Synthesis And Aggregation-Induced Emission Behaviors Of Aryl Cyclooctatetrathiophene Derivatives

The aggregation-induced emission（AIE）phenomenon is a photophysical phenomenon in which a fluorescence enhancement effect occurs due to aggregation of chromophores.In 2001,the AIE phenomenon was first reported,AIE has been received more and more attentions in the fields of chemistry,biology,materials,etc.Its relative research has developed rapidly in a wide range of applications,including optical waveguide effects,fluorescent chemical sensing,bioimaging,liquid crystal display and circularly polarized fluorescence.The AIE phenomenon is caused by the mechanism of restriction of intramolecular motions（RIM）,including the restriction of intramolecular rotations（RIR）and the restriction of intramolecular vibrations（RIV）.Among the two mechanisms,the RIV mechanism is rarely involved in application of AIEgens.The molecular structure of cyclooctatetrathiophene（COTh）containing a cored cyclooctatetetraene and four thiophene units,belongs to a cyclic thiophene oligomer with,a unique"saddle"spatial structure.COTh contains multiple active sites,which is convenient for structural derivation and functionalization.In our previous research work,it was found that COTh is a kind of AIE molecule based on RIV mechanism,but this compound has small solubility and weak AIE emission.In this thesis,aryl groups are introduced to the molecular structure of COTh to increase the AIE fluorescence intensity.By changing the type and quantity of aryl group,a series of aryl COTh derivatives were designed and synthesized,and their AIE properties were studied.In addition,the luminescence properties of aryl COTh under high pressure were investigated.The relationship between the molecular structure and luminescence property is revealed in this work.The main work of the thesis can be divided into four parts as follows:1.Synthesis of cyclooctatetrathiophene arylated derivativesStart from 3-Bromothiophene as a raw material to synthesize Br-COTh intermediate by n-BuLi lithium bromide exchange,CuCl₂ coupling,NBS bromination and other reactions,and the yields were 14%and 29%,respectively.With Br-COTh and arylboronic acid as raw materials,two series of aryl COTh derivatives were synthesized by Suzuki coupling reactions,and the monoaryl COTh derivatives（COTh-Ar）were synthesized in a total yield range of 6-11%,and tetraaryl COTh derivatives（COTh-4Ar）were synthesized in a total yield range of 8-18%.The aryl groups includingα-thienyl,β-thienyl,phenyl,a oxyphenyl group,cyanophenyl group,α-naphthyl group,β-naphthyl group,biphenyl group,and pyrene group were employed.The molecular structures of target products were confirmed by NMR spectra,HRMS,IR spectra.Three COTh-Ar compounds were further confirmed by single crystal study.From the crystal structures of the three COTh-Ar compounds,COTh-BPh,COTh-β-Np and COTh-Ph-p-OCH₃showed similar structural properties.The aryl gourp and connected thiophene group are co-planar with small dihedral angles of 6.03^o,9.13^o,20.20^o for COTh-BPh,COTh-β-Np and COTh-Ph-p-OCH₃,respectively.There are strong intermolecular interactions of S···πand S···C are observed in the crystal structures of these three COTh-Ar compounds and COTh itself.Such intermolecular interaction is the main factor causing the AIE phenomena of aryl COTh derivatives in this work.2.The AIE Properties of Aryl COTh DerivativesThe AIE phenomena of the aryl COTh derivatives were studied.The AIE behaviors in the THF-H₂O binary solvent system,the luminescence in solid state,quantum yields and fluorescence lifetimes were investigated for these aryl COTh derivatives.There are four interesting results were obtained.1)All aryl COTh derivatives showed more strong AIE emission than that of COTh,indicating that the introduction of aryl group is reasonable idea to benefit the AIE properties of COTh derivatives.2)By changing the type of aryl group,all COTh-Ar derivatives showed showed AIE emission peaked at⁵25 nm,a little bit red shift than that of COTh itself peaked at 514 nm.The weak effect of substituted aryl group in this case means that the local conjugation effect between the aryl group and the thiophene ring does not show a major role for AIE emission compared to the intermolecular interactions of S···πand S···C of COTh and COTh-Ar derivatives in their aggregation states.3)All COTh-4Ar derivatives showed stronger AIE emission than that of COTh-Ar derivatives,and peaked at longer wavelength of⁵60 nm than that of⁵25 nm for COTh-Ar derivatives due to the incease of conjugation effect in COTh-4Ar molecules.4)The electron-donating group（-OCH₃）and the electron-withdrawing group（-CN）are introduced to the o-,m-and p-positions of substituted phenyl group.The reasonable electronic effect was observed to the AIE emssiions of both COTh-Ph and COTh-4Ph derivatives.Thus,the introduction of aryl group to the molecular structure of COTh is an effective method for increasing the AIE intensity of COTh.3.High-pressure fluorescence emission study of aryl COTh derivativesThe high-pressure fluorescence tests of all COTh-Ar and COTh-4Ar derivatives under a pressure range from 0.00-12.00 GPa were investigated.The luminescence behaviors of all COTh-Ar and COTh-4Ar derivatives showed two characteristics:1)The luminescence peak showed strong red shift in a wide range of 30-160 nm for COTh-Ar derivatives,and 50-130 nm for COTh-4Ar derivatives.2)The fluorescence intensity rapidly decreased,showing a remarkable phenomenon of fluorescence quenching for all COTh-Ar and COTh-4Ar derivatives.On the one hand,under high pressure,the"saddle"molecular structure of COTh should be squeezed,the intramolecular interactions are enhanced,and the distance between meighbored molecules is getting more closed,which is one of the main factors leading to the red shift of luminescence.Further more,the intermolecular distance should be reduced,and the intermolecular π-π interaction is enhanced,which is another major factor leading to the red shift of the aggregate luminescence,and it is also the main reason for the rapid quenching of fluorescence at same time.4.Chiral resolution and characterization of COTh isomersChiral resolution of two COTh isomers,COTh-6 and COTh-7 by high performance liquid chromatography using a mixed solvent of 95%n-hexane and 5%isopropanol as mobile phase and chiral column of CHIRALPAKIBN-5 model.The optically enantiomers of COTh-6 and COTh-7 were obtained,and the yield of the resolution was as high as 80-86%.The characterization of their chiral properties,including ee and circular dichroism were observed.At the same time,the racemization processes of COTh-6 and COTh-7 enantiomers under different temperature conditions were investigated.The inversion energy barriers of COTh-6 and COTh-7 were calculated to be 23.84 and 22.55 kcal/mol,respectively.The chiral stability of COTh-6 and COTh-7 showed as COTh-6?COTh-7.