Diquat electron acceptors

The first detailed study of the driving force dependence for photoinduced ET was reported by Elliott and co-workers [89]. The dyads used in Elliott s work consist of a tris-diimine Ru(II) chromophore covalently linked to a series of quatemized 2,2 -bipyridine (diquat) electron acceptors, 19a-c (Scheme 10, Table... 

Schmehl, Elliott, and co-workers reported an extensive study of the spacer dependence of photoinduced ET in several series of metal-organic dyads consisting of a Ru(diimine) + chromophore covalently linked to a diquat electron acceptor [128]. In the present review we list only results for the series of poly-methylene linked dyads 52a-f (Scheme 23 and Table 4) the reader is directed to the original... 

The herbicidal activity of the bipyridyliums depends on their redox properties. Their abilities as one-electron acceptors of the right redox potential (-350 mV for diquat and -450 mV for paraquat) allow them to siphon electrons out of the photosynthetic electron-transport system, competing with the natural acceptors. The radical anion produced is then reoxidized by oxygen, generating the real toxicant, hydrogen peroxide, which damages plant cells. Structure-activity relationships in this series have been reviewed (60MI10701). 

While artificial photosynthetic mimics come in many manifestations, our efforts have focused predominantly on the class of molecules represented by the structure in Fig. 10.1. These molecules consist of a visible-light-absorbing chromophore in the form of a trisbipyridineruthenium(ll) complex (C) finked by flexible polymethylene chains to one or more electron donors (D) and an electron acceptor (A). The electron acceptor is anAl,W-dialkylated-2,2 -bipyridine (a so-called diquat ) and the electron donors are Al-aUcylated phenothiazines. The diquat type acceptor was chosen because... 

The most widely used electron acceptors in inorganic chromophore-quencher systems have been bipyridinium dications, often called viologens (quatemarized derivatives of 4,4 -bipyridine) or diquat (cyclic quatemarized derivatives of 2,2 -bipyridine). The classical studies of Elliott, Schmehl, and Mallouk have been concentrated on dyads of types (4) and (5). For dyads (4) [168, 169], oxidative PET takes place, with forward processes in the 80 to 1700-ps time scale and very fast (<30 ps) charge recombination. The main observations are that (i) electron transfer to the diquat quencher occurs from the directly linked bipyridine ligand (ii) fast equilibration between the MLCT excited states on the three bipyridine ligands precedes electron transfer (iii) the electron transfer rates are in the normal Marcus... 

A number of other herbicides interfere with photosynthesis in specific ways. Amitrole inhibits biosynthesis of chlorophyll and carotenoids. The affected plants present a bleached appearance before they die because of the loss of their characteristic pigments. Another herbicide, atrazine, inhibits the oxidation of water to hydrogen ion and oxygen. Still other herbicides interfere with electron transfer in the two photosystems. In photosystem II, diuron inhibits electron transfer to plastoquinone, whereas bigyridylium herbicides accept electrons by competing with the electron acceptors in photosystem I. The inhibitors active in photosystem I include diquat and paraquat. The latter substance attained some notoriety when it was used to interfere with an... 

The interaction of bipyridyl herbicides (paraquat and diquat) with photosynthesis is different from that of the electron transport inhibitors. These compounds, with highly negative redox potentials (paraquat E 446mV diquat Eq -349mV), interact in the vicinity of ferredoxin causing a diversion of electron flow from the ultimate electron acceptor NADP. This was clearly seen in paraquat-treated plant material as a progressive inhibition of carbon dioxide uptake (25)... 

Bipyridyliums with redox potentials in the range of -300 to -500 mV, such as diquat and paraquat, can accept electrons in competition with the acceptor of photosystem I (Figure 2, site 4) and have herbicidal activity. Interception of electron flow from photosystem I essentially shunts the electron transport chain. 

Covalently linked donor-acceptor molecules. One can gain greater control of the kinetics of electron transfer reactions in these systems by fixing the distance between donor and acceptor, and by introducing a third redox molecule, which serves as either a secondary donor or accrator, into the chain. For example, we have shown that a Ru(bpy)32+-diquat + donor-... 

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