A closer look at photochemical reactions of transition-metal compounds by time-resolved EPR

Martino, Debora Marcela; Kleverlaan, Cornelis J.; Slageren, Joris van; Bussandri, Alejandro; Willigen, H. van; Kiarie, C.

doi:10.1007/bf03166579

Cited by 2 publications

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“…Our TA experiments reported in 2007 clearly indicate the formation of two radicals from this photochemistry, however, only one is observed by TREPR. This is an unusual situation, and for such TREPR-detected photochemical reactions involving a Ru complex, there is only one other system reported in the literature, where alkyl radicals were observed after a direct bond cleavage reaction from Re−C or Ru−C bonds. (3) The PCET reaction in Scheme represents a new method for the formation of organic free radicals that deserves further mechanistic scrutiny.…”

Section: Introductionmentioning

confidence: 99%

Structural and pH Dependence of Excited State PCET Reactions Involving Reductive Quenching of the MLCT Excited State of [Ru^II(bpy)₂(bpz)]²⁺ by Hydroquinones

et al. 2011

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The proton-coupled electron transfer (PCET) reaction between the bpz-based photoexcited (3)MLCT state of [Ru(II)(bpy)(2)(bpz)](2+) (bpy = 2,2'-bipyridine, bpz = 2,2'-bipyrazine) and a series of substituted hydroquinones (H(2)Q) has been studied by transient absorption (TA) and time-resolved electron paramagnetic resonance (TREPR) spectroscopy at X-band. When the reaction is carried out in a CH(3)CN/H(2)O mixed solvent system with unsubstituted hydroquinone, the neutral semiquinone radical (4a) and its conjugate base, the semiquinone radical anion (4b), are both observed. Variation of the acid strength in the solvent mixture allows the acid/base dependence of the PCET reaction to be investigated. In solutions with very low acid concentrations, TREPR spectra exclusively derived from radical anion 4b are observed, while at very high acid concentrations, the spectrum is assigned to the protonated structure 4a. At intermediate acid concentrations, either a superposition of spectra is observed (slow exchange between 4a and 4b) or substantial broadening in the TREPR spectrum is observed (fast exchange between 4a and 4b). Variation of substituents on the H(2)Q ring substantially alter this acid/base dependence and provide a means to investigate electronic effects on both the ET and PT components of the PCET process. The TA results suggest a change in mechanism from PCET to direct ET quenching in strongly basic solutions and with electron withdrawing groups on the H(2)Q ring system. Changing the ligand on the Ru complex also alters the acid/base dependence of the TREPR spectra through a series of complex equilibria between protonated and deprotonated hydroquinone radicals and anions. The relative intensities of the signals from radical 4a versus 4b can be rationalized quantitatively in terms of these equilibria and the relevant pK(a) values. An observed equilibrium deuterium isotope effect supports the conclusion that the post-PCET HQ(•)/Q(•-) equilibrium is the most important in determining the 4a/4b ratio at early delay times.

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