Synthesis,Self-assembly And Functionalization Of Boron Difluoride Complexes

Boron difluoride compounds represented by BODIPY derivatives possess excellent photophysical properties such as high molar extinction coefficient,high fluorescence quantum yield,narrow emission band width and good photostability.Simple modifications on the molecular structures can change the luminescence properties,so that they can be employed to construct materials used in the fields of photosensitizer,fluorescence imaging and fluorescence labeling,etc.Supramolecular self-assembly driven by non-covalent interactions such as host-guest interaction,hydrophobic interaction,hydrogen bonding and π-π stacking is often used to construct molecular functional materials with ordered structures.The study on the supramolecular selfassembly mechanisms provides quantitative analysis for better regulating the properties and functions of supramolecular materials.Supramolecular materials based on boron difluoride compounds such as BODIPY derivatives have been widely used in chemical sensing,gels,liquid crystalline materials and luminescence materials.The stimuli-responsiveness of the apparent color of boron difluoride compounds may facilitate their further application.Although it has been reported that the BODIPY derivatives-containing thin films prepared by supramolecular chemistry which act as active layers for charge separation have been used in the field of solution-processable organic solar cells,the research in other fields of organic electronics is less explored due to the relatively poor film formation properties.Other than BODIPY and boron difluoride β-diketonates,the self-assembly properties of other types of amphiphilic five-membered-ring boron difluoride compounds are relatively less explored.Therefore,in this thesis,the self-assembly properties,including self-assembly mechanisms,of boron difluoride compounds have been investigated.The memory devices prepared from corresponding molecules showed good binary memory performance.Disubstituted BODIPY compound was employed to prepare thermochromic and vapochromic materials.The effects of the hydrophilicity and π-conjugated surface area on the self-assembly behaviors of the amphiphilic fivemembered-ring boron difluoride compounds have been explored.First,disubstituted BODIPY compounds decorated with different π-bridging groups,have been designed and synthesized.The effects of the bridging groups on the selfassembly behaviors have been investigated and the applications of the compounds for stimuli-responsive materials have been explored.The photophysical properties of the compounds in DCM have been measured.For the self-assembly properties,variabletemperature UV-vis absorption measurements of C12-BDP,C12-P-BDP,C12-BBDP and C12-T-BDP have been performed to investigate their self-assembly behaviors.The plots of the degree of aggregation versus temperature were found to be non-sigmoidal in shape,indicating that the compounds underwent a cooperative selfassembly process in THF-water mixtures.C12-P-BDP,C12-B-BDP and C12-T-BDP were found to elongate at higher temperatures due to the increased hydrophobicity and the steric hindrance.Transmission electron microscopy,scanning electron microscopy,atomic force microscopy and dynamic light scattering have been employed to study the morphologies of the aggregates.It was found that C12-B-BDP decorating with n-butyl-containing carbazole groups self-assembled into bent nanofibers,which was different from the assemblies of C12-P-BDP decorating with phenyl-containing carbazole groups.The slight modifications in the molecular structures of the compounds reduced the steric hindrance and allowed C12-B-BDP to form more regular aggregates.2D ROESY NMR measurement for C12-B-BDP confirmed that the adjacent compounds stacked with each other in a staggered manner to minimize the steric hindrance.Powder X-ray diffraction experiment for C12-BBDP revealed a lamellar packing arrangement in the self-assembled architecture.C12-BDP was found to exhibit thermochromic and vapochromic properties.Variabletemperature UV-vis absorption and variable-temperature fluorescence experiments have been employed to monitor the behavior and the powder X-ray diffraction experiments of the samples prepared before and after chromism have been used to verify the chromic properties.On the contrary,such chromic phenomena were not observed in C12-BDP and C12-P-BDP,C12-B-BDP and C12-T-BDP modified with other groups,suggesting that both the alkoxy chains and the conjugate units played important roles on such properties.Second,a series of BODIPY-containing donor-acceptor compounds substituted at different sites with carbazole moieties tethered with solubilizing triethylene glycol or alkoxy chains has been designed and synthesized.The self-assembly mechanisms of the compounds in THF-water mixtures have been investigated.Representative compounds have been employed to fabricate solution-processable organic resistive memory devices,which exhibited good binary memory performance.The photophysical properties of the compounds in DCM have been investigated and verified with the theoretical calculations.Temperature-dependent UV-vis absorption measurements have been employed to investigate the self-assembly mechanisms.The plots of the degree of aggregation versus temperature were found to be non-sigmoidal in shape,suggesting a cooperative growth mechanism.Fitting the absorption data to the nucleation-elongation model has been employed to derive the thermodynamic parameters of the self-assembly.Based on the thermodynamic parameters,the compounds with triethylene glycol chains were found to require larger nuclei and elongate at lower temperatures.Compounds with longer alkyl chains were found to experience a more exothermic aggregation in the mixtures due to their enhanced hydrophobicity,and thus a stronger hydrophobic interactions between the aggregates.Powder X-ray diffraction,grazing incidence X-ray diffraction and atomic force microscopy have been employed to examine the structure and the surface morphologies of the spin-coated films.The memory devices fabricated with compound C16-BDP-1 were found to exhibit good binary memory performance such as a low threshold voltage of about 1.35 V,high ON/OFF ratio of 105,decent stability with retention time of >8500 s,narrow threshold voltage distribution and excellent reproducibility.The devices fabricated with compounds C16-BDP-2 exhibited similar properties.The performance of the devices with the Li F interlayer were found to show that the switching of different resistance state was intrinsic to the active materials.Upon fitting the I-V curves with appropriate theoretical charge transport models,the OFF state was found to dominate by the thermionic emission process,while the ON state was found to dominate by the ohmic conduction.It could be inferred from the results of cyclic voltammetry that the hole injection from the electrode to the film was an energetically more favorable process.The switching mechanism of the devices was speculated to be charge trapping mechanism.Third,two kinds of five-membered-ring boron difluoride compounds with different conjugated structures and different amounts and lengths of oligoethylene glycol chains have been designed and synthesized.Their self-assembly behaviors in mixed solvents have been investigated.Solvent-dependent UV-vis absorption experiments have been employed to study the self-assembly processes in THF-water mixtures and the data have been fitted with the nucleation-elongation model.Only the plot of the degree of aggregation versus the volume fraction of THF of 3TEG-BF2-P,which has been modified with three triethylene glycol chains,was found to show a sigmoidalshaped curve with the cooperative factor σ of 1,indicating an isodesmic growth mechanism.Other plots of the degree of aggregation versus the THF volume fractions were found to be non-sigmoidal in shape with the cooperative factors σ of < 1,suggesting a cooperative growth mechanism.The differences suggested that the enhancement in hydrophilicity and the conjugated area of the compounds would result in the imbalance of these noncovalent interactions and thus would ultimately lead to different self-assembly mechanisms.The more negative ΔG0 of the TEG-BF2-P-HEG-BF2-P with larger conjugated π-surfaces could be attributed to the enhanced hydrophobicity.Transmission electron microscopy and atomic force microscopy have been employed to study the morphologies of the assemblies.It was also found that different hydrophilicities and conjugated π-surface areas could also be the factors governing the morphologies of the aggregates.The packing arrangements of the assemblies have been examined by powder X-ray diffraction.HEG-BF2-B-HEGBF2-P were found to exhibit lamellar structures in the assemblies.In summary,hydrophobic alkoxy and hydrophilic oligoethylene glycol chains,which were found to be associated with a better film formation ability,were introduced to the boron difluoride compounds.The self-assembly mechanisms of the compounds have been studied.Stimuli-responsive materials with thermo-/vapochromic properties have been obtained and resistive memory devices with binary memory properties have been fabricated.The effects of the hydrophilicity and the conjugated π-surfaces of the amphiphilic five-membered-ring boron difluoride compounds on the self-assembly mechanisms and morphologies have been investigated.The results of this thesis expanded the applications of boron difluoride compounds in the fields of electronics and stimuli-responsive materials and might be relevant for the preparation of new classes of organic supramolecular fluorescent functional materials.