Effective Charge Separation in Intermolecular Complexes of an Electron Donor and a Doubly Branched Triad Assembly: A Model System for Environmental Effects Controlling Electron Transfer

We describe in this Account the preparation of RuL(3) complexes and their significance as biomimetic models for the photosynthetic reation center. Their preparation from simple or more complicated bypyridine ligands L and their photophysical data, especially their stability, are reported. Biomimetic models involving three concepts of the interaction of RuL(3) with acceptors in coordinatively, mechanically, or covalently linked supramolecular assemblies are also presented. The electron transfer (ET) of the noncovalently linked assemblies of RuL(3) complexes carrying polyether chains with one or two anisyl binding sites (4 or 5) with the cyclic bisviologen was studied. Molecular modeling and NMR titration clearly show the formation of supramolecular assemblies. Time-resolved spectroscopy demonstrated that ET and charge separation in the RuL(3) complexes with two binding sites are more efficient. The more constrained RuL(3)-bisviologen-catenane (6) possesses two conformations which exhibit different efficiency in ET, creating a charge-separated state in the microsecond domain. The covalently linked Ru(bpy)(3)(2+)-viologen assemblies having one (7, diad) or two bisviologen arms (8, diad) result in efficient ET. Addition of linear polyethers, cyclic polyethers, or crowns generates new triads and tetrads of the pseudorotaxane type. Molecular modeling and NMR titration clearly indicate the formation of supramolecular assemblies. The analysis of time-resolved studies proves fast ET and especially long-lived charge-separated states in these pseudorotaxanes. These data, compared with the findings for the photosynthetic reation center, show conclusive results. The lifetimes of the charge-separated states increase clearly in the sequence for noncovalently < mechanically < and covalently linked assemblies.

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“…23a, 24,25 The supramolecular assemblies 4, 5, 11, and 12/BXV 4+ can be classified as pseudorotaxanes. The data are shown in Table 1.…”

Section: Electron-transfer Studies Of D-a Coronate Complexesmentioning

confidence: 99%

“…Therefore, model compounds of the types 8 and 9 [21][22][23][24][25]28,[30][31][32] were designed to be good models for such studies. A monobranched D-A-diad 7sin analogy to a …”

Section: Covalently Linked Systemsmentioning

confidence: 99%

Ruthenium Polypyridine Complexes. On the Route to Biomimetic Assemblies as Models for the Photosynthetic Reaction Center

2001

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“…11 Similarly, we showed that the formation of donor-acceptor supramolecular complexes between a molecular triad and electron donor results in steric rigidification of the triad that results in photoinduced vectorial electron transfer and stabilization of the redox products against back electron transfer. 12 It is established that N,N′-dialkylbipyridinium salts form donor-acceptor complexes with different electron donors. 13,14 Recently, the intermolecular complexes between dialkoxybenzenes and cyclo[bis(N,N′-p-xylylene-4,4′-bipyridinium)], BXV 4+ , were extensively studied by Stoddart and co-workers.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 1996

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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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“…Structural alignment and rigidification of the molecular arrays in these systems result in the stabilization of the redox products against back-electron transfer. Similarly, the formation of donor−acceptor supramolecular complexes between a molecular photosensitizer−acceptors triad and an electron donor was reported to result in the steric rigidification of the triad, leading to vectorial photoinduced electron transfer …”

Section: Introductionmentioning

confidence: 98%

Photoinduced Electron Transfer in π-Donor-Capped Zn(II) Porphyrins and N,N‘-Dimethyl-4,4‘-bipyridinium Supramolecular Assemblies

Kaganer¹,

Joselevich²,

Willner³

et al. 1998

J. Phys. Chem. B

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π-Donor-polyoxyethylene-capped Zn(II)-porphyrins, 1−3, form supramolecular complexes with N,N‘-dimethyl-4,4‘-bipyridinium, MV2+, 4. The association constants of the resulting adducts are 8 × 104, 1 × 104, and 1 × 103 M-1, respectively. The Zn(II)-porphyrins are quenched by MV2+ by two complementary routes that include internal electron-transfer quenching in the supramolecular complexes and diffusional electron-transfer quenching of the free chromophore.

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Effective Charge Separation in Intermolecular Complexes of an Electron Donor and a Doubly Branched Triad Assembly: A Model System for Environmental Effects Controlling Electron Transfer

Cited by 19 publications

References 29 publications

Ruthenium Polypyridine Complexes. On the Route to Biomimetic Assemblies as Models for the Photosynthetic Reaction Center

Ruthenium Polypyridine Complexes. On the Route to Biomimetic Assemblies as Models for the Photosynthetic Reaction Center

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 π-Donor-Capped Zn(II) Porphyrins and N,N‘-Dimethyl-4,4‘-bipyridinium Supramolecular Assemblies

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