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

A new mixed-ligand polypyridylruthenium(II) complex, [Ru(bpy)(2)L]Cl-2, 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 (lambda(em) = 637 nm, tau = 846 +/- 11 ns, phi = 0.046 at 295 K) whose decay involves an activated crossing to higher energy ligand field states (E(a) = 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 (lambda(em) = 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 tau = 54 +/- 1 ns (lambda(em) = 686 nm, phi(em) = 0.0036) at 280 K to tau = 75 +/- 1 ns (lambda(em) = 675 nm, phi(em) = 0.0053) at 360 K. The data are fit to the Arrhenius expression to give E(a) = -270 +/- 15 cm(-1) in H2O and E(a) = -178 +/- 10 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.