Near-infrared emissive Er(iii) and Yb(iii) molecular nanomagnets in metal–organic chains functionalized by octacyanidometallates(iv)

Abstract

The self-assembly of lanthanide(3+) ions with pyrazine N,N′-dioxide (pzdo) and bis(triphenylphosphine)iminium (PPN⁺) salts of diamagnetic octacyanidometallates of Mo^IV and W^IV results in one-dimensional (PPN)[Ln^III(pzdo)₂(MeOH)_0.3(H₂O)_3.7][M^IV(CN)₈]·7.7H₂O·2MeOH (Ln = Er and Yb; M = Mo and W) coordination networks. They are constructed of zigzag metal–organic {Ln^III(μ-pzdo)}_n chains with terminal [M^IV(CN)₈]⁴⁻ metalloligands attached to the Ln^III centres. Both Er- and Yb-containing frameworks exhibit field-induced slow magnetic relaxation originating from the field-dependent equilibrium between quantum tunnelling of magnetization and the direct process along with the temperature-dependent Raman and Orbach relaxation processes. Transition metal substitution on [M^IV(CN)₈]⁴⁻ sites modifies the intrinsic magnetic anisotropy of lanthanides, as depicted by the subtle change of thermal energy barriers of Orbach relaxation in Er-based systems, while it significantly affects Raman relaxation due to the modulated phonon mode scheme. All reported compounds exhibit strong visible light absorption due to a series of electronic transitions of pzdo ligands and [M(CN)₈]⁴⁻ ions and the appearance of low energy anion–π charge transfer band involving pzdo and octacyanidometallates. These charge transfer states were utilized in achieving the sensitized near-infrared photoluminescence of Er^III and Yb^III centers; thus, the energy transfer process is strongly dependent on the nature of the metal centre in the [M^IV(CN)₈]⁴⁻ ion. The W(IV)-pzdo system is a good sensitizer for Er^III, while the Mo(IV)-pzdo unit is better for Yb^III emission which can be rationalized in terms of the positions of energy levels of their respective donor states. Therefore, we report NIR emissive Er^III and Yb^III single-molecule magnets with tunable magnetic and optical properties, uncovering the crucial role of octacyanidometallates and their anion–π interactions with pzdo ligands.