Multifunctional luminescent magnetic cryocooler in a Gd5Mn2 pyramidal complex

Abstract

Magnetic cooling is a highly efficient refrigeration technique with the potential of replacing expensive and rare helium-3 in the field of ultra-low temperature cooling. However, the visualization of a cryogen at an extremely low temperature and in a strong magnetic field is challenging, but it is crucial for the precise positioning and in situ thermal probe measurements in potential practical applications. Here, the activation of a red-emissive Mn(II) ion using 3d/4f chemistry produces a luminescent molecule cooler, [Gd₅Mn₂(L^OMe)₂(OH)₄(Ac)₆(MeOH)₁₀Cl₂]Cl₃·2MeOH (1), with the core of an Mn(II)-anchored heptanuclear [Gd^III₅Mn^II₂] pyramid. The photoluminescence (PL) of the Mn²⁺ emission, with a large Stokes shift (λ_em ∼ 690 nm) from ⁴T₁(⁴G) → ⁶A₁(⁶S), shows not only a sensitive temperature sensing property but also reversible mechanoluminescence (ML). More attractively, these findings reveal a considerable magnetocaloric effect (MCE) coupled with a tunable emission window, opening up new opportunities in the multifunctional applications of PL, ML, and the MCE involving red-light sources, thermometers, and stress imaging. In particular, this provides a novel resolution to design visualized PL coolers.