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Ultrafast nonadiabatic dynamics of [FeII(bpy)3] 2+ in solution

Gawelda, Wojciech ; Cannizzo, Andrea ; Van Mourik, Frank ; Bressler, Christian ; Chergui, Majed and Pham, Van-Thai LU (2007) In Journal of the American Chemical Society 129(26). p.8199-8206
Abstract

The ultrafast relaxation of aqueous iron(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band ( 1MLCT) has been characterized by femtosecond fluorescence up-conversion and transient absorption (TA) studies. The fluorescence experiment shows a very short-lived broad 1MLCT emission band at ∼600 nm, which decays in ≤20 fs, and a weak emission at ∼660 nm, which we attribute to the 3MLCT, populated by intersystem crossing (ISC) from the 1MLCT state. The TA studies show a short-lived (<150 fs) excited-state absorption (ESA) below 400 nm, and a longer-lived one above 550 nm, along with the ground-state bleach (GSB). We identify the short-lived ESA as being due... (More)

The ultrafast relaxation of aqueous iron(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band ( 1MLCT) has been characterized by femtosecond fluorescence up-conversion and transient absorption (TA) studies. The fluorescence experiment shows a very short-lived broad 1MLCT emission band at ∼600 nm, which decays in ≤20 fs, and a weak emission at ∼660 nm, which we attribute to the 3MLCT, populated by intersystem crossing (ISC) from the 1MLCT state. The TA studies show a short-lived (<150 fs) excited-state absorption (ESA) below 400 nm, and a longer-lived one above 550 nm, along with the ground-state bleach (GSB). We identify the short-lived ESA as being due to the 3MLCT state. The long-lived ESA decay and the GSB recovery occur on the time scale of the lowest excited high-spin quintet state 5T2 lifetime. A singular value decomposition and a global analysis of the TA data, based on a sequential relaxation model, reveal three characteristic time scales: 120 fs, 960 fs, and 665 ps. The first is the decay of the 3MLCT, the second is identified as the population time of the 5T2 state, while the third is its decay time to the ground state. The anomalously high ISC rate is identical in [RuII(bpy) 3]2+ and is therefore independent of the spin-orbit constant of the metal atom. To reconcile these rates with the regular quasi-harmonic vibrational progression of the 1MLCT absorption, we propose a simple model of avoided crossings between singlet and triplet potential curves, induced by the strong spin-orbit interaction. The subsequent relaxation steps down to the 5T2 state dissipate ∼2000 cm-1/100 fs. This rate is discussed, and we conclude that it nevertheless can be described by the Fermi golden rule, despite its high value.

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publication status
published
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in
Journal of the American Chemical Society
volume
129
issue
26
pages
8 pages
publisher
The American Chemical Society (ACS)
external identifiers
  • scopus:34447134304
ISSN
0002-7863
DOI
10.1021/ja070454x
language
English
LU publication?
no
id
d6088d6a-f39d-4396-8d8c-bbd97b861c1c
date added to LUP
2019-06-30 10:02:33
date last changed
2022-04-02 19:49:27
@article{d6088d6a-f39d-4396-8d8c-bbd97b861c1c,
  abstract     = {{<p>The ultrafast relaxation of aqueous iron(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band ( <sup>1</sup>MLCT) has been characterized by femtosecond fluorescence up-conversion and transient absorption (TA) studies. The fluorescence experiment shows a very short-lived broad <sup>1</sup>MLCT emission band at ∼600 nm, which decays in ≤20 fs, and a weak emission at ∼660 nm, which we attribute to the <sup>3</sup>MLCT, populated by intersystem crossing (ISC) from the <sup>1</sup>MLCT state. The TA studies show a short-lived (&lt;150 fs) excited-state absorption (ESA) below 400 nm, and a longer-lived one above 550 nm, along with the ground-state bleach (GSB). We identify the short-lived ESA as being due to the <sup>3</sup>MLCT state. The long-lived ESA decay and the GSB recovery occur on the time scale of the lowest excited high-spin quintet state <sup>5</sup>T<sub>2</sub> lifetime. A singular value decomposition and a global analysis of the TA data, based on a sequential relaxation model, reveal three characteristic time scales: 120 fs, 960 fs, and 665 ps. The first is the decay of the <sup>3</sup>MLCT, the second is identified as the population time of the <sup>5</sup>T<sub>2</sub> state, while the third is its decay time to the ground state. The anomalously high ISC rate is identical in [Ru<sup>II</sup>(bpy) <sub>3</sub>]<sup>2+</sup> and is therefore independent of the spin-orbit constant of the metal atom. To reconcile these rates with the regular quasi-harmonic vibrational progression of the <sup>1</sup>MLCT absorption, we propose a simple model of avoided crossings between singlet and triplet potential curves, induced by the strong spin-orbit interaction. The subsequent relaxation steps down to the <sup>5</sup>T<sub>2</sub> state dissipate ∼2000 cm<sup>-1</sup>/100 fs. This rate is discussed, and we conclude that it nevertheless can be described by the Fermi golden rule, despite its high value.</p>}},
  author       = {{Gawelda, Wojciech and Cannizzo, Andrea and Van Mourik, Frank and Bressler, Christian and Chergui, Majed and Pham, Van-Thai}},
  issn         = {{0002-7863}},
  language     = {{eng}},
  month        = {{07}},
  number       = {{26}},
  pages        = {{8199--8206}},
  publisher    = {{The American Chemical Society (ACS)}},
  series       = {{Journal of the American Chemical Society}},
  title        = {{Ultrafast nonadiabatic dynamics of [Fe<sup>II</sup>(bpy)<sub>3</sub>] <sup>2+</sup> in solution}},
  url          = {{http://dx.doi.org/10.1021/ja070454x}},
  doi          = {{10.1021/ja070454x}},
  volume       = {{129}},
  year         = {{2007}},
}