PHOTOINDUCED ELECTRON-TRANSFER IN SUPRAMOLECULAR ASSEMBLIES COMPOSED OF ALKOXYANISYL-TETHERED RUTHENIUM(II)-TRIS(BIPYRIDAZINE) COMPLEXES AND A BIPYRIDINIUM CYCLOPHANE ELECTRON-ACCEPTOR

Photoinduced electron transfer in photosystems consisting of bis(6,6'- dimethoxy-3,3'-bipyridazine)(6,6'-bis -dioxaoctyl-1-oxy]-3,3'-bipyrida zine)ruthenium(II) dichloride (1), (6,6'-bis[8-((4-methoxyphenyl)oxy)- 3,6-dioxaoctyl- 1-oxy]-3,3'-bipyridazine)ruthenium(II) dichloride (2a) , tris(6,6'-bis[11-(4-methoxyphenyl)-3 oxa-undecyl-1-oxy]-3,3'-bipyrid azine)ruthenium(II) dichloride (2b), and tris(6-(8-hydroxy-3,6-dioxa-o ctane-1 idazine)-1,3,5-benzenetricarboxylate-ruthenium(II) dichloride (3), with bis(N,N'-p-xylylene-4,4'-bipyridinium) (BXV(4+), 4) were exa mined. The series of photosensitizers include alkoxyanisyl donor compo nents tethered to the photosensitizer sites, capable of generating don or-acceptor supramolecular complexes with BXV(4+) (4). Detailed analys es of the steady-state and time-resolved electron transfer quenching r eveal a rapid intramolecular electron transfer quenching, k(sq), withi n the supramolecular assemblies formed between the photosensitizers an d BXV(4+) (4) and a diffusional quenching, k(dq), Of the free photosen sitizers by BXV(4+) (4). A comprehensive model that describes the elec tron transfer in the different photosystems and assumes the formation of supramolecular assemblies of variable stoichiomehries, SA(n), is fo rmulated. Analysis of the experimental results according to the formul ated model indicates that supramolecular complexes between 1-3 and BXV (4+) of variable stoichiometries exist in the different photosystems. Maximal supramolecular stoichiometries between 1, 2a and 3, and BXV(4) (4), corresponding to N = 2, 6, and 3, respectively, contribute to t he electron transfer quenching paths. The derived association constant s of BXV(2+) to a single binding site in the photosensitizers 1, 2a, 2 b, and 3 are 240, 100, 100, and 140 M(-1), respectively. The back elec tron transfer of the photogenerated redox products was followed in the different photosystems. Back electron transfer proceeds via two route s that include the intramolecular recombination, k(sr), within the sup ramolecular diads and diffusional recombination, k(dr), of free redox photoproducts. Detailed analysis of the back electron transfer in the different photosystems revealed that the non-covalently linked supramo lecular assemblies, SA(n), act as static diads where electron-transfer quenching and recombination occurs in intact supramolecular structure s despite the dynamic nature of the systems; The lifetime of the redox photoproducts Ru3+-BXV(. 3+) in the various systems is relatively lon g as compared to diad assemblies (0.56- 1.20 mu s). This originates fr om electrostatic repulsive interactions of the photoproducts within th e supramolecular assemblies resulting in stretched conformations of th e diads and spatial separation of the redox products.