Photoinduced Electron Transfer in Supramolecular Assemblies Composed of Alkoxyanisyl-Tethered Ruthenium(II)−Tris(bipyridazine) Complexes and a Bipyridinium Cyclophane Electron Acceptor

Abstract

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.

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Structure and Lateral Electron Hopping in Osmium-tris-4,7-diphenylphenanthroline Perchlorate Monolayers at the Air/Water Interface

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