| Supramolecular chemistry is an emerging interdisciplinary subject in which chemistry crosses with physics,biology,materials and environmental science.It was first proposed by Jean-Marie Lehn who won the Nobel Prize in 1987.This field then received more attention and developed rapidly.Unlike traditional chemistry,which is based on covalent bonds,supramolecular chemistry focuses on non-covalent bonding between molecules,including hydrogen bonding,hydrophobic-hydrophobic interactions,π-π interactions,metal-ligand interactions,cation-π interactions,anion-π interactions and so on.The supramolecular self-assembly is formed by the non-covalent bond between molecules,and the building blocks with different structures can be obtained,such as micelles,vesicles,nanosheets and nanofibers.A functionalized supramolecular assembly can be obtained by using a functional building block.The amphiphilic supramolecular compound,which contains hydrophilic group and hydrophobic group,can undergo self-assembly to give various structures through intermolecular non-covalent bonding.Since the aggregation of amphiphilic monomers in an aqueous solution can result in various nanostructures,such as micelles and vesicles which is similar to liposome and cell membrane structure,this can be applied to the fields of biomedicine,such as drug delivery and release.The dynamic and reversible properties of non-covalent bonds allow the supramolecular system to have various stimuli responsiveness such as light,heat,pH,redox,etc.Thermal responsiveness is the most convenient and controllable stimuli responsiveness which obtained extensive attention and research in materials science.With the rapid development of supramolecular chemistry,supramolecular self-assembly has evolved from simple assembly structures to stimuli-responsive assemblies to highly ordered and complex assemblies with multiple functionalities.In order to design intelligent functional materials,an in-depth understanding of the supramolecular self-assembly mechanism is crucial.Firstly,a set of amphiphilic carbazole derivatives containing different numbers of triethylene glycol chains was designed and synthesized.The self-assembly process of compounds under temperature regulation and solvent control was studied by electron absorption spectroscopy.In 70% dimethyl sulfoxide-water mixed solution,the self-assembly of 2.1-2.4 is under the nucleation-elongation cooperative mechanism during the cooling process.Since 2.1-2.3 have the same rigid molecular skeleton,with the increase of the number of ethylene glycol chains,the molecular flexibility is enhanced,and the cooperative effect is weakened during the self-assembly process.2.4 contains two carbazole units,and has a large conjugated π-surface area,which provides hydrophobic-hydrophobic interaction and π-π interaction for molecular self-assembly,This promotes the self-assembly process in solution and is beneficial in obtaining a more ordered aggregation.2.1-2.4 in the dimethyl sulfoxide-water mixed solution under same concentration would undergo aggregation upon increasing the proportion of water,and the self-assembly process obeyed the nucleation-elongation cooperative mechanism.2.1-2.4 with triethylene glycol chains have a good thermal response.2.3 and 2.4 have a low critical solution temperature in the aqueous solution,and the cloud point decreases with the concentration increasing.The responsiveness of different ions is consistent with the LCST properties of the polymer.The heating and cooling process of 2.3 and 2.4 are similar with each other with no hysteresis.Both 2.3 and 2.4 show high reversibility after multiple cycles of cooling and heating,indicative of a stable LCST property.In view of the high phase transition temperature of 2.3 and 2.4,and a relatively low cloud point temperature of 2.3,a new series of amphiphilic carbazole derivatives 3.1-3.4 have been synthesized with the main structure similar to 2.3 and polyaromatic hydrocarbon substituents.3.1-3.4 exhibit thermally responsive properties with lower cloud point no hysteresis and high stability,and therefore can be used for designing thermal responsive molecules with LCST temperature in the range of body temperature.Amphiphilic compounds have a favorable performance during the aggregate process,and the self-assembly of 3.1-3.4 in solution was studied by electronic absorption spectroscopy.The polyaromatic hydrocarbon enhanced the intermolecular hydrophobic interaction and π-π interaction.The self-assembly of 3.4 in the 10% tetrahydrofuran-water solution aggregated under an isodesmic mechanism.The isodesmic model was used to fit the spectral change of the absorption spectra,and obtain the thermodynamic parameters in the self-assembly process.It was found that compounds with a larger aromatic π surface have a larger tendency to assemble in a more ordered manner.Thermogravimetric analysis showed that the decomposition temperature of 3.1-3.4 were above 280 °C and had a very good thermal stability.The electrochemical and memory performance of the compounds were studied.Compound 3.3 has found to show a binary memory effect,which can be used as a static random access memory,and has a low misreading rate and stable memory performance during the storage process.Self-assembly under cooperative mechanism has more possibly to obtain highly ordered complex aggregates compared with isodesmic mechanism while under the same conditions.Therefore,introduced hydrophobic alky chains to the structure of 2.3,it not only increases the hydrophobicity of the molecules,but also reduces the rigidity appropriately.The introduction of alkyl chains with different lengths could affect the aggregation process of the new set of amphiphilic carbazole derivatives 4.1-4.4.Compound 4.5 has the similar rigid structure with 4.1-4.4,replacing the flexible chains by methyl group,molecular stacked by a head-to-tail manner in the crystal structure of 4.5,indicating that head-to-tail packing way is stable,it is possible to choose this head-to-tail manner to aggregate of 4.1-4.4.The self-assembly behavior of 4.1-4.4 in solution was studied.It was found that in the tetrahydrofuran-water mixed solvent,the more hydrophobic compounds 4.1 and 4.2 were self-assembled by isodesmic model;in the non-polar n-hexane solution,more hydrophilic compounds 4.3 and 4.4 were aggregated with a nucleation-elongation cooperative mechanism.It is indicated that the change of hydrophilicity in the flexible chains of compounds affects the self-assembly mechanism.In summary,the self-assembly process and mechanism of the obtained amphiphilic carbazole derivatives in solution are studied,by changing the flexible chains in the molecular structure or introducing rigid polyaromatic hydrocarbons,and it is hoped that a highly ordered aggregates can be constructed.The amphiphilic carbazole deratives show temperature-responsive properties and exhibits stable and sensitive LCST phenomenon by the introduction of temperature-sensitive triethylene glycols.These compounds can be used to prepare temperature-sensitive smart materials.Memory performance results indicate this carbazole derivatives have potential applications in memory devices. |
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