On-Surface Synthesis and Characterization of Pentadecacene and Its Gold Complexes

Acenes are an important class of polycyclic aromatic hydrocarbons that have gained considerable attention from chemists, physicists, and material scientists, due to their exceptional potential for organic electronics. They serve as an ideal platform for studying the physical and chemical properties of sp² carbon frameworks in the one-dimensional limit and also provide a fertile playground to explore magnetism in graphenic nanostructures due to their zigzag edge topology. While higher acenes up to tridecacene have been successfully generated by means of on-surface synthesis, it is imperative to extend their synthesis toward even longer homologues to comprehensively understand the evolution of their electronic ground state. Here, we demonstrate the on-surface synthesis of pentadecacene (15ac) from a trietheno-bridged precursor via the atom-manipulation-induced dissociation of protecting groups or the elimination of adatoms in a gold-pentadecacene complex. The generated 15ac was investigated by scanning tunneling microscopy (STM)/spectroscopy (STS) and noncontact atomic force microscopy (nc-AFM), in combination with first-principles spin-polarized density functional theory (DFT) calculations. We found that 15ac exhibits an open-shell singlet ground state with an experimental singlet-triplet gap of 124 meV and an STS transport gap of ∼1.12 eV. The formation of Au-pentadecacene complexes suggested a considerable contribution of polyradical character to the electronic ground state of 15ac. Our work contributes to a fundamental understanding of the electronic properties of long acenes and to the development of a versatile STM tip-assisted methodology for the synthesis of elusive compounds.