QUENCHING OF THE EXCITED-STATES OF RUTHENIUM(II)-DIIMINE COMPLEXES BYOXYGEN

The quenching by O-2 of the MLCT excited states of 10 Ru(II)-diimine c omplexes (RuL(2+)) of the form Ru(bpy)(3-m-z)(bpm)(m)(bpz)(z)(2+) (bpy = 2,2'-bipyridine, bpm = 2,2'-bipyrimidine, bpz = 2,2'-bipyrazine, m and z = 0, 1, 2, 3 and m + z less than or equal to 3) in aqueous solut ion has been investigated using the techniques of laser flash photolys is and time-resolved and steady-state near-infrared emission spectroph otometry. Values of k(q), have been determined in H2O and D2O and rang e between 5.1 x 10(8) and 3.4 X 10(9) M(-1) s(-1). The quantum yields (Phi(Delta)) of singlet molecular oxygen, O-2((1) Delta(g)), originati ng from the energy-transfer quenching of RuL(2+) by O-2 and determine d in D2O by comparison with Phi(Delta) from the excited state of tetra kis(4-sulfonatophenyl)porphine (TPPS4-), are 0.5 for Ru(bpy)(3)(2+) an d similar to 1 for all the other complexes. It has been established th at for Ru(bpy)(3)(2+) and, presumably, all the other complexes the yie ld of electron-transfer products, RuL(3+) and O-2(.-), in bulk solutio n, is negligibly small. The lower value of Phi(Delta) for Ru(bpy)(3)(2 +) is attributed to competitive charge-transfer quenching, followed by efficient back electron transfer within the solvent cage. The values of the rate constants of energy transfer for the six complexes that co ntain at least one bpz ligand decrease with increasing driving force.