Porphyrins with multiple crown ether voids: novel systems for cation complexation studies

Abstract

Porphyrins appended with crown ether, benzo-15-crown-5, at the methine positions have been synthesized and characterized. The fully and partially substituted porphyrins and their metallo (Co, Cu, and Zn) derivatives describe one or more ether cavities in the periphery that are capable of recognizing spherical cations. The ability of these macrocycles to complex cations (Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, and NH₄⁺) is investigated by use of visible, ¹H NMR, ESR, and emission spectral studies. The tetrasubstituted crown porphyrin (TCP) exhibits very high selectivity for K⁺. The cations (K⁺, Ba²⁺, and NH⁴⁺) that require two crown ether cavities for complexation promote dimerization of the porphyrins. The ESR study of the cation-induced porphyrin dimers reveals axial symmetry with the porphyrin planes separated by ~4.2 Å. This distance increases from the fully substituted to partially substituted porphyrins. The cations (K⁺, Ba²⁺, and NH₄⁺) quench efficiently the fluorescence of the free base porphyrins and their metallo derivatives. The quenching process is attributed to the steric geometry of the dimers.