Recoordination of a metal ion in the cavity of an arylazacrown ether: Model study of the conformations and microsolvation of calcium complexes of arylazacrown ethers

Photoinduced recoordination of Ca2+ complexes of photochromic azacrown ethers is studied by the density functional method using model arylazacrown ethers and a real azacrown-containing styryl dye. Two types of previously found conformations of the crown ether-calcium complexes, axial and equatorial, are studied, taking into account water molecules in the first coordination sphere of the cation. It is found that the first coordination sphere of the cation contains no more than three water molecules, the fourth one belonging to the second coordination sphere. The presence of three or four water molecules in the coordination sphere of the cation decreases relative energies of the conformations with broken metal-nitrogen bond, thus favoring metal recoordination. In the azacrown-containing styryl dye, this leads to the ground-state equilibrium (i.e., tautomerism) between the axial and equatorial forms of the complex. (C) 2004 Wiley Periodicals, Inc.