Mechanochromic Luminescence Research Articles

Organic Mechanochromic Luminescent Materials with Self-Recovering Characters.

Recently, applied research on stimuli-responsive materials with luminescence-switching characteristics has been conducted in various fields. A representative phenomenon of stimuli-responsive luminescent materials is mechanochromic luminescence (MCL), which exhibits luminescent color change induced by mechanical stimuli such as grinding. These materials are among the most prominent candidates for security and sensing applications. Interestingly, some mechanochromic luminescent materials have shown self-recovery character, in which their original luminescent color can be recovered by just standing under ambient conditions after grinding. Although there have been more and more reports of such materials in recent years, the fundamental principles of molecular design still remain elusive. In this concept, we summarize distinctive advances in mechanochromic luminescent materials with self-recovery according to the core structures of luminescent molecules. Controlling amorphous state by introducing substituents such as alkyl or polar groups is effective method to provide self-recovering properties.

AIE-active fluorescence emitters containing tetraphenylethylene and α-cyanostilbene with mechanochromism and photo-responsive behaviors

Aggregation induced emission luminogens (AIEgens) as one of the optimal alternatives for designing stimuli-responsive fluorophores for real-life applications have sparked much current interest in smart materials design. Herein, the “Star molecules” in AIEgens of α-cyanostilbene and Tetraphenylethylene (TPE) derivatives were constructed into one molecular, namely DC-TPE-tBu and DC-TPE-CN. Both compounds exhibited significant AIE properties. Moreover, the two compounds could change their luminescent colors and intensities under the stimulation of external mechanical force and UV light, showing excellent mechanochromic luminescent (MCL) and remarkable photo-responsive behaviors, respectively. The stimuli responsive mechanisms were investigated by powder X-Ray diffractometer (PXRD) spectra and density functional theory calculations (DFT). The MLC behaviors were associated with the crystalline-to-amorphous transitions after mechanical grinding. These remarkable features make them suitable for the application of mechano/photo-sensors. The studies provide a useful strategy for developing AIE-active functional materials.

Multifunctional thermally activated delayed fluorescence emitter for both doped and non-doped organic light emitting diodes

Developing efficient smart materials that display mechanochromic luminescence (MCL) and thermally activated delayed fluorescence (TADF) remains a formidable challenge. Herein, we designed and synthesized a multifunctional emitter, 2BPy-mTC, where benzoyl pyridyl (2BPy) acts as an acceptor and 3,6-di-tert-butyl-9H-carbazole (TC) acts as a donor. 2BPy-mTC exhibits blue emission with λmax of 456 nm in pristine form and emission shifted to green with λmax of 495 nm upon grinding. Interestingly, when fumed with CH2Cl2, the emission reverts to deep blue at λmax of 440 nm. 2BPy-mTC possesses a ΔEST of 0.20 eV in neat film and 0.11 eV in doped film (5 wt% 2BPy-mTC: mCBP) and exhibits a PLQY of 42 % in neat film and 59 % in doped film. The transient photoluminescence (PL) decay curves show a delayed lifetime of 110.3 μs for the neat film and 192.9 μs for the doped film and confirm the TADF property in both doped and non-doped film. It shows an EQEmax of 16.3 % in doped device with an emission maximum of 484 nm. Conversely, it retains the EQEmax of 13.8 % in non-doped device with a slight red shift in the emission maximum (496 nm). This discovery opens a new avenue for designing multifunctional emitters with improved EL performance.

Tuning the Reversible Mechanochromic Luminescence of Triphenylethylenes by Rational Introduction of Functionalized Thiophenes Substituents

Tuning the Reversible Mechanochromic Luminescence of Triphenylethylenes by Rational Introduction of Functionalized Thiophenes Substituents

Designing High-Sensitivity Mechanochromic Luminescent Materials Through Friction-Induced Crystallization Strategy.

Despite recent significant breakthroughs in novel mechanochromic luminescent (ML) materials, developing a high-sensitivity ML material is still challengeable. Herein, a "friction-induced crystallization" strategy is proposed to realize highly sensitive transformations of luminescent signal, through an integration of polymeric chains and an aggregation-sensitive luminescent core, which act as mechanical sensors and fluochromic actuators, respectively. The coupling of these two components enables the material to crystallize in response to shear friction, thereby exhibiting blue-shift fluorescence due to a more restricted relaxation pathway. This study underscores a high-sensitivity ML material based on the precise regulation of molecular-scale motions, and also expands the scope and potential of ML materials toward user-friendly, interactive wearable devices.

Benzodifuranone Crystals with Solid-State Emission Arising from the Introduction of Bulky Substituents.

Benzodifuranone derivatives, structural analogues of widely studied diketopyrrolopyrrole, have been reported to show photoluminescence exclusively in solution states. Here, upon introducing bulky aromatic groups into benzodifuranone dyes, crystals with unprecedented solid-state emission were obtained. A crystallographic analysis revealed that the emissive properties should most likely be attributed to the absence of stacking between the dye scaffolds. In addition to the solid-state emission, the compound showed responsivity to external stimuli, i. e., luminescent mechanochromism and thermosalient effect.

Synthesis of quadruply boron-doped acenes with stimuli-responsive multicolor emission

Boron-doped acenes have attracted attention due to their unique structures and intriguing luminescent properties. However, the hitherto known boron-doped acenes have only one or two boron atoms, limiting the chemical space of this unique family of compounds and the capability to tune their optical properties. Herein, we report the synthesis of quadruply boron-doped acenes, including pentacene, heptacene, and nonacene. The importance of the boron doping level on the luminescent properties of acenes is demonstrated. The title compounds manifest enhanced Lewis acidity as compared with dihydrodiboraacenes, leading to Lewis-base-responsive emission in the solid state. Moreover, quadruply boron-doped nonacene displays mechanochromic luminescence in addition to Lewis-base-responsive properties, realizing high-contrast solid-state multicolor emission. This work greatly expands the chemistry of boron-doped acenes and offers opportunities for developing boron-based luminescent materials.

The Solid-State Multi-Color Fluorescence Switching from a [2.2]Paracyclophane-Based Triarylborane.

The fluorophores, the fluorescence of which can be switched between multi bright colors in the solid state, show promising applications not only in the sophisticated multicolor display but also in the advanced encryption and anti-counterfeiting systems. However, it is very challenging to obtain such fluorophores. Herein, we disclose such an example, g-BPhANMe2-Cp, which contains an electron-donating dimethylamino (NMe2) and an electron-accepting [(2-dimesitylboryl)phenyl]acetyl at the pseudo-gem position of [2.2]paracyclophane skeleton. This molecule can display tricolor mechanochromic luminescence (MCL) due to the different responses of the mechanically ground amorphous state to heating and solvent-fuming. Owing to the absence of intermolecular π-π interactions in the solid state, the fluorescence efficiency is very high irrespective of its morphological state (ΦF=0.60-0.87). Moreover, this molecule also displays reversible acidochromic luminescence (ACL) by protonation and deprotonation of NMe2 with trifluoroacetic acid (TFA) and triethylamine (TEA), respectively. The protonated sample fluoresces (ΦF=0.31) at much shorter wavelength due to the interruption of intramolecular charge transfer process. Therefore, with the combination of tricolor MCL and ACL properties, the solid-state emission of g-BPhANMe2-Cp can be switched among four bright fluorescence colors of yellow, green, cyan and blue via treatment with appropriate stimulus.

Asymmetric D-A-D’ type diphenylsulfone derivative modified by carbazole and tetraphenylethylene showing high solid-state luminescence efficiency, aggregation-induced emission and reversible mechanochromic luminescence

The new D-A-D’ type diphenylsulfone derivative TPE-DBS-Cz with carbazole and tetraphenylethylene units as the donors and diphenylsulfone moiety as the acceptor was prepared. Photophysical properties in solutions and theoretical calculations indicated that the D-A-D’ type molecule TPE-DBS-Cz adopted a highly twisted molecular conformation, obvious intramolecular charge transfer (ICT) and aggregation-induced emission (AIE) behaviors, which not only endowed it with high solid-state luminescence efficiency but also have an obvious fluorescent response to mechanical force. When applied by the grinding and fuming treatment, the emitting color was continuously switched between bright cyan and green. In other words, effective MCL behavior was obtained. The XRD, DSC, fluorescence lifetimes and the PL spectral analyses in different solid states indicated that the MCL nature of TPE-DBS-Cz was derived from the conversion of the crystalline and amorphous states. Moreover, the bathochromic-shift of the PL peak after grinding could be attributed to the degree of extended conjugation and the conformational planarization under external pressure.

Mechanochromic and Solvent-Induced Luminescence of a Supramolecular Pt(II)-Bipyridine Complex with Di(4-pyridylmethyl)aminedithiocarbamate.

[Pt(bpy)(DPMACS2)]2Cl2•3H2O (1•3H2O) (bpy = 2,2'-bipyridine, DPMACS2 = di(4-pyridylmethyl)aminedithiocarbamate) was synthesized and characterized by X-ray diffraction studies, and its crystal structure displayed intermolecular Pt(II)···Pt(II) contacts of 3.471 and 5.065 Å. Upon excitation, 1•3H2O showed broad luminescence at 538 nm, which was red-shifted and enhanced to 560 nm while cooling to 77 K. To this end, the B3LYP/LanL2DZ calculation results were performed to clearly explain their excited-state origin. Moreover, complex 1•3H2O displayed a dramatic mechanochromic shift from 538 to 608 nm while grinding, and the above red-shift was also observed while exposed to air within 1 day, suggestive of the simultaneous mechanochromic and solvent-induced luminescence. It is noted that the luminescence almost reverted to the original luminescence at 535-542 nm upon immersion in various solvents for the ground samples of complex 1•3H2O. In addition, the luminescence for the acetone-immersed ground samples returned to 608 nm in 1 min. The possible interactions between halogenated solvents and the free pyridyl groups in DPMACS2, which were not expected for acetone, have been proposed to be responsible for such a dramatic difference in this study.

Multistimuli Luminescence and Anthelmintic Activity of Zn(II) Complexes Based on 1H-Benzimidazole-2-yl Hydrazone Ligands

Three novel Zn(II) mononuclear complexes with the general formula ZnL2Cl2 (L = 2-(4-R-phenylmethylene)benzimidazol-2-hydrazines; R-H (1), R-CH3 (2), and R-OCH3 (3)) were synthesized and fully characterized by various means. These complexes demonstrate excitation-dependent emission, which is detected by a change in the emission color (from blue to green) upon an increase in the excitation wavelength. Moreover complex 1 shows reversible mechanochromic luminescence behavior due to the reversible loss of solvated methanol molecules upon the intense grinding of crystals. In addition, 1 exhibits vapochromic properties, which originate from the adsorption methanol vapor on the crystal surface. The strengthening of anthelmintic activity at the transition from free hydrazones to zinc-based complexes is shown.

Dynamic, multi-color, multi-level stimulus-responsive luminescence in a co-crystallized coordination polymer: Modulation of energy transfer by acid-induced ligand protonation and photo-induced radical generation

Stimulus-responsive luminescent materials (SLMs), which can respond to external stimuli by switching their luminescence properties, hold great promise for a variety of applications. Particularly, multi-level SLMs with a logical stimulus sequence and a dynamic, multi-color transition are highly desirable for anti-counterfeiting and information encryption, but an effective design strategy is lacking. Herein, a co-crystallized coordination polymer multi-level SLM [Cd(9-AC)4]·(HAD)2·(9-HAC)2 (9-HAC for anthracene-9-carboxylic acid and AD for acridine) with dual emission centers of coordination unit ([Cd(9-AC)4]2-) and exciplex ([9-HAC/HAD]+) has been successfully constructed by a stepwise solvent process. This SLM possesses a variety of stimulus-responsive properties, including mechanochromic (MC), mechanochromic luminescence (MCL), reversible acidochromic luminescence (ACL), and reversible photochromic luminescence (PCL). Notably, both ACL and PCL properties are achieved by modulating the energy transfer process between the coordination unit and the exciplex, while there is a logical sequence between these two stimulus actions. The acid-induced activation and radical-regulated PCL performance has been exploited in the development of anti-counterfeiting and information encryption applications. This work provides an effective strategy to realize dynamic, multi-color, multi-level stimuli-responsive luminescence by modulating the energy transfer through acid-induced ligand protonation and photo-induced radical generation.

Multi-component synthesis and enhanced photophysical study of novel azolo[1,5-a]pyrimidine based dyes

A series of novel fluorophores were obtained based on azolo[1,5-a]pyrimidines (APs) through multicomponent and oxidation reactions, starting from aminoazoles, morpholineacrylonitrile, and 4-(dimethylamino)benzaldehyde. Photophysical studies, as well as DFT calculations have been performed for the obtained APs. Aggregation-induced emission (AIE) phenomenon was demonstrated for fluorophores containing an aromatic substituent at the C-2 position of the azolo[1,5-a]pyrimidine core. It was observed that fluorophore 6d exhibited reversible mechanochromic luminescence. Two-photon absorption cross-section was measured for all obtained APs. In addition, cell staining resulted in identical intracellular distribution under excitation at 405 nm, 488 nm and 561 nm.

Reversible mechanochromic luminescence in a binuclear zero-dimensional Cd(II) complex through transformation between exciplex and excimer

In this work, the mechanochromic luminescence (MCL) property of a binuclear zero-dimensional cadmium complex with a variety of weak interactions is investigated. It exhibits a reversible transition between blue (445 nm) and yellow-green (470 nm) emission under the alternating treatments of grinding and CH3CN fumigation. The mechanism of MCL is proposed by combining the structural features, characterization results and theoretical calculations. During the grinding process, the ordered stacking of the structural units is disrupted and the crystalline state is transformed into an amorphous state, which is accompanied by a transition of the luminescent center from exciplex to excimer. In addition to exploring the MCL properties of a binuclear cadmium complex, this work provides feasible ideas for studying the response mechanism.

Mechanochromic Luminescence of π-Conjugated Polymer Based on Pyridylenolate Boron Complexes: Molecular Design and Application to Shear-Force Memory

Mechanochromic Luminescence of π-Conjugated Polymer Based on Pyridylenolate Boron Complexes: Molecular Design and Application to Shear-Force Memory

Engineering Single Component Luminogens to Multicomponent Charge‐transfer Co‐crystal Substrate as New Frontiers for Sensitive SERS Detection

AbstractOrganic charge‐transfer (CT) co‐crystals have demonstrated remarkable physical properties and have found applications in numerous fields. Yet their utility as a Surface‐Enhanced Raman Spectroscopy (SERS) substrate, a powerful and versatile analytical tool, has never been explored. Herein, three twisted molecular donors are synthesized, that exhibit well‐controlled switchable optical properties including aggregation‐induced emission (AIE), mechanochromic luminescence (MCL), and color‐specific polymorphism. Rapid production of charge‐transfer co‐crystals is also established with a π‐acceptor TCNQ and utilized conceptually as a SERS substrate for methylene blue (MB) detection, exhibiting a very high enhancement factor of 109 and limit of detection of 10−13 m, respectively, due to the presence of low‐lying excited state, exhibit an 80% CT character, originating from the HOMO of the co‐crystal and interacting with the LUMO of the MB molecule. This approach using CT co‐crystals as a SERS substrate presents newer frontiers that require minuscule levels of rapid detection and impact allied areas, helping us understand and optimize the fascinating properties of such multicomponent materials for newer technologies.

Effects of donor substituents on the conformational heterogeneity, photophysical, mechanochromic and electroluminescent properties of the donor-substituted fluorine-containing triphenylpyrimidines

Three derivatives of fluorinated triphenylpyrimidine with the attached carbazole, phenothiazine, or acridan donor moieties are synthesized by Buchwald-Hartwig reactions. The impact of the donor units on emissive and other properties of the compounds is reported. The compounds exhibit excellent thermal stability, competitive photophysical phenomena such as room temperature phosphorescence (RTP) appearing when compounds are molecularly dispersed in the rigid polymer matrix and thermally activated delayed fluorescence (TADF). The compounds with carbazole and phenothiazine donor moieties show the manifestation of triplet–triplet annihilation in the electroluminescence when used as emitters in organic light-emitting diodes (OLEDs). The phenothiazine-containing compound exhibit dual photoluminescence with the blue-shifted peak corresponding to the quasi-axial conformer and a red-shifted peak to the quasi-equatorial conformer. This derivative shows reversible shifts of emission spectra exceeding 100 nm due to the stable (at least 4 cycles) mechanochromic luminescence under the application of external stimuli. After grinding the emission intensity maximum is observed at 555 nm, after fuming at. ca 448 nm and after melting at 555 nm. The photoluminescence shifts and ON/OFF delayed fluorescence of the phenothiazine-based emitter occur due to the alteration between the crystalline and amorphous states. Optimization of the device structure allows to control the charge balance resulting in external quantum efficiency of up to 5.7 % observed for the OLED based on the phenothiazine-based emitter. This compound also shows the biggest response to the presence of oxygen acting as the quencher of triplet excited energy. The film of the compound doped in rigid Zeonex shows an 8.4-fold increase in emission intensity after evacuation. The optical sensor fabricated using the derivative of fluorinated triphenylpyrimidine and phenothiazine is characterized by the Stern-Volmer constant 1.37 × 10-4 ppm−1.

Intramolecular Charge Transfer and Stimuli-Responsive Emission in Cholesterol-Appended Phenothiazine-Cyanostyryl-Based Donor-Acceptor Systems.

Organic fluorescent molecules have received considerable attention owing to their various optoelectronic applications. Herein, we report the design and synthesis of two cholesterol-functionalized cyanostyrene-phenothiazine-based D-π-A systems that are emissive in both the solution and solid states. The newly synthesized cholesterol-appended phenothiazine-cyanostyrene diads PTCS-1 and PTCS-2 vary in the N-alkylation of phenothiazine, respectively, with─octyl and─hexyl chains. Both molecules are highly fluorescent and show reasonably good quantum yields in nonpolar solvents because of twisted intramolecular charge transfer (TICT). The molecules exhibit aggregation-induced emission in the solid state. Due to the presence of flexible alkyl chains in the phenothiazine and cholesterol moieties, PTCS-1 and PTCS-2 show mechanochromic luminescence switching in response to external shear stress and emission recovery under methanol vapor. Powder X-ray diffraction studies prove that the emission switching on the applied stimuli in both PTCS-1 and PTCS-2 is attributed to the reversible transformation between the crystalline and amorphous states. Time-dependent density functional theory (TD-DFT) studies are carried out to gain insight into the ICT interactions. TD-DFT analysis at the TD-M06-2X/def2-TZVP level further revealed that in both molecules, the lowest unoccupied molecular orbital (LUMO) + 2, LUMO, highest occupied molecular orbital (HOMO), and HOMO - 1 orbitals are responsible for the charge transfer interactions. These ICT interactions are identified as π-π* type interactions.

Force/Acid-Activated and Radical-Regulated photochromic luminescence in a binuclear Zero-Dimensional complex for multi-level anti-counterfeiting

Traditional static optical anti-counterfeiting technology can only output a single emission signal, which is easy to replicate and difficult to meet the needs in current high-tech fields. Multi-level stimuli-responsive luminescent materials with internal logical relationship can improve the degree of anti-counterfeiting and resist against the flood of counterfeiting. In this work, we propose to use force and acid dual stimuli to restrict the dissipation path of photogenerated free radicals, which further lead to the radical-actuated photochromic luminescence (PCL) performance. Based on this strategy, a zero-dimensional (0D) dinuclear zinc complex Zn2(9-AC)4(IM)2 (1) with blue luminescent properties (450 nm) was designed and synthesized. The interaction between 9-AC and IM (as 9-AC/IM exciplex) can be cut off by force-induced rearrangement of structural units in the spatial dimensions or acid-induced protonation of 9-AC, achieving the luminescent transformation from 9-AC/IM exciplex (447 nm) to 9-AC/9-AC excimer (480 nm) or 9-HAC (485 nm). At the same time, the PCL performance was activated and the emission intensity quenching rate increased from 4.77 % to 93.86 % or 44.93 % under UV light irradiation, respectively. The solid-state ultraviolet–visible absorption spectra, electron paramagnetic resonance spectra and theoretical calculation confirmed that the activation of PCL performance is due to the fact that the 9-AC• radicals are more difficult to dissipate upon force- or acid-induced stimuli. Combining mechanochromic luminescence (MCL) and PCL, a dynamic anti-counterfeiting quick response (QR) code with multi-level and fast-responsive logic sequence have been designed, which are promising for the application in time-resolved information encryption. This work provides an effective strategy to construct multi-level stimuli-responsive luminescence as novel anti-counterfeiting materials by changing the photogenerated free radical dissipation pathway to activate PCL properties.

Impact of substituents on luminescence and mechanochromism of two novel monometallic copper(I) complexes

Two new mononuclear Cu(I) complexes, [Cu(cbpy)(BINAP)]ClO4 (1) and [Cu(mcbpy)(BINAP)]ClO4 (2), have been synthesized and characterized (cbpy = 6-cyano-2,2ʹ-bipyridine, mcbpy = 4,4ʹ-dimethyl-6-cyano-2,2ʹ-bipyridine, BINAP = 2,2ʹ-bis(diphenylphosphino)-1,1ʹ-binaphthyl). Both Cu(I) complexes are photo-luminescent in the solid state at ambient temperature, with similar emission bands at 620 and 601 nm, respectively. Complex 2 exhibits the mechanochromic luminescence behavior with emission color changing from orange to orange-red, which is mainly due to the breakage of multiple CH⋯O hydrogen bonds between mcbpy-CH and counter-ion ClO4− under grinding. In contrast, the mechanochromic luminescence is not observed in 1 with much weaker cbpy-CH⋯O hydrogen bonds than those of 2. These clearly suggest that the methylation of 2,2ʹ-bipyridyl has an important effect on the mechanochromic luminescence properties of Cu(I) complexes. In addition, the potential applications of 2 have been investigated.