Design and synthesis of luminescence chemosensors based on alkynyl phosphine gold(i)–copper(i) aggregates

Abstract

Receptor-containing polynuclear mixed-metal complexes of gold(I)–copper(I) 1–3 based on a [{Au₃Cu₂(CCPh)₆}Au₃{PPh₂–C₆H₄–PPh₂}₃]²⁺ (Au₆Cu₂) core with benzo-15-crown-5, oligoether and urea binding sites were designed and synthesized, respectively. These complexes exhibited remarkably strong red emission at ca. 619–630 nm in dichloromethane solution at room temperature upon photoexcitation at λ > 400 nm, with the emission quantum yield in the range 0.59–0.85. The cation-binding properties of 1 and 2 and the anion-binding properties of 3 were studied using UV-vis, emission and ¹H NMR techniques. Complex 1, with six benzo-15-crown-5 pendants, was found to show a higher binding preference for K⁺, with a selectivity trend of K⁺ ≫ Cs⁺ > Na⁺ > Li⁺. The addition of metal ions (Li⁺, Na⁺, K⁺ and Cs⁺) to complex 1 led to a modest emission enhancement with a concomitant slight blue shift in energy and well-defined isoemissive points, which is attributed to the rigidity of the structure and the inhibited PET (photo-induced electron transfer) process from the oxygen to the aggregate as a result of the binding of the metal ion. The six urea receptor groups on complex 3 were found to form multiple hydrogen bonding interactions with anions, with the positive charge providing additional electrostatic interaction for anion-binding. The anion selectivity of 3 follows the trend F⁻ > Cl⁻ ≈ H₂PO₄⁻ > Br⁻ and the highest affinity towards F⁻ is attributed to the stronger basicity of F⁻, as well as its good size match with the cavity of the urea pocket.