Photoacidic and Photobasic Behavior of Transition Metal Compounds with Carboxylic Acid Group(s)

Abstract

Excited state proton transfer studies of six Ru polypyridyl compounds with carboxylic acid/carboxylate group(s) revealed that some were photoacids and some were photobases. The compounds [Ru(II)(btfmb)2(LL)](2+), [Ru(II)(dtb)2(LL)](2+), and [Ru(II)(bpy)2(LL)](2+), where bpy is 2,2'-bipyridine, btfmb is 4,4'-(CF3)2-bpy, and dtb is 4,4'-((CH3)3C)2-bpy, and LL is either dcb = 4,4'-(CO2H)2-bpy or mcb = 4-(CO2H),4'-(CO2Et)-2,2'-bpy, were synthesized and characterized. The compounds exhibited intense metal-to-ligand charge-transfer (MLCT) absorption bands in the visible region and room temperature photoluminescence (PL) with long τ > 100 ns excited state lifetimes. The mcb compounds had very similar ground state pKa's of 2.31 ± 0.07, and their characterization enabled accurate determination of the two pKa values for the commonly utilized dcb ligand, pKa1 = 2.1 ± 0.1 and pKa2 = 3.0 ± 0.2. Compounds with the btfmb ligand were photoacidic, and the other compounds were photobasic. Transient absorption spectra indicated that btfmb compounds displayed a [Ru(III)(btfmb(-))L2](2+)* localized excited state and a [Ru(III)(dcb(-))L2](2+)* formulation for all the other excited states. Time dependent PL spectral shifts provided the first kinetic data for excited state proton transfer in a transition metal compound. PL titrations, thermochemical cycles, and kinetic analysis (for the mcb compounds) provided self-consistent pKa* values. The ability to make a single ionizable group photobasic or photoacidic through ligand design was unprecedented and was understood based on the orientation of the lowest-lying MLCT excited state dipole relative to the ligand that contained the carboxylic acid group(s).

Kinetics teach that electronic coupling lowers the free-energy change that accompanies electron transfer

“…Under all conditions, the transient data fully recovered to initial values within 10 ms with no evidence of net photochemistry. The kinetic model utilized has previously been reported for excited-state acid base equilibria (22,23) and was constrained here with kinetic data from a model compound, that did not contain the pendant TPA group, which accounted for the nonexponential nature of the interfacial back-electron-transfer reaction (SI Appendix). The insets show the classical Arrhenius analysis of the k 1 and k −1 values extracted from the kinetic data.…”

“…It is recognized that this light-initiated reaction technically yields a "quasi-equilibrium" since true equilibrium is achieved only when the injected electrons recombine with the oxidized compound. Nevertheless, related photochemical strategies have been widely utilized in fluid solution to characterize excited-state "equilibrium" reactions, most notably for the determination of excited-state pK a * values of photoacids and photobases in aqueous solutions (22,23). Consequently, this kinetic approach is expected to be of general utility for characterization of free-energy changes that accompany electron transfer in chemistry and biology.…”

Ruthenium(II)–Pyridylimidazole Complexes as Photoreductants and PCET Reagents

“…The resulting p K a * values were verified by pH titration monitoring steady‐state emission as shown in the Supporting Information (Figures S8–S9). Because the π*‐orbitals of the bpy ligands are energetically lower lying than the π*‐orbitals of the pyimH ligand, electron density is withdrawn from the acidic N–H functionality, and its acidity is increased in the emissive 3 MLCT states of all four complexes (Table ) analogous to other photoacids , . Excited‐state acid‐base equilibration takes place in the pH range between p K a and p K a *, where the excited [Ru R pyimH] 2+ complexes are deprotonated by solvent or buffer molecules.…”

Organocatalytic Approach to Functional Semifluorinated Polymers Driven by Visible Light

“…CV tests were also carried out to reveal the positive effect of reducing agents and bases on our polymerization strategy, and the detailed results were presented in Figure S7c,d of the Supporting Information. Based on the detailed analysis of the results, we reasoned simple bases participated in photoredox catalysis indirectly, which regulated the pH values of the system and the corresponding redox potential ( E 1/2 *) of excited PCs* . Although not all bases were efficient for photoredox catalysis system like reducing agents, simple bases are quite suitable for bulk production without potential explosion risk brought by the utilization of large amount of reducing agents.…”