Functional Molecular Gels Based On Cyanostilbene Derivatives

Cyanostilbene and its derivatives have received great research attention becauseof their unique multi-responsive, modifiable and luminescent characteristics. Mostcyanostilbene-based compounds can produce typical aggregation-induced enhancedemission (AIEE) in the aggregated or solid states. It should be pointed out that thecyano moiety has the ability to interact with either other cyano groups or hydrogens inaromatic rings. The cyano interaction could cooperate with other intermolecularinteractions such as π-π stacking, to fabricate supramolecular architectures. On thebasis of their unique feature and easily bonded with other units or systems,cyanostilbene and its derivatives are promising to be used in optoelectronic devices,chemosensors and biological imaging. In order to obtain a new class ofsupramolecular gelators based on cyanostilbene amide derivatives, we combine thecyanostilbene with amide group to increase the intermolecular interactions.First of all, we synthesized three kinds of cyanostilbene amide derivatives withdifferent molecular structures, and studied their properties in anion recognition, AIEE,and self-assemble. It was found that these cyanostilbene amide derivatives can act ashighly selective colorimetric sensors for fluoride ion, overcoming the interference ofH2PO4, AcO and other halide ions. The deprotonation of the NH groups induced byfluoride ion is responsible for the dramatic color change from colorless to yellow. Thedifferent molecular structures did not affect the detection of fluoride ion, but played asignificant role in their AIEE and self-assembly properties. Both BPIA and TPBTAexhibited typical AIEE in the aggregated state, but the conpound BPIA showed higherfluorescence quantum efficiency than that of TPBTA with three cyanostilbene unitswhen the water volume fraction is70%. Compared with PBA and BPIA, only TPBTAcan readily self-assemble into gels with nanofibers structure in the mixed solvents ofDMSO/H2O. Interestingly, the xerogels prepared from the TPBTA organogels showeda striking property of adsorbing crystal violet dyes from water. This dye-adsorptionability of gelators can be utilized in water purification by removing toxic dyes from wastewater.Secondly, we synthesized a novel class of gelators based on V-shapedcyanostilbene amide derivatives without steroidal units and long alkyl chains throughchanging the isophthalic acid substituted group (a highly polar nitro group and a largevolume of tert-butyl group) to favor the balance between their solubility andcrystallization. The introduction of strong polar nitro group may enhance theintermolecular interactions, which make BPNIA form thermoreversible gel inDMSO/H2O and ultrasound-stimulated gel in DMSO by the cooperative interplay ofintermolecular hydrogen bonding, π–π stacking and cyano interactions. On the basisof the deprotonation of NH groups induced by fluoride ion, introducing of fluoride notonly changed the color of the system but also disrupted the preformed gel to solution.Subsequently, the addition of polar protic solvents could make the solution regelatedand the red color faded. Therefore, the reversible gel-sol transition and noticeablecolor change were controlled by the use of fluoride stimulus and proton control. Theintroduction of larger steric hindrance of tert-butyl group not only may weaken thetight molecular stacking and strong intermolecular interactions, but also could balancethe solubility and deposition, leading to the formation of the gel. Meanwhile,well-defined ultralong1D nanowires of BPBIA with lengths up to several millimetersalso have been fabricated from a simple slow evaporation approach. More importantly,the individually-dispersed nanowires with uniform morphology are independent ofsubstrates and solvents. The single-crystal X-ray analysis of compound BPBIAprovided reasonable explanation for the formation of1D nano-or microstructures. Itis worth noting that the generated nanofibrous gels and the nanowires emit strong bluelight under UV irradiation, revealing that these1D nanomaterials show potentialapplications in emitting materials as well as fluorescent chemosensors.Finally, we synthesized a class of ALS-type organic gelators based oncyanostilbene amide derivatives and studied the influence of π-conjugated structureon their gellability by changing the aromatic groups. It was found that the compoundG2with larger π-conjugated structure showed great ability to gel a variety of organic observed for compounds G2after gelation. Owing to the phenolic hydroxyl of2,7-naphthalenediol could destroy the intermolecular hydrogen bonds and cyanointeractions between gelators, the organogel of G2exhibited a high selectivitytowards2,7-naphthalenediol instead of the other phenolic derivatives, which exhibiteda two channel response through gel–sol transition and fluorescence disappearance.Such responsive systems are highly desirable for the development of sensor devices orin applications like drug delivery. Additionally, G2showed phase-selective gelationproperties in aromatic solvents–water mixtures at room temperature by introducing ofthe assistant solvent. Most importantly the gelator molecule can be recovered andreused several times, indicating its potential application in collection of aromaticsolvents and purification of water. Interestingly, the gelator G2can purify water viaphase-selective gelation process. Thus, this part of work offers a new way fordesigning new functional gelators.