Synthesis, complexation properties and spectroscopic studies of the cation-induced conformational changes of some new oligooxaethylene-spacered diporphyrin arrays

Abstract

New linear diporphyrin arrays and a heteroleptic diad have been synthesized in good yield. ¹H NMR, UV-visible and fluorescence studies reveal that the receptors bind to alkali and alkaline-earth metal ions to give a supramolecular complex in which the ion is nestled within the oligooxaethylene framework. The formation of the host–guest complexes promotes a change in the geometry of the system toward a topology in which the two tetrapyrrolic macrocycles tend to face together. This is evidenced by the mutual ring current anisotropy effect exerted by the porphyrinic platforms, and by the blue shift and broadening of the relative UV-visible bands (Soret). In particular, the changes observed allow the complexation process to be monitored at very low concentrations. The association constants lie within the range 25–1 × 10⁵ M⁻¹ (CD₃CN–CDCl₃ 1:1) depending on either the nature of the diporphyrinic receptors or that of the interacting metal ions. The heteroleptic diad shows a very efficient energy transfer process (> 99%) between the two different macrocycles in a variety of solvents that is not altered by complexation with metal ions. The systems reported present interesting properties as switchable biomimetics.