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.

CEMES CNRS 29 Rue J Marvig 31055 Toulouse France

Centro de Física de Materiales CSIC UPV EHU 20018 San Sebastián Spain

Donostia International Physics Center 20018 San Sebastián Spain

Ikerbasque Basque Foundation for Science 48009 Bilbao Spain

Institute of Physics of the Czech Academy of Sciences Cukrovarnicka 10 162 00 Praha Czech Republic

Nanomaterials and Nanotechnology Research Center CSIC UNIOVI PA 33940 El Entrego Spain

Univ Bordeaux CNRS Centre de Recherche Paul Pascal CRPP UMR 5031 33600 Pessac France

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