| Nowadays,there is a continuing quest for constructing optically active supramolecules from achiral building blocks due to the appeal of avoiding of tedious synthesis of chiral polymers.Various strategies have been successfully implemented,such as chiral liquid-crystal field,circularly polarized light?CPL?,solvent chirality transfer?SCT?,gelation.Among these,SCT has been widely used in preparing optically active supramolecules from achiral??and?-conjugated polymers.Ternary solvents or cosolvents containing a chiral solvent are commonly applied for this method,which often confuse the chirality induction mechanism with complex intermolecular interactions between solvents and polymers.Moreover,the quick phase-separation process often results in the production of nanoparticles,and makes it difficult to observe the helical structures.Therefore,using a neat chiral solvent instead is much better for investigating the morphologies of helical aggregates and exploring the mechanism of chirality induction.Focused on the key scientific problems such as construction and modulation of supramolecular chirality,the structural dependence,nature of chirality induction and chirality memory,researches were systematically studied,which are based mainly on the main-or side-chain length dependence of chiral solvation,chiral gelation and chiral sensing properties of achiral polydialkylfluorenes in chiral limonene.The detailed results are summarized as follows:?1?By using chiral limonene as the neat solvent,we successfully constructed the supramolecular chirality from achiral poly?9,9-dioctylfluorene??PF8?induced by chiral solvation.PF8/limonene solution could form optically active aggregates in nonracemic limonene by cooling at relatively low temperatures?below-10 oC?.Intriguingly,PF8/limonene solution could undergo chiral sol-gel transition at high concentration?>2mg/m L?at-20 oC,and the xerogel structure showed microporous morphologies.The process of chiral supramolecular assembly was then conducted at different temperatures?-10 oC,-20oC,-30 oC or-40 oC?,assembling time,polymer concentration and limonene enationpurity,monitored by UV-vis spectroscopy,circular dichroism?CD?,and fluorescent spectroscopy.The most appropriate assembling temperature is-20 oC.Assembling speed is too slow at higher temperature?-10 oC?,and too fast at lower temperature?-30 oC and-40 oC?accompanying with polymer precipitation.All spectra data reveal the intense CD signals contributed by?-phase formation.Atomic force microscopy?AFM?inferred helically distorted PF8 aggregate motifs responsible for the optical activity.This supramolecular chirality of PF8/limonene aggregates was then effectively transferred to solid film and perfectly memorized even near decomposition temperature?300 oC?,which could compare favorably with those formed by chiral polyfluorenes.It was further demonstrated that achiral PF8 film spin-casted from CHCl3 solution could be employed as a chiral sensor to detect nonracemic limonene molecules.?2?Conjugated homopolyfluorene with chiral pendants in side chain and its corresponding alternating copolymer?PSF and P?SF-alt-F8??were designed and synthesized by Suzuki coupling polycondensation reaction,to compare the induction ability between chiral side chain and chiral solvent.PSF and P?SF-alt-F8?showed similar solubility with that of PF8 in limonene,and their solutions could form aggregates/gels upon cooling treatment.A well-dissolved PSF solution did not show any optical activity,but its aggregates showed similar CD signs in different enatiopure limonene.It demonstrated that the optical activity was induced by chiral side chain,and could not be inversed by solvent chirality.The chirality induction ability of chiral side chain is stronger than that of chiral solvent.P?SF-alt-F8?aggregates in limonene showed no CD signals,because the alternating insertion of F8 unit in main chain prohibits the chirality transfer from chiral side chain of SF unit to polymer backbone.What's more,films casting from PSF aggregates and from CHCl3 solution exhibited mirror CD Cotton effects and dissymmetry ratios.?3?A serials of polydialkylfluorenes?PFs??PF5-PF10 and PF2/6?,substituted at the C9-position by linear alkyl chains and branched side chains,were designed and synthesized by Kumada catalyst transfer polycondensation?KCTP?and Yamamoto-type coupling polycondensation,respectively.They were obtained in a controlled manner with similar numbers of repeating units around 100 and polydispersity indices of?1.2.Among them,PFs with linear alkyl chains?PF5-PF10?are all good achiral candidates to assemble into optically active supramolecular aggregates or gels by chiral solvation.Moreover,the sol-gel transition temperature(Tsol-gelol-gel and Tgel-sol)are dependent on the side chain length.Comparably,PFs with branched alkyl chains?PF2/6?could not form gel in neat limonene at low temperature.UV-vis absorbance spectra for those aggregates validated the production of?-phase conformer,?-phase conformers in PF5,PF7,PF8,PF9 and PF10,and another?-phase conformer in PF6,responsible for the intense CD signals.Moreover,?-phase content is related with alkyl side chain length,achieving highest at PF8.There also exists an“odd-even”effect in CD signs for the chiral?-phase of PFs/limonene aggregates.The?-phase of odd-numbered and even-numbered PFs possesses the opposite preferential helicity,which indicates a complex interplay between the solvent chirality transfer and the alkyl side chain length.These results are critical for building optically active?-conjugated polymers from achiral ones,since subtle modifications in the length of the side alkyl chain can strongly influence the chirality sign.?4?A series of monodisperse oligofluorenes?PFn?were designed and synthesized by elongating the monomer conjugation length in Yamamoto coupling polycondensation reaction,to investigate the minimal main chain conjugation length required for the chiral supramolecular assembly.First,Suzuki coupling reaction between the brominated and boronic ester-functionalized dioctylfluorene units afforded the dimer?FO2?and trimer?FO3?.Further bromination of FO2 and FO3 gave the corresponding dibrominated products,the monomers FO2-2Br and FO3-2Br.The monobrominated products?FO2-Br and FO3-Br?were achieved from Suzuki coupling reaction,and used as the end-capping agents.By adjusting the feeding ratio of monomer/end-capping agents,the temperature and the addition order,the polymerization conditions were well optimized so that oligomers were mainly produced.Then the fluorene oligomers were isolated and purified by recycling preparative HPLC.The supermolecular assembly of oligofluorenes in neat limonene were then further investigated.The main results are summarized as follows.?I?When the monomer/end-capping agent feeding ratio is 1/1,the polymerization condition is optimal for preparing oligofluorenes;?II?Oligomers?FO6,FO8,FO9,FO12 and FO15?were successfully isolated and purified;?III?The longer the main chain length are,the higher?-phase content are,and more stable the assembly prepared are;?IV?The limonene-induced chirality is related to the number of fluorene repeating units?n?.When n?8,the oligomer cannot assemble in neat limonene even at very low temperature;When n>8,the oligomer can effectively assemble at low temperature,displaying a clear CD signal accompanying with an obvious?-phase absorption peak. |
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