The selection of hydrogen-bonding donors is crucial for the development of stimuli-responsive luminescent materials that rely on weak hydrogen-bonding interactions. In this study, we report two novel dinuclear Cu(I) complexes, [Cu2(μ-η2(N,N),η2(N,N)-dpa)(μ-dppm)2](ClO4) (1) and [Cu2(μ-η2(N,N),η2(N,N)-dpa)(μ-dppa)2](ClO4)·2CH3COCH3 (2), which differ in their diphosphine linkers (CH2 in dppm vs NH in dppa). X-ray crystallography reveals weak CH···O hydrogen bonds between dppm-CH2 and perchlorate-O in 1 and weak NH···O interactions between dppa-NH and acetone-O in 2. The destruction of CH···OClO3- hydrogen bonds in 1, induced by grinding, triggers mechanochromic luminescence. In 2, the disruption of NH···OC(CH3)2 hydrogen bonds, caused by heating and grinding, leads to both thermochromic and mechanochromic luminescence. It is demonstrated that differences in donor-acceptor pairing (CH2/ClO4- vs NH/acetone) influence the nature and responsiveness of hydrogen-bonding interactions. Furthermore, these results offer new insights into the design and synthesis of low-cost, multistimuli-responsive luminescent materials through the modification of hydrogen-bonding donors.

D-π-A type chalcone derivatives modified by carbazole and triphenylamine exhibiting high solid-state fluorescence efficiency and reversible mechanochromic luminescence

Phenothiazine-functionalized cyclic chalcone derivative: aggregation-induced emission enhancement, high-contrast mechanochromic luminescence beyond 100nm, and its applications in latent fingerprint extraction, anti-counterfeiting and information encryption

The arrangement of molecules plays a crucial role in governing the luminescent properties of organic crystals. While quasiracemic crystallization, based on the co-crystallization of two structurally similar but nonidentical enantiomeric molecules, offers a promising strategy for organizing distinct species within a single crystal, its potential for tuning luminescent behavior remains largely unexplored. Herein, we have prepared quasiracemic crystals from a pair of enantiomeric binaphthyl derivatives functionalized with either ethynylnaphthalene or ethynylbenzothiadiazole groups. The molecular arrangement of the quasiracemic crystal is analogous to that of the racemic ethynylnaphthalene derivative, whereas the quasiracemic crystals exhibit mechanochromic luminescence (MCL) behavior similar to that of the racemic ethynylbenzothiadiazole derivative. Notably, the quasiracemic crystals exhibit circularly polarized luminescence (CPL), and the CPL sign is inverted relative to that of the enantiopure crystal of the ethynylbenzothiadiazole derivative. These results highlight quasiracemic crystallization as a new strategy for modulating MCL and CPL properties.

Abstract Fluorescence and phosphorescence are emissions from molecules in the lowest excited singlet state (S1) and the lowest triplet state (T1), respectively. Generally, as the S1 and T1 energy levels are different, the fluorescence and phosphorescence wavelengths are also different. By switching between these emissions through mechanical stimuli, a drastic change in emission color can be achieved. However, organic molecules typically have low intersystem crossing efficiency, making it difficult for them to exhibit phosphorescence. Therefore, organic dyes that exhibit remarkable mechanochromic luminescence (MCL) via the switching between thermally activated delayed fluorescence (TADF) and room‐temperature phosphorescence (RTP) are extremely limited. In this study, heavy‐atom‐free molecules are synthesized. One of them, which has a bulky amino group to a benzodithienoborepin core, exhibited green TADF in solution and the amorphous state, and orange RTP in the crystalline state. This switching between TADF and RTP is enabled by mechanical grinding. Furthermore, DFT calculations and single‐crystal X‐ray structural analysis revealed that the twisting of the bulky amino group in the crystalline state induces the switching between TADF and RTP.

Thia-quasi[7]circulenes (TQCs) are a novel class of π-conjugated molecules exhibiting unique electronic, optical, and electrochemical properties. We report a highly efficient bottom-up synthesis of TQCs via Scholl oxidative cyclization, achieving exceptional yields and structural precision. Single-crystal X-ray analysis confirms their negatively curved geometry, featuring a central antiaromatic heptagonal core and a surrounding aromatic rim, leading to bifacial electronic properties. TQCs demonstrate stable redox behavior with tunable Highest Occupied Molecular Orbital (HOMO)-Lowest Unoccupied Molecular Orbital (LUMO) energy levels. Their remarkable electrochemical stability highlights potential as redox-active materials. TQCs exhibit blue-shifted fluorescence compared to carbo-quasi[7]circulenes, with enhanced quantum yields upon benzoannulation. Additionally, they display mechanochromic luminescence upon grinding, revealing tunable molecular characteristics. These findings establish TQCs as versatile molecular frameworks, bridging fundamental chemistry and next-generation functional materials, making them promising candidates for organic electronics, energy storage, and optoelectronic applications.

Two novel cyclometalated platinum(II) complexes based on 2-phenylpyridine (ppy) and 2,4-difluorophenylpyridine (dfppy) ligands in combination with a benzoylthiourea (4-(decyloxy)-N-((4-(decyloxy)phenyl)carbamothioyl)benzamide, BTU) functionalized with decyloxy alkyl chains as auxiliary ligands were synthesized and characterized for their mechanochromic and photophysical properties. Structural characterization was achieved through IR and NMR spectroscopy, single-crystal X-ray diffraction, and TD-DFT calculations. Both complexes exhibit significant photoluminescence with quantum yields up to 28.3% in a 1% PMMA film. The transitions in solution-phase spectra were assigned to mixed metal-to-ligand (MLCT) and intraligand (ILCT) charge-transfer characteristics. Temperature-dependent studies and thermal analyses confirm reversible phase transitions without mesomorphic behavior despite the presence of the two long alkyl chains. Both complexes displayed reversible mechanochromic and solvatochromic luminescence, with a change in emission color from green to red-orange emissions upon grinding and solvent treatment or heating at 80 °C.

Abstract Anthracene-d- and l-phenylalanine compounds (d-1, l-1) were obtained in high yields by the condensation reactions of 9-anthraldehyde with d- and l-phenylalanine and reduction with sodium borohydride. Both compounds d-1 and l-1 show luminescence in solution and solid states, exhibiting dual-state emission property. Solid samples of d-1 and l-1 exhibit mechanochromic luminescence enhancement with grinding. In respect of chiroptical property, d-1 and l-1 display mirror-image circular dichroism spectra in solution, and solid samples in KBr pellets exhibit circularly polarized luminescence with a dissymmetry factor of −1.4 × 10−4/6.0 × 10−4, respectively. The two compounds can differentiate enantiomers of free amino acids such as phenylalanine, lysine, glutamic acid and folic acid, in aqueous phase.

Four bis-alkynyl Pt(II) complexes [Pt(dtbpy)(C2-L-P(O)Ph2)2] with dtbpy = 4,4'-ditertbutyl-2,2'-bipyridine and alkynylphosphine oxide ligands (L = no linker, Pt0; phenyl, Pt1; biphenyl, Pt2; naphthyl, Pt3) have been synthesized and fully characterized by spectroscopic methods and single crystal XRD analysis. It has been found that the nature of the π-conjugated linker is a key factor in fine-tuning the emission energy of the complexes in solution and in achieving the aggregation-induced phosphorescence enhancement (AIPE) effect for complex Pt0 with the most compact linker. Phosphine oxide fragment, which can be involved in weak intermolecular interactions, promotes the existence of two solid forms with different luminescence properties. These two forms can be switched from one to another upon grinding, thus featuring distinct mechanochromic luminescence properties. TDDFT calculations are consistent with the experimental results and assign mixed 3MLCT and 3LL'CT solution emission character and 3MMLCT and 3LL'CT emission nature in supramolecular dimeric structures.

Mechanochromic luminescence molecules show great appeal in the realm of intelligent luminescent materials but face great challenges from aggregation-caused quenching and/or amorphization during mechanical processing. Integrating aggregation-induced emission (AIE) luminogens into the architecture of highly crystalline metal-organic frameworks (MOFs) could potentially address these issues. In this work, two isomorphic Zn-MOFs ({[Zn(TCPE)0.5(Lx)0.5]·guests}n, where H4TCPE = 1,1,2,2-tetra(4-carboxylphenyl)ethylene, L1 = triethylenediamine, and L2 = piperazine) are synthesized via a multiligand assembly approach. The mechanochromic luminescence behaviors observed in these Zn-MOFs have been thoroughly analyzed, with a focus on the influence of linker effects. Under mechanical grinding, the lattice contraction of Zn-MOFs is accompanied by the distortion of the benzene rings within TCPE4-, leading to altered intramolecular twisted charge transfer within the Zn-MOFs, which subsequently changes their luminescence properties. The potential application of this luminescence behavior in light-emitting diodes was preliminarily explored.

The plastic zone shields the crack tip from high stress and plays an important role in the fracture of structures. Determination of the plastic zone is a significant challenge in large-scale and complex structures. In the present work, a detection method using mechanochromic luminescent (MCL) sensing film has been proposed to detect the plastic zone near the crack tip. The deformation near the crack tip is converted into visible green fluorescence emission. A comprehensive post-processing protocol and a feature quantification scheme for fluorescence images are introduced. A significant correlation is obtained between the characteristic values of fluorescence images and the parameters of the plastic zone (i.e., maximum equivalent strain and plastic zone size), indicating that the fluorescence response provides effective characterization parameters within the forward model. The plastic zone parameters determined using the MRL-based method agree well with the results measured using the DIC method. This indicates that the plastic zone near the crack tip can be effectively analyzed by capturing loading conditions and fluorescence response.

Frontispiz: Contrasting Mechanochromic Luminescence of Enantiopure and Racemic Pyrenylprolinamides: Elucidating Solid‐State Excimer Orientation by Circularly Polarized Luminescence

Frontispiece: Contrasting Mechanochromic Luminescence of Enantiopure and Racemic Pyrenylprolinamides: Elucidating Solid‐State Excimer Orientation by Circularly Polarized Luminescence

We report synthesis and optical properties of the dimers of boron β-dialdiminate (DAI) complexes with different connection manners. The series of dimers, o-, m-, p- and d-BisDAI, were prepared, and their photophysical properties were compared with the single unit, DAI. It was found that the dimers show characteristic optical properties: o-BisDAI showed bathochromic shifts in absorption and emission spectra and emission quenching because of the through-space interactions, while the photophysical properties of m-BisDAI and DAI were almost identical. The extension of π-conjugation in p-BisDAI could induce not only the bathochromic shifts of its absorption and emission bands but also the solvatochromism originating from the symmetry-breaking charge transfer character. d-BisDAI showed the weak low-energy emission in solution and the mechanochromic luminescence which might stem from the rotational degree of freedom between the adjacent two DAI units.

This mini-review highlights the transformative potential of benzothiazole (BTz)- and benzoxazole (BOz)-based boron-complexed dyes. It represents an innovative evolution of the classic boron-dipyrromethene (BODIPY) structure, which is well established for its superior photophysical properties. Incorporating BTz- or BOz-ligands into the borane (-BR2) component, originates more electron-deficient architecture, enabling novel modes of complexation and addressing limitations such as spectral overlap and self-quenching in traditional BODIPY dyes. The review focuses on the remarkable versatility of boron-benzothiazole (BOBTz)- and boron-benzoxazole (BOBOz)-based complexes, particularly in three rapidly advancing fields: organic light emitting diode (LED) technology, bioimaging, and mechanochromic luminescence (MCL). Over the past 15 years, these complexes have demonstrated exceptional adaptability, showcasing enhanced properties like high fluorescence quantum yields, large molar extinction coefficients, and tunable emissions across visible and near-infrared spectra. The insights described in this review highlight the major role of BOBTz- and BOBOz-complexes in shaping innovative, and sustainable advanced materials while addressing emerging challenges in modern materials science. Besides, the refining of both BOBTz- and BOBOz-complexes offers exciting prospects for technological challenges such as energy-efficient lighting, non-invasive imaging, and creating stimuli-responsive materials for next-generation sensors. Moreover, the environmental sustainability of these materials, including green synthesis approaches and recyclable components represents an important frontier for future exploration.

Abstract Circularly polarized luminescence (CPL) and mechanochromic luminescence (MCL) have independently made substantial progress in recent years. However, the exploration of MCL in solid‐state CPL materials, which holds practical significance, is still in its infancy. Herein, we report the MCL properties of readily accessible chiral pyrenylprolinamides bearing tert‐butoxycarbonyl (Boc) or 2,2,2‐trichloroethoxycarbonyl (Troc) groups. Enantiopure crystals of the Boc derivative display a greater MCL wavelength shift than racemic crystals, while the Troc derivative exhibit the opposite trend. Most notably, the enantiopure crystals show mechanochromic CPL. Unlike in previous examples, where CPL is quenched upon amorphization, robust CPL spectra were observed even in the amorphous states. By applying the excimer chirality rule, we have, for the first time, acquired insights into the excited‐state structures within mechanically generated amorphous states. These findings offer a novel design strategy for developing mechanochromic CPL materials, paving the way for the future advancements in this emerging field.

Circularly polarized luminescence (CPL) and mechanochromic luminescence (MCL) have independently made substantial progress in recent years. However, the exploration of MCL in solid-state CPL materials, which holds practical significance, is still in its infancy. Herein, we report the MCL properties of readily accessible chiral pyrenylprolinamides bearing tert-butoxycarbonyl (Boc) or 2,2,2-trichloroethoxycarbonyl (Troc) groups. Enantiopure crystals of the Boc derivative display a greater MCL wavelength shift than racemic crystals, while the Troc derivative exhibit the opposite trend. Most notably, the enantiopure crystals show mechanochromic CPL. Unlike in previous examples, where CPL is quenched upon amorphization, robust CPL spectra were observed even in the amorphous states. By applying the excimer chirality rule, we have, for the first time, acquired insights into the excited-state structures within mechanically generated amorphous states. These findings offer a novel design strategy for developing mechanochromic CPL materials, paving the way for the future advancements in this emerging field.

Abstract A platinum complex that contains benzoselenophene and 2,6‐dichlorophenyl isocyanide groups [Pt(BS)2(DCPI)2; BS=2‐benzoselenophenyl, DCPI=2,6‐dichlorophenyl isocyanide; 1] forms crystalline samples that show blue or green emission. The blue‐emitting crystals showed luminescent mechanochromism, affording a green‐emitting powder due to a grinding‐induced crystal‐to‐amorphous phase transition. Despite the different emission colors of the green‐ and blue‐emitting crystals, their powder X‐ray patterns are similar, suggesting that the green‐emitting crystals contain both the blue‐emitting crystal domain and a small amount of an amorphous phase, between which energy transfer occurs to achieve the green photoluminescence from the minor domain.

MicroED reveals insights into mechanochromic luminescence in copper nanoclusters, demonstrating structural transitions from crystalline to amorphous state upon grinding, with restoration of crystallinity following solvent exposure. Luminescence switching occurs due to reversible amorphization-recrystallization rather than chemical changes, providing critical design principles for such stimuli-responsive materials.

Nowadays, benzimidazole and its derivatives are widely assembled into multifunctional materials with various properties such as mechanochromism, photochromism, thermochromism and electrochromism. Herein, two novel zinc(II) coordination compounds, [Zn2(L2)Br4]·2H2O (1) and [Zn2(L2)Cl4]·2H2O (2) (L2 = tetra(1H-benzo[d]imidazol-2-yl)ethene), have been constructed via one-pot facile synthesis from bis(1H-benzo[d]imidazol-2-yl)methane (L1) and zinc(II) salts. The ligand L2 with a CC double bond was in situ formed by C-C coupling of two sp3-C atoms of L1 in solvothermal synthesis, which provides a new strategy to generate the conjugation system conveniently. Impressively, complex 1 shows distinct fluorescence and phosphorescence dual emission due to the heavy atom effect of bromide ions. Complexes 1 and 2 exhibit high-contrast mechanochromic luminescence properties due to the transformation from a crystalline state to an amorphous state. In addition, they also exhibited excitation wavelength-dependent luminescence in crystalline states and DMF systems. As the excitation wavelength increased, their luminescence color in DMF solution changed significantly from blue to yellow. Theoretical calculations show that the complexes have multiple emission centers due to the locally excited nature and intramolecular charge transfer. Due to their excellent excitation wavelength-dependent luminescence properties, PMMA films based on complexes 1 and 2 were prepared and could be applied in anti-counterfeiting and UV protection.