Synthesis and sensing behavior of cyanoanthracene modified 1,3-alternate calix[4]benzocrown-6: a new class of Cs+ selective optical sensors

Abstract

1,3-Calix[4]bis(10-cyano-9-anthrylmethyl-o-benzocrown-6), AnCN2, and 1,3-calix[4]-25,27-bis(allyloxy)-26,28-(10-cyano-9-anthrylmethyl-o-benzocrown-6), AnCN1, were synthesized as the first generation of Cs⁺ selective optical sensors based on calixarenes. The maximum enhancement in the emission of AnCN2 and AnCN1 upon Cs⁺ complexation was 11.7 and 8.2 in a 1 ∶ 1 CH₂Cl₂–MeOH mixture. Their response to various alkali metal ions follows the order Cs⁺ > Rb⁺ ≫ K⁺ ≫ Na⁺ > Li⁺ indicating good selectivity towards caesium. The fluorescence and absorption profile for Cs⁺ complexation suggest a 1 ∶ 1 stoichiometry for the host AnCN1–Cs complex and a stability constant of 10⁷ M⁻¹. AnCN2, on the other hand, shows two plateau regions corresponding to a 1 ∶ 2 stoichiometry for complexation of Cs⁺. The first complexation constant is much higher (10⁷ M⁻¹) than the second one (10⁴ M⁻¹). The NMR studies further substantiate the complexation of two metal ions by AnCN2. This is the first report of stepwise complexation of metal ions to a 1,3-alternate calix[4]bis(crown-6) derivative.