Trans-cis Photoisomerization Research Articles

Photoresponsive Conjugated Microporous Polymer Films Fabricated by Electrochemical Deposition for Controlled Release.

Stable controlled release system, conjugated microporous polymers (CMPs) with stimuli-responsive properties can be ideal structures because their 3D microporous matrix structure and possible stimulated response provide inherent switchable acceptor sites to capture and release guest molecules. Herein, the in situ electrochemical deposition of precursors (DTCzAzo) is utilized to construct highly crosslinked photoresponsive CMP films, which can reversibly undergo the trans-to-cis isomerization alternately with irradiation by 355 and 480 nm laser beams. The size of pores in CMP films changes tremendously during the process of trans-cis photoisomerization, to controllably capture, conserve, and release the guest molecules.

Fusion of Ultraviolet-Visible and Infrared Transient Absorption Spectroscopy Data to Model Ultrafast Photoisomerization.

Ultrafast photoisomerization reactions generally start at a higher excited state with excess of internal vibrational energy and occur via conical intersections. This leads to ultrafast dynamics which are difficult to investigate with a single transient absorption spectroscopy technique, be it in the ultraviolet-visible (UV-vis) or infrared (IR) domain. On one hand, the information available in the UV-vis domain is limited as only slight spectral changes are observed for different isomers. On the other hand, the interpretation of vibrational spectra is strongly hindered by intramolecular relaxation and vibrational cooling. These limitations can be circumvented by fusing UV-vis and IR transient absorption spectroscopy data in a multiset multivariate curve resolution analysis. We apply this approach to describe the spectrodynamics of the ultrafast cis-trans photoisomerization around the C-N double bond observed for aromatic Schiff bases. Twisted intermediate states could be elucidated, and isomerization was shown to occur through a continuous complete rotation. More broadly, data fusion can be used to rationalize a vast range of ultrafast photoisomerization processes of interest in photochemistry.

Fabrication of Liquid Crystalline Polyurethane Networks with a Pendant Azobenzene Group to Access Thermal/Photoresponsive Shape-Memory Effects.

Herein, we report a novel thermal/photoresponsive shape-memory polyurethane network with a pendant azobenzene group by utilizing its anisotropic-isotropic phase transitions and photoresponsive feature concurrently. To achieve this goal, the side-chain liquid crystalline polyurethane networks based on the pendant azobenzene group [SCLCPU(AZO)-Ns] were developed in a well-defined architecture. The smectic C nature of an LC phase in the polyurethane networks was confirmed by differential scanning calorimetry, polarized optical microscopy, and one-dimensional and two-dimensional wide-angle X-ray diffraction. The well-defined architecture-made SCLCPU(AZO)-N displays two distinct transition temperatures (Ttrans) (Tg and Tcl), with a difference of about 40 °C. Consequently, the excellent triple-shape-memory effect in this network was demonstrated by cyclic thermomechanical analysis. By making full use of the trans-cis photoisomerization of azobenzene, the reversible bending and unbending behaviors were realized under the light irradiation with wavelengths of 450 and 550 nm, respectively.

Φ-order spectrophotokinetic characterisation and quantification of trans-cis oxyresveratrol reactivity, photodegradation and actinometry

A new Φ-order kinetic method was proposed in this study for the investigation of trans-cis photoisomerization reaction of Oxyresveratrol (ORVT) subjected to non-isosbestic irradiation. In ethanolic media, it has been proven that forward (ΦA→Bλirr) and reverse (ΦB→Aλirr) reaction quantum yields were dependent on the monochromatic irradiation wavelength according to sigmoid patterns over the spectral ranges of their electronic absorption (260–360nm). An 11.4- and 6.6-fold increases were recorded for ΦB→Aλirr and ΦA→Bλirr, respectively. The efficiencies of the former (ΦB→Aλirr, ranging between 2.3×10−2 and 26.3×10−2) were 33 to 60% smaller than those of the respective ΦA→Bλirr measured at the irradiation wavelengths selected. Overall, between 57 and 97% degradation of the initial trans-ORVT was observed under relatively weak light intensities, with the highest values recorded at the longest wavelengths. These findings strongly recommend protection from light in all situations of this biologically important phytomolecule that possesses therapeutic value of interest to pharmaceutical applications. The Φ-order kinetics also offered a simple way to develop a reliable actinometric method that proved ORVT to be an efficient actinometer for the dynamic range 295–360nm. The usefulness of Φ-order kinetics for the investigation and quantification of phytoproducts' photodegradation was discussed.

Photochemistry and Photophysics of Enediyne-Cored Polyphenylene Dendrimers

Abstract Two kinds of enediyne-cored polyphenylene dendrimer (mGn, pGn) were prepared as pure cis and trans isomers. In comparison to parent enediyne with no dendron substituents, the dendrimers exhibited highly efficient fluorescence emission with mutual cis–trans isomerization. Along with an increase in the generation of dendrons, decrease of Φiso and shortening of τS were observed, suggesting that dendrons affect cis–trans photoisomerization and deactivation from excited singlet state.

A pOH Jump Driven by N═N Out-of-Plane Motion in the Photoisomerization of Water-Solvated Triazabutadiene.

Utilization of photoinitiated isomerization reaction has recently emerged as a very promising platform to modulate the basicity of compounds; however, theoretical insight into its regulatory mechanism remains largely unknown and needs to be addressed. For the first time, an unexpected trans-cis photoisomerization via the N═N out of plane (NOOP) motion triggered by an in-plane inversion of N-N═N moiety was computationally demonstrated to regulate the pOH jump of water-solvated triazabutadiene by using the multiconfigurational perturbation theory together with the calculation of rate constants of protonation-deprotonation reactions. Kinetic analyses show that the dramatic pOH change can be attributed to the reinforced intramolecular hydrogen bonding resulting from water cluster reorientation and the enhanced coupling between the rotated π orbital and N lone pair of triazabutadiene in the remarkable trans-cis photoisomerization.

Synthesis and characteristics of novel azo-based diblock copolymers and their self-assembly behavior via solvents and thermal annealing

AbstractA series of azo-based diblock copolymers (DBCs) with various compositions were successfully synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization in anisole with PCAEMA-CTA (macro-CTA), DOPAM (new acrylamide monomer) and AIBN (initiator). Kinetic studies on diblock copolymerization manifested a controlled/living manner with good molecular weight control. Structures and properties of monomers and DBCs were determined by 1H nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC) and gel permeation chromatography (GPC). Liquid crystalline (LC) phases and morphological properties were investigated using optical polarizing microscope (OPM), atomic force microscopy (AFM), scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS). Experimental results demonstrated that the prepared PCAEMA-CTA and DBCs possessed low polydispersity index (≤1.37). All DBCs revealed sharp endothermic transition peaks corresponding to the smectic-to-nematic phase. DBCs with high azo contents showed batonnet textures of the smectic phase whereas DBCs of low azo segments displayed threaded textures of the nematic phase. DBC with 49 wt% of azo side-chains generated a lamellar compared to DBCs with low azo block (≤41 wt%) or non-azo block (≤38 wt%) which produced hexagonal-type nanostructures. In addition, all DBCs exhibited reversible trans-cis photoisomerization behavior under UV irradiation and dark storage at different intervals of time.

Tunable Photocontrolled Motions Using Stored Strain Energy in Malleable Azobenzene Liquid Crystalline Polymer Actuators.

A new strategy for enhancing the photoinduced mechanical force is demonstrated using a reprocessable azobenzene-containing liquid crystalline network (LCN). The basic idea is to store mechanical strain energy in the polymer beforehand so that UV light can then be used to generate a mechanical force not only from the direct light to mechanical energy conversion upon the trans-cis photoisomerization of azobenzene mesogens but also from the light-triggered release of the prestored strain energy. It is shown that the two mechanisms can add up to result in unprecedented photoindued mechanical force. Together with the malleability of the polymer stemming from the use of dynamic covalent bonds for chain crosslinking, large-size polymer photoactuators in the form of wheels or spring-like "motors" can be constructed, and, by adjusting the amount of prestored strain energy in the polymer, a variety of robust, light-driven motions with tunable rolling or moving direction and speed can be achieved. The approach of prestoring a controllable amount of strain energy to obtain a strong and tunable photoinduced mechanical force in azobenzene LCN can be further explored for applications of light-driven polymer actuators.

Trans-cis photoisomerization in nanostructured floating layers and x-ray diffraction study of Langmuir-Schaefer films of nonyloxypheylazocinnamic acid

ABSTRACTAzo-functionalised materials are of special interest due to their photochromic nature, i.e. reversible trans–cis isomerisation upon photoirradiation. The structure and properties of Langmuir monolayers of nonyloxyphenylazocinnamic acid (NOPACA) with and without exposure to UV-irradiation were studied. The NOPACA floating layer structure was determined by using a quantitative method of compression isotherm analysis. The results showed that the structure of two dimensional face-on M-nanoaggregates, the stable structural elements of floating monolayers, changes when exposed to UV-irradiation. Langmuir-Schaefer films of NOPACA were prepared by deposition of floating layers and studied using X-ray diffraction technique.

Substituent Dependence Charge Transfer and Photochemical Properties of Donor-Acceptor Substituted Ethenyl Thiophenes.

Donor-acceptor conjugated molecules with efficient light induced properties represent interesting material for electronic device application. In this context, we have calculated excited state dipole moment of three ethenyl thiophenes (1-3) bearing varied electron donor-acceptor substituent in p-phenyl unit using Lippert-Mataga, Bakhshiev and Kawski method. It is found that 1 with strong electron-withdrawing nitro substituent, is exhibiting charge transfer and highly dipolar excited state as compared to 2 and 3. Photochemical studies of 1-3, indicate towards the charge transfer dependence trans-cis photoisomerization under direct irradiation condition. Compound 1 exhibits charge transfer and less efficient towards photoisomerization, whereas 2 and 3 undergo efficient photoisomerization. Graphical Abstract Substituent dependence charge transfer and photochemical properties of donor-acceptor substituted ethenyl thiophenes.

Photonics of 18-crown-6-containing styryl dye and its complexes with metal cations

Using absorption, luminescence, 1H NMR, and laser kinetic spectroscopies, the photophysical processes and photochemical reactions of 4-[(E)-2-(2,3,5,6,8,9,11,12,14,15-decahydro-1,4,7,10,13,16-benzohexaoxacyclooctadecin-18-yl)vinyl]-1-ethylpyridinium perchlorate and its complexes with lithium, sodium, potassium, calcium, barium, silver, and lead perchlorates in MeCN have been studied. The styryl dye and its complexes with metal cations are capable of emitting normal (prompt) and delayed fluorescence and enter into the trans–cis photoisomerization reaction. The dye molecules in the triplet state participate in the processes of degradation of electronic excitation energy. Triplet–triplet absorption is observed only in the presence of lead cations with a maximum at 470 nm and the deactivation rate constant of the triplet state k = 2.5 × 104 s–1 in a deoxygenated solution.

A photochromic AgBF4 complex with cis-1,2-dicyano-1,2-bis(2′,4′,5′-trimethyl-3′-thienyl)ethene: Synthesis, structure and photochromism

A new coordination compound was isolated by reaction of AgBF4 with cis-1,2-dicyano-1,2-bis(2′,4′,5′-trimethyl-3′-thienyl) ethene (cis-dbe) in benzene with the aim of suppressing cis-trans photo-isomerization of cis-dbe by the steric hindrance from metal coordination. The complex was revealed as a dinuclear structure by single crystal X-ray diffraction, where two Ag(I) ions are bridged by two cis-dbe molecules via their four N atoms of cyano groups. The benzene, employed as crystallized solvent in synthesis, was revealed to coordinate with metal center. The ligand presents as cis configuration and the two thiophene rings adopt anti-parallel conformation with reactive C distance of 3.46Å, which is short enough for the photo-cyclization. These structural features are favorable for the ring-closure reaction. On the other hand, BF4− is existed as counter anion sandwiched between the neighboring dinuclear complexes via extensive intramolecular hydrogen bonds and Van de Waals contacts. Also, strong π-π interactions exist in the complex. These weak interactions are theoretically disadvantageous to the photo-cyclization.As expected, the cis-trans photo-isomerization was successfully suppressed by involving of cis-dbe into coordination bonds and rational selecting of 405nm excitation light as evidenced by UV–Vis spectra and 1H NMR studies. And the reversible ring-closing and ring-opening reactions with color inter-conversion between yellow and red were not prohibited in solution, solid state and PMMA films, suggesting an indistinctive effect of metal coordination on ring closure.

Photoresponsive polyelectrolyte/mesoporous silica hybrid materials with remote-controllable ionic transportation

We present a comprehensive research on the photo-induced multi-responsibility of azobenzene(azo) diblock copolymer (BPC) grafted from the surface of mesoporous silica as a light-triggered and controllable ions transport system. The surface-initiated RAFT method was used to grow sequentially a first poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) block serving as an inner layer for the pH and thermal responsibility and a second terminal azo block functionalizing the trans-cis photoisomerization to control the closure or opening of the brushes in water. We show that the ions conductivity of hybrid system could be controlled by the second block switchable between the coil-recoil brush chain states in response to pH or temperature change or exposure to light. It was confirmed a suitable composition ratio of azo BPC brushes could significantly influence the hydrophilicity and polarity of the out layer with light, resulting the protonation or gelation of PDMAEMA block. For example, the Ru(NH3)63+ cations as probing ions was employed to prototyping the controlled permselectivity triggered by the UV light exposure. This kind of hybrid material is particularly interesting in the versatile utility for drug delivery, separating, biomimic sensor and so on.

Light-addressable liquid crystal polymer dispersed liquid crystal

Scattering-free liquid crystal polymer-dispersed liquid crystal polymer (LCPDLC) films are fabricated by combining a room temperature polymerizable liquid crystal (LC) monomer with a mesogenic photosensitive LC. The morphological and photosensitive properties of the system are analysed with polarized optical microscopy and high resolution scanning and transmission electron microscopy. A two-phase morphology comprised of oriented fibril-like polymeric structures interwoven with nanoscale domains of phase separated LC exists. The nanoscale of the structures enables an absence of scattering which allows imaging through the LCPDLC sample without optical distortion. The use of a mesogenic monomer enables much smaller phase separated domains as compared to non-mesogenic systems. All-optical experiments show that the transmitted intensity, measured through parallel polarizers, can be modulated by the low power density radiation (31 mW/cm2) of a suitable wavelength (532 nm). The reversible and repeatable transmission change is due to the photoinduced trans-cis photoisomerization process. The birefringence variation (0.01) obtained by optically pumping the LCPDLC films allow their use as an all-optical phase modulator.

Oxidative coupling of hydroxy- or aminoazobenzenes with secondary phosphine chalcogenides: Towards new media-responsive molecular switches

One or two chalcogenophosphinate groups were introduced to the azobenzene scaffold via the oxidative cross-coupling reaction of 4-amino-, 4-hydroxy- and 4,4′-dihydroxyazobenzenes with secondary phosphine chalcogenides using the CCl4/Et3N system under mild conditions in 41–95% yield. Cis-trans photoisomerization of the phosphorylated azobenzenes was reversibly controlled by alternating UV/Vis light irradiation. The chalcogenophosphinate group imparts the properties of media-responsive molecular photoswitches to the synthesized azobenzenes.

Synthesis and tunable thermoresponsive solution morphologies of 2,2-bis-methylolpropionic acid dendron-azobenzene-poly(N -isopropyl acrylamide) copolymers

Amphiphilic temperature and photoresponsive linear-dendritic block copolymers comprising second generation acetonide-2,2-bis-methylolpropionic acid (bis-MPA)-based polyester dendron and linear poly(N-isopropyl acrylamide) (PNIPAM) linked by an azobenzene unit were synthesized using ATRP followed by click chemistry. Precursor linear PNIPAM was prepared from an azide-functionalized azobenzene containing ATRP initiator. Two polymers obtained by varying the chain length of PNIPAM block showed different morphologies and LCST values in aqueous solution. Complete change in morphology of the two polymers into large spherical aggregates and nanotubes, respectively, was observed upon heating the micellar solution above LCST. Azobenzene unit was found to undergo trans-cis photoisomerization in the assemblies and caused change in the microenvironment of encapsulated hydrophobic dye without any release. Acetonide groups on the dendron were deprotected to afford hydroxylated polymer that showed well-defined morphologies above the LCST and after heating-cooling cycle while significant dye encapsulation was seen only above LCST.

Lumisterol to Tachysterol Photoisomerization in EPA Glass at 77 K. A Comparative Study

We present a comparative study of the photoisomerizations of lumisterol (Lumi), previtamin (Pre), and provitamin D3 (Pro) to tachysterol (Tachy) at 77 K in EPA (5:5:2 ether, isopentane, and ethanol by volume) glass. Fluorescence, fluorescence excitation, and UV spectra, measured in the course of these reactions, were analyzed using singular value decomposition with self-modeling (SVD-SM). This represents an extension of previous work that led to the conclusion that in the EPA glass Pre exists as an s-cis,s-cis-conformer (cZc-Pre) which gives, exclusively, an unstable s-cis,s-cis-conformer of Tachy (cEc-Tachy) and Pro gives mainly the tEc-Tachy, that corresponds to a stable s-trans,s-cis-conformer. ( Redwood , C. ; et al. J. Phys. Chem. Lett. 2013 , 4 , 716 - 721 . ) The surprising result was that the major Pre photoproduct from Pro also has a tZc-Pre conformation instead of the expected cZc-Pre conformation. Accordingly, the Pre to Tachy cis-trans photoisomerization proceeds via a conformer specific one-bond-twist (OBT) process as proposed by Havinga ( Maessen , P. A. ; et al. Angew. Chem. Int. Ed. Engl. 1983 , 22 , 718 - 719 . Maessen , P. A. ; et al. Angew. Chem. Int. Ed. Engl. 1983 , 22 , 994 - 1004 . Maessen , P. A. Ph.D. Thesis, State University at Leiden, Leiden, The Netherlands, 1983. ). The role of the EPA glass in controlling conformer distributions and reaction outcomes is further explored by the extension of the studies to Lumi, whose structure differs substantially from that of its stereoisomer, Pro. Initially, the light-induced conrotatory ring openings of Pro and Lumi are expected to give cZc-Pre conformers that differ in the relative orientation of the double bond dihedral angles that define the chiral axis of the triene moiety: (-)cZ(-)c-Pre and (+)cZ(+)c-Pre, respectively. In the case of Pro, much of the cZc-Pre proceeds to tZc-Pre, the precursor of tEc-Tachy. In contrast, we show that under the same conditions most cZc-Pre formed from Lumi retains the cZc-conformation and isomerizes to cEc-Tachy. cZc-Pre from Lumi was not detected by fluorescence, but UV absorption measurements establish its formation as an essential intermediate to Tachy. Aided by theoretical calculations of conformer UV and CD spectra, we conclude that fluorescent thermodynamic Pre and nonfluorescent Pre from Lumi are both (+)cZ(+)c-Pre conformers. They differ in the orientation of the OH in the A ring, pseudoequatorial in the former and pseudoaxial in the latter. The most likely major photochemical sequences starting from Pre and Lumi are (+)cZ(+)c-Pre-eq-OH → (+)cE(+)c-Tachy-eq-OH and Lumi → (+)cZ(+)c-Pre-ax-OH → (+)cE(+)c-Tachy-eq-OH.

The novel photoresponsive oligomers containing azo derivatives of sulfamerazine for spontaneous surface relief grating inscription

In this work, a novel photoresponsive materials has been successfully developed. We have synthesized the series of azopolymers containing derivatives of sulfamerazine and studied their photochromic properties. The polymers were obtained from methacrylate azomonomers, butyl methacrylate and isobornyl methacrylate by radical polymerization. Spectral properties and photoisomerization kinetic constants were determined for thin transparent films, using UV–Vis spectroscopy. The maximum absorption of the films was observed at 432–440nm. The obtained materials showed ability to reversible trans–cis photoisomerization. Reversibility of the process was confirmed during ellipsometric measurements. The change of the real part of the complex refractive index induced by laser irradiation was between 0.010 and 0.053. Additionally, some properties of described polymers and azobenzene derivatives were calculated and were utilized in order to better understand the differences in the photochromic behaviour of the new materials. Moreover, we determined the suitability of the obtained azopolymers for spontaneous surface relief grating inscription. Spontaneous surface relief grating recording on the thin azopolymer films was carried out using set-up with one laser beam. The spontaneous SRG was successfully recorded in all investigated polymers.

Ultrafast Barrierless Photoisomerization and Strong Ultraviolet Absorption of Photoproducts in Plant Sunscreens.

Sunscreens are aimed at protecting skin from solar ultraviolet (UV) irradiation. By utilizing femtosecond transient absorption spectroscopy and time-dependent density functional theory, we explain nature's selection of sinapoyl malate rather than sinapic acid as the plant sunscreen molecule. In physiological pH conditions, the two molecules are deprotonated, and their excited ππ* states are found to relax to the ground states in a few tens of picoseconds via a barrierless trans-cis photoisomerization. After the cis-photoproduct is formed, the efficacy of sinapic acid is greatly reduced. In contrast, the efficacy of sinapoyl malate is affected only slightly because the cis-product still absorbs UV light strongly. In addition, protonated sinapic acid is found to be a good potential sunscreen molecule.

QM/MM Study of Substituent and Solvent Effects on the Excited State Dynamics of the Photoactive Yellow Protein Chromophore.

Substituent and solvent effects on the excited state dynamics of the Photoactive Yellow Protein chromophore are studied using the average solvent electrostatic potential from molecular dynamics (ASEP/MD) method. Four molecular models were considered: the ester and thioester derivatives of the p-coumaric acid anion and their methylated derivatives. We found that the solvent produces dramatic modifications on the free energy profile of the S1 state: 1) Two twisted structures that are minima in the gas phase could not be located in aqueous solution. 2) Conical intersections (CIs) associated with the rotation of the single bond adjacent to the phenyl group are found for the four derivatives in water solution but only for thio derivatives in the gas phase. 3) The relative stability of minima and CIs is reverted with respect to the gas phase values, affecting the prevalent de-excitation paths. As a consequence of these changes, three competitive de-excitation channels are open in aqueous solution: the fluorescence emission from a planar minimum on S1, the trans-cis photoisomerization through a CI that involves the rotation of the vinyl double bond, and the nonradiative, nonreactive, de-excitation through the CI associated with the rotation of the single bond adjacent to the phenyl group. In the gas phase, the minima are the structures with the lower energy, while in solution these are the conical intersections. In solution, the de-excitation prevalent path seems to be the photoisomerization for oxo compounds, while thio compounds return to the initial trans ground state without emission.