A Negative Activation Energy for Luminescence Decay: Specific Solvation Effects on the Emission Properties of Bis(2,2‘-bipyridine)(3,5-dicarboxy-2,2‘-bipyridine)ruthenium(II) Chloride

Abstract

A new mixed-ligand polypyridylruthenium(II) complex, [Ru(bpy)2L]Cl2, has been prepared where bpy = 2,2‘-bipyridine and L = 3,5-dicarboxy-2,2‘-bipyridine. The ligand L is a non-symmetrically-substituted 2,2‘-bipyridine having two hydrophilic carboxylate groups located at the 3- and 5-positions of only one of its two pyridyl rings. In acetonitrile, the photophysical properties of the metal complex include a long-lived excited state (λem = 637 nm, τ = 846 ± 11 ns, φ = 0.046 at 295 K) whose decay involves an activated crossing to higher energy ligand field states (Ea = 4170 ± 200 cm-1). This behavior is similar to that observed for other ruthenium tris(bipyridyl) compounds. In contrast, the title compound displays several unusual photophysical properties in aqueous solution. These include a strongly red-shifted emission (λem = 685 nm) having a short, pH-dependent lifetime which is quenched by an excited-state proton transfer from solvent. The completely deprotonated form of the molecule is the dominant emissive species. Surprisingly, under neutral conditions the excited-state lifetime increases with increasing temperature, from a value of τ = 54 ± 1 ns (λem = 686 nm, φem = 0.0036) at 280 K to τ = 75 ± 1 ns (λem = 675 nm, φem = 0.0053) at 360 K. The data are fit to the Arrhenius expression to give Ea = −270 ± 15 cm-1 in H2O and Ea = −178 ± 10 cm-1 in D2O. Thermochromic emission data and temperature-induced energy-gap law behavior indicate that the unique photophysical properties of this compound are due to specific interactions involving protic solvent.