Hong-Ying Gao

Decarboxylative Polymerization of 2,6-Naphthalenedicarboxylic Acid at Surfaces

Metal-catalyzed polymerization of 2,6-naphthalenedicarboxylic acid (NDCA) to form poly-2,6-naphthalenes at various surfaces is reported. Polymerizations occur via initial formal dehydrogenation of self-assembled diacids with subsequent decarboxylation to give polymeric bisnaphthyl-Cu species at elevated temperature as intermediate structures (<160 °C). Further temperature increase eventually leads to poly-naphthalenes via reductive elimination. It is demonstrated that the Cu(111) surface works most efficiently to conduct such polymerizations as compared to the Au(111), Ag(111), Cu(100), and Cu(110) surfaces. Poly-2,6-naphthalene with a chain length of over 50 nm is obtained by using this approach. The decarboxylative coupling of aromatic diacids is a very promising tool which further enlarges the portfolio of reactions allowing for on-surface polymerizations and novel organometallic systems preparations.

Ballbot-type motion of N-heterocyclic carbenes on gold surfaces

Recently, N-heterocyclic carbenes (NHCs) were introduced as alternative anchors for surface modifications and so offered many attractive features, which might render them superior to thiol-based systems. However, little effort has been made to investigate the self-organization process of NHCs on surfaces, an important aspect for the formation of self-assembled monolayers (SAMs), which requires molecular mobility. Based on investigations with scanning tunnelling microscopy and first-principles calculations, we provide an understanding of the microscopic mechanism behind the high mobility observed for NHCs. These NHCs extract a gold atom from the surface, which leads to the formation of an NHC-gold adatom complex that displays a high surface mobility by a ballbot-type motion. Together with their high desorption barrier this enables the formation of ordered and strongly bound SAMs. In addition, this mechanism allows a complementary surface-assisted synthesis of dimeric and hitherto unknown trimeric NHC gold complexes on the surface.

Tip-plasmon mediated molecular electroluminescence on the highly oriented pyrolytic graphite substrate

Well-defined molecular fluorescence is realized by tunneling electron excitations from porphyrins on highly oriented pyrolytic graphite that is non-plasmonic in the visible spectral range. The occurrence of molecular electroluminescence is found to rely critically on the plasmonic emitting state of scanning tunneling microscope tip that is pre-examined on silver. These observations, together with the selective enhancement of molecular emission bands by energy-matching tip plasmons, suggest that the plasmonic field is indispensable for the generation of molecular electroluminescence, and the tip plasmon alone is sufficient in achieving this. Excitation of molecules directly by electrons is inefficient to produce light.

Fluorescence decay of quasimonolayered porphyrins near a metal surface separated by short-chain alkanethiols

We investigated the spectral feature and fluorescence lifetime of quasimonolayered porphyrins at very short distance to metal substrates (1–2.5nm) through fine-tuning the length of alkanethiols. The ordered self-assembled monolayers of alkanethiols on Au(111) act as a uniform electronic decoupling layer and suppress the interface quenching via charge transfer. However, the fluorescence quenching via nonradiative energy transfer to the metal still prevails in the porphyrin-alkanethiol-metal sandwich structures. The decay rates are found to follow a 1∕d3 dependency on spacer thickness, which suggests that the classical electromagnetic theory appears still valid at distance down to 1nm through volume damping.

Enhancement and suppression effect of molecules on nanocavity plasmon emissions excited by tunneling electrons

We use tunneling electron induced luminescence techniques to investigate the role of adsorbed molecules in nanocavity plasmon (NCP) mediated emissions. Porphyrin molecules directly adsorbed on metals are found to suppress NCP emissions, while molecules on top of an inserted ultrathin oxide layer on the metal substrate yield enhanced NCP emissions. We attribute such difference in enhancement versus suppression to a competing mechanism of two major roles of molecules on the local field enhancement: geometrical spacer and dynamic dipole oscillator. The latter could become dominant when molecules are sufficiently decoupled from the substrate, leading to the overall enhancement of NCP emissions.

In-plane Van der Waals interactions of molecular self-assembly monolayer

We demonstrate that the Van der Waals interactions in plane are important to control molecular self-assembly structure as well their phase transition. Using precise chemical modification to mediate such in-plane cohesive interactions, we observed the spontaneous formations of 2D order or disorder molecular self-assembly structures, as well their order-disorder phase transitions by annealing. Interestingly, we identified that the side alkyl chains stand up at surfaces and form the ‘locked’ pairs/windmill structures. Moreover, we realized the covalent coupling based on ethynyl functionality before molecular desorption from metal surfaces, by enhancing the in-plane interactions.

Formation of Organometallic Intermediate States in On-Surface Ullmann Couplings

Possible origins of the formation of organometallic intermediates in on-surface Ullmann couplings have been investigated by surface tunneling microscopy (STM) and density functional theory (DFT) calculations. We consider the case of iodobenzenes on the coinage metals copper, silver, and gold. We found experimental evidence for the formation of surface vacancies and the presence of metal adatoms in these coupling reactions, which are taken as a hint for the reactive extraction of surface atoms by adsorbates. In a second step, we demonstrate by ab initio molecular dynamics calculations for aryl-iodides on copper that metal atoms can be pulled out of the surface to form metal-organic species. By contrast, a thermally activated provision of a metal atom from the surface to form an adatom is energetically unfavorable. Finally, we investigate the mechanism and energetics of the reactive extraction of surface metal atoms by means of (climbing-image) nudged elastic band density-functional theory calculations for iodobenzene on copper, silver, and gold, and analyze our results in the light of the experimental findings.

Intermolecular On-Surface σ-Bond Metathesis

Silylation and desilylation are important functional group manipulations in solution-phase organic chemistry that are heavily used to protect/deprotect different functionalities. Herein, we disclose the first examples of the σ-bond metathesis of silylated alkynes with aromatic carboxylic acids on the Ag(111) and Au(111) surfaces to give the corresponding terminal alkynes and silyl esters, which is supported by density functional theory calculations and further confirmed by X-ray photoelectron spectroscopy analysis. Such a protecting group strategy applied to on-surface chemistry allows self-assembly structures to be generated from molecules that are inherently unstable in solution and in the solid state. This is shown by the successful formation of self-assembled hexaethynylbenzene at Ag(111). Furthermore, it is also shown that on the Au(111) surface this σ-bond metathesis can be combined with Glaser coupling to fabricate covalent polymers via a cascade process.

Substrate-Mediated C–C and C–H Coupling after Dehalogenation

Intermolecular C-C coupling after cleavage of C-X (mostly, X = Br or I) bonds has been extensively studied for facilitating the synthesis of polymeric nanostructures. However, the accidental appearance of C-H coupling at the terminal carbon atoms would limit the successive extension of covalent polymers. To our knowledge, the selective C-H coupling after dehalogenation has not so far been reported, which may illuminate another interesting field of chemical synthesis on surfaces besides in situ fabrication of polymers, i.e., synthesis of novel organic molecules. By combining STM imaging, XPS analysis, and DFT calculations, we have achieved predominant C-C coupling on Au(111) and more interestingly selective C-H coupling on Ag(111), which in turn leads to selective synthesis of polymeric chains or new organic molecules.

α-Diazo Ketones in On-Surface Chemistry

Polymerization of a biphenyl bis α-diazo ketone on Cu(111) and Au(111) surfaces to provide furandiyl bridged poly-para-phenylenes is reported. Polymerization on Cu(111) occurs via initial N2 fragmentation leading to Cu-biscarbene complexes at room temperature as polymeric organometallic structure. At 135 °C, carbene coupling affords polymeric α,β-unsaturated 1,4-diketones, while analogous alkene formation on the Au(111) surface occurs at room temperature. Further temperature increase leads to deoxygenative cyclization of the 1,4-diketone moieties to provide alternating furandiyl biphenyl copolymers on Cu(111) (165 °C) and Au(111) (240 °C) surfaces. This work shows a new approach to generate Cu-biscarbene intermediates on surfaces, opening the pathway for the controlled generation of biphenyl copolymers.