Aromatic Molecules by Design

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.

Indene-annulated perylenes merge cyclopentadiene-like benzylic acidity with the multielectron redox chemistry of rylene carbonyl scaffolds, creating discrete redox–protonation (“Pourbaix”) spaces in which electron transfer and acid–base processes become intrinsically coupled. Here we map the accessible states of doubly and singly pentannulated perylenediimides and perylene tetraesters using absorption/emission spectroscopy, electrochemistry and spectroelectrochemistry, and DFT analysis.

Voltammograms of all derivatives show two quasi-reversible, largely core-centered reductions, yet the pentannulated tetraesters uniquely undergo electrochemically induced γ-deprotonation prior to core reduction, consistent with electrogenerated-base chemistry. Chemical stimulation reveals pronounced chemodivergence: potassium tert-butoxide readily produces deprotonated “dienolate” NIR chromophores in the pentannulated tetraester series, whereas the corresponding diimides first form perylene-centered radical anions/dianions that evolve into benzylic anions and higher multianions in a cation-dependent manner, with crown ether sequestering K+ and reshaping reaction trajectories.

Strong reductants enable entry into highly reduced charge states, including an X-ray-verified salt of a pentannulated diimide trianion displaying diverse potassium binding motifs, underscoring the non-innocent role of ion pairing. Oxidation of deprotonated dianions furnishes weakly coupled neutral diradicaloids with NIR absorption. These results establish indene-annulated rylenes as multistate NIR chromophores whose experimentally accessible Pourbaix subsets can be programmed by redox bias, base strength, and cation coordination, with direct implications for organic electrochromism and rylene-based energy-storage chemistries.