Synthesis And Photoelectronic Properties Of Novel Naphthalimide-Based Conjugated Polymers

Electron-transporting polymeric semiconductors exhibit great application potential in the area of organic field-effect transistors(OFETs),non-fullerene solar cells,and organic integrated circuits.However,this kind of materials still confront with some great chanllenges,such as poor electron mobility and scarce acceptor building-blocks.In this dissertation,two types of donor-acceptor(D-A)type electron-transporting polymers containing naphthalenediimide derivatives have been designed and synthesized to investigate the relationship between molecular struture and performance.And the main contexts are as follows:1.We have designed and synthesized a series of A1-D-A2-D type naphthalenediimide-based electron-transporting polymers,including thiophene-and selenophene-containing copolymers(PNBT,PNBTF,PNBS,and PNBSF),in which the second weak acceptors,such as benzothiadiazole or 5,6-difluorobenzothiadiazole,are introduced into the polymeric main-chains.The research results demonstrate that,compared with thiophene-containing copolymers(PNBT and PNBTF),selenophene-containing copolymers(PNBS and PNBSF)exhibit excellent film-forming ability and could form free-standing films.Additionally,the introduction of selenophene and fluorine atoms reduce the LUMO level of the polymer,thus increasing the electron affinity of the resulting copolymers.Compared with PNBT and PNBTF,both PNBS and PNBSF thin films exhibit a more smooth surface morphology,which is helpful for enhancing carrier transport of top-gate OFETs.On the silicon substrates,four polymer thin films adopt a kind of mixed stacking arrangement.Moreover,selenophene heterocycles endow PNBS and PNBSF with closer ?-? stacking distances as compared with thiophene-containing polymers.Generally,small ?-?stacking distance would facilitate high-performance carrier transport.Using the four poymers as the active layer,top-gate bottom-contact OFET device is fabricated.The results indicate that selenophen-containing polymers achieve much higher mobilities than those of thiophene-containing polymers.For the PNBS-based OFETs,an encouraging hole and electron mobility up to 1.7 and 8.5 cm2 V-1 s-1 have been demonstrated,respectively.When fluorine atoms are incorporated into the polymeric main-chains,the carrier-transporting characteristics of the polymers is transfered from ambipolar features to unipolar n-type ones.Moreover,PNBSF-based OFETs show excellent unipolar electron-transporting performance,thus giving a high electron mobility of 3.5 cm2 V-1 s-1.This finding indicates that heteroatom engineering strategy is an effective molecular design method,which can be used to fine-tune film-forming ability,energy level,aggregation structure,and carrier transport properties of the naphthalenediimide-based copolymers.2.We have designed and synthesized of two highly electron-deficient heterocyclic acceptor units,including naphthalenediimide-fused pyrazinyl heterocycle(NBT-2Br)and naphthalene-fused pyrazinyl heterocycle(APhTQ).On the basis of NBT-2Br and APhTQ units,three ultra-low band gap D-A type conjugated polymers(PNBT-2T,PNBT-TT,and PAPhTQ)have been designed and synthesized.Moreover,we systematically investigate the effect of diimide groups on the thermal stability,absorption spectra,and HOMO/LUMO energy levels of the polymers.The results indicate that,compared with APhTQ unit,highly electron-deficient diimide groups sharply improve the electron affinity of the NBT-2Br unit,thereby achieving much lower HOMO/LUMO energy levels,lower optical bandgap,and broader absorption band for the NBT-2Br unit.Absorption spectra reveal that,compared with the PAPhTQ(0.98 eV),both diimide-containing copolymers(PNBT-2T and PNBT-TT)obtain much lower optical band gaps of 0.82 and 0.76 eV,respectively;moreover,their maximum onset values of the thin-film spectra are extended to above 1500 nm.Additionally,the HOMO/LUMO energy levels of the three polymers are also determined by cyclic voltammetry measurements.We find that the calculated HOMO and LUMO energy levels of PAPhTQ is-5.10 and-3.74 eV,respectively.Compared with PAPhTQ,the introduction of diimide groups into the PNBT-TT and PNBT-2T effectively reduce their HOMO and LUMO energy levels,and the calculated HOMO/LUMO energy levels are-5.16/3.85 eV for PNBT-2T and-5.18/-3.89 eV for PNBT-TT,respectively.Our works demonstrate that,by introduction of highly electron-deficient diimide groups into the polymeric backbone,we not only can increase the electron affinity of the polymers,but alos can effectively reduce the optical bandgap and HOMO/LUMO energy level of the polymers.Our developed ultra-low band gap polymers exhibt great application potential in the fields of ambipolar OFET devices,organic near-infrared detectors,and near infrared-?fluorescence imaging.