New Insights into Mn–Mn Coupling Interaction-Directed Photoluminescence Quenching Mechanism in Mn²⁺-Doped Semiconductors

Abstract

Strong Mn-Mn coupling interactions (dipole-dipole and spin-exchange), predominantly determined by statistically and apparently short Mn···Mn distances in traditional heavily Mn²⁺-doped semiconductors, can promote energy transfer within randomly positioned and close-knit Mn²⁺ pairs. However, the intrinsic mechanism on controlling Mn²⁺ emission efficiency is still elusive due to the lack of precise structure information on local tetrahedrally coordinated Mn²⁺ ions. Herein, a group of Mn²⁺-containing metal-chalcogenide open frameworks (<b>MCOFs</b>), built from [Mn₄In₁₆S₃₅] nanoclusters (denoted T4-MnInS) with a precise [Mn₄S] configuration and length-variable linkers, were prepared and selected as unique models to address the above-mentioned issues. <b>MCOF-5</b> and <b>MCOF-6</b> that contained a symmetrical [Mn₄S] core with a <i>D</i>_2<i>d</i> point group and relatively long Mn···Mn distance (∼3.9645 Å) exhibited obvious red emission, while no room-temperature PL emission was observed in <b>MCOF-7</b> that contained an asymmetric [Mn₄S] configuration with a <i>C</i>₁ point group and relatively short Mn···Mn distance (∼3.9204 Å). The differences of Mn-Mn dipole-dipole and spin-exchange interactions were verified through transient photoluminescent spectroscopy, electron spin resonance (ESR), and magnetic measurements. Compared to <b>MCOF-5</b> and <b>MCOF-6</b> showing a narrower/stronger ESR signal and longer decay lifetime of microseconds, <b>MCOF-7</b> displayed a much broader/weaker ESR signal and shorter decay lifetime of nanoseconds. The results demonstrated the dominant role of distance-directed Mn-Mn dipole-dipole interactions over symmetry-directed spin-exchange interactions in modulating PL quenching behavior of Mn²⁺ emission. More importantly, the reported work offers a new pathway to elucidate Mn²⁺-site-dependent photoluminescence regulation mechanism from the perspective of atomically precise nanoclusters.