title: 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.
abstract: 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(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 (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 D(2)O it is demonstrated that Ru-CN --> Ru-bpyam PEnT occurs (PEnT time scales were in the range 10 ps-3 ns) because the (3)MLCT energy of the Ru-CN terminus is higher than that of the Ru-bpyam terminus. Changing the solvent from D(2)O to CH(3)CN results in lowering the (3)MLCT 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 (3)MLCT excited state formulated as {(bpyam)(2)Ru(3+)(L(n*-))} and by the presence of multiple conformers in solution which have different Ru...Ru separations giving rise to different energy transfer rates.
authors.affiliation: Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK.
authors.forename: Timothy L
authors.initials: TL
authors.lastname: Easun
authors.forename: Wassim Z
authors.initials: WZ
authors.lastname: Alsindi
authors.forename: Nina
authors.initials: N
authors.lastname: Deppermann
authors.forename: Michael
authors.initials: M
authors.lastname: Towrie
authors.forename: Kate L
authors.initials: KL
authors.lastname: Ronayne
authors.forename: Xue-Zhong
authors.initials: XZ
authors.lastname: Sun
authors.forename: Michael D
authors.initials: MD
authors.lastname: Ward
authors.forename: Michael W
authors.initials: MW
authors.lastname: George