Endowing Hydrochromism to Fluorans via Bioinspired Alteration of Molecular Structures and Microenvironments and Expanding Their Potential for Rewritable Paper

Hydrochromic film for dynamic information storage using cellulose nanofibers and silica nanoparticles

A hydrochromic composite film was fabricated by incorporating silica nanoparticles (SiNPs) with cellulose nanofibers (CNFs). The CNF/SiNP composite film underwent a reversible change in transparency in response to external moisture variation. The CNFs improved the dimensional stability of the CNF/SiNP composite film and induced morphological differences in SiNP agglomerates, which control the water vapor condensation in a porous film. The condensed water in the pores reduced the difference in refractive index over the CNF/SiNP film, enhancing its transparency. The selective transparency of the composite film was challenged by printing CNF/SiNP inks at different composition ratios. The differing susceptibility of the printed patterns to moisture provided selective transparency at specific patterns, which can store dynamic information such as QR or numerical codes by simple water vapor adsorption and desorption.

Oxidative stress biomarker triggered multiplexed tool for auxiliary diagnosis of atherosclerosis

Oxidative stress is integral in the development of atherosclerosis, but knowledge of how oxidative stress affects atherosclerosis remains insufficient. Here, we design a multiplexed diagnostic tool that includes two functions (photoacoustic imaging and urinalysis), for assessing intraplaque and urinary malondialdehyde (MDA), a well-recognized end-product of oxidative stress. Molecular design is conducted to develop the first near-infrared MDA-responsive molecule (MRM). Acid-unlocked ratiometric photoacoustic nanoprobe is designed to report intraplaque MDA, enabling it to reflect plaque burden. Furthermore, MRM is tailored for urinary MDA detection with excellent specificity in a blind study. Moreover, we found a significant difference in urinary MDA between healthy adults and atherosclerotic patients (more than 600 participants). Combining these two functions, such a multiplexed diagnostic tool can dynamically report intraplaque and systemic oxidative stress levels during atherosclerosis progression, pneumonia infection, and drug treatment in atherosclerotic mice, which is promising for the auxiliary diagnosis of atherosclerosis.

Design of reversibly charge-changeable rhodamine B modified magnetic nanoparticles to enrich phosphopeptides

In the present study, by employing ethylenediaminetetraacetic acid (EDTA), tetraethylene pentaamine (TEPA), and rhodamine B (Rb), we designed and synthesized a magnetic adsorbent (Fe₃O₄@EDTA@TEPA@Rb) on the basis of reversible charge change of Rb and applied to capture phosphopeptides. Rb existing in open planarized zwitterion form when stimulated by acidic loading buffer adsorbs negative phosphopeptides via electrostatic interaction. Under the stimulation of alkalic eluent, ring-closed structure of Rb is formed to elute the enriched phosphopeptides. TEPA containing rich amino groups is used as a crosslinking agent, which is also protonated in acidic loading buffer to bond phosphopeptides. Then phosphopeptides are eluted when TEPA deprotonates in alkalic eluent. Coupled with matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS) detection, phosphopeptide signals originated from 0.4 fmol/μL β-casein digests were successfully detected. In addition, Fe₃O₄@EDTA@TEPA@Rb can also efficiently enrich phosphopeptides from skimmed milk, human serum and saliva samples (26, 4, 39 phosphopeptides, respectively), opening a new gallery for phosphopeptides-related analysis. In general, the developed adsorbent has the great potential for further application in the near future.

Three-state fluorescence hydrochromism of a fluorophore-spacer-receptor system with variations in relative humidity

The uptake of atmospheric moisture by hygroscopic materials can have marked effects on a material's physical and chemical properties. This is true of materials that go on to incorporate waters of hydration in their molecular structural lattice, forming stable hydrates with fluctuations in relative humidity (RH). Nevertheless, RH remains relatively uncontrolled for a variable that can fluctuate widely depending on geographical climate, weather fluctuations, and building HVAC system stability. Herein, we report a processable 1,8-napthalimide-based fluorophore-spacer-receptor system that unexpectedly exhibited reversible three-state fluorescence hydrochromism with changes in RH due to RH-induced solid state molecular rearrangement. Care should be taken to evaluate the impact of variations in RH when characterising the solid state emission properties of charged fluorescent materials.

Fast and Reversibly Humidity-Responsive Fluorescence Based on AIEgen Proton Transfer

The construction of humidity-responsive fluorescent materials with reversibility, specificity, and sensitivity is of great importance for the development of information encryption, fluorescence patterning, and sensors. Nevertheless, to date, the application of these materials has been limited by their slow response rate and nonspecificity. Herein, a humidity-responsive fluorescence system was designed and assembled to achieve a rapid, reversible, and specific moisture response. The system comprised tetra-(4-pyridylphenyl)ethylene (TPE-4Py) as a fluorescent proton acceptor with an aggregation-induced emission (AIE) effect and poly(acrylic acid) (PAA) as a proton donor with an efficient moisture-capturing ability. The fluorescence color and intensity rapidly changed with increasing relative humidity (RH) because of TPE-4Py protonation, and TPE-4Py deprotonation resulted in recovery of the original fluorescence color in low-humidity environments. The proton transfer between the pyridyl group in TPE-4Py and the carboxyl group in PAA was reversible and chemically stable, and the humidity-responsive fluorescence system showed a high response/recovery speed, an obvious color change, good reversibility, and an outstanding specific moisture response. Because of these advantages, diverse applications of this humidity-responsive fluorescence system in transient fluorescent patterning and the encryption of information were also developed and demonstrated.

Stimuli-Responsive Electrochemical Energy Storage Devices

Electrochemical energy storage (EES) devices have been swiftly developed in recent years. Stimuli-responsive EES devices that respond to different external stimuli are considered the most advanced EES devices. The stimuli-responsive EES devices enhanced the performance and applications of the EES devices. The capability of the EES devices to respond to the various external stimuli due to produced advanced EES devices that distinguished the best performance and interactions in different situations. The stimuli-responsive EES devices have responsive behavior to different external stimuli including chemical compounds, electricity, photons, mechanical tensions, and temperature. All of these advanced responsiveness behaviors have originated from the functionality and specific structure of the EES devices. The multi-responsive EES devices have been recognized as the next generation of stimuli-responsive EES devices. There are two main steps in developing stimuli-responsive EES devices in the future. The first step is the combination of the economical, environmental, electrochemical, and multi-responsiveness priorities in an EES device. The second step is obtaining some advanced properties such as biocompatibility, flexibility, stretchability, transparency, and wearability in novel stimuli-responsive EES devices. Future studies on stimuli-responsive EES devices will be allocated to merging these significant two steps to improve the performance of the stimuli-responsive EES devices to challenge complicated situations.

Rewritable PEDOT Film Based on Water-Writing and Electroerasing

Rewritable paper has greatly promoted the sustainable development of society. However, the hydrophilicity/lipophilicity of the poly(3,4-ethylenedioxythiophene) (PEDOT) film limits its application as the rewritable paper. Herein, we constructed a repeatable writing/erasing pattern on a PEDOT film (rewritable PEDOT paper) by combining wettability control, water-induced dedoping, and an electrochemical redox reaction. The treatment with a medium-polarity/high-volatility solvent (MP/HVS) adjusted the wettability of the PEDOT film (water contact angle increased from 6.5° to 146.2°), contributing to the formation of a hydrophobic writable substrate. The treatment with a high-polarity solvent (HPS) induced the dedoping of anions in the PEDOT chain, resulting in the film's color changed from blue to purple and serving as a writing process. The intrinsic electrochemical redox (elimination of color change by doping/dedoping of lithium ions in the PEDOT chain) of the PEDOT film enabled the erasing process. This writing/erasing process can be repeated at least 10 times. The patterned PEDOT film maintained excellent stability to standing diverse solvents (low-polarity solvent (LPS) and MP/HVS), high temperatures (350 °C), and irradiation of different light wavelengths (wavelengths of 365, 380, 460, 520, and 645 nm). Additionally, the conductivity of the PEDOT film was quantitatively measured (impedance: LPS, increased 8.84%; MP/HVS, decreased 6.67%; and HPS, increased 27.97%) by fabricating a micropatterned PEDOT electrode. This work will provide a method for the fabrication of PEDOT-based optoelectronic functional materials.

Unexpected three-state hydrochromism of a donor-acceptor self-complex with fluctuations in relative humidity

Water in our environment is ever present, particularly in our atmosphere, from which it may be adsorbed by materials hygroscopically. At the molecular level, the binding of water molecules to various materials is driven by weak interactions but can have profound effects on physical properties, including the donor-acceptor interactions in charge transfer (CT) salts. Herein we present the unexpected three-state hydrochromatic switching of a bipyridinium-based donor-acceptor self-complex with changes in relative humidity (RH) and subsequent stable hydrate formation. RH is typically an overlooked variable that can vary greatly. These findings suggest that care should be taken to consider fluctuations in RH when characterizing the solid state optical band gap and CT absorption bands for organic donor-acceptor CT salt complexes.

The Unusual Photochromic and Hydrochromic Switching Behavior of Cellulose-Embedded 1,8-Naphthalimide-Viologen Derivatives in the Solid-State

Stimuli-responsive chromic materials such as photochromics, hydrochromics, thermochromics, and electrochromics have a long history of capturing the attention of scientists due to their potential industrial applications and novelty in popular culture. However, hybrid chromic materials that combine two or more stimuli-triggered color changing properties are not so well known. Herein, we report a design strategy that has led to a series of emissive 1,8-naphthalimide-viologen dyads which exhibit unusual dual photochromic and hydrochromic switching behavior in the solid-state when embedded in a cellulose matrix. This behavior manifests as reversible solid state fluorescence hydrochromism upon changes in atmospheric relative humidity (RH), and reversible solid state photochromism upon generation of a cellulose-stabilized viologen radical cation. In this design strategy, the bipyridinium unit serves as both a water-sensitive receptor for the hydrochromic fluorophore-receptor system, and a photochromic group, capable of eliciting its own visible colorimetric response, generating a fluorescence quenching radical cation with prolonged exposure to ultraviolet (UV) light. These dyes can be inkjet-printed onto cellulose paper or drop-cast as cellulose powder-based films and can be unidirectionally cycled between three different states which can be characteristically visualized under UV light or visible light. The material's photochromism, hydrochromism, and underlying mechanism of action was investigated using computational analysis, dynamic vapor sorption/desorption isotherms, electron paramagnetic resonance spectroscopy, and variable humidity UV-Vis adsorption and fluorescence spectroscopies.

Bistable Elastic Electrochromic Ionic Gels for Energy-Saving Displays

The diversification of electrochromic materials greatly expands the application fields of electrochromic devices. However, highly flexible electrochromic materials remain challenging due to the inherent limitations associated with the existing electrochromic processes. Inspired by the hydrogen bonding effect in the hydrogel structure, a highly elastic and bistable electrochromic ionic gel based on a hydrogen bonding cross-linking network is prepared by solution polymerization having excellent tensile resilience, uniform coloring, reversible switching (≤24.3 s), maximum transmittance change (≥80%), bistability (54 h), reversibility (>500 cycles), and coloration efficiency (≥85.3 cm²·C^-1). This method has been used to develop bistable electrochromic displays. The unconventional exploration of the bistable design principle may provide a new idea for the realization of bistable electrochromic devices.

Stimuli-Induced Reversible Proton Transfer for Stimuli-Responsive Materials and Devices

ConspectusStimuli-responsive materials have a great potential in various novel photoelectric devices, such as self-adaptive adjustment devices, intelligent detection, molecular computers with information storage capability, camouflage and anticounterfeiting display, various energy-saving displays, and others. However, progress in related areas has been relatively slow because of the lack of high-performance smart materials and the limitations of available reaction mechanisms currently. To address these problems fundamentally, new mechanisms need to be designed and developed, and learning from nature is an effective and intelligent method to achieve this long-awaited target, such as mimicking of proton transfer processes in nature at the molecular/supramolecular level. The stimuli-induced reversible proton transfer system is composed of materials that release or capture protons in response to stimuli and switch molecules that control color and/or fluorescence modulation by protons, and it is applied in stimuli-responsive materials and devices, including bistable electronic/electrochromic devices, electrofluorochromic devices, water-jet rewritable paper, visible-light-responsive rewritable paper, and mechanochromic materials.To help researchers gain deep insight into stimuli-induced reversible proton transfer, we attempted to summarize its reaction mechanism and design principle, and discuss strategies to design and prepare various related stimuli-responsive materials and devices. This Account discusses the different systems in which a color/fluorescence change is induced by the proton transfer process under various stimuli, including electric field, water, light, heat, and stress. Relative very promising applications as well as their performance especially for energy-saving and environmentally friendly devices are then summarized, such as energy-saving bistable electrochromic devices, water-jet rewritable paper, and visible-light-responsive rewritable paper. Meanwhile, we focus on the key influence factors and useful additives for improving the device's performance. At last, challenges and bottlenecks faced by stimuli-responsive materials and devices based on the mechanism of reversible proton transfer are proposed. Moreover, we put forward some suggestions on solving these limitations.These exciting results reveal that smart materials based on the mechanism of proton transfer are extremely attractive and possess great potential in the next generation of energy and resource saving and environmental protection display. We hope that this Account further prospers the field of intelligent stimuli-responsive discoloration materials and next-generation green displays.

Inkless Rewritable Photonic Crystals Paper Enabled by a Light-Driven Azobenzene Mesogen Switch

Rewritable paper, as an environment-friendly approach of information transmission, has potential possibility to conserve energy and promote a sustainable development of our society. Recently, photonic crystals (PCs) have become a research hotspot in the development of rewritable paper. However, there are still many shortcomings that limit the further application of PC paper, such as slow response sensitivity, short-cycle lifetime, poor storage stability, and so on. Herein, we constructed an optically rewritable azobenzene inverse opals (AZOIOs) with a thin film (ca. 1 μm) plated on an inverse opal structure based on the UV/vis switchable structure color of the sample. The top thin film acts as a protective layer to avoid the large deformation of the pore structure and the bottom inverse opal structure with refractive index/pore structure change that provides reversible structure color. Large, reversible, and rapid bandgap shift (ca. 60 nm, 2 s) of AZOIOs can be repeated more than 100 times under alternating UV/vis irradiation based on isomerization of high content of the azobenzene group. On-demand long-time preservation pattern can be obtained by the appearance of azobenzene's intrinsic color. The proof of concept for rewritable PC paper is demonstrated herein. Such inkless rewritable colorful paper paves a way for developing novel display technology.

A hybrid hydrochromic molecular crystal applicable to invisible ink with high reversibility

Highly reversible hydrochromic behavior is realized in a novel hybrid molecular crystal by controlling the gain and loss of coordinated water.

Recent Progress in Small Spirocyclic, Xanthene-Based Fluorescent Probes

The use of fluorescent probes in a multitude of applications is still an expanding field. This review covers the recent progress made in small molecular, spirocyclic xanthene-based probes containing different heteroatoms (e.g., oxygen, silicon, carbon) in position 10'. After a short introduction, we will focus on applications like the interaction of probes with enzymes and targeted labeling of organelles and proteins, detection of small molecules, as well as their use in therapeutics or diagnostics and super-resolution microscopy. Furthermore, the last part will summarize recent advances in the synthesis and understanding of their structure-behavior relationship including novel computational approaches.

Photoluminescent and Chromic Nanomaterials for Anticounterfeiting Technologies: Recent Advances and Future Challenges

Counterfeiting and inverse engineering of security and confidential documents, such as banknotes, passports, national cards, certificates, and valuable products, has significantly been increased, which is a major challenge for governments, companies, and customers. From recent global reports published in 2017, the counterfeiting market was evaluated to be $107.26 billion in 2016 and forecasted to reach $206.57 billion by 2021 at a compound annual growth rate of 14.0%. Development of anticounterfeiting and authentication technologies with multilevel securities is a powerful solution to overcome this challenge. Stimuli-chromic (photochromic, hydrochromic, and thermochromic) and photoluminescent (fluorescent and phosphorescent) compounds are the most significant and applicable materials for development of complex anticounterfeiting inks with a high-security level and fast authentication. Highly efficient anticounterfeiting and authentication technologies have been developed to reach high security and efficiency. Applicable materials for anticounterfeiting applications are generally based on photochromic and photoluminescent compounds, for which hydrochromic and thermochromic materials have extensively been used in recent decades. A wide range of materials, such as organic and inorganic metal complexes, polymer nanoparticles, quantum dots, polymer dots, carbon dots, upconverting nanoparticles, and supramolecular structures, could display all of these phenomena depending on their physical and chemical characteristics. The polymeric anticounterfeiting inks have recently received significant attention because of their high stability for printing on confidential documents. In addition, the printing technologies including hand-writing, stamping, inkjet printing, screen printing, and anticounterfeiting labels are discussed for introduction of the most efficient methods for application of different anticounterfeiting inks. This review would help scientists to design and develop the most applicable encryption, authentication, and anticounterfeiting technologies with high security, fast detection, and potential applications in security marking and information encryption on various substrates.

Dye-Encapsulated Zeolitic Imidazolate Framework (ZIF-71) for Fluorochromic Sensing of Pressure, Temperature, and Volatile Solvents

Luminescent metal-organic frameworks (MOFs) offer a multifunctional platform for creating noninvasive sensors and tunable optoelectronics. However, fluorochromic materials that are photophysically resilient and show high sensitivity toward different physical and chemical stimuli are scarce. We report a facile host-guest nanoconfinement strategy to construct a fluorescent hybrid material with multiple sensing capabilities. We design and fabricate a new Guest@MOF material: comprising a zeolitic MOF (ZIF-71) as a nanoporous host for encapsulating rhodamine B (RhB dye) guest molecules, resulting in an RhB@ZIF-71 system with mechanochromic, thermochromic, and solvatochromic sensing response. The fluorochromic sensing properties stem from the nanoconfinement effect that ZIF-71 imposes on RhB monomers, yielding the H- or J-type aggregates with tunable photophysical and photochemical properties. For mechanochromism, the external pressure causes an emission red shift in a linear fashion, switching RhB guests from H-type to J-type aggregates through a shear deformation. For thermochromism, we demonstrate a linear scaling as a function of temperature due to the spatial restriction imposed on J-type aggregates incarcerated in ZIF-71 pores. Harnessing the solvatochromism of RhB@ZIF-71, we interrogated its photochemical response by employing three diverse groups of volatile organic compounds. The multimodal sensing response paved the way to smart applications like photonic pressure sensors, noninvasive thermometers, and ultrasensitive chemosensors.

Leuco-Based Composite Resin Dosimeter Film

A polystyrene (PS)-based composite resin dosimeter (CRD) film comprising 2-(phenylamino)-6-(dipentylamino)-3-methylspiro[9H-xanthene-9,3'-phthalide] (Black305) fluoran leuco dye, cerium-doped yttrium aluminum perovskite YAlO₃:Ce (YAP:Ce) scintillator, and 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine (MBTT) photoacid generator was developed for the visualization of X-ray exposure doses in the range of 18-170 Gy. The color of the CRD film changed from yellow to black within this range based on the X-ray exposure dose. A CRD film based on Black305/YAP:Ce/MBTT/PS could be used to produce a dosimeter with various color change sensitivities by controlling the thermodynamic parameters of the constituent materials. A CRD film is a promising tool for measuring the X-ray exposure dose and can be used as an alternative to the existing radiochromic dosimeters.

Large-Area and Water Rewriteable Photonic Crystal Films Obtained by the Thermal Assisted Air-Liquid Interface Self Assembly

Compared with traditional paper, water rewritable photonic crystal (PC) paper is an environmentally friendly and low resource-consuming material for information storage. Although, recently reported PC papers have high-quality structure color showing promising prospect, the paper size, that is within several centimeters, still limits turning it from potential to reality. Here, we present a new water rewritable PC film as large as the A4 size (210 × 300 mm²) with a high-quality structure color. The material is prepared by thermal assisted self-assembly on the air-liquid interface. To fix such a large-area self-assembled PC film, we partially deform and coalesce the self-assembled nanoparticles, which have low glass transition temperature. This process causes the film to be transparent and structural colorless but still keeps the inner 3D-ordered structure. Then, utilizing the hydrophilic nature of the assembled block, the film can be switched to a structural color state by touching water. Diverse brilliant structural colors appear with different assembled particle (poly(butyl methacrylate- co-methylmethacrylate- co-butyl acrylate- co-diacetone acrylamide) named as PBMBD) sizes. The transparency-structural color transition can be performed multiple times reversibly in all or specific regions of the film. It provides a new solution for future applications of rewriteable PC paper.

A H-bond strategy to develop acid-resistant photoswitchable rhodamine spirolactams for super-resolution single-molecule localization microscopy

Rhodamine spirolactam based photoswitches have been extensively applied in super-resolution single-molecule localization microscopy (SMLM). However, the ring-opening reactions of spirolactams are cross-sensitive to acid, limiting their photoswitch use to neutral pH conditions. In addition, the ring-closing reactions of spirolactams are environment-sensitive and slow (up to hours), virtually making rhodamine spirolactams caged fluorescent dyes instead of reversible photoswitches in SMLM. Herein, by introducing hydrogen bonds to stabilize spirolactams, we report a series of acid-resistant rhodamine spirolactams with accelerated ring-closing reactions from fluorescent xanthyliums to non-fluorescent spirolactams, endowing them with good photoswitchable properties even in acidic environments. By further substitution of 6-phenylethynyl naphthalimide on the spirolactam, we shifted the photoactivation wavelength into the visible region (>400 nm). Subsequently, we have successfully applied these dyes in labeling and imaging the cell surface of Bacillus subtilis at pH 4.5 using SMLM.

Highly Tunable Multicolor Water-Jet Rewritable Paper Based on Simple New-Type Dual-Addressable Oxazolidines

Rewritable paper based on switchable molecules has attracted great attention in both academic research and marketplace. However, most available switchable dyes have single switchable color state only, which cannot meet the long-awaited multicolor reversible displays. Herein, through simple introduction of phenolic hydroxyl group, we develop a series of new oxazolidines with one switch unit, which could reversibly display two different as well as their mix-gradient colors by treating with water and mild acid, respectively, both in solution and solid substrate. The structures and mechanism for the formation of two colors had been studied in detail via UV-vis/NMR spectroscopy, skillfully designing contrast molecules, and kinetics experiments. This multiple switchable colors of the dyes have been further applied to construct a rewritable paper for ink-free printing with multi/gradient-color display.

Water assisted biomimetic synergistic process and its application in water-jet rewritable paper

The colour of water-jet rewritable paper (WJRP) is difficult to be expanded via single hydrochromic molecule, especially black. Here, inspired by the amazing phenomenon of bound-water in cells enabling various biological transformations via facilitating synergistic inter-/intra-molecular proton transfer, we present a simple strategy toward WJRP based on binary systems containing less-sensitive acidochromic dyes and mild proton donors (or developers). With such a binary system containing commercial black dye as the colouring agent, benzyl 4-hydroxybenzoate as the developer, and biomimetic bound-water as proton-transferring medium, we successfully achieve the long-awaited black WJRP. Printed images on such WJRP have excellent performances and long retaining time (>1 month). In addition, the robustness, durability and reversibility of WJRP could be increased distinctly by using polyethylene terephthalate as substrate. This strategy significantly expands hydrochromic colours to entire visible range in an eco-friendly way, which opens an avenue of smart materials for practical needs and industrialization.

Water based inks used for water-jet rewritable paper (WJRP) are an environmental friendly alternative to conventional printing, but black colour in WJRP could not be realized so far. Here the authors demonstrate black as well as other colour WJRP based on binary systems containing less-sensitive acidochromic dyes and mild proton donors.

Water-soluble and adjustable fluorescence copolymers containing a hydrochromic dye: synthesis, characterization and properties

Water solubility and adjustable fluorescence properties have been successfully implemented in the hydrochromic amino rhodamine via copolymerization. Four copolymers have been synthesized and clearly characterized by UV-Vis spectroscopy, proving greater detail than the commonly used NMR and IR technologies. The four copolymers have good solubility in pure water and in many common organic solvents, while preserving the hydrochromism of the dye monomer. Based on aggregation and dispersion of the copolymers as adjusted by solvent media and temperature, reversible fluorescence properties were successfully realized. Furthermore, their luminescence in solid state was observed. These studies are of great significance for expanding the application of hydrochromic dyes in biological fields and promoting green industrialization.

Water solubility and adjustable fluorescence adjustable properties have been successfully endowed to established in a hydrochromic dye via copolymerization.

Stimuli-responsive fluorescence switching of cyanostilbene derivatives: ultrasensitive water, acidochromism and mechanochromism

A novel donor–π–acceptor structure stimuli-responsive fluorescent material of (Z)-2-(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)-3-(pyridin-2-yl)acrylonitrile (oN-TPA) was designed and synthesized, with the cyano-group and pyridine as the acceptors (A) and triphenylamine as the donor (D). oN-TPA exhibits an obvious solvatochromic effect and the excited state is confirmed to be a hybridized local and charge-transfer (HLCT) state that simultaneously possesses the locally-excited (LE) state and charge transfer (CT) state characters. The LE state ensures relatively high fluorescence efficiency while the CT state provides multi-stimuli responsive fluorescence behaviors because it is easily tuned by the surrounding environment. Firstly, oN-TPA exhibits “on–off–on” fluorescence properties in the mixture of water/tetrahydrofuran (THF) with the increasing water content. For the “on–off” part, a good linear relationship between fluorescence intensity and water fraction is achieved, which is ascribed to the synergistic effect of protons in water and intramolecular charge-transfer (ICT) effect depending on solvent polarity. The “off–on” part demonstrates the aggregation-induced enhanced emission (AIEE) character of oN-TPA. Secondly, oN-TPA can be used as a protonic acid sensor to detect trifluoroacetic acid (TFA) in solvent and HCl vapour in the solid state due to the binding of the proton to the pyridine group. Finally, oN-TPA presents remarkable and reversible mechanochromic fluorescence switching between 552 nm and 642 nm (90 nm red-shift) during the pressurizing–depressurizing process. This work not only comprehensively demonstrates the stimuli-responsive fluorescence behaviors of oN-TPA, but also provides a D–π–A structure fluorescent material possessing potential applications in detection and sensing with remarkable fluorescence changes.

A donor–acceptor dye exhibits high luminescence efficiency and high-contrast piezochromism with HLCT properties. The doped films show the ratiometric photo-luminescence peak shift under high pressure, interestingly, making them piezo-sensors.