Multi-stimuli Responsive Materials Research Articles

Genetically Fusing Order-Promoting and Thermoresponsive Building Blocks to Design Hybrid Biomaterials.

The unique biophysical and biochemical properties of intrinsically disordered proteins (IDPs) and their recombinant derivatives, intrinsically disordered protein polymers (IDPPs) offer opportunities for producing multistimuli-responsive materials; their sequence-encoded disorder and tendency for phase separation facilitate the development of multifunctional materials. This review highlights the strategies for enhancing the structural diversity of elastin-like polypeptides (ELPs) and resilin-like polypeptides (RLPs), and their self-assembled structures via genetic fusion to ordered motifs such as helical or beta sheet domains. In particular, this review describes approaches that harness the synergistic interplay between order-promoting and thermoresponsive building blocks to design hybrid biomaterials, resulting in well-structured, stimuli-responsive supramolecular materials ordered on the nanoscale.

Multi-azobenzene moieties on rigid cyclotriphosphazene core: Synthesis, structural characterization, electrochemistry, and photoisomerization study

Multi-azobenzene derivatives capable of showing multi-stimuli responsive material can exhibit superior performance in biomedicine, the dye industry, cosmetics, and photonics. Here, a nucleophilic substitution reaction was employed to assemble two (Az2CPZ(biph)2) or four (Az4CPZ(biph)) or six (Az6CPZ) units of azobenzene moieties on the cyclotriphosphazene (CPZ) core. We have observed that the color of each compound varied by changing the number of azobenzene moieties on the CPZ core. These compounds were characterized by elemental analysis, 1H, 13C{1H}, and 31P{1H} NMR techniques. Single crystal X-ray diffraction techniques ensure the trans conformation in the azobenzene for Az2CPZ(biph)2 and Az4CPZ(biph). Electronic spectroscopy forAz2CPZ(biph)2,Az4CPZ(biph), andAz6CPZhaving π-π* transition around 325 nm show a hypsochromic shift with ∼25 nm compared to the parent molecule, 4-hydroxyazobenzene AzH. As a result of the electronic spectroscopy, the cis isomer formation enhanced upon increasing the concentration ofAz2CPZ(biph)2/Az4CPZ(biph)/Az6CPZ in DCM/DMF. The photoisomerization study confirms that these molecules having at least one azo moiety can switch from trans to cis conformation upon exposure to 365 nm light. Interestingly, the thin-layer chromatography (TLC) corroborated the switch of trans to cis conformation occurs at room temperature in a solution state even without UV irradiation. The proton NMR integration providesthe generationofthe cis-azobenzene moiety is about 12–20 % inAz2CPZ(biph)2upon exposure to 365 nm light for 15 to 60 mins. Additionally, TLC verifies the generation of multiple isomers that confirm multi-functional switch behavior when the sample with UV exposure time increases. Owing to these molecules undergoing a isomerization process (trans-azo⇌cis-azo) even without UV, their fluorescence gets quenched in the solution state. Cyclic voltammetry confirms the quasi-reversible type redox behavior of all compounds in dry dichloromethane. We believe the results presented in this work will be significant for dye and biomedical applications.

Acid–base regulated inclusion complexes of β-cyclodextrin with 1-[2-(4-fluorophenyl)-2-oxoethyl]-4,4′-bipyridinium dichloride displaying multistimuli-responsive chromic behaviors and photomodulable fluorescence

Two inclusion complexes of β-cyclodextrin with a viologen derivative display multistimuli-responsive chromic behaviors and photomodulable fluorescence, which endows them with various applications.

Multi-stimuli responsive behaviors in a new chiral hybrid nitroprusside salt (R-3-hydroxypyrrolidinium)2[Fe(CN)5(NO)

Multi-stimuli responsive materials controlled and coupled by two or more channels have a broad range of applications in the field of switches, memories, and molecular machines. The exploration of the material is currently focused on the pure organic system, which limits the development of such materials greatly. In this work, we present a new chiral organic-inorganic hybrid salt, (R-3-hydroxypyrrolidinium)2[Fe(CN)5(NO)] (1), which exhibits rare multi-stimuli responsive behaviors in thermal, mechanical and optical channels. In detail, 1 undergoes a C2-P21221 phase transition deriving from the thermal motion of organic cations with the increase of temperature, but the reverse transition can only be induced by mechanical pressure. Moreover, polycrystalline hybrid salt showed photo-responsive performance, i.e., the ground-state N-bound nitrosyl ligand adopts two configurations in excited states caused by light in 532 nm irradiation, accompanying with a photo-induced structural transformation of the anionic framework. Namely, the thermal motion characteristics of organic cations, the photoresponse characteristics of anionic inorganic skeleton and the pressure characteristics from hydrogen bonds are simultaneously integrated in 1. This unprecedented coupling mechanism of multi-stimuli responses makes 1 a potential candidate for future multichannel data storage applications.

Cyanostyrene derivative with multi-stimuli responsive properties: Multicolor- and high-color-contrast switching in response to force, heat and light

Multi-stimuli responsive luminescent materials are ideal materials for luminescent anti-counterfeiting technology. Currently, it is still challenging to develop excellent multi-stimuli-responsive material that is easy to synthesize and low-cost. Herein, a dicyanostilbenebenzene derivative (α-CFP) with a symmetrical structure was designed and successfully synthesized, which showed obvious AIE and solvent effects in solution. Two polymorphs of it were prepared by controlling the cooling rate, and it was found that green-emitting Form I and blue-emitting Form II can transform into each other by heat and mechanical force. Crystal structure analysis reveals the possible intrinsic mechanism of polymorphism-dependent emission and interconversion. Further studies found that both Form I and Form II could transform into another new polymorph Form III upon heating to 220 °C. Form III can undergo photoinduced [2 + 2] cycloaddition reaction accompanied by discoloration, and it can return to its original state by heating. Finally, three application scenarios were designed and demonstrated, which confirm their potential applications in information encryption and anti-counterfeiting.

Spatiotemporally Controllable Electrical Stimulator via Independent Photobending and Upconversion Photoluminescence Using Two Different Wavelengths of Near-Infrared/Visible Light as Dual Stimuli.

Multistimuli responsive materials are advantageous in that they can enhance the desired response or bypass unwanted reactions. Light is one of the most attractive stimuli since it allows remote spatiotemporal control and multiplexing of properties (e.g., wavelength, intensity, irradiation time, pulsed/continuous wave) for application on multiphotoresponsive materials. However, the operating wavelength for such photoresponsive systems often includes an ultraviolet (UV) range that limits its use in the biomedical field. Herein, we investigate near-infrared (NIR)/visible (Vis) light-responsive nanocomposite films composed of rare earth element (i.e., Yb, Er)-doped NaYF4 nanoparticles (NPs) embedded in azobenzene-incorporated poly(dimethylsiloxane) (AzoPDMS), silk fibroin, and silver nanowire (AgNW) layers. Photobending (PB) of the nanocomposite film is induced by a Vis light of 400-700 nm, while upconversion photoluminescence (UCPL) of embedded NPs is activated by an NIR light of 980 nm. The excitation wavelength of photoluminescence (PL) is shifted to the NIR (λ = 980 nm) range via photon upconversion in rare earth element-doped NPs. Independent operation of PB and UCPL enables both on-demand electrical switching and real-time location monitoring for spatiotemporally controlled electrical pulse stimulation. As a result, the dual-photoresponsive nanocomposite film can be utilized as a remotely controllable electrical stimulator and location indicator via different wavelengths of light.

Multistimuli-responsive fluorescence behaviours of aggregation-induced emission small-molecule and electrospun nanofibre films for acid-base vapour sensing.

Multistimuli-responsive fluorescent materials have garnered great research interest benefited from their practical applications. Two twisted-structure compounds containing tetraphenylethylene (TPE) as the aggregation-induced emission (AIE) group and a pyridine unit as the acid reaction site to obtain new multistimuli-responsive fluorescent compounds (namely, TPECNPy: TPECNPy-2 and TPECNPy-3) were successfully synthesized through a one-step Knoevenagel condensation reaction. The multiple-stimuli response process of TPECNPy was investigated by means of photoluminescence (PL) spectra and emission colour. The results showed that both TPECNPy compounds with excellent AIE abilities displayed reversible emission wavelength and colour changes in response to multiple external stimuli, including grinding-fuming by CH2 Cl2 or annealing and HCl-NH3 vapour fuming. More importantly, fluorescent nanofibre films were prepared by electrospinning a solution of TPECNPy mixed with cellulose acetate (CA), and these exhibited reversible acid-induced discolouration, even with only 1wt% TPECNPy. The results of this study may inspire strategies for designing multistimuli-responsive materials and preparing fluorescent sensing nanofibre films.

Novel salicylic acid derivatives connecting to five-membered cycle through an acyl hydrazone bond as multi-stimuli responsive fluorescent smart materials with photoswitching properties

In order to exploit novel multi-stimuli responsive fluorescent materials, a series of novel fluorescent molecules of salicylic acid derivatives were designed and synthesized via introducing pyrazole or cyclopentane to the salicylic acid scaffold through a special Schiff base–acylhydrazone, and the molecular structures of representative compounds A2 and A4 were verified via single crystal X–ray diffraction. All title molecules could exhibit obvious solvatofluorochromism from cyan to indigo in several solutions with different polarity. The fluorescence titration data exhibited compound A2 and complex A2-Cu2+ with prime detection limits to Cu2+ (0.24 μM) and S2- ions (2.83 μM). The sensitive recognition of A2 to trifluoroacetic (TFA) and A2-TFA to triethylamine (TEA) were also confirmed via fluorescent titration experiments in various solutions, respectively. What’s more, the 1H NMR and UV/Vis absorption spectra further explained the mechanism between molecules and ions or molecules and TFA/TEA. Besides, the photoswitching properties of the compounds A2 and A3 could be demonstrated via the irradiation of special wavelength light or heating accompanied with changes in quantum yields. In addition, these phenomena of multiple responses were explained via Density Functional Theory (DFT) based on the Gaussian calculations. Thus, this work provided a preliminary basis for the research of multi-stimuli responsive fluorescent molecules with photoswitching properties.

Dehydroabietic acid and triphenylamine combined BioAIEgens with an imine linker and RIM: mechanochromism and acidichromism

The petrochemical-based multi-stimuli responsive materials have been widely explored as advanced functional materials in the past several decades, but they inevitably face problems like complicated synthesis, non-renewability, and non-sustainability. On this basis, the bio-based materials used to construct multi-stimuli responsive due to the unique natural structures have come to the fore. In this work, three novel triphenylamine-based salicylideneanilines have been simply designed with aggregation-induced emission (AIE) properties. Among all, DAMB-TPAOH and DAMB-TPA possessing the natural structure of dehydroabietic acid from rosin are bioproduct-inspired AIE luminogens (BioAIEgens). Derived from the excited-state intramolecular proton transfer (ESIPT) and restriction of intramolecular motion (RIM) dual-mechanism caused by the hydroxyl group and rigidification of alicyclic moiety, respectively, the solid-state DAMB-TPAOH exhibits red-shifted (compared to DAMB-TPA) and stronger (compared to BA-TPAOH) emission, endowing this BioAIEgen with the best fluorescence behavior. Further, the introduction of the natural skeleton, propeller TPA, and imine group results in the high-contrast mechanochromic and acidichromic properties of DAMB-TPAOH. The multi-stimuli responsive properties endow DAMB-TPAOH with potential utilization in the manufacture of rewritable paper and multi-color ink. The current work contributes greatly to the development of BioAIEgens with both mechanochromism and acidichromism.

A novel multi-stimuli responsive fluorescence liquid crystal material with aggregation-induced emission effect

ABSTRACT Scientists are faced with a formidable problem in improving the smart properties of multi-stimuli responsive fluorescent materials. Here, we integrated two kinds of stimuli-responsive materials: aggregation-induced emission (AIE) and liquid crystals (LCs) into one system. An AIE-active fluorescence liquid crystal compound of α-cyanodistyrene derivate Py-CN-LBM8 was synthesised using knoevenagel condensation reaction. Its AIE effect, LC behaviour, multi-stimuli responsive properties and mechanisms were investigated by polarised optical microscopes (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), UV-visible (UV-vis) absorption spectra and photoluminescence (PL) spectra. The results showed that Py-CN-LBM8 exhibited significant physical response to external stimuli including temperature, mechanical force, solvent, pH and trace water. This work provided a strategy for designing new multi-stimuli responsive materials and made it possible to apply in the multifunction anti-counterfeit, sensor and information storage fields.

The Rainbow Arching over the Fluorescent Thienoviologen Mesophases.

Thermofluorochromic materials exhibit tunable fluorescence emission on heating or cooling. They are highly desirable for applications ranging from temperature sensing to high-security anti-counterfeiting. Luminescent matrices based on liquid crystals are very promising, particularly those based on liquid crystals with intrinsic fluorescence. However, only a few examples have been reported, suggesting ample margins for development in the field, due to the wide range of fluorophores and supramolecular organizations to be explored. Moreover, thermofluorochromic liquid crystals can be tailored with further functionalities to afford multi-stimuli responsive materials. For the first time, herein we report the thermofluorochromism of thienoviologen liquid crystals, already known to show bulk electrochromism and electrofluorochromism. In particular, we studied their photophysics in the 25 °C-220 °C range and as a function of the length of the N-linear alkyl chains, m (9 ≤ m ≤ 12 C atoms), and the type of anion, X (X = OTs-, OTf-, BF4-, NTf2-). Interestingly, by changing the parameters m, X and T, their fluorescence can be finely tuned in the whole visible spectral range up to the NIR, by switching among different mesophases. Importantly, by fixing the structural parameters m and X, an interesting thermofluorochromism can be achieved for each thienoviologen in a homologous series, leading to a switch of the emitted light from red to green and from white to blue as a consequence of the temperature-induced variation in the supramolecular interactions in the self-assembled phases.

Stimuli-responsive luminescent nanomaterial based on lanthanide modified carbon dots for multilevel information encryption

Multistimuli responsive luminescence material have tremendous potential in various advanced photonic applications, especially in high-level information encryption. Here, a facile and effective way was presented to construct multistimuli intelligent luminescent nanomaterial based on europium modified carbon dots (CDs-Eu) for advanced information encryption. This nanocomposite exhibiting controllable response behaviors upon exposure to acid-base, 2,6-dipicolinic acid (DPA) and Cu2+ hold an enormous potential in multilevel encryption application. The fluorescence (FL) color of CDs-Eu can be reversibly switched between colorless and green upon acid-base stimulation. Furthermore, in the presence of the deciphering agent (DPA), the fluorescence color of CDs-Eu exhibited a green to red color change, which can be restored upon another external stimulus (Cu2+). Combined with Morse code and anti-counterfeiting pattern, the encryption, decryption and anti-counterfeiting of important information can be realized. This stimuli-responsive luminescence material with high confidentiality shows great potential in advanced information encryption.

Multistimuli-Responsive Materials Based on Zn(II)-Viologen Coordination Polymers and Their Applications in Inkless Print and Anticounterfeiting.

Recently, stimuli-responsive materials have attracted great attention, while most of them respond to single or two stimuli. Thus, it is essential to design multifunctional stimuli-responsive materials and develop their applications. The strategy that constructing high-dimensional coordination polymers facilitates the application scope of a viologen-based photochromic system is put forward and confirmed for the first time. Herein, a novel multistimuli-responsive viologen-based Zn-MOF with a two-dimensional framework has been successfully designed and synthesized. Complex 1 exhibits chromic behavior under a variety of external stimuli such as 365 nm UV, X-rays, heat, electricity, and ethylamine. More interestingly, the crystal state of complex 1 displays dual fluorescence and room-temperature phosphorescence (RTP) emission and emits a yellow afterglow when turning off the UV lamp. In addition, Eu(III)-functionalized hybrids, Eu3+@Zn-MOF, were prepared by coordinated postsynthetic modification based on viologen complexes for the first time. The sample of Eu3+@Zn-MOF inherits the photochromic characteristics of the viologen complexes and gives the distinctive fluorescence of the europium ions. Based on the multicolor switching of 1 and Eu3+@Zn-MOF, their possible practical utilization was successfully developed in the fields of inkless, erasable print media, electrochromic information tag printing, information encryption, and anticounterfeiting.

UV and X-ray dual photochromism of a Zn(II)-viologen coordination polymer and its application in inkless and erasable printing

Multistimuli responsive materials based on the viologen coordination polymers have attracted much interest in the past few years due to the increasing demand for smart materials. In this paper, based on a viologen ligand 1-(3-carboxybenzyl)- 4,4′-bipyridinium chloride (Hcbbpy) and zinc(II) ion, a new crystalline complex, namely, {[Zn(cbbpy)Cl2]·H2O}n(1) has been designed and synthesized. In complex 1, every cbbpy ligand as a bridge links two Zn2+ ions via carboxylate O atom and pyridine N atom to extend to a 1D zigzag chain. The 1D chains are extended to 3D supramolecular structure through π···π stacking and hydrogen bonding. More importantly, 1 exhibits fast photochromic behaviors in response to UV and X-ray due to the formation of viologen radicals. Complex 1 also shows photo-modulated luminescence property in the solid state. In addition, complex 1 can be deposited on filter paper and displays the potential application as inkless print material.

Multistimuli-responsive materials based on a zinc(II) complex with high-contrast and multicolor switching.

The development of stimulus-responsive luminescent materials, especially those based on a single compound exhibiting multicolor and high-contrast (Δλem ≥ 100 nm) chromic properties, is a critical challenge. In this work, we synthesized and characterized a zinc(II) complex (1). As expected, 1 displays aggregation-induced emission enhancement (AIEE) in THF/H2O mixtures, and remarkable multicolor switching under external stimuli in the solid state. Complex 1 shows reversible mechanochromic luminescence behavior with a large wavelength shift (Δλem = 100 nm) during the grinding-fuming cycles, due to the phase transformation between the crystalline and amorphous states. More impressively, 1 exhibits obvious acidochromic properties (Δλem = 130 nm) which originate from the adsorption of vapor and a gas-solid reaction on the crystal surface. Furthermore, 1 exhibits electrochemical oxidation behavior accompanied by quenching of yellow-green emission due to the overlap of an emission band and an absorption band. The above-mentioned color changes under ambient light can also be observed by the naked eye during the mechanical, acid-base vapor and electrical stimulation. Based on the high-contrast and multicolor switching, complex 1 was successfully developed into test papers and films in the field of rapid detection of mechanical stimuli and HCl/NH3 vapors.

Stimuli Responsive Materials Supported by Orthogonal Hydrogen and Halogen Bonding or I···Alkene Interaction.

Smart materials represent an elegant class of (macro)-molecules endowed with the ability to react to chemical/physical changes in the environment. Herein, we prepared new photo responsive azobenzenes possessing halogen bond donor groups. The X-ray structures of two molecules highlight supramolecular organizations governed by unusual noncovalent bonds. In azo dye I-azo-NO2, the nitro group is engaged in orthogonal H···O···I halogen and hydrogen bonding, linking the units in parallel undulating chains. As far as compound I–azo–NH–MMA is concerned, a non-centrosymmetric pattern is formed due to a very rare I···π interaction involving the alkene group supplemented by hydrogen bonds. The Cambridge Structural Database contains only four structures showing the same I···CH2=C contact. For all compounds, an 19F-NMR spectroscopic analysis confirms the formation of halogen bonds in solution through a recognition process with chloride anion, and the reversible photo-responsiveness is demonstrated upon exposing a solution to UV light irradiation. Finally, the intermediate I–azo–NH2 also shows a pronounced color change due to pH variation. These azobenzenes are thereby attractive building blocks to design future multi-stimuli responsive materials for highly functional devices.

Tetraphenylethene functionalized quinoxaline derivative exhibiting aggregation-induced emission and multi-stimuli responsive fluorescent switching

A new tetraphenylethene functionalized quinoxaline derivative TPVSQ has been synthesized using classical Knoevenagel condensation reaction. Spectral study and quantum chemical calculation indicated that it possessed D-π-A molecular structure. TPVSQ exhibited typical ICT (intramolecular charge transfer) transition and AIE property. Interestingly, TPVSQ also showed multi-stimuli responsive behavior. The emission spectra of TPVSQ could be switched in response to not only mechanical grinding and solvent fuming but also the protonation effect. The responsive mechanism has been studied and explained. The current work provided a strategy for the molecular design of new multi-stimuli responsive materials.

Tailoring C-6-Substituted Coumarin Scaffolds for Novel Photophysical Properties and Stimuli-Responsive Chromism.

A judicious strategy was utilized to envision the substantial regio-positional effects of substituents on the photophysical properties of the 2H-chromen-2-one-3-benzothiazole scaffold-based push-pull framework, named 6-X-CUMs. Among them, 6-NEt2-CUM reveals prominent excited-state intramolecular charge transfer with a large change of dipole moment (Δμ ∼ 18.23 D), hence displaying remarkable emission solvatochromism from the green (536 nm in cyclohexane) to far-red region (714 nm in dimethyl sulfoxide) and a high-temperature sensitivity (-0.23 nm °C-1). These, together with unique basicity and acido-/vaporchromism upon acidification elucidated by NMR and photospectroscopic studies, show stark contrast to the conventional 7-NEt2-CUM. The new series of these tailored 6-X-CUMs represents a new dimension in tailoring the photophysical properties for the development of a promising class of multistimuli-responsive materials.

Mechanochromic Detection for Soft Opto-Magnetic Actuators

New multi-stimuli responsive materials are required in smart systems applications to overcome current limitations in remote actuation and to achieve versatile operation in inaccessible environments. The incorporation of detection mechanisms to quantify in real time the response to external stimuli is crucial for the development of automated systems. Here, we present the first wireless opto-magnetic actuator with mechanochromic response. The device, based on a nanostructured-iron (Fe) layer transferred onto suspended elastomer structures with a periodically corrugated backside, can be actuated both optically (in a broadband spectral range) and magnetically. The combined opto-magnetic stimulus can accurately modulate the mechanical response (strength and direction) of the device. The structural coloration generated at the corrugated back surface enables to easily map and quantify, in 2D, the mechanical deflections by analyzing in real time the hue changes of images taken using a conventional RGB smartphone camera, with a precision of 0.05°. We demonstrate the independent and synergetic optical and magnetic actuation and detection with a detection limit of 1.8 mW·cm–2 and 0.34 mT, respectively. The simple operation, versatility, and cost-effectiveness of the wireless multiactuated device with highly sensitive mechanochromic mapping paves the way to a new generation of wirelessly controlled smart systems.

Bioinspired Multi-Stimuli Responsive Actuators with Synergistic Color- and Morphing-Change Abilities.

The combination of complex perception, defense, and camouflage mechanisms is a pivotal instinctive ability that equips organisms with survival advantages. The simulations of such fascinating multi‐stimuli responsiveness, including thigmotropism, bioluminescence, color‐changing ability, and so on, are of great significance for scientists to develop novel biomimetic smart materials. However, most biomimetic color‐changing or luminescence materials can only realize a single stimulus‐response, hence the design and fabrication of multi‐stimuli responsive materials with synergistic color‐changing are still on the way. Here, a bioinspired multi‐stimuli responsive actuator with color‐ and morphing‐change abilities is developed by taking advantage of the assembled cellulose nanocrystals‐based cholesteric liquid crystal structure and its water/temperature response behaviors. The actuator exhibits superfast, reversible bi‐directional humidity and near‐infrared (NIR) light actuating ability (humidity: 9 s; NIR light: 16 s), accompanying with synergistic iridescent appearance which provides a visual cue for the movement of actuators. This work paves the way for biomimetic multi‐stimuli responsive materials and will have a wide range of applications such as optical anti‐counterfeiting devices, information storage materials, and smart soft robots.