Conformational Control of Intramolecular Electron Transfer in Calix[4]diquinones and Their Cationic Complexes

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.