Light-induced Isomerization Research Articles

Structural Changes in Wormlike Micelles on the Incorporation of Small Photoswitchable Molecules.

Chromophores susceptible to light-induced trans-cis isomerization embedded in cylindrical micelles can modify micelles and their light-responsive performance. A small chromophore (4-(phenylazo)benzoate ion) is embedded in cylindrical micelles made of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) in water. The microstructure is examined by scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). Rheological behavior and the length scales of the micellar network are determined by rheology and microrheology. The chromophore substantially modifies the micelles even without UV irradiation. The larger is the chromophore concentration, the smaller is the micellar length. Additional length scales of the micellar network do not substantially vary even when NaCl is added. Chromophore incorporation also modifies the rheology of the micellar solution, although gradient shear banding is preserved. Viscosity decreases as the chromophore concentration increases, and viscoelastic spectra are modified, but when they are correctly rescaled, they can be superimposed. The addition of the chromophore makes the fluids more Maxwellian, particularly when NaCl is also added. When the chromophore is incorporated into the micelles, there is a response after UV irradiation, although it does not produce a significant rheological change.

An all-photonic full color RGB system based on molecular photoswitches

On-command changes in the emission color of functional materials is a sought-after property in many contexts. Of particular interest are systems using light as the external trigger to induce the color changes. Here we report on a tri-component cocktail consisting of a fluorescent donor molecule and two photochromic acceptor molecules encapsulated in polymer micelles and we show that the color of the emitted fluorescence can be continuously changed from blue-to-green and from blue-to-red upon selective light-induced isomerization of the photochromic acceptors to the fluorescent forms. Interestingly, isomerization of both acceptors to different degrees allows for the generation of all emission colors within the red-green-blue (RGB) color system. The function relies on orthogonally controlled FRET reactions between the blue emitting donor and the green and red emitting acceptors, respectively.

Facile synthesis of a photoresponsive AIEgen used for monitoring UV light and photo-patterning

Photoresponsive materials with good luminescence property in solid state have shown promising applications in bio-imaging, photo-patterning, and optical switch related areas. However, most of the reported photoresponsive systems suffered from the drawbacks of either weak luminescence property in solid state or complex synthetic processes. In this work, a novel photoresponsive molecule of 2,3-diphenyl-6,7-dicyanoquinoxaline (PCQ) has been facilely prepared via one-step reaction with commercially available materials. PCQ exhibits typical aggregation-induced emission (AIE) property due to the mechanism of restricted intramolecular rotation (RIR). As a photoresponsive molecule, PCQ shows remarkable fluorescence changes both in solution and in solid state upon UV light irradiation. Mechanism study reveals that there is an UV light-induced isomerization reaction in PCQ. A good linear relationship between fluorescence intensity and UV light power density is achieved, which suggests that PCQ can be served as a sensor for monitoring UV light strength. Furthermore, characters are successfully recorded on PCQ in solid matrix, demonstrating the feasible application of PCQ in photo-patterning. The conspicuous contrast between the states before and after UV light irradiation suggests that PCQ can be potentially used as an anti-counterfeiting material. PCQ enriches the kinds of AIE luminogen (AIEgen) and, more importantly, this work provides a new strategy for constructing multifunctional photoresponsive systems with good luminescence property.

Emerging structural biology of lipid G protein-coupled receptors.

The first crystal structure of a G protein-coupled receptor (GPCR) was that of the bovine rhodopsin, solved in 2000, and is a light receptor within retina rode cells that enables vision by transducing a conformational signal from the light-induced isomerization of retinal covalently bound to the receptor. More than 7 years after this initial discovery and following more than 20 years of technological developments in GPCR expression, stabilization, and crystallography, the high-resolution structure of the adrenaline binding β2 -adrenergic receptor, a ligand diffusible receptor, was discovered. Since then, high-resolution structures of more than 53 unique GPCRs have been determined leading to a significant improvement in our understanding of the basic mechanisms of ligand-binding and ligand-mediated receptor activation that revolutionized the field of structural molecular pharmacology of GPCRs. Recently, several structures of eight unique lipid-binding receptors, one of the most difficult GPCR families to study, have been reported. This review presents the outstanding structural and pharmacological features that have emerged from these new lipid receptor structures. The impact of these findings goes beyond mechanistic insights, providing evidence of the fundamental role of GPCRs in the physiological integration of the lipid signaling system, and highlighting the importance of sustained research into the structural biology of GPCRs for the development of new therapeutics targeting lipid receptors.

Conductance Photoswitching of Metal-Organic Frameworks with Embedded Spiropyran.

Conductive metal-organic frameworks (MOFs) as well as smart, stimuli-responsive MOF materials have attracted considerable attention with respect to advanced applications in energy harvesting and storage as well as in signal processing. Here, the conductance of MOF films of type UiO-67 with embedded photoswitchable nitro-substituted spiropyrans was investigated. Under UV irradiation, the spiropyran (SP) reversibly isomerizes to the open merocyanine (MC) form, a zwitterionic molecule with an extended conjugated π-system. The light-induced SP-MC isomerization allows for remote control over the conductance of the SP@UiO-67 MOF film, and the conductance can be increased by one order of magnitude. This research has the potential to contribute to the development of a new generation of photoelectronic devices based on smart hybrid materials.

Numerical Demonstration of In-Tube Liquid-Column Migration Driven by Photoisomerization.

Droplet manipulation by light-induced isomerization was numerically demonstrated and investigated regarding the driving mechanism. Such a non-invasive manipulation of a droplet in a microchannel can be realized, for example, by the use of watery solution of photoresponsive surfactant that exhibits the isomerization. Due to variable fluid properties between the cis and trans isomers, one-side light irradiation on a liquid column in a tube would lead to some kind of imbalance between the two ends of the liquid column and then drive droplet migration. The present numerical simulations of air–liquid two-phase flow and its scalar transport of the isomer, considering the variable static contact angle, agreed quantitatively with the experimental results in terms of the migration speed. This fact supports the contention that the droplet migration is more likely to be driven by an imbalance in the wettability, or the contact angle. The migration speed was found to be less dependent on the liquid-column length, but proportional to the tube diameter.

Light-Induced Bending of Needle-Like Crystals of Naphthylvinylbenzoxazole Triggered by trans-cis Isomerization.

New diarylethene derivatives containing benzoxazole (NBO) and benzothiazole (NBT) have been synthesized. Light-induced trans-cis isomerization of NBO and NBT took place in crystals, and only induced the needle-like crystals of NBO to bend backwards away from the UV light source. The movement of the atoms was deemed to take place during the isomerization of NBO; hence, strain would be produced and accumulated rapidly in the surface of crystals exposed to UV light. The uniform release of strain led to the bending of needle-like crystals. The light-induced trans-cis isomerization efficiency of NBT was too low to drive the motion of crystals, which might have originated from the large repulsion between naphthyl and benzothiazole. These results provide a new platform for the transformation of light energy into mechanical energy in molecular crystals through the unimolecular photochemical reaction of diarylethene derivatives.

London dispersion as important factor for the stabilization of (Z)-azobenzenes in the presence of hydrogen bonding.

The understanding and control of the light-induced isomerization of azobenzenes as one of the most important classes of molecular switches is crucial for the design of light-responsive materials using this entity. Herein, we present the stabilization of metastable (Z)-azobenzenes by London dispersion interactions, even in the presence of comparably stronger hydrogen bonds in various solvents. The Z→E isomerization rates of several N-substituted 4,4′-bis(4-aminobenzyl)azobenzenes were measured. An intramolecular stabilization was observed and explained by the interplay of intramolecular amide and carbamate hydrogen bonds as well as London dispersion interactions. Whereas in toluene, 1,4-dioxane and tert-butyl methyl ether the hydrogen bonds dominate, the variation in stabilization of the different substituted azobenzenes in dimethyl sulfoxide can be rationalized by London dispersion interactions. These findings were supported by conformational analysis and DFT computations and reveal low-energy London dispersion forces to be a significant factor, even in the presence of hydrogen bonds.

Sensitive Assays by Nucleophile-Induced Rearrangement of Photoactivated Diarylethenes.

Upon light-induced isomerization, diarylethenes (DAEs) equipped with reactive aldehyde moieties rearrange selectively in the presence of amines, accompanied by decoloration. In a comprehensive study, the probe structure was optimized with regard to its inherent reactivity in the nucleophile-triggered rearrangement reaction. Detailed structure-reactivity relationships could be derived, in particular with regard to the type of integrated (het)aryl moieties as well as the location of the formyl residue, and the probes' intrinsic reactivity with primary and secondary amines was optimized. Utilizing an ancillary base, the initially formed rearrangement product can engage in a subsequent catalytic cycle, leading to an amplified decoloration process. This additional catalytic pathway allows us to enhance the sensitivity of our method and successfully discriminate between amines and thiols. Moreover, probes that exhibit strong analyte-induced fluorescence modulation have been designed to further decrease the detection limit by using a more sensitive read-out. The optimized DAE probes are promising molecular components for future programmable sensing materials and devices.

Antioxidant activity and photostability assessment of trans-resveratrol acrylate microspheres

Trans-resveratrol (RSV) was microencapsulated in Eudragit® RS100 and RL100 resin blends. Lyophilized microspheres were characterized in the solid state for their micromeritic properties and drug loading. FT-IR, PXRD, and DSC analyzes suggested that RSV formed an intimate microcrystalline dispersion within the polymer network, also confirmed by SEM analysis. This produced a reduced degradation of RSV after storage at 40 °C, compared to the neat drug, and a protection of the drug from UV light-induced trans-cis isomerization (60% intact drug was found after 60 s irradiation at 350 nm, compared to 37% for the pure drug). Solubility and in vitro dissolution studies indicated that microencapsulation did not improve the dissolution pattern of RSV in simulated gastric and intestinal aqueous fluids. Evaluation of the in vitro antioxidant activity showed that, compared to the neat drug in aqueous solution, RSV loaded in the microspheres retained for a longer time, up to 22 days of incubation, the initial ORAC capacity. The present study thus demonstrated that Eudragit® Retard resins can be used to easily produce micro-sized solid dispersions with RSV, for potential oral administration, contributing to ameliorate the physico-chemical stability and antioxidant activity of this compound.

Engineering the vibrational coherence of vision into a synthetic molecular device

The light-induced double-bond isomerization of the visual pigment rhodopsin operates a molecular-level optomechanical energy transduction, which triggers a crucial protein structure change. In fact, rhodopsin isomerization occurs according to a unique, ultrafast mechanism that preserves mode-specific vibrational coherence all the way from the reactant excited state to the primary photoproduct ground state. The engineering of such an energy-funnelling function in synthetic compounds would pave the way towards biomimetic molecular machines capable of achieving optimum light-to-mechanical energy conversion. Here we use resonance and off-resonance vibrational coherence spectroscopy to demonstrate that a rhodopsin-like isomerization operates in a biomimetic molecular switch in solution. Furthermore, by using quantum chemical simulations, we show why the observed coherent nuclear motion critically depends on minor chemical modifications capable to induce specific geometric and electronic effects. This finding provides a strategy for engineering vibrationally coherent motions in other synthetic systems.

Reversible and Efficient Light-Induced Molecular Switching on an Insulator Surface.

Prototypical molecular switches such as azobenzenes exhibit two states, i.e., trans and cis, with different characteristic physical properties. In recent years various derivatives were investigated on metallic surfaces. However, bulk insulators as supporting substrate reveal important advantages since they allow electronic decoupling from the environment, which is key to control the switching properties. Here, we report on the light-induced isomerization of an azobenzene derivative on a bulk insulator surface, in this case calcite (101̅4), studied by atomic force microscopy with submolecular resolution. Surprisingly, cis isomers appear on the surface already directly after preparation, indicating kinetic trapping. The photoisomerization process is reversible, as the use of different light sources results in specific molecular assemblies of each isomer. The process turns out to be very efficient and even comparable to molecules in solution, which we assign to the rather weak molecular interaction with the insulator surface, in contrast to metals.

Switching the Proton Conduction in Nanoporous, Crystalline Materials by Light.

Proton conducting nanoporous materials attract substantial attention with respect to applications in fuel cells, supercapacitors, chemical sensors, and information processing devices inspired by biological systems. Here, a crystalline, nanoporous material which offers dynamic remote-control over the proton conduction is presented. This is realized by using surface-mounted metal-organic frameworks (SURMOFs) with azobenzene side groups that can undergo light-induced reversible isomerization between the stable trans and cis states. The trans-cis photoisomerization results in the modulation of the interaction between MOF and guest molecules, 1,4-butanediol and 1,2,3-triazole; enabling the switching between the states with significantly increased (trans) and reduced (cis) conductivity. Quantum chemical calculations show that the trans-to-cis isomerization results in the formation of stronger hydrogen bridges of the guest molecules with the azo groups, causing stronger bonding of the guest molecules and, as a result, smaller proton conductivity. It is foreseen that photoswitchable proton-conducting materials may find its application in advanced, remote-controllable chemical sensors, and a variety of devices based on the conductivity of protons or other charged molecules, which can be interfaced with biological systems.

Shape-memory and self-healing functions of DNA-based carboxymethyl cellulose hydrogels driven by chemical or light triggers.

Photoresponsive nucleic acid-based carboxymethyl cellulose (CMC) hydrogels are synthesized, and their application as shape-memory and self-healing functional matrices are discussed. One system involves the preparation of a carboxymethyl cellulose hydrogel crosslinked by self-complementary nucleic acid duplexes and by photoresponsive trans-azobenzene/β-cyclodextrin (β-CD) supramolecular complexes. Photoisomerization of the trans-azobenzene to the cis-azobenzene results in a hydrogel exhibiting lower stiffness due to the separation of the azobenzene/β-CD bridging units. The hydrogel is switched between high and low stiffness states by the cyclic and reversible light-induced isomerization of the azobenzene units between the trans and cis states. The light-controlled stiffness properties of the hydrogel are used to develop a shape-memory hydrogel, where the duplex bridging units act as permanent memory in the quasi-liquid shapeless state of the hydrogel. A second system in the study is a carboxymethyl cellulose hydrogel crosslinked by the K+-stabilized G-quadruplex bridging units and by trans-azobenzene/β-CD complexes. The resulting hydrogel includes dual-trigger functionalities, where the trans-azobenzene/β-CD complexes can be reversibly formed and dissociated through the trans and cis photoisomerization of the azobenzene units, and the K+-stabilized G-quadruplexes can be reversibly dissociated and reformed in the presence of 18-crown-6-ether/K+-ions. The signal-responsive crosslinked hydrogel reveals controlled stiffness properties, where the hydrogel crosslinked by the trans-azobenzene/β-CD and K+-ion-stabilized G-quadruplex reveals high stiffness and the hydrogel crosslinked only by the K+-ion-stabilized G-quadruplexes or only by the trans-azobenzene/β-CD complexes reveals low stiffness properties. The controlled stiffness properties of the hydrogel are used to develop shape-memory hydrogels, where the trans-azobenzene/β-CD complexes or the K+-ion-stabilized G-quadruplexes act as permanent memories in the shapeless and quasi-liquid states of the hydrogels. In addition, the hydrogel that includes two types of stimuli-responsive crosslinking units is used as a self-healing matrix, where each of the triggers guides the self-healing processes.

New diterpenoids from the marine sponge Dactylospongia elegans

Eleganstone A (1), a rare diterpene possessing a 5/6/4/5 fused tetracyclic ring skeleton, along with its probable biogenetic precursor (2), one new dolabellane diterpene (3), and one known dolabellane diterpene (4), have been isolated from the marine sponge Dactylospongia elegans. As a pair of Z/E isomers, 2 can convert into 4 by the light-induced isomerization. Structure elucidations of all compounds were performed by spectroscopic analysis, ECD calculations, and comparison with the literature. These compounds were tested for their antibacterial and cytotoxic activities. Compounds 1–4 exhibited weak antibacterial activity against Escherichisa coli, Bacillus subtilis, and Staphylococcus aureus with MIC values of 32–64 μg/mL.

Triaryl-Boron Functionalized Dinuclear Platinum Complexes Linked by Photoisomerizable Bpe Ligand: Luminescence and Isomerism.

We report herein a series of triarylboron-functionalized Pt(II) dinuclear complexes using a photoactive bis(4-pyridyl)ethylene (bpe) bridging unit. The new Pt(II) complexes display phosphorescence at both room temperature and 77 K. Depending on the conditions under which crystals of the complexes were grown, either the cis- or trans-isomers could be obtained. The trans and cis isomers were found to undergo light-induced isomerization in solution with the trans isomer being the dominant species.

Tuning the Effects of Bacterial Membrane Permeability through Photo-Isomerization of Antimicrobial Cationic Amphiphiles.

Several important antimicrobial drugs act by permeabilizing cell membranes. In this study, we showed that the intensity of membrane permeability caused by antimicrobial cationic amphiphiles can be modified not only by their concentration but also through light-induced isomerization of their lipid segment. Two types of photo-isomerizable cationic amphiphiles were developed and the effects of photo-isomerization on bacterial growth and membrane permeability were evaluated. One photo-isomer inhibited cell growth and division, whereas the other photo-isomer led to a rapid and lethal bacterial membrane-disrupting effect. The switch from "on" to "off" can be obtained by either the cis- or trans-isomer depending on the bacterial strain and the type of cationic amphiphile. These cationic amphiphiles offer a novel tool for research and industrial applications that require light-controlled bacterial membrane permeabilization.

Photochemical Properties of the Red-shifted Channelrhodopsin Chrimson.

Color-tuned variants of channelrhodopsins allow for selective optogenetic manipulation of different host cell populations. Chrimson is the channelrhodopsin with the longest wavelength absorbance maximum. We characterize its photochemical properties at different pH values corresponding to two protonation states of the counterion for the protonated Schiff base. Both states will lead to a functional channel opening, but the route is different as reflected in the photochemical states observed spectroscopically. The light-induced isomerization kinetics change with the local electrostatic environment, becoming faster with the presence of an anionic counterion. The spectral effect is stronger on the ground-state energy surface. From the excited state, a bifurcated pathway leads to the electronic ground state resulting in a pronounced excitation wavelength dependence. The subsequent steps in the photocycles at pH 6 and pH 9.5 differ in the accumulation of states with a protonated and deprotonated Schiff base, respectively, that can be correlated with the open channel. Therefore, different protonation states are preserved in the open and the initial states. Chrimson's photocycle at alkaline pH shows features observed in other rhodopsins without an internal proton donor to the Schiff base, but it accumulates an intermediate with an even longer lifetime reflecting slow recovery of the initial state.

Light-Induced Radical Formation and Isomerization of an Aromatic Thiol in Solution Followed by Time-Resolved X-ray Absorption Spectroscopy at the Sulfur K-Edge

We applied time-resolved sulfur-1s absorption spectroscopy to a model aromatic thiol system as a promising method for tracking chemical reactions in solution. Sulfur-1s absorption spectroscopy allows tracking multiple sulfur species with a time resolution of ∼70 ps at synchrotron radiation facilities. Experimental transient spectra combined with high-level electronic structure theory allow identification of a radical and two thione isomers, which are generated upon illumination with 267 nm radiation. Moreover, the regioselectivity of the thione isomerization is explained by the resulting radical frontier orbitals. This work demonstrates the usefulness and potential of time-resolved sulfur-1s absorption spectroscopy for tracking multiple chemical reaction pathways and transient products of sulfur-containing molecules in solution.

Ultrafast isomerization in a difluoroboryl-coordinated molecular switch

Molecular switches based on light-induced isomerization cycles are promising materials for sensors and biomedical applications. Understanding the details of the isomerization photocycle and identifying the nuclear coordinates involved in the photoreaction are relevant topics. Here we characterize the cis→trans and trans→cis isomerization of a new type of BF2-coordinated azo-dye molecular switch by pump-probe spectroscopy. While cis→trans isomerization is ultrafast and proceeds via a conical intersection, the trans→cis photocycle is more complex and involves at least three reaction channels. Finally, we employ the vibrational wavepackets accompanying isomerization to infer information on the nuclear degrees of freedom involved in the photoreaction.