Reductive Quenching of the Luminescent Excited State of Tris(2,2'-bipyrazine)ruthenium(2+) Ion in Aqueous Solution

Article abstract of DOI:10.1021/j100343a044

The photodynamics of aqueous solutions containing Ru(bpz)3²⁺ (bpz=2,2'bipyrazine) and reductive quenchers (D=EDTA (ethylenediaminetetraacetic acid), C2O4^2-, TEOA (triethanolamine), ascorbate ion, and thiols (RS^-), e.g. cysteine and glutathione) in the presence and absence of MV²⁺ (methylviologen) have been evaluated by the use of time-resolved spectrofluorimetry and pulsed laser flash photolysis as a function of pH, , and 2+>; as well, we have determined Φ(MV^.+) from the continuus photolyses of these systems.Values of k_q are dependent on pH due to the acid-base properties of the quenchers and range from ca. 3E9 M^-1 s^-1 for ascorbate ion and deprotonated thiols to ca. 1E7 M^-1 s^-1 for C2O4^2-.The oxidized radicals (D_ox^.) of EDTA, C2O4^2-, and TEOA may undergo irreversible transformation into reducing radicals (D_red^.) within the quenchig solvent cage in competition with geminate-pair back electron transfer; the efficiencies of escape of the redox products (Ru(bzp)3⁺ and D_red^.) into the bulk solution (ν_ce) for these sacrifical quenchers are very high (>0.5).Thiols are semisacrifical quenchers; their oxidized radical (RS^.) must escape from the quenching cage before conversion to D_red^. (RSSR^.-)can occur; ν_ce<0.03.Ascorbate ion is a nonsacrifical quencher; its oxidized radical reacts rapidly and quantitatively with Ru(bpz)3⁺ in bulk solution.In the absence of MV²⁺, D_red^. from sacrifical and semisacrifical quenchers reacts rapidly (k>E9 M^-1 s^-1) with Ru(bzp)3²⁺, generating a second equivalent of the reduced complex.In the presence of MV²⁺, both Ru(bzp)3²⁺ and D_red^. produce MV^2.+ rapidly (k=E8-E9 M^-1 s^-1).Ru(bzp)3⁺ engages in an acid-base equilibrium (pK_a=7.1); the conjugate acid is a poorer reducing agent by ca.0.2 V than is the basic form and is unable to reduce MV²⁺.As a result, the generation of MV^.+ in acidic solution is pH- and 2+>-dependent.The values of Φ(MV^.+) from the continuous photolysis correlate very well with the rate constants and efficiencies of the various steps in the mechanism according to the following expression: Φ(MV^.+)=ν_.*ν_q*ν_ce*(ν_red+ν_red'), where ν_. is the efficiency of population of Ru(bzp)3²⁺, ν_q is the efficiency of quenching of Ru(bzp)3²⁺, and ν_red and ν_red' are the efficiencies of the reactions of Ru(bzp)3⁺ and D_red^. with MV²⁺, respectively.Values of Φ(MV^.+) as high as 1.2 have been obtained at pH 4.5 in solutions containing 0.10 M MV²⁺ and 0.26 M C2O4^2-.