Photoinduced formation of a cryptand from a coronand: an unexpected switch in cation binding affinity

McSkimming, Gordon, Tucker, James H. R., Bouas-Laurent, Henri, Desvergne, Jean-Pierre, Coles, Simon J., Hursthouse, Michael B. and Light, Mark E. (2002) Photoinduced formation of a cryptand from a coronand: an unexpected switch in cation binding affinity. Chemistry - A European Journal, 8 (15), 3331-3342. (doi:10.1002/1521-3765(20020802)8:15<3331::AID-CHEM3331>3.0.CO;2-C).

Abstract

Aza-crown ethers 2 and 3 with anthracene-containing pendant arms have been synthesised and characterised. Both compounds bind Group 1 metal cations in solution, forming complexes of 1:1 stoichiometry. The properties of compound 2 and its complexes have been studied by a range of techniques,. including NMR, UV and fluorescence spectroscopy and X-ray crystallography. The pendant arms can adopt either a cis or a trans geometry, the cis geometry favoured with larger cations. The geometry of the complex affects the fluorescence properties of the system, with larger cations giving higher excimer/monomer ratios. Upon irradiation ? > 300 nm, coronand 2 forms the cryptand 5 through a reversible intramolecular [4?+4?] cycloaddition reaction. The rates of the forward and reverse reactions of this photochromic process are cation dependent; in particular the rate of the thermal reverse reaction is decreased by smaller cations and increased by larger cations, especially Rb+. The metal binding constants in methanol for 2 and 5 have been determined, revealing that the cryptand 5 binds Na+ and Rb+ more weakly than crown ether 2 by over two orders of magnitude.

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Contributors

Author: Simon J. Coles

Author: Mark E. Light

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