Synthesis, Characterization, and Photophysical Studies
of an Iron(III) Catecholate–Nitronylnitroxide Spin-Crossover
Complex
- Publication date
- Publisher
Abstract
The
synthesis and characterization of an Fe<sup>III</sup> catecholate–nitronylnitroxide
(CAT-NN) complex (<b>1-NN</b>) that undergoes Fe<sup>III</sup> spin-crossover is described. Our aim is to determine whether the
intraligand exchange coupling of the semiquinone–nitronylnitroxide
Fe<sup>II</sup>(SQ-NN) excited state resulting from irradiation of
the CAT → Fe<sup>III</sup> LMCT band would affect either the
intrinsic photophysics or the iron spin-crossover event when compared
to the complex lacking the nitronylnitroxide radical (<b>1</b>). X-ray crystallographic analysis provides bond lengths consistent
with a ferric catecholate charge distribution. Mössbauer spectroscopy
clearly demonstrates Fe<sup>III</sup> spin-crossover, hyperfine couplings,
and a weak ferromagnetic Fe<sup>III</sup>–CAT-NN exchange,
and spin-crossover is corroborated by variable-temperature magnetic
susceptibility and electronic absorption studies. To explore the effect
of the NN radical on photophysical processes, we conducted room-temperature
transient absorption experiments. Upon excitation of the ligand-to-metal
charge transfer band, an Fe<sup>II</sup>SQ state is populated and
most likely undergoes fast intersystem crossing to the ligand field
manifold, where it rapidly decays into a metastable low-spin Fe<sup>III</sup>CAT state, followed by repopulation of the high-spin Fe<sup>III</sup>CAT ground state. The decay components of <b>1-NN</b> are slightly faster than those obtained for <b>1</b>, perhaps
due to the higher number of microstates present within the LMCT and
LF manifolds for <b>1-NN</b>. Although the effects of the NN
radical are manifest in neither the spin-crossover nor the photophysics,
our results lay the groundwork for future studies