Monitoring Fe(II) Spin–State Equilibria via Eu(III) Luminescence in Molecular Complexes: Dream or Reality?

The modulation of light emission by Fe(II) spin-crossover processes in multifunctional materials has
recently attracted major interest for the indirect and non-invasive monitoring of magnetic information
storage. In order to approach this goal at the molecular level, three segmental ligand strands L4-L6
were reacted with stoichiometric mixtures of divalent d-block cations (M(II) = Fe(II) or Zn(II)) and
trivalent lanthanides (Ln(III) = La(III), Eu(III)) in acetonitrile to give C3-symmetrical dinuclear triplestranded
helical [LnM(Lk)3]5+ cations, which can be crystallized with non-coordinating counteranions.
The divalent metal M(II) is six-coordinate in the pseudo-octahedral sites produced by the
facial wrapping of the three didentate binding units, the ligand field of which induces variable Fe(II)
spin-state properties in [LnFe(L4)3]5+ (strictly high-spin), [LnFe(L5)3]5+ (spin-crossover (SCO)
around room temperature) and [LnFe(L6)3]5+ (SCO at very low temperature). The introduction of the photophysically active Eu(III) probe in [EuFe(Lk)3]5+ results in europium-centered luminescence
modulated by variable intramolecular Eu(III)Fe(II) energy transfer processes. The kinetic analysis
implies Eu(III)Fe(II) quenching efficiencies close to 100% for the low-spin configuration and
larger than 95% for the high-spin-state. Consequently, the sensitivity of indirect luminescence
detection of Fe(II) spin-crossover is limited by the resulting weak Eu(III)-centered emission
intensities, but the dependence of the luminescence on the temperature unambiguously demonstrates
the potential of indirect lanthanide-based spin-state monitoring at the molecular scale.