Investigation On The Synthesis And Properties Of Benzobisimidazole-based D-A Copolymers

In recent years,organic semiconductor materials represented by conjugated polymers have received extensive attention because of their excellent optical properties and electrical properties.Among them,the so-called D-A polymer that contains both an electron donor(Donor)and an electron acceptor(Acceptor)unit in the main chain of the molecule often has excellent photophysical properties and carrier transport properties,which makes it becoming a research hotspot in field of organic light emitting diodes,solar cells,and field-effect transistors.In this dissertation,benzobisimidazole was used as the electron acceptor unit,thiophene was used as the electron donor unit and bridging unit.A series of D-A type polymers were designed and synthesized.Through the regulation of the polymer alkyl side chain,the cis/trans-isomers were constructed.The structure and optical and electrical properties of the polymer were characterized by means of mass spectroscopy,nuclear magnetic resonance,ultraviolet absorption,fluorescence emission,cyclic voltammetry,theoretical calculation and thermogravimetric analysis.The relationship between the cis/trans-isomerism of the polymers and their optical and electrical properties was explored.The main work of this paper is as follows:1.A series of cis/trans-isomers of 2,2’-bis(3-bromothienyl)benzobisimidazole with n-butyl,n-hexyl and n-octyl groups were designed and synthesized from4,6-dinitrobenzene-1,3-diamine.The structure of the compound was determined by mass spectrometry and nuclear magnetic protons.Four different synthetic routes were studied.The traditional synthesis routes of o-phenylenediamine derivatives and aldehydes under oxidants have poor selectivity and many by-products.The synthesis route of o-phenylenediamine derivatives and carboxylic acid compounds under strong acid and high temperature conditions is harsh and the yield is low.When1,5-dichloro-2,4-dinitrobenzene is used as a raw material,the synthesis route of the trans structure does not work.In the end,o-nitroaniline was used as a starting material,and insurance powder was used as a reducing agent to react with aldehydes to synthesize benzobisimidazoles in two steps.This method is simple to operate,with mild reaction,good selectivity,and a 20%increase in yield.2.A series of cis/trans-benzobisimidazole thiophene copolymers were synthesized by Suzuki coupling of thiophene-2,5-diboronic acid and benzobisimidazole cis/trans-isomers of different alkyl chains as monomers.It was found that the absorption spectrum of the trans-structured polymer was red-shifted with respect to the cis-structured polymer and the absorption band broadened.The cyclic voltammogram also measured that trans-structure have a smaller bandgap than the cis-structure.From the structural analysis,it is because only the trans-structure can form the quinoid type configuration,thereby reducing the bandgap of the polymers and broadening the absorption red shift.Through theoretical calculations,the trans-structured polymer can form a small dihedral angle between imidazole and thiophene rings,which is favorable for molecular conjugation,narrowing the absorption bandwidth and red shifting the spectrum.3.Different Stille coupling methods were used to design and synthesize trans-benzobisimidazole thiophene copolymers with three alkyl chains of n-octyl,n-dodecyl and n-hexadecyl.The solubility of long alkyl chains polymers is enhanced relative to short chains polymers.Moreover,through the improvement of the polymerization reaction,the degree of polymerization of the polymers have been increased,and the molecular weight has been increased by about 3 times,so that the conjugate degree of the polymers become larger.The UV-visible absorption measured the absorption edge of the polymer reaching 800 nm,and the measured cyclic voltammetric band was 1.7 eV.Density functional theory calculations show that the dihedral angle of the polymer imidazole ring and thiophene ring is 1.41°,with good coplanarity and conjugation.Thermogravimetric analysis shows that the polymer maintains its stability at 400~oC.These properties of the resulting benzobisimidazolethiophene copolymers make them potentially useful in organic optoelectronic devices.