Reductive Dynamic and Static Excited State Quenching of a Homoleptic Ruthenium Complex Bearing Aldehyde Groups.

A new homoleptic Ru polypyridyl complex bearing two aldehyde groups on each bipyridine ligand, [Ru(dab)(3)](PF6)(2), where dab is 4,4 '-dicarbaldehyde-2,2 '-bipyridine, was synthesized, characterized, and utilized for iodide photo-oxidation studies. In acetonitrile (CH3CN) solution, the complex displayed an intense metal-to-ligand charge transfer (MLCT) absorbance maximum at 475 nm (epsilon = 22,000 M-1 cm(-1)) and an infrared (IR) band at 1712 cm(-1) assigned to the pendent aldehyde groups. Visible light excitation in air-saturated solution resulted in room temperature photoluminescence (PL) with a maximum at 675 nm, a quantum yield, phi(PL) = 0.048, and an excited state lifetime, tau(omicron) = 440 ns, from which radiative and nonradiative relaxation rate constants were extracted, k(r) = 9.1 x 10(4) s(-1) and k(nr) = 1.8 x 10(6) s(-1). Pulsed visible light excitation yielded transient UV-vis and IR absorption spectra consistent with an MLCT excited state; relaxation occurred with the maintenance of two isosbestic points in the visible region, and a lifetime that agreed with that measured by time-resolved PL. Cyclic voltammetry studies in a CH3CN solution with 0.1 M TBAPF(6) electrolyte revealed a quasi-reversible oxidation, E degrees(Ru-III/II) = +1.25 V vs. Fc(+/0), and three sequential one-electron reductions at -1.10, -1.25, and -1.54 V vs. Fc(+/0). An excited state reduction potential of E degrees(Ru*(2+/+)) = +0.89 V vs. Fc(+/0) was estimated with the Rehm-Weller expression. Titration of tetrabutylammonium iodide, TBAI, into a CD3CN solution of [Ru(dab)(3)](PF6)(2) resulted in significant shifts in the aldehyde H atom and 3,3 '-biypridyl resonances that were analyzed with a 1:1 equilibrium model, from which K-eq = 460 M-1 was extracted, increasing to 5800 M-1 when the solvent was changed to acetone-d(6). Iodide titrations resulted in a significant quenching of the [Ru(dab)(3)]*(2+) lifetime and quantum yield in both CH3CN and acetone solvents. In CH3CN, the quenching was mainly dynamic and well described by the Stern-Volmer model, from which a quenching rate constant, k(q), of 4.5 x 10(10) M-1 s(-1) and an equilibrium constant, K-eq, of 8.3 x 10(3) M-1 were obtained. In acetone, the static quenching pathway by iodide was greatly enhanced, with a K-eq of 1.2 x 10(4) M-1 and a higher k(q) of 9.2 x 10(10) M-1 s(-1).