Binuclear Ruthenium−Manganese Complexes as Simple Artificial Models for Photosystem II in Green Plants

Abstract

As part of a project aimed at developing models for photosystem II (PSII) in green plants, we have prepared a series of model compounds (7, 8, and 13). In these compounds, a photosensitizer, ruthenium(II) tris(bipyridyl) complex (to mimic the function of P 680 in PSII), was covalently linked to a manganese(II) ion through different bridging ligands. The structures of the compounds were characterized by electron paramagnetic resonance measurements and electrospray ionization mass spectrometry. The interaction between the ruthenium and manganese moieties within the complex was probed by steady-state and time-resolved emission measurements. When the binuclear complexes are exposed to flash photolysis in the presence of an electron acceptor such as methylviologen (MV 2+ ), it could be shown that after the initial electron transfer from the excited state of Ru(II) in compound 7, forming Ru(III) and MV +• , an intramolecular electron transfer from coordinated Mn(II) to the photogenerated Ru(III) occurred with a first-order rate constant of 1.8 × 10 5 s -1 , regenerating Ru(II). This is believed to be the first supramolecular system where a manganese complex has been used as an electron donor to a photo-oxidized photosensitizer. Possible extensions to develop the manganese donor, and thus to approach the function of reaction center in PSII, are indicated.

Synthesis and Characterization of Dinuclear Ruthenium Complexes Covalently Linked to RuII Tris‐bipyridine: An Approach to Mimics of the Donor Side of Photosystem II

“…[13][14][15][16] With the aim to create manganese-based, biomimetic molecular catalysts for water oxidation, we have prepared and investigated several supramolecular systems in which a mono-or dinuclear manganese complex is linked to a [Ru(bpy) 3 ] 2 + -type photosensitizer. [13,14,[17][18][19][20][21][22][23][24][25] Some related work has also been reported by Wieghardt et al [15,16] In all of our linked ruthenium-manganese complexes, we have succeeded in observing light-induced, intramolecular electron transfer from the manganese moiety to the photogenerated [Ru(bpy) 3 ] 3 + center. [13,17,20,24] In addition, we were able to transfer three electrons, in a stepwise fashion, from the dinuclear manganese complexes to the ruthenium photosensitizer.…”

“…[13,14,[17][18][19][20][21][22][23][24][25] Some related work has also been reported by Wieghardt et al [15,16] In all of our linked ruthenium-manganese complexes, we have succeeded in observing light-induced, intramolecular electron transfer from the manganese moiety to the photogenerated [Ru(bpy) 3 ] 3 + center. [13,17,20,24] In addition, we were able to transfer three electrons, in a stepwise fashion, from the dinuclear manganese complexes to the ruthenium photosensitizer. [26] However, we have so far not been able to detect oxygen evolution with any of these complexes.…”

“…All the trinuclear ruthenium complexes investigated in this work have very short emission lifetimes, a feature which is quite different from the [Ru-(bpy) 3 Mn 2 ] complexes we have studied previously. [13,17] Because of the short lifetime, it seems difficult to initiate photooxidation of the excited [Ru(bpy) 3 ] 2 + by an external electron acceptor. However, the desired photoinduced multistep electron transfer was achieved by attaching the Ru(bpy) 3 unit to nanocrystalline TiO 2 as acceptor, which resulted in long-lived charge separation, albeit with a low yield.…”

Energy Transfer in a Hybrid IrIII Carbene–PtII Acetylide Assembly for Efficient Hydrogen Production

“…In this respect, of particulari nterest and inspired by nature,i st he use of ah ighly organized molecular assembly of individual functional components,t hat is, photosensitizer-catalyst conjugates,f or the development of as inglecomponent system capable of performing such process types. [6][7][8][9][10][11][12][13][14][15] Such architectures enable efficient collaboration in collecting the light energy and converting it efficiently into stored chemical energy.G reat efforts are being directedt ot he design of simple integrated-photocatalyst molecular assemblies carrying two or more spatially well-defined metal-based components, whichd emonstrates the feasibility of creating artificial photosynthetic devices.H owever,t hese single-component systems are still hampered by relativelyl ow efficiency for hydrogen evolution, and only af ew systems have reached at urnover number (TON) on the order of hundreds versus the catalysts, av alue that is still far lesst han that achieved with the multicomponent systems.[16] Therefore, it is highly desirable to seek new active assemblies that allow their implementation into originals upramolecular light-harvesting devices with the goal of improving the efficiency of light-to-chemical energy conversion.In the development of metallosupramolecular assemblies capable of giving rise to fuel production, we werei nterested in merging the properties of the cyclometalated iridium chromophore with platinum acetylide fragments. Such ap latinum species, in which the carbonÀplatinum bond of the acetylide linkage may lead to effective electronic coupling with the adjacent fragments, wass elected for its potentiala bility to drive photocatalytic hydrogen production.…”

“…www.chemeurj.org systems. [6][7][8][9][10][11][12][13][14] Figure S3 (see the SupportingI nformation) shows the quantum efficiency of the hydrogenp roduction by 1 as af unction of the incident monochromatic light wavelength in the ranges of 400-550 nm. The maximum efficiency of 1.32 % was achieved under irradiation at 450 nm after 10 ho fr eaction.…”