Abstract
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.
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