Photoinduced electron transfer in supramolecular assemblies composed of one-shell and two-shell dialkoxybenzene-tethered Ru(II)-tris(bipyridine) derivatives and a bipyridinium cyclophane

Photoinduced electron transfer reactions are characterized in supramolecular assemblies consisting of a series of Ru(II)-bipyridine complexes 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 luminescence quenching experiments and time-resolved studies reveal that for the one-shell photosensitizers 3 and 5 that include six and two dialkoxybenzene units, respectively, supramolecular photosensitizer-BXV4+ assemblies of maximal stoichiometries corresponding to six and two, respectively, coexist with lower supramolecular stoichiometries and free photosensitizers 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 stoichiometries corresponding to 6 and 2 are derived. The association constant of BXV4+ to the functionalized branch of the two-shell photosensitizer is ca. 10-fold higher than that of the one-shell photosensitizer. The higher affinity of the two-shell photosensitizers to form supramolecular complexes with BXV4+ is attributed to the cooperative participation of two dialkoxybenzene sites in the association of one BXV4+ unit. The higher association constants of BXV4+ to the two-shell photosensitizers 2 and 4, yields improved electron transfer quenching as compared to the one-shell chromophores 3 and 5. The photogenerated redox-products formed in the supramolecular assemblies Ru3+-bipyridine and BXV.3+, recombine within the non-covalent-bound structures without dissociation. The back electron transfer rate of the photogenerated redox products in the dyads and polyads is relatively slow due to their spatial separation by repulsive electrostatic interactions.