystallized and analyzed by X-ray diffraction, showing the highest torsion angle of the perylene core with a value of up to 30.3° in the family of PBIs carrying the same di-tert-butylsilanediol bridge.

Combined Photopolymerization and Localized Photochromism by Aza-Diarylethene and Hemiindigo Synergy

Molecular photoswitches produce light-controlled changes at the nanometer scale and can therefore be used to alter the states and behavior of materials in a truly bottom-up fashion. Here an escalating photonic complexity of material property control with light is shown using a recently developed aza-diarylethene in combination with hemiindigo (HI) photoswitches. First, aza-diarylethene can be used as a photoswitch in polystyrene (PS) to reversibly inscribe relief-type 3D structures into PS. Second, aza-diarylethene can further be used as a photoinitiator for light-induced polymerization of methyl acrylate (MA), demonstrating for the first time light-controlled chemical reactivity control with its zwitterionic switching state. Third, aza-diarylethene and HIs are implemented into aza-diarylethene polymerized MA, generating photochromic polymers. At the fourth level, a binary mixture allows to synergize aza-diarylethene-induced photopolymerization with localized photochromism changes of the simultaneously entrapped functional HI. With such multilevel light response, the utility of this particular photoswitch combination for applications in advanced photonic materials is demonstrated.

Rearrangement of C2-Spirooxindoles: Conversion to the 2-Hydroxyhemi-Indigo and Chromenoindole

This study demonstrates the rearrangement of C2-spirooxindoles to the 2-hydroxyhemi-indigo and chromenoindole. The N-H-spirooxindole exhibits double proton translocation and its conversion to the (Z)-2-hydroxyhemi-indigo photoswitch with trifluoroacetic acid, while the N-methyl-spirooxindole undergoes structural rearrangement to the chromenoindole. The mechanism of the reactions was proposed, and the structure of the products was confirmed by one-dimensional (1D) and two-dimensional (2D) NMR spectra and X-ray structure analysis. The photoswitching performance of (Z)-2-hydroxyhemi-indigo, which allows the stabilization of the E-switched form by intramolecular hydrogen bonding, has been studied in solvents of different polarities. It was found that in the less polar solvents, the E-switched metastable isomer is characterized by high stability.

A photoactivated chiral molecular clamp rotated by selective anion binding

Developing chiral molecular platforms that respond to external fields provides opportunities for designing smart chiroptical materials. Herein, we introduce a molecular clamp whose chiral properties can be turned on by photoactivation. Selective anion binding achieves rational tuning of the conformations and chiroptical properties of the clamp, including circular dichroism and circularly polarized luminescence. Cyanostilbene segments were conjugated to chiral amines with a rotatable axis. Negligible chiroptical signals were significantly enhanced through a light illumination-induced isomerization. Binding with halide ions (F-, Cl- and Br-) enables chiroptical inversion and subsequent amplification of the resulting opposite handedness state by photo treatment. In contrast, the larger I- and NO3 - ions failed to achieve chiroptical inversion. Also the handedness inversion was hampered in conformationally locked amines. Density-functional theory-based computational studies and experimental results reveal a structural transformation that proceeds from a butterfly-like open geometry to a closed V-shaped state initiated by four hydrogen bonds and the rotatable axis. This work illustrates design protocols for use in smart chiroptical molecular platforms mediated by photo treatment and anion binding.

Rhodanine-Based Chromophores: Fast Access to Capable Photoswitches and Application in Light-Induced Apoptosis

Molecular photoswitches are highly desirable in all chemistry-related areas of research. They provide effective outside control over geometric and electronic changes at the nanoscale using an easy to apply, waste-free stimulus. However, simple and effective access to such molecular tools is typically not granted, and elaborate syntheses and substitution schemes are needed in order to obtain efficient photoswitching properties. Here we present a series of rhodanine-based photoswitches that can be prepared in one simple synthetic step without requiring elaborate purification. Photoswitching is induced by UV and visible light in both switching directions, and thermal stabilities of the metastable states as well as quantum yields are very high. An additional benefit is the hydrogen-bonding capacity of the rhodanine fragment, which enables applications in supramolecular or medicinal chemistry. We further show that the known rhodanine-based inhibitor SMI-16a is a photoswitchable apoptosis inducer. The biological activity of SMI-16a can effectively be switched ON or OFF by reversible photoisomerization between the inactive E and the active Z isomer. Rhodanine-based photoswitches therefore represent an easy to access and highly valuable molecular toolbox for implementing light responsiveness to the breadth of functional molecular systems.

Near-Infrared, Organic Chiroptic Switch with High Dissymmetry Factors

Here we unveil a chiral molecular redox switch derived from PDI-based twistacenes─chPDI[2] that has the remarkable attributes of high-intensity and a broadband chiral response. This material exhibits facile, stable, and reversible multistate chiroptical switching behavior over a broad active wavelength range close to 700 nm, encompassing ultraviolet, visible, and near-infrared regions. Upon reduction, chPDI[2] exhibits a substantial increase in the amplitude of its circular dichroic response, with an outstanding |ΔΔε| > 300 M-1 cm-1 and a high dissymmetry factor of 3 × 10-2 at 960 nm. DFT calculations suggest that the long wavelength CD signal for doubly reduced chPDI[2] originates from excitation of the PDI backbone to the π* orbital of the bridging alkene. Importantly, the dimer's molecular contortion facilitates ionic diffusion, enabling chiral switching in solid state films. The high dissymmetry factors and near-infrared response establish chPDI[2] as a unique chiroptic switch.

Diaryl-hemiindigos as visible light, pH, and heat responsive four-state switches and application in photochromic transparent polymers

Photoswitches are indispensable tools for responsive chemical nanosystems and are used today in almost all areas of the natural sciences. Hemiindigo (HI) derivatives have recently been introduced as potent photoswitches, but their full applicability has been hampered by the limited possibilities of their functionalization and structural modification. Here we report on a short and easy to diversify synthesis yielding diaryl-HIs bearing one additional aromatic residue at the central double bond. The resulting chromophores offer an advantageous property profile combining red-light responsiveness, high thermal bistability, strong isomer accumulations in both switching directions, strong photochromism, tunable acid responsiveness, and acid gating. With this progress, a broader structural realm becomes accessible for HI photoswitches, which can now be synthetically tailored for advanced future applications, e.g., in research on molecular machines and switches, in studies of photoisomerization mechanisms, or in the generation of smart and addressable materials. To showcase the potential of these distinct light-responsive molecular tools, we demonstrate four-state switching, chemical fueling, and reversible inscription into transparent polymers using green and red light as well as acid/base stimuli, in addition to a comprehensive photochemical study of all compounds.

Supramolecular Strategy to Achieve Distinct Optical Characteristics and Boosted Chiroptical Enhancement Based on the Closed Conformation of Platinum(II) Complexes

Synthesis of chiral molecules for understanding and revealing the expression, transfer, and amplification of chirality is beneficial to explore effective chiral medicines and high-performance chiroptical materials. Herein, we report a series of square-planar phosphorescent platinum(II) complexes adopting a dominantly closed conformation that exhibit efficient chiroptical transfer and enhancement due to the nonclassical intramolecular C-H···O or C-H···F hydrogen bonds between bipyridyl chelating and alkynyl auxiliary ligands as well as the intermolecular π-π stacking and metal-metal interactions. The spectroscopic and theoretical calculation results demonstrate that the chirality and optic properties are regulated from the molecular level to hierarchical assemblies. Notably, a 154 times larger g_abs value of the circular dichroism signals is obtained. This study provides a feasible design principle to achieve large chiropticity and control the expression and transfer of the chirality.

High-throughput determination of enantiopurity in atroposelective synthesis of aryl triazoles

Atropisomeric scaffolds are a common design element found in pharmaceuticals, many deriving from an N-C axis of chirality. The handedness associated with atropisomeric drugs is oftentimes crucial for their efficacy and/or safety. With the increased use of high-throughput screening (HTS) for drug discovery, the need for rapid enantiomeric excess (ee) analysis is needed to keep up with the fast workflow. Here, we describe a circular dichroism (CD) based assay that could be applied to the ee determination of N-C axially chiral triazole derivatives. Analytical samples for CD were prepared from crude mixtures by three sequential steps: liquid-liquid extraction (LLE), a wash-elute, and complexation with Cu(ii) triflate. The initial ee measurement of five samples of atropisomer 2 was conducted by the use of a CD spectropolarimeter with a 6-position cell changer, resulting in errors of less than 1% ee. High-throughput ee determination was performed on a CD plate reader using a 96-well plate. A total of 28 atropisomeric samples (14 for 2 and 14 for 3) were screened for ee. The CD readings were completed in 60 seconds with average absolute errors of ±7.2% and 5.7% ee for 2 and 3, respectively.

A cross-conjugation approach for high-performance diaryl-hemithioindigo photoswitches

Diaryl-hemithioindigos (diaryl-HTIs) are derivatives of a novel class of highly functionalized indigoid chromophores. In this work a systematic study of the electronic effects on their photoswitching reveals the design principles for achieving an excellent property profile. Two key elements need to be invoked for perfect diaryl-HTI performance, first introduction of strong electron donors and second establishment of cross-conjugation. The resulting photoswitches combine high thermal stability, large extinction coefficients, red-light responsiveness, pronounced photochromism, and strong isomer accumulation in the photostationary states with precise geometry changes. By using the inherent basicity of their strong electron donor moiety, diaryl-HTIs can be rendered into very potent tools for molecular logic applications. We demonstrate a variety of binary logic setups as well as sophisticated three- and four-input keypad locks for sequential logic operations. Three distinct states and up to four different stimuli are invoked for this multi-level molecular information processing. Diaryl-HTIs have thus entered the stage as very capable and promising photoswitch motives for anyone interested in reversible visible- and red-light as well as multi-stimuli responsive molecular behavior.

Nanoporous Films with Oriented Arrays of Molecular Motors for Photoswitching the Guest Adsorption and Diffusion

Molecular motors are fascinating nanomachines. However, constructing smart materials from such functional molecules presents a severe challenge in material science. Here, we present a bottom-up layer-by-layer assembly of oriented overcrowded-alkene molecular motors forming a crystalline metal-organic framework thin film. While all stator parts of the overcrowded-alkene motors are oriented perpendicular to the substrate, the rotors point into the pores, which are large enough allowing for the light-induced molecular rotation. Taking advantage of the thin film's transparency, the motor rotation and its activation energy are determined by UV/Vis spectroscopy. As shown by gravimetric uptake experiments, molecular motors in crystalline porous materials are used, for the first time, to control the adsorption and diffusion properties of guest molecules in the pores, here, by switching with light between the (meta-)stable states. The work demonstrates the potential of designed materials with molecular motors and indicates a path for the future development of smart materials.

Mechanistic Insights into the Photoisomerization of N,N'-Disubstituted Indigos

N,N'-disubstituted indigos are photoswitchable molecules that have recently caught the attention due to their addressability by red-light. When alkyl and aryl groups are utilized as the N-substituents, the thermal half-lives of Z isomers can be tuned independently while maintaining the advantageous red-shifted absorption spectra. To utilize these molecules in real-world applications, it is of immense importance to understand how their molecular structures as well as the environment influence their switching properties. To this end, we probed their photoisomerization mechanism by carrying out photophysical and computational studies in solvents of different polarities. The fluorescence and transient absorption experiments suggest for more polar excited and transition states, which explains the bathochromic shifts of absorption spectra and shorter thermal half-lives. On the other hand, the quantum chemical calculations reveal that in contrast to N-carbonyl groups, N-alkyl and N-aryl substituents are not strongly conjugated with the indigo chromophore and can thus serve as a tool for tuning the thermal stability of Z isomers. Both approaches are combined to provide in-depth understandings of how indigos undergo photoswitching as well as how they are influenced by N-substituent and the chemical surroundings. These mechanistic insights will serve as guiding principles for designing molecules eyeing broader applications.

Conformational Properties of New Thiosemicarbazone and Thiocarbohydrazone Derivatives and Their Possible Targets

The structure assignment and conformational analysis of thiosemicarbazone KKI15 and thiocarbohydrazone KKI18 were performed through homonuclear and heteronuclear 2D Nuclear Magnetic Resonance (NMR) spectroscopy (2D-COSY, 2D-NOESY, 2D-HSQC, and 2D-HMBC) and quantum mechanics (QM) calculations using Functional Density Theory (DFT). After the structure identification of the compounds, various conformations of the two compounds were calculated using DFT. The two molecules showed the most energy-favorable values when their two double bonds adopted the E configuration. These configurations were compatible with the spatial correlations observed in the 2D-NOESY spectrum. In addition, due to the various isomers that occurred, the energy of the transition states from one isomer to another was calculated. Finally, molecular binding experiments were performed to detect potential targets for KKI15 and KKI18 derived from SwissAdme. In silico molecular binding experiments showed favorable binding energy values for all four enzymes studied. The strongest binding energy was observed in the enzyme butyrylcholinesterase. ADMET calculations using the preADMET and pKCSm software showed that the two molecules appear as possible drug leads.

Visible light activated BINOL-derived chiroptical switches based on boron integrated hydrazone complexes

Chiral optical switches, which use light to control chirality in a reversible manner, offer unique properties and fascinating prospects in the areas of molecular switching and responsive systems, new photochromic materials and molecular data processing and storage. Herein, we report visible light responsive chiroptical switches based on tetrahedral boron coordination towards an easily accessible hydrazone ligand and optically pure BINOL. Upon instalment of a non-planar dibenzo[a,d]-cycloheptene moiety in the hydrazone ligand's lower half, the enantiopure boron complex shows major chiroptical changes in the CD read-out after visible light irradiation. The thermal isomerization barrier in these chiroptical switching systems showed to be easily adjustable by the introduction of substituents onto the olefinic bond of the cycloheptene ring, giving profound control over their thermal stability. The control over their thermal stability in combination with excellent reversibility, photochemical properties and overall robustness of the complexes makes these BINOL-derived chiroptical switches attractive candidates for usage in advanced applications, e.g. photonic materials and nanotechnology.

Chiroptical switches, which use light to control chirality in a reversible manner, offer unique properties and fascinating prospects in the areas of molecular responsive systems, new photochromic materials and molecular data processing and storage.

Induced axial chirality by a tight belt: naphthalene chromophores fixed in a 2,5-substituted cofacial para-phenylene-ethynylene framework

We report the design of a synthetically easy accessible axial chirality-inducing framework for a chromophore of choice. The scaffold consists of two basic para-phenylene-ethynylene backbones separated by laterally placed corner units. Substitution with an inherently achiral chromophore at the 2 and 5 positions of the central phenylene excitonically couples the chromophore associated transition and thereby results in chiroptical properties. Using 6-methoxynaphthalene as a model chromophore, we present the synthesis, structural analysis and spectroscopic investigation of the framework. The chiral framework was synthesized in three straightforward synthetic steps and fully characterized. The obtained racemic compounds were resolved using HPLC and assignment of the absolute configuration was performed using the exciton chirality method, crystallography and DFT calculations. This simple yet potent framework might prove useful to enrich the structural diversity of chiral materials.

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.

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.

Chirality Remote Control in Nanoporous Materials by Circularly Polarized Light

The ability to dynamically control chirality remains a grand challenge in chemistry. Although many molecules possess chiral isomers, lacking their isolation, for instance during photoisomerization, results in racemic mixtures with suppressed enantiospecific chiral properties. Here, we present a nanoporous solid in which chirality and enantioselective enrichment is induced by circularly polarized light (CPL). The material is based on photoswitchable fluorinated azobenzenes attached to the scaffold of a crystalline metal-organic framework (MOF). The azobenzene undergoes trans-to-cis-photoisomerization upon irradiation with green light and reverts back to trans upon violet light. While each moiety in cis conformation is chiral, we show the trans isomer also possesses a nonplanar, chiral conformation. During photoisomerization with unpolarized light, no enantiomeric enrichment is observed and both isomers, R- and S-cis as well as R- and S-trans, respectively, are formed in identical quantities. In contrast, CPL causes chiral photoresolution, resulting in an optically active material. Right-CPL selectively excites R-cis and R-trans enantiomers, producing a MOF with enriched S-enantiomers, and vice versa. The induction of optical activity is reversible and only depends on the light-handedness. As shown by first-principle DFT calculations, while both, trans and cis, are stabilized in nonplanar, chiral conformations in the MOF, the trans isomer adopts a planar, achiral form in solution, as verified experimentally. This shows that the chiral photoresolution is enabled by the linker reticulation in the MOF. Our study demonstrates the induction of chirality and optical activity in solid materials by CPL and opens new opportunities for chiral resolution and information storage with CPL.

Cyclopentadienone Derivative Dimers as Tunable Photoswitches

A series of photoswitchable cyclopentadienone derivative dimers bearing bromo, thienyl, 4-(dimethylamino)phenyl, 3-pyridinyl, 4-nitrophenyl and cyano groups was designed and facilely synthesized. Photoswitching properties such as the photoconversions in the photostationary state (PSS), the thermal kinetics and thermal half-lives of photoisomers were systematically investigated. These photoswitches show high fatigue resistance and large photoconversions in the PSS. This work proves that the photoswitching properties of photoswitches based on cyclopentadienone dimers can be tuned by substitution groups and also pave the way to functionalize the cyclopentadienone derivative dimer-based photoswitch, which is important for its future applications.

Consequences of Chirality in Directing the Pathway of Cholesteric Helix Inversion of π-Conjugated Polymers by Light

Control over main-chain motion of chiral π-conjugated polymers can lead to unexpected new functionalities. Here, it is shown that by combining photoswitchable azobenzene units in conjugation with chiral fluorene comonomers and appropriate plasticizers, the polymer organization and chiroptical properties of these alternating copolymers steered by light and its state of polarization can be dynamically controlled. The configuration of the stereogenic centers in the side chains of the fluorene units determines the handedness of the cholesteric organization in thermally annealed films, indicating cooperative behavior. The polymer alignment and helicity of the supramolecular arrangement can be switched by irradiating with linearly and circularly polarized light, respectively. Intriguingly, when switching the handedness of thermally induced cholesteric organizations by illuminating with circularly polarized light that is opposite to the handedness of the cholesteric phases, a nematic-like intermediate state is observed during helix interconversion. By the sequence of irradiation with left and right circularly polarized light followed by thermal annealing, an asymmetric motion, reminiscent of that seen in molecular motors is observed. These findings suggest that functional conjugated polymers can exhibit emergent properties at mesoscopic scale.

Efficient Conversion of Light to Chemical Energy: Directional, Chiral Photoswitches with Very High Quantum Yields

Photochromic systems have been used to achieve a number of engineering functions such as light energy conversion, molecular motors, pumps, actuators, and sensors. Key to practical applications is a high efficiency in the conversion of light to chemical energy, a rigid structure for the transmission of force to the environment, and directed motion during isomerization. We present a novel type of photochromic system (diindane diazocines) that converts visible light with an efficiency of 18 % to chemical energy. Quantum yields are exceptionally high with >70 % for the cis-trans isomerization and 90 % for the back-reaction and thus higher than the biochemical system rhodopsin (64 %). Two diastereomers (meso and racemate) were obtained in only two steps in high yields. Both isomers are directional switches with high conversion rates (76-99 %). No fatigue was observed after several thousands of switching cycles in both systems.

Of Twists and Curves: Electronics, Photophysics, and Upcoming Applications of Non-Planar Conjugated Organic Molecules

Non-planar conjugated organic molecules (NPCOMs) contain π-conjugation across their length and also exhibit asymmetry in their conformation. In other words, certain molecular fragments in NPCOMs are either twisted or curved out of planarity. This conformational asymmetry in NPCOMs leads to non-uniform charge-distribution across the molecule, with important photophysical and electronic consequences such as altered thermodynamic stability, chemical reactivity, as well as materials properties. Majorly, NPCOMs can be classified as having either Fused or Rotatable architectures. NPCOMs have been the focus of significant scientific attention in the recent past due to their exciting photophysical behavior that includes intramolecular charge-transfer (ICT), thermally activated delayed fluorescence (TADF) and long-lived charge-separated states. In addition, they also have many useful materials characteristics such as biradical character, semi-conductivity, dynamic conformations, and mechanochromism. As a result, rational design of NPCOMs and mapping their structure-property correlations has become imperative. Researchers have executed conformational changes in NPCOMs through a variety of external stimuli such as pH, temperature, anions-cations, solvent, electric potential, and mechanical force in order to tailor their photophysical, optoelectronic and magnetic properties. Converging to these points, this review highlights the lucrative electronic features, photophysical traits and upcoming applications of NPCOMs by a selective survey of the recent scientific literature.

Reversible Photoswitching of Isolated Ionic Hemiindigos with Visible Light

Indigoid chromophores have emerged as versatile molecular photoswitches, offering efficient reversible photoisomerization upon exposure to visible light. Here we report synthesis of a new class of permanently charged hemiindigos (HIs) and characterization of photochemical properties in gas phase and solution. Gas-phase studies, which involve exposing mobility-selected ions in a tandem ion mobility mass spectrometer to tunable wavelength laser radiation, demonstrate that the isolated HI ions are photochromic and can be reversibly photoswitched between Z and E isomers. The Z and E isomers have distinct photoisomerization response spectra with maxima separated by 40-80 nm, consistent with theoretical predictions for their absorption spectra. Solvation of the HI molecules in acetonitrile displaces the absorption bands to lower energy. Together, gas-phase action spectroscopy and solution NMR and UV/Vis absorption spectroscopy represent a powerful approach for studying the intrinsic photochemical properties of HI molecular switches.

Unveiling how intramolecular stacking modes of covalently linked dimers dictate photoswitching properties

Covalently linked π-stacked dimers represent the most significant platform for elucidating the relationship between molecular alignments and their properties. Here, we present the one-pot synthesis of two intramolecularly π-stacked dimers and disclose how intramolecular stacking modes dictate photoswitching properties. The dimer, which features cofacially stacked chromophores and geometrically favours intramolecular photochemical [2 + 2] cycloadditions, displays a nearly irreversible photoswitching behaviour. By contrast, the dimer, bearing crosswise stacked chromophores, is geometrically unfavourable for the cycloaddition and exhibits a highly reversible photoswitching process, in which the homolysis and reformation of carbon−carbon single bonds are involved. Moreover, the chiral carbon centres of both dimers endow these photoswitches with chirality and the separated enantiomers exhibit tuneable chiroptical properties by photoswitching. This work reveals that intramolecular stacking modes significantly influence the photochemical properties of π-stacked dimers and offers a design strategy toward chiral photoswitchable materials.

Covalently bridged π-stacked dimers are excellent molecular platforms for understanding the relationship between stacking orientation and properties. Here, the authors synthesize a pair of π-stacked dimers that are aligned either cofacially or crosswise, allowing them to compare how the intramolecular stacking mode affects each dimer’s photoswitching properties.

Design, synthesis and investigation of water-soluble hemi-indigo photoswitches for bioapplications

A series of hemi-indigo derivatives was synthesized and their photoswitching properties in aqueous medium were studied. The dimethoxy hemi-indigo derivative with the best photochromic performance in water was identified as a promising platform for the development of photoswitchable binders for biomolecules. The synthetic approach towards the introduction of the alkylamino pendant to the dimethoxy hemi-indigo core was developed that allowed to obtain an RNA-binding hemi-indigo derivative with photoswitchable fluorescent properties.

Chiroptical inversion of a planar chiral redox-switchable rotaxane

A tetrathiafulvalene (TTF)-containing crown ether macrocycle with C s symmetry was designed to implement planar chirality into a redox-active [2]rotaxane. The directionality of the macrocycle atom sequence together with the non-symmetric axle renders the corresponding [2]rotaxane mechanically planar chiral. Enantiomeric separation of the [2]rotaxane was achieved by chiral HPLC. The electrochemical properties - caused by the reversible oxidation of the TTF - are similar to a non-chiral control. Reversible inversion of the main band in the ECD spectra for the individual enantiomers was observed after oxidation. Experimental evidence, conformational analysis and DFT calculations of the neutral and doubly oxidised species indicate that mainly electronic effects of the oxidation are responsible for the chiroptical switching. This is the first electrochemically switchable rotaxane with a reversible inversion of the main ECD band.

A [2]Rotaxane-Based Circularly Polarized Luminescence Switch

A rotaxane-based molecular shuttle has been synthesized in which the switching of the position of a fluorescent macrocycle on the thread turns “on” or “off” the circularly polarized luminescence (CPL) of the system while maintaining similar fluorescence profiles and quantum yields in both states. The chiroptical activity relies on the chiral information transfer from an ammonium salt incorporating d- or l-phenylalanine residues as chiral stereogenic covalent units to an otherwise achiral crown ether macrocycle bearing a luminescent 2,2′-bipyrene unit when they interact through hydrogen bonding. Each enantiomeric thread induces CPL responses of opposite signs on the macrocycle. Upon addition of base, the switching of the position of the macrocycle to a triazolium group disables the chiral information transfer to the macrocycle, switching “off” the CPL response. The in situ switching upon several acid/base cycles is also demonstrated.

Visible-range hemi-indigo photoswitch: ON-OFF fluorescent binder for HIV-1 RNA

A proof-of-principle for the application of hemi-indigo derivatives as RNA binders with photocontrollable fluorescence is presented. The photoswitch binds to the human immunodeficiency virus type 1 (HIV-1) RNA with a significant light-up effect. The fluorescence of the RNA-bound ligand can be reversibly switched ON and OFF by light without destroying the ligand-RNA associates.

Regulation of Axial Chirality through Dynamic Covalent Bond Constrained Biaryls

A strategy of dynamic covalent chemistry within constrained biaryls was developed for the modulation of axial chirality. The ring fusion partners of amide and aldehyde allowed the manipulation of ring/chain equilibrium and chirality transfer within cyclic diastereomeric hemiaminal. Dynamic covalent reactions (DCRs) with alcohols, thiols, and secondary amines further enabled the reversal of chirality relay and thereby regulation of axial chirality. Moreover, a combination of NMR, X-ray, and density functional theory results shed light on the structural basis of chirality transfer, exhibiting modest to excellent diastereoselectivity under thermodynamic control. The critical role of the amide unit in the modulation of axial chirality was also corroborated. Finally, the chiroptical signal was controlled through changing solvents, DCRs, and stimuli-responsive switching of DCRs.

Radical cations of twisted acenes: chiroptical properties and spin delocalization

We introduce the first series of enantiopure twistacene radical cations, which form reversibly upon chemical or electrochemical oxidation. Their vis-NIR chiroptical properties (Cotton effect and anisotropy factor) increase systematically with the backbone twist. The hyperfine constants observed by EPR demonstrate significant spin delocalization even for large backbone twist angles.

Symmetric and nonsymmetric bis-hemithioindigos – precise visible light controlled shape-shifters

A series of bis-hemithioindigo photoswitches with different molecular setups are presented allowing precise manipulation of molecular shapes with visible light.