Mechanochromic Luminescence Research Articles

Cationic Zinc(II) Complexes with Carbazole-Type Counter-Anions: Intracomplex Donor/Acceptor Pairs Affording Exciplexes with Thermally Activated Delayed Fluorescence.

Two cationic zinc(II) complexes with carbazole-type counter-anions, namely, [Zn(tpy)2]2+[CAZ-p-BF3-]2 (Zn-p) and [Zn(tpy)2]2+[CAZ-o-BF3-]2 (Zn-o), have been designed and synthesized, where tpy is 2,2':6',2″-terpyridine, CAZ-p-BF3- is 4-((9H-carbazol-9-yl)phenyl)trifluoroborate, and CAZ-o-BF3- is (2-(9H-carbazol-9-yl)phenyl)trifluoroborate. The complex cation [Zn(tpy)2]2+ (as the acceptor) and the carbazole-type counter-anion CAZ-p-BF3- or CAZ-o-BF3- (as the donor) form an intracomplex donor/acceptor pair. Single-crystal structures reveal that compared to Zn-p, Zn-o exhibits a stronger π-π stacking interaction between the carbazole group (as the donor unit) of the counter-anion and the tpy ligand (as the acceptor unit) of [Zn(tpy)2]2+ because of the different anchoring position of the BF3- anion in the counter-anion. In a doped film, Zn-p and Zn-o afford an isolated exciplex formed between the carbazole group and the tpy ligand within the single complex, which gives green-yellow emission with a thermally activated delayed fluorescence (TADF) feature. In crystalline states, Zn-p and Zn-o afford exciplexes with blue emission centered at 468 nm and green-blue emission centered at 508 nm, respectively. The Zn-p crystalline sample shows a relatively large singlet-triplet energy gap (ΔEST) (0.33 eV) and no TADF, whereas the Zn-o crystalline sample exhibits a small ΔEST (0.06 eV) and distinct TADF, with a reverse intersystem crossing rate at 3.3 × 105 s-1. Zn-p and Zn-o both exhibit intriguing mechanochromic luminescence, with largely red-shifted (by over 70 nm) emission and modulated TADF properties upon mechanically grinding the crystalline samples. The work demonstrates that donor/acceptor pairs affording exciplexes can be formed within cationic metal complexes using counter-anions with donor nature, which opens a new avenue toward photo-active metal complexes with rich photophysical properties.

Two Calcium Coordination Polymers with Multi‐Stimuli‐Responsive Properties

AbstractTwo skeleton isomers, [CaL(H2O)(DMF)2]•DMF (1) (H2L=5‐(1,3‐dioxo‐1H‐benzo[de]isoquinolin‐2(3H)‐yl)isophthalic acid) with a 1 dimensional chain structure and [CaL(DMF)1.72]• (DMF)0.28 (2) with a 2 dimensional layer structure were synthesized. Mechanochromic luminescence studies reveal that 1 and 2 exhibit luminescent red‐shift and blue‐shift respectively under grinding stimuli. The different influence of grinding stimuli on the π⋅⋅⋅π interaction in the two structures are proposed to be the main reason for such different responses. In addition, 1 has a rare broad sensing ability for halogenated hydrocarbons and can distinguish seven halogenated hydrocarbons out of 17 solvents. 1 can also be used for rapid detection of trace water in DMF with a calculated detection limit of 0.0078 v/v %. The relevant recognition mechanism is the decomposition of the structure during the recognition process due to the water instability of the ionic bond. The above results reveal that both compounds have the potential to be used as multi‐stimulus‐responsive materials.

Bisisocyanide cyclometallated platinum(II) complexes: synthesis, structure, photophysical properties, and mechanochromic behavior

A series of cyclometallated platinum(II) complexes [Pt(ppy){CNAr}2]X with two isocyanide ligands (Hppy = 2-phenylpyridine, Ar = C6H2-2,4,6-Me3, C6H3-2-Cl-6-Me , C6H3-2,6-Cl2, C6H4-4-NMe2, C6H4-4-Me, C6H4 4-Cl, C6H4-4-Br, C6H4-4-I, C6H4-4-CF3, C6H4-3-CF3; X = BF4, OTf) was synthesized by the reaction of the [{Pt(ppy)Cl}2] dimer with isocyanides (yield 52-70%). The structure of the resulting complexes was determined using mass spectrometry, 1H, 13C{1H}, 195Pt{1H}, 1H-1H COSY, 1H-1H NOESY, 1H-13C HSQC, and 1H-13C HMBC NMR spectroscopy in solution and solid-state CP/MAS 13C and 195Pt NMR spectroscopy, IR spectroscopy and X-ray diffraction analysis in the solid phase. The photophysical properties of the obtained complexes in the solid phase and the mechanochromic luminescence behavior were studied. In the solid phase, all synthesized compounds phosphoresce in the green or orange range of visible light, while photoluminescence quantum yields reach 26%. Green phosphors exhibit a reversible mechanochromic luminescence change achieved by mechanical grinding (green to orange) and solvent adsorption (orange to green).

Molecular engineering to enhance the reactive oxygen species generation of AIEgens and exploration of their versatile applications.

Fluorescent dyes with aggregation-induced emission (AIE) characteristics have shown potential applications in the fields of biological imaging, photodynamic therapy and photothermal therapy, in which photosensitizers (PSs) play a crucial role. However, how to design high-quality PSs with high reactive oxygen species (ROS) generation efficiency remains unclear. In this contribution, an effective molecular design strategy to improve the ROS generation efficiency of AIE PSs was proposed. A series of tetraphenylethylene derivatives containing the pyridine ring or pyridinium with different substituents were designed and synthesized. All the molecules were weakly emissive when molecularly dissolved in solution but displayed intense emission upon aggregation, demonstrating a phenomenon of AIE characteristic. Pyridinium molecules could be used as visualization agents to specifically stain the mitochondria in living cells, while most of the molecules failed to generate ROS upon white light irradiation. In contrast, TPE-Pys-BP containing benzophenone produced ˙OH and 1O2 efficiently in the presence of light due to its large spin-orbit coupling constant to promote efficient intersystem crossing. Such a property allowed TPE-Pys-BP to serve as a PS to kill cancer cells using photodynamic therapy. TPE-Pys-BP also exhibited mechanochromic luminescence (ML), and its emission could be reversibly switched between two distinct colors through repeated grinding and fuming processes. A security paper was fabricated using the ML properties of TPE-Pys-BP.

Solution-state mechanochromic luminescence of Pt(ii)-complexes displayed within micellar aromatic capsules

Whereas mechanochromic luminescence (MCL) is observed only in the solid state, solution-state MCL from Pt(ii)-complexes was generated upon encapsulation by aromatic capsules. The resultant host–guest composite exhibited strong red MCL (Φ = 33%) via metal–metal interactions in water.

Pressure-shortened delayed fluorescence lifetime of solid-state thermally activated delayed fluorescent 4CzIPN: the structure evolution.

The incorporation of mechanochromic luminescence into thermally activated delayed fluorescence (TADF) molecules is a promising strategy for developing multifunctional mechanochromic luminescent materials. Nevertheless, due to the difficulties in systematic design, it is still challenging to controllably exploit the versatility of TADF molecules. In this work, we were surprised to find that the delayed fluorescence lifetime of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene crystals was continuously shortened with increasing pressure, which was ascribed to the increasing HOMO/LUMO overlap by planarization of the molecular conformation, as well as the pressure-induced emission enhancement and obvious multicolor emission at high pressure (green to red), owing to the formation of new interactions and the part-planarization of the molecular conformation, respectively. This study not only developed a new function of TADF molecules, but also provided a route to shorten the delayed fluorescence lifetime, which is beneficial for designing TADF-OLEDs with small efficiency roll-off.

Engineering TADF, mechanochromism, and second harmonic up-conversion properties in regioisomeric substitution space

Fine-tuning of TADF, mechanochromic luminescence (MCL), and second harmonic generation (SHG) properties among three donor–acceptor based regio-isomers have been explored in this research article based on both theoretical and experimental outcomes.

A convenient route to a vinylogous dicyano aryl based AIEgen with switchable mechanochromic luminescence properties.

Molecules with solid state luminescence and mechanochromic luminescence properties have attracted immense interest owing to their potential application in the areas of organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), optoelectronic devices, fluorescence switches, mechano-sensors and data storage. Herein we report a convenient two step synthetic protocol to obtain a couple of luminescent molecules. Using these, a comparative study has been performed to showcase the importance of the weak π⋯π interactions to observe the aggregation induced emission (AIE) and solid-state mechanochromic luminescence. The most fascinating part of this report is to observe the switchable fluorescent dark and bright states of the solid AIEgen. We have also demonstrated the use of the AIEgen to detect volatile organic compounds.

Solvates of a dianisyl-substituted donor–acceptor-type benzothiadiazole: mechanochromic, vapochromic, and acid-responsive multicolor luminescence

Solvates of a donor–acceptor-type fluorophore exhibited multicolor luminescence that can respond to mechanical stimuli, solvent and acid vapors.

Mechanochromic luminescence of phase-separated hydrogels that contain cyclophane mechanophores

Phase-separated hydrogels that contain cyclophane mechanophores exhibit mechanochromic luminescence.

Bridged o-carborane–anthracene dyads as dual state emission luminogens: synthesis, characterization, and mechanochromic properties

Bridged carborane–anthracene dyads 1 and 2 exhibit emissions both in solution and in solid state, and blue-shifted mechanochromic luminescence.

Luminescent Pt(II) Complexes Containing (1-Aza-15-crown-5)dithiocarbamate and (1-Aza-18-crown-6)dithiocarbamate: Mechanochromic and Solvent-Induced Luminescence.

The strong tendency to stack in the solid state and rich luminescence for the Pt(II) complexes makes them potential candidates as new mechanochromic materials and sensing applications. Six mononuclear complexes [Pt(ppy)(O4NCS2)] (1), [Pt(bpy)(O4NCS2)]ClO4 (2), [Pt(ppy)(O5NCS2)] (3), [Pt(phen)(O4NCS2)]ClO4·CH3OH (5a), [Pt(phen)(O4NCS2)]ClO4 (5b), and [Pt(phen)(O5NCS2)]ClO4 (6a), one dinuclear complex [Pt2(phen)2(NaO5NCS2)2(ClO4)3]ClO4 (6b), and one one-dimensional (1-D) coordination polymer {[Pt2(bpy)2(NaO5NCS2)2(ClO4)2](ClO4)2}n (4) were synthesized by reacting [Pt(ppy)Cl]2, Pt(bpy)Cl2, and Pt(phen)Cl2 (ppy = 2-phenylpyridine, bpy = 2,2'-bipyridine, and phen = 1,10-phenanthroline) with (1-aza-15-crown-5)dithiocarbamate (O4NCS2) or (1-aza-18-crown-6)dithiocarbamate (O5NCS2), respectively, which have been isolated and structurally characterized by X-ray diffraction. Neutral complexes 1 and 3 contain no intermolecular Pt(II)···Pt(II) contact, whereas cationic complexes 2, 5a, 5b, and 6a with ClO4- as counteranions show alternative intermolecular Pt(II)···Pt(II) contacts of 3.535/4.091, 3.480/5.001, 3.527/4.571, and 3.446/4.987 Å in the solid state, respectively. Interestingly, complex 4 forms a 1-D coordination polymer through coordination between the encapsulated Na+ ions inside the azacrown ether rings of O5NCS2 and ClO4- anions with respective intra- and intermolecular Pt(II)···Pt(II) contacts of 3.402 and 3.847 Å in crystal lattices, whereas a dinuclear complex 6b was surprisingly formed and also connected by the encapsulated Na+ ions and ClO4- anions with alternative intra- and intermolecular Pt(II)···Pt(II) contacts of 3.650 and 3.677/4.4.372 Å, respectively. Upon excitation, complexes 1 and 3 showed similar vibronic luminescence at 507, 534, and 502, 532 nm, respectively, and the other complexes 2 and 4-6 showed broad luminescence with maxima at 537-567 nm. The B3LYP/LanL2DZ calculation was carried out and used to clarify their excited-state properties. In addition, the powder samples for complexes 1-4 almost showed no energy shift for the luminescence and significantly those of complexes 5-6 exhibited the mechanochromic luminescence upon grinding. It is noted that complexes 5a and 6a only showed minor red shifts (i.e., from 544 to 556 nm for complex 5a and from 551 to 565 nm for complex 6a), whereas complex 6b exhibited a remarkable red shift from 558 to 603 nm upon grinding. Besides, their luminescence reversibility was also examined toward various solvents.

Precise Molecular Design for Two Parent BN‐Pentacyclic Isomers Through Stitching Engineering Toward Extremely Sensitive Explosive Detection

AbstractThe incorporation of BN units into polycyclic aromatic hydrocarbons (PAHs) has been ascertained to be a promising strategy for creating novel materials with unique functions and properties. However, it poses a formidable challenge to achieve precise construction of BN‐PAHs with variable skeleton module, which conversely, enhances greater understanding of the structure‐property relationship. Herein, two BN‐pentacyclic isomers (P‐BN‐BT and I‐BN‐BT) with different stitching patterns are addressed comprehensively, in which the integrated analysis reveals that the geometric structure, aromaticity, and photophysical properties indeed vary with the variational stitching models. More interestingly, further investigations show that the I‐BN‐BT with twisted molecular configuration displays a low fluorescence quantum yield, while the P‐BN‐BT with fully planar conformation exhibits high quantum yield and exceptional mechanochromic luminescent (MCL) properties. More importantly, P‐BN‐BT shows extreme sensitivity toward 2,4,6‐trinitrophenol (TNP). Visual detection of TNP is handily performed by fabrication of the test strips, which reaches ppm level under daylight. This work provides systematic insight into the effects of BN doping and highlights the importance of accurate control of the stitching patterns in conjugated materials.

Aggregation induced emission and mechanochromic luminescence by cyanostilbene-based organic luminophores

In this manuscript, we report novel cyanostilbene based luminogens, 1CF3 and 1F2 that exhibit pronounced “aggregation induced emission” (AIE) characteristics and mechanochromic luminescence (MCL) switching in solid state. A poorly emissive methanol solution of 1CF3 or 1F2 can be highly emissive after addition of poor solvent. Our morphological study reveals that the molecules self-assemble to higher crystalline state at during AIE process. When the single crystals of 1CF3 or 1F2 were subjected to mechanical pressure, a prominent mechanochromic switching from greenish yellow emission to red emission was observed. The red emissive material can be switched back to its initial greenish yellow emissive material by solvent vapour fumigation. Powder X-ray diffraction and single crystal X-ray diffraction study revealed that the mechanochromic colour switching is due to the packing change induced by mechanical pressure. The practical applicability of the MCL materials was demonstrated for ink-free writing and solvent vapour erasing.

Mechanical Force-Induced Blue-Shifted and Enhanced Emission for AIEgens.

Mechanochromic (MC) luminescence of organic molecules has been emerging as a promising smart material for optical recording and memory devices. At the same time, pressure-induced blue-shifted and enhanced luminescence are rarely reported now. Herein, a series of cyanostilbene-based AIEgens with different substituents were synthesized to evaluate the influence of morphology transformation and push-pull electronic effect on the MC luminescence. Among these luminophores, compound 1 with one cyano group and diethylamino group was more susceptible to mechanical stimuli and obtained blue-shifted and enhanced fluorescence in response to anisotropic grinding. Powder X-ray diffraction patterns indicated that the MC behaviors were ascribed to the solid-state morphology transition from crystal-to-crystal. Analysis of crystal structures revealed that loose molecular packing is a key factor for high high-contrast MC luminescence. The smart molecular design, together with the excellent performance, verified that luminophores with twisted structures are ideal candidates for MC luminogens.

Achieving dual-state emission and high-contrast mechanochromic luminescence based on donor-acceptor type phenothiazine methanone derivatives

The development of fluorescent materials that can achieve dual-state emission (DSE) and high-contrast mechanochromic luminescence has attracted more and more attention. However, such materials are very rare, and there are no clear molecular design principles to follow. In current work, two novel DSE emitters are designed and synthesized by modifying phenothiazine and benzene or naphthalene rings on both sides of the methanone. Their photophysical properties in both dilute solution and the solid state are studied and compared. Optical researches have shown that D-A type phenothiazine methanone derivatives exhibit well-defined ICT (intramolecular charge transfer) characteristics. At the same time, they show intense luminescent efficiencies in both solution (ΦPTZ-CO-B = 0.645; ΦPTZ-CO-NA = 0.624) and solid states (ΦPTZ-CO-B = 0.386; ΦPTZ-CO-NA = 0.357), indicating their DSE properties. It is noteworthy that compounds PTZ-CO-B and PTZ-CO-NA exhibit significant fluorescence response to mechanical force, and the emitting wavelengths are red-shifted by 33 nm and 46 nm, respectively. XRD patterns and DSC curves confirm that the mechanochromism results from reversible changes in stacking pattern. The current work develops materials that can achieve DSE emission and high-contrast mechanochromism, laying the foundation for the development of such materials.

Ultralong room temperature phosphorescence and reversible mechanochromic luminescence in ionic crystals with structural isomerism

Photofunctional ionic crystals have become more and more appealing in the applications of light-emitting devices, optical imaging, and anti-counterfeiting. However, it is still limited to achieve ultralong room temperature phosphorescence (RTP) and stimuli-responsive property in ionic crystals due to the lack of valid design principles. Herein, we developed a new class of organic ionic crystals (sodium o-, m-, p-carboxyphenylborate, o-, m-, p-Nacpb) by a structural isomerization strategy, which could exhibit ultralong RTP and mechanochromic emission. Among the three isomers, m-Nacpb crystal exhibits the longest RTP lifetime up to 732.8 ms, which is ca. 120 times longer than that (6.1 ms) of o-Nacpb crystal. Interestingly, o-Nacpb could display mechano-responsive luminescence with tunable fluorescence-phosphorescence emissions and longer phosphorescence lifetime (increased by about 20-fold). Both experimental and theoretical studies indicate that the distinct RTP and mechanochromic luminescence properties of the organic ionic crystals are attributed to the alternation of molecular conformations and spatial packing modes. Moreover, potential applications including data encryption and decoration are demonstrated, benefiting from the varied phosphorescence lifetimes and good processability. Therefore, this work not only provides a rational isomerization route to prepare various organic ionic crystals with ultralong RTP and stimuli-responsive emission, but also supplies a facile way to develop the advanced information encryption applications.

A simple molecular design towards the conversion of a MCL backbone to a multifunctional emitter exhibiting polymorphism, AIE, TADF and MCL

Compared with the large number of single-function materials such as aggregation-induced emission (AIE), mechanochromic luminescence (MCL), or thermally activated delayed fluorescence (TADF) emitters, multifunctional emitting materials offer more opportunities in practical applications. In this report, we provide a simple molecular design strategy towards the conversion of a MCL building block to a multifunctional emitter. Through altering the substituent sites and increasing the number of electron donors and steric hindrance on a normal MCL backbone benzo[d,e]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one, a novel multifunctional material 10,11-bis-(4-diphenylamino-phenyl)-benzo[d,e]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one (10,11-2TPA-BBI) is designed and synthesized. 10,11-2TPA-BBI exhibits simultaneous polymorphism, AIE, MCL and TADF properties. It can form four different aggregate species: yellow solid (YS) and orange solid (OS), orange flake-shaped crystal (OC), and red prism-like crystal (RC). Among them, because of the small energy gaps (ΔESTs < 0.3 eV) between the singlet and triplet excited states, OS, OC and RC exhibit TADF properties, while YS show normal fluorescence characteristics with a large ΔEST of 0.33 eV. OS can be reversibly transformed into YS upon external stimuli, which can be attributed to the emission switch between local excited state and charge transfer state. Crystallographic study indicates that the bulky structure and weak intermolecular interactions account for polymorphism and AIE properties. This work will provide a simple molecular design strategy for multifunctional materials.

Back Cover: Excited State Charge‐Transfer Complexes Enable Fluorescence Color Changes in a Supramolecular Cyclophane Mechanophore (Angew. Chem. Int. Ed. 42/2022)

Mechanochromic mechanophores visualize stress applied to polymers. In their Communication (e202209225), Christoph Weder, Yoshimitsu Sagara et al. demonstrate the first cyclophane mechanophore utilizing charge–transfer complex emission. Polyurethane elastomer films in which the mechanophore is covalently embedded exhibit instantly reversible emission color change between orange and blue upon deformation. The spatial separation of the luminophore and quencher leads to vivid mechanochromic luminescence.

Thermo-, Mechano-, and Vapochromic Dinuclear Cuprous-Emissive Complexes with a Switchable CH3CN-Cu Bond.

A thermo-, mechano-, and vapochromic bimetallic cuprous-emissive complex has been reported, and the origin and application of its tri-stimuli-responsive luminescence have been explored. As revealed by single-crystal structure analysis, thermo- and vapochromic luminescence adjusted by heating at 60 °C and CH3CN vapor fuming, accompanied by a crystalline-to-crystalline transition, is due to the breaking and rebuilding of the CH3CN-Cu bond, as supported by 1H nuclear magnetic resonance (NMR), Fourier-transform infrared (FT-IR) spectroscopy, powder X-ray diffraction (PXRD), thermogravimetry (TG), and time-dependent density functional theory (TD-DFT) analyses of the CH3CN-coordinated species [Cu2(μ-dppa)2(μ-η1(N)η2(N,N)-fptz)(CH3CN)](ClO4)·H2O (1) and its CH3CN-removed derivative [Cu2(μ-dppa)2(μ-η1(N)η2(N,N)-fptz)](ClO4)·H2O (2). Luminescence mechanochromism, mixed with a crystalline-to-amorphous transition where the initial crystalline is different for 1 and 2, is mainly assigned as the destruction of the CH3CN-Cu bonding and/or the O···HNdppa and OH···Ntriazolyl hydrogen bonds. It is also suggested that a rational use of switchable coordination such as weak metal-solvent bonding is a feasible approach to develop multi-stimuli-responsive luminescent materials and devices.