Reversible Trans-cis Research Articles

Modulation of room temperature emission characteristics of heteroleptic ruthenium-terpyridine complexes via aggregation

In this work, we have thoroughly investigated the modulation of room temperature luminescence characteristics of our recently reported stilbene-appended heteroleptic Ru(II)-terpyridine complexes of the type, [(tpy-PhCH3)Ru(tpy-pvp-X)](ClO4)2, wherein X =naphthalene, anthracene, and pyrene moiety via aggregation through the use of appropriate solvent mixtures. Taking profit of the polyaromatic motif, the complexes display both aggregation-induced emission quenching as well as emission augmentation, depending upon nature of the solvent mixtures. By virtue of the presence of stilbene unit, the complexes undergo trans–cis isomerization upon irradiation of light. We have previously investigated the reversible trans–cis isomerization of the non-aggregated form of the complexes upon alternative treatment of visible and UV light. In the present study, we have also undertaken the trans → cis photoisomerization of the aggregated forms of the complexes. The rate constant and quantum yield of photoisomerization in the aggregated form of the complexes are also estimated and compared with their non-aggregated counterparts.

A switchable zinc-based metal organic framework for the light triggered absorption and release of organic dyes

Due to its adjustable pore structure, the metal–organic frameworks (MOFs) have attracted much attention in the treatment of organic dye molecules in wastewater. However, the general tuning of the pore channels of MOFs is mainly by changing the chain length of the organic ligands and metal nodes. This approach not only limits the types of MOFs, but also limits the treatment of a large number of dye methylene blue (MB) and methyl orange (MO) dye molecules in the wastewater. Hence, we synthesized photoresponsive Zn-AzDC/TPA MOF as adsorbents to adsorb and release organic dye molecules in a photo-controlled manner. The photoresponsive Zn-AzDC/TPA was prepared using a mixed ligand strategy, in which the azobenzene carboxylic acid derivative (AzDC) served as a photoresponsive ligand and terephthalic acid (TPA) served as a non-active ligand in coordination with zinc ion. The optical and structural properties of the synthesized Zn-AzDC/TPA was characterized by UV–vis, FT-IR, PXRD, SEM, TEM, TGA, and pore analysis. Interestingly, it was found that the photoresponsive AzDC unit of Zn-AzDC/TPA demonstrated reversible trans–cis isomerization under the alternating UV and visible light, resulting in reversible changes in the pore size of the Zn-AzDC/TPA. Its photoresponse properties can trapp and release dye molecules under light-driven conditions. This result provides a new direction for the application of photoresponsive MOF and also lays a foundation for the study of diversified optically responsive materials.

Photo-Healable Fabrics: Achieving Structural Control via Photochemical Solid-Liquid Transitions of Polystyrene/Azobenzene-Containing Polymer Blends.

While polymer fabrics are integral to a wide range of applications, their vulnerability to mechanical damage limits their sustainability and practicality. Addressing this challenge, our study introduces a versatile strategy to develop photohealable fabrics, utilizing a composite of polystyrene (PS) and an azobenzene-containing polymer (PAzo). This combination leverages the structural stability of PS to compensate for the mechanical weaknesses of PAzo, forming the fiber structures. Key to our approach is the reversible trans-cis photoisomerization of azobenzene groups within the PAzo under UV light exposure, enabling controlled morphological alterations in the PS/PAzo blend fibers. The transition of PAzo sections from a solid to a liquid state at a low glass transition temperature (Tg ∼ 13.7 °C) is followed by solidification under visible light, thus stabilizing the altered fiber structures. In this study, we explore various PS/PAzo blend ratios to optimize surface roughness and mechanical properties. Additionally, we demonstrate the capability of these fibers for photoinduced self-healing. When damaged fabrics are clamped and subjected to UV irradiation for 20 min and pressed for 24 h, the mobility of the cis-form PAzo sections facilitates healing while retaining the overall fabric structure. This innovative approach not only addresses the critical issue of durability in polymer fabrics but also offers a sustainable and practical solution, paving the way for its application in smart clothing and advanced fabric-based materials.

Luminescent Ruthenium-Terpyridine Complexes Coupled with Stilbene-Appended Naphthalene, Anthracene, and Pyrene Motifs Demonstrate Fluoride Ion Sensing and Reversible Trans-Cis Photoisomerization.

A new family of luminescent heteroleptic Ru(II)-terpyridine complexes coupled with stilbene-appended naphthalene, anthracene, and pyrene motifs is reported. Each of the complexes features moderately intense emission at room temperature having a lifetime of 16.7 ns for naphthalene and 11.4 ns for anthracene, while a substantially elevated lifetime of 8.3 μs was observed for the pyrene derivative. All the three complexes display a reversible couple in the positive potential window due to Ru2+/Ru3+ oxidation but multiple reversible and/or quasi-reversible peaks in the negative potential domain because of the reduction of the terpyridine moieties. All the complexes selectively sense F- among the studied anions via the intermediary of different noncovalent interactions. The interaction event is monitored through absorption, emission, and 1H and 19F NMR spectroscopy. Additionally, upon utilizing the stilbene motif, reversible trans-cis isomerization of the complexes has been undertaken upon alternate treatment of visible and UV light so that the complexes can act as potential photomolecular switches. We also carried out the anion sensing characterization of the cis form of the complexes. Theoretical calculation employing density functional theory is also executed for a selective complex (naphthalene derivative) to elucidate different noncovalent interactions that are operative during the complex-fluoride interplay.

Photochromic Azobenzene Inverse Opal Film toward Dynamic Anti-Fake Pattern.

Azobenzene mesogens have garnered considerable research attention in the realm of photo-responsive materials due to their reversible trans-cis isomerization. In this paper, we demonstrate an azobenzene inverse opal film synthesized via photo-polymerization from a SiO2 opal template. The proposed design exhibits intriguing optical properties, including dynamic fluorescent features, distinct fluorescent enhancement, and an anti-fake micropattern with a switchable structure color. This work holds significant importance for advancing the development of novel optical devices.

Optocapacitive current of non-thermal origin.

Optocapacitive effects are based on sudden jumps in membrane capacitance due to light adsorption. Thus far, photothermal effects are exploited, i.e., in the first step, illumination heats light-absorbing nanoparticles, which transfer the heat to the nearby membrane in the second step [1]. We now propose simplifying the approach by omitting the nanomaterials and using photolipids instead. Their acyl chains undergo a reversible trans-cis photoisomerization due to the photo-susceptibility of the azobenzene moiety. In our hands, the associated capacitance changes of planar lipid bilayers [2] and in HEK cells are large enough to elicit substantial optocapacitive current spikes. The optocapacitive current changes with the (i) surface potential and (ii) the dipole potential of bilayers folded from lipid monolayers of different compositions. Our observation that the sizes of the optocapacitive currents induced by photothermal effects and lipid photoisomerization are comparable paves the way for using photolipids for neuronal stimulation. The work was supported by grants from the FWF (Austrian Science Fund): TAI 181 and the NIH: GM030376. 1. Pinto et al. Biophysical Reviews, 2022. 14(2): p. 569-577. 2. Pfeffermann et al., Journal of Photochemistry and Photobiology B: Biology, 2021: p. 112320.

Meta-stable initial condition for improving the switching probability in azobenzene derivatives on surface

The trans isomer of azobenzene (AB) and its derivatives is the most abundant isomer at thermodynamic equilibrium and is known to switch between its trans and cis isomers when triggered by light, electrons/holes and electrical field in adlayers on surfaces. However, the equilibrium initial condition limits the operation of molecular switches on surface due to large activation barrier for switching (trans to cis), hence the switching probability is very low. We show in this article, condensation of meta-stable (non-equilibrium) cis state of azobenzene derivatives on graphite. When electrons/holes induced switching is performed on these molecular switches from the meta-stable initial condition, the switching probability is enhanced by several folds. The molecules are switching between two associated cis states, instead of trans–cis switching. The switching between the cis states has relatively low energy barrier as revealed by density functional theory (DFT) calculations is at the origin of the enhanced switching probability. We also observe that reversible trans–cis and cis–cis’ switching on graphite is mediated through a stable charged intermediate state, leading to a low threshold voltage for switching compared to neutral molecules.

Anisotropic fluid with phototunable dielectric permittivity

Dielectric permittivity, a measure of polarisability, is a fundamental parameter that dominates various physical phenomena and properties of materials. However, it remains a challenge to control the dielectric permittivity of materials reversibly over a large range. Herein, we report an anisotropic fluid with photoresponsive dielectric permittivity (200 < ε < 18,000) consisting of a fluorinated liquid-crystalline molecule (96 wt%) and an azobenzene-tethered phototrigger (4 wt%). The reversible trans-cis isomerisation of the phototrigger under blue and green light irradiation causes a switch between two liquid-crystalline phases that exhibit different dielectric permittivities, with a rapid response time (<30 s) and excellent reversibility (~100 cycles). This anisotropic fluid can be used as a flexible photovariable capacitor that, for example, allows the reversible modulation of the sound frequency over a wide range (100 < f < 8500 Hz) in a remote manner using blue and green wavelengths.

Light-controllable chiral dopant based on azo-fragment: synthesis and characterisation

ABSTRACT We present the synthesised chiral dopant 2-[(2-isopropyl-5-methylcyclohexyl)oxy]-2-oxoethyl 4-{(E)-[4-(decyloxy)phenyl]diazenyl}benzoate (ChD-3501), consisting of azo- and aliphatic fragments together with a chiral centre based on l-menthol as a reversible light-controllable chiral dopant. To assess the effects of UV–vis irradiation and temperature in the isotropic and liquid crystalline (LC) states, we studied the spectral kinetics of ethanol solution of ChD-3501, as well as induction of the cholesteric helix when it was dissolved in nematic LC (E7) as a chiral dopant. The concentration dependence of the helical pitch of the induced cholesterics was studied by means on Grandjean–Cano method, and the helical twisting power of ChD-3501 in the nematic host E7 was determined. The reversible trans-cis isomerisation of chiral dopant ChD-3501 in E7 under UV–vis irradiation was studied, and it has been found that the storage of the cis-isomer at certain constant temperature also leads to the reversible isomerisation. Possible applications of the studied system are discussed.

Visible light, temperature, and electric field-driven rotation of diffraction gratings enabled by an axially chiral molecular switch

A visible light-driven chiral molecular switch shows reversibletrans–cisphotoisomerization, good compatibility, and high helical twisting power in achiral liquid crystals. Multi-stimuli responsive rotatable in-plane beam steering and chromatic dispersion are accomplished.

Identification of Trans- and Cis-2-Methylcyclopropanecarboxylic acid using EVV 2DIR spectroscopy: A theoretical study

Due to the reversible trans–cis geometric isomerization reaction between trans- and cis-2-methylcyclopropanecarboxylic acid (MCA) in gas-phase, it is impossible to distinguish them by linear infrared spectroscopy experiments. We show that the spectral signatures between trans- and cis-MCA are well resolved in ab initio calculated gas-phase electron-vibration–vibration two-dimensional infrared (EVV 2DIR) spectra. We further analyse the vibration mode pairs that mainly contribute to the intensity of these characteristic cross-peaks. Our work suggests that the EVV 2DIR technique can be used to in situ detect the concentration ratio of each species in the reversible reaction processes.

Guest‐Driven Light‐Induced Spin Change in an Azobenzene Loaded Metal–Organic Framework

AbstractStimuli‐responsive materials that can be reversibly switched by light are of immense interest. Among them, photo‐responsive spin crossover (SCO) complexes have great promises to combine the photoactive inputs with multifaceted outputs into switchable materials and devices. However, the reversible control the spin‐state change by photochromic guests is still challenging. Herein, we report an unprecedented guest‐driven light‐induced spin change (GD‐LISC) in a Hofmann‐type metal–organic framework (MOF), [Fe(bpn){Ag(CN)2}2]⋅azobenzene. (1, bpn=1,4‐bis(4‐pyridyl)naphthalene). The reversible trans–cis photoisomerization of azobenzene guest upon UV/Vis irradiation in the solid‐state results in the remarkable magnetic changes in a wide temperature range of 10–180 K. This finding not only establishes a new switching mechanism for SCO complexes, but also paves the way toward the development of new generation of photo‐responsive magnetic materials.

Guest-Driven Light-Induced Spin Change in an Azobenzene Loaded Metal-Organic Framework.

Stimuli-responsive materials that can be reversibly switched by light are of immense interest. Among them, photo-responsive spin crossover (SCO) complexes have great promises to combine the photoactive inputs with multifaceted outputs into switchable materials and devices. However, the reversible control the spin-state change by photochromic guests is still challenging. Herein, we report an unprecedented guest-driven light-induced spin change (GD-LISC) in a Hofmann-type metal-organic framework (MOF), [Fe(bpn){Ag(CN)2 }2 ]⋅azobenzene. (1, bpn=1,4-bis(4-pyridyl)naphthalene). The reversible trans-cis photoisomerization of azobenzene guest upon UV/Vis irradiation in the solid-state results in the remarkable magnetic changes in a wide temperature range of 10-180 K. This finding not only establishes a new switching mechanism for SCO complexes, but also paves the way toward the development of new generation of photo-responsive magnetic materials.

Liquid‐Phase Super Photoactuator through the Synergetic Effects of a Janus Structure and Solvent/Thermal/Photo Responses

AbstractAzobenzene actuator has attracted wide research attention in the fields of soft robots, artificial muscles, etc., owing to the typical photoresponsive material based on its reversible trans–cis isomerization. However, it remains challenging to enhance the actuation performance of azobenzene actuators through simple methods and can work in complex and variable environments. In contrast to complex molecular functional design, this study presents a Janus azobenzene inverse opal actuator: one side of a monodomain azobenzene polymer and the other side of a polydomain azobenzene inverse opal structure. The proposed design can significantly enhance the photoactuation performance in the liquid phase based on the synergetic effects of the Janus structure and the plastic influence of solvent/thermal/photo responses on the polymer segment. Promising applications of photo‐driven ring rolling in the liquid phase are demonstrated. The results of this study are of great significance for the design and fabrication of novel‐type azobenzene actuators in the liquid phase.

Emission Switching in the Near-Infrared by Reversible Trans-Cis Photoisomerization of Styrylbenzene-Conjugated Osmium Terpyridine Complexes.

A new array of homoleptic osmium(II) complexes based on styrylbenzene-conjugated terpyridine ligands (tpy-pvp-X) were synthesized and their photophysical, electrochemical, and photoisomerization behaviors thoroughly investigated in this work. Both electron-donating and -withdrawing substituents were incorporated onto a tpy-pvp-X (X = H, Me, Cl, NO2, and Ph) moiety to tune the optical properties and also the rate of photoisomerization behaviors in the complexes. All complexes display strong spin-allowed singlet metal-to-ligand charge-transfer bands in the visible (495-506 nm) and weak singlet ground state to triplet metal-to-ligand charge-transfer (3MLCT) broad bands within the 600-700 nm range. The complexes also exhibit strong phosphorescence emission from their 3MLCT state in the near-infrared domain (737-752 nm) at room temperature with excited-state lifetimes spanning between 107 and 165 ns. Two styrylbenzene units promote reversible trans-trans to trans-cis/cis-cis isomerization induced by light. The rate constants and quantum yields of photoisomerization were found to vary linearly with the Hammett σp parameters of the substituents. The rate and quantum yields were also found to decrease with increasing polarity of the solvents. Considerable modulation of the optical behavior along with luminescence switching in the complexes has been achieved upon photoisomerization. Moreover, the optical outputs as a function of two photonic stimuli inputs were used to demonstrate the binary function of a two-input IMPLICATION logic gate. In conjunction with the experimental study, computational investigations were also carried out in all three conformations of the complexes (trans-trans, trans-cis, and cis-cis) to have a perception of their electronic structures and for correct assignment of their absorption and emission spectral bands.

Norbornenyl-based amphiphilic ABA-triblock azobenzene copolymers: Synthesis, photoresponsive and self-assembly properties

A series of well-defined ABA-triblock copolymers P(NB-PtBA)x-b-P(NB-PFP)y-b-P(NB-PtBA)x were synthesized by living ring-opening metathesis polymerization (ROMP) of norbornenyl-poly(tert-butyl acrylate) (NB-PtBA), norbornenyl-pentafluorophenyl (NB-PFP) and NB-PtBA in a sequential manner by the use of Grubbs’ third generation catalyst (G3). The corresponding amphiphilic ABA-triblock azobenzene copolymers P(NB-PAA)x-b-P(NB-Azo)y-b-P(NB-PAA)x were obtained in high yields after subsequent post-polymerization modifications, including the reaction of active esters with amine-functionalized azobenzene to introduce the azobenzene unit, and the hydrolysis reaction to remove the tert-butyl groups. Under alternating UV and visible light illumination, the light response behaviors of the polymers were demonstrated by UV–Vis spectroscopy, and the reversible changes were observed as a result of the reversible trans-cis photoisomerization of azobenzene mesogens. These amphiphilic copolymers can also spontaneously self-assemble into spherical nanoparticles in an aqueous medium, and the hydrodynamic diameter increases as the hydrophobic NB-Azo content increases.

Stereoregular hybrid azobenzene-cyclosiloxanes with photoinduced reversible solid to liquid transition properties

A new type of stereoregular hybrid azobenzene-organocyclosiloxane discrete molecular structures with photoactive azobenzene groups unidirectionally connected to rigid cyclosiloxane frames through alkyl spacers of different lengths were developed for the first time.The kinetics and thermodynamics of nondegradative reversible trans-cis photoisomerization processes of the synthesized azobenzene-cyclosiloxane conjugates investigated using UV/Vis spectroscopy, DSC and XRD methods showed dependency on their molecular structures.It has been observed that room temperature UV (365 nm) irradiation of the synthesized hybrids leads to photoliquefaction effect consisting of a photoinduced phase transition of crystal solids into liquids. The photoliquefaction process was observed and confirmed with DSC, XRD and POM methods. Backward solidification process was generated with visible light and by heating.The tetra-azobenzene substituted cyclosiloxane single molecule compounds described in this work can be regarded as promising candidates for the development of new hybrid smart materials with many potential high-tech applications.

Photo-switchable iron-terpyridine complexes functionalized with styrylbenzene unit

The manuscript deals with synthesis, characterization, photophysics, and reversible trans-cis photoisomerization behaviours of three homoleptic Fe(II)-terpyridine complexes, ([Fe(tpy-pvp-X)2]2+ where XH, Me, and NO2) covalently coupled with photo-active styrylbeneze moiety. The complexes underwent trans-trans to cis-cis isomerization upon the action of both visible and UV light with remarkable change in their absorption and emission spectral profiles. The isomerization studies were performed in four different solvents, viz. DCM, MeCN, MeOH and DMSO with both UV and visible light sources. The reversal from cis-cis to trans-trans isomerization also proceeds, albeit very slowly, on keeping and can be accelerated upon heating. The rate, rate constant and quantum yield of photoisomerization were determined in all the solvents. The rate and quantum yield of isomerization found to be much higher in presence of visible light source in all solvents than that of UV light. To better understand the electronic structures and correctly assign the experimental optical spectral bands, DFT and TD-DFT computational studied were also carried out in their trans-trans, trans-cis and cis-cis forms. Reasonably good correlation between the experimental and calculated bands could lead us to assign the main absorption spectral bands of the compounds.

Ultralarge Contraction Directed by Light‐Driven Unlocking of Prestored Strain Energy in Linear Liquid Crystal Polymer Fibers

AbstractAnisotropic 1D contraction motion of polymeric actuating materials has drawn growing interests in fields ranging from soft robotics to biomimetic muscles. Although light‐driven liquid crystal polymers (LCPs) represent promising candidates to realize contraction (&lt;20%) triggered remotely and spatially, there remain multitudes of challenges to develop an LCP system possessing ultralarge contraction rate. Here, a novel strategy combining shape memory effect and photochemical phase transition is presented to realize light‐driven contraction as large as 81% in a newly designed linear liquid crystal copolymer, where the eutectic mesogens of azobenzene and phenyl benzoate self‐organize into the smectic B phase. Importantly, this highly ordered structure as the switching segment firmly locks the stress‐induced strain energy, which is rapidly released by reversible trans–cis photoisomerization that destroys the lamellar liquid crystal phase, therefore leading to such ultralarge contraction. Fibers serve as light‐driven building blocks to achieve precise origami, to mimic the recovery of a “broken” spider web and to screen objects in different sizes, laying new ground for advanced applications of light‐driven LCPs from biomimetic robots to human assists.

A Sandwich Azobenzene-Diamide Dimer for Photoregulated Chloride Transport.

There has been a tremendous evolution for artificial ion transport systems, especially gated synthetic systems, which closely mimic their natural congeners. Herein, we demonstrate a trans-azobenzene-based photoregulatory anionophoric system that transports chloride by forming a sandwich dimeric complex. Further studies confirmed a carrier-mediated chloride-anion antiport mechanism, and the supramolecular interactions involved in chloride recognition within the sandwich complex were revealed from theoretical studies. Reversible trans-cis photoisomerization of the azobenzene was achieved without any significant contribution from the thermal cis→trans isomerization at room temperature. Photoregulatory transport activity across the lipid bilayer membrane inferred an outstanding off-on response of the azobenzene photoswitch.