Flash-Quench Studies on the One-Electron Reduction of Triiodide

Abstract

The one-electron reduction of triiodide (I(3)(-)) by a series of reduced ruthenium polypyridyl compounds was studied in an acetonitrile solution at room temperature using the flash-quench technique. Reductive quenching of the metal-to-ligand charge-transfer excited state of [Ru(bpy)(2)(deeb)](2+), [Ru(deeb)(2)(bpy)](2+), or [Ru(deeb)(3)](2+), where bpy is 2,2'-bipyridine and deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine, by iodide generated the reduced ruthenium compounds and diiodide (I(2)(•-)). Charge recombination of the reduced ruthenium compounds and I(2)(•-) occurred with rate constants near the calculated diffusion limit of 2.6 × 10(10) M(-1) s(-1). The reaction of the reduced ruthenium compounds with I(3)(-) was characterized spectroscopically through the addition of I(3)(-) into the experimental solution prior to the laser flash. Transient absorption data indicated that I(2)(•-) was a reaction product of I(3)(-) reduction and appeared with an average second-order rate constant of (5.0 ± 0.6) × 10(9) M(-1) s(-1) for all three compounds. The insensitivity of the rate constants for I(3)(-) reduction over an 80 meV change in the driving force was unexpected. The relevance of these findings to solar energy conversion within dye-sensitized solar cells is discussed.

Hole injection dynamics from two structurally related Ru–bipyridine complexes into NiOx is determined by the substitution pattern of the ligands

“…21,47 On the other hand, the electrochemically reduced dye Ru2 shows a ground state absorption at 527 nm. 29 The two 25 Notably the formation of a photoinduced transient with the absorption here, which is indicative of hole injection, is not observed for Ru1 on the NiO x surface.…”

“…4D) correlates with the spectral features of excited Ru2 molecules but has also been observed in spectro-electrochemical UV/vis-absorption spectroscopy experiments on NiO x nanoparticles. 47 This finding indicates that after completion of the ultrafast photophysics a fraction of the initially photoexcited Ru2 ensemble remained in the long-lived triplet state (luminescence lifetime t lum = 2.1 ms for Ru2 in degassed ACN solution 29 ) and does not contribute to the fast photoinduced hole injection processes.…”

“…21,47 On the other hand, the electrochemically reduced dye Ru2 shows a ground state absorption at 527 nm. 29 The two In order to quantitatively analyze the photoinduced kinetics, a multiexponential model was fit globally to the transient absorption data. By judging the systematic residuals (see ESI †) a bi-exponential function including an offset to reflect any longlived component was found to be sufficient to fit the data.…”

Halide Photoredox Chemistry

“…To better understand the one electron reduction of I 3 – , flash-quench studies were performed (Figure ). , In these experiments, the MLCT excited state was reduced by iodide in solutions that contained a large excess of I 3 – . In this manner, the reduced Ru product was most likely to undergo collisional reactions with I 3 – .…”

“…The flash-quench cycle where Ru 2+ is the chromophore, I – is the electron donor, and I 3 – is the electron acceptor is indicated. Reproduced with permission from ref . Copyright 2013 American Chemical Society.…”