The synthesis is described of a photoactive molecular dyad comprising a luminescent ruthenium(II) tris(2,2‘-bipyridyl) fragment covalently attached, via a methylene bridge, to a calix[4]arene receptor in which two of the four walls are benzoquinone functions. A distribution of interconverting conformers is expected for the calix[4]arene platform, including cone, partial cone, and 1,3-alternated forms, according to molecular dynamics simulations and NMR studies. Luminescence from the metal complex is quenched because of light-induced electron transfer from the triplet state to a nearby quinone. The MD simulations indicate that the redox-active subunits approach within orbital contact of each other and that the noncone conformations adopt coparallel arrangements of the redox partners that appear to be highly favorable for electron transfer. Cations, such as barium(II), are held at the lower rim of the receptor by coordination to the four O atoms and by two N atoms provided by an ancillary 2,2‘-bipyridine that is appended to the receptor. Cation complexation increases slightly the thermodynamic driving force for light-induced electron transfer but restores luminescence from the ruthenium(II) tris(2,2‘-bipyridyl) fragment. This apparent contradiction is explained in terms of the cation inducing a major structural modification of the supermolecule. Both MD and NMR studies indicate that the cation forces the luminophore away from the quinone and, by favoring the cone conformation of the calix[4]arene, destabilizes cofacial orientations between closely spaced reactants. Consequently, the system functions as a sensitive photoionic detector.

Values for the protonation constants of myo-inositol hexakis (ph0sphate) have been determined at 25°C and 0.1 M ionic strength by potentiometry in media containing two different concentrations of Li+, Na', K + and Cs+ salts.The Alog K,,,, values, which correspond to the differences between the constants obtained with and without alkali-metal cations, are considered as being related to the binding ability of the Iigand towards these cations. 703Copyright 0 1989 hy Marcel Dekker, Inc Downloaded by [McMaster University] at 11:35 22 December 2014 7 04 BIETH ET AL.Their relative behaviour is then analysed and some conclusions are drawn about the conformations which might be adopted by the complexes in different p H regions.

Based on molecular dynamics simulations, we describe the F- versus Cl- complexation by an hexaprotonated cryptand L6+ in aqueous solution, in order to elucidate their structures, solvation properties and the status of external halide counterions. In water, F- and Cl- simulated inclusive complexes adopt a structure somewhat different from the solid state structure of the F- complex: The anion binding involves two diammonium bridges only, and the accompanying counterions are dissociated from the +5 charged complex. A remarkable result is obtained for the dissociated L6+,3F- ,3Cl- system, where spontaneous complexation of F- (the anion which forms the most stable complex with L6+) takes place during the dynamics. The resulting complex is of facial type; this suggests that the equilibrium involves multiple binding modes and structures in aqueous solution. The question of F-/Cl- binding selectivity is investigated by free energy perturbations simulations which nicely reproduce the spectacular preference for F- over Cl-. Two different methodologies used for the treatment of electrostatics (standard versus Ewald calculations) yield similar conclusions.

Binding of K or Ba ions attenuates intramolecular electron transfer between a chromophore and an electron-affinic macrocyclic receptor (see schematic representation), and simultaneously amplifies luminescence. In the complexed state, photons absorbed by the dicationic chromophore are emitted as luminescence, whereas illumination of the uncomplexed superstructure causes rapid reduction of the receptor.

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