On‐Surface Metalation and 2D Self‐Assembly of Pyrphyrin Molecules Into Metal‐Coordinated Networks on Cu(111)

The metalation of the tetradentate molecule pyrphyrin by copper substrate atoms on a Cu(111) surface is studied. Pyrphyrin, in contrast to porphyrin, consists of four fused pyridine groups instead of pyrrol groups. Using thermal desorption spectroscopy (TDS ) and N 1s X‐ray photoelectron spectroscopy (XPS ), we show that metalation of the monolayer of pyrphyrin with Cu atoms from the Cu(111) surface occurs at 377 K. The formation of an extended two‐dimensional (2D) network is observed with scanning tunneling microscopy (STM ). A honeycomb‐like lattice of metalated pyrphyrin molecules is formed by intermolecular connection via the two cyano groups at the periphery of pyrphyrin as well as Cu adatoms. Dehydrogenation at the periphery of the molecule is observed during annealing at 520 K. The surface‐adsorbed metal‐pyrphyrin has the potential to serve as a molecular catalyst.     

Internal Architecture and Adsorption Sites of Violet Lander Molecules Assembled on Native and KBr‐Passivated InSb(001) Surfaces

The adsorption of individual Violet Lander molecules self‐assembled on the c(8×2) reconstructed InSb(001) surface in its native form and on the surface passivated with one to three monolayers of KBr is investigated by means of low‐temperature scanning tunneling microscopy (STM). Preferred adsorption sites of the molecules are found on flat terraces as well as at atomic step edges. For molecules immobilized on flat terraces, several different conformations are identified from STM images acquired with submolecular resolution and are explained by the rotation of the 3,5‐di‐tert‐butylphenyl groups around σ bonds, which allows adjustment of the molecular geometry to the anisotropic substrate structure. Formation of ordered molecular chains is found at steps running along substrate reconstruction rows, whereas at the steps oriented perpendicularly no intermolecular ordering is recorded. It is also shown that the molecules deposited at two or more monolayers of the epitaxial KBr spacer do not have any stable adsorption sites recorded with STM. Prospects for the manipulation of single molecules by using the STM tip on highly anisotropic substrates are also explored, and demonstrate the feasibility of controlled lateral displacement in all directions.     

Adsorption characteristic of self‐assembled corrole dimers on HOPG

Our study first focus on two types of corrole dimers oxidized and reduced forms on highly oriented pyrolytic graphite (HOPG) surface. Scanning tunneling microscopy (STM), X‐ray photoelectron spectroscopy (XPS) and contact angle measurement (CAM) were used to investigate the self‐assembled monolayers of corrole dimers adsorbed on HOPG surfaces at room temperature in air. XPS and CAM results have confirmed both two molecules adsorbed on an HOPG surface and formed self‐assembled films, and STM experiments found that the corrole dimers adsorbed on HOPG surfaces form similar lobes. The different stable space structure of the oxidized form molecule (OFM) and reduced form molecule (RFM), led to the diversity of the tetramer structural dimensions. The occurrence of molecular aggregations and assembly was controlled by the interactions between molecular–molecular and molecule–substrate. The electrostatic interactions between the molecules control the geometrical sizes and molecule–substrate interactions determine topographical shapes of the self‐assembled corrole dimers on HOPG surface. Copyright © 2009 John Wiley & Sons, Ltd.     

Comparing Ullmann Coupling on Noble Metal Surfaces: On‐Surface Polymerization of 1,3,6,8‐Tetrabromopyrene on Cu(111) and Au(111)

The on‐surface polymerization of 1,3,6,8‐tetrabromopyrene (Br₄Py) on Cu(111) and Au(111) surfaces under ultrahigh vacuum conditions was investigated by a combination of scanning tunneling microscopy (STM), X‐ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. Deposition of Br₄Py on Cu(111) held at 300 K resulted in a spontaneous debromination reaction, generating the formation of a branched coordination polymer network stabilized by C?Cu?C bonds. After annealing at 473 K, the C?Cu?C bonds were converted to covalent C?C bonds, leading to the formation of a covalently linked molecular network of short oligomers. In contrast, highly ordered self‐assembled two‐dimensional (2D) patterns stabilized by both Br?Br halogen and Br?H hydrogen bonds were observed upon deposition of Br₄Py on Au(111) held at 300 K. Subsequent annealing of the sample at 473 K led to a dissociation of the C?Br bonds and the formation of disordered metal‐coordinated molecular networks. Further annealing at 573 K resulted in the formation of covalently linked disordered networks. Importantly, we found that the chosen substrate not only plays an important role as catalyst for the Ullmann reaction, but also influences the formation of different types of intermolecular bonds and thus, determines the final polymer network morphology. DFT calculations further support our experimental findings obtained by STM and XPS and add complementary information on the reaction pathway of Br₄Py on the different substrates.     

Surface‐Confined Electropolymerization of Methylene Blue on Gold Electrodes

We report on the electrochemical behaviour and electropolymerization of self‐assembled monolayers (SAMs) of methylene blue (MB) on gold electrodes. The SAMs of MB on gold electrodes were prepared by immersing the substrates into a solution of 1.0 mM MB in absolute ethanol for different times at room temperature. Cyclic voltammetry experiments exhibited that reductive desorption of MB monolayer takes place at three different potentials on polycrystalline gold electrodes, while reductive desorption of MB monolayer consists of only one peak on single crystal Au(111) substrates. Calculated charge densities for different immersion times indicated that optimal immersion time for self‐assembly of MB is 96 h. Electropolymerization of SAMs of MB on gold electrode was achieved by applying 0.95 V for 1 s in 0.1 M borate buffer solution (pH: 9.0). It was observed that poly(MB) monolayers are highly stable in acidic media. ATR‐FTIR and UV‐vis spectra exhibited differences between monomer and polymer monolayers, which are attributed to surface‐confined electropolymerization. STM image of poly(MB) monolayer on Au(111) substrate revealed a surface that is covered by well‐ordered, collateral nanowires with an average size of 3 nm.     

Photochemical Reactions in Self‐Assembled Organic Monolayers Characterized by using Scanning Tunneling Microscopy

Research on the supramolecular self‐assembly behavior at interfaces is of great importance to improving the performance of nanodevices that are based on optical functional materials. In this Minireview, several photoinduced isomerization and polymerization reactions in self‐assembled organic monolayers on surfaces are discussed. Typical organic molecules contain azobenzene, alkynyl, or olefins groups. The feature surface base is a highly oriented pyrolytic graphite (HOPG) surface or a gold surface. Scanning tunneling microscopy (STM) is used as a strong tool to characterize new species’ structures before and after illumination.     

2H‐Tetrakis(3,5‐di‐tert‐butyl)phenylporphyrin on a Cu(110) Surface: Room‐Temperature Self‐Metalation and Surface‐Reconstruction‐Facilitated Self‐Assembly

The adsorption behavior of 2H‐tetrakis(3,5‐di‐tert‐butyl)phenylporphyrin (2HTTBPP) on Cu(110) and Cu(110)–(2×1)O surfaces have been investigated by using variable‐temperature scanning tunneling microscopy (STM) under ultrahigh vacuum conditions. On the bare Cu(110) surface, individual 2HTTBPP molecules are observed. These molecules are immobilized on the surface with a particular orientation with respect to the crystallographic directions of the Cu(110) surface and do not form supramolecular aggregates up to full monolayer coverage. In contrast, a chiral supramolecular structure is formed on the Cu(110)–(2×1)O surface, which is stabilized by van der Waals interactions between the tert‐butyl groups of neighboring molecules. These findings are explained by weakened molecule–substrate interactions on the Cu(110)–(2×1)O surface relative to the bare Cu(110) surface. By comparison with the corresponding results of Cu–tetrakis(3,5‐di‐tert‐butyl)phenylporphyrin (CuTTBPP) on Cu(110) and Cu(110)–(2×1)O surfaces, we find that the 2HTTBPP molecules can self‐metalate on both surfaces with copper atoms from the substrate at room temperature (RT). The possible origins of the self‐metalation reaction at RT are discussed. Finally, peculiar irreversible temperature‐dependent switching of the intramolecular conformations of the investigated molecules on the Cu(110) surface was observed and interpreted.     

Surface microscopic structure and electrochemical rectification of a branched alkanethiol self-assembled monolayer

The tert-butanethiol self-assembled monolayers (SAMs) on Au(111) surfaces were prepared from various solvents and investigated by a combination of scanning tunneling microscopy (STM) and electrochemistry in aqueous environments. High-resolution STM images reveal a (radical(7) x radical(7))R19 degrees surface lattice structure, in contrast with the conventional lattice (radical(3) x radical(3))R30 degrees structure for straight-chain alkanethiol SAMs. Interestingly, such a branched monolayer shows electrochemical rectification toward redox probes. We suggest that electrochemical rectification could be a general characteristic of short-chain branched alkanthiol SAMs, and originate in localized electronic effects.     

Physically controlled,free‐radical polymerization of vaporized fluoromonomer on solid surfaces

Gas/vapor‐deposition polymerization (GDP) of vinyl monomer is expected to exhibit a unique polymerization behavior different from its polymerization in the liquid phase. Free‐radical GDP of 2,2,3,3,3‐pentafluoropropyl methacrylate (FMA) was carried out with a conventional free‐radical initiator (azobisisobutyronitrile) on substrate surfaces. A linear relationship between the number‐average molecular weight and polymer yield was observed, and the consecutive copolymerization of methyl methacrylate (MMA) and FMA led to the formation of block copolymer P(MMA‐block‐FMA). These results suggested that the GDP process on substrate surfaces has a living nature. During the process, the active species at growing chain ends may be immobilized on the deposit surface and restricted from the chain‐transfer reactions, resulting in a continuation of the propagation reaction. The GDP on substrate surfaces is therefore a physically controlled polymerization process. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2621–2630, 2004     

Solid‐state polymerization of 2,3,6,7,10,11‐hexa(methacrylate) triphenylene

A new monomer, 2,3,6,7,10,11‐hexa(methacrylate) triphenylene (HMTP), and its crystals have been successfully synthesized, and the solid‐state polymerization under UV irradiation has been investigated. The photo polymerization of HMTP in solid was confirmed by the reduction of vinyl bonds in the FT‐IR and UV spectra of PHMTP in comparison with the corresponding spectra of its precursor. Thus, IR spectroscope was used to follow the polymerization of HMTP crystals under UV irradiation, and kinetic studies show a first‐order reaction with rate constant of 6.12 × 10^?3 min^?1. This value is slightly larger than that measured by the weight method. The polarizing optical microscope and X‐ray diffraction were used to study the crystal structure difference between the polymers and its monomer. The results show that the polymers' crystals obtained from photo polymerization kept the monomer crystal lattice. Because of strong overlap between the π‐electron of the triphenylene, the monomer and polymer crystals showed different fluorescence properties. All these results proved that the photo polymerization of HMTP crystals is governed by the packing structure of monomer molecules; in other words, this reaction is just lattice controlled polymerization. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1526–1534, 2005     

On‐Surface Self‐Assembly of a C3‐Symmetric π‐Conjugated Molecule Family Studied by STM: Two‐Dimensional Nanoporous Frameworks

The on‐surface self‐assembled behavior of four C ₃‐symmetric π‐conjugated planar molecules ( Tp , T12 , T18 , and Ex ) has been investigated. These molecules are excellent building blocks for the construction of noncovalent organic frameworks in the bulk phase. Their hydrogen‐bonded 2D on‐surface self‐assemblies are observed under STM at the solid/liquid interface; these structures are very different to those in the bulk crystal. Upon combining the results of STM measurements and DFT calculations, the formation mechanism of different assemblies is revealed; in particular, the critical role of hydrogen bonding in the assemblies. This research provides us with not only a deep insight into the self‐assembled behavior of these novel functional molecules, but also a convenient approach toward the construction of 2D multiporous networks.     

Strain‐Promoted Double Azide Addition to Octadehydrodibenzo[12]annulene Derivatives

Octadehydrodibenzo[12]annulenes (DBAs), readily available by the oxidative acetylenic coupling of 1,2‐diethynylbenzene derivatives, were reacted with organic azides. As compared to the well‐known strain‐promoted azide‐alkyne cycloaddition (SpAAC) of 5,6,11,12‐tetradehydrodibenzo[a,e][8]annulene, the reactivity of the DBA alkynes was lower due to the lower strain energy. However, the regioselective double azide addition occurred without any side reactions under mild conditions, yielding bis‐triazole products. The structures of the products were confirmed by an X‐ray crystal structure analysis, and the reaction mechanism was studied by ¹H‐NMR spectroscopy and computational studies. It was also found that the DBAs were hardly fluorescent, while the bis‐triazole products showed a green fluorescence with quantum yields up to 5.1 %. Finally, the new strain‐promoted double azide addition to the DBAs was used for step‐growth polymerization, successfully producing a high molecular weight triazole polymer.     

Hydrogen‐Bonding‐Induced Polymorphous Phase Transitions in 2D Organic Nanostructures

The 2D self‐assembly of various 2‐hydroxy‐7‐alkoxy‐9‐fluorenone (HAF) molecules has been investigated by scanning tunneling microscopy (STM) at the liquid/solid interface. A systematic study revealed that HAF molecules with different numbers of carbon atoms in their alkoxy chains could form two or three different kinds of nanostructures, that is, less‐ordered, flower‐like, and zig‐zag patterns, owing to the formation of different types of intermolecular hydrogen bonds. The observed structural transition was found to be driven by molecular thermodynamics, surface diffusion, and the voltage pulse that was applied to the STM tip. The zig‐zag pattern was the most stable of these configurations. An odd–even effect on the flower‐like structure, as induced by the odd and even number of carbon atoms in the side chain, was observed by STM. The influence of the odd–even effect on the melting point has a close relationship with the molecular self‐assembled pattern. Our results are significant for understanding the influence of hydrogen‐bonding interactions on the dominant adsorption behavior on the surface and provide a new visual approach for observing the influence of the odd–even effect on the phase transition.     

Substrate‐Independent Magnetic Bistability in Monolayers of the Single‐Molecule Magnet Dy2ScN@C80 on Metals and Insulators

Magnetic hysteresis is demonstrated for monolayers of the single‐molecule magnet (SMM) Dy₂ScN@C₈₀ deposited on Au(111), Ag(100), and MgO|Ag(100) surfaces by vacuum sublimation. The topography and electronic structure of Dy₂ScN@C₈₀ adsorbed on Au(111) were studied by STM. X‐ray magnetic CD studies show that the Dy₂ScN@C₈₀ monolayers exhibit similarly broad magnetic hysteresis independent on the substrate used, but the orientation of the Dy₂ScN cluster depends strongly on the surface. DFT calculations show that the extent of the electronic interaction of the fullerene molecules with the surface is increasing dramatically from MgO to Au(111) and Ag(100). However, the charge redistribution at the fullerene‐surface interface is fully absorbed by the carbon cage, leaving the state of the endohedral cluster intact. This Faraday cage effect of the fullerene preserves the magnetic bistability of fullerene‐SMMs on conducting substrates and facilitates their application in molecular spintronics.     

The study of paramagnetic self‐assembled monolayers formed by nitroxide radical (TEMPO)3O3P on HOPG

The studies of (TEMPO)₃O₃P molecules (tri‐(2,2,6,6‐tetramethyl‐1‐oxyl‐4‐piperidyl) phosphite) and their monolayers formed on highly oriented pyrolytic graphite (HOPG) are presented. (TEMPO)₃O₃P is synthesized from TEMPOL precursor (4‐hydroxy‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl). We focused on electron paramagnetic resonance (EPR) and scanning tunnelling microscopy (STM) investigations. The EPR experiments demonstrate the paramagnetism of (TEMPO)₃O₃P in powdered form and in solution. Furthermore, these paramagnetic properties of (TEMPO)₃O₃P are preserved after the deposition on HOPG substrate. The STM studies show that (TEMPO)₃O₃P molecules have a strong tendency to form self‐assembled monolayers (SAMs). The detailed STM images let us to explore the structure of (TEMPO)₃O₃P on HOPG and propose a structural model for the observed SAMs. Copyright © 2013 John Wiley & Sons, Ltd.     

XPS and AFM characterization of the self‐assembled molecular monolayers of a 3‐aminopropyltrimethoxysilane on silicon surface,and effects of substrate pretreatment by UV‐irradiation

The self‐assembled (SA) molecular monolayers of a 3‐aminopropyltrimethoxysilane (3‐APTS) on a silicon (111) surface, and the effects of ultraviolet (UV) pre‐treatment for substrates on the assembling process have been studied using XPS and atomic force microscopy (AFM). The results show that the SA 3‐APTS molecules are bonded to the substrate surface in a nearly vertical orientation, with a thickness of the monolayer of about 0.8–1.5 nm. The SA molecular monolayers show a substantial degree of disorder in molecular packing, which are believed to result from the interactions of amine tails in the silane molecules used with surface functionalities of the substrates, and the oxygen‐containing species from the ambient. UV irradiation for silicon substrates prior to the self‐assembly reaction can enhance the assembly process and hence, significantly increase the coverage of the monolayer assembled for the substrates. Copyright © 2010 John Wiley & Sons, Ltd.     

A Diacetylene‐Containing Wedge‐Shaped Compound: Synthesis,Morphology, and Photopolymerization

A novel wedge‐shaped compound containing two diacetylene tails, namely, methyl 3,5‐bis(trideca‐2,4‐diyn‐1yloxyl)benzoate (DDABM), was synthesized. As shown by UV/Vis spectroscopy this compound can be polymerized under UV irradiation. The crystalline structure of DDABM was investigated by grazing‐incidence wide‐angle X‐ray diffraction on oriented crystalline films deposited on PTFE‐rubbed silicon wafer substrates. Furthermore, the spherulites formed in thicker films were analyzed by wide‐angle X‐ray diffraction. A molecular packing model of DDABM based on the X‐ray diffraction data is proposed. The diacetylene units are oriented along a defined lattice direction with a reticular distance of 4.85 Å, which fulfills the requirements for topochemical polymerization. It was observed that UV polymerization does not affect the phase behavior of the compound, but mainly alters its optical properties.     

Square Tiling by Square Macrocycles at the Liquid/Solid Interface: Co‐crystallisation with One‐ or Two‐Dimensional Order

We have systematically investigated the self‐assembled monolayers of seven bimolecular mixtures of square‐shaped pyridinophanes and cyclophanes bearing alkoxy or alkoxycarbonyl substituents in the presence of the tropylium ion as a marker of pyridinophanes at liquid/graphite interfaces by means of scanning tunnelling microscopy (STM). The purpose of this work was to elucidate the mixing behaviour of these macrocycles highlighting the formation of one‐ or two‐dimensionally ordered square tilings consisting of alternating alignments of different macrocycles as a result of attractive dipole–dipole or hydrogen‐bonding interactions; four co‐crystals differing in the dimensionality of the ordering of pyridinophane and cyclophane were observed. The different modes of interaction between the functional groups (ether or carbonyl group) in the side‐chains of the pyridinophanes and cyclophanes lead to the formation of co‐crystals with dimensionally different orderings of the two macrocycles. These observations revealed that a slight modification of the molecular structure may dramatically change the mixing behaviour and structures of the co‐crystals.     

On‐Surface Synthesis of NBN‐Doped Zigzag‐Edged Graphene Nanoribbons

We report the first bottom‐up synthesis of NBN‐doped zigzag‐edged GNRs (NBN‐ZGNR1 and NBN‐ZGNR2) through surface‐assisted polymerization and cyclodehydrogenation based on two U‐shaped molecular precursors with an NBN unit preinstalled at the zigzag edge. The resultant zigzag‐edge topologies of GNRs are elucidated by high‐resolution scanning tunneling microscopy (STM) in combination with noncontact atomic force microscopy (nc‐AFM). Scanning tunneling spectroscopy (STS) measurements and density functional theory (DFT) calculations reveal that the electronic structures of NBN‐ZGNR1 and NBN‐ZGNR2 are significantly different from those of their corresponding pristine fully‐carbon‐based ZGNRs. Additionally, DFT calculations predict that the electronic structures of NBN‐ZGNRs can be further tailored to be gapless and metallic through one‐electron oxidation of each NBN unit into the corresponding radical cations. This work reported herein provides a feasible strategy for the synthesis of GNRs with stable zigzag edges yet tunable electronic properties.