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 (kobs=k0 exp(−Ea/RT)) to give values of k0=(2.6±0.3)×108 s−1 and Ea=+330±20 cm−1. In contrast the emission of 2 measured in ethanol displays longer lifetimes at higher temperatures to give k0=(2.9±0.3)×106 s−1 and Ea=−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.