Eu(III)-dilution of Er(III) and Yb(III) molecular nanomagnets as a route for improving magnetic features and linking with optical thermometry

Abstract

Lanthanide(III) single-molecule magnets (SMMs) are a tool for the combination of molecular nanomagnetism with luminescent thermometry, opening the pathway not only for the broadened multifunctionality but also for optical selfmonitoring of temperature in SMM-based systems. Usually, this goal is realized by exploring the intrinsic properties of a single lanthanide (Ln) ion. We present an innovative strategy based on embedding the magnetically anisotropic Ln(III) centers into a red-emissive coordination system based on Eu(III) complexes of a diamagnetic ground state. This concept is presented on novel cyanido-bridged chains, {[Ln^III(dppmO₂)₃][Ag^I(CN)₂]}[OTf]₂ (Ln = Eu, EuAg; Ln = Er, ErAg; Ln = Yb, YbAg; Ln = Er_0.05Eu_0.95, Er@EuAg; Ln = Yb_0.04Eu_0.96; Yb@EuAg; dppmO₂ = bis(diphenylphosphino)methane dioxide; OTf = trifluoromethanesulfonate). The Ln(III) coordination sphere, consisting of O,O-bidentate dppmO₂ ligands occupying the equatorial positions and two axially aligned cyanido bridges, generates the distinct SMM characteristics of Er(III)/Yb(III) centers. On the other hand, the strong photoluminescence of Eu(III) in EuAg exhibits the pronounced thermal variation of their excitation spectra, which was employed for ratiometric optical thermometry, revealing the relative thermal sensitivity (S_r) up to 2.1% K^-1 at 55 K and good thermometric response below 110 K. The incorporation of Er(III)/Yb(III) centers in the Eu(III)-based framework leads to heterotrimetallic Er@EuAg and Yb@EuAg systems, which link the SMM features with luminescent thermometry. Both physical properties are enhanced upon Ln(III) centers mixing, as depicted by the slowdown of magnetic relaxation, especially for Er(III), due to the Eu(III)-induced weakening of quantum tunneling of magnetization, which is accopanied by the improvement of thermometric response, including the increase of the maximal S_r to 3.3% K^-1 and broadening of its operating range to 150 K, due to the structural distortion induced by Er(III)/Yb(III) centers.