Thermal Cis-trans Isomerization Research Articles

Combined Liposome-Gold Nanoparticles from Honey: The Catalytic Effect of Cassyopea® Gold on the Thermal Isomerization of a Resonance-Activated Azobenzene.

Gold nanoparticles (AuNPs) have been synthesized directly inside liposomes using honey as a reducing agent. The obtained aggregates, named Cassyopea® Gold due to the method used for their preparation, show remarkable properties as reactors and carriers of the investigated AuNPs. A mean size of about 150 nm and negative surface charge of -46 mV were measured for Cassyopea® Gold through dynamic light scattering and zeta potential measurements, respectively. The formation of the investigated gold nanoparticles into Cassyopea® liposomes was spectroscopically confirmed by the presence of their typical absorption band at 516 nm. The catalytic activity of the combined liposome-AuNP nanocomposites was tested via the thermal cis-trans isomerization of resonance-activated 4-methoxyazobenzene (MeO-AB). The kinetic rate constants (kobs) determined at 25 °C in the AuNP aqueous solution and in the Cassyopea® Gold samples were one thousand times higher than the values obtained when performing MeO-AB cis-trans conversion in the presence of pure Cassyopea®. The results reported herein are unprecedented and point to the high versatility of Cassyopea® as a reactor and carrier of metal nanoparticles in chemical, biological, and technological applications.

Cis-Trans Reisomerization Preceding Reprotonation of the Retinal Chromophore Is Common to the Schizorhodopsin Family: A Simple and Rational Mechanism for Inward Proton Pumping.

The creation of unidirectional ion transporters across membranes represents one of the greatest challenges in chemistry. Proton-pumping rhodopsins are composed of seven transmembrane helices with a retinal chromophore bound to a lysine side chain via a Schiff base linkage and provide valuable insights for designing such transporters. What makes these transporters particularly intriguing is the discovery of both outward and inward proton-pumping rhodopsins. Surprisingly, despite sharing identical overall structures and membrane topologies, these proteins facilitate proton transport in opposite directions, implying an underlying rational mechanism that can transport protons in different directions within similar protein structures. In this study, we unraveled this mechanism by examining the chromophore structures of deprotonated intermediates in schizorhodopsins, a recently discovered subfamily of inward proton-pumping rhodopsins, using time-resolved resonance Raman spectroscopy. The photocycle of schizorhodopsins revealed the cis-trans thermal isomerization that precedes reprotonation at the Schiff base of the retinal chromophore. Notably, this order has not been observed in other proton-pumping rhodopsins, but here, it was observed in all seven schizorhodopsins studied across the archaeal domain, strongly suggesting that cis-trans thermal isomerization preceding reprotonation is a universal feature of the schizorhodopsin family. Based on these findings, we propose a structural basis for the remarkable order of events crucial for facilitating inward proton transport. The mechanism underlying inward proton transport by schizorhodopsins is straightforward and rational. The insights obtained from this study hold great promise for the design of transmembrane unidirectional ion transporters.

Sequence effects on the thermal cis–trans isomerization of side‐chain stearate‐containing azobenzene polymers

AbstractPhotochromic azobenzenes undergo light‐mediated trans–cis isomerization. The cis isomer reverts to the trans isomer thermally. To investigate the effect of different monomer sequences on the thermal stability of cis‐azobenzene, a series of copolymers, namely, block, and random structures containing stearic acid and azobenzene moieties as their side chains have been synthesized through reversible addition–fragmentation chain transfer (RAFT) polymerization. In this study, we investigate the photoisomerization of the trans and cis forms of the polymers and also the thermal reversal of the cis‐azobenzene photochromic systems. The isomerization data revealed significant differences in the isomerization timescales between the polymers and the corresponding monomer. It was observed that the local polarity around the azobenzene units within a polymer was significantly influenced by the chain segment depending on whether it was in the vicinity of the hydrophobic alkyl chains or other azobenzene units. This local environment of the azobenzene units regulates the stabilization of the transition states during the cis–trans thermal isomerization, consequently affecting the half‐life of the cis isomer.

Comprehensive investigations of trans-cis-trans isomerization in the solid state for azo polyimides

This work describes the thermal cis-trans isomerization in the solid-state for poly(ether imide)s and poly(ester imide)s with azobenzene or azo pyridine moieties. The cis-trans relaxation was monitored for covalently functionalized polyimides (main-chain, side-chain, „T-type”) and “guest-host” azo systems. The obtained results from the investigations allow us to formulate some general relationships between material aspects such as polyimide structure, its molar mass, the content of azo dye, the value of d-spacing, the thickness of polymer film, and thermal cis-trans isomerization. The reversibility of the trans-cis-trans reaction was studied in 10 subsequent cycles and 100 cycles for the selected polymers. Studies of the reversibility of the trans-cis-trans reaction were realized by irradiation using two wavelengths of radiation λ = 365 nm (P = 2.9 W) and λ = 460 nm (P = 850 mW). To the best of our knowledge, this is the first article that comprehensively presents solid-state cis-trans isomerization for azo polyimides.

Combined calorimetric, spectroscopic and microscopic investigation on the inclusion complex from cyclocurcumin and sulfobutylether-β-cyclodextrin in aqueous solution and Kinetics of thermal cis-trans isomerization

The complexation reaction between cyclocurcumin (CyCur), a natural curcuminoid with bioactive effects, and sulfobutylether-β-cyclodextrin (SBE-βCD), one of the more versatile and tolerate β-cyclodextrin derivates, was investigated in aqueous solution at 298 K. The UV–vis spectral changes of CyCur in the presence of SBE-βCD indicate the formation of the host-guest inclusion complex according to a 1:1 stoichiometry. The binding constant (Kb) determined by applying the Benesi-Hildebrand model was comparable to the value obtained by Isothermal Titration Calorimetry (ITC) measurements. Moreover, the thermodynamic parameters of the SBE-βCD/CyCur interactions demonstrate that the complexation process is enthalpically driven. Interestingly, microscopy analysis highlights the tendency of cyclocurcumin to aggregate into spherical fluorescent structures able to change their aspect and morphology in the presence of SBE-βCD. Finally, the thermal cis-trans isomerization rates of CyCur in the temperature range 294–314 K and the energetic parameters calculated by Arrhenius and Eyring plots were spectroscopically determined in SBE-βCD aqueous solution.

Characterization of DAG Binding to TRPC Channels by Target-Dependent cis-trans Isomerization of OptoDArG.

Azobenzene-based photochromic lipids are valuable probes for the analysis of ion channel–lipid interactions. Rapid photoisomerization of these molecules enables the analysis of lipid gating kinetics and provides information on lipid sensing. Thermal relaxation of the metastable cis conformation to the trans conformation of azobenzene photolipids is rather slow in the dark and may be modified by ligand–protein interactions. Cis photolipid-induced changes in pure lipid membranes as visualized from the morphological response of giant unilamellar vesicles indicated that thermal cis–trans isomerization of both PhoDAG-1 and OptoDArG is essentially slow in the lipid bilayer environment. While the currents activated by cis PhoDAG remained stable upon termination of UV light exposure (dark, UV-OFF), cis OptoDArG-induced TRPC3/6/7 activity displayed a striking isoform-dependent exponential decay. The deactivation kinetics of cis OptoDArG-induced currents in the dark was sensitive to mutations in the L2 lipid coordination site of TRPC channels. We conclude that the binding of cis OptoDArG to TRPC channels promotes transition of cis OptoDArG to the trans conformation. This process is suggested to provide valuable information on DAG–ion channel interactions and may enable highly selective photopharmacological interventions.

Novel Azocoumarin Derivatives-Synthesis and Characterization.

The paper presents synthesis and characterization of nine new thiazolyl-(phenyldiazenyl)-2H-chromen-2-one dyes. The impact of substituent structure in thiazole ring in the synthesized azocoumarin derivatives on electrochemical properties, photoisomerization process and photovoltaic response was examined. The dyes were electrochemically active and undergo reduction and oxidation processes. They showed low electrochemically estimated energy band gap in the range of 1.71–2.13 eV. Photoisomerization process of the synthesized molecules was studied in various solvents such as ethanol, chloroform and N,N-dimethylformamide (DMF) upon the UV illumination. It was found that novel azodyes showed reversible trans-cis-trans isomerization and exhibited long thermal back to the trans form, that was even 7 days in DMF. Selected azocoumarin were molecularly dispersed in polystyrene for preparation of guest-host azopolymer systems to study the cis-trans thermal isomerization of obtained dyes in solid state. The photovoltaic activity of the azochromophores was tested in bulk-heterojunction solar cells. They acting as weak donors in device with structure ITO/PEDOT:PSS/dye:PC70BM/Al. No photovoltaic response of cells with azocoumarin derivatives bearing 4-fluorobenzene, 3,4-dichlorobenzene, or 4-(1-adamantyl) unit was found. Additionally, dye which showed the best activity was examined in three-component solar cells ITO/PEDOT:PSS/PTB7:PC70BM:dye/PFN/Al.

New Photoresponsive Poly(meth)acrylates Bearing Azobenzene Moieties Obtained via ATRP Polymerization Exhibiting Liquid-Crystalline Behavior.

Here, we report our studies on photoresponsive poly(meth)acrylates containing azobenzene groups connected to a polymer backbone via a short methylene linker. A series of side-chain azobenzene polymers was synthesized via the atom transfer radical polymerization (ATRP) technique using several catalytic systems. The polymers synthesized under the optimized conditions were characterized by a narrow polydispersity (Đ ≤ 1.35), and they underwent a reversible transformation of their structure under light illumination. The fabricated polymers can store and release energy accumulated during the UV-light illumination by the thermal cis-trans isomerization of the chromophore groups. The enthalpy of the process (determined from DSC) was relatively high and equaled 61.9 J∙g−1 (17 Wh∙kg−1), indicating a high potential of these materials in energy storage applications. The liquid-crystalline behavior of the synthesized poly(meth)acrylates was demonstrated by the birefringent optical textures as observed in thin-films and X-ray scattering studies.

An insight into cyclocurcumin cis–trans isomerization: Kinetics in solution and in the presence of silver nanoparticles

Cyclocurcumin (CyCUR) is a non-diarylheptanoid curcuminoid characterized by an α,β-unsaturated dihydropyranone moiety generally existing as trans isomer in the ground state but able to convert into the cis form by photoisomerization. Herein the kinetics of thermal cis–trans isomerization of cyclocurcumin was spectroscopically determined in ethanol, pure water and silver nanoparticles (AgNPs) aqueous solution. Energetic parameters were calculated by Arrhenius and Eyring plots in the temperature range 294–314 K. The presence of AgNPs increases the rate of the reaction, according to the polarizability of the environment. The solvatochromism of cyclocurcumin was also studied by using the Catalán empirical solvent parameters scale in a series of organic solvents. The bathochromic shift of CyCUR suggests that polarizability of the medium plays the major role in the investigated conditions.

Reconfiguring Gaussian Curvature of Hydrogel Sheets with Photoswitchable Host-Guest Interactions.

Photoinduced shape morphing has implications in fields ranging from soft robotics to biomedical devices. Despite considerable effort in this area, it remains a challenge to design materials that can be both rapidly deployed and reconfigured into multiple different three-dimensional forms, particularly in aqueous environments. In this work, we present a simple method to program and rewrite spatial variations in swelling and, therefore, Gaussian curvature in thin sheets of hydrogels using photoswitchable supramolecular complexation of azobenzene pendent groups with dissolved α-cyclodextrin. We show that the extent of swelling can be programmed via the proportion of azobenzene isomers, with a 60% decrease in areal swelling from the all trans to the predominantly cis state near room temperature. The use of thin gel sheets provides fast response times in the range of a few tens of seconds, while the shape change is persistent in the absence of light thanks to the slow rate of thermal cis–trans isomerization. Finally, we demonstrate that a single gel sheet can be programmed with a first swelling pattern via spatially defined illumination with ultraviolet light, then erased with white light, and finally redeployed with a different swelling pattern.

A family of azoquinoline derivatives: Effect of the substituent at azo linkage on thermal cis-trans isomerization based on an experimental and computational approach

The subject of the research is a series of azo-dyes based on the quinoline structure, which are practically omitted as materials which use photoinduced optical anisotropy for applications. The work presents the kinetics of thermal cis-trans isomerization reaction of 5-azo-8-hydroxy-2-methylquinoline and 5,5'-((1E,1′E)-[1,1′-biphenyl]-4,4′-diylbis(diazene-2,1-diyl))bis(2-methylquinolin-8-ol) compounds. The impact of the structure of the substituent at the azo linkage on the cis-trans isomerization kinetics was investigated experimentally and theoretically using the UV–Vis spectroscopy and the density-functional theory (DFT). The kinetics of the dark-return of the cis-to trans-isomer of azoquinolines was investigated after UV-irradiation. It was found that attaching the nitro substituent to the azoquinolines structure retards the photoisomerization process in comparison to the other substituents (i.e. F, Cl, Br, N atoms or CH3, OH, COOH, OCH3 groups). The calculated rate constants were in qualitative agreement with the experimental values. To the best of our knowledge, the cis-trans isomerization kinetics for azoquinolines has not been studied yet.

Photochromic properties of some azomaleimide derivatives and DFT quantum chemical study of thermal cis-trans isomerization pathways

A systematic study on the dynamics of trans-cis, thermal cis-trans backward and light induced cis-trans isomerization of some azobismaleimide derivatives was performed. Systems under study were N-[4-(carbonylaminoazobenzene)-phenyl]-maleimide (AAzM) and a styrene-maleimide copolymer containing azo units in the side chain (AAzMC). The isomerization reactions were followed by electronic absorption spectra. The kinetics of trans-cis photoisomerization was investigated in six solvents with different polarities. Results revealed that the content of the photostationary state depended on the solvent nature. In turn, the chemical environment had a minor influence on the photoisomerization rate constants. The kinetic and thermodynamic activation parameters were evaluated according to Arrhenius and Eyring equations. The theoretical calculations based on density functional theory (DFT) were employed to identify the mechanism of the thermal isomerization in AAzM. In our results, the thermal cis-trans isomerization of azobismaleimide AAzM followed an inversion assisted by rotation pathway in the S0 state.

Silver nanoparticles as interactive media for the azobenzenes isomerization in aqueous solution: From linear to stretched kinetics

The thermal cis-trans isomerization of azobenzene and 4‑methoxyazobenzene has been investigated in aqueous solution in the presence of electrochemically synthesized silver nanoparticles (AgNPs) in the concentration range 0.32–32 ppm. The samples in which the investigated molecules in trans state have been converted into the cis isomer by irradiation immediately after their addition to the AgNPs solution show different kinetic behavior: in the case of azobenzene the cis-trans isomerization does not occur; in the case of 4‑methoxyazobenzene the reaction follows a first order decay. The samples in which the molecules have been added in trans state and left in solution for 120 h before their conversion into the cis isomers by irradiation show a similar behavior by which the cis-trans isomerization occurs according to the Kohlrausch-Williams-Watts stretched exponential kinetics. The UV–vis spectra and kinetics measurements performed at 25 ± 0.1 °C supported by zeta potential analysis reflect the existence of different microenvironments for the cis and trans isomers of azobenzene and 4‑methoxyazobenzene in solution in the presence of AgNPs in the investigated silver concentration range.

Fast dark cis-trans isomerization of azopyridine derivatives in comparison to their azobenzene analogues: Experimental and computational study

Thermal cis-trans isomerization of azobenzene and azopyridine derivatives was studied. The impact of the structural features of azo derivatives on its isomerization kinetics being important for molecular switching materials was investigated experimentally and theoretically based on the density-functional theory (DFT). Eleven azo-dyes in which the phenyl rings were substituted in the para-position to NN– linkage were prepared. Additionally, their thermal properties were evaluated by thermogravimetric analysis and differential scanning calorimetry. DFT calculations were also used for optimization ground and transition state geometries, the density of states, electronic structures of studied azo-dyes. The kinetics of the dark-return cis to trans-form of azo-compounds was investigated after UV-irradiation. It was found that attaching hydroxyalkoxy-substituents to the azo-dyes structure retarding the photoisomerization process in comparison to the hydroxyl group. Moreover, dark-return cis to trans-isomer was ca. twice times faster for azopyridine derivatives than for their azobenzene analogues. The calculated rate constants were in qualitative agreement with the experimental values.

Thermal Isomerization of Hydroxyazobenzenes as a Platform for Vapor Sensing.

Photoisomerization of azobenzene derivatives is a versatile tool for devising light-responsive materials for a broad range of applications in photonics, robotics, microfabrication, and biomaterials science. Some applications rely on fast isomerization kinetics, while for others, bistable azobenzenes are preferred. However, solid-state materials where the isomerization kinetics depends on the environmental conditions have been largely overlooked. Herein, an approach to utilize the environmental sensitivity of isomerization kinetics is developed. It is demonstrated that thin polymer films containing hydroxyazobenzenes offer a conceptually novel platform for sensing hydrogen-bonding vapors in the environment. The concept is based on accelerating the thermal cis–trans isomerization rate through hydrogen-bond-catalyzed changes in the thermal isomerization pathway, which allows for devising a relative humidity sensor with high sensitivity and quick response to relative humidity changes. The approach is also applicable for detecting other hydrogen-bonding vapors such as methanol and ethanol. Employing isomerization kinetics of azobenzenes for vapor sensing opens new intriguing possibilities for using azobenzene molecules in the future.

Catalysis of Ground State cis[Formula: see text] trans Isomerization of Bacteriorhodopsin's Retinal Chromophore by a Hydrogen-Bond Network.

For the photocycle of the membrane protein bacteriorhodopsin to proceed efficiently, the thermal 13-cis to all-trans back-isomerization of the retinal chromophore must return the protein to its resting state on a time-scale of milliseconds. Here, we report on quantum mechanical/molecular mechanical energy calculations examining the structural and energetic determinants of the retinal cis-trans isomerization in the protein environment. The results suggest that a hydrogen-bonded network consisting of the retinal Schiff base, active site amino acid residues, and water molecules can stabilize the twisted retinal, thus reducing the intrinsic energy cost of the cis-trans thermal isomerization barrier.

Kinetics of thermal cis-trans isomerization in a phototropic azobenzene-based single-component liquid crystal in its nematic and isotropic phases.

Single-component azobenzene-based phototropic liquid crystals (PtLC) are promising materials that have started to be explored for photonic applications. One of the essential factors determining the applicability of these materials is the rate of the thermally driven cis-trans isomerization. In this paper, the kinetics of the thermal back cis-to-trans reaction in a pure 4-hexyl-4'-methoxyazobenzene (6-AB-O1) compound in its isotropic liquid and nematic phases is studied (the undoped LC). The reaction rate constants, activation energies and thermal activation parameters were determined based on spectroscopic studies. The reaction kinetics is compared to that measured for the compound dissolved in chloroform. The results demonstrate that the thermal back reaction depends on the phase and molecular environment of the cis-isomer. Moreover, the effect of temperature on the absorption spectra of 6-AB-O1 in its isotropic, nematic and crystalline phases is examined. The changes in the compound's absorption spectra in the respective phases have been correlated to the positional order parameter S.

Tautomerism-Induced Cis-Trans Isomerization of Pyridylethenyl N-Confused Porphyrin.

Pyridylethenyl-substituted N-confused porphyrins (NCPs) were synthesized, and their cis-trans isomerization was studied. Among four possible isomers, trans-3H and cis-2H types of structures, of which aromaticity and absorption/emission properties differ largely, were isolated. The cis-isomer was largely stabilized by the intramolecular hydrogen bonding between the pyrrolic-NH and the pyridinic-N in the vicinity. The thermal cis-trans isomerization proceeded even at 30 °C, which was significantly accelerated by the pyridine added to the system. The kinetic studies revealed that the isomerization reaction was second-order and the activation energy of the thermal isomerization from cis to trans isomer was ΔG0⧧cis→trans = 35.7 kcal/mol at 298 K, which is significantly smaller than that of Ni complex (42.3 kcal/mol). An intermolecular proton transfer induced cis-trans isomerization mechanism was proposed.

Kinetics and Energetics of Thermal Cis-Trans Isomerization of a Resonance-Activated Azobenzene in BMIM-Based Ionic Liquids for PF₆-/Tf₂N- Comparison.

BMIM PF6 (1-butyl-3-methylimidazolium hexafluorophosphate) and BMIM Tf2N (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) are two conventional room-temperature ionic liquids widely employed and investigated as reaction media. Despite the presence of the same imidazolium ring in their structure they are different in many chemical and physical properties due to the nature of the anions. The thermal cis-trans isomerization of an electronically activated azobenzene have been used as reaction model to compare the behavior of PF6− and Tf2N−. Rotation is the mechanism by which the investigated azobenzene is converted into the trans isomer spontaneously in the dark both in BMIM PF6 and in BMIM Tf2N. The kinetic rate constants of the process have been determined at different temperatures and the activation energies of the reaction have been calculated according to the Arrhenius and Eyring equations. The results presented herein highlight different solute-solvent interactions involving the PF6− and Tf2N− anions during the cis-trans isomerization.

A theoretical study on the thermal cis–trans isomerization of azoheteroarene photoswitches

Azoheteroarene photoswitches exhibit superior properties and the thermal cis–trans isomerization processes are sensitive to the different substitution patterns.