Control of nanopore wetting by a photochromic spiropyran: a light-controlled valve and electrical switch

By modifying the surface of nanoporous alumina membranes using mixtures of a photochromic spiropyran and hydrophobic molecules, it is possible to control the admission of water into the membrane using light. When the spiropyran is in the thermally stable, relatively hydrophobic closed form, the membrane is not wet by an aqueous solution. Upon exposure to UV light, the spiropyran photoisomerizes to the more polar merocyanine form, allowing water to enter the pores and cross the membrane. Thus, the photosensitive membrane acts as a burst valve, allowing the transport of water and ions across the membrane. If the aqueous solution contains ions, then the membrane acts as an electrical switch; photoisomerization leads to a two-order-of-magnitude increase in ionic conductance, allowing a current to flow across the membrane. Exposure to visible light causes photoisomerization of the merocyanine back to the closed, spiro form, but dewetting of the membrane does not occur spontaneously, due to a high activation barrier.     

A Novel Design of Multi‐Mechanoresponsive and Mechanically Strong Hydrogels

A newly developed polyacrylamide‐co ‐methyl acrylate/spiropyran (SP) hydrogel crosslinked by SP mechanophore demonstrates multi‐stimuli‐responsive and mechanically strong properties. The hydrogels not only exhibit thermo‐, photo‐, and mechano‐induced color changes, but also achieve super‐strong mechanical properties (tensile stress of 1.45 MPa, tensile strain of ≈600%, and fracture energy of 7300 J m^?2). Due to a reversible structural transformation between spiropyran (a ring‐close) and merocyanine (a ring‐open) states, simple exposure of the hydrogels to white light can reverse color changes and restore mechanical properties. The new design approach for a new mechanoresponsive hydrogel is easily transformative to the development of other mechanophore‐based hydrogels for sensing, imaging, and display applications.     

Photoreversible Fluorescent Modulation of Nanoparticles via One‐Step Miniemulsion Polymerization

A nitrobenzoxadiazolyl(NBD)‐based fluorescent dye and a photochromic spiropyran derivative are incorporated into polymeric nanoparticles via a one‐step miniemulsion polymerization. The diameter of the nanoparticles can be varied from approximately 40 nm to 80 nm by adjusting the polymerization conditions. The prepared nanoparticles exhibit the spectral properties of both NBD dye and spiropyran, indicating that the two chromophores are incorporated into the nanoparticles. The determined amount of NBD and spiropyran in the nanoparticles are about ≈85–90% of the feed amount, while the determined weight ratios of spiropyran to NBD in nanoparticles are very close to that of feed ratios, suggesting the miniemulsion polymerization is a suitable approach for incorporating multiple chromophores into individual nanoparticles with controlled amounts (content) and ratio. UV and visible light can be applied to modulate the fluorescence emission of NBD dye in nanoparticles. Upon UV irradiation, the spiropyran moieties in nanoparticles are converted to the open‐ring (McH form) structure and upon visible‐light irradiation they return to the closed‐ring (SP form) structure; as a result, the fluorescence of NBD can be reversibly “switched off” and “switched on”. Fluorescence resonance energy transfer from the excited NBD dye molecules to the McH form of the spiropyran moieties is the drives the fluorescence modulation. The nanoparticles display fairly good photoreversibility, photostability, and relatively fast photoresponsivity upon alternate UV/Vis irradiation. This class of photoresponsive nanoparticles may find applications in biological fields, such as labeling and imaging, as well as in optical fields, for example, individually light‐addressable nanoscale devices.     

Formation of organic heterostructures into p-n heterojunctions by surface pressure control

A surface-active rhodamine dye, combined with arachidic acid or with arachidic acid and merocyanine dye, is squeezed out from the monolayer at the air- water interface above a certain surface pressure and forms a two-storey structure with each component possessing an ordered molecular arrangement. Photoelectrical measurements have revealed that the two-storey structure composed of rhodamine, merocyanine and arachidic acid can be deposited on solid substrates to form an alternating p-n hetero Langmuir-Blodgett film, which is an entirely new way of constructing p-n junction heterostructures.     

Controls of photochromic reactions in spiropyran Langmuir-Blodgett films

The possibility of controlling photochromic reactions has been investigated in Langmuir-Blodgett (LB) films. The formation of J aggregates of a spiropyran (SP1822) with two long chains has been found previously. We have studied another possibility using LB films of 1′-octadecyl-3′,3′-dimethyl-6-nitro-8-methoxy- spiro[2H-1-benzopyran-2,2′-indoline] (SP1801). SP1801 at the air-water interface gave a monolayer which showed the photomerocyanine form. The LB film of the compound returned to the spiropyran form in the dark. The LB film of the hetero structure with SP1801 and stearic acid, however, turned to J aggregates from the photomerocyanine form spontaneously in the dark. The absorption of the J aggregates was a sharp and intense band, and the half-decay period in the dark was 10⁴ times longer than that of conventional spiropyrans. SP1801, in addition to SP1822, is suggested as a candidate for a multifrequency optical memory system with several J-aggregated photochromic compounds having different sharp absorption bands.     

N‐Type Superconductivity in an Organic Mott Insulator Induced by Light‐Driven Electron‐Doping

The presence of interface dipoles in self‐assembled monolayers (SAMs) gives rise to electric‐field effects at the device interfaces. SAMs of spiropyran derivatives can be used as photoactive interface dipole layer in field‐effect transistors because the photochromism of spiropyrans involves a large dipole moment switching. Recently, light‐induced p‐type superconductivity in an organic Mott insulator, κ‐(BEDT‐TTF)₂Cu[N(CN)₂]Br (κ‐Br: BEDT‐TTF = bis(ethylenedithio)tetrathiafulvalene) has been realized, thanks to the hole carriers induced by significant interface dipole variation in the spiropyran‐SAM. This report explores the converse situation by designing a new type of spiropyran monolayer in which light‐induced electron‐doping into κ‐Br and accompanying n‐type superconducting transition have been observed. These results open new possibilities for novel electronics utilizing a photoactive SAMs, which can design not only the magnitude but also the direction of photoinduced electric‐fields at the device interfaces.     

Light‐Driven Proton Transfer for Cyclic and Temporal Switching of Enzymatic Nanoreactors

Temporal activation of biological processes by visible light and subsequent return to an inactive state in the absence of light is an essential characteristic of photoreceptor cells. Inspired by these phenomena, light‐responsive materials are very attractive due to the high spatiotemporal control of light irradiation, with light being able to precisely orchestrate processes repeatedly over many cycles. Herein, it is reported that light‐driven proton transfer triggered by a merocyanine‐based photoacid can be used to modulate the permeability of pH‐responsive polymersomes through cyclic, temporally controlled protonation and deprotonation of the polymersome membrane. The membranes can undergo repeated light‐driven swelling–contraction cycles without losing functional effectiveness. When applied to enzyme loaded‐nanoreactors, this membrane responsiveness is used for the reversible control of enzymatic reactions. This combination of the merocyanine‐based photoacid and pH‐switchable nanoreactors results in rapidly responding and versatile supramolecular systems successfully used to switch enzymatic reactions ON and OFF on demand.     

Generation and transmission of a surface pressure impulse in monolayers

Surface pressure impulse propagation through a monolayer at the air-water interface has been investigated. The pressure impulse confined to the monolayer plane is produced by the photoisomerization of an amphiphilic spiropyran. Arachidic acid, dimyristoylphosphatidylcholine and monomethyloctadecanedioate have been used as transmitting layers. The pressure impulse is detected with a microphone-type sensor at various distances. The longitudinal pulse velocity is in the 50–260 cm s⁻¹ range. The results are interpreted in terms of a simple model in which a thin water layer under the monolayer moves along with it. The thickness of this layer is estimated to be about 100 μm, independent of the transmitting layer. It is also concluded that the dynamical compression modulus is up to four times larger than the stationary compression modulus.     

Thermoresponsive copolymer containing a coumarin-spiropyran conjugate: reusable fluorescent sensor for cyanide anion detection in water

A simple copolymer consisting of N-isopropylacrylamide and coumarin-conjugated spiropyran (CS) units, poly(NIPAM-co-CS), has been synthesized. This polymer enables selective fluorometric detection of cyanide anion (CN(-)) in water at room temperature. The polymer itself shows almost no fluorescence, but shows a strong blue fluorescence in the presence of CN(-) under irradiation of UV light. The fluorescence enhancement occurs via a nucleophilic interaction between CN(-) and the photoformed merocyanine form of the CS unit, leading to a localization of π-electrons on the coumarin moiety. The polymer enables accurate determination of very low levels of CN(-) (>0.5 μM). The polymer can be recovered from water by simple centrifugation at high temperature (>40 °C), due to the heat-induced aggregation of the polymer. In addition, the polymer is regenerated by simple acid treatment, and the resulting polymer is successfully reused for further CN(-) sensing without loss of sensitivity.     

Multifunctional photochromic spiropyran dendrimers and their relaxation behaviors of photochromism

First and second generation dendrimers that exhibit unique photochromic behavior were synthesized through multi-step reactions. Their photochromic behavior under illumination of monochromatic ultraviolet light (λ = 365 nm) was investigated. The colorless dendritic dye solution and its film exhibit typical ring opening and E/Z geometrical transformation from spiropyran to merocyanine. The stability of photochromism in the two dendrimers was much better than that observed in the dye-attached polymer or dye-doped polymer system. Particularly, blend material system of photochromic dendrimer and photocrosslinkable dendrimer showed much better stability of the photochromism.     

Perhydropolysilazane-derived silica-polymethylmethacrylate hybrid thin films highly doped with spiropyran: Effects of polymethylmethacrylate on the hardness, chemical durability and photochromic properties

Polymethylmethacrylate (PMMA)-perhydropolysilazane (PHPS) hybrid thin films doped with spiropyran were prepared by spin-coating, which were then converted into 0.26-1.7 μm thick, spiropyran-doped PMMA-silica hybrid films by exposure treatment over aqueous ammonia. The spiropyran/(spiropyran + PHPS + PMMA) mass ratio was fixed at a high value of 0.2 so that the films exhibit visual photochromic changes in color, while the PMMA/(PMMA + PHPS) mass ratio, r, was varied. The spiropyran molecules in the as-prepared films were in merocyanine (MC) and spiro (SP) forms, with and without an optical absorption at 500 nm, at low (r ≤ 0.2) and high (r ≥ 0.4) PMMA contents, respectively. When PMMA content r was increased from 0 to 0.2, the degree of the MC-to-SP conversion on vis light illumination was enhanced, while at higher r's the spiropyran molecules underwent photodegradation. When the silica film (r = 0) was soaked in xylene under vis light, the spiropyran molecules were almost totally leached out, while not on soaking in the dark. On the other hand, no leaching occurred for the film of r = 0.2 either in the presence or absence of vis light. These suggest that the introduction of PMMA is effective in improving the chemical durability of the films, while the silica film (r = 0) is an interesting material with a photoresponsive controlled-release ability. The pencil hardness of the films decreased with increasing PMMA content, but remained over 9H at r ≤ 0.4.     

Strain and stress mapping by mechanochemical activation of spiropyran in poly(methyl methacrylate)

The relationship between fracture‐induced mechanophore activation and the strain and stress ahead of a propagating crack in poly(methyl methacrylate) (PMMA) is studied. The mechanophore spiropyran is used as a secondary cross‐linker in rubber toughened PMMA, and the spiropyran‐linked material is subjected to fracture testing. Mechanophore activation is detected and analysed by fluorescence imaging. Digital image correlation is used to measure the strain field ahead of the crack tip, whereas the corresponding stress field is calculated using the Hutchinson–Rice–Rosengren singularity field equations. Mechanophore activation follows a power law dependence on distance from the crack tip and provides both a qualitative and quantitative measure of the strain and stress fields ahead of the crack.     

Controlling Plasmon‐Enhanced Fluorescence via Intersystem Crossing in Photoswitchable Molecules

By harnessing photoswitchable intersystem crossing (ISC) in spiropyran (SP) molecules, active control of plasmon‐enhanced fluorescence in the hybrid systems of SP molecules and plasmonic nanostructures is achieved. Specifically, SP‐derived merocyanine (MC) molecules formed by photochemical ring‐opening reaction display efficient ISC due to their zwitterionic character. In contrast, ISC in quinoidal MC molecules formed by thermal ring‐opening reaction is negligible. The high ISC rate can improve fluorescence quantum yield of the plasmon‐modified spontaneous emission, only when the plasmonic electromagnetic field enhancement is sufficiently high. Along this line, extensive photomodulation of fluorescence is demonstrated by switching the ISC in MC molecules at Au nanoparticle aggregates, where strongly enhanced plasmonic hot spots exist. The ISC‐mediated plasmon‐enhanced fluorescence represents a new approach toward controlling the spontaneous emission of fluorophores near plasmonic nanostructures, which expands the applications of active molecular plasmonics in information processing, biosensing, and bioimaging.     

Application of photoresponsive polymers carrying crown ether and spirobenzopyran side chains to photochemical valve

A vinyl copolymer carrying crown ether and spirobenzopyran side chains, which undergoes significant photoinduced rheology changes, i.e., contraction and extension of its polymer chain, was applied to a material for photocontrol system of solvent permeation rate, so-called, photochemical valve. Macroporous polyethylene membranes coated by the crown ether-spirobenzopyran copolymer can work as a functional membrane controlling solvent permeation rate photochemically. UV-light irradiation on the photoresponsive membrane decreased the permeation rate of hexane, due to the increased polarity of the membrane pore, which was in turn derived from the photoisomerization of its spirobenzopyran moiety to the corresponding ionic merocyanine form. The following visible-light irradiation on the membrane restored the permeation rate by isomerization back to the electrically neutral spiropyran form. To the contrary, the permeation of ethanol through the membrane was enhanced by UV-light irradiation due to the increase in the apparent membrane pore size induced by the polymer chain contraction and vice versa by visible light. Similar photoresponses in the permeation rate of nonpolar and polar solvents were also observed with a sintered glass filter modified chemically by both silane-coupling reagents containing crown ether and spirobenzopyran moieties.     

Simultaneous nucleophilic-substituted and electrostatic interactions for thermal switching of spiropyran: a new approach for rapid and selective colorimetric detection of thiol-containing amino acids

Complementary electrostatic interaction between the zwitterionic merocyanine and dipolar molecules has emerged as a common strategy for reversibly structural conversion of spiropyrans. Herein, we report a concept-new approach for thermal switching of a spiropyran that is based on simultaneous nucleophilic-substitution reaction and electrostatic interaction. The nucleophilic-substitution at spiro-carbon atom of a spiropyran is promoted due to electron-deficient interaction induced by 6- and 8-nitro groups, which is responsible for the isomerization of the spiropyran by interacting with thiol-containing amino acids. Further, the electrostatic interaction between the zwitterionic merocyanine and the amino acids would accelerate the structural conversion. As proof-of-principle, we outline the route to glutathione (GSH)-induced ring-opening of 6,8-dinitro-1',3',3'-trimethylspiro [2H-1-benzopyran-2,2'-indoline] (1) and its application for rapid and sensitive colorimetric detection of GSH. In ethanol-water (1:99, v/v) solution at pH 8.0, the free 1 exhibited slight-yellow color, but the color changed clearly from slight-yellow to orange-yellow when GSH was introduced into the solution. Ring-opening rate of 1 upon accession of GSH in the dark is 0.45 s(-1), which is 4 orders of magnitude faster in comparison with the rate of the spontaneous thermal isomerization. The absorbance enhancement of 1 at 480 nm was in proportion to the GSH concentration of 2.5 × 10(-8)-5.0 × 10(-6) M with a detection limit of 1.0 × 10(-8) M. Furthermore, due to the specific chemical reaction between the probe and target, color change of 1 is highly selective for thiol-containing amino acids; interferences from other biologically active amino acids or anions are minimal.     

Molecular switching in nano-structured photochromic films of biopolymers

This paper reports the investigation of the photochromic and conformational behaviour of a poly (l-glutamic acid) (PSG) chemically modified with 85% spiropyran units in the side chains, PSG. We preliminary studied the behaviour of PSG in solution and then we performed a characterisation of the polymer in bidimensional systems. PSG monolayer at the water–air interface was characterised by means of surface pressure–molecular area isotherms and UV–Vis Spectroscopy and the photochromic behaviour was illustrated. Moreover, we transferred, by means of the Langmuir–Blodgett (LB) technique, polypeptide monolayers onto solid support in order to obtain ordered and nano-organized systems whose spectroscopic properties were investigated.     

Efficient and Layer‐Dependent Exciton Pumping across Atomically Thin Organic–Inorganic Type‐I Heterostructures

The fundamental light–matter interactions in monolayer transition metal dichalcogenides might be significantly engineered by hybridization with their organic counterparts, enabling intriguing optoelectronic applications. Here, atomically thin organic–inorganic (O–I) heterostructures, comprising monolayer MoSe₂ and mono‐/few‐layer single‐crystal pentacene samples, are fabricated. These heterostructures show type‐I band alignments, allowing efficient and layer‐dependent exciton pumping across the O–I interfaces. The interfacial exciton pumping has much higher efficiency (>86 times) than the photoexcitation process in MoSe₂, although the pentacene layer has much lower optical absorption than MoSe₂. This highly enhanced pumping efficiency is attributed to the high quantum yield in pentacene and the ultrafast energy transfer between the O–I interface. Furthermore, those organic counterparts significantly modulate the bindings of charged excitons in monolayer MoSe₂ via their precise dielectric environment engineering. The results open new avenues for exploring fundamental phenomena and novel optoelectronic applications using atomically thin O–I heterostructures.     

Switchable dual-wavelength fiber laser mode-locked by monolayer graphene on D-shaped fiber

A switchable mode-locking fiber laser is demonstrated by means of a monolayer graphene saturable absorber (SA) based on a D-shaped fiber. The monolayer graphene, which is grown by chemical vapor deposition, is transferred onto the D-shaped fiber and then the light–graphene interaction via the evanescent field of the fiber is enhanced greatly. Using such a graphene-based SA, the single-wavelength mode locking can be switched from 1531.5 to 1559.1 nm by appropriately adjusting the polarization controller (PC). In addition, the stable dual-wavelength mode-locking operation is also observed at the proper state of PC.     

Dynamically Tuning Particle Interactions and Assemblies at Soft Interfaces: Reversible Order–Disorder Transitions in 2D Particle Monolayers

Particles trapped at fluid interfaces experience long‐range interactions that determine their assembly behavior. Because particle interactions at fluid interfaces tend to be unusually strong, once particles organize themselves into a 2D assembly, it is challenging to induce changes in their microstructure. In this report, a new approach is presented to induce reversible order–disorder transitions (ODTs) in the 2D monolayer of colloidal particles trapped at a soft gel–fluid interface. Particles at the soft interface, consisting of a nonpolar superphase and a weakly gelled subphase, initially form a monolayer with a highly ordered structure. The structure of this monolayer can be dynamically varied by the addition or removal of the oil phase. Upon removing the oil via evaporation, the initially ordered particle monolayer undergoes ODT, driven by capillary attractions. The ordered monolayer can be recovered through disorder‐to‐order transition by simply adding oil atop the particle‐laden soft interface. The possibility to dynamically tune the interparticle interactions using soft interfaces can potentially enable control of the transport and mechanical properties of particle‐laden interfaces and provide model systems to study particle‐laden soft interfaces that are relevant to biological tissues or organs.     

The enhanced assumed strain method for the isogeometric analysis of nearly incompressible deformation of solids

Isogeometric analysis has recently become very popular for the numerical modeling of structures and fluids. Among other potential features, advantages of using a non‐uniform rational B‐splines (NURBS)‐based isogeometric analysis over the traditional finite element method include the possibility of using higher‐order polynomials for the basis functions of the approximation space, which may be easily built on a recursive (hierarchical) fashion as well as higher convergence ratio. Nevertheless, NURBS‐based isogeometric analysis suffers from the same problems depicted by other methods when it comes to reproduce isochoric deformations, that is, it shows volumetric locking, especially for low‐order basis functions. Similar remedies as those that have been proposed for the finite element method may be appropriate for integration in the NURBS‐based isogeometric analysis and some have already been tried with success. In this work, the analysis of the underlying space of incompressible deformations of a NURBS‐based isogeometric approximation is performed with the main objective of understanding the likelihood of volumetric locking. As a remedy, the enhanced assumed strain methodology is blended with the NURBS‐based isogeometric analysis to alleviate the volumetric locking associated with incompressible deformations. The solution includes a stabilization term derived directly from a penalized form of the classical Veubeke–Hu–Washizu three‐field variational principle. Copyright © 2012 John Wiley & Sons, Ltd.