Valence Tautomerism in Co–Dioxolene Complexes:
Static and Time-Resolved Infrared Spectroscopy Study
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Abstract
In this work, we studied the valence
tautomerism process on two
different Co–dioxolene complexes by means of transient infrared
spectroscopy (TRIR). The molecules investigated are <i>ls</i>-Co<sup>III</sup>(Cat-N-BQ)(Cat-N-SQ) (<b>DQ</b><sub><b>2</b></sub>) and [<i>ls</i>-Co<sup>III</sup>(tpy)(Cat-N-SQ)]PF<sub>6</sub> (<b>tpy</b>), where Cat-NBQ = 2-(2-hydroxy-3,5-ditert-butylphenyl-imino)-4,6-ditert-butylcyclohexa-3,5-dienone,
Cat-N-SQ is the dianionic radical analogue, and tpy = 2,2′-6-2″-terpyridine.
DFT calculations of the harmonic frequencies for the two complexes
allow us to pinpoint the normal modes to be used as markers of the
semiquinonate and benzoquinonate isomers. The photoinduced one-electron
charge transfer process from the radical semiquinonate ligand to the
metal center leads to a <i>ls</i>-Co<sup>II</sup>(<i>x</i>)(Cat-N-BQ) electronic state (where <i>x</i> is
the other ligand). Following this first step, an ultrafast ISC process
(τ < 200 fs) takes places, yielding the benzoquinonate isomer
(<i>hs</i>-Co<sup>II</sup>(<i>x</i>)(Cat-N-BQ)).
In the experiments, we employed different excitation wavelengths on
resonance with different absorption bands of the two samples. Excitation
in the ligand-to-metal charge transfer (LMCT) band at ∼520
nm and in the semiquinonate band at ∼1000 nm induces the valence
tautomerism (VT) in both samples. From the time evolution of the TRIR
spectra, we determine the time constants of the vibrational cooling
in the tautomeric state (7–14 ps) and the ground state recovery
times (∼350 ps for <b>tpy</b> and ∼450 ps for <b>DQ</b><sub><b>2</b></sub>). In contrast, when the pump frequency
is set at 712 nm, on resonance with the benzoquinonate absorption
band of the second active ligand of the <b>DQ</b><sub><b>2</b></sub>, no electron transfer takes place: the TRIR spectra
basically show only ground state bleaching bands and no marker band
of the tautomeric conversion shows up