New DTPA-derived bis-naphthalenophanes: fluorescence, protonation, and complexation with aromatic amines

Two fluorescent, water-soluble bis-naphthalenophane isomers with six carboxylate arms, abbreviated as (bis-dtpa14nap)H₆ and (bis-dtpa15nap)H₆, were synthesized, which consist of two 1,4- or 1,5-substituted naphthalene rings interlinked by two diethylenetriaminepentaacetic (DTPA) chains through amide-linkages. Both DTPA-based macrocycles exhibit intense excimer and monomer emission bands, which sensitively respond to pH in three protonation steps; more sensitive is the 1,4-naphthyl isomer. The full pH-emission profiles have confirmed that the mono-protonated anion (bis-dtpanap)H₅ ⁻ is the major protonation species at the physiological pH. Fluorometric titrations at pH 7.2 have proven that the 1,4-naphthalenophane anion forms 1:1-complexes with cationic phenethylamine (formation constant, 5700 M⁻¹) and histamine (3000 M⁻¹), excluding tryptamine cation, whereas the 1,5-isomer does not react with any of the three amines. The primary binding forces are electrostatic interactions between the CH₂CO₂ ⁻ arms of 1,4-naphthalenophane and the CH₂CH₂NH₃ ⁺ chain of an aromatic amine. The resulting ion-pair is stabilized by encapsulation of the guest molecule in 1,4-napthalenophane cavity, while the 1,5-isomer cannot encapsulate. NMR studies have demonstrated that 1,4-napthalenophane has a higher freedom in reorientation of naphthalene rings. Such geometrical properties controlled by selection of naphthalene units are the feature of the new naphthalenophanes, and are responsible for the pH‒emission profiles and the complexation.