Allosteric fluoride anion recognition by a doubly strapped porphyrin.

A highly selective allosteric fluoride recognition system is described consisting of a doubly strapped porphyrin (I) that contains 2 small hydrogen-bonding cavities not being able to bind larger anions. Compd. I was prepd. by condensation of strap moieties with a $alpha$$beta$$alpha$$beta$-tetrakis(2-aminophenyl)porphyrin under high diln. conditions. By comparison with related strapped porphyrins contg. 4 linkers the cofacial distance between the phenyls in the straps and the porphyrin was estd. to be 3-4 AA creating a small pocket suitable for F- binding but excluding larger anions. The proximate location of the strap moieties to the porphyrin in I was confirmed by NMR spectroscopy in CDCl3. The Soret band of I in dichloromethane was split into 2 bands of equal intensity indicating that the strap presented a significant perturbation caused by $π$-$π$ interactions of the bis(dithienyl)-benzene moieties with the porphyrin. Upon addn. of F- to a soln. of I in DMSO the split Soret band shifted to a single band with a simultaneous red shift of the Q band whereas no absorbance changes were obsd. upon exposure of larger anions including Cl-, Br-, J-, CN-, and H2PO3-. The plots of absorbance changes vs. TBAF showed a sigmoidal curve indicating that the F- binding to I is cooperative. This binding was analyzed with the Hill equation and further characterized using Scatchard plots. Given the obsd. 1:2 binding stoichiometry energy-minimized geometries were computed suggesting that the cavities in I are contracted. The calcd. structure with 2 bound F- caused an expansion of the cavity that sepd. the planes of the porphyrin and the straps by 4.9 AA. From studies of the influence of a second coexisting halide ion on the UV/Vis spectrum of the I(F2-) complex it was estd. that the affinity of I to F- was ∼104 times higher than for other halides. A conducting polymer based upon I displays both electrochem. and cond. responses to F- and no response to Cl-. [on SciFinder(R)]