Cis Isomerization Research Articles

Photopyroelectric Investigation of the trans–cis Isomerization Effect on the Nematic- Isotropic Phase Transition of a Liquid Crystalline Azobenzene

Photopyroelectric calorimetry has been applied to the study of the effects of different concentration of photo-induced cis isomeric molecules on the characteristics of the nematic – isotropic phase transition in p,p’-diheptylazobenzene (7AB) samples. The investigations could be carried out by monitoring the behavior of the thermal diffusivity over the phase transition before and during the sample irradiation with UV light of varying intensity. Among other, it is shown that the monitoring of the time dependence of the thermal diffusivity at fixed sample temperature enabled the comparative study of the dynamics of the trans–cis and cis–trans isomeric transitions. It was shown that the increasing cis isomers concentration caused an increase in the fraction of material undergoing the phase transitions and increased the cis–trans conversion rate. Finally, polarization microscopy observations of the sample texture, which could be carried out together with the calorimetric evaluations proved useful for a more comprehensive analysis of the obtained results.

Computational and experimental investigation of photoresponsive behavior of 4,4′-dihydroxyazobenzene diglycidyl ether

The photochromic properties of 4,4′-dihydroxyazobenzene diglycidyl ether have been investigated to gain further insight into the photoisomerization pathways. The light induced trans–cis isomerization, thermal cis–trans recovery and light induced cis–trans isomerization were monitored using electronic absorption spectroscopy. The pohotoisomerization and thermal relaxation kinetics of azobenzene chromophores were investigated in solution. It was found that the trans–cis isomerization process occurs very fast and a high content of cis isomer was obtained (94 %). The optimization of the ground and transition state geometries as well as the electronic structure of the azo epoxy derivative was performed by the density functional theory (DFT) and time-dependent DFT with PBE0 and CAM-B3LYP functionals, using 6-31 + G(d,p) as basis set. Also, the theoretical UV–vis absorption spectra were calculated. The mechanism of isomerization along the S2, S1 and S0 energy levels has been described. It was shown how each internal coordinate contributes to the isomerization mechanism. TD-DFT and DFT methods revealed that modification of the NN bond length was the internal coordinate responsible for the trans to cis as well as cis to trans conversion.

Monitoring the stability of the xanthophyll fucoxanthin in microalga and seaweed biomasses, and extracts stored at low temperatures

The analysis of fucoxanthin (FCX) in algal biomasses presents some limitations, especially regarding the sample preservation after collection until extraction of the pigment. FCX stability was evaluated in biomass samples of one diatom and three brown seaweed species (Phaeophyceae) and their extracts stored at -20ºC and -80ºC, for 60 days. Fucoxanthin was quantified through UHPLC-DAD. The methanolic extracts of the micro/macroalga biomasses stored at -20ºC and -80ºC were more effective in preserving the xanthophyll, comparatively to the algal biomasses. Besides, the chromatographic analysis showed an increase in the FCX’s peak retention time along the sample storage, possibly due to the trans→cis isomerization of the molecule. Such information is relevant for choosing the best protocol for storing and preserving at most FCX, because changes in the geometric conformations of the molecule over storage strongly affects its biological activities.

Long-Range Proton Transfer in 7-Hydroxy-Quinoline-Based Azomethine Dyes: A Hidden Reason for the Low Efficiency

In the tautomeric Schiff bases, derived from 7-hydroxyquinoline, two competitive channels are possible upon excitation of the enol tautomer, namely proton transfer (PT) through intramolecular hydrogen bonding to the corresponding keto form and trans-cis isomerization around the azomethine double bond. The former leads to switching, based on twist-assisted excited state intramolecular PT, where the long-range proton transfer can occur as a targeted process. The latter, determined by the flexibility of the crane part, reduces the efficiency of the main targeted process. In previously studied molecular switches based on the 7-hydroxyquinoline skeleton, only the intramolecular PT photo-process undergoing from the excited enol form towards the keto tautomer, which is in most cases barrierless, has been discussed. Therefore, in the current study, the ground state PT properties and isomerization of (E)-8-((phenylimino)methyl)quinolin-7-ol and (E)-8-(((pentafluorophenyl)imino)methyl)quinolin-7-ol are investigated in depth using the MP2 methodology, while the excited state energy profiles are calculated with the ADC(2) method. The obtained results are discussed in light of the existing experimental data.

DFT Study of Mechanism and Stereochemistry of Nickel-Catalyzed trans-Arylative Desymmetrizing Cyclization of Alkyne-Tethered Malononitriles.

Present here is a density functional theory (DFT) study of the mechanism and origin of enantioselectivity of Ni-catalyzed desymmetric cyclization of alkyne-tethered malononitriles and aryl boronic acids. The reaction starts from transmetalation and arylnickel addition, followed by trans to cis isomerization to give cis-alkenyl nickel species. The stereodetermining step is the CN insertion, which prefers a transition state with the bystander CN group staying away from the ligand to reduce steric repulsion, and gives the final (R)-product.

Thermal-induced impact on physicochemical property and bioaccessibility of β-carotene in aqueous suspensions fabricated by wet-milling approach

The physicochemical properties of β-carotene in aqueous system were scarcely explored before. Herein, β-carotene aqueous suspensions were fabricated by wet-milling, and the impact of milling temperature on thermal degradation kinetics, coloring mechanism, recrystallization behavior and bioaccessibility of β-carotene in aqueous system was investigated. The thermodynamic parameters indicated that the degradation of total β-carotene and all-trans isomer in aqueous suspensions was endothermic and energetically unfavorable. The hue of aqueous suspensions was dependent on the particle size and cis isomer ratio of β-carotene (especially in the formation of 13-cis isomer). The reduced conjugation effect of CC groups was responsible to the hypochromicity and amorphization of isomerized β-carotene. The recrystallization rate of β-carotene in aqueous suspensions was decreased owing to the presence of higher magnitude of steric hindrance between the isomerized β-carotene. The bioaccessibility of β-carotene was significantly increased through the nanoencapsulation and elevating the level of cis isomers of β-carotene.

NH3[sbnd]NO interaction at low-temperatures: An experimental and modeling study

The present work provides new insight into NH3NO interaction under low-temperature conditions. The oxidation process of neat NH3 and NH3 doped with NO (450, 800 ppm) was experimentally investigated in a Jet Stirred Flow Reactor at atmospheric pressure for the temperature range 900–1350 K. Results showed NO concentration is entirely controlled by DeNOx reactions in the temperature range 1100–1250 K, while NH3NO interaction does not develop through a sensitizing NO effect, for these operating conditions.A detailed kinetic model was developed by systematically updating rate constants of controlling reactions and declaring new reactions for N2H2 isomers (cis and trans). The proposed mechanism well captures target species as NO and H2 profiles. For NH3NO mixtures, NO profiles were properly reproduced through updated DeNOx chemistry, while NH2 recombination reactions were found to be essential for predicting the formation of H2. The role of ammonia as a third-body species is implemented in the updated mechanism, with remarkable effects on species predictions. For neat NH3 mixture, the reaction H+O2(+M)=HO2(+M) was crucial to predict NO formation via the reaction NH2+HO2H2NO+OH.

Phenoxy-Amidine Ligands: Toward Lactic Acid-Tolerant Catalysts for Lactide Ring-Opening Polymerization

The replacement of the imine functionality in the ubiquitous phenoxy-imine (FI) ligands by a more robust and donor N,N,N′-trisubstituted amidine function was examined and gave rise to the synthesis of five new phenoxy-amidine (FA) ligands (L1–L5). The solid-state structure of four proligands has been determined by X-ray diffraction analysis and showed that the amidine moiety is in a trans-configuration. The reaction of the phenol-amidine proligands with AlMe3 afforded mononuclear (L1–L5)AlMe2 (1a–5a). A similar alkane elimination route was used from ZnEt2 and led to dinuclear [(L1–L5)ZnEt]2 complexes (1b–5b) or to homoleptic (L2/L4)2Zn complexes (2b′, 4b′) depending on the metal/ligand ratio used. The structure of these complexes has been determined by NMR spectroscopy (1H, 13C, HMBC, HSQC, DOSY, and NOESY experiments) and X-ray diffraction study for seven of them. The crystal structure of the Al complexes showed FA ligands coordinated in a chelate fashion via the O atom of the aryloxy group and the imino-N atom, indicating that the amidine function has undergone trans–cis isomerization upon coordination. A similar chelating coordination mode was observed for the FA ligands with Zn metal ions. These complexes were used as initiators for the ring-opening polymerization of rac-lactide. FA–Zn complexes gave the best performance, affording polylactic acid with a narrow molecular weight distribution and heterotactic bias (Pr up to 0.75). Remarkably, some of these complexes were able to tolerate the presence of a large amount of lactic acid to the point of using it as a co-initiator during the polymerization reaction.

The shackling photoisomerization effect on self-assembly of azobenzene-containing side-chain homopolymers

We herein demonstrate a simple method for designing and synthesizing a series of azobenzene-containing side-chain homopolymers via acyclic diene metathesis polymerization, which display the distinct photoresponsive properties from azobenzene units under alternating irradiation of UV and Vis light. The trans–cis isomerization of azobenzene pendants in one of the homopolymers is limited, thus obtains more trans-isomers in photostationary state, leading to the strong non-covalent π–π interaction between azobenzene pendants that can further drive self-assembly to form the spherical micelles. Moreover, the size of these micelles can be modulated through increasing the molecular weight of homopolymer. Importantly, we can realize the morphological transitions of these micelles from the spherical nanoparticles to shuttle-like nanoaggregates by UV light irradiation. All these results will not only provide a direct and efficient strategy for constructing photosensitive homopolymers, but also expand the self-assembly behaviors and of optical properties azobenzene-containing homopolymers.

Photoswitchable assembly of long-lived azobenzenes in water using visible light

HypothesisSwitchable assemblies relevant for bio-applications may be accessed from water-soluble tetra-ortho-substituted azobenzenes that reversibly self-assemble and form complexes with β-cyclodextrin under visible light. ExperimentsTwo azobenzenes bearing either four fluorines or two chlorines and two fluorines in the ortho positions were synthesised with short poly(ethylene oxide) tails for water solubility. Photophysical properties were determined by UV–vis and 1H NMR spectroscopies, complexation with β-cyclodextrin was assessed by 1H NMR spectroscopy, and self-assembly in water was investigated by static and dynamic light scattering. FindingsBoth molecules underwent trans–cis isomerization at 530 nm and cis–trans isomerization at 415 nm, with the cis forms exhibiting thermal half-lives > 300 days at room temperature. Both molecules formed inclusion complexes with β-cyclodextrin in water, with cis-4F-AZO-PEO binding 3-fold stronger than trans, and 2Cl2F-AZO-PEO binding significantly weaker. Self-assembly of pure 2Cl2F-AZO-PEO in water showed an open association process regardless of configuration, while 4F-AZO-PEO showed an open association process for cis (Nagg increasing from 30 to 1000) but a closed association process for trans (Nagg stable at ∼ 170). Aqueous solutions of 2Cl2F-AZO-PEO showed cloud points close to 45 °C, while the 4F-AZO-PEO isomers presented well-separated cloud points allowing reversible and all-visible transition between clear and turbid states at room temperature.

Transdermal diffusion of resveratrol by multilamellar liposomes: Effect of encapsulation on its stability

Trans-Resveratrol (RV) was encapsulated in multi-lamellar liposomes (MLLs) composed of a mixture of phosphatidylcholine (P100), Tween 80 (T80) and water. The P100-to-T80 ratio as well as their water content were chosen based on a previous study to promote transdermal transport of RV. The effect of RV on the size, elasticity, and charge of MLLs was evaluated as well as the effect of encapsulation on the apparent solubility of RV. The diffusion of RV and MLLs in artificial skin, namely Strat-M™, was monitored by confocal Raman imaging, and compared with that of a RV solution and empty MLLs, while the transdermal passage rate was measured by UV–vis spectrophotometry on both artificial and excised human skin. RV was found to remain localized in the outer layer of the skin with less than 3% passing through it over a 24-h period in both skin types. Encapsulation in MLLs drastically increased its transdermal passage: 73 ± 10% after 3 h of incubation on excised human skin, and 10% on Strat-M™ after 9 h. Whereas RV in its free form underwent cis isomerization, MLLs protected it up to 9h before undergoing chemical degradation after 24h.

Opto-electronic properties of isomers of azobispyridine

The density-functional theoretical studies are carried out to examine the isomerization and opto-electronic properties of heteroaryl azobispyridine derivatives (AzoPy). Among the isomers, trans o-o-AzoPy is the lowest in energy. Gibbs free energy change associated with the trans–cis isomerization of AzoPy via rotation is found to be 34–38 kcal/mol, whereas the reverse transformation occurs with a relatively low barrier (21–24 kcal/mol). The dipole moment and length of the molecule are significantly changed due to isomerization suggesting their use in molecular machines. In dimer, trans o-o-AzoPy adopts a parallel-slipped orientation. The charge carrier mobilities indicate the molecules as electron transporters.

Efficient Strategy for Investigating the Third-Order Nonlinear Optical (NLO) Properties of Solid-State Coordination Polymers.

The investigation of third-order nonlinear optical (NLO) properties of coordination polymers (CPs) based on solid samples is very difficult but is crucial for practical applications. Herein, we show a method for preparing high optical quality CP films in a polymer matrix to study the third-order NLO performance of solid-state CPs. Two novel azobenzene-based CPs, [CdL(DMAc)(H2O)]n (1) and {[CuL(4,4'-azobpy)]·3H2O}n (2) (H2L = 5-((4-(phenyldiazenyl)phenoxy)methyl)isophthalic acid), were selected as study subjects. The corresponding microcrystals with a grain size of around 3 μm were doped into poly(vinyl alcohol) (PVA), forming CP films (1-MC/PVA and 2-MC/PVA). 1-MC/PVA and 2-MC/PVA exhibit NLO absorption switching behavior from saturable absorption (SA) to reverse saturable absorption (RSA) with increasing pulse energy. Moreover, their NLO properties can also be efficiently modulated by photostimulation energy due to the trans → cis isomerization of an azobenzene moiety. The density functional theory (DFT) results show that the narrower the band gap between the conduction band minimum and the valence band maximum, the denser the electron density distribution in the central mental and coordination atoms, which is beneficial to exhibit better third-order NLO performance. This work provides a feasible method for the wider practical application of solid materials with excellent third-order NLO performance.

Coenzyme Q10 in the eye isomerizes by sunlight irradiation

Photoisomerization of lipids has been well studied. As for the eyes, photoisomerization from 11-cis isomer to all-trans-retinal is well-known as the first step of the visual transduction in the photoreceptors. In addition to that, there would be other ocular lipids that undergo photoisomerization, which may be involved in ocular health and function. To explore any photoisomerizable lipids in the eyes, the nonirradiated and sunlight-irradiated eyeball extracts were subjected to liquid chromatography-mass spectrometry analysis, followed by the identification of the decreased lipid species in the irradiated extracts. Surprisingly, more than nine hundred lipid species were decreased in the irradiated extracts. Three lipid species, coenzyme Q10 (CoQ10), triglyceride(58:4), and coenzyme Q9, were decreased both significantly (p < 0.05) and by more than two-fold, where CoQ10 showed the most significant decrease. Later, photoisomerization was identified as the prominent cause underlying the decrease of CoQ10. Interestingly, CoQ10 in the sunlight-irradiated fresh eyeballs was also isomerized. Both the visible light and ultraviolet radiation were capable of producing CoQ10 isomer, while the latter showed rapid action. This study is believed to enhance our understanding of the biochemistry and photodamage of the eye and can potentially contribute to the advancement of opto-lipidomics.

Light-Modulated Rheological Properties in Green Innovative Formulations

The addition of azorubine to a viscoelastic aqueous dispersion of sodium oleate (NaOL, 0.43 M, 13% w/w) and KCl (up to 4% w/w) leads to a green gel-like system whose rheological behavior can be efficiently and reversibly triggered from remote by using UV light. Rheology, Differential Scanning Calorimetry (DSC) measurements and phase behavior studies indicate that the original texture of the NaOL dispersion is significantly hardened upon UV irradiation for 8 hours in the presence of azorubine, showing a seven hundred-fold increase in viscosity. The UV treatment brings about the trans to cis isomerization of azorubine, which modifies the structure of the NaOL wormlike micellar system, leading to a more entangled, close-textured network. The cooperative effect of KCl on the fluid viscosity is found to be concentration-dependent. The system slowly reverts to its original rheological behaviour after standing for about 1 day. These results are relevant for the development of stimuli-responsive innovative systems based on biocompatible, non expensive and commercially available materials that can be used in a wide range of applications, such as in drug delivery or enhanced oil recovery, where a quick change in the physico-chemical features of the system is required but difficult to be performed.

Intramolecular vibrational redistribution in formic acid and its deuterated forms.

The intramolecular vibrational relaxation dynamics of formic acid and its deuterated isotopologues is simulated on the full-dimensional potential energy surface of Richter and Carbonnière [J. Chem. Phys. 148, 064303 (2018)] using the Heidelberg MCTDH package. We focus on couplings with the torsion vibrational modes close to the trans-cis isomerization coordinate from the dynamics of artificially excited vibrational mode overtones. The bright C-O stretch vibrational mode is coupled to the out-of-the plane torsion mode in HCOOH, where this coupling could be exploited for laser-induced trans-to-cis isomerization. Strong isotopic effects are observed: deuteration of the hydroxyl group, i.e., in HCOOD and DCOOD, destroys the C-O stretch to torsion mode coupling whereas in DCOOH, little to no effect is observed.

Near-Infrared Light Regulation of Capture and Release of ctDNA Platforms Based on the DNA Assembly System.

Despite recent progress, a challenge remains on how to gently release and recover viable ctDNA captured on DNA probe-based devices. Here, a reusable detector was successfully manufactured for the capture and release of ctDNA by means of an UCNPs@SiO2-Azo/CD-probe. Biocompatible NIR light is used to excite UCNPs and convert into local UV light. Continuous irradiation induces a rapid release of the entire ctDNA-probe–CD complex from the functionalized surface via the trans−cis isomerization of azo units without disrupting the ctDNA-structure receptor. Specifically, these composite chips allow reloading DNA probes for reusable ctDNA detection with no obvious influence on their efficiency. The results of our study demonstrated the potential application of this platform for the quantitative detection of ctDNA and the individualized analysis of cancer patients.

Structure-Performance Relationships for Tail Substituted Zwitterionic Betaine-Azobenzene Surfactants.

Azobenzene-containing surfactants (azo-surfactants) have garnered significant attention for their use in generating photoresponsive foams, interfaces, and colloidal systems. The photoresponsive behavior of azo-surfactants is driven by the conformational and electronic changes that occur when the azobenzene chromophore undergoes light-induced trans ⇌ cis isomerization. Effective design of surfactants and targeting of their properties requires a robust understanding of how the azobenzene functionality interacts with surfactant structure and influences overall surfactant behavior. Herein, a library of tail substituted azo-surfactants were synthesized and studied to better understand how surfactant structure can be tailored to exploit the azobenzene photoswitch. This work shows that tail group structure (length and branching) has a profound influence on the critical micelle concentration of azo-surfactants and their properties once adsorbed to an air-water interface. Neutron scattering studies revealed the unique role that intermolecular π-π azobenzene interactions have on the self-assembly of azo-surfactants, and how the influence of these interactions can be tuned using tail group structure to target specific aqueous aggregate morphologies.

Optochemical Control of Therapeutic Agents through Photocatalyzed Isomerization

AbstractA Ru(bpy)3Cl2 photocatalyst is applied to the rapid trans to cis isomerization of a range of alkene‐containing pharmacological agents, including combretastatin A‐4 (CA‐4), a clinical candidate in oncology, and resveratrol derivatives, switching their configuration from inactive substances to potent cytotoxic agents. Selective in cellulo activation of the CA‐4 analog Res‐3M is demonstrated, along with its potent cytotoxicity and inhibition of microtubule dynamics.

Optochemical Control of Therapeutic Agents through Photocatalyzed Isomerization.

A Ru(bpy)3Cl2 photocatalyst is applied to the rapid trans to cis isomerization of a range of alkene‐containing pharmacological agents, including combretastatin A‐4 (CA‐4), a clinical candidate in oncology, and resveratrol derivatives, switching their configuration from inactive substances to potent cytotoxic agents. Selective in cellulo activation of the CA‐4 analog Res‐3M is demonstrated, along with its potent cytotoxicity and inhibition of microtubule dynamics.