Metal-organic dyads

Photoinduced Electron Transfer in Metal-Organic Dyads Kirk S. Schanze and Keith A. Walters... 

Photoinduced Electron Transfer in Metal-Organic Dyads... 

In most of the metal-organic dyads described in this review the metal center has a d6 electronic configuration. Further, the lowest excited state typically has a metal-to-ligand charge transfer (MLCT) configuration arising from promotion of a metal centered -electron into a ligand based -tt level, e.g.,... 

Figure 1 Photoinduced electron transfer schemes for type 1 and type 2 metal-organic dyads. Key L is a diimine ligand such as 2,2 -bipyridine M is a transition metal dn indicates the electron count in the valence shell d-orbitals of M A is an organic electron acceptor D is an organic electron donor FET is forward ET BET is back ET.

The first example of a covalently linked metal-organic dyad is reported by Meyer and co-workers in a communication that appeared in 1978 [80]. This manuscript describe the synthesis, electrochemistry, and photophysics of 8 (Scheme 4), which consists of a pair of /V-methyl-4,4 -bipyridinium (monoquat) electron acceptors coordinated directly to the (bpy)2Run chromophore. This complex is an inner-sphere analog of the Ru(bpy) +/paraquat (3) system which has received significant attention in studies of bimolecular photoinduced ET. Complex 8 displays electrochemical features typical of both the Ru-bpy unit and the monoquat electron acceptor. The photophysical results indicate that in fluid solution the lowest excited state is based on a Ru — monoquat MLCT transition. 

VI. PHOTOINDUCED INTRAMOLECULAR ELECTRON TRANSFER IN METAL-ORGANIC DYADS ... 

A significant amount of work has been carried out on complexes that are analogous to 8 in that they contain pyridinium acceptors directly coordinated to a photoactive metal center [82-85]. As noted above, in these complexes electronic coupling between the metal center and the pyridinium acceptor is comparatively large, and as a result the dynamics of photoinduced forward and back ET are best considered by using excited state decay theory [86]. In any event, these complexes have figured prominently in the study of ET in metal-organic dyads and some of the important discoveries made with them are briefly reviewed in this section. The Re(I) complex 10a (Scheme 5) has been featured in much of this work... 

Figure 3 Transient absorption difference spectra obtained 40 ns following 355-nm pulsed excitation (10-ns pulse width). (A) Model complex 11 (B) metal-organic dyad 10a. Reprinted with permission from Ref. 83.

Inspection of the literature uncovers ET rate data for three other type 1 metal-organic dyad systems (Scheme 11). The first system comprises complex 21, which features a p-benzoquinone acceptor linked to a polypyridine-Ru(II) chromophore via a 3-atom amide-based spacer [94]. Photoinduced forward ET occurs... 

The first successful and detailed study of photoinduced ET in a metal-organic dyad was carried out on type 2 dyad 27c (Scheme 13, Table 2) which... 

Gray and co-workers investigated the metal-organic dyad system 34 (Scheme 15 and Table 3) which comprises a dimeric iridium(I) complex (Ir2) as the... 

In an entirely new approach to the design of metal-organic dyads, G. Meyer... 

Figure 9 Transient absorption difference spectrum of metal-organic dyad 40 obtained 20 ns after 417-nm excitation pulse (10-ns pulse width). (Figure provided by Professor G. J. Meyer, Johns Hopkins University [110].)...

Figure 10 Plot of log vs. AG for metal-organic dyads. Data from Tables 1 and 2.

Schanze and Sauer were the first to report a detailed study of long-range photoinduced ET in a series of metal-organic dyads [94]. These authors synthesized dyads 50a-e (Scheme 22 and Table 4) that comprise a Ru(diimine) + chromophore covalently linked to a p-benzoquinone acceptor using a series of oligo-L-proline peptide spacers. Oligo-proline peptide spacers are used because previous studies suggest that these peptides are conformationally restricted and... 

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... 

Figure 13 Plot of logA as a function of approximate separation distance for metal-organic dyads data from Table 4. Figure legend (0) 53a-d (back ET) ( ) 53a-g (forward ET) (A) 50a-e ( ) 52a-f ( ) 51a-c (o) 54a-f.

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