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Polycyclic Aromatic Chains on Metals and Insulating Layers By ARTICLE https://doi.org/10.1038/s41467-020-15210-2 OPEN Polycyclic aromatic chains on metals and insulating layers by repetitive [3+2] cycloadditions ✉ Alexander Riss 1 , Marcus Richter2, Alejandro Pereź Paz 3,4,5, Xiao-Ye Wang 6,7, Rajesh Raju 6, Yuanqin He1, Jacob Ducke1, Eduardo Corral1, Michael Wuttke6, Knud Seufert 1, Manuela Garnica1,8, Angel Rubio5,9,10, Johannes V. Barth1, Akimitsu Narita 6,11, Klaus Müllen6,12, Reinhard Berger2, ✉ Xinliang Feng 2, Carlos-Andres Palma1,13 & Willi Auwärter 1 1234567890():,; The vast potential of organic materials for electronic, optoelectronic and spintronic devices entails substantial interest in the fabrication of π-conjugated systems with tailored func- tionality directly at insulating interfaces. On-surface fabrication of such materials on non- metal surfaces remains to be demonstrated with high yield and selectivity. Here we present the synthesis of polyaromatic chains on metallic substrates, insulating layers, and in the solid state. Scanning probe microscopy shows the formation of azaullazine repeating units on Au(111), Ag(111), and h-BN/Cu(111), stemming from intermolecular homo-coupling via cycloaddition reactions of CN-substituted polycyclic aromatic azomethine ylide (PAMY) intermediates followed by subsequent dehydrogenation. Matrix-assisted laser desorption/ ionization (MALDI) mass spectrometry demonstrates that the reaction also takes place in the solid state in the absence of any catalyst. Such intermolecular cycloaddition reactions are promising methods for direct synthesis of regioregular polyaromatic polymers on arbitrary insulating surfaces. 1 Physics Department E20, Technical University of Munich, James-Franck-Str. 1, 85748 Garching, Germany. 2 Department for Molecular Functional Materials, Center for Advancing Electronics Dresden (cfaed), Faculty of Chemistry and Food Chemistry, Dresden University of Technology, Mommsenstr. 4, 01062 Dresden, Germany. 3 School of Physical Sciences and Nanotechnology, Yachay Tech University, 100119 Urcuquí, Ecuador. 4 Chemistry Department, College of Science, United Arab Emirates University (UAEU), P.O. Box 15551, Al Ain, United Arab Emirates. 5 Nano-Bio Spectroscopy Group and ETSF, Universidad del País Vasco, 20018 San Sebastián, Spain. 6 Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany. 7 State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, 300071 Tianjin, China. 8 Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nanociencia), 28049 Madrid, Spain. 9 Max Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149, 22761 Hamburg, Germany. 10 Center for Free−Electron Laser Science and Department of Physics, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany. 11 Organic and Carbon Nanomaterials Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son, Kunigami, Okinawa 904-0495, Japan. 12 Institute of Physical Chemistry, Johannes Gutenberg-University Mainz, Duesbergweg 10-14, D-55128 ✉ Mainz, Germany. 13 Institute of Physics, Chinese Academy of Sciences, 100190 Beijing, China. email: [email protected]; [email protected] NATURE COMMUNICATIONS | (2020)11:1490 | https://doi.org/10.1038/s41467-020-15210-2 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-15210-2 xtended covalently bonded polyaromatic (polycyclic aro- Here, we present a covalent intermolecular cycloaddition matic) systems hold great promise for molecular devices reaction of CN-functionalized PAMY with itself, into one- E 26 due to their increased structural and thermal stability and dimensional polyaromatic chains with azaullazine repeating favorable electronic properties1–8. In many areas of technological units on metals, insulating layers, and in the solid state. Scanning applications and fundamental research, it is crucial to obtain tunneling microscopy (STM) and bond-resolved atomic force covalently bonded fused ring systems (such as graphene and microscopy (AFM) with CO-functionalized tips demonstrates graphene nanoribbons) on insulating surfaces, to leverage device that on all investigated substrates, i.e., Au(111), Ag(111), and fabrication and integration of active layers onto dielectric sub- hexagonal boron nitride (h-BN) on Cu(111), phenanthridinium strates. In some cases, ex situ synthesis of such materials and precursors predominantly form covalently coupled chains with subsequent processing and transfer onto insulating surfaces is repeating units of 1-azadibenzo[d,k]ullazine via the cycloaddition possible. In situ synthesis, on the other hand, provides an reaction27,28 between the cyano group and the azomethine ylide alternative pathway towards functional nano-architectures and is moiety, as well as subsequent dehydrogenation (see Fig. 1)24,29–34. expected to decrease costs, favor miniaturization, improve the The reaction did not proceed under solution reaction conditions, purity and enhance the performance of polyaromatic frame- but was found to occur in the solid state as evidenced by MALDI works. However, only few reaction types for aryl-aryl coupling measurements. This result suggests that the catalytic effect of the have been realized on insulating surfaces9–19. The control of substrate is not crucial, and instead, the intrinsic reactivity of the covalent coupling reactions on nonmetallic surfaces proves dif- molecules plays the dominant role, thus facilitating the cycload- ficult due to the lack of metal-catalyzed carbon-hydrogen or dition reaction in the absence of catalysts, i.e., on insulating and carbon-halogen bond activation20,21. Coupling and, in particular, inert substrates. (aromatic) ring formation reactions on insulators thus often require harsher reaction conditions, the addition of catalysts, or external stimuli16,18,22. Such environments commonly lead to Results impurities and (randomly coupled) side products that remain on Cycloaddition on metal substrates. To afford repetitive inter- the surface, substantially hampering the quality of the fabricated molecular coupling we pursued a head-to-tail approach employing architectures. Creation of atomically precise surface-supported phenanthridinium precursors with a sterically accessible cyano- organic materials for devices prompts catalyst-free polymeriza- group (“tail”). The phenanthridinium precursor can form a reactive tion23 and ring-formation reactions with high selectivity and azomethine ylide group (“head”)insitubyprotoncleavage29,30,34. preferably surface-independent reactivity. For instance, we The bifunctionalized precursor, 2-cyano-8H-isoquinolino[4,3,2-de] recently introduced PAMY as a powerful halogen-free building phenanthridin-9-ium tetrafluoroborate (1), was synthesized as block for the surface-assisted dimerization into a conjugated described in our previous work24 and deposited onto a Au(111) polycyclic hydrocarbon, namely pyrazine-embedded hexa-peri- surface held at 600 K. STM measurements revealed units that 24 hexabenzocoronene (N2-HBC) . Therefore, further develop- resemble the shape of the polyaromatic ring system of the cation ment of PAMY chemistry might grant access to previously and the cycloaddition products of head-to-tail coupled molecules unknown materials and in situ fabrication on device interfaces, (Fig. 2a–c). No evidence of the tetrafluoroborate counter-anions such as solar cells and light-emitting devices25. near their molecular units was found, suggesting that the anions or H H on Au(111), Ag(111), h-BN/Cu(111) C N C N N and in the solid state N H n Azaullazine 1 2 –H+ –(2H)n PAMY intermediate H H H H C N C N N C N N C N N H H H N Cyano n Azomethine ylide "tail" H "head" 1a 1a 1a 2a ... ... Fig. 1 Conceptual route for the reported synthesis of polyaromatic azaullazine chains. The thermally induced head-to-tail coupling of 1 was observed on Au(111), Ag(111), h-BN/Cu(111), as well as in the solid state. Bottom: Reactive PAMY species 1a as potential intermediates with affinity towards the cyano moiety, yielding intermolecular head-to-tail cycloaddition product 2a. Subsequent dehydrogenation affords polyaromatic azaullazine chains 2. 2 NATURE COMMUNICATIONS | (2020)11:1490 | https://doi.org/10.1038/s41467-020-15210-2 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-15210-2 ARTICLE gh i k H H ≡ N H C H H H 5 + 4 H N N –HBF C 1 +N C ≡ H 4 C ≡ N +N C ≡ N N +N –2H N C N 3 N C C 2 n N N – N N BF4 1 2 Fig. 2 Head-to-tail coupling of 1 on a Au(111) substrate held at 600 K. a–c STM images of monomeric, dimeric, and trimeric species. d–f Bond-resolved AFM images of the monomer, dimer, and trimer shown in a–c. The images show the formation of an imidazole ring via cycloaddition of the –CN group of one monomer towards the phenanthridinium-derived azomethine ylide moiety of another monomer. g–i Chemical structures of the monomer, dimer, and trimer. The formed imidazole ring is highlighted in blue color. j Overview STM image shows a coexistence of covalently coupled (yellow solid circle) and noncovalently assembled (green dotted and red dashed circles) motifs. k Reaction scheme showing head-to-tail polymerization of 1 upon imidazole ring formation. Scan parameters: (a, b) Sample bias VS = 20 mV, tunneling current setpoint I = 5 pA; (c) VS = 20 mV, I = 20 pA; (d–f) VS = 0 V, oscillation amplitude AOSC = 40 pm, constant height; (j) VS = 20 mV, I = 5 pA. Frequency shift ranges, dark to bright: (d) −10.2 to 6.7 Hz; (e) −10.1 to 3.1 Hz; (f) −9.9 to 7.2 Hz. anion-derived species exhibit rather low sticking probability or/and Comparative studies on Ag(111) revealed that on this substrate are highly mobile on the surface (the anions could also not be moderate temperatures of 520 K analogously induce head-to-tail detected when 1 was deposited onto a sample held at room tem- coupling via imidazole ring formation (Fig. 4). In contrast to Au perature; see Supplementary Fig. 1 for more details)35.TheSTM (111), longer oligomers units were observed on Ag(111). Many of images in Fig. 2b, c show increased local density of states (LDOS) the terminal –CN groups were found to be abstracted (Fig.
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