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)2L n ]2+ (Ru-bpyam) unit (bpyam = 4,4′-diethylamido-2,2′-bipyridine; L n = 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 (L1) and the other (2) contains a more flexible bis-bipyridyl bridging ligand with only one covalent linkage between the two bipyridyl binding sites (L2). 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 3MLCT 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 3MLCT 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 3MLCT excited state formulated as {(bpyam)2Ru3+(L n●−)} and by the presence of multiple conformers in solution which have different Ru···Ru separations giving rise to different energy transfer rates.