Reversible Mechanochromic Luminescence Research Articles

Dithienopyrrole compound with twisted triphenylamine termini: Reversible near-infrared electrochromic and mechanochromic dual-responsive characteristics

A dithieno[3,2-b:2′,3′-d]pyrrole-based luminogen 1 has been synthesized by appending two twisted triphenylamine units to the rigid conjugated dithieno[3,2-b:2′,3′-d]pyrrole core. This compound exhibits reversible mechanochromic luminescence in the solid state and near-infrared electrochromic switching behavior in solution. A reversible switching in fluorescence color between yellow-green and green can be observed through mechanical grinding and vapor fuming of solid powdered 1, which involves an interconversion between a crystalline form and an amorphous phase according to X-ray diffraction analysis. The associated oxidized species of 1 show intense redox-switchable near-infrared absorption and different fluorescence colors in solution in spectroelectrochemical and luminescence measurements. Density functional theory calculations have confirmed that there is considerable electron delocalization between the dithieno[3,2-b:2′,3′-d]pyrrole linker and two triphenylamine redox-active termini during the oxidation process.

Binuclear Complexes and Extended Chains Featuring Pt(II)-Tl(I) Bonds: Influence of the Pyridine-2-Thiolate and Cyclometalated Ligands on the Self-Assembly and Luminescent Behavior.

Platinum solvate complexes [Pt(C6F5)(C^N)(S)] [C^N = phenylpyridinyl (ppy), S = dimethyl sulfoxide (DMSO) (A); C^N = benzoquinolinyl (bzq), S = CH3COCH3 (B)] react with [Tl(Spy)] (Spy = 2-pyridinethiolate) to afford binuclear [{Pt(C6F5)(C^N)}Tl(Spy)] [C^N = ppy (1) and bzq (2)] species containing a Pt-Tl bonding interaction, supported by a μ-Spy-κN,S bridging ligand, as confirmed by X-ray diffraction. However, the related reactions with [Tl(SpyCF3-5)] [SpyCF3-5 = 5-(trifluoromethyl)-2-pyridinethiolate] give neutral extended chains [{Pt(C6F5)(C^N)}Tl(SpyCF3-5)]n [C^N = ppy (3) and bzq (4)]. 3 features a zigzag -Pt-Tl···S-Pt- chain, generated by Pt-Tl and Tl···S bonds, with the SpyCF3 acting as a μ-κN:κ(2)S bridging ligand, whereas 4 displays an unsupported ···Tl-Pt···Tl-Pt··· backbone (angle of ca. 158.7°). The lowest-energy absorption bands in the UV-vis spectra in CH2Cl2, associated with (1)L'LCT transitions with minor (1)LC/(1)MLCT (L' = Spy or SpyCF3-5; L = C^N) character, are similar for all complexes 1-4, demonstrating that for 3 and 4 the chains break down in solution to yield similar bimetallic Pt-Tl units. For 2, two different forms, 2-o (orange) and 2-y (yellow), exhibiting different colors and emissions were found depending on the isolation conditions. Slow crystallization favors formation of the thermodynamically more stable yellow form (2-y), which exhibits a high-energy (HE) structured emission band, whereas fast crystallization gives rise to the orange form (2-o), with a remarkably lower energy structureless emission. Complexes 1 and 3 exhibit dual luminescence in the solid state at 298 K: an unstructured low-energy band associated with (3)ππ* excimeric emission due to π···π (C^N) interactions and a more structured HE band, assigned, with support of density functional theory calculations, to an intraligand (3)LC (C^N) excited state mixed with some ligand (SPy)/platinum-to-ligand (C^N)(3)[(L' + M)LCT] charge transfer. Chain 4 only shows a HE band at 298 K, attributed to a (3)L'LCT (SpyCF3 → bzq) excited state mixed with a minor (3)MLCT/(3)MM'CT (M = Pt; M' = Tl) contribution. At 77 K, the ππ*-stacking emission is predominant in all complexes, except in the form 2-y. Interestingly, 2-4 exhibit reversible mechanochromic color and luminescence changes, with remarkable red shift and increased quantum yields, and upon exposure to solvents, they are restored to their original color and emission. On the basis of powder X-ray diffraction studies, a plausible mechanism of the mechanochromic processes is proposed, involving reversible crystalline-to-amorphous phase transitions.

Modulating Mechanochromic Luminescence Quenching of Alkylated Iodo Difluoroboron Dibenzoylmethane Materials

Difluoroboron β-diketonate compounds exhibit solid-state luminescence phenomena. Among these are reversible mechanochromic luminescence (ML), aggregation induced emission (AIE), and mechanochromic luminescence quenching (MLQ). These properties can be tuned by alterations to the molecular structure. Dyes with varying halide substituents exhibit tunable ML, MLQ, and solid-state emission with high quantum yields. A series of difluoroboron dibenzoylmethane (BF2dbm) dyes with iodide and alkoxyl substituents (BF2dbm(I)OR) were synthesized where R = CH3 (C1), C5H11 (C5), C6H13 (C6), C12H25 (C12), and C18H37 (C18)). The 4-iodo parent compound BF2dbm(I) (H) was made for comparison. By keeping the heavy atom static, the dependence of ML properties on alkyl chain length was probed. The hydrogen derivative is only weakly emissive as a solid and exhibited minimal mechanoresponsive behavior. In contrast, alkoxy dyes exhibited tunable ML and MLQ properties depending on the length of the alkyl chain. Longer chain dyes co...

Bismuth‐Based Coordination Polymers with Efficient Aggregation‐Induced Phosphorescence and Reversible Mechanochromic Luminescence

AbstractTwo bismuth coordination polymers (CPs), (TBA)[BiBr4(bp4mo)] (TBA=tetrabutylammonium) and [BiBr3(bp4mo)2], which are based on the rarely used simple ditopic ligand N‐oxide‐4,4′‐bipyridine (bp4mo), show mechanochromic luminescence (MCL). High solid‐state phosphorescence quantum yields of up to 85 % were determined for (TBA)[BiBr4(bp4mo)] (λem=540 nm). Thorough investigations of the luminescence properties combined with DFT and TDDFT calculations revealed that the emission is due to aggregation‐induced phosphorescence (AIP). Upon grinding, both samples became amorphous, and their luminescence changed from yellow to orange and red, respectively. Heating or exposure to water vapor led to the recovery of the initial luminescence. These materials are the first examples of mechanochromic phosphors based on bismuth(III).

Bismuth‐Based Coordination Polymers with Efficient Aggregation‐Induced Phosphorescence and Reversible Mechanochromic Luminescence

Two bismuth coordination polymers (CPs), (TBA)[BiBr4 (bp4mo)] (TBA=tetrabutylammonium) and [BiBr3 (bp4mo)2 ], which are based on the rarely used simple ditopic ligand N-oxide-4,4'-bipyridine (bp4mo), show mechanochromic luminescence (MCL). High solid-state phosphorescence quantum yields of up to 85 % were determined for (TBA)[BiBr4 (bp4mo)] (λem =540 nm). Thorough investigations of the luminescence properties combined with DFT and TDDFT calculations revealed that the emission is due to aggregation-induced phosphorescence (AIP). Upon grinding, both samples became amorphous, and their luminescence changed from yellow to orange and red, respectively. Heating or exposure to water vapor led to the recovery of the initial luminescence. These materials are the first examples of mechanochromic phosphors based on bismuth(III).

Aggregation-induced emission and reversible mechanochromic luminescence of carbazole-based triphenylacrylonitrile derivatives

Conjugated organic molecules exhibiting AIE are characterized by the strongly twisted conjugated skeleton, which could provide loose intermolecular stacking and weak π–π interactions in the solid state to generate stimuli-responsive materials.

Process-dependent reversible mechanochromic luminescence of bismuth based polymorphs

The reversibility of the mechanochromic luminescence (MCL) properties of bismuth based polymorphs depends on how the ground amorphous sample is treated.

High-contrast reversible mechanochromic luminescence of a conjugated anthracene-based molecule

We report a solid-emissive anthracene-based compound with rigid conjugated structure, which exhibits strong blue fluorescence in the crystalline state. Grinding the crystals turns its initial blue emission to green with higher emission efficiency, and the blue emission could be recovered by fume with isobutanol. The phenomenon was suggested to be due to the regulation of π–π stacking between the aromatic rings by mechanical and fume stimuli.

Anion-, Solvent-, Temperature-, and Mechano-Responsive Photoluminescence in Gold(I) Diphosphine-Based Dimers.

A series of [Au2 (nixantphos)2](X)2 (nixantphos=4,6-bis(diphenylphosphino)-phenoxazine; X=NO3, 1; CF3 COO, 2; CF3 SO3, 3; [Au(CN)2], 4; and BF4, 5) complexes that exhibit intriguing anion-switchable and stimuli-responsive luminescent photophysical properties have been synthesized and characterized. Depending on their anions, these complexes display yellow (3), orange (4 and 5), and red (1 and 2) emission colors. They exhibit reversible thermo-, mechano-, and vapochromic luminescence changes readily perceivable by the naked eye. Single-crystal X-ray studies show that the [Au2 (nixantphos)2](2+) cations with short intramolecular Au⋅⋅⋅Au interactions are involved as donors in an infinite N-H⋅⋅⋅X (X=O and N) hydrogen-bonded chain formation with CF3 COO(-) (2 C) and aurophilically linked [Au(CN)2](-) counterions (4 C). Both crystals show thermochromic luminescence; their room temperature red (2 C) and orange (4 C) emission turns into yellow upon cooling to 77 K. They also exhibit reversible mechanochromic luminescence by changing their emission color from red to dark (2 C), and orange to red (4 C). Compounds 1-5 also display reversible mechanochromic luminescence, altering their emission colors between orange (1) or red (2) to dark, as well as between yellow (3) or orange (4 and 5) to red. Detailed photophysical investigations and correlation with solid-state structural data established the significant role of NH⋅⋅⋅X interactions in the stimuli-responsive luminescent behavior.

High-contrast and reversible mechanochromic luminescence of a D–π–A compound with a twisted molecular conformation

By integrating a triphenylamino donor (D) and a (2,2-dicyanovinyl)phenyl acceptor (A) with an anthryl π-spacer, a mechanochromic luminescent (ML) D–π–A compound, TPAANDCV, has been constructed. The single crystals of TPAANDCV display efficient green emission with a good fluorescence quantum yield (ΦF) of 0.35. Upon grinding the crystals, a dramatic emission-colour variation from green to orange-red has been observed, and such a variation can be erased through annealing or solvent-vapour fuming. Moreover, the emission of the ground sample is also bright with a ΦF value of 0.11. The powder XRD measurements indicate that the emission-colour change upon grinding is caused by a partial conversion of the ordered solid-state structures to an amorphous state. The high-contrast and reversible fluorescent response of TPAANDCV towards mechanical stress, as well as the bright emission of both the unground and ground states, enable TPAANDCV to be a good ML material.

Reversible mechanochromic luminescence of phenothiazine-based 10,10′-bianthracene derivatives with different lengths of alkyl chains

The phenothiazine-based bianthracene derivatives exhibited mechanofluorochromic behaviours, and their fluorescence spectrum shifts and thermal recovery times were dependent on the length of the alkyl chain.

Mechanochromic luminescence and aggregation induced emission for a metal-free β-diketone.

Reversible mechanochromic luminescence (ML) is known for difluoroboron β-diketonates (BF2bdks). Here, we report ML for the methoxy-substituted dinaphthoylmethane (dnmOMe) ligand even without coordination to boron. In addition, dnmOMe shows solvatochromism and aggregation-induced emission (AIE). Diketone thin films showed high contrast emission when smeared, and rapid recovery of the pre-smeared state at room temperature.

Photophysical responses in Pt2Pb clusters driven by solvent interactions and structural changes in the Pb(II) environment.

Two types of Pt2Pb luminescent clusters were successfully prepared by the reaction of [Pt(C6F5)(bzq)(OCMe2)] (1) and [Pt(C6F5)(ppy)(dmso)] (2) with [Pb(SpyR-5)2] (R = H, CF3). Thus, whereas 5 (ppy, Spy) is generated through coordination of the pyridine-N atoms to the Pt centers, the formation of 3, 4 (bzq), and 6 (ppy, SpyCF3) is accompanied by a formal thiolate transfer from Pb(II) to Pt(II), keeping the two N atoms in the primary environment of the lead. In 5, the neutral Pb center adopts a rather stable and symmetrical "Pt2S2" coordination sphere supplemented by two Pb··Fo contacts, whereas for the remaining species several pseudopolymorphs were found depending on the solvent (3, 4) and crystallization conditions (6). This structural diversity relies on changes in the coordination mode of the SpyR ligands (μ-κS,N/μ-κ(3)S,N,S), intermetallic Pt-Pb bonds, and secondary intra- and intermolecular contacts induced by Pb-solvent binding. Notably, the changes, which entail a slight tuning of the stereochemical activity of the lone pair, have also a remarkable impact on the emissive state ((3)L'CCT/(3)L'LCT, SpyR → Pb,Pt/(C(∧)N) in nature). Clusters 3 and 4 display a distinct and fast reversible blue shift vapoluminescent response (4 shows also color changes) to donor solvents, correlated with changes in the environment of the Pb(II) ion (asymmetric hemidirected to more symmetric holodirected) upon solvent binding and, additionally, in 4 with modifications in the crystal packing, as confirmed by XRD and supported by TD-DFT calculations. 5 and 6 do not show a vapoluminescent response. However, for 6, three different and interconvertible forms, a symmetrical form (yellow 6-y) and two asymmetrical forms with a rather short Pt-Pb bond (pale orange 6·acetone and orange 6-o), exhibiting different emissions were found. Notably, slow crystallization and low concentration favor the formation of the thermodynamically more stable yellow form, whereas fast crystallization gives rise to orange solids with a remarkable red shift of the emission. Interestingly, 6 also exhibits reversible mechanochromic color and luminescence changes.

Reversible Mechanochromic Luminescence at Room Temperature in Cationic Platinum(II) Terpyridyl Complexes

Reversible mechanochromic luminescence in cationic platinum(II) terpyridyl complexes is described. The complexes [Pt(Nttpy)Cl]X2 (Nttpy = 4'-(p-nicotinamide-N-methylphenyl)-2,2':6',2″-terpyridine, X = PF6 (1), SbF6 (2), Cl (3), ClO4 (4), OTf (5), BF4 (6)) exhibit different colors under ambient light in the solid state, going from red to orange to yellow. All of these complexes are brightly luminescent at both room temperature and 77 K. Upon gentle grinding, the yellow complexes (4-6) turn orange and exhibit bright red luminescence. The red luminescence can be changed back to yellow by the addition of a few drops of acetonitrile to the sample. Crystallographic studies of the yellow and red forms of complex 5 suggest that the mechanochromic response is likely the result of a change in intermolecular Pt···Pt distances upon grinding.

The Relationship of Solid‐State Plasticity to Mechanochromic Luminescence in Difluoroboron Avobenzone Polymorphs

AbstractIn solid‐state mechanochromic luminescence (ML) materials, it remains a challenge to establish the origin of fluorescence color changes upon mechanical action and to determine why only some fluorophores exhibit ML behavior. The study of mechanical properties by nanoindentation, followed by ML experiments on green‐ and cyan‐emitting polymorphs of difluoroboron avobenzone reveals that upon smearing, the plastically deformable cyan form shows a prominent color change to yellow, while in the harder green form the redshifted emission is barely detectable. Crystal structure analysis reveals the presence of slip planes in the softer cyan form that can facilitate the formation of recoverable and low energy defects in the structure. Hence, the cyan form exhibits prominent and reversible ML behavior. This suggests a potential design strategy for efficient ML materials.

A mechanistic investigation of mechanochromic luminescent organoboron materials

Mechanochromic luminescence (ML) refers to the luminescence color and/or intensity change of solid-state materials induced by mechanical perturbations. For organic molecular solids, this phenomenon is related to the specific packing modes and orientations of individual fluorophores, which could give rise to different excited-state interactions. The molecular solids of difluoroboron dibenzoylmethane (BF2dbm) derivatives were previously found to exhibit reversible ML at room temperature and are promising as self-healing optical materials. In this report, we aim to shed some light on the mechanism of BF2dbm ML by trying to understand the excited-state interactions among solid-state BF2dbm molecules and elucidate how these interactions change upon mechanical stimulation. We first investigated the optical properties of monomeric, dimeric, and polymeric BF2dbm derivatives in optically dilute solutions and demonstrated unambiguously that BF2dbm moieties have a propensity to form H-aggregates. Next, we studied the physical properties of these boron complexes in the solid state including their crystal structures, fluorescence emissions, and mechanochromic luminescence. By correlating solution data with the solid-state characterization results, it was concluded that two coupled processes, force-induced emissive H-aggregate formation and energy transfer to the emissive H-aggregates, are responsible for the observed BF2dbm ML in the solid state.

Arene effects on difluoroboron β-diketonate mechanochromic luminescence

Difluoroboron β-diketonate (BF2bdk) dyes display reversible mechanochromic luminescence (ML) in the solid state. A series of BF2bdk dyes with methyl, phenyl, naphthyl and anthracyl groups (i.e. arene substituents and bdk ligands: Me–Ph = mbm, Ph–Ph = dbm, Np–Ph = nbm, An–Ph = abm) were prepared to test the effects of π conjugation length and arene size on ML properties. Solid-state emission spectra were recorded for powders, spin-cast films, and dye coated weighing paper. The materials emit at various wavelengths from blue to red, depending on the conjugation length. Additionally, emission spectra were recorded for smeared solids and their recovery was tracked at room temperature over time. All dyes except for BF2mbm show emission changes upon mechanical perturbation, with increasing π conjugation correlating with more dramatic, redshifted fluorescence, however, their recovery is significantly affected by the aromatic substituents. The thermal and structural properties of the dyes in the solid state were also investigated by differential scanning calorimetry (DSC) and atomic force microscopy (AFM).

Alkyl chain length effects on solid-state difluoroboron β-diketonate mechanochromic luminescence

Solid-state difluoroboron β-diketonate dyes display reversible mechanochromic luminescence (ML). To test the effects of alkyl chain length on solid state photoluminescence and ML, a series of dyes, BF2dbmOR, with dibenzoylmethane (dbm) ligands and alkoxyl substituents (–OR) were prepared, where R = CnH2n+1 and n = 1, 2, 3, 5, 6, 12, 14, 16, 18. Emission properties were investigated in solution and in the solid state. Fluorescence spectra and lifetimes were nearly identical for dyes in CH2Cl2 solution; whereas, in the solid state, as powders, thin films or spin cast films, emission maxima, and lifetimes were different among the samples. Solid-state ML emission spectra were monitored at room temperature as a function of time for smeared powders on quartz surfaces. The recovery time generally increased with alkyl chain length, ranging from minutes (n = 3) to days (n = 18). Longer chain analogues (n = 6, 12, 14, 16, 18) did not fully return to the original annealed emissive state even after months on quartz, though the dynamics are substrate dependent. Solid-state dyes were also investigated by XRD and DSC (powders), and by AFM (spin cast films).

Reversible Mechanochromic Luminescence of [(C6F5Au)2(μ-1,4-Diisocyanobenzene)

Reversible mechanochromic luminescence of [(C6F5Au)2(mu-1,4-diisocyanobenzene)] is reported. Grinding of the complex induced a photoluminescent color change, which was restored by exposure to a solvent. This cycle was repeated 20 times with no color degradation in the emissions. Their optical properties, X-ray crystallographic analysis, IR, and XRD measurements strongly suggested that the change in the molecular arrangement is responsible for this mechanochromic property. Intermolecular aurophilic bondings presumably play a key role in the altered emission.