Mechanochromic Luminescence Research Articles

Ligand Detachment—New Insight into the Mechanochromic Luminescence Mechanism of Copper Iodide Complexes with Thermally Activated Delayed Fluorescence

AbstractMechanochromic luminescence materials have recently attracted increasing interest due to its promising applications, but the intensive inquiry of the mechanochromic luminescence mechanism is still insufficient. Herein, an in‐depth study of mechanochromic luminescent copper iodide complexes exhibiting ligand detachment and structural transformation is reported. The yellowish–green emissive CuI(dmpz) (dmpz = 2,6‐dimethylpyrazine) is ground into orange–red emissive CuI(dmpz)0.5 and the detailed structural transformation process is revealed by the thorough analysis of the crystalline structure before and after grinding. More importantly, new insight into the mechanochromic luminescence mechanism is proposed, that is, under the external force of grinding, the dmpz ligand detachment triggers the structural transformation from 1D chain to 2D network, which leads to the reduction of band gap energy and contraction of Cu─Cu bonds, together resulting in the red shift of luminescence ultimately. Moreover, the two complexes are also found to have thermochromic luminescence properties, attributing to their thermally activated delayed fluorescence process. The anti‐counterfeiting application of CuI(dmpz) is tentatively explored, demonstrating the favorable application prospect of CuI(dmpz). This study provides new insights involved in the mechanochromic luminescence mechanism of copper iodide complexes, and may provide a new understanding for rational molecular design and structural regulation of intelligent responsive luminescence.

TPE[2]arenes: Luminescent macrocyclic arenes with aggregation-induced emission and Al3+ detection

Luminescent macrocyclic arenes have attracted interest owing to their unique cyclic geometry and adaptive emission. Herein, a series of fluorescent macrocyclic arenes, denoted as TPE [2]arenes, were easily synthesized by the one-pot condensation of tetraphenylethene (TPE) moiety and paraformaldehyde in the presence of BF3·OEt2 and readily derivatized to yield cyclic arenes with diverse substitution patterns. The macrocyclic arenes shared similar cyclic skeleton with resorcin [4]arene, and existed as two enantiomers with boat conformation confirmed via single crystal analysis. Furthermore, TPE [2]arenes all retained the aggregation-induced emission properties owing to the incorporation of TPE moiety and obvious mechanochromic luminescence behavior due to the relative flexible skeleton. The carboxylic acid substituted analogues, namely TPE [2]arene-COOH, exhibited specific “turn-on” detection of Al3+ ion in aqueous media.

A Reversible Mechanochromic AIEgen Based on Triphenylamine for the Selective Detection of Vitamin B2 (Riboflavin) in Aqueous Media and Histotoxicity.

(E)-Ethyl 2-cyano-3-(4'-(diphenyl amino)-[1,1'-biphenyl]-4-yl) acrylate (RSJ) is a novel luminogen based on triphenylamine. It has been fully synthesized and characterized, exhibiting an incredible photophysical phenomenon known as aggregation-induced emission (AIE). This work describes a fluorescent sensor that detects vitamin B2 in mixed aqueous media with high selectivity and a low limit of detection as well as a mechanism for reversible mechanochromic luminescence. Moreover, the molecule was validated for its nontoxicity in water using a histotoxicological study. Fish subjected to two different concentrations of the "novel luminogen" that displayed photophysical phenomena during sensing of vitamin B2 (riboflavin) in mixed aqueous media did not exhibit any significant differences in the structural makeup of their liver, kidney, gills, brain, and muscle tissues when compared with the control group.

AIE-active dyes based on Tröger's base: Synthesis and mechanochromic luminescence

Three fluorescent dyes (DI-TB, ICZ-TB and ITPA-TB) were synthesized based on 1,3-indindanedione substituted Tröger's base (TB). All the dyes exhibited AIE properties in the mixed system of chloroform and n-hexane. The compounds showed reversible MCL properties with obvious fluorescence changes, which were confirmed by XRD and DSC. The fluorescence of ITPA-TB showed high contrast (68 nm) before and after grinding, and the fluorescence changed from yellow (546 nm) to red (614 nm). In addition, the three solid compounds showed good thermal stability by thermogravimetric analysis. The TB dyes further opened up new ideas for the development of TB-based mechanochromic luminescent materials.

Tetraphenylethylene fused pyrrolo[3,2-b]pyrrole derivatives with reversible mechanofluorochromic and aggregation-induced emission enhancement characteristics

Modification on typical of molecular unit with aggregation-induced emission (AIE)/aggregation-induced emission enhancement (AIEE) can play a important role in obtaining solid-state fluorescence materials and mechanofluorochromic (MFC) materials. We utilize the multicomponent reaction of TPE-containing aldehyde, arylamine and butane-2,3-dione in the presence of iron(III) perchlorate to design and synthesize two novel MFC materials by combination of the tetraphenylethylene (TPE) and pyrrolo [3,2-b]pyrrole core units. Investigation of CHW-1 and CHW-2 aggregation-induced emission behaviors indicated that these materials all exhibited AIEE properties. Subsequent mechanochromic luminescence tests exhibited reversible mechanochromism natures that involve fluorescent color change from green to yellow (Grinding-induced spectral shift (△λ): 35 nm and 38 nm, respectively). Then the datas (including SEM, PXRD and DSC) revealed that this mechanofluorochromism was ascribed to the transformation of crystal state to the amorphous form. Moreover, CHW-1 and CHW-2 could be applied for optical recording devices and security printing.

Tunable mechanochromic luminescence of benzofuran-fused pyrazine: effects of alkyl chain length and branching pattern

Tuning the alkyl chains of bisbenzofuropyrazines allowed control over the on/off switching of mechanochromic luminescence and self-recovery rates.

Multistimuli-responsive multicolor solid-state luminescence tuned by NH-dependent switchable hydrogen bonds.

Revealing the stimuli-responsive mechanism is the key to the accurate design of stimuli-responsive luminescent materials. We report herein the multistimuli-responsive multicolor solid-state luminescence of a new dicopper(I) complex [{Cu(bpmtzH)}2(μ-dppa)2](ClO4)2 (1), and the multistimuli-responsive mechanism is clarified by investigating its four different solvated compounds 1·2CH3COCH3·2H2O, 1·2DMSO·2H2O, 1·4CH3OH, and 1·4CH2Cl2. It is shown that luminescence mechanochromism is associated with the breakage of the hydrogen bonds of bmptzH-NH with counter-ions such as ClO4- induced by grinding, while luminescence vapochromism is attributable to the breaking and forming of hydrogen bonds of dppa-NH with solvents, such as acetone, dimethylsulfoxide, and methanol, caused by heating and vapor fuming. In addition, those results might provide new insights into the design and synthesis of multistimuli-responsive multicolor luminescent materials by using various structure-sensitive functional groups, such as distinct N-H ones, to construct switchable hydrogen bonds.

Ring-size dependent ratiometric photoluminescence of cyclophane mechanophores

Ring size determines the mechanochromic luminescence properties of cyclophane mechanophores whose mechanism relies on the dissociation of the CT complexes between a 9,10-bis(phenylethynyl)anthracene and a pyromellitic diimide.

Near-infrared aggregation-induced emission characteristics of new o-carborane fluorophores with large stokes shifts and self-recovering mechanochromic luminescence

o-Carborane fluorophores with large Stokes shifts (>14 000 cm−1) near-infrared AIE characteristics and self-recovering mechanochromic luminescence.

Mechanochromic aromatic hydrocarbons that bear one simple substituent.

Structurally simple aromatic hydrocarbons that possess only one isocyano group show luminescent mechanochromism. The structural isomers of these aromatic hydrocarbons exhibit blue- and red-shifted emission bands upon mechanical stress. Their low molecular weight enables their sublimation under mild conditions.

Realizing mechano-tunable dual emission in a twisted thianthrene derivative

The twisted molecule, TATPO, which exhibited dual emissions of fluorescence (FL) and room-temperature phosphorescence (RTP) in both crystal types, were designed, synthesized and characterized. The dual-emissive bands were combined by fluorescence (ca. 439 nm) and phosphorescence (ca. 514 nm). Intriguingly, these two kinds of luminescence ratios (FL and RTP) can be tunable by external stimuli, such as grinding or fuming. Experimental and theoretical studies have shown that twisted molecular structure and the folded dihedral angle of thianthrene can promote spin-orbit coupling (SOC) and thus enhance phosphorescence. In addition, the folded dihedral angle of thianthrene group is the main reason that this molecule could realize tunable luminescence between FL and RTP. This work not only described a twisted molecule for pure organic multifunctional materials with mechanochromic luminescence properties, but also provided a novel strategy to design stimuli-responsive materials with tunable FL/RTP ratios.

Promoting intersystem crossing by charge transfer state of dimers for persistent room temperature phosphorescence

Efficient intersystem crossing (ISC) is essential for phosphorescence emission. The charge transfer (CT) state is traditionally constructed by highly twisted donor-acceptor type structures to facilitate the ISC process. However, the highly twisted structural properties limit the diversity of room-temperature phosphorescence (RTP) molecules. By establishing the CT state in the intermolecular dimers structure, the dependence on the distorted donor-acceptor in the single-molecule design can be well eliminated. Based on this strategy, three novel pure organic luminescent molecules with long afterglow and mechanochromic luminescence (MCL) property were designed and synthesized. All of those exhibited triple emission, including prompt fluorescence (PF), delayed fluorescence (DF) and persistent RTP (pRTP) emission. In particular, CzC4-PhCN showed great potential for application in the field of anti-counterfeiting with its ultralong phosphorescent lifetime of 862 ms. Single crystal analysis and theoretical calculation revealed that the formation of CT states of intermolecular dimers effectively reduced ΔEST and promoted ISC process. This study would be highly instructive for realizing efficient ISC process and high-efficiency pRTP emission in pure organic systems.

Achieving aggregation-induced emission and remarkable mechanochromic luminescence based on a tetraphenylethene modified quinoline derivative

A simple tetraphenylethene (TPE) modified quinoline derivative TPE-QL was designed and prepared. It possessed donor-acceptor molecular structure and highly twisted conformation, which could be proved by spectral studies and theoretical calculatiuons. Due to the introduction of TPE unit, TPE-QL exhibited typical aggregation-induced emission (AIE) characteristic. Interestinly, TPE-QL also showed obvious fluorescence response to mechanical force by changing the emission wavelength from 440 nm to 485 nm upon grinding. Moreover, reversible mechanochromic behavior was detected after repeated grinding and fuming treatment. This work not only discovered a new AIE-active mechanochromic material, but also provided possibility for its application in the filed of ink-free rewritable papers for security decumentation purposes.

Investigation of Mechanochromic Luminescence of Pyrene-based Aggregation-Induced Emission Luminogens: Correlation between Molecular Packing and Luminescence Behavior.

To better understand the correlation between molecular structure and optical properties such as aggregation-induced emission (AIE) and mechanochromic luminescence (MCL) emission, two new pyrene-based derivatives with substitutions at the 4- and 5-positions (1HH) and at the 4-, 5-, 9-, and 10-positions (2HH) were designed and synthesized. Cyano groups were introduced at the periphery of the synthesized compounds (1HCN, 1OCN, 1BCN, 2HCN, 2OCN, and 2BCN) to investigate the influence of these groups on the emission properties of the pyrene derivatives both in solution and in the solid state. The fluorescence emission performance of these compounds in water/acetone mixtures was simultaneously studied, revealing outstanding aggregation-induced emission properties. The typical shift in emission maxima to higher values was attributed to J-aggregate formation in the aggregate state. Careful investigation of the crystal structures demonstrated abundant and intense intermolecular interactions, such as C-H…π and C-H…N hydrogen bonds, contributing to the remarkable mechanochromic luminescence performance of these compounds. The MCL properties of all the compounds were investigated using powder X-ray diffraction, and the remarkable mechanochromic properties were attributed to J-aggregate phenomena in the solid state. These results provide valuable insights into the structure-property relationship of organic MCL materials, guiding the design of efficient organic MCL materials.

A multifunctional emitter with synergistical adjustment of rigidity and flexibility for high-performance data-recording and organic light-emitting devices with “hot exciton” channel

An intramolecular dual-locking design is proposed for organic luminescent materials, aiming at synergistically adjusting the molecular rigidity and flexibility for applications in data storage and light-emitting devices. According to this concept, a multifunctional donor–acceptor type emitter, LPCz-TRZ, is developed, featuring a “soft” diphenylmethane lock and a “hard” single-bond lock in its donor moiety. The highly locked structure results in appreciable rigidity that endows LPCz-TRZ with high luminescence efficiency, narrow emission, promoted charge conductivity and triplet harvesting “hot exciton” channels, allowing high-performance deep-blue organic light-emitting diodes (OLEDs) with an external quantum efficiency as high as 7.88%, which is among the best deep-blue “hot exciton” OLEDs. Notably, the structure of LPCz-TRZ is semi-flexible, leading to unique mechanochromic luminescence (MCL) behavior. Compared to the conventional MCL emitters, the orchestration of rigidity and flexibility in LPCz-TRZ affords high-contrast, fatigue-resistance MCL, as evidenced by a simple, rewritable data-recording demonstration with negligible fatigue after up to 15 wiring-erasing cycles.

Crystal structure and luminescent mechanochromism of a quinoline-appended acylhydrazone ligand and its Zn(II) complex

In the work, a quinoline-appended acylhydrazone H2L was designed and prepared via a Schiff base condensation reaction of quinoline-2-formaldehyde and salicylhydrazide. Further, a new zinc(II) complex, [Zn(HL)2]·H2O was synthesized by the reacting of the ligand H2L and Zn(NO3)2·6H2O in dichloromethane/ethanol solution. The ligand and its Zn(II) complex were elucidated via UV–Vis, FT-IR, DFT calculation, Hirshfeld surfaces, MEP & IRI analyses, and X-ray single-crystal diffractions. The results of Hershfield surface analysis showed that there are obvious hydrogen bond interactions and CH···π mutual effect in the Zn(II) complex. In addition, solid-state photoluminescence properties indicated that H2L and its Zn(II) complex represent uncommon luminescent mechanochromism tuned via intermolecular hydrogen bonds.

Mechanochromism and circularly polarized luminescence of chiral cyclometalated platinum(II) complexes with different configurations and aggregated forms

Using a new synthesis method, we have prepared two different coordination configurations of chiral cyclometalated N^C^N-coordinated and C^N-coordinated Pt(II) complexes by controlling the Cl− source. Single-crystal X-ray diffraction determines the tridentate and bidentate structures. All N^C^N-coordinated and C^N-coordinated Pt(II) complexes emit green-yellow light in CH2Cl2 solutions. Square-planar N^C^N-Coordinated complexes (−)-1 and (−)-2 show remarkable aggregation-induced emission behaviors in mixed CH2Cl2/n-hexane solutions with the emission quantum yield (Φem) intensified significantly. In addition, the luminescent color could change from green-yellow to red by varying the volumetric fraction of n-hexane. Complex (−)-2 grafted with long alkyl chains could form helical stacking aggregates, inducing enhanced ECD signals and measurable CPL activity. The yellow and orange solids of complex (−)-1 can be separated, and a high-contrast mechanochromic luminescence is found for the yellow form. Furthermore, the potential utility has been attempted in anti-counterfeiting applications based on the yellow solid of (−)-1. This study provides a new design strategy for the preparation of chiral cyclometalated Pt(II) complexes with specific configurations and aggregated forms that have various optical and electronic properties.

Intramolecular hydrogen bonding-induced conformation-locked multifunctional emitters for non-doped OLEDs

Organic multifunctional triplet-harvesting emitters based organic light emitting diodes (OLEDs) are a relatively unexplored area, primarily due to the challenging requirements of achieving high rates of intersystem crossing (ISC; kISC > 107 s−1), reverse intersystem crossing (RISC; kRISC > 106 s−1), radiative decay (kr > 107 s−1), and low non-radiative decay (knr < 106 s−1) within non-doped systems. In this study, we synthesized three asymmetric heavy-atom-free (HAF) multifunctional triplet-harvesting emitters. The incorporation of intramolecular hydrogen bonding (IHB) induced conformational locking and distortion, resulting in a quasi-planar structure that effectively reduces reorganization energy (λ). Consequently, Qx-Py-2DMAC exhibited improved radiative characteristics with high quantum yields (0.94) in neat film state. Significantly, the existence of IHB interactions was confirmed through various methods, including DFT calculations, single-crystal XRD, and 2D-GIWAXD measurements, which verified the effect in both single molecules and solid states. Subsequently, the IHB-induced conformation-locked structure exhibited a preferred orientation with a large coherence length (Lc) for dense packing and efficient emission in aggregated state. Consequently, due to the IHB-induced unique aggregation properties, Qx-Py-2DMAC realized significant multifunctionality of aggregation-induced emission (AIE), mechanochromic luminescence (MCL), thermally activated delayed fluorescence (TADF), and room-temperature phosphorescence (RTP) in solid state. In this regard, non-doped OLEDs were successfully realized.

Alkynylphosphonium Pt(II) Complexes: Synthesis, Characterization, and Features of Photophysical Properties in Solution and in the Solid State.

A series of heteroleptic bis-alkynyl-diimine mononuclear Pt(II) complexes with alkynylphosphonium and di-tert-butyl-2,2'-bipyridine (dtbpy) ligands have been prepared and characterized by spectroscopic methods and single-crystal XRD. The Pt(II) complexes obtained in the present study demonstrate triplet emission in solution, which originates from 3MLCT/3LC states where the nature of the π-conjugated linker in the alkynylphosphonium ligand manages the contributions of each transition, and this conclusion is supported by DFT calculations. Additionally, the presence of the phosphonium group connected to alkynyl through the π-conjugated linker enhances nonlinear optical properties of the Pt(II) complexes increasing two-photon absorption cross section up to 400 GM. In the solid state, the Pt(II) complexes demonstrate emission that is attributed to 3MMLCT transitions due to the presence of Pt-Pt metallophilic interactions, and the reversible assembly and disassembly of these interactions by grinding and solvent treatment are responsible for the mechanochromic luminescence. It has been experimentally shown that stimuli-responsive emission of the Pt(II) complexes is the result of a "monomer/dimer" transformation; this conclusion is confirmed by DFT calculations for discrete complexes and different dimers with or without Pt-Pt interactions.

Stimuli-Responsive Properties on a Bisbenzofuropyrazine Core: Mechanochromism and Concentration-Controlled Vapochromism.

Stimulus-responsive organic materials with luminescence switching properties have attracted considerable attention for their practical applications in sensing, security, and display devices. In this paper, bent-type bisbenzofuropyrazine derivatives, Bent-H and Bent-sBu, with good solubilities were synthesized, and their physical and optical properties were investigated in detail. Bent-H gave three crystalline polymorphs, and they showed different luminescence properties depending on their crystal packing structures. In addition, Bent-H exhibited mechanochromic luminescence in spite of its rigid skeleton. Bent-sBu exhibited unique concentration-dependent vapochromic luminescence. Ground Bent-sBu was converted to blue-emissive, green-emissive, and green-emissive high-viscosity solution states at low, moderate, and high concentrations of CHCl3 vapor, respectively. This finding represents a concentration-dependent multi-phase transition with an organic solvent, which is of potent interest for application in sensing systems.