Aromatic Molecules by Design

Bisallyl diradicals are central intermediates in allene dimerization, yet they are normally too short-lived to observe under thermal conditions. Here, π-extension of Staudinger-type allene dimers weakens the central cyclobutane bond sufficiently to make reversible ring opening accessible near room temperature. Variable-temperature NMR spectroscopy and ROESY exchange correlations reveal repeated cleavage and reformation of this bond, with activation free energies of 15.77 and 13.79 kcal mol⁻¹ for the Staudinger dimer and its π-expanded analogue, respectively. Calculations reproduce this trend and identify low-population open-shell singlet bisallyls as the intermediates, while explaining the absence of neutral-state EPR signals. Reduction reverses the relative stability of the closed and open valence isomers. Voltammetry and chemical reduction support a sequence of ring-opened radical-anion, dianion, and tetraanion states. The dianion is characterized by NMR spectroscopy and X-ray diffraction, while spectroscopy and calculations reveal progressive bond redistribution from mixed-valence allyl-radical/allyl-anion character to two allyl-anion units and, finally, four fluorenyl-anion-like termini. These results connect hidden neutral diradicals with persistent reduced bisallyl frameworks through charge-controlled covalent-bond reorganization.

Two π-extended core-modified porphyrinoids were synthesized and structurally characterized to probe boron-induced geometry switching and modulation of electron delocalization. trans-Vinylene-bridged hexathiadodecaphyrin forms the largest fully characterized porphyrinoid BODIPY-type complex while retaining the parent open-form geometry, whereas octathiahexadecaphyrin does not support boron coordination. Notably, a smaller figure-of-eight tetrathiaoctaphyrin analogue exhibits unusual direct C–B bond formation, highlighting the 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.