Surface‐Assisted Cyclodehydrogenation and Annulative Dimerization on TiO 2 (110): Formation of an Azacoronene and Its Octagon‐Fused Dimer

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ABSTRACT Surface‐assisted reactions on nonmetallic substrates remain much less developed than their counterparts on metals, especially for heteroatom‐doped precursors and intermolecular coupling processes. Here we show that TiO 2 (110) can support not only cyclodehydrogenation of a nitrogen‐rich nanographene precursor but also annulative intermolecular coupling on a semiconducting oxide surface. Under ultrahigh vacuum, hexapyrrolylbenzene (HPB) undergoes cyclodehydrogenation on rutile TiO 2 (110) to give unsubstituted hexapyrrolohexaazacoronene (HPHAC, 1 ). Upon further annealing, 1 undergoes dehydrogenative dimerization to form a cyclooctatetraene‐annulated HPHAC dimer ( 2 ), as supported by scanning tunneling microscopy and spectroscopy (STM/STS), density functional theory (DFT), and simulated STM images. The dimer, but not the monomer, undergoes single‐electron transfer to the substrate to form [ 2 ] •+ , consistent with STM/STS observations, its lower ionization threshold, and computed frontier orbital energies. Gas‐phase calculations are consistent with a plausible arenium‐type pathway for annulative dimer formation and further indicate that oxidation increases global aromatic character across the fused framework.

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