Temperature dependent iodide oxidation by MLCT excited states

Abstract

The metal-to-ligand charge transfer (MLCT) excited states of two related heteroleptic Ru(ii) compounds [Ru(bpy)2(deeb)](2+) and [Ru(bpy)2(deebq)](2+), where bpy is 2,2'-bipyridine, deeb is 4,4'-(CO2CH2CH3)2-2,2'-bipyridine and deebq is 4,4'-(CO2CH2CH3)2-2,2'-biquinoline, were characterized in fluid acetonitrile by temperature dependent photoluminescence spectroscopies as well as quenching by iodide ions. Photoluminescence emanates from a manifold of thermally equilibrated excited states referred to as the thexi states. Evidence for activated internal conversion to a 4(th) MLCT excited state was garnered from an Arrhenius analysis of temperature dependent lifetime data. The activation energy was found to be 550 cm(-1) for [Ru(bpy)2(deeb)](2+)* and 1200 cm(-1) for [Ru(bpy)2(deebq)](2+)*. The pre-exponential factor abstracted from the Arrhenius analysis of the [Ru(bpy)2(deebq)](2+)* data suggested that ligand field excited states might be populated, however there was no evidence for ligand loss photochemistry under the conditions studied. The excited states were found to quench iodide by a dynamic process in good agreement with the Stern-Volmer model. Transient absorption data showed that the quenching mechanism was electron transfer to generate an iodine atom and a reduced ruthenium compound as products. The quenching rate constants abstracted from temperature dependent Stern-Volmer quenching data were corrected for diffusion and activated complex formation to yield electron transfer rate constants that were found to increase markedly with temperature. An Arrhenius analysis of the electron transfer data revealed that electron transfer from iodide to the d-orbitals of the excited state was an activated process with an Ea of 2400 cm(-1) for [Ru(bpy)2(deeb)](2+) and 3300 cm(-1) for [Ru(bpy)2(deebq)](2+).

Electric Fields Detected on Dye-Sensitized TiO2 Interfaces: Influence of Electrolyte Composition and Ruthenium Polypyridyl Anchoring Group Type

“…In particular, they found that this enhanced delocalization increased the charge-transfer distance and therefore increased Δμ . Some evidence for delocalization over diethyl ester functional groups in the 4,4′-position has also been reported for [Ru­(bpy) 2 (deeb)] 2+ , where deeb = 4,4′-(CO 2 Et) 2 -2,2′-bipyridine . To investigate whether a similar enhancement of excited-state delocalization occurs to a greater extent for RuLP or RuLC, excited-state properties were determined for both complexes in acetonitrile solutions (Table ).…”

“…41 Some evidence for delocalization over diethyl ester functional groups in the 4,4′-position has also been reported for [Ru(bpy) 2 (deeb)] 2+ , where deeb = 4,4′-(CO 2 Et) 2 -2,2′-bipyridine. 42 To investigate whether a similar enhancement of excited-state delocalization occurs to a greater extent for RuLP or RuLC, excited-state properties were determined for both complexes in acetonitrile solutions (Table 2). Comparison of the photophysical properties of RuLP and RuLC did not reveal anomalously different radiative rate constants or lifetimes for either sensitizer molecule.…”

Bromide Photo-oxidation Sensitized to Visible Light in Consecutive Ion Pairs

“…Hence in a standard quenching experiment, bromide first quenched, then enhanced, and then had no further influence on the excited state. This behavior is highly unusual particularly since it has been reported that both bromide and iodide typically quench this and related excited states in a manner expected and compliant with the Stern–Volmer model. ,− For bromide oxidation a strong photo-oxidant is necessary, and replacing the bpz ligands with 2,2′-bipyridine in [Ru­(deeb)­(bpz) 2 ] 2+ resulted in an excited state that did not oxidize bromide . To better understand the ion-pairing behavior, 1 H NMR and computational methods were utilized to quantify the detailed nature of the ground-state ion pairs and the influence on excited-state electron transfer as described below.…”

Unique Optical Oxygen-Sensing Performance of [Ru(IP)2(HNAIP)]2+ during the Groove-Binding-Induced B-to-Z DNA Conformational Transition

“…On the time scale of photoluminescence measurement, the emission quenching of Ru1 and Ru2 by oxygen in the absence and presence of DNA conforms to the Stern−Volmer equation. 55 However, as depicted by Figure 1, Ru1 can bind to the groove surface of DNA to induce the B-to-Z conformation transition, resulting in the Z-DNA stabilized by Ru1. Because the Z-DNA is quite different from the B-DNA, in which B-DNA has major and minor grooves with widths of 1.17 and 0.57 nm, and Z-DNA only shows basically consistent minor grooves with B-DNA, 56 the oxygen binding rates across the two kinds of conformational DNA would be different.…”