Luminescence and Time-Resolved Infrared Study of Dyads Containing (Diimine)Ru(4,4′-diethylamido-2,2′-bipyridine)2 and (Diimine)Ru(CN)4 Moieties: Solvent-Induced Reversal of the Direction of Photoinduced Energy-Transfer

The exploitation of the dramatic negative solvatochromism of the [Ru(bipy)(CN)(4)] moiety (bipy = 2,2'-bipyridine) allows a change in solvent to reverse the direction of photoinduced energy transfer (PEnT) in two related dinuclear complexes. Both dyads consist of a [Ru(bpyam)(2)L-n](2+) (Ru-bpyam) unit (bpyam = 4,4'-diethylamido-2,2'-bipyridine; L = bis-bipyridyl-based bridging ligand) and a [Ru(L-n(CN)(4)](2-) (Ru-CN) unit. Both termini have IR-active spectroscopic handles (amide carbonyl or cyanide, respectively) allowing the excited-state dynamics to be studied by time-resolved IR (TRIR) spectroscopy. One dyad (1) contains a relatively rigid exoditopic macrocyclic bis-bipyridyl bridging ligand (L-1) and the other (2) contains a more flexible bis-bipyridyl bridging ligand with only one covalent linkage between the two bipyridyl binding sites (L-2). The conformational effects on PEnT rates in these dyads are probed using a combination of luminescence and TRIR studies. In both 1 and 2 in D2O it is demonstrated that Ru-CN -> Ru-bpyam PEnT occurs (PEnT time scales were in the range 10 ps-3 ns) because the (MLCT)-M-3 energy of the Ru-CN terminus is higher than that of the Ru-bpyam terminus. Changing the solvent from D2O to CH3CN results in lowering the (MLCT)-M-3 energy of the Ru-CN unit below that of the Ru-bpyam unit such that in both dyads a reversal in the direction of PEnT to Ru-bpyam -> Ru-CN (time scales of 10 ps-2 ns) occurs, Complex kinetic behavior results from the presence of a dark (MLCT)-M-3 excited state formulated as {(bpyam)(2)RU3+ (Ln circle-)} and by the presence of multiple conformers in solution which have different Ru...Ru separations giving rise to different energy transfer rates.