Acid-base Vapor Research Articles

An azine‐based halochromic molecular chameleon

Halochromism has a plethora of uses in the textile industry, including wound treatments and protective apparel. Reactive dye’s fastness and a wide variety of hues, from bright to dull, make it ideal for coloring cotton and other regenerated cellulose fibers. It is also fundamentally paramount importance to determine the freshness of packaging foods. In this study, a halochromic probe, 4,4′-((1E,1′E,3E,3′E)-hydrazine-1,2-diylidenebis(prop-1-en-1-yl-3-ylidene))bis(N, N-dimethylaniline), HDBD, has been introduced utilizing a simple condensation reaction. Through the protonation and deprotonation process with the addition of acid and base in the non-aqueous medium, HDBD shows visual colorimetric as well as ratiometric UV–visible absorption spectral change. A colorimetric paper strip-based experiment has been demonstrated to detect trace amounts of acid and its reversibility with bases in non-aqueous solvents. Further, a dip-stick experiment was also carried out for the detection of acid-base vapor in a wide range of concentrations. Furthermore, the overlapping indicator method is explored to estimate acid dissociation constants in the non-aqueous medium. Moreover, we have constructed the INHIBIT (INH) and IMPLICATION (IMP) molecular logic gates exploring the reversibility of HDBD due to the cascade introduction of acid-base as chemical encoded inputs. Utilizing a reversible and reproducible detecting method, we have constructed a molecular-scale additional memory device that demonstrates binary logic “Writing-Reading-Erasing-Reading” and “Multi-write” functions. This finding provides a new way for designing acid-base indicators, which could estimate the acid dissociation constants of various acids in the non-aqueous environment which is fundamentally important in the field of acid-catalyzed organic synthesis.

A triphenylamine derivative with multistimuli responsive behavior and high-contrast vapochromism of its Zn(II) complex

In this work, we designed and synthesized a novel multifunctional molecule, (E)-2-(4-(diphenylamino)styryl)-5-(4-pyridyl)-8-quinolinol (HL), which possessed D-π-A structure and exhibited optical responses to acid-base vapor and Zn(II) ions. Zn(II) complex of HL (W1) was obtained and characterized by SC-XRD. Spectral experiments demonstrated that complex W1 displayed aggregation-induced emission enhancement (AIEE) property, which was exact opposite to the aggregation-caused quenching effect of HL. The quantum yield of W1 (15%) was much higher than that of HL (2.1%) in the solid state. Both HL and complex W1 involved sp2 hybrid N atoms, whose protonation and deprotonation gave rise to reversible acidochromism with high contrast. The mechanism was clarified by XPS, 1H NMR, infrared spectroscopy, elemental analysis, PXRD and thermogravimetry results. Benefiting from excited state intramolecular proton transfer in HL together with AIEE of W1, HL could recognize Zn(II) sensitively in aqueous media. The detection limit of HL for Zn(II) reached as low as 5.02 × 10−8 M. An anti-counterfeiting application was realized using HL-loaded test strip, which exhibited fluorescence response, but no color change upon exposure to Zn(II) solution. In addition, HL and W1 solids were combined to develop a stimuli induced encryption/decryption strategy successfully based on their reversible acidochromic behaviors.

Electrochemical synthesis of donor-acceptor triazine based polymers with halochromic and electrochromic properties

Two triazine monomers substituted with aromatic α,β unsaturated ketones (chalcones) as connecting arms, holding triphenylamine or carbazole ending groups, were by the first time synthesized using commercial and inexpensive starting materials in just two steps. Triphenylamine and carbazole groups were strategically introduced in order to allow electrochemical polymerization and redox activity of the obtained films. The chalcone, together with the triphenylamine or carbazole outer groups, give rise to the formation of a donor-acceptor systems, allowing the generation of photoinduced intramolecular charge transfer (ICT) electronic transitions and providing basic sites for protonation. Because of the star-like chemical structure of the used triazine, electrochemical polymerization of these monomers conducted to the formation of hyperbranched structures. The polymeric materials exhibited not only electrochromic properties but also showed a halochromic behavior. Spectroelectrochemical studies of the films showed different colorations in the three redox states (neutral, semioxidized and fully oxidized). The color changes are fast and easily detected by the naked eye. Electrochemical parameters such as optical contrast, coloration efficiency, optical memory, and long-term stability for both films were obtained and compared. A quasi-solid window-type electrochromic device was assembled using the electrogenerated polymer with the best electrochromic performance and stability. Moreover, the triphenylamine based polymer showed reversible color changes upon acid-base vapor exposure cycles, demonstrating that this polymer can be used not only in electrochromic devices but also in acid vapor sensors.

Self-healing luminescent polyurethane with ultrahigh toughness resulting from lanthanide-bipyridine segment and hydrogen bonds

Synthesis and manufacturing a new generation of luminescent elastomers that simultaneously features self-healing efficiency with superior mechanical properties remains a challenge due to the compromise between toughness and self-healing ability. Here, a new class of transparent luminescent materials was synthesized via coordinating lanthanide (Ln3+) to bipyridine (Bpy) units introduced into the polymer backbone prepared by polymerization of Polytetramethylene ether glycol (PTMEG) and 4-methylenedicyclohexyl diisocyanate (HMDI). Relying on collaborative reinforcement of a great number of loosely packed Ln3+-ligand coordination bonds and hydrogen bonds (H-bonds), the prepared luminescent polymers showed an ultrahigh toughness of 373.12 MJ m−3 and a mechanical ultrahigh strength of 33.73 MPa. In addition, the multi-color emission behavior of the luminescent materials can be simply adjusted via tailoring the molar ratio of Eu3+/Tb3+. The luminescent self-healing materials can reversibly respond to acid-base vapor because of protonation/deprotonation of Bpy ligands. We believe luminescent polymers obtained by us with excellent mechanical properties and luminescence off/on switching ability show great potential applications as acid-base detector.

Phosphoric acid-catalyzed synthesis of fused pyridocoumarins and their acidochromic applications

Pyridocoumarins are well known to exhibit various potent applications in optoelectronics, fluorescent probes, and sensors due to their unique photophysical properties. A concise and effective approach has been demonstrated to synthesize pyridocoumarins via a relatively inexpensive phosphoric acid-catalyzed Povarov reaction and their photophysical properties has also been investigated. The structures of all the synthesized compounds were well characterized by using 1H NMR, 13C NMR, IR, HRMS, and single crystal X-ray structure analysis. Detailed spectroscopic studies revealed that all the synthesized pyridocoumarins displayed acidochromic properties in solutions, which could be utilized as acid-base vapor sensors, and anti-counterfeiting agents. The detection limit was also calculated for all the synthesized acid sensors, which were found to be in the range of 41–103.8 ppm (parts per million). DFT and TDDFT studies were also performed to understand the protonation sites and the electronic transitions involved in these compounds. HOMO–LUMO energy gap of the synthesized pyridocoumarins ranges from 3.50 to 3.90 eV.

White light generation by regulating hydrogen bond-sensitive ESPT of naphthalimide dyes

White light emitting systems of pure organic materials have attracted extensive research interest due to their better compatibility and functional scalability. The reported organic white light materials are mainly based on the multi-channel emission regulation of the compound itself or the mixing of multicolor luminescence materials, but studies on the dependence between multicolor luminescence and the external environment are lacking, which limits the application of these materials in areas such as identification and sensing. This paper reports that the 4- or 3‑hydroxyl-substituted naphthalimides NapH1 and NapH2 form intermolecular hydrogen bonds with adjacent molecules in the environment, and undergo excited-state intermolecular proton transfer under irradiation, resulting in blue-yellow or blue-red dual fluorescence emission, respectively. Since the two compounds have different two-color luminescence channels and the two-color intensity ratio is affected by the environment, and the intermolecular hydrogen bond is determined by the hydrogen bond receptor, polarity, and temperature in the environment, the full spectrum from blue to red light and white light emission can be obtained by adjusting the mixing ratio of the two dyes and the solvent polarity and the ambient temperature. This environmentally sensitive white emission is used to detect the alkalinity of different papers, and the dyed paper can be used as a test strip for acid-base vapor detection.

Three-arm star-shaped aniline derivatives: Tunable photoluminescence, aggregation-induced emission and reversible acid-base vapor fluorescence response

The aggregation-induced emission (AIE) molecules demonstrate unique luminous performance in aggregation state, which shows great potential in optoelectronic devices, chemical sensors, bioimaging and so on. Linear aniline derivatives own excellent photoelectric properties, good processing properties, outstanding designability and pH sensitivity. Most reported linear aniline derivatives are typical aggregation caused quenching (ACQ) molecules. One commonly adopted strategy to achieve ACQ-AIE conversion of aniline derivatives is introducing functional groups onto the backbone of aniline chain, which requires multi-step synthesis and post-processing. Herein, we propose a novel “one-step” strategy to achieve the ACQ-AIE conversion of the aniline derivatives through varying the molecular geometry and modification of functional groups on the backbone of the aniline. A series of aniline derivatives in three-arm star-shaped structure was then designed by adjusting the position of aniline units. Furthermore, derivatives containing terminal groups with different electron-withdrawing abilities, including methoxy, bromine, phenyl and cyanophenyl groups, were synthesized in succession. The absorption and emission behaviors of the derivatives could be effectively adjusted. All the synthesized derivatives exhibit AIE behaviors, implying that the ACQ-AIE conversion of aniline derivatives through structural adjustment was feasible. Theoretical simulation was further used to calculate the electronic and geometric structures of the derivatives. The results proved that the distorted conformation was responsible for the AIE characteristics. In addition, the powder of four derivatives showed a reversible fluorescence response to acid-base vapor which indicated potential for anti-counterfeiting applications.

Excited-State Intramolecular Proton Transfer in 2-(2'-Hydroxyphenyl)pyrimidines: Synthesis, Optical Properties, and Theoretical Studies.

The development of fluorescence materials with switched on/off emission has attracted great attention owing to the potential application of these materials in chemical sensing. In this work, the photophysical properties of a series of original 2-(2′-hydroxyphenyl)pyrimidines were thoroughly studied. The compounds were prepared by following well-established and straightforward methodologies and showed very little or null photoluminescence both in solution and in the solid state. This absence of emission can be explained by a fast proton transfer from the OH group to the nitrogen atoms of the pyrimidine ring to yield an excited tautomer that deactivates through a nonradiative pathway. The key role of the OH group in the emission quenching was demonstrated by the preparation of 2′-unsubstituted derivatives, all of which exhibited violet or blue luminescence. Single crystals of some compounds suitable for an X-ray diffraction analysis could be obtained, which permitted us to investigate inter- and intramolecular interactions and molecular packing structures. The protonation of the pyrimidine ring by an addition of trifluoroacetic acid inhibited the excited-state intramolecular proton transfer (ESIPT) process, causing a reversible switch on fluorescence response detectable by the naked eye. This acidochromic behavior allows 2-(2′-hydroxyphenyl)pyrimidines to be used as solid-state acid–base vapor sensors and anticounterfeiting agents. Extensive density functional theory and its time-dependent counterpart calculations at the M06-2X/6-31+G** level of theory were performed to rationalize all the experimental results and understand the impact of protonation on the different optical transitions.

Self-healing metallo-supramolecular polymers showing luminescence off/on switching based on lanthanide ions and terpyridine moieties

Multi-color luminescence stimulus-responsive materials based on lanthanide (Ln3+) complexes have attracted increasing interest because of their considerable potential applications in various fields. However, enabling materials with rapidly stimulus-responsive property remains a challenge. Herein, luminescent elastomers were prepared via the coordination of Ln3+ ions to nitrogen atoms from terpyridine moieties tethered to the polymer chains composed of polydimethylsiloxane (PDMS) and hexamethylene diisocyanate (HDI) segments. The emitting color of the luminescent elastomers can be simply modulated by changing the excitation wavelengths and altering the Eu3+/Tb3+ molar ratio. The break/recovery of “antenna effect” between Ln3+ and Terpyridyl upon acid-base vapor endow the materials with fast luminescence off/on switching ability. Furthermore, the luminescent elastomers display fast self-healing capability at room temperature due to the attendance of the dynamic Ln-ligand coordination and sufficient mobility of the flexible polymer chains. This work opens up new possibilities for future information encryption to carry more codes by combining the multi-color ability as well as luminescence off/on switching property, and showing the potential application as a powerful tool in the base-acid detector fields.

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.

Multi-functional materials based on α-cyanostilbene: Response to mechanical stimuli and selective detection of PNA

Two α-cyanostilbene derivatives NCS1 and NCS2 were designed and synthesized. Both two compounds exhibited AIE properties. Solid compounds showed excellent mechanochromism with 29/26 nm redshift and distinct acid-base vapor chromotropism, which could be applied for relevant sensors. XRSD and PXRD spectra revealed the intrinsic relationship between mechanochromism and molecular packing mode. The twisted molecular conformation became more planar with an increase in conjugation after grinding, resulting in the redshift in fluorescence spectra. Moreover, the emission spectra of NCSs showed obvious fluorescence quenching for p-nitroaniline (PNA) with excellent selectivity. The fluorescence intensity showed better linear relationship at low concentration of PNA. The limit of detections were as low as 0.42 μmol L−1 and 0.60 μmol L−1, respectively. DFT theory proved that this sensing mechanism was attributed to the process of photoinduced electron transfer (PET). Importantly, NCSs were successfully used for PNA detection in real environments. We provide the working basis for developing high-performance multi-functional materials.

Acid-base Vapor Sensing Enabled by ESIPT-attributed Cd(II) Coordination Polymer with Switchable Luminescence

Fluorescence materials based on excited state intramolecular proton transfer(ESIPT) have attracted great attention due to the unique four-level energy states. Herein, we report the assembly of a Cd-LF coordination polymer from purposely designed LF(H2hpi2cf) ligand, which can present switchable luminescence behavior by gain or loss protons originated in uncoordinated ESIPT sites and serve as acid-base vapor sensors. Fabricated into in-situ grown film or transparent ink by simple methods, Cd-LF presents facile and portable amine sensor for food spoilage detection and fluorescent anti-counterfeiting ink applications.

Synthesis, optical properties, acid-base vapochromism and anti-counterfeiting of novel π-extended pyridine fused coumarins

Coumarin and its derivatives have potential applications in sensing, laser dyes, fluorescent probes, and dye sensitized solar cells due to their unique photophysical properties. However, synthesis and studies of linear pyridine-fused-π-extended coumarins remained unexplored. Here, we synthesised two π-expanded coumarin derivatives (PC1, PC2) in which molecular backbone is linearly extended with pyridyl ring where 5,7-positions are substituted with aryl groups. Single crystal X-ray structure analysis confirmed the molecular geometry. We found in spectroscopic studies that both coumarins in solutions show fluorescence via predominant radiative decay of charge transfer (CT) state due to rotational motion associated with the molecules. In addition, detailed spectroscopic analysis of both coumarins in presence of acid and base revealed the acidochromic property in solutions. Based on the acid-base sensing property of PC1, we could show its use as solid-state acid-base vapor sensor and anti-counterfeiting agent.

Reversible stimulus-responsive coordination polymers mainly involving conversion between the lone-pair–π and cation–π interactions

A new 3-position substituted imidazo[1,2-a]pyridine ligand, 4-(2-chloroimidazo[1,2-a] pyridine-3-carboxamido)phthalic acid (H2L), was conveniently synthesized and acts as a three-connecting T-shaped building block. Based on this ligand, two isostructural coordination polymers, [ML(H2O)] n (M = Zn (1) and Co (2)) with a rare one-dimensional (1D) ladder-like chain structure, were solvothermally synthesized and structurally characterized. There exist hydrogen bond and lone-pair–π interactions between two adjacent ladder chains. Compared to the H2L ligand, coordination polymer 1 reveals a blue-shifted emission stemming from ligand-to-metal charge transfer, meanwhile, coordination polymer 2 presents a dominant antiferromagnetism. Interestingly, the carboxylate anion (COO−) in coordination polymers 1 and 2 could be protonated into carboxylic acid (–COOH), upon exposure to HCl vapor. As a result, coordination polymers 1 and 2 display reversible luminescent switching in the response to acid-base vapor stimuli, which mainly originates from the conversion between lone-pair–π and cation–π interactions mediated by the protonation-deprotonation processes. Such stimulus-responsive conversion between lone-pair–π and cation–π interactions is first reported in coordination polymers, and provides a promising approach to design and construct smart coordination polymer materials.

Dyes encapsulated in a novel flexible metal−organic framework show tunable and stimuli-responsive phosphorescence

A novel flexible metal-organic framework (MOF), [ZnL]∙H2O∙DMA (USTC-1), with interesting topology has been constructed based on a new T-shaped ligand, 5-(2-chloroimidazo[1,2-a]pyridine-3-carboxamido)isophthalic acid (H2L). This MOF was characterized by IR spectroscopy, thermogravimetry, single-crystal, and powder X-ray diffraction methods. The single-crystal analysis reveals four carboxylate groups connect two zinc ions to generate a paddlewheel secondary building unit (SBU). Each paddlewheel SBU is six-connected to six L2− ligands, and each L2− is three-connected to three SBUs, thus, the framework can be described as a binodal (3,6)-connected three-dimensional (3D) flexible network. The low N2 and CO2 gas uptake should be attributed to the flexible vibration of amide chains at ligands that close the pore window, however, the closed pore window can be opened by dye molecules, such as Rhodamine B (RB), Methyl Orange (MO) or Methylene Blue (MB). Two methods for dye encapsulation were investigated: soaking and in situ encapsulation. PXRD data showed that the framework in USTC-1 was unchanged after encapsulation of dyes. Interestingly, RB@USTC-1 simultaneously displayed the characteristic emissions of both the RB dye and the MOF, resulting in red/blue two-color luminescence with intensity ratio of 4.5 and tunable phosphorescence color. Moreover, RB@USTC-1 and MO@USTC-1 could undergo protonation upon exposure to HCl vapor, and reveal reversible phosphorescent color switching in the response to acid-base vapor stimuli. The present work provides a promising approach for synthesizing novel flexible MOFs and a new access to develop the phosphorescent and stimuli-responsive MOF materials via encapsulation of various guests.

The positional isomeric effects induced various phosphorescence: Switchable properties through acid-base vapor stimulation

Three novel positional isomers, namely (E)-3-(2-chloroimidazo[1,2-a] pyridin-3-yl)- 1-(pyridin-2-yl)prop-2-en-1-one (1), (E)-3-(2-chloroimidazo[1,2-a]pyridin-3-yl)-1- (pyridin-3-yl)prop-2-en-1-one (2) and (E)-3-(2-chloroimidazo[1,2-a]pyridin-3-yl)- 1-(pyridin-4-yl)prop-2-en-1-one (3), were obtained through a mild approach. Powder X-ray diffraction patterns demonstrate their various stacking structures, attributed to positional isomeric effects. Furthermore, these positional isomers exhibit different phosphorescent colors and quantum yields. Interestingly, these positional isomers also reveal reversible phosphorescent color switching in the response to acid-base vapor stimuli. The present work provides a promising approach for synthesizing organic materials and a new access to develop dynamic functional materials which can be reversibly switched between different phosphorescence based on external acid-base vapor stimuli.

Self-healing hydrogel containing Eu-polyoxometalate as acid-base vapor modulated luminescent switch

The integration of self-healing ability into luminescent switches have attracted increasing interest in designing smart optical devices. In this paper, an acid-base vapor modulated luminescent switch of hydrogels based on Eu-containing polyoxometalate (EuPOM) and poly(2-acrylamido-2-methyl-1-propanesulfonicacid) (PAMPSA) was readily prepared, which can self-heal within only about few minutes under ambient atmosphere. Besides, the hydrogels can easily be shaped into any desired form by pouring into molds. These remarkable features enable the resulting hydrogel as a promising “smart” material with use in smart devices, sensors, etc.

Switchable luminescence and morphology through acid-base vapor annealing in organic materials

Through simple and green strategy, we unprecedentedly synthesized a novel organic compound through methyl-directed aromatic electrophilic substitution reactions. This work indicates that only introduction of methyl group at molecular framework can construct novel seven-membered ring organic materials. Interestingly, this organic material not only reveals the enhanced luminescence, but also the reversible luminescent color (red↔green) switching in the response to acid-base vapor stimuli, which is accompanied by the reversible morphological transformation (3D microbrick↔2D hexangular microflake). The present work provides a promising approach for synthesizing organic materials and a new access to develop dynamic functional materials which can be reversibly switched between different luminescence and morphology based on external acid-base vapor stimuli.

Acid–base vapor induced reversible morphological transformation of organic microcrystals: revealing photophysical “on/off” switches

In the present work, the morphology of organic microcrystals can be reversibly transformed between three-dimensional microprism and two-dimensional hexangular microflake by external acid–base vapor stimuli, which is accompanied by the reversible phosphorescent color and near-infrared (NIR) absorption intensity transformation. Other unusual features are that the fumigation of such microcrystals with HCl–NH3 vapors exhibits NIR and middle-infrared (mid-IR) absorption “on/off” switches. Thus, interesting NIR absorption (at 1950nm) and mid-IR absorption (at 3728, 3892 and 4005nm) “on/off” switches have been first realized by using the present organic microcrystal materials which might have a significant effect on the design strategy of molecular device and switch. Present work will provide new organic microcrystals for promising candidates for applications in NIR and mid-IR absorption switch materials.

Reversible On–Off Luminescence Switching in Self-Healable Hydrogels

We present herein an easy way to prepare novel responsive hydrogels by simply doping lanthanide complexes into a polymer hydrogel, poly(2-acrylamido-2-methyl-1-propanesulfonicacid) (PAMPSA). The resulting hybrid hydrogels can be readily processed into a range of shapes. Both the on-off luminescence switching and the healable properties are simultaneously achieved in the resulting responsive hybrid hydrogels. They exhibit effectively self-healing performance without any external stimulus and reversible "on-off" luminescence switching triggered by exposure to acid-base vapor. The key to this on-off luminescence switching behavior is that the protonation of the organic ligands compete with full coordination to Ln(3+) and that incomplete coordination affects the luminescence yield. The high proton strength in the resulting hydrogels makes the doped lanthanide complexes unstable, and ammonia (or triethylamine) vapor can dramatically decrease the proton strength through neutralization, driving the full coordination of the ligand to Ln(3+).