Mechanofluorochromic Properties Research Articles

Unique mechanofluorochromism and reversible multicolor switching of triphenylamine-functionalized sulfonyl hydrazone derivatives

Two new simple triphenylamine-based sulfonyl hydrazone derivatives DBBS ((E)-N'-(4-(diphenylamino)benzylidene)benzenesulfonohydrazide) and DBMS ((E)-N'-(4-(diphenylamino)benzylidene)-4-methylbenzenesulfonohydrazide) were designed and synthesized via one-step aldimine condensation reaction to investigate the influence of terminal methyl group on their photophysical, crystal stacking and mechanofluorochromic (MFC) properties. Interestingly, reversible multi-color fluorescence tuning and switching mechanofluorochromism was found for DBBS due to the nature of polymorphs, and DBMS exhibited reversible bicolor switching mechanofluorochromism. Additionally, the reversible MFC behavior was ascribed to the phase transition between the crystalline and amorphous states, which have been confirmed by powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FESEM) and differential scanning calorimetry (DSC) results. The obtained results showed that terminal methyl group did not only regulate molecular conformations and/or intermolecular interactions in crystals, but also affect the force-stimuli responsiveness. The present study provided insight into the development of multicolor switching materials by rational modification of classic luminogens.

The effect of –CF3 substitution position on the aggregation-induced emission, solvatofluorochromism and mechanofluorochromism properties of the conjugated 9-butyl-9H-carbazole phenylacrylonitrile derivatives

A series of 9-butyl-9H-carbazole derivatives with intense solid-state emission were synthesised by the Knoevenagel reaction of 9-butyl-9H-carbazole-3-carbaldehyde with phenyl acetonitrile derivatives with –CF3 group at different substituent positions of the phenyl group. They exhibit remarkable aggregation-induced emission (AIE), solvatofluorochromism and mechanofluorochromic (MFC) properties depending on the –CF3 substitution site. The ortho-substituted derivative shows enhanced charge transfer properties with a distinct dual-state fluorescence emission. This peculiar property is further utilised for data encryption and anti-counterfeiting applications. The para-substituted derivative exhibits an increased solid-state fluorescence quantum yield of 88% with switchable MFC properties. The ground and excited state photophysical properties of the synthesised luminogens were explored using various spectroscopic techniques. The study provides an efficient strategy to understand the effect of the –CF3 substitution position in tuning and modifying the luminescence properties of the organic solids via subtle modifications in the molecular scaffolds.

Fluorine-tuned dual-state luminogens for versatile applications in mechanofluorochromism, trace water analysis, and live cell imaging

Dual-state luminogens (DSEgens) are organic luminescent molecules that exhibit strong emissions in both molecularly dispersed and aggregated states. The introduction of fluorine into organic conjugates has been shown to influence their photophysical properties, but its effect on DSEgens remains unexplored. Herein, we synthesized three naphthalimide-arylimidazole luminogens, namely NIBH, NIBPF, and NIBOF, with fluorinated or nonfluorinated phenyl groups on the pyrrole-like nitrogen. We systematically investigated their photophysical properties using spectroscopic studies, theoretical calculations, and single crystallography. Our results demonstrate that fluorine substitution in DSEgens leads to shortened electronic conjugation, highly twisted molecular conformations, and loose three-dimensional packing arrangements. Consequently, fluorinated DSEgens, such as NIBPF and NIBOF, exhibited a significant spectral blueshift and more pronounced mechanofluorochromic properties compared to the non-fluorinated NIBH. Intriguingly, NIBPF with a para-position fluorine substituent displayed relatively redshifted and stronger emission in tetrahydrofuran solutions, while showing a blueshifted and lower emission in the solid state compared to NIBOF, which has an ortho-position fluorine substituent. These findings confirm the efficacy of fluorine as a small substituent to tune the photophysical properties of DSEgens in both dilute solutions and the solid state. Furthermore, the three bright DSEgens were employed for mechanofluorochromism, trace water analysis, and live-cell imaging applications.

Mechanofluorochromic behaviours of functionalized carbazole-based difluoroboron compounds with different aryl substituents moieties.

To better understand the relationship between molecular structure and mechanofluorochromic characteristics, three carbazole-based N^O-chelated difluoroboron compounds (Cz-S-BF2 , Cz-PhNp-S-BF2 , and Cz-BNp-S-BF2 ) with different aryl substituents moieties were designed and synthesized. The mechanofluorochromic behaviours of Cz-S-BF2 (luminescence from bluish-green to yellowish-green, emission from 504 to 535 nm) without aryl substitution and Cz-PhNp-S-BF2 (luminescence from green and yellow, emission from 521 to 557 nm) with a phenyl-naphthalene group underwent reversible conversion using the grinding-fuming process. For Cz-BNp-S-BF2 this was not apparent due to the well coplanarity of the binaphthalene moiety. Mechanofluorochromic properties were demonstrated through XRD patterns measurement. We envisage that this study will provide a practicable reference to acquire organic molecules with mechanofluorochromic characteristics.

Crystal packing and mechanofluorochrmism of distyrylanthracene derivatives modulated by hydrogen bonds

Two distyrylanthracene derivatives with ester and carboxyl moieties (AnDMB and AnDBA) were synthesized to investigate the influence of terminal group on their photophysical, crystal stacking and mechanofluorochromic (MFC) properties. Spectral and theory calculation results indicate that two compounds have similar absorption because their similar energy levels although they have different terminal groups. Both compounds exhibited crystallization-induced emission enhancement (CIEE). AnDMB crystals are rod-like and emitted strong blue-green fluorescence because of weak π-stacking. The blocky AnDBA crystals with DMAC (N, N-dimethylacetamide) molecules have a red-shifted emission. Moreover, instead of forming a hydrogen bond dimer between the carboxyl groups, a strong hydrogen bond is formed between the carbonyl group of DMAC and the carboxyl group. Crystal structure indicates that enhanced π-stacking should be responsible for its longer wavelength emission. Moreover, the crystals of two compounds could change their emission colors under force stimuli, but AnDMB exhibited higher color contrast than that of AnDBA. A shift of 40 nm might be observed when the AnDMB crystals were ground, while an emission spectral shift of 18 nm emerged after AnDBA crystals were ground. These results indicated that hydrogen bond did not only regulate molecular stacking and fluorescence properties in crystals, but also adjusted force-stimuli responsiveness.

Asymmetric tetraphenyl-substituted butadienes with aggregation-induced emission characteristic: Synthesis, properties and applications

Six asymmetric tetraphenyl-1,3-butadiene derivatives were firstly designed and synthesized. Their photophysical properties showed the six derivatives having aggregation-induced emission (AIE) characteristics. The single crystal structure of indanedione-containing tetraphenyl-1,3-butadiene showed that the distorted structure and a variety of strong intermolecular interactions limited the intramolecular movement to enhance the fluorescence emission. Cyanoacetate-containing tetraphenyl-1,3-butadiene (TPB-NC) and dimethylbarbituric-containing tetraphenyl-1,3-butadiene (TPB-BC) exhibited typical mechanofluorochromic (MFC) properties. Powder X-ray diffraction and polarizing microscope experiments were carried out to reveal the mechanochemical mechanism, and the results showed that MFC was mainly attributed to a transition from crystalline state to amorphous state. In addition, TPB-NC could selectively respond to butyl cholinesterase (BuChE) and glycyrrhetinic acid (GA) in real-time. The low limit of detection for BuChE and GA were 1.61 μg/mL and 0.03 μg/mL according to IUPAC-based approach, respectively. The work has significant role in guiding the design of AIE molecules and enlarging their applications.

Novel fluorescent probes based on NBD-substituted imidazole amino to sequentially detect H2S and Zn2+

Hydrogen sulfide (H2S), as an important participant in physiological processes, affects the function of many cells and plays a significant role in freshness indicator of protein foods. In order to improve the detection of H2S in vitro and in vivo, fluorescent ‘off-on’ chemodosimeter for sequential detecting H2S and Zn2+ with low detection limit was designed and synthesized in this study. Results showed that, in the detection of H2S, Probe 2 released Probe 1 with “AIE + ESIPT” and reversible mechanofluorochromic properties with the addition of Zn2+. Furthermore, the Probe 2-based test kit was successfully prepared to detect the freshness of pork. As Probe 2 could be used to detect H2S and Probe 1 could be used to detect Zn2+ with strong fluorescence change in both water and living cells, Probe 1 and Probe 2 could be great candidates in the detection of H2S and Zn2+ both in vitro and in vivo.

Synthesis, structure and mechanofluorochromic properties of phenothiazine-S-oxide and phenothiazine-S,S-dioxide derivatives

In this work, two novel luminescent molecules containing distorted phenothiazine-S-oxide and phenothiazine-S,S dioxide skeletons were synthesized by oxidation reactions using different oxidants (m-chloroperoxybenzoic acid, acetic acid /hydrogen peroxide). The target compounds were all confirmed by 1H NMR, 13C NMR and EI-MS. Combined with the results of UV–vis absorption spectra and fluorescence emission spectra, we found that the different oxidation states of S-atom, from sulfide (+2) to sulfoxide (+4) and sulfone (+6), led to the blue, yellow-green and yellowish fluorescence of these compounds in the solid states. Subsequent studies showed that the molecule containing the phenothiazine-S-oxide skeleton exhibited obvious solvatochromism, and the increase of solvent polarity induced a red-shift in the emission wavelength. Moreover, this molecule also exhibited a rare self-recovery mechanochromatic behavior. In addition, these properties were further confirmed by theoretical calculations and X-ray single-crystal diffraction analysis.

Dual-state emission and mechanofluorochromic properties of coumarin derivatives containing α-cyanostilbene

Four novel coumarin derivatives (CM-H, CM-F, CM-CH3 and CM-CF3) functionalized by α-cyanostilbene unit were designed and synthesized. The molecular spatial structure and photophysical performances were systematically studied and discussed. The fluorescent spectra indicate that these coumarin derivatives display dual-state emission (DSE) characteristic. In dilute organic solutions, strong fluorescent emissions originated from intramolecular charge transfer (ICT) process, and in aggregated states, the bright fluorescent emissions are attributed to the collective effects of restriction of intramolecular motions and J-aggregation stacking. Meanwhile, these coumarin derivatives all exhibited mechanofluorochromic (MFC) property, in particular, compound CM-CF3 exhibit the reversible conversion between yellow and red emission. The XRD and SEM experimental results confirm that the MFC mechanism is based on the conversion of degree of crystallinity. Our study provides a reasonable approach for designing DSE materials with the MFC property.

Intramolecular H-type dimer and excimer formation in dibenzoylmethanatoboron difluoride dyads connected via diphenylsiloxane linkers

A series of new dibenzoylmethanatoboron difluoride (DBMBF2) and methoxy-substituted DBMBF2 dyads linked via flexible diphenylsiloxane linkers of various length have been synthesized and characterized. The structure and optical properties of these compounds were studied by X-ray single-crystal measurements, electron absorption, and steady-state and time-resolved fluorescence spectroscopy. In the dyads under study, which are in an equilibrium between the open and folded conformations, the formation of intramolecular H-dimers in the ground state are observed. For the dyads in dilute solutions in various solvents, both monomer and excimer (through intramolecular excimer formation) emissions are observed at room temperature. The effect of solvent polarity and linker length on the formation of intramolecular excimers has been studied. The contribution of excimers into the total fluorescence spectrum decreases with increasing the solvent polarity, and the length of the siloxane linker. The studied dyads in hexane and cyclohexane solutions have CIE chromaticity coordinates very close to the white color point. The mechanofluorochromic properties of the synthesized dyads were studied.

Aggregation-induced emission and mechanofluorochromic materials based on the structural modification of 4,5,6-triaryl-4H-pyran skeleton

Although aggregation-induced emission (AIE)-active molecules with twisted structures have become the mainstream of mechanofluorochromic (MFC) materials, twisted molecular conformations do not necessarily lead to MFC properties. There have been some examples of failure based on 4,5,6-triaryl-4H-pyran skeleton. Herein, a series of 4,5,6-triaryl-4H-pyran derivatives were designed and synthesized to obtain fluorescent materials with both AIE and MFC characteristics. The results reveal that the introduction of triphenylamine endows all these propeller-type compounds with typical AIE properties originating from the restriction of intramolecular rotation, while the introduction of halogen atoms and the imidization of amino group are both conducive to the generation of the MFC activity by reducing the crystallization ability of derivatives. Furthermore, the red-shifted MFC phenomena are demonstrated to be ascribed to the planarization of molecular conformation caused by the crystalline-to-amorphous transition upon external pressure. This work confirms that appropriate crystallinity is very important for the candidate AIE-active compounds to exhibit MFC activities, which can be achieved by precise structural modification.

D-A-D type cyclohexyl and cycloheptyl-modified benzo[d]imidazole derivatives with different aggregation-induced emission enhancement (AIEE) and mechanofluorochromic properties

Four tetraphenylethylene or triphenylamine-functionalized donor-acceptor-donor type benzo[d]imidazole derivatives 1–4 with different cycloalkyl substituents were successfully developed and characterized. All of these fluorogenic compounds showed remarkable aggregation-induced emission enhancement characteristics, and these luminogens emitted high brightness orange or green fluorescence in the aggregation state. Furthermore, these cyclohexyl or cycloheptyl-modified benzo[d]imidazole molecules exhibited tunable solid-state fluorescence effect, and their solids demonstrated contrasting mechanically triggered fluorescence responses. More specifically, luminogens 1–3 showed no observable fluorescence changes under mechanical stimulation, while tetraphenylethylene-based cycloheptyl-modified benzo[d]imidazole luminogen 4 displayed reversible mechanochromic fluorescence conversion between green and yellowish-green emission colors. The mechanofluorochromic mechanism of solid 4 was revealed by the powder X-ray diffraction measurements, and the experimental results proved that the phase interconversion between crystalline and amorphous states was responsible for the noticed switchable mechanoresponsive fluorescence. In addition, the mechanical force-sensitive emission of fluorogenic molecule 4 afforded its promising application in ink-free writing.

Mechanochromism and crystallization-induced emission enhancement of carbazole derivatives with different terminal groups

Two D-A carbazole derivatives (TBCA and NCA) were synthesized to investigate the influence of terminal group on the photophysical and mechanofluorochromic (MFC) properties in solution and crystal state. Results suggest that TBCA with t-butylphenyl as terminal group has a slightly smaller optical bandgap than that of NCA with naphthyl as terminal group because NCA adopts a more distorted conformation. Both compounds exhibited crystallization-induced emission enhancement (CIEE). TBCA crystals are rod-like and emitted strong blue fluorescence and the lamellar NCA crystals have a red-shifted emission in comparison with that of TBCA. The single-crystal structure analysis indicates that the two molecules adopt a more distorted conformation in their crystal. CN⋯H–CC hydrogen bonds help TBCA molecules to form a 1D chain stacking, but no strong π-stacking between TBCA molecules exists. Obvious π-π interaction is present between the carbazole units in NCA crystal, which results in a longer emission wavelength that of TBCA. Furthermore, two compounds exhibited reversible MFC behaviors. Force stimuli promoted a larger bathochromic shift for TBCA than that of NCA because a more pronounced planarization for TBCA existed after force damaged ordered stacking in crystal.

Design, synthesis, and mechanochromic luminescence of twisted donor–acceptor bisarylic methanone derivatives

Two bisarylic methanone derivatives, DBF-BZ-POZ and DBF-BZ-PTZ, which have phenoxazine or phenothiazine as electron donors and carbonyl group as electron acceptor are presented. Both molecules have highly distorted molecular conformation and exhibit obvious intramolecular charge transfer (ICT), effective solid-state emissions, and high-contrast mechanofluorochromic (MFC) properties. Both DBF-BZ-POZ and DBF-BZ-PTZ have good solid-state luminescence, efficiencies that reach 33.5% and 38.9%, respectively. The fluorescence colors of the as-prepared powders for both molecules are bright blue-green. The colors change to bright yellow with external mechanical stimulation and red shifts of 104 and 82 nm are observed in the photoluminescence (PL) spectra. Moreover, the MFC behavior of DBF-BZ-POZ and DBF-BZ-PTZ are reversible when their ground samples are subjected to fuming with the DCM vapor. Powder X-ray diffraction results indicate that the MFC phenomenon of DBF-BZ-POZ and DBF-BZ-PTZ may be attributed to crystalline-to-amorphous transitions in the material. The red shifts observed in the PL spectra after the material is ground are due to the smaller band gap induced by the planar intramolecular charge transfer (PICT) and the increased π-π interactions, the increase of exciton coupling, as well as the increased orbital overlap between neighboring molecules.

D-π-A type carbazole and triphenylamine derivatives with different π-conjugated units: Tunable aggregation-induced emission (AIE) and mechanofluorochromic properties

Eight donor-π-acceptor (D-π-A) skeleton high brightness luminogens 1–8 with various π-conjugated units are designed, synthesized, and fully characterized. Among synthetic luminophores, 7 and 8 possess typical aggregation-induced emission properties, and these reported fluorophores display different fluorescence in the solid state. Meanwhile, their solid-state emission behaviors can be effectively regulated by mechanical grinding except for solids 5 and 7. More specifically, the solid-state fluorescence of D-π-A type compounds 1, 3, 4, and 6 can be reversibly switched by successive mechanical grinding and solvent fuming, while D-π-A type compounds 2 and 8 display irreversible mechanofluorochromism behaviors. Remarkably, when the pristine powders of these mechanical force-responsive fluorophores are ground, 1 and 2 exhibit an obvious blue-shift, while 3, 4, 6, and 8 show a red-shift. The powder X-ray diffraction measurements demonstrate that these observed diverse mechanoresponsive fluorescence phenomena are attributed to the different morphological interconersion between the crystalline and amorphous states.

Highly emissive D-A-π-D type aggregation-induced emission (AIE) or aggregation-induced emission enhancement (AIEE)-active benzothiadiazole derivatives with contrasting mechanofluorochromic features

Mechanochromic luminophors with strong solid-state emission are promising candidates for high-contrast mechanochromic luminescence materials. Meanwhile, mechanically responsive luminogenic molecules with tricolor switching are highly desirable but are seldom reported. In this work, three anthracene-based donor–acceptor-π-donor (D-A-π-D) type benzothiadiazole derivatives were designed and synthesized. These luminogens showed remarkable aggregation-induced emission (AIE) or aggregation-induced emission enhancement (AIEE) effect. Furthermore, these luminogens exhibited bright and different solid state fluorescence involving yellow-green, yellow and orange colors, and the fluorescence of their solids could be effectively regulated by mechanical grinding. For luminogen 1, its solid displayed reversible two-color mechanofluorochromic property. As for luminogens 2 and 3, their solids displayed fluorescent colors change from yellow to yellow-green upon slight grinding, and the yellow-green light-emitting solids were converted into orange fluorescent solids after heavy grinding, demonstrating interesting three-color mechanofluorochromism features.

Amphiphilic α-cyanostilbene mesogens with oligooxyethylene polar tails: syntheses and properties

ABSTRACT A series of amphiphilic rodlike α-cyanostilbene mesogens bearing an α-cyanostilbene core, with one/three lipophilic and flexible alkyl chains at one end and a polar oligooxyethylene (EO) at the opposite end, have been synthesized and investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), X-ray scattering, scanning electron microscope (SEM), UV-vis spectroscopy and photoluminescence as well as CV measurements. These compounds can self-assemble into photoresponsive liquid crystalline phases and possess AIEE characteristics, mechanofluorochromic properties and Fe3+ recognition properties. The conductivity of the lithium cation complex was also measured to be 2.6 × 10−7 S m−1. Such compounds can be considered as a kind of multifunctional materials.

Mechano-responsive D–A luminogen based on bisarylic methanone derivative with brightly tricolored mechanochromic luminescence

A new luminogen PTZ-BZ-DBF bearing an electron donor phenothiazine unit and an electron acceptor carbonyl group is designed and synthesized. This molecule adopts highly twisted molecular conformation and exhibits intramolecular charge transfer (ICT), effective solid-state luminescence, and high contrast mechanofluorochromic (MFC) properties. The as-prepared powders show bright yellow-green fluorescence. Under conditions of external mechanical stimulation, the color changes to bright orange-red, and a red shift of 87 nm is observed in the emission spectral profile. Meanwhile, the solid-state luminescence efficiency increases from 20.5% to 28.5%. It is different from general MFC materials, when fumed using DCM vapor or annealed at 150 °C, the fluorescence peaks blue shift to approximately 560 nm, and the yellowish-emitting Y-powders are obtained. The ground solids and Y-powders can be converted to the as-prepared solid powders following the process of solvent recrystallization. Thus, tricolor switching is realized. The results obtained using the PXRD and DSC techniques indicate that the MFC phenomenon can be attributed to the process of crystalline-to-amorphous transition. The red shift of the PL spectra is attributed to the planar intramolecular charge transfer (PICT) process induced by conformational planarization in response to external forces.

Construction of Mechanofluorochromic and Aggregation-Induced Emission Materials Based on 4-Substituted Isoquinoline Derivatives.

Although many organic mechanofluorochromic (MFC) materials have been reported, the design strategy and formation mechanism are still unclear, and thus some of their discoveries are based on chance or a result of blind screening of some compounds. Herein, we show a strategy for constructing MFC materials from a multisubstituted isoquinoline compound with excellent crystallinity. The introduction of bromine atom at 4-position realizes the morphological change from crystalline state to amorphous state under the stimulation of external pressure, but it is not enough to generate MFC phenomenon. Similarly, although naphthalene, benzofuran, and benzothiophene endow the resultant isoquinolines with twisted molecular conformations and aggregation-induced emission (AIE) activities, no obvious solid-state emission color changes are observed. However, the introduction of 2-benzylidenemalononitrile and (E)-2-cyano-3-phenylacrylate leads not only to the AIE activities of IQ-M and IQ-A, but also to their outstanding and reversible MFC properties. The MFC activities of these two derivatives are demonstrated to come from more twisted molecular conformations, larger deformation spaces, and stronger intramolecular charge transfer compared with the other isoquinolines. This work offers important reference value for the construction of MFC and AIE materials from traditional fluorophores.

AIE and reversible mechanofluorochromism characteristics of new imidazole-based donor-π-acceptor dyes.

Four new imidazole-based donor–π–acceptor 2a–2d dyes have been synthesized, and their solvatochromism, aggregation-induced emission (AIE) and mechanofluorochromic (MFC) properties were investigated. The new dyes 2a–2d were designed to have 1,4,5-triphenyl-1H-imidazole as an electron donor (D) and 1-indanone, 1,3-indandione, 2-phenylacetonitrile and 2-thiopheneacetonitrile as electron acceptors (A) linked through a phenyl bridge. The maximum absorption wavelength of 2a–2d dyes in DCM solution appeared at 376, 437, 368, and 375 nm, respectively. The dyes exhibit a high molar extinction coefficient (ε) and large Stokes shift, making them useful in optoelectronic applications. Solvatochromic properties of dyes 2a–2d have been studied and showed bathochromic changes in emission wavelengths, from 449 to 550 nm for 2a, 476 to 599 nm for 2b, 438 to 520 nm for 2c, and from 439 to 529 nm for 2d, as the solvent polarity increased from n-hexane to acetonitrile. Moreover, in dioxane/water mixture systems, AIE behaviors were observed, and the emission intensity of 2b–2d dyes increased by around 5, 3, and 3 times in the mixed solvent (dioxane : water = 10 : 90) in contrast to pure dioxane. In addition, the XRD data of the 2a–2d dyes in pristine, ground, and fumed states illustrate that the transition between the ordered crystalline and disordered amorphous phases is the primary cause of MFC behaviors mechanism. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) showed that the highest occupied molecular orbital (HOMO) of dyes is distributed on the donor unit. In contrast, the lowest unoccupied molecular orbital (LUMO) is mainly placed on the acceptor unit to reveal that the HOMO–LUMO transition has a great ICT character.