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Structural and pH dependence of excited state PCET reactions involving reductive quenching of the MLCT excited state of [RuII(bpy)2(bpz)]2+ by hydroquinones.

Publication ,  Journal Article
Lebedeva, NV; Schmidt, RD; Concepcion, JJ; Brennaman, MK; Stanton, IN; Therien, MJ; Meyer, TJ; Forbes, MDE
Published in: The journal of physical chemistry. A
April 2011

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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Published In

The journal of physical chemistry. A

DOI

EISSN

1520-5215

ISSN

1089-5639

Publication Date

April 2011

Volume

115

Issue

15

Start / End Page

3346 / 3356

Related Subject Headings

  • Stereoisomerism
  • Ruthenium
  • Quantum Theory
  • Oxidation-Reduction
  • Organometallic Compounds
  • Molecular Structure
  • Molecular Conformation
  • Hydroquinones
  • Hydrogen-Ion Concentration
  • 5102 Atomic, molecular and optical physics
 

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Lebedeva, N. V., Schmidt, R. D., Concepcion, J. J., Brennaman, M. K., Stanton, I. N., Therien, M. J., … Forbes, M. D. E. (2011). Structural and pH dependence of excited state PCET reactions involving reductive quenching of the MLCT excited state of [RuII(bpy)2(bpz)]2+ by hydroquinones. The Journal of Physical Chemistry. A, 115(15), 3346–3356. https://doi.org/10.1021/jp200381n
Lebedeva, Natalia V., Robert D. Schmidt, Javier J. Concepcion, M Kyle Brennaman, Ian N. Stanton, Michael J. Therien, Thomas J. Meyer, and Malcolm D. E. Forbes. “Structural and pH dependence of excited state PCET reactions involving reductive quenching of the MLCT excited state of [RuII(bpy)2(bpz)]2+ by hydroquinones.The Journal of Physical Chemistry. A 115, no. 15 (April 2011): 3346–56. https://doi.org/10.1021/jp200381n.
Lebedeva NV, Schmidt RD, Concepcion JJ, Brennaman MK, Stanton IN, Therien MJ, et al. Structural and pH dependence of excited state PCET reactions involving reductive quenching of the MLCT excited state of [RuII(bpy)2(bpz)]2+ by hydroquinones. The journal of physical chemistry A. 2011 Apr;115(15):3346–56.
Lebedeva, Natalia V., et al. “Structural and pH dependence of excited state PCET reactions involving reductive quenching of the MLCT excited state of [RuII(bpy)2(bpz)]2+ by hydroquinones.The Journal of Physical Chemistry. A, vol. 115, no. 15, Apr. 2011, pp. 3346–56. Epmc, doi:10.1021/jp200381n.
Lebedeva NV, Schmidt RD, Concepcion JJ, Brennaman MK, Stanton IN, Therien MJ, Meyer TJ, Forbes MDE. Structural and pH dependence of excited state PCET reactions involving reductive quenching of the MLCT excited state of [RuII(bpy)2(bpz)]2+ by hydroquinones. The journal of physical chemistry A. 2011 Apr;115(15):3346–3356.
Journal cover image

Published In

The journal of physical chemistry. A

DOI

EISSN

1520-5215

ISSN

1089-5639

Publication Date

April 2011

Volume

115

Issue

15

Start / End Page

3346 / 3356

Related Subject Headings

  • Stereoisomerism
  • Ruthenium
  • Quantum Theory
  • Oxidation-Reduction
  • Organometallic Compounds
  • Molecular Structure
  • Molecular Conformation
  • Hydroquinones
  • Hydrogen-Ion Concentration
  • 5102 Atomic, molecular and optical physics