PHOTOINDUCED ELECTRON-TRANSFER IN SUPRAMOLECULAR ASSEMBLIES COMPOSED OF ONE-SHELL AND 2-SHELL DIALKOXYBENZENE-TETHERED RU(II)-TRIS(BIPYRIDINE) DERIVATIVES AND A BIPYRIDINIUM CYCLOPHANE

Photoinduced electron transfer reactions are characterized in supramol ecular assemblies consisting of a series of Ru(II)-bipyridine complexe s that include tethered dialkoxybenzene units 2-5 and cyclo[bis(N,N'-p -xylylene-4,4'-bipyridinium)], BXV4+ (1). Formation of supramolecular complexes between BXV4+ and the dialkoxybenzene pi-donor sites, linked to the photosensitizers, yields effective electron transfer quenching in the non-covalent-bound dyads and polyads. Steady-state luminescenc e quenching experiments and time-resolved studies reveal that for the one-shell photosensitizers 3 and 5 that include six and two dialkoxybe nzene units, respectively, supramolecular photosensitizer-BXV4+ assemb lies of maximal stoichiometries corresponding to six and two, respecti vely, coexist with lower supramolecular stoichiometries and free photo sensitizers in the systems. For the two-shell dialkoxybenzene-tethered photosensitizers 2 and 4 that include 12 and 4 pi-donor binding sites , respectively, supramolecular assemblies with BXV4+ of maximal stoich iometries corresponding to 6 and 2 are derived. The association consta nt of BXV4+ to the functionalized branch of the two-shell photosensiti zer is ca. 10-fold higher than that of the one-shell photosensitizer. The higher affinity of the two-shell photosensitizers to form supramol ecular complexes with BXV4+ is attributed to the cooperative participa tion of two dialkoxybenzene sites in the association of one BXV4+ unit . The higher association constants of BXV4+ to the two-shell photosens itizers 2 and 4, yields improved electron transfer quenching as compar ed to the one-shell chromophores 3 and 5. The photogenerated redox-pro ducts formed in the supramolecular assemblies Ru3+-bipyridine and BXV. 3+, recombine within the non-covalent-bound structures without dissoci ation. The back electron transfer rate of the photogenerated redox pro ducts in the dyads and polyads is relatively slow due to their spatial separation by repulsive electrostatic interactions.