A twisted molecular conformation is known to be beneficial to the formation of mechanochromic (MC) phenomenon for organic fluorescent molecules, but it is sometimes not a decisive factor. Up to now, the clear design guidance for MC-active materials based on these twisted molecules is still lacking. Herein, four twisted 1,4-dihydropyridine derivatives (3a–3d) with a simple structure were synthesized to investigate the effect of N-substituted groups on their potential MC activities. Compound 3d containing 2-phenylethyl group presented reversible high-contrast MC properties, while 3a with an ethyl group and 3c with a 1-phenylethyl group were MC-inactive. Moreover, 3b with a benzyl group had two different crystalline forms, interestingly, the blue-emitting 3b-B was MC-active, whereas the green-emitting 3b-G was MC-inactive. The results indicated that N-substituted groups played a decisive role in determining whether or not these compounds had MC activities. Through the careful analyses of their X-ray diffraction and differential scanning calorimetry experiments, it was found that the phase transformation between different crystalline states was responsible for the reversible MC properties of 3b-B and 3d. Furthermore, the red shift of their fluorescence spectra was ascribed to the planarization of molecular conformation and the resultant enhancement of the degree of π-electron conjugation. This work provides a feasible design direction for the development of efficient MC materials based on twisted fluorescent molecules.

A series of 3-, 4-, and 5-position indole-substituted dicyanomethylene-4H-pyran (I3PM, I4PM, and I5PM) derivatives with different alkyl chains were synthesized and their mechanofluorochromic (MFC) properties were investigated. Compared with the alkyl chains, the isomerization of the indole unit plays a more important role in regulating the MFC properties of these compounds. The increase of the alkyl chain length and the isomerization of the alkyl chain are favorable to the effective mechanofluorochromism of the I3PM derivatives, however, the former exhibits the opposite effect on that of the I4PM derivatives. The opposite alkyl length-dependent MFC behavior is closely related to the degree of distortion of the molecular conformation, namely the more twisted the molecular conformation, the more pronounced the MFC phenomenon. In particular, the introduction of the alkyl chains is completely detrimental to the MFC properties of the I5PM derivatives. X-Ray diffraction experiments indicate that the MFC properties of some of these compounds should be mainly attributed to the transformation from a crystalline state to an amorphous state because the weak intermolecular interactions and loose molecular packing modes in their solid-state samples can be easily damaged under external force stimuli as revealed by the single crystal X-ray diffraction analysis.

Herein, we report the synthesis of a new 1,4-dihydropyridine derivative containing 2,2-dimethyl-1,3-dioxane-4,6-dione and N,N-dimethylaniline. This compound exhibits aggregation-induced emission properties due to its highly twisted conformation and it has three crystalline polymorphs, which exhibit yellow (DHPM-y), orange (DHPM-o), and red (DHPM-r) fluorescence. The different emissions of the polymorphs mainly depend on their molecular conformations due to the weak intermolecular interactions, as revealed by single crystal structural analysis. More importantly, these three crystalline polymorphs show different mechanochromic phenomena under varying pressure. The fluorescent color changes in DHPM-o and DHPM-r upon gentle grinding are attributed to a crystal-to-crystal transformation which alters their molecular conformations, whereas those of DHPM-y and DHPM-o upon strong grinding are ascribed to the transformation from a crystalline state to an amorphous state. Additionally, DHPM-y and DHPM-o also display thermochromic properties. These results will provide useful information for obtaining intriguing multifunctional fluorescent materials by rational modification of classic luminogens.

Organic compounds exhibiting polymorphic and/or mechanochromic (MC) properties are promising for applications in multiple areas. However, the design strategy for such compounds is not very clear. Herein, several series of N-alkylated 1,4-dihydropyridine (DHP) derivatives incorporating different electron-withdrawing end groups were synthesized and compared. The electron-withdrawing groups were responsible for their polymorphic and MC properties. Additionally, a number of polymorphs of these DHP derivatives showed a decreasing trend as length of the alkyl chain increased, indicating that a longer alkyl chain was not conducive to formation of the polymorphs. Although differences in emissions of the polymorphs were mainly attributed to their different intermolecular interactions and molecular packing patterns, a subtle difference in the distances of their intermolecular interactions could also be a key factor in the formation of specific polymorphs. Different polymorphs of DHP derivatives could be interconverted by a simple recrystallization process with a specific solvent or through the application of pressure and vapor stimuli. Additionally, the MC properties of these DHP derivatives were ascribed to a phase transition between different crystalline states, instead of the more common transformation between crystalline and amorphous states.

•Synthesis of three AIE-active 1,4-dihydropyridine derivatives.•The compound based on 2-methylene-1H-indene-1,3(2H)-dione exhibits reversible piezochromism.•These compounds show different acid/base-induced solid-state fluorescence switching properties.•These compounds can be used for MCF-7 cell imaging.Three D-π-A 1,4-dihydropyridine derivatives with aggregation-induced emission characteristics were synthesized. The molecules comprise of a 4-(dimethylamino)styryl group as the electron donor group and different end groups, dicyanomethylene, vinylcyanoacetate, and 2-methylene-1H-indene-1,3(2H)-dione, as the electron acceptor, respectively. These target compounds display different stimulus-responsive fluorescent properties in the solid state. The original samples with the dicyanomethylene and vinylcyanoacetate groups do not show fluorescence color changes in response to external force stimuli, whereas the compound containing the 2-methylene-1H-indene-1,3(2H)-dione unit exhibits reversible piezochromism and solvent-induced emission changes due to the transformation between the crystalline and amorphous states, which can be ascribed to their different molecular stacking mode in the solid state. Furthermore, these compounds show different acid/base-induced solid-state fluorescence switching properties due to the different sites of the protonation. Additionally, all of the derivatives can be fabricated into biocompatible fluorescent nanoparticles and used for MCF-7 cell imaging.

•Synthesis and characterization of novel D-π-A benzo[c][1,2,5]selenadiazole-based derivatives.•The new compounds exhibit positive solvatochromism.•The band gaps of the new compounds are in the range of 2.21–2.70 eV.•Application as fluorescent sensors using benzo[c][1,2,5]selenadiazole as an acceptor or a fluorophore.A series of novel D-π-A benzo[c][1,2,5]selenadiazole (BSe)-based derivatives 7a–i including electron-donating/electron-withdrawing groups were prepared by Pd-catalyzed Sonogashira–Hagihara reaction of 4,7-diethynylbenzoselenadiazole with various diiodo aryl compounds. Photophysical properties of these compounds were investigated by absorption and emission spectra analyses. Compound 7c containing methoxy group exhibited positive solvatochromism and its solvatochromic properties were analyzed by the Lippert–Mataga equation and Kamlet–Taft equation, respectively. The resulting compounds showed tunable band gaps in the range of 2.21–2.70 eV using theoretical calculations, which was in good agreement with the results derived from the absorption of UV–vis spectra. In addition, fluorescent sensors 7c and 7d showed highly sensitive response for Hg2+ and traces of water using BSe as an acceptor or a fluorophore in real-time detection, respectively. These findings indicate that BSe-based molecules can be developed as excellent fluorophores for fluorescent material applications.

A series of dicyanomethylene-1,4-dihydropyridine (DCMP) derivatives with aggregation-induced emission (AIE) were designed and synthesized. These target compounds emit low fluorescence in THF solutions because of the free rotation of the phenyl and dihydropyridine rings about the axes of the olefinic double bonds and the resultant nonradiative decay process, but they exhibit strong fluorescence in the aggregate state because of the restriction of intramolecular rotation (RIR), as confirmed by solution thickening and cooling experiments. Compared with the almost-planar dicyanomethylene-4H-pyran (DCM) derivative DCM-1 with aggregation-caused quenching (ACQ), DCMP-5 with AIE takes a highly twisted conformation by replacing the oxygen atom of the DCM skeleton with an N-ethyl group, as revealed by crystallographic data and theoretical calculations. Depending on the different electron-donating substituents, the solids of these compounds emit yellow or orange fluorescence. Moreover, the cyano group endows DCMP-5 with a strong self-assembly ability under proper conditions, and the obtained regular microparticles emit bright orange fluorescence. These materials will broaden the new family of AIE-exhibiting fluorophores.

Three donor (D)-π-acceptor (πA) indene-1,3-dionemethylene-1,4-dihydropyridine (IDM-DHP) derivatives with triphenylamine (TPA)/bis(diphenylamino)triphenylamine (BDPA-TPA) end groups were designed and synthesized. These target compounds with highly twisted conformations showed aggregation-induced emission enhancement properties in their THF/water mixtures due to the restriction of intramolecular rotation, and distinct piezofluorochromic (PFC) properties in the solid state. Interestingly, solvent-induced emission changes similar to those resulting from PFC properties can be achieved by a simple dissolution–desolvation process in different solvent systems, such as chloroform, THF, and dichloromethane. X-ray diffraction experiments revealed that the transformations between crystalline and amorphous states were responsible for the PFC properties and solvent-induced emission changes. Moreover, these compounds exhibited remarkable and reversible acid/base-induced fluorescence switching properties in both solution and the solid state. The results indicate that the IDM-DHP derivatives with a TPA/BDPA-TPA unit exhibit intriguing multi-stimulus-responsive fluorescent behaviors. The current study will help researchers to design and synthesize more aggregation-induced emission/aggregation-induced emission enhancement-active multifunctional stimulus-responsive fluorescent materials.

A conjugated polymer P-1 based on benzoselenadiazole was synthesized by the polymerization of 1,4-bis[3′-(N,N-diethylamino)-1′-oxapropyl]-2,5-diiodobenzene (M-1) and 4,7-diethynylbenzoselenadiazole (M-2) via Pd-catalyzed Sonogashira–Hagihara coupling reaction. The water-soluble conjugated polyelectrolyte P-2 was obtained by the reaction of P-1 with ethyl bromide. The responsive optical properties of P-1 and P-2 on various metal ions were investigated by fluorescence spectroscopy in real-time detection. P-1 showed highly sensitive and selective detection of Fe3+ by ‘turn-on’ fluorescence enhancement in DMSO/H2O. P-2 exhibited outstanding sense of Hg2+ using BSe as receptor by ‘turn-off’ fluorescence quench type and Fe3+ using unionized triethylamino group as receptor by ‘turn-on’ fluorescence enhancement type in MeOH/H2O, respectively. Additionally, the addition of EDTA to the P-2–metal complexes could result in the fluorescence recovery of the P-2 solution. Fluorescence spectral responses as output signals and two independent molecular input signals based on P-2 could be used to demonstrate important logic operation like INHIBIT and IMPLICATION gates.

Reports about fluorescent sensors for the highly sensitive and selective detection of Ni2+ are rare compared with Hg2+, Ag+, Cd2+, Pb2+, Cr3+, etc. Herein, we report the synthesis and application of two conjugated polymer fluorescent sensors, P-1 and P-2, using benzochalcogendiazole and triazole as cooperative receptors of Ni2+. The synthesis was carried out by the polymerization of 4,7-diethynylbenzoselenadiazole (M-2) and 4,7-diethynylbenzothiadiazole (M-3) with 2,7-diazido-9,9-dioctyl-9H-fluorene (M-1)via copper-catalyzed azide–alkyne cycloaddition. P-1 and P-2 exhibit emission peaks at 535 nm and 514 nm, and show yellow and green fluorescence, respectively. These two polymers showed outstanding fluorescence response behavior toward Ni2+ during real-time detection. Compared with benzothiadiazole-based P-2, benzoselenadiazole-based P-1 showed higher sensitivity and selectivity, and its detection limit could reach 2.4 nM, which might be due to the specific electronegativity of selenium. This work increases the application scope of the click reaction in regard to the design and synthesis of novel conjugated polymer fluorescent sensors based on the benzochalcogendiazole unit.

A series of novel indeno[2,1-a]phenalene derivatives 4a–j including electron-donating or electron-withdrawing groups were prepared by Pd-catalyzed Sonogashira–Hagihara reaction of 7-iodo-12-phenylindeno[2,1-a]phenalene with various arylacetylene. Photoelectric properties of these compounds were investigated by UV–vis absorption and fluorescence. The fluorescent emission of 4d containing dimethyl amino group is correlated with solvent polarity and environmental acidity, which indicates that this kind of compound can be used as solvent polarity sensor and pH sensor. In addition, the band gaps of these indeno[2,1-a]phenalene derivatives can be tuned in the range 1.78–2.09 eV by using different substituted groups. These findings indicate that indeno[2,1-a]phenalene-based molecule can be developed as an excellent fluorophore for optoelectronic material applications.Graphical abstractHighlights► Synthesis and characterization of novel indeno [2,1-a] phenalene derivatives. ► The emission spectra show solvent polarity and environmental acidity dependence. ► The band gaps of the new compounds are in the range of 1.78–2.09 eV.

Two coumarin-type fluorescent sensors were synthesized and their fluorescence response to pH value was investigated. The fluorescence intensity of sensor 3 and sensor 4 is obviously enhanced along with the increase of pH from 7 to 12 and the reduction of pH value from 8 to 1, respectively. Possible mechanism for these fluorescence recovery systems is proposed. Intramolecular hydrogen bond could be formed under different condition, which blocks electron transferring route from nitrogen atom to fluorophore. The blue fluorescence color change of the two sensory systems could be directly detected by naked eyes under UV-lamp for pH values.Figure optionsDownload full-size imageDownload as PowerPoint slide

Three novel donor-π-acceptor (D-π-A) type conjugated polymers P-1, P-2, and P-3 based on benzo[c][1,2,5]selenadiazole moiety and phenyl or naphthyl group via alkyne module were firstly prepared by Sonogashira–Hagihara reaction of various diiodo aryl compounds with the key monomer 4,7-diethynylbenzo[c] [1,2,5]selenadiazole (M-1), which was synthesized by a strategy of firstly introducing the trimethylsilylacetylene flexible group, and then introducing the selenium atom. The polymers displayed obvious absorption peaks at the region from 503 to 510 nm and narrow orange-red or red fluorescence in the range of 576–595 nm because benzo[c][1,2,5]selenadiazole unit can effectively reduce the lowest unoccupied molecular orbital (LUMO) energy levels of these polymers. The band gaps of these polymers can be tuned in the range of 1.37–1.76 eV by using different aryl donor groups. These findings indicate that these benzo[c][1,2,5]selenadiazole-based conjugated polymers can be developed for excellent fluorescent materials.

•Synthesis and characterization of four AIE-active 4H-pyran derivatives.•Two pairs of positional isomers exhibit different stimuli-responsive solid-state fluorescent properties.•These compounds show mechanochromic and/or acidochromic properties.•These fluorescent sensors can be used to establish molecular logic gates.Several 1- and 2-position naphthylethylene-substituted 4H-pyran derivatives comprising two different end groups were synthesized. These two pairs of positional isomers with aggregation-induced emission properties exhibit different stimuli-responsive solid-state fluorescent properties. Both the linking position of the naphthylethylene and the electron-withdrawing end group have a significant impact on the mechanochromic properties of the compounds, whereas, the acidochromic properties are only related to the latter. The transformation between amorphous and crystalline states accounts for the mechanochromic properties and is attributed to loose molecular packing with weak intermolecular interactions, as revealed by crystallographic data. The reversible acidochromic properties are mainly ascribed to protonation/deprotonation of the carbonyl group, which is confirmed by 1H NMR experiments. Additionally, because the emission of these 4H-pyran derivatives in solid state can be tuned by external stimuli, several molecular logic gates were established by using appropriate stimuli operations as the inputs and fluorescent colors as the outputs.

Organic compounds exhibiting polymorphic and/or mechanochromic (MC) properties are promising for applications in multiple areas. However, the design strategy for such compounds is not very clear. Herein, several series of N-alkylated 1,4-dihydropyridine (DHP) derivatives incorporating different electron-withdrawing end groups were synthesized and compared. The electron-withdrawing groups were responsible for their polymorphic and MC properties. Additionally, a number of polymorphs of these DHP derivatives showed a decreasing trend as length of the alkyl chain increased, indicating that a longer alkyl chain was not conducive to formation of the polymorphs. Although differences in emissions of the polymorphs were mainly attributed to their different intermolecular interactions and molecular packing patterns, a subtle difference in the distances of their intermolecular interactions could also be a key factor in the formation of specific polymorphs. Different polymorphs of DHP derivatives could be interconverted by a simple recrystallization process with a specific solvent or through the application of pressure and vapor stimuli. Additionally, the MC properties of these DHP derivatives were ascribed to a phase transition between different crystalline states, instead of the more common transformation between crystalline and amorphous states.

Reports about fluorescent sensors for the highly sensitive and selective detection of Ni2+ are rare compared with Hg2+, Ag+, Cd2+, Pb2+, Cr3+, etc. Herein, we report the synthesis and application of two conjugated polymer fluorescent sensors, P-1 and P-2, using benzochalcogendiazole and triazole as cooperative receptors of Ni2+. The synthesis was carried out by the polymerization of 4,7-diethynylbenzoselenadiazole (M-2) and 4,7-diethynylbenzothiadiazole (M-3) with 2,7-diazido-9,9-dioctyl-9H-fluorene (M-1)via copper-catalyzed azide–alkyne cycloaddition. P-1 and P-2 exhibit emission peaks at 535 nm and 514 nm, and show yellow and green fluorescence, respectively. These two polymers showed outstanding fluorescence response behavior toward Ni2+ during real-time detection. Compared with benzothiadiazole-based P-2, benzoselenadiazole-based P-1 showed higher sensitivity and selectivity, and its detection limit could reach 2.4 nM, which might be due to the specific electronegativity of selenium. This work increases the application scope of the click reaction in regard to the design and synthesis of novel conjugated polymer fluorescent sensors based on the benzochalcogendiazole unit.