Photoinduced energy transfer in a conformationally flexible Re(I)/Ru(II) dyad probed by time-resolved infrared spectroscopy: Effects of conformation and spatial localization of excited states

The dyad RuLRe contains {Re(bpy)(CO)(3)Cl} and {Ru(bpy)(bpyam)(2)}(2+) termini (bpy = 2,2'-bipyridine; bpyam = 4,4'-diethylamido-2,2'-bipyridine) separated by a flexible ethylene spacer. Luminescence studies reveal the expected Re -> Ru photoinduced energy transfer, with partial quenching of Re'-based triplet metal-to-ligand charge-transfer ((MLCT)-M-3) luminescence and consequent sensitization of the Ru-II-based (MLCT)-M-3 luminescence, which has a component with a grow-in lifetime of 0.76 (+/- 0.2) ns. The presence of IR-active spectroscopic handles on both termini [CO ligands directly attached to Re-I and amide carbonyl substituents on the bpy ligands coordinated to Ru-II] allowed the excited-state dynamics to be studied by time-resolved IR (TRIR) spectroscopy in much more detail than allowed by luminescence methods. A combination of picosecond- and nanosecond-time-scale TRIR studies revealed the presence of at least three distinct Re -> Ru energy-transfer processes, with lifetimes of ca. 20 ps and 1 and 13 ns. This complex behavior occurs because of a combination of two different Ru-based (MLCT)-M-3 states (Ru -> L and Ru -> bpyam), which are sensitized by energy transfer from the Re-I donor at different rates; and the presence of at least two conformers of the flexible molecule Ru center dot center dot center dot Re, which have different Re center dot center dot center dot Ru separations.