Molecular Fe(II)-Ln(III) dyads for luminescence reading of spin-state equilibria at the molecular level

Due to the primogenic effect, the valence shells of divalent iron Fe(II) ([Ar]3d⁶) and trivalent lanthanides Ln(III) ([Xe]4f) are compact enough to induce spin-state equilibrium for the 3d-block metal and atom-like luminescence for the 4f-block partner in Fe(II)-Ln(III) dyads. In the specific case of homoleptic pseudo-octahedral [Fe(II)N₆] units, programming spin crossover (SCO) around room temperature at normal pressure requires the design of unsymmetrical didentate five-membered ring chelating N^∩N' ligands, in which a five-membered (benz)imidazole heterocycle (N) is connected to a six-membered pyrimidine heterocycle (N'). Benefiting from the trans influence, the facial isomer fac-[Fe(II)(N^∩N')₃]²⁺ is suitable for inducing SCO properties at room temperature in solution. Its connection to luminescent [LnN₆O₃] chromophores working as non-covalent podates in the triple-stranded [Fe(II)Ln(L10)₃]⁵⁺ helicates (Ln = Nd, Eu) controls the facial arrangement around Fe(II). The iron-based SCO behaviour of the 3d-4f complex mirrors that programmed in the mononuclear scaffold. Because of the different electronic structures of high-spin and low-spin [Fe(II)N₆] units, their associated absorption spectra are different and modulate the luminescence of the appended lanthanide luminophore via intramolecular intermetallic energy transfers. It thus becomes possible to detect the spin state of the Fe(II) center, encoded by an external perturbation (i.e. writing), by lanthanide light emission (i.e. reading) in a single molecule and without disturbance. Shifting from visible emission (Ln = Eu) to the near-infrared domain (Ln = Nd) further transforms a wavy emitted signal intensity into a linear one, a protocol highly desirable for future applications in data storage and thermometry.