Surface-Confined Polymerization of Halogenated Polyacenes: The

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Surface-Confined Polymerization of Halogenated Polyacenes: The Case of Dibromotetracene on Ag(110) Igor Píš, Lara Ferrighi, Thanh Hai Nguyen, Silvia Nappini, Luca Vaghi, Andrea Basagni, Elena Magnano, Antonio Papagni, Francesco Sedona, Cristiana Di Valentin, Stefano Agnoli, and Federica Bondino J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12047 • Publication Date (Web): 12 Feb 2016 Downloaded from http://pubs.acs.org on February 14, 2016

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The Journal of Physical Chemistry

Surface-Confined Polymerization of Halogenated Polyacenes: The Case of Dibromotetracene on Ag(110) Igor Píš,†§* Lara Ferrighi, ‡ Thanh Hai Nguyen,|| Silvia Nappini,§ Luca Vaghi, ‡ Andrea Basagni,|| Elena Magnano,§# Antonio Papagni,‡ Francesco Sedona,|| Cristiana Di Valentin,‡ Stefano Agnoli,|| and Federica Bondino§* †

Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 Km 163.5, I-34149 Basovizza (TS), Italy

§

IOM CNR, LABORATORIO TASC - S.S. 14 Km. 163,5 I-34149 Basovizza (TS), Italy

‡

Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Via Cozzi 55, 20125

Milano, Italy ||

Department of Chemical Sciences, University of Padova, Via Marzolo 1, 35131 Padova, Italy

#

Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South

Africa

KEYWORDS Nanostructures, organic thin films, coordinated polymers, surface synthesis, surface chemistry

ACS Paragon Plus Environment

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ABSTRACT On-surface synthesis of thin organic and organometallic films in a bottom-up fashion has become a promising approach for the development of new nanotechnologies. In this work we studied 5,11-dibromotetracene (C18H10Br2) as a prototypical case of rodlike polyaromatic molecules functionalized with two bromine atoms on the sides. The adsorption and temperature-stimulated transformations of dibromotetracene assemblies on Ag(110) have been investigated by a combination of synchrotron radiation X-ray photoemission spectroscopy (XPS), near-edge X-ray absorption spectroscopy (NEXAFS), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. Upon the contact with the Ag substrate, the Br–C bonds are promptly cleaved at room temperature and Ag-coordinated protopolymers are formed along the [001] substrate direction. The organometallic dimers and trimers remain on the surface up to 523 K. The stabilization of the protopolymers is driven by the substrate anisotropy and weak interactions with nearby Br atoms. The short oligomers formed at elevated temperatures are weakly bounded to the substrate and desorb before covalent structures can be formed.

1. Introduction Controlled growth of highly ordered thin organic films is a subject of ongoing research, having many potential applications.1 Understanding of the organization of organic molecules on surfaces and of their electronic interaction is essential for the development of molecular nanotechnologies, organic electronics and optoelectronics,2 such as organic light-emitting devices (OLEDs), organic photovoltaic cell (OFVC) or organic field-effect transistor (OFET). Ultra-thin films made up of aromatic molecules (e.g., acenes,3 nanographenes,4 rylenes,5 porphyrins6,7) can exhibit specific properties that are connected not only to the chemical nature of the molecular units, but also to their connectivity.8–13


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One of the most promising strategies to control the architectures of molecular nanostructures and tailor their functional properties is surface-assisted polymerization of halogenated molecular precursors.14–19 A chemically stable central molecular unit, possessing a specific function (e.g., having electronic, magnetic or catalytic properties), can be functionalized by introduction of carbon–halogen bonds that can be dissociated on a catalytic metal surface, creating reactive sites at their positions. The surface-stabilized radicals can subsequently diffuse on the surface and connect one with another through C–C Ullmann coupling reaction.20–22 The ability to design and synthesize molecules with multiple combinations of the number and position of the halogen atoms allows the construction of various topologies such as dimers, linear chains, and 2D arrays.17,19,23,24 It has been demonstrated that surface-confined polymerization of aromatic19,25 and heteroaromatic building blocks26 is a viable route to the bottom-up synthesis of graphene nanostructures with tailored band gaps. Of great interest is on-surface synthesis of armchair graphene nanoribbons (GNR) with atomic precision via sequential catalytic activation of C–X and C–H bonds. GNRs are semiconductive with band gap opening dependent on their width and edge shape. Variety of GNRs with different electronic properties can be synthesized by appropriate design of the precursor structure.19 On-surface synthesis of organic assemblies and networks can strongly depend on the chosen substrate. Coinage metals are generally used because their interaction with the molecules is sufficiently strong to cleave the bonds between carbon atoms and halogen substituents and adequately weak to allow the migration of the reaction partners on the metal surface. The crystal orientation of the substrate influences the on-surface reactions, too. Surface anisotropy in particular affects the molecular mobility, it can direct the growth and stimulate preferred orientations of the molecular nanostructures.18,27 Moreover, the substrate can directly participate


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in the molecular-assembly process by providing additional coordinating centers.28,29 In several cases, before the on-surface radicals bond covalently, they are first interconnected by substrate supplied metal adatoms and form metal-coordination complexes or so-called protopolymers.30 Despite great strides were made in the field of on-surface covalent coupling, the expansion of onsurface chemistry and the control of the nanostructures orientation on solid substrates have remained a challenge. Several fundamental points are still left to be understood: among others, rate-limiting reaction steps, reaction parameters affecting long-range order, the role of dissociated halogen atoms chemisorbed on the metal surface29,31 or the optimal conditions for the transformation of organometallic intermediates into polymeric networks.32,33 Thus, temperaturedependent studies under well defined conditions on single-crystals in ultrahigh vacuum (UHV) are important for understanding the fundamental aspects of the fabrication of functional organic polymers. Herein, 5,11-dibromotetracene (DBT, Scheme 1) on Ag(110) substrate has been studied. DBT has been chosen as a prototypical planar halogenated polyacene derivative, for which the C–C coupling step is strongly affected by steric hindrance. The molecular structure of the DBT precursor predetermines its potential to form quasi-1D conjugated polymers, such as GNRs with mixed armchair and cove-type edges.34 We will show that the particular choice of the positions of the halogen atoms within the molecular structure affects not only the interconnection between the monomer units, but also the interaction with the metal substrate, and thus the chemical and topographic structure of the resulting polymeric network.


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Scheme 1. Molecular structure of 5,11-dibromotetracene. Grey, green and red balls represent carbon, bromine and hydrogen atoms, respectively. The previous study of DBT polymerization on the Cu(110) surface revealed an intriguing polymerization scenario.35 The DBT molecules underwent easy debromination at room temperature (RT), leading to the creation of organometallic monomers aligned along the 110 direction, and anchored in the troughs between the close-packed substrate atomic rows. Organometallic oligomers interlinked by Cu adatoms along the [001] direction were formed upon annealing at the temperature of 573 K. Remarkably, further annealing at 673 K stimulated migration of the active sites to more stable positions on the tetracene scaffold and subsequent formation of intermolecular covalent bonds along the 110 direction. Here, we will show how the choice of different metal substrate can affect DBT polymerization, going from Cu to Ag surfaces. We present a combined study using synchrotron-excited X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and density functional theory (DFT) calculations that unveils relations between the observed morphology and the chemical transformations of DBT structures on Ag(110).


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2. Experimental Methods and Computational Details The synthesis of 5,11-dibromotetracene has been carried out by electrophilic bromination of tetracene with N-bromosuccinimide in a CHCl3/DMF mixture, as described by Müllen.36,37 Under such conditions, the 5,12-dibromotetracene regioisomer is formed in less than 10%. Pure 5,11-dibromotetracene can be obtained by crystallization from toluene and oxidation by-products (tetracene–quinone) can be easily removed by filtration of the crude mixture over a bed of SiO2. The Ag(110) single-crystal was cleaned by repeated cycles of 1 keV Ar+ ion sputtering and annealing in vacuum at 675 K for 20 minutes until the sharp Ag(110)-(1 × 1) LEED pattern was visible and no presence of carbon nor oxygen were observed in photoemission spectra. DBT molecules were deposited on the Ag(110) kept at RT by thermal sublimation from a resistively heated crucible placed in front of the substrate. The crucible was heated to 360 K during the DBT evaporation and the base pressure during the deposition was 1.5 × 10-9 mbar. Sub-monolayer and monolayer DBT coverage were studied. The sample with sub-monolayer coverage was annealed in a stepwise manner from RT to 673 K, by steps of 50 K. The temperature at each step was maintained for 5–10 minutes. Temperature was measured by a thermocouple attached to the sample holder near the crystal. Absolute error in the temperature determination is estimated to be less than 5 K. All the measurements have been performed at RT. The monolayer coverage is defined as the amount of DBT left on the surface after heating DBT multilayer to 423 K, which corresponds to 0.75 ± 0.15 DBT molecules/nm2. The absolute value of the surface coverage used during XPS measurements was calibrated by comparing the C 1s intensity of the equivalent amount of DBT deposited on Cu(110) substrate with C 1s and O 1s intensity of CO adsorbed on Cu(110) and O 1s intensity of Cu(110)-(2 × 1)O surface oxide.38


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High-resolution synchrotron-excited XPS and near-edge X-ray absorption spectroscopy (NEXAFS) were carried out at the BACH beamline at Elettra synchrotron, Trieste, Italy, in an UHV chamber with the base pressure of

Surface-Confined Polymerization of Halogenated Polyacenes: The

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