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

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 of local tetrahedrally-coordinated Mn2+ ions. Herein, a group of Mn2+-containing metal-chalcogenide open frameworks (MCOFs), 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 abovementioned issues. MCOF-5 and MCOF-6 that contained a symmetrical [Mn4S] core with a D2d point group and relatively long Mn∙∙∙Mn distance (~ 3.9645 Å) exhibited obvious red emission, while no room-temperature PL emission was observed in MCOF-7 that contained an asymmetric [Mn4S] configuration with a C1 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 MCOF-5 and MCOF-6 showing a narrower/stronger ESR signal and longer decay lifetime of microseconds, MCOF-7 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.