4,4 -Bipyridinium ligands

The molecule [ReI(MQ+)(CO)3(dmb)]2+ has been a model system for studying intramolecular electron transfer over the last two decades. Here, MQ+ is the monodentate ligand Af-methyl-4,4/-bipyridinium, dmb is the bidentate ligand 4,4r-dimethyl-2,2r-bipyridine, and the three CO ligands are facially coordinated. Irradiation of this complex at room temperature in solution with near-UV light leads to a sequence of intramolecular electron-transfer events as shown in Fig. 7. 

Dicarboxypyridinium chlorochromate (2,6-DCPCC)392 possesses an acidic character that allows the in situ deprotection and oxidation of alcohols, protected as tetrahydropyranyl and trimethylsilyl ethers. 2,2 -Bipyridinium chlorochromate (BPCC)393 contains a ligand that complexes efficiently with the reduced chromium species, generated during the oxidation of alcohols, allowing for a substantial simplification of the work-ups. For this reason, it enjoys a popularity among chlorochromates surpassed by only PCC. 

In remarkable work, Benjamin Coe of the University of Manchester, UK, has produced the first genuinely switchable NLO device. Coe and co-workers were able to show that the Ru(II) complex 11.74 exhibits a very large ji coefficient of 1112 X 10 30 esu associated with intense low-energy MLCT excitations. Furthermore, these / values may be tuned by manipulation of the substituents on the bipyridinium portion of the molecule and exchange of the amine ligands for, for example, pyridyls. The key to the complexes switching ability lies in the observation that the NLO behaviour of the system is reduced by approximately 10-20-fold by oxidation of the metal centre to Ru(III) by... 

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... 

In the potential window examined (-2.2/-I-2.1 V versus SCE), L/+ and L2 can reversibly accept five and, respectively, six electrons, with processes localized on their bipyridinium- and bpy-type units. The catenane ligand L4" +, besides the re-... 

TRIR and TR3 spectra were used to study the dynamics and mechanism of metal-to-ligand and interligand electron transfer in /uc-[Re(-MQ+)(CO)3(dmb)]2+, where MQ+ = V-methyl-4,4 -bipyridinium, dmb = 4,4,-dimethyl-2,2,-bipyridine.191,192... 

Systematic studies by Coe et al. with salts of the form franx-[Ru (NH3)4(L°) (L )][PFg]n (L° = an electron-rich ligand = an electron acceptor-substituted ligand n = 2 or 3) initially focused on species where = 4-acetylpyridine, ethylisonicotinate or V-mcthyl-4,4 -bipyridinium (MeQ ) [29]. The intense, visible Ru° MLCT absorptions of these complexes are strongly solva-... 

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