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Issue 3, 2010
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Vibrational coherence in the excited state dynamics of Cr(acac)3: probing the reaction coordinate for ultrafast intersystem crossing

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Abstract

Vibrational coherence was observed following excitation into the lowest-energy spin-allowed 4A24T2 ligand-field absorption of Cr(acac)3. The transient kinetics were fit to a rapidly damped 164 cm−1 oscillatory component, the frequency of which is not associated with the ground state of the molecule. The signal is assigned as an excited-state vibrational coherence; the timescale of the event suggests that this vibrational coherence is retained during the 4T22E intersystem crossing that immediately follows 4A24T2 excitation. DFT calculations indicate that the 164 cm−1 oscillation likely corresponds to a combination of Cr–O bond stretching in the ligand-field excited state as well as large amplitude motion of the ligand backbone. This hypothesis is supported by ultrafast time-resolved absorption measurements on Cr(t-Bu-acac)3 (where t-Bu-acac is the monoanionic form of 2,2,6,6-tetramethyl-3,5-heptanedione) – an electronically similar but more sterically encumbered molecule – which exhibits a 4T22E conversion that is more than an order of magnitude slower than that observed for Cr(acac)3. These results provide important insights into the nature of the reaction coordinate that underlies ultrafast excited-state evolution in this prototypical coordination complex.

Graphical abstract: Vibrational coherence in the excited state dynamics of Cr(acac)3: probing the reaction coordinate for ultrafast intersystem crossing

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Publication details

The article was received on 12 Apr 2010, accepted on 29 May 2010 and first published on 25 Jun 2010


Article type: Edge Article
DOI: 10.1039/C0SC00262C
Citation: Chem. Sci., 2010,1, 405-410
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    Vibrational coherence in the excited state dynamics of Cr(acac)3: probing the reaction coordinate for ultrafast intersystem crossing

    J. N. Schrauben, K. L. Dillman, W. F. Beck and J. K. McCusker, Chem. Sci., 2010, 1, 405
    DOI: 10.1039/C0SC00262C

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