Highly sensitive and selective "turn-on" calcium cation sensing from a dendronic terthiophene tetraethylene glycol (TEG) molecule

A molecule consisting of a branched or dendronic terthiophene with tetraethylene glycol moiety (3T5O) was synthesized for highly sensitive and selective calcium ion detection in solution. The sulfur atoms of the terthiophene dendron moiety contribute to the metal-ligand complexation of calcium cations. The modification of the terthiophene dendrons with a hydroxyl-terminated tetraethylene glycol (TEG) group produces a twisted intramolecular charge transfer (TICT) event in which the terthiophene moiety is conformationally twisted, affecting its chromophoric and fluorophoric activity. This twisted conformation resulted in an increase in emission intensity at 320 nm, at the same time producing a blue shift in the absorption spectra. This process enabled a "turn-on" system for calcium ion detection compared to most "turn-off" systems involving a quenching route. The intensity of the emission is related to saturation of the terthiophene moiety at a concentration of 3T5O:[Ca²⁺] 1:4, of ∼1 × 10^-8 M, which indicates high sensitivity for calcium cations. No response with other cations such as Na⁺, K⁺, and Mg²⁺ was observed, which also indicates high selectivity for the system. Conformational analysis of the 3T5O and the twisted terthiophene dendron geometry was done by semiempirical PM3 modeling. In an effort to distinguish specific interactions between monovalent and divalent cations and the 3T5O, restricted Hartree-Fock level calculations using the STO-3G basis set were also performed. The calculations correlated with the experimental results.