Bandgap evolution in nanographene assemblies

Recently cycloarene has been experimentally obtained in a self-assembled structure, forming graphene-like monoatomic layered systems. Here, we established bandgap engineering/prediction in cycloarene assemblies within a combination of density functional theory and tight-binding Hamiltonians. Our results show that the inter-molecule bond density rules the bandgap. The increase in such bond density increases the valence/conduction bandwidth decreasing the energy gap linearly. We derived an effective model that allows the interpretation of the arising energy gap for general particle-hole symmetric molecular arrangements based on inter-molecular bond strength.

Unraveling the molecular conformations of a single ruthenium complex adsorbed on the Ag(111) surface by calculations

The tris(dibenzoylmethanato)ruthenium (Ru(dbm)3) molecule has recently been characterized by scanning tunneling microscopy (STM) experiments upon adsorption on Ag(111). The adsorbed Ru(dbm)3 molecule shows two conformations with respect to the [11[combining macron]0] direction of the substrate, one with a three-lobed feature and the other one with a bi-lobed structure. For each of these structures, the molecule can take two geometries (states). Molecular mechanics calculations in a semi-empirical framework and STM calculated images reveal that these states on the substrate originate from the enantiomer of the Ru(dbm)3 molecule in the case of three-lobed structure and from the rotation of the two phenyls in the top dbm moities for the bi-lobed form.

In vacuo interfacial tetrapyrrole metallation

The metallation of tetrapyrroles at well-defined surfaces under ultra-high vacuum conditions represents an unconventional synthesis approach to achieve tetrapyrrole-based metal-organic complexes and architectures. Different protocols, pioneered over the last decade, and now widely applied in several fields, provide an elegant route to metallo-tetrapyrrole systems often elusive to conventional procedures and give access and exquisite insight into on-surface tetrapyrrole chemistry. As highlighted by the functionality of metallo-porphyrins in biological or other environments and by the eminent role of metallo-phthalocyanines in synthetic materials, the control on the metal centres incorporated into the macrocycle is of utmost importance to achieve tailored properties in tetrapyrrole-based nanosystems. In the on-surface scenario, precise metallation pathways were developed, including reactions of tetrapyrroles with metals supplied by physical vapour deposition, chemical vapour deposition or the tip of a scanning tunnelling microscope, and self-metallation by atoms of an underlying support. Herein, we provide a comprehensive overview of in vacuo tetrapyrrole metallation, addressing two-dimensional as well as three-dimensional systems. Furthermore, we comparatively assess the available library of on-surface metallation protocols and elaborate on the state-of-the-art methodology.

Single-molecule insight into Wurtz reactions on metal surfaces

Wurtz reactions feature the dehalogenated coupling of alkyl halides. In comparison to their widely investigated counterparts, Ullmann reactions, Wurtz reactions have however been scarcely explored on surfaces. Herein, by combining high-resolution STM imaging and DFT calculations, we have systematically investigated Wurtz reactions on three chemically different metal surfaces including Cu(110), Ag(110) and Au(111). We find that the Wurtz reactions could be achieved on all three surfaces, and the temperatures for triggering the reactions are in the order of Cu(110) > Ag(110) > Au(111). Moreover, DFT calculations have been performed to unravel the pathways of on-surface Wurtz reactions and identify three basic steps of the reactions including debromination, diffusion and coupling processes. Interestingly, we found that the mechanism of the on-surface Wurtz reaction is intrinsically different from the Ullmann reaction and it is revealed that the coupling process is the rate-limiting step of Wurtz reactions on three different substrates. These findings have given a comprehensive picture of Wurtz reactions on metal surfaces and demonstrated that such a reaction could be an alternative reaction scheme for advanced on-surface synthesis.

On-surface formation of two-dimensional polymer via direct C–H activation of metal phthalocyanine

From high-resolution UHV-STM imaging and DFT calculations, we successfully obtained a 2D polymer structure formed through direct C–H activation followed by an aryl–aryl coupling of a metal-phthalocyanine (CoPc) on Ag(110).