Magnetic-Bipolaronic Luminescence Enhancement in a Zero-Dimensional Rare-Earth Metal Ion Doped Manganese Chloride

Abstract

Polaron formation has emerged as an important mechanism for unconventional luminescence in metal halide semiconductors. Here, we show that incomplete rare-earth-ion doping in the zero-dimensional hybrid halide (TMBA)₂MnCl₄ induces magnetic-polaron-mediated emission, rather than conventional local d-d or f-f transitions. Single-crystal structural analysis reveals that the compounds consist of isolated [MnCl₄]^2– tetrahedra separated by TMBA⁺ cations, forming a zero-dimensional framework. Upon rare-earth incorporation, Mn²⁺ ions retain tetrahedral coordination without the formation of octahedral [MnCl₆] units. Despite preserving an invariant tetrahedral coordination framework over the entire temperature range, the lattice exhibits a pronounced anisotropic arrangement, giving rise to an effective one-dimensional orientational motif embedded within the 0D structure. This structural feature enables cooperative excited-state coupling and leads to antithermal-quenching luminescence governed by phonon-assisted, photoinduced magnetic correlations. Multiple emissive channels are observed, including Ln–Cl charge-transfer-related excitonic emission (360–385 nm), Mn-centered green emission (∼550 nm) arising from the ⁴T₁→⁶A₁ transition and associated with an exciton magnetic bipolaron, and an orange-red emission (∼600 nm) emerging exclusively in Ln (La or Ce)-incorporated lattices. In Mn–Ce coexisted samples, cooperative coupling between Mn-centered magnetic polarons and Ce-stabilized lattice/electronic polarons produces an enhanced low-energy radiative channel with pronounced antithermal-quenching behavior. Spin-polarized density functional theory calculations support the photoinduced localized magnetic alignment underlying these polaron-related emissive states. Moreover, the coexistence of Mn–Ce coupled emissive channels enables ratiometric optical emission humidity sensing with good reversibility and stability, in addition to their spin-related luminescence and display functionalities, highlighting the broad application potential of these magnetic bipolaronic systems.