Design,synthesis And Characterization Of Low Bandgap Conjugated Semiconductor Materials With Strong Near-infrared Absorption

Low bandgap conjugated semiconductor materials with good stability have great potential in the application fields of photovoltaics,photodetectors and biological therapies.However,the fact of lacking variety leaves this kind of materials still to be developed.Quinoidal compounds have the advantages of high rigidity from planar structure favorable for charge transport,low molecular orbital energy levels for good air stability and low bandgap and high molar extinction coefficient for strong absorption in Near-Infrared（NIR）.Based on these advantages,a series of quinoidal small molecules with S,N-fused ring central core of different conjugation lengths and 1,3-indandione end-cappers（QSN3-QSN6）were synthesized.The dependence of properties of these molecules on the conjugation length was systematically studied.Furthermore,acceptor units based on the above quinoidal structures were employed to construct low bandgap conjugated polymers.Detailed investigation on basic physical,chemical and charge transport properties of the polymers was carried out.First,all synthesized quinoidal small molecules had a stable closed-shell structure and exhibited obvious quinoid characteristics.All small molecules had similar lowest unoccupied molecular orbital energy levels（LUMO～-4.17 e V）.Along with the increase of the conjugation length in this series of molecules,the highest occupied molecular orbital（HOMO）energy levels became higher,the bandgaps turned smaller and the electron mobility was increased.Among them,QSN6,with the longest conjugation,exhibited the smallest optical bandgap（0.74 e V）,the strongest absorption in Near-Infrared I（NIR I,780-1000 nm）and II（NIR II,1000-1700 nm）,and the highest electron mobility(0.012 cm² V^-1 s^-1).And QSN6 had good air stability.The simulation calculation results showed that the absorption spectrum of small quinoid molecules obtained by continuing to extend the conjugation length of the S,N-fused central core can be further red-shifted.Besides,the quinoidal small molecules QSN5 and QSN6 were functionalized with bromo groups and used as the acceptor units to copolymerize with diketopyrrolopyrrole（DPP）derivatives to obtain low bandgap conjugated polymers P1 and P2.Both P1 and P2 exhibited good thermal stability.The optical bandgaps of the two polymers were1.03 and 0.96 e V,respectively,and they both showed strong absorption in NIR range.P1 and P2 also displayed n-type transport property,with maximum electron mobilities of 0.0067 and 0.0058 cm² V^-1 s^-1 for P1 and P2,respectively.The results in this thesis demonstrated that the optical and semiconducting properties of quinoidal small molecules and polymers could be effectively controlled by regulating the conjugation length of the central core and end-cappers.This structural design strategy is promising for obtaining of semiconductor materials with good stability and strong NIR absorption.