Katarzyna Ślusarek-Zając

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.

Two viologen-like macrocyclic receptors, [1]²⁺ and [2]²⁺, merging a bent oligophenylene loop with a (para-xylylene)bispyridinium moiety were designed and synthesized. Both dications were derived from the same precursor: the more strained cycloparaphenylene analogue [1]²⁺ was obtained using a reductive aromatization, whereas the less curved, meta-phenylene-containing [2]²⁺ was generated using a double eliminative rearrangement. [1]²⁺ is active as a receptor toward pyrene-1,3,6,8-tetrasulfonate, displaying a 2:1 binding interaction, with K₁₁ = 7.4(3) × 10³ M⁻¹. In the solid state, [1][TFA]₂ shows differential sorption of C₆ hydrocarbons and shows reversible sorption of water. [1]²⁺ displays yellow emission in solution (λ_em,max = 544 nm in DMF), which is considerably red shifted in the solid state (λ_em,max = 644 nm for dry samples). Both [1]²⁺ and [2]²⁺ undergo electrochemical reduction, yielding transient viologen-like radical cations and diradicals.