Cation–Anion Interaction‐Directed Molecular Design Strategy for Mechanochromic Luminescence

Mechanofluorochromic materials have great potential for a wide variety of applications such as sensors, memory devices, motion systems, security systems, and so forth. However, only few design principles have been disclosed, which greatly impedes the growth of mechanofluorochromic dyes. Here, a strategy of molecular design for mechanochromic luminescence is reported, based on the cation–anion interaction‐directed switching of molecular stacking. On the basis of this strategy, a series of common N‐heteroaromatic onium fluorophores such as imidazolium, 1,2,4‐triazolium, triazolopyridinium, benzoimidazolium, γ‐carbolinium, and pyridinium salts have been designed and proved to have striking reversible mechanofluorochromic behaviors. The simple attachment of a non‐fluorescent imidazolium unit to the pyrene scaffold through a flexible carbon chain can even trigger the mechanofluorochromic phenomenon, which gives a consummate interpretation that the cation–anion interaction can be considered as an important general tool to design organic mechanochromic luminescent materials.     

A Double‐Layer Mechanochromic Hydrogel with Multidirectional Force Sensing and Encryption Capability

Hydrogel‐based soft mechanochromic materials that display colorimetric changes upon mechanical stimuli have attracted wide interest in sensors and display device applications. A common strategy to produce mechanochromic hydrogels is through photonic structures, in which mechanochromism is obtained by strain‐dependent diffraction of light. Here, a distinct concept and simple fabrication strategy is presented to produce luminescent mechanochromic hydrogels based on a double‐layer design. The two layers contain different luminescent species—carbon dots and lanthanide ions—with overlapped excitation spectra and distinct emission spectra. The mechanochromism is rendered by strain‐dependent transmittance of the top‐layer, which regulates light emission from the bottom‐layer to control the overall hydrogel luminescence. An analytical model is developed to predict the initial luminescence color and color changes as a function of uniaxial strain. Finally, this study demonstrates proof‐of‐concept applications of the mechanochromic hydrogel for pressure and contact force sensors as well as for encryption devices.     

Two‐Component Molecular Materials of 2,5‐Diphenyloxazole Exhibiting Tunable Ultraviolet/Blue Polarized Emission,Pump‐enhanced Luminescence,and Mechanochromic Response

The development of π‐conjugated molecular systems with high‐efficiency generation of UV and blue light plays an important role in the fields of light‐emitting diodes, fluorescent imaging, and information storage. Herein, supramolecular construction of solid‐state UV/blue luminescent materials are assembled using 2,5‐diphenyloxazole (DPO) with four typical co‐assembled building blocks (1,4‐diiodotetrafluorobenzene, 4‐bromotetrafluorobenzene carboxylic acid, pentafluorophenol, and octafluoronaphthalene). Compared with the pristine DPO sample, the as‐prepared two‐component molecular materials feature ease of crystallization, high crystallinity, enhanced thermal stability and tunable luminescence properties (such as emissive wavelength, color, fluorescence lifetime, and photoluminescence quantum yield) as well as multicolor polarized emission in the UV/blue region. Moreover, pump‐enhanced luminescence and reversible mechanochromic fluorescence (MCF) properties can also be obtained for these molecular solids, which are absent for the pristine DPO sample. Therefore, this work provides a procedure for the facile self‐assembly of ordered two‐component molecular materials with tunable UV/blue luminescence properties, which have potential application in the areas of light‐emitting displays, polarized emission, frequency doubling, and luminescent sensors.     

Non‐Covalent Functionalization of Individual Nanotubes with Spiropyran‐Based Molecular Switches

Single‐walled carbon nanotubes (SWCNTs) are functionalized with a spiropyran derivative, which is attached non‐covalently to the SWCNT's sidewall via a pyrene anchor group. Using this non‐covalent functionalization strategy, individual SWCNTs can be stabilized in solution without the need for additional surfactants. Bright luminescence confirms the presence of individual tubes in the thus‐prepared samples. In these samples, the majority of pyrene‐spiropyran molecules are attached to the walls of the SWCNTs. Upon complex formation with the SWCNT, the switching moiety retains its ability to switch, i.e., to undergo reversible transformations between the closed spiropyran and the opened merocyanine form, and is stable over many cycles of operation.     

Multistimuli Response and Polymorphism of a Novel Tetraphenylethylene Derivative

Multi‐stimuli‐responsive fluorescent molecule tetraphenylethylene (TPE) derivative 1 is synthesized, which contains a TPE skeleton and peripheries of two methoxyl groups and one carboxyl group. It shows a typical aggregation‐induced emission behavior and also exhibits fluorescence responses to pH change and amine vapors, and multicolored mechanochromic properties. The emission of 1 can be reversibly switched among blue (1p‐f, 462 nm, Φ_f = 7.4%), bright cyan (1p‐h, 482 nm, Φ_f = 82.3%), and yellow (1p‐g, 496 nm, Φ_f = 10.5%) with high contrast through solvent fuming, heating, or grinding. Molecule 1 occurs in four crystalline states (1c‐a, 1c‐b, 1c‐c, and 1c‐d) after crystallization from different solvents. The multicolored mechanochromic property is related to the different interactions and packing modes of molecules in the crystals. The crystals with weak fluorescent emission have characteristic porous structures and corresponding intensive XRD diffraction peaks, which are absent in the crystal with intensive fluorescent emission. The porous structures are critical for the fluorescence intensity of the molecules. Upon heating, the porous structures of crystals are damaged and fluorescence is significantly enhanced.     

Room‐Temperature Phosphorescence From Films of Isolated Water‐Soluble Conjugated Polymers in Hydrogen‐Bonded Matrices

It is well known that luminescent conjugated polymers suffer serious loss of photoluminescence quantum yield (PLQY) in the solid state compared to dilute solution. This is due to efficient exciton migration in the solid, which enables the excitons to readily find low energy quenching sites. Here a new method to fabricate solid films with densely packed non‐interacting luminescent polymer chains, which yield very high PLQY and more astonishingly room temperature phosphorescence, is reported. Using water‐soluble conjugated polymers (WSCP) and polymeric surfactants such as poly(vinyl alcohol) (PVA) and poly(vinyl‐pyrrolidone) (PVP), films at 1:1 wt% or higher WSCP are produced and show room temperature phosphorescence; such behavior has never been observed before and clearly shows the very high degree of chain isolation that can be achieved in these hosts. The PVA or PVP not only breaks up WSCP aggregates in solution as an effective surfactant, PVA‐PVA or PVP‐PVP hydrogen bond formation upon drying locks in the isolation of the WSCP, avoiding segregation and yielding long time stability to these polymer/polymer nanomixtures. The method is found to work with a wide variety of WSCPs.     

Solvent‐Controlled Assembly of Aromatic Glutamic Dendrimers for Efficient Luminescent Color Conversion

A binary supramolecular system where self‐sorting and coassembly behavior can be switched by changing the solvent polarity is hereby reported. Glutamic dendron is separately conjugated with pyrene and naphthalimide luminophores through an alkyl spacer. The resulting structurally similar building units can self‐assemble into one‐dimensional micro/nanostructures with hexagonal and lamellar packing, respectively. Varying solvents from polar aqueous solution to nonpolar decane is evidenced to profoundly inverse the superchirality and switch self‐sorted assembly to coassembly of the two building blocks. The moisture sensitivity of the naphthalimide moiety is considered the primary driving force for the self‐sorting phenomenon in aqueous solution, resulting in inevitable hydration to repel its stacking with hydrophobic pyrene moiety. On the other hand, the naphthalimide unit can integrate segmentally with the pyrene unit in decane, greatly facilitating the nanofiber growth and supramolecular gel formation along with improved energy transfer efficiency between luminophores. As a result, the coassembly‐based thin films show efficient luminescent color conversion upon the UV light irradiation. This research presents a useful route for the fabrication of controllable solution‐processed light emitting devices from self‐assembled multicomponent systems.     

Improved Quantum Yield and Excellent Luminescence Stability of Europium‐Incorporated Polymeric Hydrogen‐Bonded Heptazine Frameworks Due to an Efficient Hydrogen‐Bonding Effect

Two of the most persistent challenges for the high‐end application of luminescent lanthanide (Ln) compounds are a low quantum yield and luminescence quenching caused by a liquid medium. In this work, a type of polymeric hydrogen‐bonded heptazine framework is developed incorporating trivalent europium ions (P‐HHF‐Eu) via a low‐cost and facile low‐temperature thermal condensation reaction. Structural characterization clearly reveals that the solid‐phase pyrolyzation reaction results in the formation of P‐HHF‐Eu. Using time‐resolved and steady state photoluminescence (PL) spectroscopies, the photophysics and photochemistry of P‐HHF‐Eu at different hydration degrees are investigated and the role of hydrogen bonding in the significant enhancement of the emission properties is demonstrated. Furthermore, the P‐HHF‐Eu particles suspended in polyvinyl alcohol hydrogel exhibit excellent luminescence stability with a high PL quantum yield of up to ≈46% and wavelength responsive color‐tunable emission, which holds potential for security applications.     

The Relationship of Solid‐State Plasticity to Mechanochromic Luminescence in Difluoroboron Avobenzone Polymorphs

In solid‐state mechanochromic luminescence (ML) materials, it remains a challenge to establish the origin of fluorescence color changes upon mechanical action and to determine why only some fluorophores exhibit ML behavior. The study of mechanical properties by nanoindentation, followed by ML experiments on green‐ and cyan‐emitting polymorphs of difluoroboron avobenzone reveals that upon smearing, the plastically deformable cyan form shows a prominent color change to yellow, while in the harder green form the redshifted emission is barely detectable. Crystal structure analysis reveals the presence of slip planes in the softer cyan form that can facilitate the formation of recoverable and low energy defects in the structure. Hence, the cyan form exhibits prominent and reversible ML behavior. This suggests a potential design strategy for efficient ML materials.     

A Stimuli‐Responsive,Photoluminescent, Anthracene‐Based Liquid Crystal: Emission Color Determined by Thermal and Mechanical Processes

Here, a photoluminescent liquid crystal that exhibits a change of emission color on the metastable–stable phase transition induced by external stimuli is prepared. A 2,6‐diethynylanthracene derivative with amide groups and dendritic side chains exhibits a columnar phase on slow cooling from the isotropic phase and shows blue emission in this columnar phase. In contrast, a cubic phase is obtained by rapid cooling from the isotropic phase. In the cubic phase, the 2,6‐diethynylanthracene cores form excimers, resulting in yellow emission. While the columnar phase is a stable liquid‐crystalline (LC) phase, the cubic phase is a metastable LC phase. It is found that a change of the photoluminescent color from yellow to blue is observed on the cubic‐columnar phase transition induced by heating or mechanical shearing for this 2,6‐diethynylanthracene derivative in the cubic phase. This change of photoluminescent color is ascribed to the inhibition of excimer formation on the metastable–stable LC phase transition.     

Molecular Gelation‐Induced Functional Phase Separation in Polymer Film for Energy Transfer Spectral Conversion

A new strategy for creating the energy transfer spectral conversion thin film by using fluorophore‐functionalized molecular gelation is proposed. This is based on the facts that nanofibrillar phase separation of the self‐assembling pyrene derivative as a fluorophore is formed in a bulk polymer‐containing organic gel, and consequently that the phase‐separated nano domain in a polymer thin film is enough small to keep the transparency but also extremely high Storks shift is gained by efficient excimer formation through highly ordered stacking among the pyrene moieties. When the phase separation‐mediated functional polymer is applied as spectral conversion films (SCFs) for copper–indium–gallium–selenide (CIGS) solar cell, the SCF‐covered solar cell exhibits significant improvement of power conversion efficiency by increase of photocurrent. In this paper, the FRET efficiency and emission wavelength are also demonstrated to be thermotropically switchable since order‐to‐disordered transitions are essential characteristics of as non‐covalent low molecular assembling.     

Co‐deposited Cu(I) Complex for Tri‐layered Yellow and White Organic Light‐Emitting Diodes

Four compounds 4‐[3,6‐di(carbazol‐9‐yl)carbazol‐9‐yl]isoquinoline (TCIQ), 3‐[3,6‐di(carbazol‐9‐yl)carbazol‐9‐yl]pyridine (TCPy), 4‐(carbazol‐9‐yl)isoquinoline (4CIQ), and 3‐(carbazol‐9‐yl)pyridine (CPy) containing pyridyl or isoquinolyl were designed and synthesized to co‐deposition with copper iodide (CuI) to form luminescent Cu(I) complex doped film in situ, which could be utilized as the emissive layer in organic light‐emitting diodes (OLEDs). It is found that simple tri‐layered yellow and white OLEDs can be achieved by co‐depositing CuI and TCIQ with tuning ratios. The compound TCIQ serves a dual role as both a ligand for forming the emissive Cu(I) complex and as a host matrix for the formed emitter in yellow OLEDs, and a third role as a blue emitter in white OLEDs.     

Sunlight‐Induced Cross‐Linked Luminescent Films Based on Polysiloxanes and d‐Limonene via Thiol–ene “Click” Chemistry

The increasing pursuit of biocontained elastic materials led the investigation of the potential use of the monoterpene limonene in film synthesis via thiol–ene reaction. Poly[(mercaptopropyl)methylsiloxane] (PMMS) is first synthesized. By controlling the molar ratio of PMMS and functional monomers, such as polyethylene glycol allyl methyl ether or rhodamine‐B, PMMS is partially functionalized while leaving spare mercapto groups that could be further used as cross‐linking sites. On the basis of the functionalized PMMS, novel transparent silicone luminescent films with hydrophilic tunable properties are prepared by natural‐sunlight‐triggered thiol–ene “click” chemistry by using d ‐limonene as a cross‐linker. Their structures and properties are thoroughly characterized. Transparent luminescent films are coated on commercially available UV‐light emitting diode (LED) cell from solution medium followed by an in situ cross‐linking step; a colorful LED cell is obtained through this facile and efficient method. The UV‐LED coated by films show very intense photoluminescence under normal visible light or the light is on, and has very high coloric purity.     

Simultaneous and Dual Emissive Imaging by Micro‐Contact Printing on the Surface of Electrostatically Assembled Water‐Soluble Poly(p‐phenylene) Using FRET

Poly{[2,5‐bis(3‐sulfonatobutoxy)‐1,4‐phenylene sodium salt]‐alt‐(1,4‐phenylene)}, which is an anionically charged, water‐soluble poly(para‐phenylene) derivative with aldehyde groups at both chain ends, is prepared via the Suzuki coupling reaction in order to develop a FRET energy donor, while simultaneously dual‐fluorescence‐patterning the protein. Regardless of the end‐capping, the synthesized polymer exhibits a good solubility in water with an absorption maximum at 338 nm and a photoluminescence maximum at 417 nm, similar to those of the the end‐capped polymer. The emission spectrum of the polymer overlaps the absorption spectrum of fluorescein, and therefore, the polymer can be used as an energy donor with fluorescein as the energy acceptor in the FRET mechanism. This polymer design not only takes advantage of the introduction of biotin at both chain ends (through a reaction with the aldehyde end groups) to realize the facile interaction with streptavidin, but also brings into play the electrostatic features of the anionic sulfonate groups to fabricate an electrostatic self‐assembly with polycation for the pattern substrate. The micropattern of fluorescein‐labeled streptavidin is fabricated on the polymer‐coated substrate through micro‐contact printing using a polydimethylsiloxane mold. As a result, the polymer substrate exhibits a dual fluorescence micropattern, which results from the blue emission color from the energy donor and the FRET‐amplified green emission from the energy acceptor. The high‐resolution patterning is carried out for the application of multiplexing by simultaneously imaging the patterned green‐emitting fluorescein by FRET and the surrounding blue‐emitting polymer according to an optical detection scheme.     

Highly Luminescent Organosilane‐Functionalized Carbon Dots

The first use of an organosilane as a coordinating solvent to synthesize highly luminescent (quantum yield = 47%) amorphous carbon dots (CDs) in one minute is reported. The CDs, which benefit from surface methoxysilyl groups, have a diameter of ~0.9 nm and can easily be fabricated into pure CD fluorescent films or monoliths simply by heating them at 80 ºC for 24 h. Moreover, the non‐water‐stable CDs can be further transformed into water‐soluble CDs/silica particles, which are biocompatible with and nontoxic to the selected cell lines in our preliminary evaluation. The proposed novel synthetic route is believed to provide an alternative synthesis route and should inspire more research into the origin and applications of CDs, as well as delivering CD‐based materials.     

Luminescent Graphene Oxide with a Peptide‐Quencher Complex for Optical Detection of Cell‐Secreted Proteases by a Turn‐On Response

Graphene oxide (GO) is an emerging luminescent nanomaterial with photostable and unique photoluminescence (PL) in the visible and near‐infrared region. Herein, a GO PL‐based optical biosensor consisting of a luminescent GO donor covalently linked with a peptide‐quencher complex is reported for the simple, rapid, and sensitive detection of proteases. To this end, the quenching efficiency of various candidate quenchers of GO fluorescence, such as metalloprotoporphyrins and QXL₅₇₀, are examined and their quenching mechanisms investigated. A fluorescence resonance energy transfer‐based quencher, QXL₅₇₀, is found to be much more effective for quenching the intrinsic fluorescence of GO than other charge transfer‐based quenchers. The designed GO–peptide–QXL system is then able to sensitively detect specific proteases—chymotrypsin and matrix metalloproteinase‐2—via a “turn‐on” response of quenched GO fluorescence after proteolytic cleavage of the quencher. Finally, the GO–peptide–QXL hybrid successfully detects MMP‐2 secreted from living cells—human hepatocytes HepG2—with high sensitivity.     

Polymeric Alkoxy PBD [2‐(4‐Biphenylyl)‐5‐Phenyl‐1,3,4‐Oxadiazole] for Light‐Emitting Diodes

The syntheses are reported of the title polymeric alkoxyPBD derivative 5 and the dipyridyl analogue 12 using Suzuki coupling reactions of 1,4‐dialkoxybenzene‐2,5‐diboronic acid with 2,5‐bis(4‐bromophenyl)‐1,3,5‐oxadiazole, and its dipyridyl analogue, respectively. Thermal gravimetric analysis shows that polymers 5 and 12 are stable up to 370 °C and 334 °C, respectively. Films of polymer 5 spun from chloroform solution show an absorption at λ_max = 367 nm, and a weaker band at 312 nm, and strong blue photoluminescence at λ_max = 444 nm. The photoluminescence quantum yield (PLQY) was found to be 27 ± 3 %. For polymer 12 , the absorption spectra reveal bands of equal intensity at λ_max = 374 and 312 nm, with PL at λ_max = 475 nm. Device studies using polymer 12 were hampered by its instability under illumination and/or electrical excitation. Polymer 5 is stable under these conditions and acts as an efficient electron‐transporting/hole‐blocking layer. For devices of configuration ITO/PEDOT/MEH‐PPV/polymer 5 /Al an external quantum efficiency of 0.26 % and brightness of 800 cd/m² was readily achieved: orange emission was observed, identical to the MEH‐PPV electroluminescence.     

1,2‐Dithienylethene Photochromes and Non‐destructive Erasable Memory

Monitoring changes in ultraviolet‐visible (UV‐vis) absorption is not a viable method to process information for photochromic memory media due to the readout signal interfering with the photochromism. Only by monitoring the changes in other photophysical properties accompanying the photoisomerization reaction (refractive index, optical rotation, or luminescence, for example) can non‐destructive, all photon‐mode photochromic memory be realized. We have investigated several such systems based on 1,2‐dithienylcyclopentene derivatives, which have a backbone that we consider to be currently the most promising of the photochromes. The two readout signals highlighted in this article are luminescence and optical rotation. The luminescent systems rely on porphyrinic chromophores tethered to the photochrome directly or through dative bonds. When the macrocycles are irradiated with light at wavelengths outside the absorption range of the photochrome, luminescence is only observed when the 1,2‐dithienylcyclopentene backbone exists in its open‐state. The self‐assembly of a chiral photochromic metallo‐helicate allows for stereoselective ring‐closing of the 1,2‐dithienylcyclopentene backbone providing a change in optical rotation that can be used as a readout signal. In the article, we also describe the use of ring‐opening metathesis polymerization (ROMP) to fabricate well‐ordered photochromic homopolymers possessing identical photochromic properties as their monomers.     

2,5‐Functionalized Spiro‐Bisiloles as Highly Efficient Yellow‐Light Emitters in Electroluminescent Devices

New spiro‐bisilole molecules functionalized with nitrogen‐containing heterocyclic groups including 7‐azaindolyl, indolyl, and 2,2′‐dipyridylamino have been synthesized. These molecules are found to display good chemical and thermal stability. They are luminescent in solution and in the solid state with an emission color ranging from blue–green to yellow, depending on the functional group. In the solid state, they display high photoluminescence quantum efficiency (32–40 %). The electroluminescence properties for one of the new molecules, 2,3,3′,4,4′,5‐hexaphenyl‐2′,5′‐bis(p‐2,2′‐dipyridylaminophenyl)spiro‐bisilole, have been investigated by fabricating single‐layer and double‐layer electroluminescent devices. The double‐layer device, in which N,N′‐bis(1‐naphthyl)‐N,N′‐diphenylbenzidine acts as the hole‐transport layer and the functionalized spiro‐bisilole functions as the emitter (emission wavelength = 566 nm) and the electron‐transport layer, displays a brightness of 8440 cd m^–2 at 9 V with a current efficiency of 1.71 cd A^–1. No evidence of exiplex emission is observed.     

Elastoplastic Inverse Opals as Power‐Free Mechanochromic Sensors for Force Recording

Light‐weight, power‐free mechanochromic sensors that can change and record the reflective color depending on the magnitude and rate of the applied force are fabricated from inverse opals by infiltrating the colloidal crystals of silica particles with uncrosslinked SU‐8, followed by removal of the colloidal templates. The mechanical sensing range of the materials is high, 17.6–20.4 MPa. Due to elastoplastic deformation of the SU‐8 films, the deformed structures and thus colors can be locked after the removal of the load, therefore establishing a quantitative relationship between the mechanical force and optical responses. In comparison, mechanochromic photonic gels reported in the literature typically detect force in the range of 10–100 kPa; once the load is removed, the structure and color return back to the original ones. The mechanochromic sensors are highly sensitive: the ratio of shift in the stopband wavelength to the change in applied strain is up to 5.7 nm per percent, the highest among literature. Comparison of finite element simulations with experiments confirms the elastoplastic deformation of the films and highlights that reconfiguration of pore shape under compression plays a key role in the mechanochromic response.