Synthesis And Properties Of Conjugate-extended Benzodipyrrolidones And Their Copolymers

D-A(Donor-Acceptor)copolymerswith both electron-donating and electron-withdrawing units in the molecular main chains have been widely used in organic solar cells(OSCs),field effect transistors and other fields because of their excellent carrier transport characteristics and narrow band gaps.Benzodipyrrolidone(BDP)with amide structure has the characteristics of high planarity,high conjugated structure and strong electron-withdrawing ability,so it is an ideal acceptor unit for constructing narrow band gap D-A copolymers.Through different conjugate expansion strategies for benzodipyrrolidone,BDP monomers modified by different bridging units,namely BDPP,BDPT and BDPTh,were designed and synthesized.These acceptor units were polymerized with indacenodithiophene(IDT)and naphthodithiophene(NDT)donor units,respectively,to obtain a series of D-A copolymers,and the properties of these polymers were systematically studied.(1)Study on the properties of the copolymers of benzodipyrrolidone and indacenodithiophene.Three dibrominated BDP monomers were synthesized,namely,BDPP(benzene modification),BDPT(thiophene modification),and BDPTh(alkylthiophene modification).Under the catalysis of Pd₂(dba)₃/P(o-tol)₃,three D-A copolymers were synthesized by Stille polymerization of these acceptor units and bistrimethyltinylated IDT donor unit,namely PBDPP-IDT,PBDPT-IDT and PBDPTh-IDT.The number-average molecular weights of PBDPP-IDT,PBDPT-IDT and PBDPTh-IDT are 66.8 k Da,18.4 k Da and 21.7 k Da,respectively.Due to the introduction of solubilized alkyl chains,the three polymers showed good solubility in conventional solvents.The optical and electrochemical properties of polymers can be adjusted by changing the conjugated expansion structure of BDP.The absorption spectra in solution show that PBDPP-IDT,PBDPT-IDT and PBDPTh-IDT have absorption ranges of 300-850 nm,300-1100 nm and 300-950 nm,respectively.Furthermore,all polymers exhibit different absorption properties in different solvents.In the film state,the absorption peaks of PBDPP-IDT,PBDPTh-IDT and PBDPT-IDT in the low energy region are located at 667 nm,750 nm and 880 nm,respectively,showing an obvious red shift compared with the corresponding absorption spectra in the solution.The three polymers all show narrow optical band gaps and electrochemical energy gaps.The optical band gaps of PBDPP-IDT,PBDPTh-IDT and PBDPT-IDT are 1.55 e V,1.37 e V and 1.21 e V,respectively,and the electrochemical energy gaps are 1.61 e V,1.28 e V and 1.12 e V,respectively.Using the electron-donating ability of the conjugated extension unit and the modulating of the molecular configuration by the alkyl chain,the effective tuning of the molecular orbitals and band gaps of the polymers is achieved.The results of frontier molecular orbital distributions and energy levels of polymers simulated by DFT show that BDP has strong elctro-acceptor properties,and the orbital energy gaps of these polymers have the same trends as that of electrochemical energy gaps.The thermal decomposition temperatures of PBDPP-IDT,PBDPT-IDT and PBDPTh-IDT are387?,372?and 353?,respectively,showing good thermal stability.(2)Study on the properties of the copolymers of benzodipyrrolidone and naphthodithiophene.Using BDPP and BDPTh as acceptor units and NDT as donor unit,two D-A copolymers PBDPP-NDT and PBDTPh-NDT were synthesized by Stille polymerization,and their number-average molecular weights were 18.8 k Da and12.8 k Da,respectively.The two polymers have good solubility in conventional solvents.With the enhanced electron-donating property of the conjugated extension unit,the polymer exhibits stronger intramolecular charge transfer transition(ICT),which leads to a red-shift of the absorption peak in the low-energy region of the polymer and a narrower band gap.The absorption spectrum in solution shows that the absorption range of PBDPP-NDT is about 300-750 nm,and that of PBDPTh-IDT is about 300-850 nm.The maxima of the low-energy absorption peaks of the polymers PBDPP-NDT and PBDTPh-NDT in the thin film state are located at 624 nm and 698nm,respectively,and the optical band gaps of PBDPP-NDT and PBDTPh-NDT are1.52 e V and 1.43 e V,respectively.The energy gaps are 1.36 e V and 1.31 e V,respectively.The distribution and energy levels of frontier molecular orbitals of the polymers are calculated by density functional theory(DFT),and the simulated values of orbital energy levels are close to the electrochemical test values.Both polymers have good thermal stability,and the thermal decomposition temperatures of PBDPP-NDT and PBDPTh-NDT are 395°C and 328°C,respectively.