Journal
INORGANIC CHEMISTRY COMMUNICATIONS
Volume 4, Issue 1, Pages 30-32Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/S1387-7003(00)00187-8
Keywords
osmium; polypyridine; solvation; non-radiative decay; negative activation energy
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The temperature dependence of the emission lifetimes of two structural isomers of an osmium polypyridyl complex are reported. The compounds [Os(bpy)(2)(4, 4'-dcbpy)] (1) and [Os(bpy)(2)(3, 5-dcbpy)] (2), where 4, 4'-dcbpy = 4, 4'-dicarboxy-2, 2'-bipyridine and 3, 5-dcbpy = 3, 5-dicarboxy-2, 2'-bipyridine. differ only in the placement of their two hydrophilic carboxylate groups within the unique bipyridine ligand. In argon-saturated water, the emission life-time of 1 decreases with increasing temperature and can be fit to the Arrhenius expression (k(obs) = k(0) exp(-E-a/RT)) to give values of k(0) = (2.6 +/- 0.3) x 10(8) s(-1) and E-a = +330 +/- 20 cm(-1). In contrast the emission of 2 measured in ethanol displays longer lifetimes at higher temperatures to give k(0) = (2.9 +/- 0.3) x 10(6) s(-1) and E-a = -590 +/- 20 cm(-1). Thus, the specific localization of the hydrophilic carboxylate groups in these molecules significantly alters their photophysical properties and can produce negatively activated emission lifetimes. (C) 2001 Elsevier Science B.V. All rights reserved.
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