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 (ε = 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, ϕ_PL = 0.048, and an excited state lifetime, τ_ο = 440 ns, from which radiative and nonradiative relaxation rate constants were extracted, k_r = 9.1 × 10⁴ s^-1 and k_nr = 1.8 × 10⁶ 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 TBAPF₆ electrolyte revealed a quasi-reversible oxidation, E°(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°(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-d₆. 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 × 10¹⁰ M^-1 s^-1 and an equilibrium constant, K_eq, of 8.3 × 10³ M^-1 were obtained. In acetone, the static quenching pathway by iodide was greatly enhanced, with a K_eq of 1.2 × 10⁴ M^-1 and a higher k_q of 9.2 × 10¹⁰ M^-1 s^-1.