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[8-((4- methoxyphenyl)oxy)-3,6-dioxaoctyl-1-oxy]-3,3′-bipyridazine)ruthenium(II) dichloride (1), tris(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)-3,6,9-trioxa-undecyl-1-oxy]-3,3′- bipyridazine)ruthenium(II) dichloride (2b), and tris(6-(8-hydroxy-3,6-dioxa-octane-1-oxy)-6′-[8-((4-methoxyphenyl)oxy)-3, 6-dioxaoctyl-1-oxy]-3,3′-bipyridazine)-1,3,5-benzenetricarboxylate- ruthenium(II) dichloride (3), with bis(N,N′-p-xylylene-4,4′-bipyridinium) (BXV⁴⁺, 4) were examined. The series of photosensitizers include alkoxyanisyl donor components tethered to the photosensitizer sites, capable of generating donor - acceptor supramolecular complexes with BXV⁴⁺ (4). Detailed analyses of the steady-state and time-resolved electron transfer quenching reveal a rapid intramolecular electron transfer quenching, k_sq, within the supramolecular assemblies formed between the photosensitizers and BXV⁴⁺ (4) and a diffusional quenching, k_dq, of the free photosensitizers by BXV⁴⁺ (4). A comprehensive model that describes the electron transfer in the different photosystems and assumes the formation of supramolecular assemblies of variable stoichiometries, SA_n, is formulated. Analysis of the experimental results according to the formulated model indicates that supramolecular complexes between 1-3 and BXV₄₊ of variable stoichiometries exist in the different photosystems. Maximal supramolecular stoichiometries between 1, 2a and 3, and BXV⁴⁺ (4), corresponding to N = 2, 6, and 3, respectively, contribute to the electron transfer quenching paths. The derived association constants of BXV²⁺ to a single binding site in the photosensitizers 1, 2a, 2b, and 3 are 240, 100, 100, and 140 M^-1, respectively. The back electron transfer of the photogenerated redox products was followed in the different photosystems. Back electron transfer proceeds via two routes that include the intramolecular recombination, k_sr, within the supramolecular 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 supramolecular assemblies, SA_n, act as static diads where electron-transfer quenching and recombination occurs in intact supramolecular structures despite the dynamic nature of the systems. The lifetime of the redox photoproducts Ru³⁺-BXV^•3+ in the various systems is relatively long as compared to diad assemblies (0.56-1.20 μs). This originates from electrostatic repulsive interactions of the photoproducts within the supramolecular assemblies resulting in stretched conformations of the diads and spatial separation of the redox products.