Mechanofluorochromic Properties Research Articles

Mechanofluorochromism of D-A typed phenothiazine derivative

A D-A typed phenothiazine derivative MPTPM has been designed and synthesized via a Friedel-Crafts reaction, which contains two benzoyl groups. It was found that MPTPM gave strong emission in solution and in solid state, and the solid fluorescence quantum yield was as high as 0.60. Moreover, an excellent reversible mechanofluorochromic behavior was realized upon the treatment of grinding/fuming with CH2Cl2 or heating in spite of a very simple molecular structure. The emission color of MPTPM changed from green to orange-yellow after grinding and could be restored via fuming with CH2Cl2 or heating with a spectra shift of ∼63 nm. Photophysical, XRD, DSC studies indicated the mechanofluorochromic properties of MPTPM originating from the transformation between crystalline phase and amorphous phase.

Mechanofluorochromic properties of aggregation-induced emission-active tetraphenylethene-containing cruciform luminophores

Two twisted donor–acceptor cruciform luminophores (DHCS-TPE and DMCS-TPE) that were composed of two π-conjugated segments connected via a central benzene core have been prepared. Such cruciforms showed unique intramolecular charge transfer (ICT) emission, obvious aggregation-induced emission (AIE) properties (αAIE = 19 and 20, respectively) and high solid state efficiency (0.913 and 0.424, respectively). Especially, the two compounds exhibited substituent-dependent mechanofluorochromic (MFC) behavior. The as-prepared powders of the more twisted DMCS-TPE could emit strong blue-green light centered at 474 nm, and the fluorescence color changed into yellowish green (531 nm) after grinding, a red shift of 57 nm was observed. Such mechanochromism was reversible upon the treatment of grinding and fuming with DCM. The high contrast MFC behavior of DMCS-TPE originated from the planarization of the molecular conformation and subsequent planar intramolecular charge transfer (PICT) under external force. By sharp contrast, the less twisted DHCS-TPE exhibited no MFC behavior. The reason is that the less twisted molecular conformation of DHCS-TPE generates strong intermolecular forces and close packing. Thus compared to DMCS-TPE, the crystalline solid powders of DHCS-TPE possess higher lattice energy and better structural stability. The grinding of the crystalline solid powders of DHCS-TPE cannot lead to amorphization and mechanochromic luminescence.

Non-conjugated anthracene derivatives and their mechanofluorochromic properties

A series of non-conjugated methylene–anthracene Schiff base derivatives (DNCAs) were designed and synthesized. Photoluminescence emission spectra indicated that DNCA-4 and DNCA-12 showed obviously mechanofluorochromic properties, and distinctive 26- and 37-nm hypochromic shifts were observed, respectively. The experiment results revealed that there is no specific relationship between alkoxy chain lengths and their stimuli–response behavior. The PXRD profiles demonstrated a transformation from crystalline to amorphous state upon grinding.

Pyrene spiropyran dyad: solvato-, acido- and mechanofluorochromic properties and its application in acid sensing and reversible fluorescent display

Linking pyrene and spiropyran results in a new molecule that exhibits multistimuli responsive emission switching properties both in solution and in solid state.

Twisted donor–acceptor cruciform fluorophores exhibiting strong solid emission, efficient aggregation-induced emission and high contrast mechanofluorochromism

Two twisted π-conjugated cruciform luminophores DMTCS-CC6 and DBTCS-CC6 characterized by rigid X-shaped geometry with two conjugated donor and acceptor intersecting at the central aromatic core have been rationally designed and successfully synthesized. Spectral analysis and theoretical calculations studies indicated that the two cross-conjugated compounds showed unique intramolecular charge-transfer (ICT) process from the donor (carbazole) to the acceptor (dicyanodistyrylbenzene) axis in the excited state. Especially, DMTCS-CC6 and DBTCS-CC6 exhibited efficient aggregation-induced emission (AIE) and high contrast mechanofluorochromic (MFC) properties. By simple grinding, the as-prepared powders of DMTCS-CC6 and DBTCS-CC6 emitting strong yellowish-green (540 nm) and yellow light (560 nm) under UV irradiation could be transferred into powders emitting yellow (582 nm) and red (609 nm), respectively, the red shifts of 42 and 49 nm were observed. Such mechanochromism was reversible upon the treatment of grinding and fuming with DCM. The XRD patterns suggested that the MFC behavior originated from the transition between the crystalline and amorphous states. More interestingly, DBTCS-CC6 exhibited more obvious AIE and more contrast MFC behavior relative to DMTCS-CC6. This is a result of the quite larger space hindrance induced by 3,5-bis(trifluoromethyl)phenyl with two trifluoromethyl groups bonded at meta positions could make DMTCS-CC6 exhibit more twisted spatial conformation, leading more efficient solid state fluorescence and better structural transformability.

Mechanofluorochromic properties of tert-butylcarbazole-based AIE-active D-π-A fluorescent dye

Two new non-planar carbazole derivative DBTNC and NC were designed and synthesized. The D-π-A type conjugated compounds showed obvious intramolecular charge transfer (ICT) properties, which were evidenced by theoretical calculations and spectral analysis. Both of them exhibited evident AIE characteristic with high fluorescence quantum yield in the solid state with the fluorescence quantum yields of 0.37 and 0.33, respectively. DBTNC and NC were quite weakly fluorescent in THF, while a significant AIE effect were observed in water/THF mixtures with AIE factors of 10.7 and 9.7, respectively. Interestingly, DBTNC showed reversible mechanofluorochromism (MFC). The as-prepared solid of DBTNC emitted strong yellowish-green light and could be transformed into the powders emitting orange-yellow light after grinding with the fluorescence quantum yield decreased to 0.35, and the fluorescence could recover the original station when the ground powders were heated or fumed with DCM for 1 min. The powder XRD and DSC studies suggested that the mechanofluorochromic properties of DBTNC originating not from chemical changes but from the transformation of a crystalline state into an amorphous state. We deduced that tert-butyl can lead to the incompact packing of luminophore, which was more sensitive to external stimuli, so that multi-color solid-state luminescence can be afforded during mechanofluorochromic process. Herein, we provided the strategy for fabricating new mechanofluorochromic dyes via the introduction of non-planar π-skeletons and steric effect, which may possess potential applications in mechanosensors and anti-faked materials.

Aggregation-induced emission and mechanofluorochromism of tetraphenylbutadiene modified β-ketoiminate boron complexes

Two new benzo[d]oxazole- and benzo[d]thiazole-fused β-ketoiminate boron complexes (BF2-TBO and BF2-TBT) modified by tetraphenylbutadiene have been designed and successfully prepared, and their intramolecular charge-transfer (ICT), aggregation-induced emission (AIE) and mechanofluorochromic (MFC) properties were investigated. The results showed that the two D-π-A type compounds exhibited typical ICT emission, evident AIE characteristics (αAIE = 89 and 33, respectively), and high state emission intensity (up to 0.445 and 0.367, respectively). More importantly, BF2-TBO and BF2-TBT showed the reversible MFC behavior with their emitting colors changing from yellowish green (located at 528 and 530 nm) to yellow and orange (located at 552 and 572 nm) upon grinding, respectively, with red-shifts of 24 and 42 nm were obtained. The X-ray diffraction analyses confirmed that the MFC properties of BF2-TBO and BF2-TBT should be derived from a phase transition between crystalline and amorphous states. It was noticed that benzo[d]thiazole-fused BF2-TBT showed more contrasting MFC behavior relative to benzo[d]oxazole-fused BF2-TBO. The reason is that the existence of sulfur atom in BF2-TBT could lead to a larger degree of ICT in the excited state than that of BF2-TBO, which endowed it with greater planar intramolecular charge transfer (PICT) after grinding, resulting in a larger red-shift in the PL spectrum.

Effects of alkyl chain length on aggregation-induced emission, self-assembly and mechanofluorochromism of tetraphenylethene modified multifunctional β-diketonate boron complexes

Two tetraphenylethene functionalized β-diketonate boron complexes C2TPE-CAR and C16TPE-CAR that were bridged by carbazole bearing alkyl chains of different lengths, have been designed and successfully synthesized. The two donor–acceptor luminogens showed typical twisted intramolecular charge-transfer (TICT) emission. In particular, they exhibited alkyl-dependent aggregation-induced emission (AIE), self-assembly and mechanofluorochromic (MFC) properties. Results showed that C16TPE-CAR with a long hexadecyl chain formed gels in certain solvents and also exhibited gelation-induced emission enhancement. H-aggregates were formed driven by the synergistic effect of π-stacking interaction and van der Waals force in the gel phase. However, C2TPE-CAR, which possessed the short ethyl group, exhibited no self-assembly behavior. Simple mechanical force could change the emission color of C2TPE-CAR and C16TPE-CAR powders from initial bright yellow (571 and 558 nm) to final red (617 and 610 nm), thus high contrast mechanofluorochromism with large spectral shifts of up to 46 and 52 nm, respectively, was observed. The powder XRD studies showed that the MFC behavior might be switched reversibly under the stimulus of external force because of crystalline-amorphous phase transformation.

Polymorphism, Mechanofluorochromism, and Photophysical Characterization of a Carbonyl Substituted Difluoroboron-β-Diketone Derivative

Difluoroboron-β-diketones are known to exhibit very promising mechanochromic luminescence but a greater understanding of these materials and their mechanoresponsive properties is still desirable. Here we demonstrate that a combination of spectroscopic and physicochemical techniques is necessary to understand the variation of solid-state fluorescence observed under mechanical stress and thermal annealing. For this study, we decided to focus on a new fluorescent compound showing a polymorphic behavior. Our investigation on the mechanofluorochromic properties is based on a thorough spectroscopic study (steady-state and time-resolved) on powder samples and thin films deposited on paper and coverglass substrates. Three different states were identified: two crystalline states (a stable green-emissive and a metastable yellow-emissive one) and an amorphous phase (yellow orange emission). The detailed photophysical properties highlight the dynamic excimer formation processes, which can take place in the yellow-emi...

Mechanofluorochromic and thermochromic properties of simple tetraphenylethylene derivatives with fused fluorine containing 1,4-dioxocane rings

A series of tetraphenylethylenes (TPE) derivatives in which one or more of the phenyl rings have been replaced by a fused fluorine containing 2,3,4,5-tetrahydrobenzo[b][1,4]dioxocane unit have been synthesized. The analysis of the photoluminescence emission under application of mechanical grinding and thermal treatment show that in addition to the expected aggregation induced emission behavior observed in solution, these simple modifications of the TPE system confer mechanofluorochromic properties to the corresponding materials. The results of X-ray diffraction and theoretical calculations suggest that the introduction of tetrafluorobutylenedioxy “loops” control the balance between weak intermolecular interactions and thus the interconversion between “macro” and “micro-aggregates” which is proposed as the basic mechanism for the observed MFC properties.

Effect of molecular conformation on the mechanofluorochromic properties based on DDIF

Mechanofluorochromic (MFC) materials are smart materials in that their absorption and/or emission can respond to mechanical stimuli. They have received much attention recently. Although there have been several new material systems designed, little work has been done regarding the influence of molecular conformation on MFC properties. Herein, to disclose the relationship between molecular conformation and MFC properties, two molecules based on a 6, 12-Dihydro-6, 12-diaza-indeno[1,2-b]fluorine (DDIF) building block with thienyl linker, BDDIF-Th and BDDIF-BTh, have been designed and synthesized. Optical and electrochemical properties have been studied by UV–vis spectrometer and cyclic voltammetry measurements. Weak aggregation-induced emission (AIE) phenomena were obtained in the tetrahydrofuran (THF)/water solution. MFC behaviors suggest that BDDIF-Th is more sensible to the external mechanical forces than BDDIF-BTh. The color change could be attributed to the appearance of new emission peak instead of a bathochromic or hypsochromic effect. Theoretical calculations reveal that MFC performance is highly related to the molecular conformation, meaning that the BDDIF-BTh with perpendicular conformation is more difficult to flatten than the comparatively planar BDDIF-Th.

Tetraphenylethene modified β-ketoiminate boron complexes bearing aggregation-induced emission and mechanofluorochromism

Two new thiazole-based β-ketoiminate boron complexes, namelyTTPE-HandTTPE-CN, were designed and successfully synthesized. The two compounds exhibited strong aggregation-induced emission (AIE) and evident mechanofluorochromic (MFC) properties.

Mechanofluorochromic properties of fluorescent molecules based on a dicyanomethylene-4H-pyran and indole isomer containing different alkyl chains via an alkene module

The length and isomerization of the alkyl chains exhibit different effects on the mechanofluorochromic properties of the dicyanomethylene-4H-pyran derivatives when the substitution position of the indole unit is different.

Synthesis, crystal structure and reversible mechanofluorochromic properties of a novel phenothiazine derivative

A dibenzoyl phenothiazine derivative containing two benzoyl groups has been synthesized via a Friedel-Crafts reaction. The new phenothiazine gave strong emission in solution and in solid state, and the solid fluorescence quantum yield was as high as 0.63. The single crystal structure of the phenothiazine revealed that the hydrogen bonds CH⋯C (2.864 Å), CH⋯C (2.850 Å), CH⋯O (2.652 Å) would lock the molecular conformation, and the twisted conformation of the molecule lead to poor solid molecular packing, yielding highly emissive behavior in its crystal. Moreover, reversible mechanofluorochromism of the phenothiazine was realized upon grinding/fuming or annealing. The emission color of the phenothiazine changed from green to yellow after grinding and could be restored via fuming with CH2Cl2 or heating. The XRD results show that the mechanofluorochromic nature is generated through crystalline-amorphous phase transformation under external pressure.

Alkyl length dependent mechanofluorochromism of AIE-based phenothiazinyl fluorophenyl acrylonitrile derivatives

Four alkyl phenothiazinyl fluorophenyl acrylonitrile derivatives (PhC3F, PhC6F, PhC10F, and PhC12F) with different length of alkyl chains were synthesized in high yield. The compounds showed obvious aggregation-induced emission (AIE) and alkyl length dependent mechanofluorochromic (MFC) properties. PhC3F with propyl chain and PhC12F with dodecyl chain showed obvious and reversible MFC behaviours, while PhC6F with hexyl chain and PhC10F with decyl chain exhibited little MFC property. Small and wide-angle X-ray scattering indicated the MFC behaviour of PhC3F and PhC12F were attributed to the phase transformation from crystalline to amorphous state, while PhC6F maintained excellent crystalline performance before and after grinding, which was also evidenced by differential scanning calorimetry result. Quantum mechanical computations further revealed PhC6F had high molecular polarity, which greatly improved its crystalline performance, and made its original aggregated morphology hard to damage, then brought about little MFC diversification. PhC10F showed the highest molecular polarity and least MFC behaviour. We concluded that obvious MFC feature derived from appropriate crystalline performance, favourable molecular polarity and energy gap.

Halogen effect on mechanofluorochromic properties of alkyl phenothiazinyl phenylacrylonitrile derivatives

Three novel alkyl phenothiazinyl phenylacrylonitrile derivatives (C12F, C12Cl, and C12Br) with different halogen end groups (fluorine, chlorine, and bromine) were synthesized with ultra-high yield (>90%) and successfully confirmed according to standard spectroscopic methods. All these compounds were demonstrated with apparent twisted intramolecular charge transfer (TICT) and aggregation-induced emission (AIE) features. The halogen effect rendered them different electronic donor-acceptor behaviours, and gave birth to peculiar different mechanofluorochromic properties. The fluorine-substituted compound (C12F) showed obvious red-shifted mechanofluorochromic feature, while almost no mechanofluorochromic characteristic existed in the chlorine- (C12Cl) and bromine-substituted (C12Br) compounds. The mechanofluorochromic mechanism of C12F was investigated and attributed to the phase transformation from crystalline to amorphous state between the original and ground samples, easy crystallinity of the compound, straight conformation of alkyl chain, higher energy gap, and significant decrease of weighted mean lifetime. Moreover, C12F showed reversible mechanofluorochromic behaviour and reproducibility by annealing the ground sample, making it promising dual responsive and smart fluorescent materials for fluorescence switches and mechanosensors. The discussion of halogen effect on mechanofluorochromic properties in this work would provide a new way to adjust the fluorescent feature of mechanofluorochromic materials.

A multi-stimuli responsive “AIE” active salicylaldehyde-based Schiff base for sensitive detection of fluoride

In this work, (E)-2-[(pyridin-2-ylimino)methyl]phenol, a Schiff-base compound was synthesized and the investigation of solid state emission property was carried out. The observed bright emission in solid state was identified it as an ‘aggregation induced emission (AIE)’ active compound. The mechanism of AIE property was investigated and proposed to occur as J-aggregation mediated ‘excited state intramolecular proton transfer (ESIPT)’. It was interestingly observed that the compound exhibited mechanofluorochromic (MFC) property and changed its emission color from yellow to greenish-yellow after grinding. Furthermore, a very sensitive detection of F− was achieved with turn-on fluorescent emission with using the same compound. The detection limit was calculated and found to be 14.0pM. The 1:1 binding of analyte with the fluorophore (host/guest) was supported with UV–vis and 1H NMR spectroscopes. Several important experimental findings were further supported theoretically by DFT and TD-DFT calculations.

Modulation of sensitivity to mechanical stimulus in mechanofluorochromic properties by altering substituent positions in solid-state emissive diiodo boron diiminates

This manuscript describes modulation of the sensitivity to mechanical forces in diiodo boron diiminates by altering the substituent position of iodine groups.

Indene-1,3-dionemethylene-4H-pyran derivatives containing alkoxy chains of various lengths: aggregation-induced emission enhancement, mechanofluorochromic properties and solvent-induced emission changes

Indene-1,3-dionemethylene-4H-pyran derivatives exhibiting alkoxy-chain-length-dependent optical properties.

The effect of N-alkyl chain length on the photophysical properties of indene-1,3-dionemethylene-1,4-dihydropyridine derivatives

AIEE-active indene-1,3-dionemethylene-1,4-dihydropyridine derivatives exhibit reversible alkyl-length-dependent mechanofluorochromic properties.