Embedding pyridine units in acceptors to construct donor-acceptor conjugated polymers

Zi-Yuan Wang,Jie-Yu Wang*,Jian Pei

Beijing National Laboratory for Molecular Sciences,Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education,Key Laboratory of Polymer Chemistry and Physics of Ministry of Education,Center for Soft Matter Science and Engineering,College of Chemistry and Molecular Engineering,Peking University,Beijing 100871,China

Key words:Donor-acceptor Conjugated polymers Coplanarity Pyridine-embedded Organic electronics

ABSTRACT The development of donor-acceptor(D-A)conjugated polymers greatly promotes the device performance in organic electronics.Recently,the strategy of embedding pyridine units into D-A conjugated polymer backboneshasattracted much attention due to the resulted lowered LUMOlevels.In addition,the possible non-bonding interactions resulted from the nitrogen atoms also improve the coplanarity of the polymer backbones.All these factors have great contribution to enhance the device performance.In this review,we summarized the recent development of pyridine-embedded D-A conjugated polymers and their applications in organic fi eld-effect transistors(OFETs).

1.Introduction

Over the past few decades,donor-acceptor(D-A)conjugated polymers[1,2]have been promising candidates for low-cost,fl exible and large-scale engineering materials[3,4]of the nextgeneration electronics[5],due to their solution-processability and good mechanic properties.D-A conjugated polymers usually consist of tw o parts:the donor units and the acceptor units.The acceptor units have great impact on the lowest unoccupied molecular orbital(LUMO)energy levels,as well as the molecular conformation.To the best of our knowledge,there have been many electron-de fi cient aromatic units such as isoindigo(IID)[6–8],diketopyrrolopyrrole(DPP)[9,10],benzothiadiazole(BT)[11–13],and naphthalene diimide(NDI)[14,15],which are proved to have the potential to enhance the optoelectronic properties of conjugated polymers.Although a number of acceptor units have been rapidly developed,strategies to construct high-performance acceptor units are still limited.

A common strategy to achieve electron-de fi cient acceptors is introducing electron-withdraw ing groups(such as F,Cl,CN)onto the conjugated backbones to lower their LUMO levels[16–18].Besides this,embedding sp2-nitrogen in benzene ring to form pyridine ring in acceptors[19–21]can also further lower the energy levels of the acceptor moiety of D-A conjugated polymers.Swager et al fi rst developed a promising class of w ater and/or methanol soluble n-type poly(pyridinium phenylene)s[1,2],which may fi nd utility in photovoltaic devices w hen blended with donor conjugated polymers.Introduction of electron-withdraw ing sp2-nitrogen groups onto molecular backbones[22–24]may not only tune the frontier molecular orbital energy levels of the polymers,but also lock the conformation of conjugated polymer backbones through intramolecular noncovalent interactions[25],which is bene fi cial to charge transport[26].In this review,we summarize the recent development of D-A conjugated polymers with acceptors containing pyridine units,which mainly relates to three types of acceptors:azaisoindigos(7DNIID,5DNIID),azaBDOPV(AzaBDOPV),and pyridal[2,1,3]thiadiazole(PT),as show n in Fig.1.

2.D-A conjugated polymers based on azaisoindigo

2.1.Design of azaisoindigo-based conjugated polymers

Fig.1.Molecular structures of electron-de fi cient acceptors containing pyridine units.

Fig.2.(A)Molecular structures and(B)the optimized structures and energy levels of isoindigo derivatives according to density functional theory at the B3LYP/6-31G*level.

Recently,isoindigo has been extensively explored as electronaccepting moiety in organic fi eld-effect transistors(OFETs)and organic light-emitting diodes(OLEDs)materials[6,27,28].According to the calculation results,isoindigo has a torsion angle of ca.20?–40?due to C??H ???O?Csteric interactionsbetween the phenyl ring and the neighboring moiety[29](Fig.2B).In order to improve the conjugation of isoindigo,several strategies have been designed to enhance the coplanarity[30].Ashraf and coworkers replaced the benzene units in isoindigo with thiophene units to decrease the steric repulsion[31].Introducing thiophene units into isoindigo is favorable for efficient charge transport due to the increased planarity.The resulted new isoindigo derivative was applied to copolymers,which showed both good hole and electron mobilities up to 0.1cm2V?1s?1.Afterw ards,this strategy is extensively explored by Wan and Jassen to construct conjugated polymers for electronics[32,33].Moreover,fl uorinated isoindigos having more planar conformation were proved by Pei and others[34–36]using the “conformational lock”strategy.

To achieve high-performance electronic materials,there is a grow ing demand to develop new strategies to achieve planar isoindigo derivatives.Inspired by the application of azaisoindigo in medicinal chemistry[37],Pei and Yu developed several azaisoindigo-based D-A conjugated polymers, respectively. The conformation and energy levels of these azaisoindigo units were calculated to elucidate the influence of the different nitrogen positions on the properties of azaisoindigos.According to the computational results(Fig.2B),the different positions of nitrogen atoms in azaisoindigos obviously influence their planarity and LUMO levels.Compared to isoindigo,azaisoindigos with nitrogen at 5,6,7 positions show more planar conformation with torsion angles of 10.4?,4.9?and 0?in 5,50-diazaisoindigo(5DNIID),6,60-diazaisoindigo(6DNIID)and 7,70-diazaisoindigo(7DNIID),respectively.Notably,all the azaisoindigos show lower LUMO levels as compared to isoindigo,indicating that embedding sp2-nitrogen in benzene ring to form pyridine ring isindeed an effective strategy to develop newelectron-de fi cient acceptors.Several synthetic routes have been designed to synthesize azaisoindigo and their derivatives,which usually involve brominaton and oxidation reactions[38–40].However,because of the high reactivity of 4DNIID and 6DNIID,only 5-and 7-azaisoindigos were explored in D-A conjugated polymers[29,40–42].

In 2016,Yu et al.synthesized tw o D-A conjugated polymers,PAIID-BT-C1and PAIID-BT-C3,with 7DNIID as acceptor and bithiophene asdonor[29],and fabricated OFETsbased on these polymers with top-gate/bottom-contact(TGBC)and bottom-gate/bottomecontact(BGBC)con fi gurations through spin-coating.They presented that PAIID-BT-C3(Fig.3A)achieved hole mobilities as high as 7.28 cm2V?1s?1in BGBC con fi guration and ambipolar mobilities of 2.33/0.78cm2V?1s?1in TGBC con fi guration,indicating that PAIID-BT-C3 is an excellent organic bipolar semiconducting material.

In 2017,Peiet al.reported the fi rst example of D-A conjugated polymer based on 5DNIID.They successfully copolymerized 5DNIID with bithiophene unit to construct a D-A polymer 5DNIID-2T(Fig.3A)and demonstrated its application in OFETs[40].Compared with the isoindigo-based conjugated polymer IIDDT,5DNIID-2T exhibited a lower LUMO level as expected.

The molecular weights(Mn)of IIDDT[6]and 5DNIID-2T were measured to be 33.7 k Da and 33.46 k Da,respectively,by hightemperature gel permeation chromatography(GPC)using 1,2,4-trichlorobenzene as the eluent at 150?C.The UV–vis spectra of both polymers(IIDDT,5DNIID-2T)showed tw o characteristic bands(Fig.3B),w here the high-energy band from 300 nm to 450 nm was attributed to the p-p*transition of the IID and5DNIID units,and the low-energy band from 450 nm to 900 nm originated from the intramolecular charge transfer(ICT)absorption.In addition,grazing incidence X-ray diffraction(GIXD)was used to investigate polymer packing in fi lm(Fig.3C).IIDDT showed four diffraction peaks in out-of-plane direction with a strong diffraction peak at 2u=3.58?,corresponding to a d-spacing of 19.88?(l=1.240?).However,5DNIID-2Tshowed a weaker diffraction peak at 2u=3.00?,which correspondsto a d-spacing of 23.68?(l=1.2398 ?).Tw o other weak diffraction peaks were also observed in 5DNIID-2T fi lm which were ascribed to(200)and(300)diffractions.Different from the crystalline fi brillar intercalating network of IIDDT fi lm[6],the fi lm of 5DNIIT-2T was more amorphous as show n in the tapping-mode atomic force microscopy(AFM)images.As a result,5DNIID-2T showed lower carrier mobility than IIDDT.Further chemical modification of 5DNIID-2T might provide the opportunity to achieve better charge transport properties.In addition,5DNIID-2T showed obvious different charge transport behaviors in OFETs with PAIID-BT-C3,which may be related to the different device con fi gurations.

2.2.Structure-property relationships of AzaBDOPV-based conjugated polymers

Fig.3.A)Molecular structures of D-A conjugated polymers based on azaisoindigos;B)UV–vis-NIR absorption spectra of IIDDT(left)and 5DNIID-2T(right);Copied with permission for IIDDTfrom Ref[6].Copyright 2011,American Chemical Society.C)2D-GIXDpatternsof IIDDT(left)and 5DNIID-2T(right).Copied with permission for 5DNIID-2T from Ref[40].Copyright 2017,Wiley-VCH.

In 2016,Peiet al.fi rst embedded sp2-nitrogen atoms into benzodifurandione-based oligo(p-phenylene vinylene)(BDOPV)backbone[41]to construct a new acceptor AzaBDOPV,and developed a D-Aconjugated polymer AzaBDOPV-2T(Fig.4),which showed high electron mobility of 3.22 cm2V?1s?1under ambient conditions.

Compared with BDOPV-2T,AzaBDOPV-2T exhibited a more planar backbone and lower LUMO level of?4.37[13_TD DIFF]eV(Fig.5),which could be inferred from the absorption spectra.The absorption spectra of both BDOPV-2Tand AzaBDOPV-2T displayed dual-band absorption and obvious 0??0 and 0??1 vibrational peaks both in solution and in fi lms.However,the 0??0 vibrational peak of AzaBDOPV-2T was more obvious in solution,suggesting a more planar conformation.GIXD and tapping-mode AFM experiments indicated a distinct edge-on lamellar packing and stronger crystallinity in AzaBDOPV-2T fi lms,which could be attributed to stronger interchain interactions and more ordered microstructures.These results demonstrated that AzaBDOPVwas an excellent acceptor for constructing high-performance D-A conjugated polymers.

In 2017,Cho and his coworkers applied alkylated bithiophene and(E)-2-(2-(thiophen-2-yl)vinyl)thiophene as donor unit[42]to synthesize tw o AzaBDOPV-based conjugated polymers PBABDFDT and PBABDF-TVT(Fig.4).The LUMO levels of PBABDF-DTand PBABDF-TVT were?4.04eV and?3.99 eV according to the cyclic voltammetry experiments,which were 0.34eV and 0.29 eV lower than that of BDOPV-2T,but higher than that of AzaBDOPV-2T.OFETs were fabricated with bottom-gate/top-contact(BGTC)con fi guration through spin-coating.These tw o polymers displayed electron mobilities up to 1.86 cm2V?1s?1and 1.56cm2V?1s?1after annealing process.Grazing incident X-ray diffraction showed the p-p stacking distance of PBABDF-DT and PBABDF-TVT were 3.52? and 3.50 ?,respectively.The relatively higher LUMO levels and larger p-p distance might result in the poorer performance of these tw o polymers than that of AzaBDOPV-2T.Recently,Yuet al.reported tw o ambipolar conjugated polymers,PNBDOPV-DTBT and PNBDOPV-DTF2BT,based on AzaBDOPV[44].OFETs based on PNDBOPV-DTBTexhibited well-balanced high hole/electron mobilities of 4.68/4.7 cm2V?1s?1.

3.D-A conjugated polymers based on pyridal[2,1,3]thiadiazole

Replacing the benzene ring in benzo[2,1,3]thiadiazole with pyridine was fi rst proposed in the bulk heterojunction(BHJ)polymer solar cells,w here the active layer was composed by donor materialsand acceptor materials.In order to lower the bandgaps of donor materials to achieve high short-circuit currrent(Jsc),incorporating a more electron-de fi cient acceptor to construct DA polymers or small molecules was an effective method.In 2008,Leclerc et al.synthesized several polycarbazole analogues[45](Fig.6A),and explored the different performance between benzo[2,1,3]thiadiazole and pyridal2,1,3]thiadiazole(PT)-based conjugated polymers.However,the polymerization reactions of pyridine-embedded monomer were hard to achieve high molecular weight,which limited the performance in BHJsolar cells.

In 2010,Youet al.reported a series of“weak donor-strong acceptor”[46]conjugated polymers based on pyridal[2,1,3]thiadiazole.The molecular weight was determined by gel permeation chromatography(GPC)in 1,2,4-trichlorobenzene at 135?C.These three polymers PNDT-DTPyT,PQDT-DTPy T,and PBn DT-DTPy T(Fig.6B)showed an Mnof 17.1,21.7 and 104.4kg/mol,respectively.Compared with those of benzothiadiazolebased polymers,obviously reduced LUMO levels and slightly decreased HOMOlevels were observed by cyclic voltammetry.The optical bandgaps of PNDT-DTPy T,PQDT-DTPy T and PBn DT-DTPy T deduced from UV–vis absorption spectra were 1.53,1.56 and 1.51eV,ca.0.09–0.19 eV smaller than those of benzothiadiazole counterparts.Therefore,this strategy was proved to be effective in designing narrow-bandgap materials.

In order to further lower the bandgap of D-A conjugated polymers based on pyridal[2,1,3]thiadiazole,Bazan[23_TD DIFF]et al.utilized Lew is acids to coordinate with the nitrogen atoms in pyridal[2,1,3]thiadiazole[47].For example,coordination of the pyridal[2,1,3]thiadiazole with electron-de fi cient boron reagent(such as B(C6F5)3)lowered both HOMO and LUMO levels,which was more pronounced for the LUMO levels.

Fig.5.UV–vis-NIRspectrum,2D-GIXD patterns and AFM images of BDOPV-2T(A)and AzaBDOPV-2T(B).Reproduced with permission[43].Copyright 2013,Wiley-VCH.

In addition,Bazan[24_TD DIFF]et al.reported the fi rst regioregular PT-based copolymers[48],which exhibited the highest hole mobility of 0.6[25_TD DIFF]cm2V?1s?1due to the high degree of structural order within fi lms.They designed tw o types of regioregular structures,with the pyridal nitrogen atoms of one type pointed in the same direction,while another type showed the pyridal units alternated in orientation(Fig.7A).They proposed that the additional dipole moment caused by embedding nitrogen atoms w ould affect the molecular packing in fi lm.P1-P3 represented the fi rst type,the second type and regiorandom copolymers,respectively.The UV–vis-NIR absorption spectra(Figs.7B and C)of these polymers’solution at 110?C showed hypsochromic shifts compared with those at room temperature,which resulted from the breakup of aggregated polymer chains.To reveal the impact of regioregularity on the charge mobility,bottom-gate/top-contact OFETs were fabricated through spin-casting.As a result,the hole mobilities of regioregularP1 and P2 were tw o orders of magnitude higher than that of regiorandom P3.These results revealed the importance of dipole moment on achieving highly ordered molecular orientation.

Fig.6.Molecular structures of polycarbazole analogues(A)and pyridal[2,1,3]thiadiazole-based conjugated polymers(B).

Afterw ards,Bazan et al.performed a detailed study on the surface topography of copolymer P2[30_TD DIFF]via atomic force microscopy(AFM)[11,49].To achieve high mobility,P2 with high molecular weight(Mn)of 300 k Da and polydispersity(PDI)of 1.5 was synthesized.Figs.8A–D show the surface morphology of gate dielectrics before and after scratching with diamond nanoparticles.For the untreated dielectric surface,the root-meansquare(RMS)roughness was 0.2 nm.After scratching,the RMS increased to 0.48,0.49 and 0.86nm for 100,250 and 500nm diamond nanoparticles.In addition,the uniformity was poorer and grooves were observed on the surface.Figs.8E–H show the morphology of P2 thin fi lms on the unscratched and scratched dielectrics.Long-range orientation and alignment were observed w hen P2 fi lms were deposited on the nanostructured dielectric substrates.OFETs fabricated on structured dielectric substrates were explored as a function of annealing temperature,and the highest hole mobility of 6.7cm2V?1s?1was achieved after optimizing several parameters.

Subsequently,they proposed a new method[50]to orientate P2 in fi lm by utilizing capillary action.The capillary action contributed to highly oriented crystalline fi lms with a compact lamellar structure,which leadsto the hole mobilities of 36.3[cm2V?1s?1for P2(Mn=140 k Da)fi lms w hen the channel length was 140[38_TD DIFF]m m.

In addition,Bazan[39_TD DIFF]et al.also explored the relationship between charge mobilities and different mixing ratios of polystyrene(PS)and P2[51].They obtained mobilities as high as 2.7[40_TD DIFF]cm2V?1s?1w hen mixing 90 w t%PS,which was among the highest reported mobilities for majority-insulator blend systems.

Recently,Liu and coworkers reported the fi rst ambipolar pyridal[2,1,3]thiadiazole-based D-Asemiconducting polymer(PBPTV)[52],which utilized bispyridal[2,1,3]thiadiazole(BPT)asthe acceptor unit and alkylated(E)-2-(2-(thiophen-2-yl)vinyl)-thiophene(TVT)as the donor moiety.PBPTV had a molecular weight(Mn)of 22.7[41_TD DIFF]k Da and narrow PDIof 2.23.Cyclic voltammetry measurementsrevealed its HOMOlevel of?5.61 eV and LUMOlevel of?3.66 eV,which are suitable for ambipolar charge transport.Top-gate/bottom-contact OFETdevices were fabricated,and the hole and electron mobilities of 4.47 and 5.25 cm2V?1s?1were obtained respectively for the ascast thin fi lms(Fig.9).Further annealing of the thin fi lms gave the best performance with the average hole and electron mobilities of 6.64 and 8.13[42_TD DIFF]cm2V?1s?1.AFM images of PBPTVthin fi lms showed small surface roughness.Moreover,2D grazing-incidence w ideangle X-ray scattering(GIWAXS)indicated that the polymer chains arranged mainly in an edge-on orientation and the annealing treatment led to higher degree ordering,which is bene fi cial to the carrier transport.

Fig.7.(A)Regiochemically precise PT-containing alternating copolymer structures.(B,C)UV–vis-NIRabsorption spectra of P1,P2 and P3(1.0?10?5 g/m L)at 25?Cand 110?C in o-DCB.Copied with perm ission[48].Copyright 2011,American Chemical Society.

Fig.8.AFM images of dielectric substrate surfaces and related polymer fiber morphology without and with nanostructures:(A)Surface without structures,(B)100-nmstructured surface,(C)250-nm-structured surface,(D)500-nm-structured surface,and(E)polymer fibers on nonstructured surface,(F)polymer fibers on 100-nm-structured surface,(G)polymer fibers on 250-nm-structured surface,(H)polymer fibers on 500-nm-structured surface.Reproduced with permission[11].Copyright 2012,American Chemical Society.

Fig.9.(A)Molecular structures of BPTacceptor and its copolymer PBPTV;(B)Transfer and output curvesof the hero FETdevice.Copied with permission[52].Copyright 2017,American Chemical Society.

4.Sum m ary and persp ective

In this review,we outlined several design strategies and recent progress in pyridine-embedded D-A conjugated polymers.Based on these results,we can conclude that introducing sp2-nitrogen atoms into polymer backbones has several advantages:i)lowering the LUMOlevels to realize the ideal charge injection;ii)enhancing the coplanarity via noncovalent interactions to lock the conformation;iii)providing coordination sites with Lew is acids to lower the energy levels,especially the LUMO levels;iv)producing dipole moment to achieve ordered molecular orientation.However,some challenges still exist,such as designing new types of pyridinecontaining D-A conjugated polymers,better controlling the molecular packing,and substantially improving the device performance.Therefore,effort on molecular design and device processing engineering is still needed to apply this strategy in organic electronics.

Acknow ledgments

This work was supported by National Key R&DProgram of China(No.2017YFA0204701),National Natural Science Foundation of China(Nos.21722201,21790360,21420102005),and the Major State Basic Research Development Program(No.2015CB856505)from the MOST.