Study On Microwave-assisted Direct C-H/c-H Coupling Polycondensation For The Synthesis Of Carboxylate Ester Substituted Polythiophene Derivatives

Polythiophene and its derivatives are an important class of semiconductor materials,which are widely used in organic photovoltaics,field-effect transistors,light-emitting diodes,sensors and other fields.The introduction of ester groups onto the thiophene ring of polythiophene can not only regulate its HOMO and LUMO energy levels and band gaps,but also affect its aggregation and solid-state stacking behavior,thereby adjusting its optical and electrical properties.Currently,polythiophene ester derivatives are mainly prepared by Stille coupling or Suzuki coupling.It is required to pre-prepare organotin compounds or organoborides,and then react with thiophene bromides to obtain polymers in these mentioned methods.Due to the poor stability of organometallic reagents and difficulty in purification,the cost of monomer synthesis is relatively high.In addition,some toxic by-products,such as organotin derivatives,will be generated during the polymerization process.Previously,our research group successfully prepared polythiophene ester derivatives through direct C-H/C-H coupling polycondensation catalyzed by transition metals.This polymerization protocol with high atom economy,not to require any pre-functionalization of monomers,is a new alternative for preparing polythiophene esters,showing remarkbale advantages of few synthesis steps and low cost.However,long reaction time（usually 48 h-72 h）is required for this method,and only low-molecular-weight polythiophene esters were obtained.This dissertation aims to introduce microwave technology into the direct C-H/C-H coupling polycondensation reaction to improve the reaction efficiency,shorten the reaction time,and increase the molecular weight of the polymers.Microwave technology has beenwidely used in small molecule C-C coupling reactions and conjugated polymer synthesis.Numerous studies have shown that,compared with traditional heating,the unique thermal effect of microwave can significantly increase C-H bond activity,increase molecular weight,and enhance reaction yield.Moreover,the reaction selectivity can be significantly improved,side reactions can be reduced,the reaction can be greatly accelerated,and the reaction time can be shortened.With the advantages of microwave technology,this dissertation explored microwave-assisted direct C-H/C-H coupling polycondensation for the synthesis of carboxylate ester substituted polythiophene,and investigated the influence of reaction conditions on polymerization.Further,the optical and electrical properties of the as-prepared polythiophene ester derivatives were characterized by UV-Vis absorption spectrometer,fluorescence spectrophotometer and electrochemical workstation etc.their OLED properties were also evaluated.The specific research content is as follows:1.Microwave-assisted direct C-H/C-H coupling polycondensation for the synthesis of carboxylate ester substituted polythiophene derivativesTaking the polymerization of bis（2-butyloctyl）[2,2’:5’,2”-terthiophene]-4,4”-dicarboxylate（M3）as a model reaction,effect of microwave mode,microwave power,catalyst type,additive type,monomer concentration,reaction temperature and reaction time on the C-H/C-H coupling polymerization of M3 was systematically studied.The optimized polymerization reaction conditions were screened out.Then,under these conditions,using bis（2-butyloctyl）[2,2’-bithiophene]-4,4’-dicarboxylate（M1）,bis（2-butyloctyl）[2,2’:5’,2”-Terthiophene]-4,4”-Dicarboxylate（M2）and M3 as monomers,polythiophene derivatives（P1-P3）were successfully prepared.The chemical structure of the polymers were determined by nuclear magnetic resonance spectroscopy（NMR）and fourier transform infrared spectroscopy（FT-IR）,and their molecular weight and distribution were determined by gel permeation chromatography（GPC）.2.Evaluation of photoelectric properties and OLED performance of polythiophene ester derivativesUsing UV-Vis absorption spectrometer,fluorescence spectrometer,electrochemical workstation,four-probe resistance tester,X-ray diffractometer,thermogravimetric analyzer（TGA）and other analytical methods,the optical properties,electrochemical properties,the crystallinity and thermal stability were characterized.The results show that as the length of the aromatic spacer group between the two thiophene esters on the main chain increases,the effective conjugation length increases,and the UV absorption peaks of the solution and the film of the corresponding polymers gradually become red-shift.In addition,compared with P3HT,the introduction of ester groups significantly reduces the HOMO energy level of the polymer（P1:-6.46eV,P2:-6.12eV,P3:-5.99eV,P3HT:-5.06eV,respectively）.Furthermore,we prepared OLED devices（the device structure was ITO/PEDOT:PSS/Polymer/TPBi/LiF/Al）with P1-P3 as the light-emitting layer,separately to study their electroluminescence characteristics.The OLED devices with P1-P3 as the light-emitting layer emit green,yellow-green and orange-red fluorescence,respectively.Their CIE coordinates are（0.53,0.42）for P1,（0.36,0.59）for P2,and（0.53,0.42）for P3,respectively.The maximum emission peaks(λ_ma^EL _x)of the devices based P1-P3 are of 544 nm,564 nm,and 600 nm,respectively.OLED with P3 as the light-emitting layer has obtained better OLED performance,which may be due to P3 having better charge mobility than P1 and P2.