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Indacenodipyrene containing small molecules and ladder polymers

Open AccessPublished:July 13, 2022DOI:https://doi.org/10.1016/j.tchem.2022.100019

      Abstract

      A series of s-indaceno[1,2,3-cd:5,6,7-c'd']dipyrene-containing small molecules and ladder polymers were prepared using a palladium catalyzed arylation reaction. Precursor polymers and their resulting ladder polymers with molecular weights up to 13 ​kDa were prepared. The rigid, planar materials possessed highest occupied molecular orbital (HOMO) energies of −5.39 to −5.23 ​eV, lowest unoccupied molecular orbitals (LUMO) energies of −2.42 ​eV to −2.98 ​eV, and optical gaps of 1.68–2.03 ​eV. Organic field effect transistors were prepared with derivatives giving hole mobilities up to 2.5 X 10−5 cm2V−1s−1.

      Graphical abstract

      Keywords

      1. Introduction

      The demand of renewable and sustainable energy has grown exponentially due to industrialization, urbanization, geopolitical changes, and environmental concerns [
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      • Stafford-Smith M.
      • Gaffney O.
      • Rockström J.
      • Öhman M.C.
      • Shyamsundar P.
      • Steffen W.
      • Glaser G.
      • Kanie N.
      • Noble I.
      Sustainable development goals for people and planet.
      ,
      • Lu L.
      • Zheng T.
      • Wu Q.
      • Schneider A.M.
      • Zhao D.
      • Yu L.
      Recent advances in bulk heterojunction polymer solar cells.
      ,

      Polman, A.; Knight, M.; Garnett, E. C.; Ehrler, B.; Sinke, W. C. Photovoltaic materials: present efficiencies and future challenges. Science 352 (6283), 307–317.

      ]. Robust organic semiconducting materials that are thermally and oxidatively stable while possessing high charge carrier mobilities are worthy targets for advancing technologies including organic field effect transistors and solar cells [
      • Anthony J.E.
      Functionalized acenes and heteroacenes for organic electronics.
      ,
      • Wu J.
      • Pisula W.
      • Müllen K.
      Graphenes as potential material for electronics.
      ]. Among the variety of organic materials possible for adaptation to these technologies, conjugated ladder polymer based semiconductors offer opportunities for improved thermal stability and greater interchain pi-stacking interactions owing to the lack of bond rotation along the polymer backbone [
      • Schlüter A.-D.
      Ladder polymers: the new generation.
      ,
      • Scherf U.
      • Müllen K.
      Polyarylenes and poly(arylenevinylenes), 7. A soluble ladder polymer via bridging of functionalized poly(p-phenylene)-precursors.
      ,
      • Schlüter A.-D.
      • Löffler M.
      • Enkelmann V.
      Synthesis of a fully unsaturated all-carbon ladder polymer.
      ,
      • Goldfinger M.B.
      • Swager T.M.
      Fused polycyclic aromatics via electrophile-induced cyclization reactions: application to the synthesis of graphite ribbons.
      ,
      • Scherf U.
      • Müllen K.
      The synthesis of ladder polymers.
      ,
      • Goldfinger M.B.
      • Crawford K.B.
      • Swager T.M.
      Directed electrophilic cyclizations: efficient methodology for the synthesis of fused polycyclic aromatics.
      ,
      • Ohashi K.
      • Kubo T.
      • Masui T.
      • Yamamoto K.
      • Nakasuji K.
      • Takui T.
      • Kai Y.
      • Murata I.
      4,8,12,16-Tetra-Tert-Butyl-s-Indaceno[1,2,3-Cd:5,6,7-c‘d‘]Diphenalene: a four-stage amphoteric redox system.
      ,
      • Babel A.
      • Jenekhe S.A.
      High electron mobility in ladder polymer field-effect transistors.
      ,
      • Xu C.
      • Wakamiya A.
      • Yamaguchi S.
      Ladder oligo(p-phenylenevinylene)s with silicon and carbon bridges.
      ,
      • Chen Z.
      • Amara J.P.
      • Thomas S.W.
      • Swager T.M.
      Synthesis of a novel poly(iptycene) ladder polymer.
      ,
      • Briseno A.L.
      • Mannsfeld S.C.B.
      • Shamberger P.J.
      • Ohuchi F.S.
      • Bao Z.
      • Jenekhe S.A.
      • Xia Y.
      Self-assembly, molecular packing, and electron transport in n-type polymer semiconductor nanobelts.
      ,
      • Yang X.
      • Dou X.
      • Rouhanipour A.
      • Zhi L.
      • Räder H.J.
      • Müllen K.
      Two-dimensional graphene nanoribbons.
      ,
      • Plumhof J.D.
      • Stöferle T.
      • Mai L.
      • Scherf U.
      • Mahrt R.F.
      Room-temperature bose–einstein condensation of cavity exciton–polaritons in a polymer.
      ,
      • Lee J.
      • Rajeeva B.B.
      • Yuan T.
      • Guo Z.-H.
      • Lin Y.-H.
      • Al-Hashimi M.
      • Zheng Y.
      • Fang L.
      Thermodynamic synthesis of solution processable ladder polymers.
      ,
      • Yang W.
      • Lucotti A.
      • Tommasini M.
      • Chalifoux W.A.
      Bottom-up synthesis of soluble and narrow graphene nanoribbons using alkyne benzannulations.
      ,
      • Gao J.
      • Uribe-Romo F.J.
      • Saathoff J.D.
      • Arslan H.
      • Crick C.R.
      • Hein S.J.
      • Itin B.
      • Clancy P.
      • Dichtel W.R.
      • Loo Y.-L.
      Ambipolar transport in solution-synthesized graphene nanoribbons.
      ,
      • Lee J.
      • Kalin A.J.
      • Yuan T.
      • Al-Hashimi M.
      • Fang L.
      Fully conjugated ladder polymers.
      ,
      • Zeng W.
      • Phan H.
      • Herng T.S.
      • Gopalakrishna T.Y.
      • Aratani N.
      • Zeng Z.
      • Yamada H.
      • Ding J.
      • Wu J.
      Rylene ribbons with unusual diradical character.
      ,

      Cao, Z.; Leng, M.; Cao, Y.; Gu, X.; Fang, L. How Rigid Are Conjugated Non-Ladder and Ladder Polymers? Journal of Polymer Science n/a (n/a). https://doi.org/10.1002/pol.20210550.

      ,
      • Wang Y.
      • Huang Y.
      • Huang T.
      • Zhang J.
      • Luo T.
      • Ni Y.
      • Li B.
      • Xie S.
      • Zeng Z.
      Perylene-based linear nonalternant nanoribbons with bright emission and ambipolar redox behavior.
      ]. The defining feature of a ladder polymer is the linkage between monomer units consists of two or more separate bonds instead of a single linkage location. The result of this bonding motif is ring-like structures linking aromatic segments along the polymer backbone. The rigid-coplanar structures often offer higher thermal stability and extended conjugation lengths and electron delocalization [
      • Teo Y.C.
      • Lai H.W.H.
      • Xia Y.
      Synthesis of ladder polymers: developments, challenges, and opportunities.
      ]. New synthetic pathways that enable the synthesis of unique carbon backbones in conjugated ladder polymers can provide opportunities to probe the structure-function properties and enable advancement in this field.
      In this contribution, we have synthesized a series of rigid conjugated small molecules and conjugated ladder polymers based on an indacenodipyrene skeleton (Fig. 1). The pyrene chromophores provide a large surface area aromatic chromophore that is well-studied in its monomeric form. In addition, this work builds upon a different pyrene-fused s-indacene regioisomer recently reported [
      • Melidonie J.
      • Liu J.
      • Fu Y.
      • Weigand J.J.
      • Berger R.
      • Feng X.
      Pyrene-Fused s-Indacene.
      ]. A palladium-catalyzed arylation reaction was employed as an efficient rigidification reaction that ultimately forms five-membered rings as the new ladder rungs linking the pyrene chromophores in the small molecules as well as polymeric materials.
      Fig. 1
      Fig. 1Conjugated small molecules and conjugated ladder polymers based on indacenodipyrene.

      2. Results and discussion

      To optimize reaction conditions for the preparation of the conjugated ladder polymers, small molecule derivatives were first prepared. Initial attempts to access the fused indacenodipyrene scaffold followed a Scholl cyclodehydrogenation strategy (Scheme 1) [
      • King B.T.
      • Kroulík J.
      • Robertson C.R.
      • Rempala P.
      • Hilton C.L.
      • Korinek J.D.
      • Gortari L.M.
      Controlling the Scholl reaction.
      ,
      • Grzybowski M.
      • Skonieczny K.
      • Butenschön H.
      • Gryko D.T.
      Comparison of oxidative aromatic coupling and the Scholl reaction.
      ]. Singly borylated pyrene 3 was cross-coupled with 1,4-dibromo-2,5-bis(dodecyloxy)benzene 4 to give precursor 5 in good yields. However, Scholl cyclodehydrogenation reactions utilizing either FeCl3 [
      • Horibe T.
      • Ohmura S.
      • Ishihara K.
      Structure and reactivity of aromatic radical cations generated by FeCl3.
      ] or DDQ [
      • Zhai L.
      • Shukla R.
      • Wadumethrige S.H.
      • Rathore R.
      Probing the arenium-ion (ProtonTransfer) versus the cation-radical (electron transfer) mechanism of Scholl reaction using DDQ as oxidant.
      ] with strong organic acids (triflic acid and methyl sulfonic acid) were found to not give the desired product 6, but instead resulted in retrieval of starting material. As an alternative, we prepared brominated precursors to enable a palladium-catalyzed arylation reaction strategy [
      • Rice J.E.
      • Cai Z.W.
      An intramolecular arene-triflate coupling reaction for the regiospecific synthesis of substituted benzofluoranthenes.
      ,
      • Reisch H.A.
      • Bratcher M.S.
      • Scott L.T.
      Imposing curvature on a polyarene by intramolecular palladium-catalyzed arylation reactions: a simple synthesis of dibenzo[a,g]Corannulene.
      ]. Mono-brominated pyrene derivatives 79 with varying substituents on the pyrene core were prepared (Supporting Information) and reacted with 2,2'-(2,5-dibromo-1,4-phenylene)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) 10 [
      • Higashino T.
      • Ishida K.
      • Sakurai T.
      • Seki S.
      • Konishi T.
      • Kamada K.
      • Kamada K.
      • Imahori H.
      Pluripotent features of doubly thiophene-fused benzodiphospholes as organic functional materials.
      ] to give precursors 1113 in modest yields of 29–35% (Scheme 2). Here, [Pd(PCy3)2Cl2] as the catalyst and DBU as base was employed to create the five-membered ring bridges to give 1a-c in isolated yields of 78–90%. It should be noted that a preliminary route to build up a library of diverse substituted structures from a common precursor was investigated. For example, attempts were made to brominate 1a, which would have provided a scaffold to easily build a library of structures through cross-coupling chemistry (e.g., 1b and 1c). However, these attempts were unsuccessful in preparing pure brominated materials for further utilization as an intractable mixture of products were found during chromatography. This preliminary difficulty led to the less convenient, yet operational method, of stepwise functionalization we report here.
      Scheme 1
      Scheme 1Synthesis of small molecule precursor via Suzuki cross coupling.
      Scheme 2
      Scheme 2Synthesis of small molecule derivatives via Suzuki cross coupling and Heck reaction.
      With the small molecule derivatives prepared, the reaction chemistry was applied to preparing ladder conjugated polymers. Pyrene precursors were modified from the single bromine containing monomers 79 to dibromo-containing monomers 14 and 15 (Scheme 3). Polymerization conditions utilized the same Suzuki-Miyaura conditions with monomer 10 to build up the pyrene containing materials. Recycling preparative gel permeation chromatography (GPC) was used to exclude lower molecular weight material and resulted in collected samples of 10 ​kDa and 13 ​kDa (based on GPC with polystyrene standards), for 16 and 17, respectively (supporting information). The palladium-catalyzed arylation reaction was applied to 16 and 17 to form the laddered polymers 2a and 2b. As expected, the solubility of the resulting materials was significantly reduced owing to the extended planarized surfaces. Ladder polymer 2a was found to be totally insoluble in common organic solvents such as toluene, chloroform, dichloromethane, tetrahydrofuran and dichloroethane at higher temperatures. However, 2b was found to give some solubility (∼0.05 ​mg/mL) in hot chloroform for limited processing and characterization.
      Scheme 3
      Scheme 3Synthesis of conjugated ladder polymers via Suzuki cross coupling and arylation reaction.
      Solution based UV/Vis absorption spectra of the indacenodipyrene based materials are shown in Fig. 2. Overall, the three small molecule derivatives (1a-c) were similar, but with small bathochromic shifts with substitutions consisting of the anisole (8 ​nm) and thiophene (18 ​nm). The optical band gaps of the pyrene-based materials, as determined by the onset of film-based absorption from the diffuse reflectance absorption spectra (supporting information) were 2.03 ​eV, 1.98 ​eV and 1.90 ​eV for 1a, 1b and 1c, respectively. The precursor polymer 17 onset is significantly hypsochromically shifted in relation to the ring closed compounds 1a-c. However, upon the arylation reaction, polymer 2b shows significant bathochromic shift with an onset of 693 ​nm in solution with the lowest energy transition (470–600 ​nm) overlapping with the small molecule analogs. The absorption tails to lower energies and suggests expanded delocalization along the polymer backbone in relation to the dimeric pyrene small molecule derivatives.
      Fig. 2
      Fig. 2Absorption spectra of small molecules 1a-c and polymers 17 and 2b in chloroform.
      To further probe the electronic properties of 1a-c, 17, and 2b, cyclic voltammograms (CV) of the solid films were obtained. Organic films for CV were prepared by drop casting chloroform solutions on a glassy carbon electrode. Analysis of the CV shows the materials have both irreversible oxidation and reduction signals (Fig. 3). The most significant oxidation was found in 1c, which would arise from thiophene radical cation electropolymerization at the open 2-position [
      • Roncali J.
      Conjugated poly(thiophenes): synthesis, functionalization, and applications.
      ]. It is unknown the exact reactive radical species that leads to the irreversibility in these system, however, the dimerization of pyrene and quinone formation are known pathways in other pyrene systems [
      • Jeftic L.
      • Adams R.N.
      Electrochemical oxidation pathways of benzo[a]Pyrene.
      ]. Utilizing ferrocene as an internal standard, the highest occupied molecular level (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of the materials were evaluated. The HOMO levels ranged from −5.28 ​eV to −5.39 ​eV, while the LUMO energies varied between −2.42 to −2.98 ​eV (Table 1). The optical and electrochemical gaps diverged in absolute value and is presumably owing to the exciton binding energies being considerably different in these systems [
      • Sariciftci N.S.
      Primary Photoexcitations in Conjugated Polymers: Molecular Excitons vs Semiconductor Band Model.
      ].
      Fig. 3
      Fig. 3Thin film cyclic voltammetry of 1a-c, 17 and 2b in acetonitrile with 0.05 ​M tetrabutyl ammonium hexafluorophosphate, glassy carbon working electrode, platinum counter electrode, and an Ag/AgCl reference electrode. Scan rate ​= ​100 ​mV/s. Ferrocene was added as an internal standard and referenced to 0 ​V.
      Table 1Summary of molecular weight and optoelectronic properties of compounds 1a-c, 17 and 2b.
      CmpdEox/onset (V)Ered/onset (V)HOMOLUMOE-chemλonsetOptical
      (eV)(eV)gap (eV)(nm)gap (eV)
      1a0.59−2.36−5.39−2.442.956102.03
      1b0.43−2.38−5.23−2.422.816251.98
      1c0.48−2.01−5.28−2.792.496511.90
      170.58−2.31−5.38−2.492.894712.63
      2b0.58−1.82−5.38−2.982.407391.68
      a ​Potentials measured relative to a ferrocenium/ferrocene redox couple used as an internal standard (Fig. 3). Eox/onset ​is the onset of oxidation potential, Ered/onset ​is the onset of reduction potential. HOMO and LUMO values calculated on the basis of the oxidation of the ferrocene reference in vacuum (−4.8 ​eV). Optical gap taken from λonset of film (SI).
      The newly prepared materials were then evaluated in regard to their charge carrier mobilities. Organic field effect transistors with bottom gate and bottom contact arrangements were prepared. Traditional gold contacts deposited on octadecyltrichlorosilane functionalized Si/SiO2 substrates were utilized. Films were cast by spin-coating chloroform solutions at 3000 ​rpm onto the prepared substrates. Typical output and transfer plots of the resulting devices can be found in Fig. 4 and Fig. 5. The average (n ​= ​5) charge carrier mobilities 1b and 1c, as tested in air, were found to be an average of 2.5X10−5 (±2.1X10−6) and 7.4X10−5 (±1.7 X10−5) cm2 V−1 s−1, respectively. No appreciable current was observed for 1a or polymer 2b, and were presumed to be owing to inconsistent film formation resulting from poor solubilities. Although the charge carrier mobilities in these systems are relatively small, they do demonstrate the materials are active as charge carriers.
      Fig. 4
      Fig. 4Typical output curves for 1b showing p-type semiconductor behavior.
      Fig. 5
      Fig. 5Typical transfer plot for the p-type semiconductor 1b. VG ​= ​−80 ​V. Average mobility (h+ ​= ​2.5 ​× ​10−5 ​cm2 ​V−1 s−1 ​± ​2.1 ​× ​10−6).
      In conclusion, we have synthesized a series of new indacenodipyrene based small molecule and conjugated ladder polymers by an efficient palladium catalyzed arylation of a dipyrenyl benzene precursor. The small molecule reaction optimization was applied to make conjugated-ladder polymers that possessed bathochromically shifted optical properties and less soluble materials in comparison to the small molecule analogs. While the conjugated polymers in this study did not show charge conduction, the small molecule derivatives did show modest charge carrier mobilities.

      Declaration of competing interest

      The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

      Acknowledgments

      This work was supported by a National Science Foundation CAREER grant ( CHE-1352431 ).

      Appendix A. Supplementary data

      The following is the Supplementary data to this article:

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