D. Garciafresnadillo et al., SINGLET-OXYGEN ((1)DELTA(G)) PRODUCTION BY RUTHENIUM(II) COMPLEXES CONTAINING POLYAZAHETEROCYCLIC LIGANDS IN METHANOL AND IN WATER, Helvetica Chimica Acta, 79(4), 1996, pp. 1222-1238
In the context of our studies on ruthenium(II) complexes containing po
lyazaheterocyclic ligands, we have determined the rate constants of qu
enching by molecular oxygen (k(q)) of the metal-to-ligand charge-trans
fer-excited state of a series of homoleptic [RuL(3)] complexes (where
L stands for 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), 2,2'-b
ipyrazine (bpz), 4,7-diphenyl-1,10-phenanthroline (dip), diphenyl-1,10
-phennnthroline-4,7-disulfonate (dpds), and 1,10-phenanthroline-5-octa
decanamide (poda)) in H2O and in MeOH. These compounds are singlet-oxy
gen (O-2((1) Delta(g))) sensitizers, and quantum yields of singlet-oxy
gen production (Phi(Delta)) in both solvents are also reported. Values
of k(q) and Phi(Delta) depend on the nature of the ligand L and on th
e solvent, Phi(Delta), values showing a large range of variation (0.2
to 1.0). In MeOH, the only pathway for quenching of the excited [RuL(3
)] complexes by molecular oxygen is energy transfer: the fraction of q
uenched excited states yielding singlet oxygen (f(Delta)(T)) is unity
for all compounds in the series investigated. Changing from MeOH to H2
O has several remarkable effects: higher k(q) and lower Phi(Delta) val
ues are observed; f(Delta)(T) drops to ca. 0.5 except for [Ru(bpz)(3)]
(2+). In fact, [Ru(bpz)(3)](2+) is by far the weakest reductant in the
series and behaves differently from the other complexes, with lowest
k(q) and Phi(Delta) values and a f(Delta)(T) equal to 1 in both solven
ts. Results are interpreted on the basis of the role played by charge-
transfer interactions between the sensitizer excited state and molecul
ar oxygen in the quenching mechanism. Ru-II Complexes based on the 4,7
-diphenyl-1,10-phenanthroline (dip) ligand are very efficient and stab
le singlet-oxygen sensitizers with Phi(Delta) values close to unity in
air-saturated MeOH.