Cis Isomerization Process Research Articles

Ultrafast non-adiabatic dynamics of stilbene-based plant-derived sunscreens with cis–trans isomerization structures

In this work, we investigated the potential UV protection mechanism of the natural compounds hydroxy resveratrol and pterostilbene by combining theoretical calculations and femtosecond transient absorption spectra (FTAS). The UV absorption spectra showed that the two compounds exhibited strong absorption properties and high photostability. We found two molecules will reach the S1 state or an even higher excited state after UV exposure and molecules in S1 will cross a lower energy barrier to reach the conical intersection. The adiabatic trans–cis isomerization process happened and finally return to the ground. Meanwhile, FTAS clarified the time scale of trans–cis isomerization of two molecules was ∼ 10 ps, which also met the requirement of fast energy relaxation. This work also provides theoretical guidance for developing new sunscreen molecules from natural stilbene.

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.

Photoisomerization Kinetics of Photoswitchable Thin Films Based on Nanostructure/Molecular Layers of AlN‐AO7

The photoisomerization kinetics of photoswitchable thin films based on nanostructure/molecular layers of AlN-AO7 have been studied, investigated and reported. The trans → cis isomerization process occurs by UV-light irradiation. The cis-isomer could be turned back to the trans-isomer by either thermal or optical relaxation. The kinetics and time-evolution of the photoisomerization and reverse isomerization mechanism of AlN-AO7 thin films are investigated by UV-Vis absorbance spectra using relevant models. All phases of AlN-AO7 thin film, initial trans-, cis-, optical trans-, thermal trans-phases, were investigated using UV-Vis absorbance spectra, FTIR spectra, XRD and SEM. Transforming AlN-AO7 thin film from the initial trans-phase into cis-phase leads to curvature in the AO7 leaves and increases in the strain inside the structure. Going back to the trans-phase by either optical or thermal relaxation leads to vanishing the curvature and decreasing the structure's strain. Finally, the energy storage capacity was calculated using DSC and was found to be 36.38 J g-1 , simultaneously realizing the multisource solar energy storage and environmental heat.

Rational design and synthesis of a task-specific porous organic framework featured azobenzene as a photoresponsive low-energy CO2 adsorbent

An azobenzene-containing porous organic framework (POF) was rationally designed and synthesized via Sonogashira-Hagihara coupling reaction. By virtue of high porous skeleton and azobenzene as light responsive group, this task-specific POF exhibits the reversible stimuli-responsive adsorption property triggered by UV irradiation and heat treatment. The initial Brunauer-Emmet-Teller (BET) surface area of this porous material is 571 ​m2 ​g−1, while the BET surface area after UV irradiation decreases to 550 ​m2 ​g−1 along with the reduction of micropores around 0.5 ​nm and 1.3 ​nm during the trans to cis isomerization process. Furthermore, CO2 sorption isotherms show a slight decrease, which is caused by the trans to cis conversion of azobenzene side group. It is worth mentioning that CO2 uptakes of POFs are almost constant for three cycles via alternating external stimuli to confirm the excellently reversible switchability of trans-to-cis isomerization.

Trans-cis C-Pd-C rearrangement in hemichelates.

Kinetically unstable heteroleptic trans-bispalladacycles were isolated by using hemichelation. Three structures of trans isomers and five of cis isomers were characterized by X-ray diffraction analysis. The ready trans-to-cis isomerization of such hemichelates that was monitored by variable temperature NMR experiments is facilitated dynamically because the Pd(ii) center can preserve its square planar coordination in a rather low lying transition state, which was localized by methods of the density functional theory. This process is not achievable in the isomerization of conventional homoleptic trans-bispalladacycles since it involves the preliminary partial chelate decoordination and an unfavorable high-lying planar trigonal coordinated - or Y-shaped-Pd(ii) transition state according to DFT investigations.

Influence of supramolecular interactions on photoresponsive behavior of azobenzene poly(amide imide)s

Abstract This study presents the comparison between the photoresponsive behavior of a series of six poly(amide imide)s bearing azobenzene-derivative chromophores as side groups. The solubility, molecular weight, glass transition temperature and thermal stability of the polymers are reported. The chosen azopolymers contain one or two azochromophores in the repeating units attached to the main chain at different locations, i.e., one of the aromatic cores of azobenzene is a part of backbone connected in the 2 and 4 position with either the amide or/and imide groups. UV–vis spectroscopy shows that the chromophore position affects the polymer absorption band. The photoinduced birefringence measurements reveal large differences in the photoresponsive behavior of the materials. The birefringence of order of 0.02 and 0.05 is achieved for the polymers of moderate molecular weight containing one azo-dye per structural unit attached between the amide groups or for the polymers with two chromophores per structural unit placed between both the amide and imide groups, respectively. The lack of optical anisotropy is observed for the polymers of a low molecular weight containing the azochromophores located between the imide groups, indicating a very low efficiency of the trans–cis isomerization process in these materials. The explanation of the observed relationship based on H-bond formation is confirmed by FTIR and 1H NMR analysis.

Comprehensive evaluation of the photo-transformation routes of trans-resveratrol

Liquid chromatography (LC) combined with accurate mass spectrometry (MS), based on the use of a hybrid quadrupole time-of-flight (TOF) MS system, is employed to systematically investigate the photo-transformation routes of trans-resveratrol. Experiments were performed in quartz tubes, containing ethanolic solutions (12% v/v) of the precursor compound, exposed to different ultraviolet (UV) sources and to solar light. Time-courses of trans-resveratrol and transformation products (TPs) were investigated by direct injection of different reaction times aliquots in the LC–QTOF–MS system. Structural elucidation of detected TPs was derived from interpretation of their accurate product ion scan spectra. Trans-resveratrol labelled with 13C6 in the mono-hydroxylated ring was also employed to further confirm the exact positions of some substituents in the generated TPs. In addition to the well-known trans-/cis-isomerization process, three different main reactions pathways were noticed under all the investigated conditions: (1) water addition to the exocycle double bond followed by oxidation to a ketone and cleavage of the molecule, (2) intramolecular cyclization to render a trihydroxylated phenanthrene, and (3) oxidation of the phenanthrene-like derivative to generate an orto-diquinone. Both, the trihydroxylated phenanthrene and the orto-diquinone underwent further aromatic hydroxylation reactions. The above transformation routes were also noticed for cis-resveratrol and the two analogue phytoalexins piceid and piceatannol. In addition to above transformation pathways, under solar light exposure, resveratrol underwent a molecular re-arrangement rendering the so-called resveratrone, whose structure consists of two fused aromatic rings bonded to a linear chain containing a carbonyl group conjugated with a double bond.

Targeted synthesis of porous aromatic frameworks with stimuli-responsive adsorption properties

Two porous aromatic frameworks, PAF-36 and PAF-37, containing azo moieties, were synthesized via Sonogashira–Hagihara coupling reactions. Gas sorption measurements indicated that reversible stimuli-responsive adsorption properties were triggered by ultraviolet (UV) irradiation and heat treatment, because of the presence of azo functional groups. The initial Brunauer–Emmett–Teller (BET) surface areas of PAF-36 and PAF-37 were 325 and 443 m2 g−1, respectively. After UV irradiation, the BET surface areas increased during the trans - cis isomerization process, and the micropore sizes, around 6 and 8 A, also increased. In addition, the CO2 adsorption capacities increased slightly because of trans - cis conversion of azo groups. It is worth mentioning that the CO2 uptakes of the polymers were almost constant during multiple cycles of alternating external stimuli, displaying high switchability of the trans - cis isomerization.

Photomechanical response of disperse red 1 azobenzene dye-doped PMMA polymer fiber

Disperse red 1 azobenzene (DR1) doped poly(methyl methacrylate) (PMMA) optical fiber has been shown to have a fast photomechanical response upon 633nm laser irradiation originating in photo-isomerization of the dopants between the cis and trans forms. In this work, laser light of 355nm wavelength is used to investigate the trans to cis isomerization process, which should result in length contraction. A three-point-contact optically actuated beam-controlling mount is made of dye doped polymer fiber segments and metal-coated microscope coverslips to measure the photomechanical response. The length change of the fiber is determined from a quadrant photodetector reading upon beam deflection. The fiber is observed to elongate upon UV irradiation. We find that for DR1 dye in PMMA polymer, the dominant mechanism of the photomechanical effect is photo-thermally stimulated isomerization rather than direct photoisomerization.

Single Dye Molecule Behavior in Fluorescent Core–Shell Silica Nanoparticles

Understanding the parameters that control the intermolecular interactions of chromophores encapsulated within nanoparticles is of fundamental importance to various fields of nanoscience. Employing single-molecule time and spectral domains, we studied a red-absorbing hemicyanine analogue (DY-630) covalently encapsulated in the core of ∼20 and ∼30 nm core–shell silica nanoparticles. We find that on average 4 and 7 dyes are encapsulated within these particles, respectively. Steady state and fluorescence correlation spectroscopy show unusually strong enhancements (up to 16 times) in the relative fluorescence efficiency of the nanoparticles as compared to the free dye in aqueous solution. This increase is explained in terms of restriction of the trans–cis isomerization process due to the more rigid local environment provided by the silica, and protection from the solute–solvent interaction, while preserving the spectral characteristics of the constituent dye. Single molecule measurements reveal that the majori...

Cation Enhanced trans-cis Photoisomerization in Lariat Azacrown Ethers

We describe the synthesis and effect of complexation on photochemical E (trans) to Z (cis) isomerization of lariat azacrown ethers. Lariat azacrown ethers-Ca2+ complexes are characterized by UV-Vis, 1H-NMR and fluorescence techniques. Photostationary state composition of E-Z isomers upon irradiation under direct excitation and complexation with Ca(ClO4)2, quantum yield of photoisomerization, and fluorescence properties are described. Anomalous increase in the quantum yields were observed for lariat azacrown ether-Ca2+ complexes. Fluorescence studies indicated that the highly polarized/charge transfer nature of the singlet excited state is involved in E (trans) to Z (cis) isomerization process.

On the spectra and isomerization of azobenzene attached non-covalently to an armchair (8,8) single-walled carbon nanotube

The structure, energies, electronic spectra and thermal trans– cis isomerization of azobenzene (AB) attached non-covalently to the sidewall of CNT(8,8) were calculated using the ONIOM(B3LYP/6–31 + G*:UFF) level of theory. We found that some subtle differences existed between the structures of the attached AB and the isolated one. The combination of AB and CNT(8,8) favored in energy because of their negative interaction energies. The attached trans-AB (TAB) was more stable than the physisorbed cis-AB (CAB). The electronic spectrum calculations showed that the absorption wavelengths of the attached CAB were red-shifted for the lowest three singlet states. The trans– cis isomerization process of the AB attached non-covalently to the CNT(8,8) involved mainly the bending of CNN angle. The π–π interaction between AB and CNT(8,8) was found to restrain the trans- to- cis isomerization of AB, but had little influence on the cis- to- trans backward reaction.

AB INITIO STUDY OF THE MOLECULAR STRUCTURE AND THERMAL CIS–TRANS ISOMERIZATION OF 3,3′-DIAMINO-4,4′-AZOFURAZAN AND 3,3′-DIAMINO-4,4′-AZOXYFURAZAN

In this work, we carried out the density functional theory (DFT) calculations in order to understand the molecular structures and thermal trans–cis isomerization of 3,3′-diamino-4,4′-azofurazan (DAAzF) and 3,3′-diamino-4,4′-azoxyfurazan (DAAF). We found that the azo group oxidation of DAAzF leading to the formation of DAAF induced evidently the twist of azofurazan molecule and weakened one of CN bonds connecting the furazan rings and azo group. This means that DAAzF is more stable than DAAF. In addition, the oxidation of azo group has a significant influence on the energy difference between trans- and cis-isomer of azofurazan. The energy difference between the isomers of DAAF is only about half of that of DAAzF. Furthermore, the thermal trans–cis isomerization of DAAzF and DAAF was found to follow a different mechanism: an inversion mechanism for DAAzF and a rotation mechanism for DAAF. By analyzing the reversible trans–cis isomerization process, we firstly proposed that a self-desensitization effect could be responsible for the low sensitivity of azofurazan. This new desensitization mechanism may be useful in the design of novel high energetic density material.

Theoretical study on the trans– cis isomerization and initial decomposition of energetic azofurazan and azoxyfurazan

In this work, we carried out the hybrid density functional theory (DFT) calculations in order to understand the thermal trans– cis isomerization and initial thermal decomposition of 3,3′-diamino-4,4′-azofurazan (DAAzF), 3,3′-diamino-4,4′-azoxyfurazan (DAAF), 3,3′-dinitro-4,4′-azofurazan (DAAF) and 3,3′-dinitro-4,4′-azoxyfurazan (DAAzF). The relative energy between the trans- and cis-isomer was also calculated at the B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) level of theory. We found that a negative correlation existed between the relative energy and the sensitivity for these energetic azofurazan and azoxyfurazan compounds, where the higher relative energy means the lower the sensitivity. It was also found that the oxidation of azo-group could cause the decreasing in the relative energy between the trans- and cis-isomer, as well as the alteration of the isomerization mechanism. An inversion mechanism operates for azofurazan compounds (DAAzF and DNAF) while a rotation mechanism works for azoxyfurazan compounds (DAAF and DNOAF). Compared with the thermal trans– cis isomerization, the homolytic cleavage of C–N bond needs to overcome a much higher energy barrier, which indicates that the energy of the external stimulus should firstly trigger the trans– cis isomerization, rather than the breakage of C–N bond. A self-desensitization effect caused by the reversible thermal trans– cis isomerization process was firstly proposed to explain that the azofurazan and azoxyfurazan compounds are class of energetic materials with lower sensitivity. This new concept (self-desensitization effect) is expected to be useful to design the novel high density, insensitive energetic material.

Water-assisted enol-to-keto tautomerism of a simple peptide model: A computational investigation

Computational investigations on the enol-to-keto tautomerism of N-methylacetamide (NMA), a simple peptide model, in water are performed by B3LYP and MP2 methods with the 6-311+G(d,p) and 6-311++G(2df,2pd) basis sets. Calculated results indicate that the enol/ trans form is less stable than the keto/ trans form both in the gas-phase and in water. The effects of solvent water are described using the combined supramolecular/continuum approach based on the SCRF/PCM model, in which one, two, and three water molecules are, respectively, used to assist this tautomerism of NMA. Two possible enol-to-keto tautomeric pathways are considered in this study. The first type of pathway is a stepwise process and characterized by the cis/ trans isomerization of the C–N bond. The second type of tautomeric pathway is a concerted mechanism that the hydrogen atom of the enol/ trans NMA migrates directly from O to N atom to form the stable keto/ trans structure. The results show that the rate-limiting step in the stepwise pathway is the enol/ trans-enol/ cis isomerization process where the transition state presents a linear conformation approximatively and the reaction energy barrier is little affected by the solvent. In the concerted pathway, at least three water molecules are needed to assist the proton transfer from O to N atom in NMA. It seems that the concerted process with the participation of three water molecules is the most favorable path for the enol-to-keto tautomerism of NMA, because its Gibbs free energy barrier of activation of 11.9 kcal/mol is lower than those of the other pathways.

INTRAMOLECULAR CHARGE TRANSFER AND PHOTOISOMERIZATION OF THE DCM STYRENE DYE: A THEORETICAL STUDY

Structures and properties of the low-lying states in 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) have been investigated theoretically. Calculations show that the dimethylamino and dimethylanilino twisted conformations of DCM on the potential energy surface of the first excited state ( S 1) have relatively high stabilities and remarkable intramolecular charge transfers (ICT). Both structures can serve as candidates for the red-shifted emissive state in polar solvent. In particular, the dimethylanilino twisted ICT state has been predicted to have a dipole moment increment of 20 Debye with respect to the ground state by CASSCF calculations, in good agreement with the suggested experimental values. The optimized geometry of the S 1 state exhibits a long central CC bond of 1.458 Å, which makes the trans–cis isomerization quite facile through intramolecular rotation around the central CC bond on the S 1 potential surface. The S 1 state is a precursor to the formation of the ICT emissive state and photoinduced trans–cis isomerization. The S 1/ S 0 crossing in polar solvent and avoid-crossing in the gas phase as well as in non-polar solvent are involved in the trans–cis isomerization process. The presence of an early S 1/ S 0 crossing in the strong polar solvent reduces the isomerization efficiency.

Synthesis and photochromic behavior of new polyimides containing azobenzene side groups

Aromatic polyimides containing side azobenzene groups have been synthesized by low-temperature solution polycondensation of certain aromatic dianhydrides with aromatic diamines containing preformed side azobenzene groups followed by chemical imidization at 100°C in the presence of pyridine and acetic anhydride. The weight average molecular weight of these polymers is in the range of 16,000–129,000. The glass transition temperature of these polyimides is in the range of 185–230°C. The polymer architecture presents a special characteristic, one of the azobenzene aromatic units being in the main chain of the polymer. This situation is intermediary between main-chain and side-chain azobenzene-containing polymers. The photochromic behavior, determined by the trans–cis isomerization process of azo-groups, in solution and in solid state, was evaluated. The synthesized polyimides were studied by spectral methods (UV, IR), thermal analysis and molecular modeling. A preliminary study concerning the surface structuration capacity of the polyimides was accomplished. Good results were obtained using Nd:YAG laser at 355nm, at an incident fluence of 35mJ/cm2 (situated below the laser ablation limit).

Diamagnetic 8-amidoquinoline and 8-(trialkylsilylamido)quinoline complexes of divalent nickel and zinc

Transamination of Zn[N(SiMe 3) 2] 2 with 8-(triisopropylsilylamino) ( 1a) and 8-(tert-butyldimethylsilylamino)quinoline ( 1b) yields monomeric heteroleptic bis(trimethylsilyl)amido zinc 8-(trialkylsilylamido)quinoline ( 2a, 2b). The reaction of Zn[N(SiMe 3) 2] 2 with 8-aminoquinoline leads to the formation of heteroleptic dimeric 8-amidoquinoline zinc bis(trimethylsilyl)amide ( 3). This compound shows fluxionality on the NMR time scale due to a transannular trans– cis isomerization process and also a strong temperature dependency of the ratio of these two isomers. Variation of the stoichiometry allows the synthesis and isolation of homoleptic zinc bis(8-amidoquinoline) ( 4) with the zinc atoms in a distorted tetrahedral environment. The metallation of 8-aminoquinoline with (tmeda)NiMe 2 yields diamagnetic nickel bis(8-amidoquinoline) ( 5) with a nickel center showing a distorted square planar coordination sphere. Dinuclear zinc complexes are accessible employing tetra-dentate di(8-aminoquinolyl)diphenylsilane ( 6). The metallation of 6 with dimethylzinc yields dimeric zinc di(8-amidoquinolyl)diphenylsilane ( 7) with tetrahedrally coordinated zinc atoms. A comparison of the molecular structures of 1a, 2b, 3, 4, 5, 6, and 7 shows nearly no dependency of the structural data of the quinolyl unit on the substitution pattern of the amino function. The metal–nitrogen bond lengths depend on the coordination numbers of the metal and the nitrogen atoms (terminal or bridging position) as well as the electrostatic attraction between the metal cations and the amino bases.

Photo-induced isomerization of poly 4′-(6-acryloxy) hexyloxy-4-methoxyazobenzene ultra-thin solid film prepared by Langmuir–Blodgett technique

Abstract The photo-induced response of an ultra thin polymeric film of poly 4′-(6-acryloxy) hexyloxy-4-methoxyazobenzene (P5) is investigated. A monolayer of P5 at a gas–water interface possesses a mean molecular area of 28.0 A 2 /monomer-repeat. Multilayer films of P5 were prepared by horizontal deposition at a surface pressure of 25 mN/m 2 . The uniformity of the transfer process is shown by UV–vis absorption spectra where a linear relationship between the absorption maxima and the number of transferred layer was observed. The average layer thickness of the transferred film determined by XRD measurements is 34.0 A. This is longer than the length of the azobenzene side group. The transferred film shows a blue shift of the π–π ∗ transition from 357 nm for the P5 in solution to 340 nm for the P5 in the film. This suggests the formation of H-aggregate with a head-to-head arrangement of the dipole within the film. The optical property of the transferred film is changed by the irradiation of the film with the UV light at 385 nm. An irreversible change in its molecular packing in the film is seen in the shift of the UV–vis absorption maxima and the change in morphology as observed by AFM. The film morphology changes from being a smooth film into an island-like surface when exposed to the UV irradiation. The layer structure in the film is destroyed. A mass transport is observed during the cis–trans thermal back isomerization process. This suggests that movement of the P5 took place in both the trans–cis isomerization process and the cis–trans back isomerization process. The first movement leads to a molecular expansion while the second, to a molecular contraction.

Initial steps of signal generation in photoactive yellow protein revealed with femtosecond mid-infrared spectroscopy.

Photoactive yellow protein (PYP) is a bacterial blue light sensor that induces Halorhodospira halophila to swim away from intense blue light. Light absorption by PYP's intrinsic chromophore, p-coumaric acid, leads to the initiation of a photocycle that comprises several distinct intermediates. Here we describe the initial structural changes of the chromophore and its nearby amino acids, using visible pump/mid-infrared probe spectroscopy. Upon photoexcitation, the trans bands of the chromophore are bleached, and shifts of the phenol ring bands occur. The latter are ascribed to charge translocation, which probably plays an essential role in driving the trans to cis isomerization process. We conclude that breaking of the hydrogen bond of the chromophore's C=O group with amino acid Cys69 and formation of a stable cis ground state occur in approximately 2 ps. Dynamic changes also include rearrangements of the hydrogen-bonding network of the amino acids around the chromophore. Relaxation of the coumaryl tail of the chromophore occurs in 0.9-1 ns, which event we identify with the I(0) to I(1) transition observed in visible spectroscopy.