Reversible piezo- and photochromic behaviors accompanied by emission color switching of two anthracene-containing organic molecules

Synergism of Light-Induced [4 + 4] Cycloaddition and Electron Transfer Toward Switchable Photoluminescence and Single-Molecule Magnet Behavior in a Dy4 Cubane

Molecular materials possessing switchable magneto-optical properties are of great interest due to their potential applications in spintronics and molecular devices. However, switching their photoluminescence (PL) and single-molecule magnet (SMM) behavior via light-induced structural changes still constitutes a formidable challenge. Here, a series of cubane structures were synthesized via self-assembly of 9-anthracene carboxylic acid (HAC) and rare-earth ions. All complexes exhibited obvious photochromic phenomena and complete PL quenching upon Xe lamp irradiation, which were realized via the synergistic effect of photogenerated radicals and [4 + 4] photocycloaddition of the AC components. The quenched PL showed the largest fluorescence intensity change (99.72%) in electron-transfer photochromic materials. A reversible decoloration process was realized via mechanical grinding, which is unexpectedly in the electron-transfer photochromic materials. Importantly, an SMM behavior of the Dy analog was observed after room-temperature irradiation due to the photocycloaddition of AC ligands and the photogenerated stable radicals changed the electrostatic ligand field and magnetic coupling. Moreover, based on the remarkably photochromic and photoluminescent properties of these compounds, 2 demos were applied to support their application in information anti-counterfeiting and inkless printing. This work, for the first time utilizing the simultaneous modulation of photocycloaddition and photogenerated radicals in one system, realizes complete PL quenching and light-induced SMM behavior, providing a dynamical switch for the construction of multifunctional polymorphic materials with optical response and optical storage devices.

Stimuli-fluorochromic smart organic materials

Smart materials based on stimuli-fluorochromic π-conjugated solids (SFCSs) have aroused significant interest due to their versatile and exciting properties, leading to advanced applications. In this review, we highlight the recent developments in SFCS-based smart materials, expanding beyond organometallic compounds and light-responsive organic luminescent materials, with a discussion on the design strategies, exciting properties and stimuli-fluorochromic mechanisms along with their potential applications in the exciting fields of encryption, sensors, data storage, display, green printing, etc. The review comprehensively covers single-component and multi-component SFCSs as well as their stimuli-fluorochromic behaviors under external stimuli. We also provide insights into current achievements, limitations, and major challenges as well as future opportunities, aiming to inspire further investigation in this field in the near future. We expect this review to inspire more innovative research on SFCSs and their advanced applications so as to promote further development of smart materials and devices.

Mechanofluorochromic Properties of 1,4-Diphenylanthracene Derivatives with Hypsochromic Shift

Several types of 1,4-diphenylanthracene derivatives 1-4 were prepared, and their photophysical properties were observed in the solid and solution states. Interestingly, the CN-group-substituted 1,4-diphenylanthracene derivative 2 was found to exhibit a higher fluorescence quantum yield (ϕf = 0.71) in the solid state than in the solution state, probably due to the formation of an intermolecular Ar-CN⋯H-Ar hydrogen bond and antiparallel type locked packing structure in the solid state. Furthermore, for some derivatives, an increase in the fluorescence quantum yield was observed in the PMMA film (1 wt%) over both the solid state and the solution state. More interestingly, some of the 1,4-diphenylanthracene derivatives exhibited unusual mechanofluorochromic properties with a "hypsochromic shift" in luminous color depending on the substituents of the phenyl group, and with the derivatives having CF3, OMe, CN, and two F substituents (1d-1f, 2-4) showing a significant luminous color change with a "hypsochromic shift" after grinding. However, no change in the luminous color was observed for the derivatives having H, Me, and one F substituent (1a-1c), and especially for some of the CN-substituted derivatives, a reversible luminous color change with a "hypsochromic shift" was observed, probably due to the formation of an antiparallel type packing structure. These "hypsochromic" anthracene derivatives could probably be utilized as new mechanofluorochromic materials.

Structural tuning of β-enamino diketones: exploration of solution and crystalline state photochromism

A library of β-enamino diketones was prepared via base-mediated, three-component reaction of 4-hydroxycoumarins with various aromatic/aliphatic amines and β-nitrostyrenes under microwave irradiation conditions to investigate their photochemical properties. Among the prepared compounds, a thiophene derived β-enamino diketone was found to be light-sensitive and to exhibit unique photochromic behavior, that is, positive photochromism in solution and negative photochromism in crystalline phase. In addition, this prepared photochromic compound was further covalently linked to a structure-related, piezochromic β-enamino diketone moiety to explore its potential multi-stimuli responsive properties.

Mechanical and Thermal ON-OFF Switching of the Vapochromic Behavior of a Luminescent Polymorphic Pt(II) Complex

Vapochromic materials that exhibit color/luminescence changes induced by vapor exposure have attracted considerable attention. Herein, we report the grinding- and heating-induced ON-OFF switching of the vapochromic behavior of [Pt(ppyCl₂)(Clacac)] (1; ppyCl₂ = 2-(3-chlorophenyl)-4-chloropyridinato, Clacac = 3-chloroacetylacetonato). 1 formed yellow and orange polymorphs (1-Y and 1-O), and 1-Y could be converted to 1-Og, which showed a very similar crystal structure but with a broadened X-ray diffraction pattern compared with that of 1-O. Moreover, 1-Og can be reversibly transformed into 1-O via heating and grinding. Notably, 1-Og underwent a N,N-dimethylacetamide vapor-induced transformation to 1-Y, whereas 1-O did not undergo such a transformation. These results indicate the ON-OFF switching of vapochromic behavior induced via grinding and heating. This finding will be beneficial for developing intelligent molecular devices.

Position of Biphenyl Group Turning the Structure and Photophysical Property of D-π- π -A Prototype Fluorescent Material

Three novel D-π-π-A prototype compounds, namely, (E)-2-(3-([1,1'-biphenyl]-2-yl)-1-(9H-fluoren-2-yl)allylidene)malononitri-le (2-BAM), (E)-2-(3-([1,1'-biphenyl]-3-yl)-1-(9H-fluoren-2-yl)allylidene)malononitri-le (3-BAM), and (E)-2-(3-([1,1'-biphenyl]-4-yl)-1-(9H-fluoren-2-yl)allylidene)malononitri-le (4-BAM) were synthesized. Furthermore, the structures and photophysical properties of three compounds were compared. Molecules of 2-BAM were packed into a 1D column structure with H-aggregation. However, both of 3-BAM and 4-BAM were packed into 3D layer structures with J-aggregation, respectively. Although all three compounds showed highly twisted molecular geometries, their respective molecular packing and intermolecular interactions were different. Because of the differences in electronic structures of molecules, three compounds displayed different emission behaviors in solid and dilute solution states. This study indicated that changing the position of biphenyl groups is an effective way for turning the structures and photophysical properties of such D-π-π-A prototype fluorescent materials.

New Rofecoxib-Based Mechanochromic Luminescent Materials and Investigations on Their Aggregation-Induced Emission, Acidochromism, and LD-Specific Bioimaging

Development of new mechanochromic luminescent (MCL) materials from aggregation-induced emission luminogens (AIEgens) has attracted wide attention due to their potential application in multiple areas. However, rational design and crafting of new MCL materials from the simple AIEgens skeleton is still a big challenge because of the undesirable concentration quenching effect. In this study, we have constructed a new class of MCL materials by adding one phenyl as a new rotator and incorporating one pair of electron donor (D) and acceptor (A) into the system of rofecoxib skeleton. This strategy endowed the compounds (Y1-Y8) with tunable emission behavior and some of them with the AIE effect and reversible MCL behavior. These properties may be caused by the highly twisted conformation and loosely molecular packing modes, which were elucidated clearly by analyzing the data of single-crystal X-ray diffraction, powder X-ray diffraction, and differential scanning calorimetry. Further investigation revealed that Y7 displayed acidochromic property due to the protonation of the nitrogen atom. Moreover, Y7, as a typical compound, showed its potential applications in the area of anticounterfeiting, pH sensor, and LD-specific bioimaging.

Reversible luminescence tuning behavior in a thermal-stimuli-responsive anthracene-based coordination polymer

A new coordination complex presented a series of structural changes accompanied by photo-fluorescence (PL) changes under different temperature stimuli.

Vapochromic crystals: understanding vapochromism from the perspective of crystal engineering

Vapochromic materials, which undergo colour and/or emission changes upon exposure to certain vapours or gases, have received increasing attention recently because of their wide range of applications in, e.g., chemical sensors, light-emitting diodes, and environmental monitors. Vapochromic crystals, as a specific kind of vapochromic materials, can be investigated from the perspective of crystal engineering to understand the mechanism of vapochromism. Moreover, understanding the vapochromism mechanism will be beneficial to design and prepare task-specific vapochromic crystals as one kind of low-cost 'electronic nose' to detect toxic gases or volatile organic compounds. This review provides important information in a broad scientific context to develop new vapochromic materials, which covers organometallic or coordination complexes and organic crystals, as well as the different mechanisms of the related vapochromic behaviour. In addition, recent examples of supramolecular vapochromic crystals and metal-organic-framework (MOFs) vapochromic crystals are introduced.

Triphenylamine-based stimuli-responsive solid state fluorescent materials

Molecular engineering of triphenylamine (TPA) units produced multi-stimuli-responsive solid state fluorescent materials.

Polymorphous Luminescent Materials Based on 'T'-Shaped Molecules Bearing 4,7-Diphenylbenzo[c][1,2,5]thiadiazole Skeletons: Effect of Substituents on the Photophysical Properties

Polymorphism, the intrinsic character of one chemical compound with at least two distinct phase arrangements, plays a very key role in the photophysical properties. In this contribution, four 'T'-shaped molecules bearing the 2,1,3-benzothiadiazole (BTD) skeleton, named 5 a-5 d, were prepared and characterized. All compounds exhibited excellent thermal stability and polymorphism in the solid state, evident from thermogravimetric analysis, differential scanning calorimetry, and polarized optical microscopy results. Intense emissions with high photoluminescent quantum yields were achieved both in solution (56-97 %) and neat films (33-98 %). All compounds possessed clearly pH-dependent luminescence properties in solution. Additionally, compound 5 d showed useful mechanochromic luminescence owing to the transformation between the crystal and amorphous state. Employing compounds 5 a-5 d as the dopant, solution-processable organic light-emitting diodes (OLEDs) were fabricated and presented a highest external quantum efficiency of 6.15 %, which is higher than the theoretical value of fluorescence-based OLEDs (∼5 %). This research provided a novel strategy for designing high-efficiency BTD-based polymorphic luminescent materials.

Polymorph-Dependent Luminescence Response to Acid Vapors and Its Application in Safety Protection of File Information

A Schiff base, (E)-1-(phenylimino)methyl-2-hydroxylnaphthalene (PIHN), was found to form both nonluminescent and luminescent polymorphs. A unique luminescence "turn on" behavior was observed when the nonluminescent polymorph was fumed with the vapor of aliphatic acids with two or three carbons in the main chain, while the luminescent polymorphs almost did not change the emission color under the same condition. As we know, this is the first report on polymorph-dependent acid response which discloses the influence of crystalline phase on acid-responsive behavior. The formation of hydrogen bonds between PIHN and aliphatic acid is proposed to be the reason for the responsive behavior of the nonemissive polymorph, and such a mechanism is different from the common protonation mechanism. A novel safety protection method of file information has been developed based on the polymorph-dependent luminescence response of PIHN. In addition, we disclose that a crystalline form could show multiple responsive behaviors toward different acids, which benefits the further design of novel acid sensors, such as the sensors that can qualitatively analyze the species of the acid source in an acidic environment.

A phototunable anion receptor for C-HX interactions with benzoate anions

A supramolecular receptor consisting of two anthracene moieties with binding motifs for binding of benzoate anions is reported here. NMR studies indicate that the binding involves π-π interactions and CHX interactions. Upon exposure to >350 nm light, the receptor undergoes a [4 + 4] photoelectrocyclization restricting the access to the binding site for benzoate. The reverse reaction works in the presence of the dual stimuli of 254 nm light and the benzoate anions. The work thus demonstrates a light mediated dynamic control of the binding pocket of a supramolecular anion receptor.

A mechano-responsive fluorescent xerogel based on an anthracene-substituted acylhydrazone derivative

AHBP-8 can form a stable organogel and enhanced fluorescence emission has been observed after gelation. The AHBP-8 xerogel exhibited favorable mechano-fluorochromism behavior.

Synthesis of tunable, red fluorescent aggregation-enhanced emissive organic fluorophores: stimuli-responsive high contrast off–on fluorescence switching

Molecular conformation controlled tunable and stimuli-responsive off–on fluorescence switching.

Construction of Luminogen Exhibiting Multicolored Emission Switching through Combination of Twisted Conjugation Core and Donor-Acceptor Units

Stimuli responsive luminescent materials, especially those exhibiting multicolor emission switching, have potential application in sensor, optical recording, security ink, and anti-counterfeit label. Through combination of twisted conjugation core and donor and acceptor units, a luminogen (2-(bis(4-(carbazol-9-yl)phenyl)methylene)malononitrile (1) was synthesized. Luminogen 1 can form three kinds of crystals emitting green (1GC, λ_em = 506 nm, Φ_F = 19.8%), yellow-green (1YC, λ_em = 537 nm, Φ_F = 17.8%), and orange (1OC, λ_em = 585 nm, Φ_F = 30.0%) light upon 365 nm UV illumination. The emission of amorphous solid of 1 (1Am) overlaps with that of 1OC (λ_em = 585 nm), with quantum yield of 13.9%, which is seldom reported. Emission of 1 can be switched among green, yellow-green, and orange through morphology modulation upon exposure to thermal, solvent vapor, or mechanical stimuli. Finally, its potential application in optical recording was also investigated.

High contrast stimuli-responsive luminescence switching of pyrene-1-carboxylic esters triggered by a crystal-to-crystal transition

Unexpected high contrast mechanochromic, thermochromic and vaporchromic luminescence has been achieved through a simple introduction of an ester group to a pyrene skeleton and a series of stimuli-responsive materials based on pyrene-1-carboxylic esters were efficiently developed.

Mechanoresponsive Luminescent Molecular Assemblies: An Emerging Class of Materials

The possibility to change the molecular assembled structures of organic and organometallic materials through mechanical stimulation is emerging as a general and powerful concept for the design of functional materials. In particular, the photophysical properties such as photoluminescence color, quantum yield, and emission lifetime of organic and organometallic fluorophores can significantly depend on the molecular packing, enabling the development of molecular materials with mechanoresponsive luminescence characteristics. Indeed, an increasing number of studies have shown in recent years that mechanical force can be utilized to change the molecular arrangement, and thereby the optical response, of luminescent molecular assemblies of π-conjugated organic or organometallic molecules. Here, the development of such mechanoresponsive luminescent (MRL) molecular assemblies consisting of organic or organometallic molecules is reviewed and emerging trends in this research field are summarized. After a brief introduction of mechanoresponsive luminescence observed in molecular assemblies, the concept of "luminescent molecular domino" is introduced, before molecular materials that show turn-on/off of photoluminescence in response to mechanical stimulation are reviewed. Mechanically stimulated multicolor changes and water-soluble MRL materials are also highlighted and approaches that combine the concept of MRL molecular assemblies with other materials types are presented in the last part of this progress report.

Preparation of organic mechanochromic fluorophores with simple structures and promising mechanochromic luminescence properties

An efficient method to design and synthesize simple organic mechanochromic molecules based on pyrene skeleton is reported. Based on this method, a broad range of phosphonium materials with promising mechanochromic luminescence have been developed.

2-(Anthracenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole: regulatable stacking structures, reversible grinding- and heating-induced emission switching, and solid-state photodimerization behavior

The molecular assembly of anbdtiH leads to three distinct solid-state structures: anbdtiH·CH₃Cl (1), anbdtiH·2CH₃OH (2) and anbdtiH₂·CF₃COO·CH₃OH·H₂O (3), which have orange-red, blue and green emissions, respectively.

2-(Anthracenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole (anbdtiH) has been synthesized. Its three solid-state structures, anbdtiH·CH₃Cl (1), anbdtiH·2CH₃OH (2) and anbdtiH₂·CF₃COO·CH₃OH·H₂O (3), have been constructed by skillfully choosing CHCl₃ or CH₃OH as the solvent and using or not using CF₃COOH, with the aim of modifying the intermolecular hydrogen bonds and/or π···π stacking interactions. The three distinct structures show significantly different solid-state luminescence behaviors, an orange-red emission at 603 nm for 1, a blue emission at 453 nm for 2, and a green emission at 533 nm for 3. Upon grinding, these emission wavelengths exhibit evident variations, a blue-shift of Δλ = 83 nm for 1, a red-shift of Δλ = 20 nm for 2, and a blue-shift of Δλ = 54 nm for 3. The emission color of 1 can be reversibly switched between orange-red and green upon regulation of the intermolecular (N–H)_imidazole···N_imidazole hydrogen bonds by a grinding–heating process. Moreover, compound 3 can undergo a solid-state [4π + 4π] photodimerization reaction upon irradiation with sunlight, forming 3-dimer. Based on the crystal structures of 1–3, this work discusses the relationship between the molecular stacking mode and the luminescence behavior/photochemical reactivity.

Mechanochromism triggered fluorescent color switching among polymorphs of a natural fluorescence pigment

The solid-state study of emodin demonstrates the feasibility of building new MCL systems via controlling the solid-state transition of natural fluorescent dyes.

Reversible Piezofluorochromic Property and Intrinsic Structure Changes of Tetra(4-methoxyphenyl)ethylene under High Pressure

During the past decade, luminescent mechanochromism has received much attention. Despite the garnered attention, only a few studies have reported the effect of internal molecular structure change on the performance of mechanochromic fluorescence. Here, we chose tetra(4-methoxyphenyl)ethylene (TMOE) as a model molecule to study the correlation between structure and fluorescence property under a hydrostatic pressure produced by a diamond anvil cell (DAC). TMOE is a methoxy-substituted tetraphenylethylene (TPE) derivative and has a nearly centrosymmetric structure and a natural propeller shape. Ultraviolet-visible absorption and fluorescence spectra of TMOE and TPE in solution proved that the presence of methoxy groups in TMOE is responsible for the difference in fluorescence emissions of TMOE and TPE. Under a hydrostatic pressure, the in situ fluorescence spectra of TMOE at different concentrations show that the fluorescence intensity gradually weakens, accompanied by an obvious redshift. The Raman peak intensities decrease gradually, and the peaks disappear eventually with the pressure increasing. These spectral changes are attributed to the changes in the intramolecular conformation, that is, the strengthening of the weak C-H···O hydrogen bonds in TMOE molecules, which is caused by the twisted dihedral angle between the benzene ring and the carbon rigid plane of ethylene. Density functional theory simulation further confirms that the decreased dihedral angle could weaken Raman peak intensity, which is consistent with our experimental results.

Comparison of shearing force and hydrostatic pressure on molecular structures of triphenylamine by fluorescence and Raman spectroscopies

Luminescent mechanochromism (e.g., shearing force and hydrostatic pressure) has been intensively studied in recent years. However, there are few reported studies on the difference of the molecular configuration changes induced by these stresses. In this study, we chose triphenylamine, C18H6N (TPA), as a model molecule to study different molecular configuration changes under shearing force and hydrostatic pressure. Triphenylamine is an organic optoelectric functional molecule with a propeller-shaped configuration, a large conjugate structure, and a single molecular fluorescence material. Fluorescence and Raman spectra of TPA were recorded in situ under different pressures (0-1.9 GPa) produced by the mechanical grinding or using a diamond anvil cell (DAC). Our results show that the crystal phase of TPA transformed to the amorphous phase by grinding, whereas no obvious phase transition was observed under hydrostatic pressure up to 1.9 GPa, indicating the stability of TPA. Hydrostatic pressure by DAC induces molecular conformation changes, and the pressure-induced emission enhancement phenomenon of TPA is observed. By analyzing the Raman spectra at high pressure, we suggest that the molecular conformation changes under pressure are caused by the twisted dihedral angle between the benzene and the nitrogen atom, which is different from the phase transformation induced by the shearing force of grinding.

Reversible piezochromism in a molecular wine-rack

Coupling of a ‘wine-rack’ lattice motif with molecular flexibility leads to a large piezochromic response in an organic material.

The facile realization of RGB luminescence based on one yellow emissive four-coordinate organoboron material

The present study not only provides a model of the facile realization of RGB luminescence based on one compound by an external stimulating approach but also gives a design guidance towards smart luminescent organic materials.

Regulating the piezofluorochromism of 9,10-bis(butoxystyryl)anthracenes by isomerization of butyl groups

Isomerization of alkyl chains can effectively regulate the piezofluorochromism of AIE-active 9,10-bis(alkoxystyryl)anthracenes.

Stimuli responsive reversible high contrast off–on fluorescence switching of simple aryl-ether amine based aggregation-induced enhanced emission materials

Aryl-ether amine based simple Schiff base molecules showed rare stimuli responsive off–on fluorescence switching with high contrast.

Efficient green-red piezofluorochromism of bisanthracene-modified dibenzofulvene

The bisanthracene modified dibenzofulvene exhibits efficient piezofluorochromic property with the fluorescence reversibly altered between green and red.

Polymorphism-dependent and piezochromic luminescence based on molecular packing of a conjugated molecule

We report a unique example of polymorphism-dependent and piezochromic fluorescence, based on 2-(anthracen-9-yl)-4,5-diphenyl-1H-imidazole.

Piezochromic phenomena of nanometer voids formed by mono-dispersed nanometer powders compacting process

Piezochromism describes a tendency of certain materials changing colors when they are subjected to various pressure levels. It occurs particularly in some polymers or inorganic materials, such as in palladium complexes. However, piezochromism is generally believed to work at high pressure range of 0.1-10 GPa. This research work focused on unique piezochromism responses of the nanometer voids formed by the 5-20 nm inorganic ISOH nanometer powders. It was discovered that microstructures of the nanometer voids could change color at very low pressures of only 0.002-0.01 GPa; its sensitivity to pressure was increased by tens of times. It is believed that the uniform microstructures of nanometer powders contributed to the material's high sensitivity of piezochromic phenomena. One factor which quantum optical change caused by nanometer voids affected the quantum confinement effect; another is surface Plasmon Resonance of great difference dielectric property between conductive ITO powder and insulation hydroxide.