Aromatic Molecules by Design

Two π-extended core-modified porphyrinoids were synthetized and structurally characterized to probe boron-induced geometry switching and modulation of electron delocalization. trans-Vinylene-bridged hexathia-dodecaphyrin forms the largest, fully characterized porphyrinoid BODIPY-type complex while retaining its parent open-form geometry, whereas octathia-hexadecaphyrin does not support boron coordination. Notably, a smaller, figure-of-eight tetrathia-octaphyrin analogue exhibits unusual direct C–B bond formation, highlighting pronounced structural sensitivity of expanded porphyrinoids and their utility in accessing unconventional bonding and coordination modes.

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 TiO₂(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 TiO₂(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.

Acid–base equilibria can be shifted when binding events alter the local electrostatic and hydrogen-bonding environment of an ionizable site. Implementing such binding-linked acidity changes in a discrete synthetic system would enable regulation of proton activity without changing molecular composition or solvent. Here we describe a new naphthalimide-fused analogue of cyclo[8]pyrrole in which anion recognition dramatically shifts protonation equilibria. Whereas established cyclo[8]pyrroles preferentially form globally aromatic dications, the electron-deficient macrocycle shows an unusual preference for a persistent neutral redox state, while simultaneously revealing exceptionally high Brønsted acidity in organic solvents. The neutral macrocycle retains key anion-recognition characteristics of dicationic cyclo[8]pyrroles, including strong sulfate affinity. Sulfate binding increases the effective first pK_a in DMSO by roughly four units, providing an orthosteric, anion-actuated acid switch and expanding the design space of switchable acidity beyond host–guest pK_a shifts and light-addressable photoacids.