Acenes are polycyclic aromatic hydrocarbons made up of linearly fused benzene rings. They are the smallest units of graphene and graphene nanoribbons, and have been extensively studied as organic semiconductor materials. Higher acenes are expected to have smaller band gaps required for practical use as organic electronic materials, but those with more than six benzene rings are poorly soluble in common organic solvents and unstable in aerobic condition to be synthesized and purified with traditional organic synthesis method. To overcome the obstacles, a precursor approach is a useful technique, where the soluble and stable precursors are purified enough and then converted to less-soluble and unstable target acenes in-situ.[1,2]

Recently surface-assisted synthesis of higher acenes using precursors have attracted much attentions. Under ultra-high vacuum atmosphere, unstable acenes can be prepared from precursors by annealing, photoirradiation, or scanning tunneling microscopy (STM)-tip treatment on metal surface. We were successful to synthesize bis-diketone precursors of heptacene and nonacene. [3-6] The precursors were vacuum-deposited on Au(111) surface and converted to heptacene and nonacene by photoirradiation. Through combined STM, non-contact atomic force microscopy(nc-AFM) and scanning tunneling spectroscopy (STS) measurements, together with state-of-the-art first principles calculations, insight into the chemical and electronic structure of these elusive compounds was successful. It was revealed that nonacene has open-shell biradical structure in the ground state on Au(111). We were also successful to prepare tetraazaundecacene on Au(111) surface from bicyclo[2.2.2]octadiene (BCOD) precursor. [7] The tip-induced release of the protecting group can be employed to produce the targeted tetraazaundecacene species. The experimental frontier orbital gap, derived from the difference between PIR and NIR, is 1.35 eV. In this talk, the attraction and possibility of the “precursor approach” for the synthesis and application of acene compounds will be overviewed.

References

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