ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Synthesis, redox, photophysical, and photochemical properties of Ru(NN)2+3 complexes NN = 2-((2′-pyridyl)thiazole (pyth), 2-(2′-pyrazyl)thiazole (pzth), 2,2′-bithiazole (bth), 5-(2′-pyridyl)-1,2,4-thiadiazole (pytda), 2-(2′-pyridyl)imidazole (pyim), 1-methyl-2-(2′-pyridyl)imidazole (Mepyim), and 2-(2′-pyridyl)oxazole (pyox)) are described. Oxidation potentials for the Ru3+/2+ couples in MeCN varied from about 0.80 V to 1.60 V vs. NHE. Three reduction waves were observed in all the cases except for Ru(pyim)2+3 and Ru(Mepyim)2+3 complexes and asigned to the one-electron reduction of each bidentate ligand. Absorption spectra contained bands in the UV (280-325 nm) and VIS (437-481 nm) regions which have been assigned to ligand-centered π-π* and metal-to-ligand charge-transfer dπ-π* transitions, respectively. Emission spectra at 77 K were determined for all the complexes presenting maxima in the 580-650-nm region, with vibrational progression in some of them. Only pyth, pzth, bth, and pytda tris-chelates showed luminescence at room temperature in aqueous solution, with quantum yields ranging from 0.0013 to 0.0095 and excited-state lifetimes from 55 to 390 ns, as determined from pulsed laser techniques. Their E0-0 spetroscopic energies have been estimated from emission wavelength maxima at 77 K which, in turn, have allowed calculation of excited-state redox potentials. A plot of E0-0 vs. ΔE1/2, where ΔE1/2 = E1/2(3+/2+) - E1/2(2+/+), was linear with a slope of ca. 1.1 and a correlation coefficient of 0.999, demonstrating an identical nature of the orbital involved in spectroscopic and electrochemical processes. Photochemical properties of Ru(NN)2+3 complexes have been tested using methyl viologen (MV2+) in Ar-purged aqueous solution at pH 5. Stern-Volmer treatment has led to the determination of bimolecular quenching constants (0.5 to 2 × 109m-1·s-1) which parallel electron-transfer free-energy changes. Homogeneous back-reaction of primarily produced MV+· and Ru(NN)3+3 has been measured resulting to be slightly higher than diffusion control and independent of ligand nature. Rate constants for the scavenging of Ru(NN)3+3 by added edta have been also determined (1.7 to 8.2 × 108M-1 · S-1). Under such conditions, net production of MV+· is attained with quantum yields varying from 0.003 to 0.038 (single-shot laser results).
Additional Material:
4 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19870700813
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