Preparation and study of photoswitchable fluorescence nanoparticles based on spirobenzopyran

The preparation and performance characterization of〈50 nm spirobenzopyran-based photochromic nanocomposites with photoswitchable fluorescence are presented.The nanocomposites were fabricated by means of a modifed miniemulsion polymerization process,in which the hydrophobic spirobenzopyran was covalently attached to the polymer chains and the matched fluorescent dyes were noncovalently embedded in the nanoscale cross-linked polymeric matrix,respectively.The obtained nanocomposites with a high relative fluorescence quantum yield（Q）exhibited superior fluorescent photoswitchable performance due to the effective photo-induced intermolecular energy transfer.The stability of photomerocyanine was also improved.     

Functionalization of aminocrown ethers with [2H]-chromene units by Buchwald-Hartwig C-N coupling for constructing of photoswitchable molecular devices

Functionalization of the diaminodibenzocrown ethers with 2H-[1]benzopyrane and 3H-naphtho[2,1-b]pyrane units by Buchwald-Hartwig C-N coupling was developed. It is demonstrated that the novel molecule possesses photochromic properties that can be modified by the complex formation process.     

Nature of charge separation and recombination processes within an organic dyad having short spacer

With the help of electrochemical, steady state and time resolved fluorescence (fluorescence lifetimes by using time correlated single photon counting technique) and nanosecond laser flash photolysis methods, the nature of charge separation along with the energy wasting charge recombination processes within a short-chained organic dyad 1-(4-Bromo-phenyl)-3-(2-methoxy-naphthalen-1-yl)-propenone (MNBA) has been revealed. In MNBA, the donor 2-methoxynaphthalene (2MNT) is connected with the acceptor p-bromoacetophenone (PBA) by an unsaturated olefinic bond. Though in the ground state elongated type structure (E-form) is observable from NMR spectra but on photoexcitation, another conformers possibly of the nature of folded type isomeric species (termed as Z-isomer) were also apparent from time resolved fluorescence measurements. However, preponderance of elongated form in the excited singlet state has been established from this time resolved measurements. NMR study on photoirradiated sample and theoretical predictions from computations using CIS method with Lanl2DZ basis set also indicate in favor of the propositions made on the formations of the two possible conformers and the stability of elongated isomeric species in the electronic excited state from the experimental results. The energy wasting charge recombination rate, k_CR, determined from the transient absorption measurements by nanosecond laser flash photolysis (LFP) technique was found to be (k_CR, ∼1.9×10⁵ s⁻¹) significantly lower than the charge-separation rate, k_cs∼9.4×10⁷ s⁻¹, measured from the time resolved fluorescence. This observation demonstrates that MNBA may serve as an efficient candidate to construct artificial light energy conversion devices or components of molecular photovoltaic cells.     

Control of the Structure and Functions of Biomaterials by Light

Vision and other light-triggered biochemical transformations in plants and living organisms represent a sophisticated biological processes in which optical signals are recorded and transduced as (physico)chemical events. Photoswitchable biomaterials are a new class of substances in which optical signals generate discrete “On” and “Off” states of biological functions, resembling logic gates that flip between 0 and 1 states in response to the changes in electric currents in computers. The (photo)chemistry of photochromic materials has been extensively developed in the past four decades. These materials isomerize reversibly upon light absorption, and the discrete photoisomeric states exhibit distinct spectral and chemical features. Integration of photoisomerizable (or photochromic) units into biomaterials allow their secondary functions such as biocatalysis, binding, and electron transfer to be tailored so that they can be switched on or off. This can be accomplished by chemical modification of the biomaterial by photoisomerizable units and by integration of biomaterials in photoisomerizable microenvironments such as monolayers or polymers. The photoswitchable properties of chemically modified biomaterials originate from the light-induced generation or perturbation of the biologically active site, whereas in photoisomerizable matrices they depend upon the regulation of the physical or chemical features of the photoisomerizable assemblies of polymers, monolayers, or membranes. Light-triggered activation of catalytic biomaterials provides a means of amplifying the recorded optical signal by biochemical transformations, and photostimulated biochemical redox switches allow its electrochemical transduction and amplification. The field of photoswitches based on biomaterials has developed extensively in the past few years within the general context of molecular switching devices and micromachinery. The extensive knowledge on the manipulation of biomaterials through genetic engineering and the fabrication of surfaces modified by biologically active materials enables us to prepare biomaterials with improved optical-switching features. Their application in optoelectronic or bioelectronic devices has been transformed from fantasy to reality. The use of photoswitchable biomaterials in information storage and processing devices (biocomputers), sensors, reversible immunosensors, and biological amplifiers of optical signals has already been demonstrated, but still leaves important future challenges.     

Azopolyesters with Intrinsic Crystallinity and Photoswitchable Reversible Solid-to-Liquid Transitions

Herein, we introduce a variety of azopolyesters (azobenzene-based polyesters) with remarkable intrinsic crystallinity and photoinduced reversible solid-to-liquid transition abilities from copolymerization of azobenzene-based epoxides with cyclic anhydrides. The length of the soft alkyl side-chain inlaid with azobenzenes and stereoregularity of main-chain of azopolymers have tremendous effects on crystallization properties of the resulting polyesters with melting temperature (T_m) in the range of 51–251 °C. Moreover, some of azopolyesters possess excellently photoinduced reversible solid-to-liquid transition performance thanks to trans-cis photoisomerization of azobenzenes. Trans-azopolyesters are yellow solids with T_ms or glass transition temperatures (T_gs) above room temperature, whereas cis-polymers are red liquids with T_gs below −20 °C. These azopolyesters could be applied as novel light-switchable adhesives for quartz/quartz, wood/wood and quartz/wood adhesion, with the strength in the range of 0.73–0.89 MPa for trans-polymers. Conversely, the adhesion strength of liquefied cis-azopolyesters generated from the irradiation of trans-polymers by UV light was about 0.1 MPa, which shows light enable to control the adhesion process with high spatiotemporal resolution.     

Photoswitchable and Reversible Fluorescent Eutectogels for Conformal Information Encryption

Smart fluorescent materials that can respond to environmental stimuli are of great importance in the fields of information encryption and anti-counterfeiting. However, traditional fluorescent materials usually face problems such as lack of tunable fluorescence and insufficient surface-adaptive adhesion, hindering their practical applications. Herein, inspired by the glowing sucker octopus, we present a novel strategy to fabricate a reversible fluorescent eutectogel with high transparency, adhesive and self-healing performance for conformal information encryption and anti-counterfeiting. Using anthracene as luminescent unit, the eutectogel exhibits photoswitchable fluorescence and can therefore be reversibly written/erased with patterns by non-contact stimulation. Additionally, different from mechanically irreversible adhesion via glue, the eutectogel can adhere to various irregular substrates over a wide temperature range (−20 to 65 °C) and conformally deform more than 1000 times without peeling off. Furthermore, by exploiting surface-adaptive adhesion, high transparency and good stretchability of the eutectogel, dual encryption can be achieved under UV and stretching conditions to further improve the security level. This study should provide a promising strategy for the future development of advanced intelligent anti-counterfeiting materials.     

Spiropyran-based advanced photoswitchable materials: A fascinating pathway to the future stimuli-responsive devices

Recent years have witnessed tremendous progress and developments of the photoswitchable spiropyran-based polymers, owing to distinctive and particular physicochemical properties of their isomers upon a variety of triggers, and especially light illumination. Light is a fascinating and green stimulus because of its remote control, micron- or submicron-sized focusing area with controllable wavelength and energy, non-invasiveness and non-destructive nature, precisely controlled direction, and availability. In this review, we have emphasized on and summarized the most recent observations and efforts in the progress of photoswitchable spiropyran-based materials and their applications as sensors for heavy metal cations, anions, pH, acid and base vapors, wettability and humidity. Other items include data recording and anticounterfeiting devices, photorheological fluids, optically reversible switching membranes, photoregulating surface plasmon resonance, photomodulation of ion conductivity and mechanoresponsive polymers. The bio-based field is another interesting subject that is discussed here and consists of reversible cell sheet engineering, photodynamic therapy, switchable fluorescence labeling, controlled drug delivery and biological ion channels. On the other hand, limited light penetration inside the living tissues and hazards of high-energy ultraviolet irradiation for initiating photochemical transformations have limited the use of such light-controlled systems in medicinal and therapeutic means. Those spiropyran-based materials which are susceptible to being triggered by low energy near IR (NIR) two-photon light irradiation and upconversion nanoparticles are recently under serious explorations and have been reviewed as a new perspective for their advanced applications.     

A spiropyran with enhanced fluorescence: A bright,photostable and red-emitting calcium sensor

A rationally designed, pyrene-spiropyran hybrid Ca²⁺ sensor (Py-1) with enhanced fluorescence intensity compared to a standalone spiropyran analogue is presented. Importantly, Py-1 retains the characteristic red emission profile of the spiropyran, while fibre-based photostability studies show the sensor is stable after multiple cycles of photoswitching, without any sign of photodegradation. Such properties are of real advantage for cell-based sensing applications. An interesting observation is that, Py-1 presents with two excitation options; direct green excitation (532 nm) of the photoswitch for a red emission, and UV excitation (344 nm) of the component pyrene, which gives rise to distinct blue and red emissions. This proof-of-concept hybrid sensing system presents as a more general approach to brighter spiropyran-based sensors.     

Rational Design in Photopharmacology with Molecular Photoswitches

Photopharmacology is an attractive approach for achieving targeted drug action with the use of light. In photopharmacology, molecular photoswitches are introduced into the structure of biologically active small molecules to allow for the optical control of their potency. Going beyond trial and error, photopharmacology has progressively applied rational drug design methodologies to devise light-controlled bioactive ligands. In this review, we categorize photopharmacological efforts from the standpoint of medicinal chemistry strategies, focusing on diffusible photochromic ligands modified with photoswitches that operate through E-Z bond isomerization. In the vast majority of cases, photoswitchable ligands are designed as analogs of existing compounds, through a variety of approaches. By analyzing in detail a comprehensive list of instructive examples, we describe the state of the art and discuss future opportunities for rational design in photopharmacology.     

Photoswitchable Enantioselective and Helix-Sense Controlled Living Polymerization

A pair of enantiomeric photoswitchable Pd^II catalysts, alkyne-Pd^II/ L ^R−azo and alkyne-Pd^II/ L ^S−azo, were prepared via the coordination of alkyne-Pd^II and azobenzene-modified phosphine ligands L ^R−azo and L ^S−azo. Owing to the cis-trans photoisomerization of the azobenzene moiety, alkyne-Pd^II/ L ^R−azo and alkyne-Pd^II/ L ^S−azo exhibited different polymerization activities, helix-sense selectivities, and enantioselectivities during the polymerization of isocyanide monomers under irradiation of different wavelength lights. Furthermore, the achiral isocyanide monomer A- 1 could be polymerized efficiently using alkyne-Pd^II/ L ^R−azo under dark condition in a living/controlled manner. Further, it generated single right-handed helical poly-A- 1 _m( L ^R−azo), confirmed by the circular dichroism spectra and atomic force microscopy images. However, the polymerization of A- 1 almost could not be initiated under 420 nm light in identical conditions of dark condition. Moreover, the photoswitchable catalyst alkyne-Pd^II/ L ^R−azo exhibited high enantioselectivity for the polymerization of the racemates of L- 1 and D- 1 , respectively. D- 1 was polymerized preferentially under dark condition with a D- 1 /L- 1 rate ratio of 70, yielding single right-handed polyisocyanides. Additionally, reversible enantioselectivity was observed under 420 nm light using alkyne-Pd^II/ L ^R−azo, and the calculated polymerization rate ratio of L- 1 /D- 1 was 57 because of the isomerization of the azobenzene moiety of the catalyst. Furthermore, alkyne-Pd^II/ L ^S−azo showed opposite enantioselectivity and helix-sense selectivity during the polymerization of the racemates of L- 1 and D- 1 .