Design Of Poly(M-Phenylene Diethynylene)-Based Foldamer And Its Binding Behaviors With Guest Molecules

The biomacromolecules such as nucleic acids and proteins possess the highly ordered and thermodynamically stable helical structures,which are closely related to molecular recognition,catalysis,replication and transcription in living systems.Inspired by biological helical structures,chemists have tried to design various artificial foldamers in recent years.Among these,aromatic foldamers play a dominant role in foldamer chemistry field because they not only have the stable and predictable helical structures but also can provide the binding sites including the side chains,helical grooves and cavities for molecules or ions.However,some formidable challenges still remain.First of all,most aromatic foldamers are only soluble in organic solvents,which limits their potential applications in biological systems and green chemistry.Second,researches nowadays are mostly focused on helix induction in foldamers,but it is difficult to modulate the helical conformations.Third,the types of guest molecules are unitary and their binding modes with foldamers are often uncontrollable.To solve the above-mentioned problems,a water soluble poly(m-phenylene diethynylene)-based foldamer is used in this dissertation to recognize and bind with chiral amines/amino alcohols,cyanine dye and hemicyanine dye to investigate the diverse properties and functions of these systems,which can offer new approaches for the research and application of artificial foldamers.Firstly,we have designed and synthesized an amphiphilic poly(m-phenylene diethynylene)-based foldamer(Poly-1)bearing L-alanine sodium pendants.Due to the amphiphilic nature of Poly-1,its helical folding in water is driven by hydrophobic effect.Meanwhile,the behavior occurs in a preferred-handed screw sense directed by L-alanine sodium pendants.The resultant preferred-handed helical conformation is further stabilized by intramolecular?-?stacking interaction between non-adjacent phenylene diethynylene units and electrostatic repulsion between non-adjacent negatively charged pendants.The Poly-1 helix features?-?stacked aromatic residues similar to the?-?stacked bases in DNA and the negatively charged pendants like the phosphate groups on DNA.Moreover,the helical folding behavior of Poly-1 can be regulated by changing the dimethyl sulfoxide/water or methanol/water volume ratios,p H and polymer molecular weight.Secondly,constructing and regulating the helical conformations of artificial foldamers have been pursued in recent years because of their great potential applications in chiral recognition,asymmetric catalysis,circularly polarized luminescence,etc.On the one hand,stabilizing the helical conformations of foldamers can usually enhance the chirality by decreasing the pitches.On the other hand,the extended helix can be obtained when the?-?stacking interaction in the main chain is weakened.In general,the former aspect is inconsistent with the later.Herein,chiral 1-phenylethylamines,2-phenylglycinols and leucinols are used as the guests to modulate the folded helix of Poly-1.With the aid of chiral center,the steric hindrance of chiral guests can increase the distances between the pitches.Meanwhile,a more ordered chain arrangement is induced by such steric effect.As a result,we successfully endow Poly-1 with an extended helix and enhanced chirality at the same time.Thirdly,constructing and modulating the cyanine dye aggregates have garnered much attention due to their wide applications in photosensitizers,biological fluorescence probes,nonlinear optics,etc.However,to realize such a process using a single additive still remains a formidable challenge.Interestingly,the aggregation and disassembly of positively charged3,3'-diethyloxadicarbocyanine iodide(Di OC₂(5))have been demonstrated just by changing the concentration of Poly-1.Upon mixing equal Di OC₂(5)and Poly-1,the dye molecules are mainly enriched on Poly-1 surface driven by electrostatic interaction and arranged along the main chain of Poly-1,resulting in the formation of chiral H-aggregates.However,the excessive Poly-1 can attract some dye molecules in H-aggregates via electrostatic interaction.The isolated dye molecules are orderly dispersed into the helical grooves of Poly-1 due to hydrophobic interaction.Meanwhile,in the dye aggregation and disassembly,the solution color changes from peachblow to purple red and then to purple and finally to navy blue.Finally,aggregation-induced emission(AIE)luminogens are weakly emissive at the dispersed state,but become highly luminescent upon aggregate formation.Herein,we demonstrate an effective strategy to make the positively charged AIE-based hemicyanine dye(TPEBe-I)more emissive via complexing with Poly-1 than its aggregated state.The AIE of TPEBe-I is awoken although TPEBe-I alone shows an aggregation-caused quenching(ACQ)phenomenon in aqueous buffer solution due to the heavy atom effect of iodide ion.Upon mixing equal TPEBe-I and Poly-1,the dye molecules dock on Poly-1 surface to form an aggregation complex and the intramolecular motions are partially restricted,giving rise to a weak red fluorescence.Interestingly,the adequate Poly-1 promotes the disassembly and the isolated dye molecules dispersedly intercalate into the cavities with a limited space,which causes sufficient restricted intramolecular motions and further fluorescence enhancement.At last,every cavity can bind with around two dye molecules and a strong orange fluorescence with 160-fold enhancement is observed.