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.