Yellow-light sensitization of a ligand photosubstitution reaction in a ruthenium polypyridyl complex covalently bound to a rhodamine dye.

The ruthenium complex [Ru(terpy)(bpy)(Hmte)](2+) ([1](2+)), where terpy is 2,2';6',2''-terpyridine, bpy is 2,2'-bipyridine, and Hmte is 2-methylthioethan-1-ol, poorly absorbs yellow light, and although its quantum yield for the photosubstitution of Hmte by water is comparable at 570 nm and at 452 nm (0.011(4) vs. 0.016(4) at 298 K at neutral pH), the photoreaction using yellow photons is very slow. Complex [1](2+) was thus functionalized with rhodamine B, an organic dye known for its high extinction coefficient for yellow light. Complex [Ru(Rterpy)(bpy)(Hmte)](3+) ([2](3+)) was synthesized, where Rterpy is a terpyridine ligand covalently bound to rhodamine B via a short saturated linker. [2]Cl3 shows a very high extinction coefficient at 570 nm (44,000 M(-1) cm(-1)), but its luminescence upon irradiation at 570 nm is completely quenched in aqueous solution. The quantum yield for the photosubstitution of Hmte by water in [2](3+) was comparable to that in [1](2+) at 570 nm (0.0085(6) vs. 0.011(4), respectively), which, in combination with the much better photon collection, resulted in a higher photosubstitution rate constant for [2](3+) than for [1](2+). The energy of yellow photons is thus transferred efficiently from the rhodamine antenna to the ruthenium center, leading to efficient photosubstitution of Hmte. These results bring new opportunities for extending the photoactivation of polypyridyl ruthenium complexes towards longer wavelengths.