Journal of the American Chemical Society, Vol.142, No.14, 6649-6660, 2020
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 center dot center dot center dot 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 (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 above-mentioned issues. MCOF-5 and MCOF-6 that contained a symmetrical [Mn4S] core with a D-2d point group and relatively long Mn center dot center dot center dot Mn distance (similar to 3.9645 angstrom) 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 center dot center dot center dot Mn distance (similar to 3.9204 angstrom). 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.