Phenylene-diimine capped conjugate of lower rim 1, 3-calix[4]arene as molecular receptor for Mg²⁺ via arm conformational changes followed by aggregation and mimicking the species by molecular mechanics

Abstract

A phenylene-diimine-capped conjugate of lower rim 1,3-calix[4]arene (L) was synthesized, characterized, and shown to selectively bind to Mg²⁺ using its capped arms. This results in a selective recognition of Mg²⁺ through eliciting fluorescence enhancement of ∼70 fold with a detection limit of 40 ± 5 ppb. However, in the presence of blood serum, the lowest detection limit is 209 ± 10 ppb (0.2 μM). The binding of Mg²⁺ to L is authenticated by absorption and ¹H NMR data. The Job’s plot derived on the basis of the absorption data showed 1:1 stoichiometry between the receptor and Mg²⁺. The 1:1 species was further confirmed through ESI MS, that is, being supported by the isotope peak pattern authenticating the presence of Mg²⁺ in the complex. The L binds Mg²⁺ octahedrally using the tetradentate L^2- and two additional acetic acid moieties by bringing conformational changes as studied on the basis of MM computations. The conformational changes that occur in the capped arms upon Mg²⁺ binding were supported experimentally by NOESY. AFM and SEM studies showed that spherical particles of L are modified into flower and chain type aggregates upon complexation with Mg²⁺, confirming the supramolecular behavior of the species formed.