How do fluoride ions bind to tetrathiacalix[2]arene[2]triazines?

Two novel tetrathiacalix[2]arene[2]triazines 1a,b have been synthesized and investigated as halide ionophores. UV-vis titrations showed that these macrocycles have a high affinity for fluoride over other halide ions; 1H NMR spectroscopy revealed that only upon fluoride (and not chloride or bromide) ion addition does the spectrum undergo a dramatic loss of symmetry. Finally, a comprehensive DFT (B3LYP/6-311+G(d,p)) investigation suggested that fluoride ions likely form σ-complexes upon binding to 1a and 1b.