Mechanofluorochromism of a Series of Benzofuro[2,3-c]oxazolo[4,5-a]carbazole-Type Fluorescent Dyes

Stimuli-responsive luminescence from polar cyano/isocyano-derived luminophores via structural tailoring and self-assembly

Polar cyano fragments and their isomeric isocyano counterparts have attracted great attention as stimuli-responsive luminescent materials in a wide range of fields including organic light-emitting diode devices, chemical fluorescent sensors, photoelectric semiconductors, anti-counterfeit products, etc., mainly because of their typical electron-deficient activity, noncovalent recognition ability, and variable coordination capacity. The electron-deficient and polar nature of these blocks have significant effects on the properties of the cyano/isocyano-based luminophore materials, especially concerning their condensed state-dependent electronic structures. Among them, donor-acceptor (D-A) derived unimolecular and co-assembled luminophores have attracted more attention because their large delocalized structures and noncovalent interaction recognition sites can rebuild the electronic transfer character in the aggregative state, thus endowing them with outstanding stimuli-responsive luminescent behavior via intermolecular and intramolecular charge transfer in polytropic morphologies. In this perspective paper, we give a brief introduction on stimuli-responsive organic and coordinated luminophores and the documented typical design concepts and applications in recent years. It is expected that this perspective article will not only summarize the recent developments of polar cyano/isocyano-derived luminophores and their coordination compounds via structural tailoring and self-assembly but also throw light on the future of the design of more sophisticated stimuli-responsive architectures and their versatile properties.

Grinding-Induced Water Solubility Exhibited by Mechanochromic Luminescent Supramolecular Fibers

Most mechanochromic luminescent compounds are crystalline and highly hydrophobic; however, mechanochromic luminescent molecular assemblies comprising amphiphilic molecules have rarely been explored. This study investigated mechanochromic luminescent supramolecular fibers composed of dumbbell-shaped 9,10-bis(phenylethynyl)anthracene-based amphiphiles without any tetraethylene glycol (TEG) substituents or with two TEG substituents. Both amphiphiles formed water-insoluble supramolecular fibers via linear hydrogen bond formation. Both compounds acquired water solubility when solid samples composed of supramolecular fibers are ground. Grinding induces the conversion of 1D supramolecular fibers into micellar assemblies where fluorophores can form excimers, thereby resulting in a large redshift in the fluorescence spectra. Excimer emission from the ground amphiphile without TEG chains is retained after dissolution in water. The micelles are stable in water because hydrophilic dendrons surround the hydrophobic luminophores. By contrast, when water is added to a ground amphiphile having TEG substituents, fragmented supramolecular fibers with the same molecular arrangement as the initial supramolecular fibers are observed, because fragmented fibers are thermodynamically preferable to micelles as the hydrophobic arrays of fluorophores are covered with hydrophilic TEG chains. This leads to the recovery of the initial fluorescent properties for the latter amphiphile. These supramolecular fibers can be used as practical mechanosensors to detect forces at the mesoscale.

Control of Fluorescence of Organic Dyes in the Solid-State by Supramolecular Interactions

Fluorescent organic dyes play an essential role in the creation of new "smart" materials. Fragments and functional groups capable of free rotation around single bonds can significantly change the fluorescent organic dye's electronic structure under analyte effects, phase state transitions, or changes in temperature, pressure, and media polarity. Dependencies between steric and electronic structures become highly important in transition from a solution to a solid-state. Such transitions are accompanied by a significant increase in the dye molecular structure's rigidity due to supramolecular associates' formation such as H-bonding, π···π and dipole-dipole interactions. Among those supramolecular effects, H-bonding interactions, first of all, lead to significant molecular packing changes between loose or rigid structures, thus affecting the fluorescent dye's electronic states' energy and configuration, its fluorescent signal's position and intensity. All the functional groups and heteroatoms that are met in the organic dyes seem to be involved in the control of fluorescence via H-bonding: C-H···N, C-H···π, S = O···H-C, P = O···H, C-H···O, NH···N, C - H···C, C - H···Se, N-H···O, C - H···F, C-F···H. Effects of molecular packing of fluorescent organic dyes are successfully used in developing mechano-, piezo-, thermo- fluorochromes materials for their applications in the optical recording of information, sensors, security items, memory elements, organic light-emitting diodes (OLEDs) technologies.

Mechanofluorochromism of (D-π-)2A-type azine-based fluorescent dyes

Bathochromic or hypsochromic shift-type mechanofluorochromism (b-MFC or h-MFC) was found for (D–π–)₂A-type azine-based fluorescent dyes OUY-2, OUK-2, and OUJ-2 possessing intramolecular charge-transfer (ICT) characteristics from two (diphenylamino)carbazole–thiophene units as D (electron-donating group)–π (π-conjugated bridge) moieties to a pyridine, pyrazine, or triazine ring as A (electron-withdrawing group): grinding of the recrystallized dyes induced red or blue shifts of the fluorescent colors, that is, bathochromic or hypsochromic shifts of the fluorescence maximum wavelengths (λ^fl-solid_max). The degrees of MFC evaluated by the absolute value of differences (Δλ^fl-solid_max) in λ^fl-solid_max before and after grinding of the recrystallized dyes increased in the order of OUY-2 (+7 nm) < OUK-2 (−17 nm) < OUJ-2 (+45 nm), so that OUJ-2 exhibits obvious b-MFC, but OUK-2 exhibits h-MFC. X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC) demonstrated that the recrystallized dyes were in the crystalline state but the ground dyes were in the amorphous state. When the ground solids were heated above their crystallization temperatures (T_c), the colors and fluorescent colors recovered to the original ones before grinding or converted to other ones, that is, heating the ground solids in the amorphous state induced the recrystallization to recover the original microcrystals or to form other microcrystals due to polymorph transformation. However, (D–π–)₂Ph-type fluorescent dye OTK-2 having a phenyl group as a substitute for the azine rings exhibited non-obvious MFC. Molecular orbital (MO) calculations indicated that the values of the dipole moments (μ_g) in the ground state were 4.0 debye, 1.4 debye, 3.2 debye, and 2.9 debye for OTK-2, OUY-2, OUK-2, and OUJ-2, respectively. Consequently, on the basis of experimental results and MO calculations, we have demonstrated that the MFC of the (D–π–)₂A-type azine-based fluorescent dyes is attributed to reversible switching between the crystalline state of the recrystallized dyes and the amorphous state of the ground dyes with changes in the intermolecular dipole–dipole and π–π interactions before and after grinding. Moreover, this work reveals that (D–π–)₂A fluorescent dyes possessing dipole moments of ca. 3 debye as well as moderate or intense ICT characteristics make it possible to activate the MFC.

Bathochromic or hypsochromic shift-type mechanofluorochromism (b-MFC of h-MFC) was found for (D–π–)₂A-type azine-based fluorescent dyes: grinding of the recrystallized dyes induced bathochromic or hypsochromic shifts of the fluorescence bands.

Mechanochromic Luminescence (MCL) of Purely Organic Two-Component Dyes: Wide-Range MCL over 300 nm and Two-Step MCL by Charge-Transfer Complexation

Despite recent extensive studies on mechanochromic luminescence (MCL), rational control over the magnitude of the emission-wavelength shift in response to mechanical stimuli remains challenging. In the present study, a two-component donor-acceptor approach has been applied to create a variety of organic MCL composites that exhibit remarkable emission-wavelength switching. Dibenzofuran-based bis(1-pyrenylmethyl)diamine and typical organic fluorophores have been employed as donor and acceptor dyes, respectively. Outstanding wide-range MCL with an emission-wavelength shift of over 300 nm has been achieved by mixing the diamine with 3,4,9,10-perylenetetracarboxylic diimide. Unprecedented two-step MCL in response to mechanical stimuli of different intensity has also been realized for a two-component mixture with 9,10-anthraquinone. Fluorescence microscopy observations at the single-particle level revealed that the segregation and mixing of the two-component dyes contribute to the stimuli-responsive emission-color switching of the MCL composites.

Multi-color mechanochromic luminescence of three polymorphic crystals of a donor–acceptor-type benzothiadiazole derivative

The three polymorphic crystals of a donor–acceptor dye exhibited different luminescence colors, which changed in response to mechanical grinding.

A mechanochromic cyclemetalated cationic Ir(iii) complex with AIE activity by strategic modification of ligands

Two new aggregation-induced emission (AIE)-active cyclemetalated cationic Ir(iii) complexes have been rationally designed and synthesized by introducing O-H substituents into Schiff base ligands. π-Hydrogen bonding is successfully exploited for the first time to realize the mechanochromic luminescence (MCL) property by the synergistic effect of O-H and F substituents in complex 1. An X-ray crystal structure analysis of the two complexes suggests that the intramolecular hydrogen bonding effectively restricted the molecular motions, thus causing typical AIE characteristics. More importantly, a loosely packed structure constructed from intermolecular hydrogen bonding interactions (O-Hπ and C-HF) is obtained, and it is susceptible to mechanical stimulation. Powder X-ray diffraction (PXRD) studies also prove that the MCL behavior of complex 1 is caused by the reversible phase transition from crystalline to amorphous state under grinding and solvent recrystallization, leading to a change in emission colors. A re-writable phosphorescence data recording device was fabricated using complex 1 as the active material. Our molecular design strategies provide a new avenue for achieving efficient phosphorescence materials with AIE and MCL properties.

Mechanochromic Luminescence from Crystals Consisting of Intermolecular Hydrogen-Bonded Sheets

Introduction of functional groups that can form intermolecular hydrogen bonds into highly-emissive luminophores is a promising way to induce mechanochromic luminescence. Herein, we report that a 9,10-bis(phenylethynyl)anthracene derivative featuring two amide groups forms green-emissive crystals based on two-dimensional hydrogen-bonded molecular sheets. Mechanical grinding changed the emission from green to yellow, owing to a transition from a crystalline to an amorphous phase. Infrared spectroscopy revealed that mechanical stimuli disrupted the linear hydrogen-bonding formation. A thermal treatment recovered the original green photoluminescence.

Stimuli-responsive aggregation-induced fluorescence in a series of biphenyl-based Knoevenagel products: effects of substituent active methylene groups on π-π interactions

A series of three biphenyl-based Knoevenagel products (denoted 1a, 1b, 1c) with active methylene groups has been synthesized. Compounds 1a and 1b show strong solid-state fluorescence, whereas 1c displays low emission. Effects of substituent groups in condensed phase packing of the molecules have been investigated and correlated with their photophysical properties. Interestingly, compound 1a exhibits mechanofluorochromism with emission color changes from yellow to green (wavelength shift of 40 nm) after mechanical grinding. Furthermore, fluorescence of 1a and 1b is turned off under alkaline conditions, making them potential candidates for aggregation-enhanced emission-based pH sensors.

Tricolor mechanochromic luminescence of an organic two-component dye: visualization of a crystalline state and two amorphous states

The tricolor mechanochromic luminescence (MCL) of a two-component mixture of a poor MCL dye and a non-MCL dye is described.

Efficient solid-state emission and reversible mechanofluorochromism of a tetraphenylethene-pyrene-based β-diketonate boron complex

A new twisted donor–acceptor (D–A) dye (BF₂-TP) that was composed of tetraphenylethene and pyrene connected with a β-diketonate boron moiety has been synthesized and characterized. Such a dye showed unique intramolecular charge transfer (ICT) features, which were evidenced by spectral analysis and theoretical calculations. More importantly, BF₂-TP solid samples exhibited an obvious mechanofluorochromic (MFC) behavior. Upon grinding with a spatula, the as-prepared powder sample illustrated a remarkable red shift of 62 nm, with considerable color contrast from yellow (562 nm) to orange red (624 nm). Its fluorescence color can be reversibly switched by repeating both the grinding–fuming and grinding–annealing processes. The mechanochromism is attributed to the phase transformation between amorphous and crystalline states. The results obtained would be helpful for designing novel MFC materials.

A new twisted dye (BF₂-TP) was synthesized, it possesses distinct mechanofluorochromism with large spectral shift of 62 nm.

Sequential halochromic/mechanochromic luminescence of pyridyl-substituted solid-state emissive dyes: thermally controlled stepwise recovery of the original emission color

A stepwise temperature-controlled emission-color switch has been achieved in a system that combines halochromic and mechanochromic luminescence in series.

Thermally activated delayed fluorescent phenothiazine-dibenzo[a,j]phenazine-phenothiazine triads exhibiting tricolor-changing mechanochromic luminescence

Novel U-shaped donor-acceptor-donor (D-A-D) π-conjugated multi-functional molecules comprising dibenzo[a,j]phenazine (DBPHZ) as an acceptor and phenothiazines (PTZ) as donors have been developed. Most importantly, the D-A-D compounds exhibit not only distinct tricolor-changeable mechanochromic luminescence (MCL) properties but also efficient thermally activated delayed fluorescence (TADF). Quantum chemical calculations, X-ray diffraction analysis, and systematic studies on the photophysical properties indicated that the "two-conformation-switchable" PTZ units play a highly important role in achieving multi-color-changing MCL. Time-resolved photophysical measurements revealed that the developed D-A-D compounds also exhibit efficient orange-TADF. Furthermore, organic light-emitting diode (OLED) devices fabricated with the new TADF emitters have achieved high external quantum efficiencies (EQEs) up to 16.8%, which significantly exceeds the theoretical maximum (∼5%) of conventional fluorescent emitters.

Iron-catalyzed synthesis of benzoxazoles by oxidative coupling/cyclization of phenol derivatives with benzoyl aldehyde oximes

An iron-catalyzed oxidative coupling/cyclization reaction for the synthesis of benzoxazoles at room temperature is reported.

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.

Multi-stimuli responsive fluorescent behaviors of a donor–π–acceptor phenothiazine modified benzothiazole derivative

Donor–π–acceptor type phenothiazine modified benzothiazole derivative PVBT exhibited multi-stimuli responsive fluorescent properties.

Mechanofluorochromism of 1-Alkanoylaminopyrenes

To create a new series of mechanofluorochromic materials and to elucidate the mechanism of the phenomenon of mechanofluorochromism, 1-alkanoylaminopyrenes including 1-acetylaminopyrene (AAPy), 1-octanoylaminopyrene (OAPy), and 1-stearoylaminopyren (SAPy) were prepared. It was found that these materials exhibited mechanofluorochromism with emission colors in the crystalline samples changing reversibly from bluish purple to yellowish green, which could be induced by mechanical grinding. X-ray crystal structure analysis, electronic absorption, and fluorescence spectroscopies, as well as fluorescence lifetime analysis and powder X-ray diffraction analysis of AAPy suggested that the present mechanofluorochromism was caused by developing crystal defects through grinding. Intermolecular hydrogen bonds were suggested to play an important role in the occurrence of mechanofluorochromism, suppressing the face-to-face overlapping of pyrene moieties to form excimers in the pristine crystal.

Mechanochromic Luminescence of Aggregation-Induced Emission Luminogens

Mechanochromic (MC) luminogens have found promising applications in mechanosensors, security papers, and optical storage for their change in emission behaviors in response to mechanical stimuli. Examples on MC luminescent materials are rare before the discovery of MC luminescence in aggregation-induced emission (AIE) luminogens. The twisted conformations of AIE luminogens (AIEgens) with appropriate crystallization capability afford loosely packing patterns, which facilitates their phase transformation in the solid state. The amorphous films of AIEgens exhibit enhanced emission intensity upon pressurization due to the increased molecular interactions, whereas crystals of AIEgens exhibit MC luminescence due to their amorphization by mechanical stimuli. AIEgens enrich the type of MC luminogens but those showing high emission contrast and multicolor emission switching and those working in a turn-on emission mode are seldom reported. Disclosure of the design strategy of high performance MC luminogens and exploration of their high-tech applications may be the future research directions for MC luminogens.

Stress acidulated amphoteric molecules and mechanochromism via reversible intermolecular proton transfer

Stress has been proved to acidulate amphoteric molecules and promote an intermolecular proton transfer, which results in a significant absorption and emission change. The stress acidulated amphoteric molecules open a new avenue for developing mechanochromic materials and anticipate many broad applications such as stress/pressure sensors and rewritable media.

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.

Mechanical control of nanomaterials and nanosystems

In situations of power outage or shortage, such as periods just following a seismic disaster, the only reliable power source available is the most fundamental of forces i.e., manual mechanical stimuli. Although there are many macroscopic mechanical tools, mechanical control of nanomaterials and nanosystems has not been an easy subject to develop even by using advanced nanotechnological concepts. However, this challenge has now become a hot topic and many new ideas and strategies have been proposed recently. This report summarizes recent research examples of mechanical control of nanomaterials and nanosystems. Creation of macroscopic mechanical outputs by efficient accumulation of molecular-level phenomena is first briefly introduced. We will then introduce the main subject: control of molecular systems by macroscopic mechanical stimuli. The research described is categorized according to the respective areas of mechanical control of molecular structure, molecular orientation, molecular interaction including cleavage and healing, and biological and micron-level phenomena. Finally, we will introduce two more advanced approaches, namely, mechanical strategies for microdevice fabrication and mechanical control of molecular machines. As mechanical forces are much more reliable and widely applicable than other stimuli, we believe that development of mechanically responsive nanomaterials and nanosystems will make a significant contribution to fundamental improvements in our lifestyles and help to maintain and stabilize our society.