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)₃Cl} 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^I-based triplet metal-to-ligand charge-transfer (³MLCT) luminescence and consequent sensitization of the Ru^II-based ³MLCT 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 ³MLCT 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 RuLRe, which have different Re-Ru separations.