Photoisomerization Kinetics Research Articles

Photoresponsive and fluorescence behaviors of azobenzene‐containing amphiphilic block copolymers

ABSTRACTThe effects of solvency and mole fraction of azobenzene moieties (fPAzoMA) on the photoresponsive and fluorescence behaviors of poly(acrylic acid)‐block‐poly(6‐[4‐(4′‐methoxyphenylazo)phenoxy]hexyl methacrylate) (PAA‐PAzoMA) amphiphilic diblock copolymers were investigated using UV–vis spectroscopy and fluorescence spectroscopy. The photoresponsive behavior depended strongly on the solvency and fPAzoMA. When dissolved in a PAA‐selective solvent, PAA‐PAzoMA formed micelles with PAzoMA in the micelle core. The confinement of azobenzene moieties caused a steric hindrance, thereby markedly reducing the kinetics of photoisomerization compared with that of the unconfined PAA‐PAzoMA in a nonselective solvent. Additionally, PAA‐PAzoMA dissolved in the PAA‐selective solvent caused a blue shift of the maximum absorbance, suggesting the formation of H‐aggregates of azobenzene mesogens. The high H‐aggregate content substantially reduced the fluorescence emission. Consequently, the fluorescence emission of PAA‐PAzoMA in the nonselective solvent was more intense than that in the PAA‐selective solvent. Upon UV irradiation, the enhanced bent‐shaped cis isomers disturbed the compact packing of azobenzene mesogens, which substantially enhanced the fluorescence emission. Both the photoisomerization rate and fluorescence emission decreased with an increase in fPAzoMA. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 793–803

Photoisomerization Kinetics and Mechanical Stress in Azobenzene-Containing Materials.

Kinetics of photoisomerization and time evolution of ordering in azobenzene-containing materials are studied theoretically and by using computer simulations. Starting from kinetic equations of photoisomerization, we show that the influence of light is equivalent to the action of the effective potential, which reorients chromophores perpendicularly to polarization direction. The strength of the potential is defined by optical and viscous characteristics of the material. The potential generates photomechanical stress of giant values ∼GPa, in accordance with recent experimental findings for azobenzene materials deep in a glassy state. The proposed approach has a great predictive strength for deeper understanding and further development of the photocontrollable smart compounds.

Determination of the kinetics of photo-isomerization and equilibrium constant of resveratrol by high performance liquid chromatography

Determination of the kinetics of photo-isomerization and equilibrium constant of resveratrol by high performance liquid chromatography

UV/Vis Spectroscopy Studies of the Photoisomerization Kinetics in Self-Assembled Azobenzene-Containing Adlayers

Direct comparative studies of the photoisomerization of azobenzene derivatives in self-assembled adlayers on Au and as free molecules in dichloromethane solution were performed using UV/vis spectroscopy. For all studied systems a highly reversible trans-cis isomerization in the adlayer is observed. Quantitative studies of the absorbance changes and photoisomerization kinetics reveal that in azobenzenes mounted as freestanding vertical groups on the surface via triazatriangulene-based molecular platforms photoswitching is nearly uninhibited by the local environment in the adlayer. The blue-shift of the π-π* transition in adlayers of these molecules is in good agreement with theoretical studies of the effect of excitonic coupling between the molecules. In contrast, in azobenzene-containing thiol self-assembled monolayers the fraction of photoswitching molecules and the photoisomerization kinetics are significantly reduced compared to free molecules in solution.

Adjusting the Ion Permeability of Polyelectrolyte Multilayers through Layer-by-Layer Assembly under a High Gravity Field.

The layer-by-layer (LbL) assembled multilayer has been widely used as good barrier film or capsule due to the advantages of its flexible tailoring of film permeability and compactness. Although many specific systems have been proposed for film design, developing a versatile strategy to control film compactness remains a challenge. We introduced the simple mechanical energy of a high gravity field to the LbL assembly process to tailor the multilayer permeability through adjusting film compactness. By taking poly(diallyldimethylammonium chloride) (PDDA) and poly{1-4[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl sodium salt} (PAzo) as a model system, we investigated the LbL assembly process under a high gravity field. The results showed that the high gravity field introduced effectively accelerated the multilayer deposition process by 20-fold compared with conventional dipping assembly; the adsorption rate was positively dependent on the rotating speed of the high gravity equipment and the concentration of the building block solutions. More interestingly, the film compactness of the PDDA/PAzo multilayer prepared under the high gravity field increased remarkably with the growing rotational speed of the high gravity equipment, as demonstrated through comparisons of surface morphology, cyclic voltammetry curves, and photoisomerization kinetics of PDDA/PAzo multilayers fabricated through the conventional dipping method and through LbL assembly under a high gravity field, respectively. In this way, we have introduced a simple and versatile external form of mechanical energy into the LbL assembling process to improve film compactness, which should be useful for further applications in controlled ion permeability, anticorrosion, and drug loading.

Synthesis, photochemical properties, and self-assembly of diblock copolymer bearing azobenzene moieties

This study describes the synthesis of an azobenzene-containing diblock copolymer, poly(StO50-b-Azo7), by using the macro-chain transfer agent StO macro-CTA, and employing sequential reverse addition-fragmentation transfer (RAFT) polymerization. We studied the effects of azobenzene (azo) unit on E/Z photoisomerization, the phase transition temperature, and self-assembly behavior of diblock copolymer, and evaluated the characteristic time involved in the decay process of the photoisomerization kinetics of diblock copolymer. Diblock copolymer poly(StO50-b-Azo7) exhibited moderate thermal stability, with thermal decomposition temperature of 5% weight loss at approximately 340.9 °C, suggesting that the enhancement of the thermal stability was attributed to the incorporation of azo segments into block copolymer. The diblock copolymer showed lower E–Z photoisomerization rates (T1 = 68.0 s) compared with azo monomer (T1 = 10.95 s). Gradually adding water to the tetrahydrofuran (THF) solution of poly(StO50-b-Azo7) produced spherical micelles. Spherical aggregates of poly(StO50-b-Azo7) were obtained (mean diameter = approximately 181.4 nm) by diluting the polymer disperse in a mixture of THF/H2O (water content = 10 vol%), and are shown in TEM images of the diblock copolymer. The results of this study contribute to research on the development of photoresponsive polymer materials.

Light-Controlled “Molecular Zippers” Based on Azobenzene Main Chain Polymers

Single strands of azobenzene main chain polymers exhibiting alkyl side chains can be largely and reversibly contracted and extended with light. We show that upon self-assembly in a thin layered film they act as “molecular zippers” that can be opened and closed with UV- and blue light, respectively. Simultaneously in situ recorded time-resolved X-ray diffraction and optical spectroscopy measurements, together with scanning force microscopy show that upon the light-induced E → Z isomerization of the main chain azobenzenes the layered film morphology remains, while the initially highly ordered alkyl side chains become disordered. Already the E → Z isomerization of about 20% of all azobenzene chromophores triggers a complete disorder of the alkyl chains. The kinetics of this partial amorphization of the film is about 18 times slower than the ensemble kinetics of the initial azobenzene photoisomerization. This is the first demonstration of a rigid main chain polymer film with reversibly photoswitchable side chain crystallinity.

Preparation and self‐assembly of stimuli‐responsive azobenzene‐containing diblock copolymers through microwave‐assisted RAFT polymerization

ABSTRACTThis article reports on studies regarding the photoisomerization kinetics and self‐assembly behaviors of two photoresponsive diblock copolymers, poly(4‐acetoxystyrene)‐block‐poly[6‐(4‐methoxy‐azobenzene‐4′‐oxy) hexyl acrylate] (poly(StO54‐b‐Cazo9) and poly(StO54‐b‐Cazo5)), which dissolved in a THF/H2O solution through two‐step reverse addition‐fragmentation transfer polymerization. We examined the effect of heating methods (i.e., conventional and microwave heating) on the polymerization kinetics of a 4‐acetoxystyrene‐based macrochain transfer agent (StO macro‐CTA). The kinetics studies on the homopolymerization of StO by using microwave heating demonstrated controllable characteristics with relatively narrow polydispersities at ∼1.14. The diblock copolymers exhibited moderate thermal stability, with thermal decomposition temperatures greater than 343.3 °C, suggesting that the enhancement of the thermal stability was due to the incorporation of azobenzene segments into block copolymers. Poly(StO54‐b‐Cazo9) showed lower photoisomerization rate constants (kt = 0.039 s−1) compared with Cazo monomer (kt = 0.097 s−1). Micellar aggregates with a mean diameter of approximately 238.3 nm were formed by gradually adding water to the THF solution (water content = 10 vol %), and are shown in SEM and TEM images of the diblock copolymer. The results of this study contribute to the literature regarding the development of photoresponsive polymer materials. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3107–3117

Star-shaped molecules with polyhedral oligomeric silsesquioxane core and azobenzene dye arms.

We synthesized a series of hybrid nanomaterials combining organic dyes with polyhedral oligomeric silsesquioxanes (POSS) based on three different azobenzenes: monoazobenzene (4-phenylazophenol), bis-azobenzene (Disperse Yellow 7 and Fast Garnet derivative), and push-pull azobenzene (Disperse Red 1) via hydrosilylation coupling. The azo-functionalized POSS compounds possess high thermal stability, and their branched architecture resulted in effective suppression of molecular aggregation and allowed for direct imaging of individual dye-POSS structures with expected molecular dimensions. Stable, uniform, smooth, and ultrathin nanocomposite films with mixed silica-organic composition and relatively low refractive indices can be fabricated from all azo-POSS branched conjugates. Finally, the photoisomerization behavior of POSS-conjugated 4-phenylazophenol was investigated in solution as well as in ultrathin nanocomposite film. We found that conjugation to POSS core did not affect the kinetics of trans-cis photoisomerization and thermal cis-trans relaxation. Furthermore, rapid and reversible photoisomerization was observed in azo-POSS nanocomposite films. We suggest that the highly stable branched azo-POSS conjugates with high dye grafting density described here can be considered for nanometer-sized photoswitches, active layer material with optical-limiting properties, and a medium with photoinduced anisotropy for optical storage.

Photoisomerization Kinetics of IR125 and HDITCP in Ionic Liquids with Different Cation Alkyl Chain Lengths

The photoisomerization kinetics of IR125 and HDITCP in ionic liquids with different cation alkyl chain lengths were investigated by measuring their fluorescence lifetimes and quantum yields using steady-state absorption and fluorescence spectroscopies, and time-correlated single-photon counting experiments. It was found that the photoisomerization rate constants for IR125 and HDITCP in all the selected ionic liquids were almost identical and did not change with increasing ionic liquid viscosity. A comparison of the photoisomerization rate constants of IR125 and HDITCP in isoviscous aqueous glycerol solutions with those in ionic liquids showed that the photoisomerization energy barriers of IR125 and HDITCP in ionic liquids were about 2 kJ∙mol-1 higher than those in the isoviscous aqueous glycerol solutions, indicating that specific interactions between IR125 or HDITCP and the ionic liquid restrain their respective photoisomerization processes in highly viscous ionic liquids.

Details of the Ultrafast DNA Photo-Cross-Linking Reaction of 3-Cyanovinylcarbazole Nucleoside: Cis–Trans Isomeric Effect and the Application for SNP-Based Genotyping

To clarify the cis-trans isomeric effect on the ultrafast DNA photo-cross-linking reaction of 3-cyanovinylcarbazole nucleoside ((CNV)K) in the DNA duplex, which gives a single photodimer on the reversed-phase HPLC chromatogram, the kinetics of the cis-trans photoisomerization of (CNV)K in double-stranded DNA was evaluated. Since the photoisomerization rate constant for cis to trans isomerization in double-stranded DNA was significantly larger than that for trans to cis isomerization, and the thermodynamic stability of the trans isomer was higher than that of the cis isomer, it was strongly suggested that the trans isomer of (CNV)K is a reactive species of the photo-cross-linking reaction. (1)H-(1)H NOESY analysis of the photoadduct consisting of (CNV)K and T also supported the trans-mediated photo-cross-linking reaction of (CNV)K. By using this ultrafast photo-cross-linking reaction for the molecular beacon-based SNPs typing, four individual Japanese rice strains were clearly distinguishable with simple photoirradiation and fluorescence imaging using double-stranded target DNAs.

Fluorescence Modulation and Photochromism in Azobismaleimide Derivatives

A fluorophore-photochrome system incorporating an aryleneimine type fluorophore and an azobismaleimide photochrome was developed and the photochemical properties of this system were investigated. The photoisomerization of trans-azoaromatic chromophore leads to the increase of the fluorescence intensity of fluorophore. The cis azobismaleimide isomers revert photochemically to the trans form and the emission intensity decreases. The fluorescence intensity of the imine fluorophore can be modulated under irradiation with UV and visible (436nm) light due to reversible trans-cis-trans photoisomerization of azobismaleimide partner. The photoisomerization kinetics was obeyed a first-order relationship with a rate constant of 1.95 × 10(-2)s(-1) for azobismaleimide/imine fluorophore system and for polyazothioetherimide/imine derivative the kinetics was described by a biexponential equation.

Steric Hindrance of Photoswitching in Self-Assembled Monolayers of Azobenzene and Alkane Thiols

Surface-bound azobenzenes exhibit reversible photoswitching via trans-cis photoisomerization and have been proposed for a variety of applications such as photowritable optical media, liquid crystal displays, molecular electronics, and smart wetting surfaces. We report a novel synthetic route using simple protection chemistry to form azobenzene-functionalized SAMs on gold and present a mechanistic study of the molecular order, orientation, and conformation in these self-assembled monolayers (SAMs). We use vibrational sum-frequency generation (VSFG) to characterize their vibrational modes, molecular orientation, and photoisomerization kinetics. Trans-cis conformational change of azobenzene leads to the change in the orientation of the nitrile marker group detected by VSFG. Mixed SAMs of azobenzene and alkane thiols are used to investigate the steric hindrance effects. While 100% azobenzene SAMs do not exhibit photoisomerization due to tight packing, we observe reversible switching (>10 cycles) in mixed SAMs with only 34% and 50% of alkane thiol spacers.

Localized Surface Plasmon Resonance Investigations of Photoswitching in Azobenzene-Functionalized Self-Assembled Monolayers on Au

Localized plasmon resonance (LSPR) spectroscopy, employing gold nanodisk substrates, is applied for studies of photoswitching in self-assembled monolayers of azobenzene-containing thiols. By choosing customized samples in which the sharp LSPR resonance is well separated from the spectral regime of the molecular absorption bands, the photoisomerization kinetics of the adlayer can be monitored in real time. Quantitative data on the photoinduced trans-cis and cis-trans isomerization processes in inert gas atmosphere were obtained as a function of irradiation intensity and temperature, demonstrating the high sensitivity of this technique to such processes in functional adlayers.

The Synthesis, X-ray Structure Analysis, and Photoisomerization Assessment of the Acyclic Triazenes 4-[(E)-2-(biphenyl-4-yl)diazenyl]-morpholine and 1,3-Bis(biphenyl-4-yl)triazene

The acyclic triazenes 4-[(E)-2-(biphenyl-4-yl)diazenyl]-morpholine (1) and 1,3-bis(biphenyl-4-yl)triazene (2) were prepared and their X-ray crystal structures determined. Compound 1 crystallized in the monoclinic space group P21/c with a = 29.153 (5) A, b = 6.0444 (14) A, c = 7.5997 (15) A, and β = 96.03 (1)°. Compound 2 crystallized as the 1/4 cyclohexane solvate in the triclinic space group P $$\bar{1}$$ with a = 10.3228 (10) A, b = 13.4541 (14) A, c = 15.4323 (15) A, α = 87.158 (4)°, β = 76.030 (4)° and γ = 71.492 (5)°. The N–N double bond in 1 and 2 adopted an (E)-configuration and had bond angles that ranged from 112.19 (9)° to 114.01 (13)°, which deviated from optimal trigonal planar geometry by 6°–8°. A degree of π-conjugation across the acyclic triazene moieties in compounds 1 and 2 was indicated by N–N double and single bond lengths that were respectively longer and shorter than the literature values for these types of bonds. The room temperature electronic absorption spectra of 1 and 2 in various solvents were both dominated by an intense band above 300 nm that was assigned to a combination of π → π* and n → π* transitions. Compounds 1 and 2 in acetonitrile and methanol solution were found to undergo trans → cis photoisomerization. The magnitudes of trans → cis rate constants were rationalized in terms of the degree of solvent stabilization of the dipolar resonance hybrid structure of the acyclic triazene subunits in 1 and 2. Irradiation of chloroform solutions of 1 and 2 caused trans → cis photoisomerization and radical decomposition to occur. The X-ray structures of compounds 1 and 2 were determined and their photoisomerization kinetics explored.

Liquid‐Crystalline Nano‐optomechanical Actuator

AbstractThe synthesis and characterization of organic nanoparticles composed of a polymer network of azobenzene moieties and capable of reproducible, photoinduced mechanical actuation are reported. The molecules within the nanoparticles undergo co‐ordinated, reversible isomerization between cis‐ and trans‐conformations in response to ultraviolet and visible electromagnetic radiation, resulting in a reversible 20% height contraction of nanoparticles adsorbed on a substrate. The kinetics of the actuation response as a function of light intensity and duration are reported and closely match the molecular kinetics of azobenzene photoisomerization. The results support the proposed mechanism of co‐ordinated molecular conformational changes resulting in observable nanoscale actuation.

Molecular orientation and photoswitching kinetics on single-walled carbon nanotubes by optical second harmonic generation

Electrical conduction through chromophore-functionalized nanotubes can be modulated by light with wavelengths expected to isomerize the chromophores. Here, we use second harmonic generation to directly measure the orientation and photoisomerization kinetics of azo-benzene chromophores on single-walled carbon nanotubes. We find a net chromophore orientation with an average chromophore tilt angle of 40° ± 3°. We show that this angle can be reduced effectively to zero with an applied corona field. Periodic illumination with unpolarized 495 nm light induces reversible trans-cis switching, enabling the extraction of switching time scales both with and without an applied electric field.

Specific and Nondestructive Detection of Different Diarylethene Isomers by NIR-SERS

This paper describes specific and nondestructive detection of two isomers of diarylethene derivative, namely 1,2-bis(5′-ethoxy-2′-(2″-pyridyl)thiazolyl)perfluorocyclopentene (DE) by surface-enhanced Raman scattering (SERS). UV and visible light irradiations transform reversibly DE between open-form (OF) and closed-form (CF) isomers, respectively. A mixture of poly(methyl methacrylate) and DE was spin-coated onto gold nanoparticles (GNP) aggregates, which support localized surface plasmon (LSP) in the near-infrared (NIR) region, leading to significant SERS enhancement when 785 nm laser excitation was used. SERS experiments showed that the SERS spectrum of the CF isomer is clearly distinguished from that of the OF isomer with a strong contrast. A comparison between photoisomerization kinetics obtained from SERS measurements recorded at 633 and 785 nm excitation revealed that excitation in the NIR enables less destructive detection of isomeric states compared to visible excitation. However, weak photoisomeri...

Photo‐regulated phase transition of poly(ethylene glycol) derivative containing azobenzene group

A photo‐responsive and thermo‐responsive poly(ethylene glycol) derivative containing azobenzene group was synthesized. Its photoisomerization kinetics was investigated by the UV–vis spectra, and the pseudo‐first‐order rate constants (kt and kc) were determined to be 1.5 × 10−2 and 9.3 × 10−2 s−1, respectively. It is found that the phase transition behavior can be tuned theoretically through photoirradiation between 26.6°C and 36.7°C. Finally, it is worthy to note that the changes of the lower critical solution temperature occur repeatedly upon successive irradiation of UV and visible light. Copyright © 2012 John Wiley & Sons, Ltd.

Solvent Effects on the Radiative and Nonradiative Decay of a Model of the Rhodopsin Chromophore.

The radiative and nonradiative decay of a model with five double bonds of the 11-cis-retinal protonated Schiff base was studied both in vacuum and in methanol solution using an extended version of the averaged solvent electrostatic potential from molecular dynamics data (ASEP/MD) method that allows the location of crossing points between free energy surfaces both in equilibrium and in frozen solvent conditions. The multireference quantum method CASSCF was used for the description of the states of interest, while the solvent structure was obtained from molecular dynamics simulations. Electron dynamic correlation corrections to the energy were included at CASPT2 level. Unlike in gas phase, where only two states seem to be implicated, in methanol solution, three states are necessary to describe the photoisomerization process. At the Franck-Condon point the S1 and S2 states are almost degenerate; consequently, the S1 surface has a region with an ionic character ((1)Bu-like) and another one with a covalent character ((2)Ag-like). Emission from the ionic minima is responsible for the low-frequency part of the fluorescence band, while emission from the covalent minima originates the high-frequency part. The ionic minimum is separated from the conical intersection yielding the all-trans isomer by an energy barrier that was estimated in 0.7 kcal/mol. The geometry of the optimized conical intersection was found at a torsion angle of the central double bond close to 90° both in vacuum and in methanol solution. This large torsion in addition to the accompanying charge displacements forces a strong solvent reorganization during the de-excitation process which slows down the photoisomerization kinetics in methanol with respect to the gas phase. Solvent fluctuations modulate the minima depth and the barrier height and could explain the multiexponential relaxation time observed in the experiments.