The localization of hydrophilic sites within an osmium polypyridyl compound can produce a negative activation energy for emission decay

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.