[19]Starphene: Combined In-Solution and On-Surface Synthesis Towards the Largest Starphene.

Starphenes are structurally appealing three-fold symmetric polycyclic aromatic compounds with potential interesting applications in molecular electronics and nanotechnology. This family of star-shaped polyarenes can be regarded as three acenes that are connected through a single benzene ring. In fact, just like acenes, unsubstituted large starphenes are poorly soluble and highly reactive molecules under ambient conditions making their synthesis difficult to achieve. Herein, we report two different synthetic strategies to obtain a starphene formed by 19 cata-fused benzene rings distributed within three hexacene branches. This molecule, which is the largest starphene that has been obtained to date, was prepared by combining solution-phase and on-surface synthesis. [19]Starphene was characterized by high-resolution scanning tunneling microscopy (STM) and spectroscopy (STS) showing a remarkable small HOMO-LUMO transport gap (0.9 eV).

Tuning the Diradical Character of Pentacene Derivatives via Non-Benzenoid Coupling Motifs.

The development of functional organic molecules requires structures of increasing size and complexity, which are typically obtained by the covalent coupling of smaller building blocks. Herein, with the aid of high-resolution scanning tunneling microscopy/spectroscopy and density functional theory, the coupling of a sterically demanded pentacene derivative on Au(111) into fused dimers connected by non-benzenoid rings was studied. The diradical character of the products was tuned according to the coupling section. In particular, the antiaromaticity of cyclobutadiene as the coupling motif and its position within the structure play a decisive role in shifting the natural orbital occupancies toward a stronger diradical electronic character. Understanding these structure-property relations is desirable not only for fundamental reasons but also for designing new complex and functional molecular structures.

Room-Temperature C-C σ-Bond Activation of Biphenylene Derivatives on Cu(111).

Activating the strong C-C σ-bond is a central problem in organic synthesis. Directly generating activated C centers by metalation of structures containing strained four-membered rings is one maneuver often employed in multistep syntheses. This usually requires high temperatures and/or precious transition metals. In this paper, we report an unprecedented C-C σ-bond activation at room temperature on Cu(111). By using bond-resolving scanning probe microscopy, we show the breaking of one of the C-C σ-bonds of a biphenylene derivative, followed by insertion of Cu from the substrate. Chemical characterization of the generated species was complemented by X-ray photoemission spectroscopy, and their reactivity was explained by density functional theory calculations. To gain further insight into this unique reactivity on other coinage metals, the reaction pathway on Ag(111) was also investigated and the results were compared with those on Cu(111). This study offers new synthetic routes that may be employed in the in situ generation of activated species for the on-surface synthesis of novel C-based nanostructures.

Circumventing the stability problems of graphene nanoribbon zigzag edges.

Carbon nanostructures with zigzag edges exhibit unique properties-such as localized electronic states and spins-with exciting potential applications. Such nanostructures however are generally synthesized under vacuum because their zigzag edges are unstable under ambient conditions: a barrier that must be surmounted to achieve their scalable integration into devices for practical purposes. Here we show two chemical protection/deprotection strategies, demonstrated on labile, air-sensitive chiral graphene nanoribbons. Upon hydrogenation, the chiral graphene nanoribbons survive exposure to air, after which they are easily converted back to their original structure by annealing. We also approach the problem from another angle by synthesizing a form of the chiral graphene nanoribbons that is functionalized with ketone side groups. This oxidized form is chemically stable and can be converted to the pristine hydrocarbon form by hydrogenation and annealing. In both cases, the deprotected chiral graphene nanoribbons regain electronic properties similar to those of the pristine nanoribbons. We believe both approaches may be extended to other graphene nanoribbons and carbon-based nanostructures.