Biscrown fluoroionophores: has the dye conformation an influence upon the formation of sandwich complexes in the ketocyanine series?

Abstract

The effect of the sensor conformation upon cation recognition, and more precisely upon the formation of 2∶1 (host-guest) sandwich complexes, was studied with three new fluoroionophores. These are symmetrical ketocyanine dyes substituted with two crown ethers. The first member of this series, 2,5-bis{[4-(N-aza-15-crown-5)-phenyl]methylene}cyclopentanone (1) has a planar and rigid structure. In compound 2, 2,6-bis{[4-(N-aza-15-crown-5)-phenyl]methylene}cyclohexanone, the two phenyl groups both are twisted by 40° with respect to the core of the molecule. Compound 3, 1,5-bis[4-(N-aza-15-crown-5)phenyl]penta-1,4-dien-3-one, displays possibilities of rotation about the carbonyl carbon quasi-single bond and was previously reported to exist in solution as a mixture of two conformers. For the sake of comparison, a monoazacrown dye of the cyclopentanone series (4) was also investigated. The behaviour of these dyes in acetonitrile in the presence of Ca²⁺ and Ba²⁺ perchlorates was studied by infrared spectroscopy, mass spectrometry, NMR and UV/vis absorption spectroscopy. The absorption data were fitted by taking into account three different stoichiometries and the corresponding association constants were calculated. Very stable 2∶1 sandwich complexes were observed for all three symmetrical dyes, meaning that small twists and conformational variations of the sensors hardly affect the formation of such structures. The monocrown dye also leads to sandwich complexes, but the corresponding association constants were slightly weaker than for biscrown dyes.