Jinyu Zhao

Carbazole modified salicylaldimines and their difluoroboron complexes: effect of the tert-butyl and trifluoromethyl terminal groups on organogelation and piezofluorochromism

New carbazole modified salicylaldimine derivatives 1a–3a and their difluoroboron complexes 1b–3b were synthesized. It was found that 3a and 3b bearing both tert-butyl and trifluoromethyl could form stable gels in some organic solvents, while no gel could be formed from 1a–1b and 2a–2b without tert-butyl or trifluoromethyl, respectively. It suggested that both tert-butyl and trifluoromethyl played key roles in the gel formation because they could tune the solubility of the compounds and adjust the strength of intermolecular interactions. It should be noted that there is no traditional gelation moiety involved in gelators 3a and 3b. The main driving force for the gelation of such nontraditional π-gelators was π–π interaction. Meanwhile, aggregation-induced emission (AIE) was detected during the organogelation process of 3a, and the gel–sol transition took place upon adding F−. Interestingly, complexes 1b and 3b exhibited reversible piezofluorochromic (PFC) behaviors upon grinding/fuming, which originated from the transformation between crystalline and amorphous states. Herein, we provided a new strategy for designing new stimuli-responsive multifunctional soft materials.

Bending, Curling, Rolling, and Salient Behavior of Molecular Crystals Driven by [2+2] Cycloaddition of a Styrylbenzoxazole Derivative

We report interesting photomechanical behaviors of the dynamic molecular crystals of (E)-2-(2,4-dichlorostyryl)benzo[d]oxazole (BOACl24). The photosalient effect of the rod-like crystal based on a metal-free olefin driven by photodimerization is observed. Moreover, the needle-like crystals of BOACl24 exhibit a reversible bending away from a UV light source. The nanofibers curl easily under UV irradiation in an organogel, in which the photo-induced rolling of a small slice occurs. This suggests that the rapid release of the accumulated strain during photodimerization may lead to a photosalient effect, and the bending or curling happens when the strain is released slowly. Notably, [2+2] cycloaddition takes place between two different conformational isomers of BOACl24 on account of the rotation of the benzoxazole ring around the C-C bond in an excited state before photodimerization. Such topo-photochemical reaction has not been reported elsewhere.

Aggregation-induced emission nanofiber as a dual sensor for aromatic amine and acid vapor

Two cyano-substituted vinylacridine derivatives linked with N-dodecyl-L-phenylalaninamide through succinyl (PC2VA) and glutaryl (PC3VA) groups were synthesized and were found to exhibit aggregation-induced emission properties. These two compounds could form gels in some solvents at low concentrations and exhibit fluorescence enhancement during gelation. The gelation abilities and microstructure intermolecular interactions of these compounds were different, although their molecular structures varied by only one methylene group. Moreover, a uniform fibrous film that could be used as an effective dual sensor on glass plates was prepared using PC2VA wet gels to quantitatively detect aromatic amine and volatile acid vapors caused by the low HOMO energy level of the fluorophore and the existence of the acridine moiety as an acid binding site. The response times and detection limits (DL) for the vapors are short and low, respectively. The DL of aniline reaches 1.3 μg m−3 because an amplifying fluorescence quenching exists within the long nanofibers induced by efficient exciton diffusion along the π–π stacking direction. Moreover, the DL for trifluoroacetic acid is low and determined to be 2.3 mg m−3.

Pyrimidine-containing β-iminoenolate difluoroboron complexes acting as non-traditional π-gelators and mechanofluorochromic dyes

New pyrimidine-containing β-iminoenolates and their difluoroboron complexes were synthesized. It was found that the ligand and the complex bearing tert-butyl could self-assemble into organogels in methylcyclohexane/1,4-dioxane and other solvents, but no organogel could be formed from the carbazole derivatives. This suggested that tert-butyl played a key role in the gel formation. It should be noted that no traditional gelation moiety was involved in the fully conjugated TCPPA ((Z)-2-(6-chloropyrimidin-4-yl)-1-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)et-hen-1-ol) and TCPPAB (6-chloro-3-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-1,1-diflu-oro-1H-1l4,9l4-pyrimido[1,6-c][1,3,2]oxazaborinine), and π–π interactions were found to be the main driving forces for the gelation. Moreover, the xerogel-based film of TCPPAB emitting strong yellow light could be used as a “turn-off” fluorescent sensor to detect TFA vapor with the decay time and detection limit of 0.8 s and 260 ppb, respectively. On the other hand, the β-iminoenolate difluoroboron complexes exhibited reversible mechanofluorochromic behavior. The as-synthesized crystals of TCPPAB emitted strong green light and could be transformed into powders emitting orange light upon grinding, and the fluorescence could be recovered when the ground powders were fumed with CH2Cl2. Such reversible MFC processes for TCPPAB were suggested to have resulted from the transformation between crystalline and amorphous states. Herein, we provide a strategy for designing new multi-stimuli-responsive soft materials.

H-bonding and C-H···π assisted mechanofluorochromism of triphenylamine-containing vinylheterocycles bearing cyano and methyl

New D–π–A-type triphenylamine-containing benzimidazoles, benzoxazoles and benzothiazoles have been synthesized. It was found that the introduction of a cyano group in vinyl and methyl in triphenylamine was favorable for producing mechanofluorochromism of the fused heterocyclic derivatives. The single crystal X-ray diffraction data revealed that the intermolecular interactions of the H-bonding formed from the cyano group and the C–H⋯π interaction formed from the methyl group would rigidify the molecular conformations, leading to strong solid-state emission. Interestingly, the solid-state emission of the synthesized triphenylamine-containing fused heterocycles bearing cyano and/or methyl groups could be tuned by external mechanical forces on account of the transition between crystalline and amorphous states or between a highly ordered crystalline state and a less ordered state.

Light-Induced Bending of Needle-Like Crystals of Naphthylvinylbenzoxazole Triggered by trans-cis Isomerization

New diarylethene derivatives containing benzoxazole (NBO) and benzothiazole (NBT) have been synthesized. Light-induced trans-cis isomerization of NBO and NBT took place in crystals, and only induced the needle-like crystals of NBO to bend backwards away from the UV light source. The movement of the atoms was deemed to take place during the isomerization of NBO; hence, strain would be produced and accumulated rapidly in the surface of crystals exposed to UV light. The uniform release of strain led to the bending of needle-like crystals. The light-induced trans-cis isomerization efficiency of NBT was too low to drive the motion of crystals, which might have originated from the large repulsion between naphthyl and benzothiazole. These results provide a new platform for the transformation of light energy into mechanical energy in molecular crystals through the unimolecular photochemical reaction of diarylethene derivatives.