Surface Synthesis of 2D Branched Polymer Nanostructures

Abstract

Molecular nanostructures formed by bottom-up self-organization [1] are model systems for advanced functional surfaces with a broad range of applications, such as sensors or coatings, molecular electronics, and heterogeneous catalysis. Supramolecular structures formed on surfaces under ultrahighvacuum (UHV) conditions through exploitation of noncovalent interactions, such as van der Waals forces, [2] dipole-dipole interactions, [3] hydrogen bonding, [4] or metal complexation, [5] have been studied extensively with scanning tunneling microscopy (STM). Structures stabilized by stronger covalent bonds between the molecular building blocks are anticipated to have an improved thermal and chemical stability, and are thus likely to be more useful for practical applications. However, investigations into covalently interlinked molecular structures on surfaces under UHV conditions are only just emerging.[6]Thin films produced by vapor-deposition polymerization [7] have been studied by STM, as has photoinduced or STM-tipinduced polymerization of diacetylene.[8] Macromolecules have been deposited at surfaces using the pulse injection technique [9,10] and characterized at modest resolution, and polymer architecture and folding have been studied upon electropolymerization [11] or drop-casting.[12] Although polymers deposited or formed in UHV [6,8,13] and at the liquid/solid interface [11,12] have been observed, no detailed high-resolution STM studies of connectivity and branching exist.Herein, we demonstrate the formation of two-component polymeric nanostructures on a Au(111) surface under UHV conditions. The branched surface polymer, which contains pores about 3-10 nm in dimension, is characterized by highresolution STM and it is shown that its connectivity can be controlled by varying the kinetic parameters of the preparation procedure.Figure 1 a shows the investigated condensation polymerization reaction between an aromatic trisalicylaldehyde [14,15] (trialdehyde) and 1,6-diaminohexane (diamine), which results in a polymer connected by imine bonds. In solution the trialdehyde is known to form a cross-linked polymer by reaction with ethylenediamine.[15] Covalent interlinking of similar two-spoke salicylaldehydes and octylamine on Au-(111) under UHV conditions was recently demonstrated by STM and synchrotron-based X-ray spectroscopy. [16] STM images of the reactants adsorbed individually on the Au(111)-(22 ffiffi ffi 3 p ) surface are shown in Figure 1 b and c. Upon co-deposition followed by annealing above 400 K, open filamentous structures are formed (Figure 2 a). The local bonding pattern is revealed from high-resolution STM images 2 ) of b) a close-packed island of trialdehydes [14] (I t = 0.60 nA, V t = 1.05 V) and c) the lamellar structure of 1,6-diaminohexanes (I t = 0.34 nA, V t = À1.9 V). Molecular models are superimposed. I t = tunneling current, V t = tunneling voltage.

Formation of extended covalently bonded Ni porphyrin networks on the Au(111) surface

“…A strategy to overcome this low stabilization energy relies on inducing covalent reactions between the molecular components, thus forming twodimensional covalently bonded networks. The formation of covalent bonds between complementary molecular components is an appealing approach in the fabrication of nanoscale structures, such as nanomeshes and nanolines, because of their high selectivity, strength, and directionality [3,[14][15][16][17][18][19][20][21][22][23][24][25][26]. These nanostructures are attractive for their intrinsic properties; for their potential applications such as in novel sensing, energy conversion or catalytic devices; for their ability to "trap" other molecules such as fullerenes, creating even more interesting complexes and for their use as templates to direct the growth of, for example, metal clusters with interesting catalytic or magnetic properties [26,27].…”

Synthesis of One‐Dimensional Schiff Base Polymers that Contain an Oligothiophene Building Block on the Graphite Surface

“…[39][40][41][42] The Schiff base reaction has been widely applied in dynamic covalent chemistry, [43,44] and it has also been investigated both under UHV and ambient conditions by means of scanning tunneling microscopy (STM). [23][24][25][32][33][34][35] The reactions, particularly those that result in small-molecule imine compounds, can proceed under room temperature, [32,43] whereas for the preparation of high-quality surface 2D polymers, mild heating is normally necessary. An in-depth understanding of the mechanism of the surface-confined reaction-for instance, the effect of monomer diffusion, polarity of the solvent, and so forth-is crucial for improving the quality and properties of the obtained nanostructures.…”

“…Studies revealed that the number and position of reactive groups, and the symmetry and rigidity of backbones of the precursors play important roles in the resultant structures and morphologies. For instance, selecting proper reactive components can fabricate 1D or branched polymers, [25,[30][31][32][33][34] 2D chiral architectures, [35] and networks with different periods and pore sizes. [21,24] Highly or-dered large-scale 2D surface covalent organic frameworks (COFs) on highly oriented pyrolytic graphite (HOPG) have also been prepared by means of a self-limiting solid-vapor interface reaction method, mild heating under open or closed environments, [36,37] or even by reaction at the solid-liquid interface at room temperature.…”

“…[7] Therefore, recently much attention has been focused on designing and synthesizing different dimensional structures by covalent linkages between the molecular building blocks owing to their enhanced thermal and chemical stability. [8][9][10][11][12] A variety of radical coupling and homo-or hetero-condensation reactions that occur directly on surfaces-for instance, Ullmann radical coupling, [13][14][15][16] polyimide formation, [17,18] boronic dehydration reaction, [19,20] imine coupling, [21][22][23][24][25] and so forth-have been studied and resulted in intriguing perspectives on many fundamental and applied research areas. For a fundamental understanding of the details of the polymerization process, direct observation of covalent polymerization on the single molecule scale is of significant importance to the design and on-surface synthesis of polymers.…”

Thermogelling of highly branched poly(N‐isopropylacrylamide)

“…[1][2][3] Recently, a huge number of works have been done for their potential application on biomedicine and catalysis. [4][5][6] The syntheses of the branched or hyperbranched polymers are still an attracting topic today. The radical solution copolymerization of a vinyl monomer with a divinyl monomer is one of the simple routes.…”