Angewandte Communications Chemie International Edition:DOI:10.1002/anie.201814154 Surface Chemistry German Edition:DOI:10.1002/ange.201814154 On-Surface Synthesis of Ethynylene-Bridged Anthracene Polymers Ana Sµnchez-Grande,Bruno de la Torre,JosØ Santos,Borja Cirera, Koen Lauwaet, Taras Chutora, Shayan Edalatmanesh, Pingo Mutombo,Johanna Rosen, Radek Zborˇil, Rodolfo Miranda, Jonas Bjçrk,* Pavel Jelínek,* Nazario Martín,* and David Écija* Abstract: Engineering low-band-gap p-conjugated polymers conjugated nanomaterials to be synthesized by wet chemis- is agrowing area in basic and applied research. The main try.[2,3] synthetic challenge lies in the solubility of the starting materials, On-surface synthesis has become apowerful discipline to which precludes advancements in the field. Here,wereport an design many novel molecular compounds,polymers,and on-surface synthesis protocol to overcome such difficulties and nanomaterials with atomistic precision,[4–12] some of them not produce poly(p-anthracene ethynylene) molecular wires on accessible by standard synthetic methods.Additionally,on- Au(111). To this aim, aquinoid anthracene precursor with surface chemistry enables the structural and electronic =CBr2 moieties is deposited and annealed to 400 K, resulting in characterization of the designed products with advanced anthracene-based polymers.High-resolution nc-AFM meas- surface-science techniques.[10,13,14] Recently,and within the urements confirm the nature of the ethynylene-bridge bond scope of on-surface synthesis,particular success has been between the anthracene moieties.Theoretical simulations achieved in the field of oligoacenes,where anthracene,[15,16] illustrate the mechanism of the chemical reaction, highlighting tetracene,[17] hexacene,[18] heptacene,[19, 20] nonacene,[21] deca- three major steps:dehalogenation, diffusion of surface-stabi- cene,[22] and undecacene[20] precursors have been deposited on lized carbenes,and homocoupling,which enables the forma- coinage metals and transformed into oligoacene derivatives tion of an ethynylene bridge.Our results introduce anovel through external stimuli. However,despite the great potential chemical protocol to design p-conjugated polymers based on of such acene compounds for plastic optoelectronics,the oligoacene precursors and pave new avenues for advancing the design of high-quality p-conjugated polymers exclusively emerging field of on-surface synthesis. based on oligoacene building units remains difficult. On- surface chemistry opens the gate to obtain acene-based The design and study of p-conjugated polymers has received polymers.Although their predicted intrinsic insolubility may great attention during the last decades due to the relevant hinder their practical application at first glance,state-of-the- optical and electronic properties that emerge from the art substrate-to-substrate transfer techniques already permit delocalization of the p-electrons.Such materials find use in the implementation of highly insoluble nanomaterials into different applications including light-emitting devices,solar molecular electronic devices.[23] cells,organic field-effect transistors,photocatalysis,and Here,wereport acomprehensive scanning tunneling biosensors.[1–3] However,despite great synthetic advances in microscopy (STM), non-contact atomic force microscopy (nc- the field, there are enormous efforts coming from emerging AFM), and density functional theory (DFT) study of the on- areas of chemistry targeting to overcome the accompanying surface synthesis of poly(p-anthracene ethynylene) molecular limitation of solubility,which precludes highly interesting wires on Au(111). Our novel chemical approach is based on [*] A. Sµnchez-Grande, Dr.J.Santos, Dr.B.Cirera, K. Lauwaet, Prof. R. Miranda Prof. R. Miranda, Prof. N. Martín, Prof. D. Écija Departamento de Física de la Materia Condensada IMDEA Nanociencia,C/Faraday 9 Facultad de Ciencias, Universidad Autónoma de Madrid Ciudad UniversitariadeCantoblanco 28049 Madrid (Spain) 28049 Madrid (Spain) Prof. N. Martín E-mail:[email protected] Departamento de Química Orgµnica [email protected] Facultad de Ciencias Químicas, Universidad Complutense Dr.B.delaTorre, T. Chutora,Prof. R. Zborˇil, Dr.P.Jelínek 28040 Madrid (Spain) Regional Centre of Advanced Technologies and Materials Supportinginformation and the ORCID identification number(s) for Palacky´ University Olomouc the author(s) of this article can be found under: ˇ Slechtitelu˚ 27, 78371 Olomouc (Czech Republic) https://doi.org/10.1002/anie.201814154. Dr.B.delaTorre, S. Edalatmanesh, P. Mutombo, Dr.P.Jelínek 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. Institute of Physics, The Czech AcademyofSciences KGaA. This is an open access article under the terms of the Creative Cukrovarnickµ 10, 16200 Prague 6(Czech Republic) Commons Attribution-NonCommercialLicense, which permits use, E-mail:[email protected] distribution and reproduction in any medium, provided the original Prof. J. Rosen, Dr.J.Bjçrk work is properly cited and is not used for commercialpurposes. Department of Physics,Chemistry and Biology,IFM LinkçpingUniversity 58183 Linkçping(Sweden) E-mail:[email protected] Angew.Chem. Int.Ed. 2019, 58,6559 –6563 2019The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim 6559 Angewandte Communications Chemie the dehalogenation, homocoupling,and aromatization of aquinoid anthracene precursor endowed with=CBr2 moieties at their 9- and 10- positions.The deposition of this precursor (11,11,12,12-tetrabromoanthraquinodimethane), abbreviated 4BrAn,onAu(111) gives rise to aclose-packed assembly. Annealing to 400 Kenables debromination, and, after diffusion, long molecular wires based on ethynylene bridges are formed, which is unambiguously confirmed by the excellent agreement of the experimental non-contact atomic force microscopy measurements with the theoretical simu- lations.Afinal step of annealing to 500 Kinduces the removal of peripheral bromine atoms and the subsequent emergence of isolated, high-quality,robust, and long anthracene-based polymers.Scanning probe microscopy reveals the shape and energy of the frontier orbitals,which leads to aband gap of 1.5 eV.The theoretical study of the reaction pathways, complemented by high-resolution scanning probe microscopy measurements with aCOtip,illustrates that the homocou- pling process is based on an efficient dehalogenation of the molecular precursors and diffusion of the surface-stabilized carbenes,finally leading to coupling and aromatization of the polymeric chain. Our study introduces anovel strategy to homocouple anthracene precursors,which could be extended to the whole family of oligoacenes.Weenvision that these results will notably contribute to the development of the field of on- surface chemistry while providing novel avenues to design p- conjugated polymers in the form of molecular chains,with aspecial emphasis in low-band gap nanomaterials. Scheme 1illustrates the suitably synthesized molecular Figure 1. a) Large-scale STM image of the parallelly aligned ethynylene- linked anthracene polymers coexisting with bromine atoms after precursor 4BrAn used for building up the desired anthracene- deposition of asubmonolayer coverageof4BrAn on Au(111) and based polymers.Notably,itincorporates =CBr2 moieties at subsequent annealing at 400 K(Vbias = 100 mV, I = 10 pA, image the positions 9and 10 of the anthraquinone backbone to size =14.9 8.4 nm2). b) Magnified constant-height nc-AFM and c) STM image (size = 3.6 3.2 nm2)ofaselected area of (a) acquired at the same time. d) Chemical structure of an ethynylene-bridged anthracene moiety.e)Constant-heightSTM and f) nc-AFM image resolving the ethylene bond, which matchesvery well with g) anc-AFM simulation (1.31.3 nm2). h) Constant-height nc-AFM image of alinear (size = 12 2nm2)and i) acurved (size = 12 4nm2)ethyny- lene-linkedanthracene polymer after annealing (a) to 500 K. Scheme 1. Reaction sequenceofthe 4BrAn (11,11,12,12-tetrabromoan- Supporting Information, Figure SI2). As illustrated by high- thraquinodimethane) precursor after deposition on Au(111) and sub- sequent annealing. resolution AFM imaging with aCOtip,the polymers are formed by anthracene moieties that are linked by ethynylene bridges.Atanadequate distance between tip and sample,the control the dehalogenation and subsequent homocoupling triple bond can be unambiguously distinguished as abright upon thermal annealing[24] by undergoing an unprecedented protrusion (Figure 1b,f and Supporting Information, Fig- reaction pathway as detailed below.The deposition of ure SI3), which is in agreement with recently synthesized asubmonolayer coverage of 4BrAn on Au(111) results in poly(p-phenylene ethynylene) molecular wires on Au(111).[25] the formation of close-packed islands in arectangular unit cell Theexcellent match between the experimental and simulated (Supporting Information, Figure SI1). At this stage of the AFM images confirms the bridge as an ethynylene moiety reaction, no isolated bromine atoms are detected on the (see Figure 1f,g), which differs from the previously reported surface,which highlights the integrity of the molecular formation of cumulene bridges by dehalogenative homocou- precursors.Afirst step of annealing to 400 Kleads to the pling of alkenyl gem-dibromides.[24] emergence of long polymeric wires (Figure 1aand Supporting At this step of annealing,asmall fraction of ethynylene- Information, Figure SI2). Themolecular precursors have now bridged anthracene dimers and trimers is present.