Upconverting‐Nanoparticle‐Assisted Photochemistry Induced by Low‐Intensity Near‐Infrared Light: How Low Can We Go?

Upconverting nanoparticles (UCNPs) convert near‐infrared (NIR) light into UV or visible light that can trigger photoreactions of photosensitive compounds. In this paper, we demonstrate how to reduce the intensity of NIR light for UCNP‐assisted photochemistry. We synthesized two types of UCNPs with different emission bands and five photosensitive compounds with different absorption bands. A λ=974 nm laser was used to induce photoreactions in all of the investigated photosensitive compounds in the presence of the UCNPs. The excitation thresholds of the photoreactions induced by λ=974 nm light were measured. The lowest threshold was 0.5 W cm^?2, which is lower than the maximum permissible exposure of skin (0.726 W cm^?2). We demonstrate that low‐intensity NIR light can induce photoreactions after passing through a piece of tissue without damaging the tissue. Our results indicate that the threshold for UCNP‐ assisted photochemistry can be reduced by using highly photosensitive compounds that absorb upconverted visible light. Low excitation intensity in UCNP‐assisted photochemistry is important for biomedical applications because it minimizes the overheating problems of NIR light and causes less photodamage to biomaterials.     

光敏性BZ反应的时空动力学

作为研究非线性时空动力学最理想的化学反应体系之一,三联(2,2'-联吡啶)钌(Ⅱ)(Ru(bpy)₃²⁺)为催化剂的Belousov-Zhabotinsky (BZ)振荡反应具有独特的光敏特性并能呈现丰富的时空动力学行为。研究光控BZ反应有助于我们对一系列物理、化学和生命体系中复杂动力学现象的理解。本文综述了不同实验条件下光效应对钌催化BZ反应均相复杂振荡和空间反应扩散化学波的影响, 以及光响应BZ反应与软物质耦合体系的复杂动力学行为,在此基础上介绍光抑制和光诱导反应机理和模型。对光控BZ反应体系存在的问题和发展方向进行了探讨。     

Interactive propagation of photosensitive chemical waves on two circular routes

The propagation of chemical waves in the photosensitive Belousov-Zhabotinsky (BZ) reaction was investigated using an excitable field composed of two rings in slight contact, which were drawn using computer software and then projected on a film soaked with BZ solution using a liquid-crystal projector. When the initial phase difference between the two chemical waves in the individual rings was smaller than a critical value, this initial value was maintained after collision of the chemical waves. However, when the initial phase difference was larger than this critical value, the phase difference converged to the same value after the second collision. The critical value increased with an increase in the thickness of the rings. These experimental results on the geometry of the excitable field are discussed in relation to the nature of chemical wave propagation. These results suggest that the photosensitive BZ reaction may be useful for creating spatiotemporal patterns that depend on the geometric arrangement of excitable fields.     

Model study of synchronization and other phenomena in light perturbation of the Briggs–Rauscher reaction

Calculations are performed to model several phenomena observed when a photosensitive oscillating chemical reaction, the Briggs–Rauscher (BR) reaction, is subject to irradiation in a continuous-flow stirred-tank reactor. The BR mechanism proposed earlier by De Kepper and Epstein is supplemented by additional steps involving the photogeneration and subsequent reaction of iodine atoms. The calculations show qualitative agreement with the following experimentally observed phenomena: (a) change in the period and amplitude of oscillation as a function of the intensity of constant illumination, (b) synchronization between periodically varying illumination and the period of chemical oscillation, (c) phase shifts induced by single light pulses, and (d) dark steady states which are excitable by single light pulses and become oscillatory at appropriate levels of constant illumination.     

Experimental Investigation of the Dynamical Modes of Four Pulse-Coupled Chemical Micro-Oscillators

We present an experimental system of four identical microreactors (MRs) in which the photosensitive oscillatory Belousov-Zhabotinsky (BZ) reaction occurs. The inhibitory coupling of these BZ MRs is organized via pulses of light coming to each MR from a computer projector. These pulses are induced by spike(s) in other MR(s) of the same network. Time delay between the spike in one BZ MR and the pulsed perturbation of the other BZ MR(s), the amplitude of light pulses, their duration, and the connectivity of the MRs are controlled by the LabVIEW software. Recording the dynamics of the BZ reaction in the MRs via a microscope equipped with a CCD camera, we observe all the main dynamical modes of our network of MRs, which are the IP (in-phase), AP (anti-phase), W (walk), and WR (walk reverse) for the unidirectional coupling, and the IP, two-cluster, three-cluster, and splay modes for the all-to-all coupling. Our software detects all the modes of the network automatically and makes it possible to switch between them on demand using a few special “switching” pulses. As the result of the present work, the experimental implementation of the adaptive behaviour of the pulse-coupled chemical micro-oscillator networks becomes available.     

Propagation of photosensitive chemical waves on the circular routes

The propagation of chemical waves in the photosensitive Belousov-Zhabotinsky (BZ) reaction was investigated using an excitable field in the shape of a circular ring or figure "8" that was drawn by computer software and then projected on a film soaked with BZ solution using a liquid-crystal projector. For a chemical wave in a circular reaction field, the shape of the chemical wave was investigated depending on the ratio of the inner and outer radii. When two chemical waves were generated on a field shaped like a figure "8" (one chemical wave in each circle) as the initial condition, the location of the collision of the waves either was constant or alternated depending on the degree of overlap of the two circular rings. These experimental results were analyzed on the basis of a geometrical discussion and theoretically reproduced on the basis of a reaction-diffusion system using a modified Oregonator model. These results suggest that the photosensitive BZ reaction may be useful for creating spatio-temporal patterns depending on the geometric arrangement of excitable fields.     

Coexistence of wave propagation and oscillation in the photosensitive Belousov-Zhabotinsky reaction on a circular route

The photosensitive Belousov-Zhabotinsky (BZ) reaction was investigated on a circular ring, which was drawn using computer software and then projected on a film soaked with BZ solution using a liquid-crystal projector. Under the initial conditions, a chemical wave propagated with a constant velocity on the black ring under a bright background. When the background was rapidly changed to dark, coexistence of the oscillation on part of the ring and propagation of the chemical wave on the other part was observed. These experimental results are discussed in relation to the nature of the photosensitive BZ reaction and theoretically reproduced based on a reaction-diffusion system using the modified Oregonator model.     

Photochemical control of network structure in gels and photo-induced changes in their viscoelastic properties

Poly(amide acid) gels containing photosensitive azobenzene groups in the main chain have been synthesized and their viscoelastic properties before and after light irradiation have been investigated by dynamic viscoelasticity measurements. It was found that 405 nm light induced a local volume change and a two-fold increase in the storage modulus of the gels. We discuss the change in storage modulus upon light irradiation quite simply in terms of classical rubber elasticity theory, which cannot explain this large increase in storage modulus. The photo-induced increase in storage modulus may result from an increase in entanglement interactions of topological constraints fixed in the network structure, caused by photoisomerization of the azobenzene moieties. We suggest that topological constraints in the network structure of the gels were realized by light irradiation and calculate the resulting slip link ratio (index of the topological constraints) in the gel network.     

Light‐Modulated Intermittent Wave Groups in a Diffusively Fed Reactive Gel

Inspired by the biological growth that takes place in time‐varying external fields such as light or temperature, we design an open reaction‐diffusion system in order to investigate growth dynamics. The system is composed of the Belousov–Zhabotinsky (BZ) oscillatory reaction coupled with a copolymer gel consisting of NIPAAm and a photosensitive ruthenium catalyst. When subject to a unidirectional flow of the BZ reactants, the system displays groups of chemical waves whose structure depends upon the period and amplitude of illumination. Simulations of a modified six‐variable Oregonator model exhibit all the complex wave groups found in our experiments. Studying this growth structure may aid in understanding the influence of periodic environmental variation on complex growth processes in living systems.     

光驱动一维活性凝胶自发定向运动

许多生物都具备一种自发响应环境刺激进行自动运动的本能。光照是一种强有力的外部刺激,且对生物系统具有正负驱动的双重效应;本实验设计了一个一维活性BZ凝胶体系,利用钌催化BZ反应的光敏性,控制凝胶振荡频率,驱动凝胶进行定向自发运动。同时,我们修正了V. V. Yashin 提出的BZ响应胶模型活性凝胶光强频率曲线进行模拟,揭示光驱动活性凝胶运动的本质。这有助于研发新型仿生智能机器人来对生物系统响应外部刺激的自发运动进行研究。     

Rose bengal-grafted biodegradable microcapsules: singlet-oxygen generation and cancer-cell incapacitation

Rose bengal-grafted chitosan (RB-CHI), synthesized through dehydration between amino and carboxyl functional groups under mild conditions, was coated onto the outer layer of preformed biodegradable microcapsules consisting of sodium alginate and chitosan. The fabricated photosensitive microcapsules were characterized by optical microscopy, scanning electron microscopy, and confocal laser scanning microscopy. The assembled materials maintained intact spherical morphology and thus showed good ability to form thin films. Electron spin resonance spectroscopy allowed direct observation of the generation of singlet oxygen ((1)O(2)) from photosensitive microcapsules under light excitation at about 545 nm. Furthermore, with increasing light radiation, the content of (1)O(2) increased, as detected by a chemical probe. In vitro cellular toxicity assays showed that RB-CHI-coated photosensitive microcapsules exhibit good biocompatibility in darkness and high cytotoxicity after irradiation, and could provide new photoresponsive drug-delivery vehicles.     

Photoswitching CO2 Capture and Release in a Photochromic Diarylethene Metal–Organic Framework

We demonstrate herein a promising pathway towards low‐energy CO₂ capture and release triggered by UV and visible light. A photosensitive diarylethene ligand was used to construct a photochromic diarylethene metal–organic framework (DMOF). A local photochromic reaction originating from the framework movement induced by the photoswitchable diarylethene unit resulted in record CO₂‐desorption capacity of 75 % under static irradiation and 76 % under dynamic irradiation.     

Phase separation of polymer mixtures induced by polarization‐selective chemical reactions: spatial symmetry breaking and mode selection

Phase separation of polystyrene/poly (vinyl methyl ether) (PS/PVME) blends was induced and controlled by irradiation with linearly polarized light. The PS component was made photosensitive by chemically labeled with either anthracene or trans‐stilbene. The former was used to crosslink the PS component whereas the latter induces phase separation by changing polymer segmental volumes. The phase separation and reaction kinetics were observed and discussed in terms of mode‐selection process.     

无皂乳液聚合制备光敏聚合物微球及其光响应性

利用红外光谱(IR)和核磁共振(1H NMR)对所制备的光敏可聚合单体重氮萘醌苯磺酸甲基丙烯酸羟乙酯(DNQMA)进行了表征. 采用无皂乳液聚合方法制备DNQMA与甲基丙烯酸叔丁酯(t-BMA)的共聚合光敏微球. 利用紫外光谱(UV/Vis)、动态光散射(DLS)、扫描电镜(SEM)对光敏微球在光照前后形态的变化进行了表征. DLS的研究结果表明在水分散液中, 光敏微球的尺寸随光照时间的增加, 先增大, 然后达到平衡. 这与紫外光谱中DNQMA特征吸收达到饱和的时间相一致. SEM研究结果表明光敏微球的形态在光照后破坏很严重, 并基本解体.     

Four-center type photopolymerization in the solid state. II. Polymerization mechanism of 2,5-distyrylpyrazine

The polymerization mechanism of trans,trans-2,5-distyrylpyrazine (DSP) has been investigated and some crystal changes along with the polymerization process have been observed through polarizing microscope and x-ray diffraction pattern. Information has been obtained on the active species, polymerization reaction type, and other factors such as light intensity, reaction temperature, or crystalline state. The polymerization of DSP occurs only in the solid state by photoirradiation. Reduced viscosity increases gradually with the increase of conversion and increases sharply above 80% conversion. Polymerization rate increases with the increase of light intensity and temperature. On the other hand, reduced viscosity decreases with the increase of temperature but does not depend on light intensity within the range investigated. The polymer obtained at low conversion as well as at high conversion has high crystallinity, and the direction of polymer axes is simply related to that of monomer crystal. It was concluded that the four-center type polymerization of DSP proceeds topochemically by a photochemically induced stepwise mechanism.     

Photoinduced textural and optical changes in a cholesteric copolymer with azobenzene-containing side groups

Textural changes induced by UV irradiation of planarly oriented films of a cholesteric copolymer were investigated. The copolymer is composed of photosensitive ethoxyazobenzene and chiral cholesterol-containing side groups, and displays a chiral nematic phase with a left-handed helical supramolecular structure in the temperature range 84-128°C. UV light with a wavelength of 365 nm leads to a deformation of the planar texture at temperatures higher than 100°C and to a reduction of the absorbance peak for left-handed circularly polarized light. Polarizing optical microscopy revealed the formation of a focal-conic texture in the irradiated regions. The texture induced by irradiation is very stable but the initial planarly oriented state can be recovered by mechanical stress. The observed phenomenon is attributed to the E-Z photoisomerization of azobenzene groups leading apparently to the formation of a thin isotropic layer in the films which in turn causes a destabilization of the planar texture. At temperatures below 100°C the textural transformations are accompanied by small shifts of the selective reflection maxima to longer wavelengths. The kinetics of the observed phenomenon were studied in detail and the influence of different parameters, such as light intensity and temperature, examined.     

Photoinduced fluorescence enhancement in mono- and multilayer films of CdSe/ZnS quantum dots: dependence on intensity and wavelength of excitation light

Photoinduced fluorescence enhancement (PFE) behavior in mono- and multilayer films of CdSe/ZnS core/shell quantum dots (QDs) on glass substrates was investigated using various intensities and wavelengths of excitation light. CdSe/ZnS QDs capped with tri-n-octylphosphine oxide (TOPO) were produced using colloidal chemical synthesis, and mono- and multilayer QD films were fabricated on glass substrates by spin coating. The fluorescence quantum yield (QY) of the QD monolayer was greatly enhanced by continuous irradiation in a dry nitrogen atmosphere, whereas the QD multilayer showed a small enhancement of the QY or fluorescence intensity decay. In addition, the shorter the excitation wavelength, the more pronounced the PFE. The rate of increase of the QY increased with decreasing excitation intensities at any wavelength. These dependences were observed in both mono- and multilayer films. Our results suggest that the photoejection of electrons to the substrate is the origin of PFE. Assuming the charging effect of electrons trapped in the substrate, a phenomenological model is proposed to explain all of the experimental results, that is, the dependence on the intensity and wavelength of excitation light and the qualitative difference in PFE behavior between mono- and multilayer films.     

Photoisomerization and thermoisomerization I: Unusual photochromism of N-(3,5-di-tert-butyl-salicylidene) amines

Unusually photosensitive Schiff bases were prepared from 3,5-di-tert-butylsalicylaldehyde and amines; the aniline derivative exhibits both normal and reverse photochromism on irradiation with UV light followed by either storage in the dark or irradiation with visible light. From a consistent kinetics explanation of the thermal back reaction, two courses for the photo-bleaching and the thermal fading of the photochromer, which is assigned to a quinoid amine form, are illustrated.     

Photo-orientation phenomena in photosensitive chiral nematic copolymers

Photo-orientational phenomena have been studied for two comb-shaped cholesteric copolyacrylates containing azobenzene side groups. Copolymer^I contains nematogenic phenyl benzoate groups and photosensitive chiral menthyl-containing azobenzene side groups. Copolymer^II is composed of nematogenic phenyl benzoate groups, photosensitive cyanoazobenzene groups and chiral photochromic benzylidene-^p-methan-3-one fragments. Under the action of polarized Ar⁺ laser light (488 nm), orientation of the side groups of the copolymers takes place, and this orientation is perpendicular to the vector of the electric field of the incident light. This process shows a co-operative character; that is, it involves both photosensitive azobenzene and phenyl benzoate groups. The kinetics of growth of the photoinduced orientational order parameter were studied as a function of film thickness, incident light intensity, and preliminary UV irradiation. For the planar oriented films of the copolymers, irradiation with polarized light leads to the development of photoinduced birefringence Δⁿ; maximum values of Δⁿ reach 0.01. The photo-optical properties of copolymers^I and ^II are compared. Such materials may be used for 'dual' data recording by varying the helix pitch, selective light reflection maximum, and photoinduced birefringence or linear dichroism.     

Photodecarbonylation of ruthenium carbonyl octaethylporphyrin via stepwise two-photon absorption of visible light

The photoinduced decarbonylation of a ruthenium(II) carbonyl octaethylporphyrin (RuOEP(CO)(Py)) complex has been shown to occur upon nanosecond pulse laser irradiation as a visible light excitation source, which is reasonably interpreted by a stepwise two-photon absorption process.