Identification of Singlet Self-Trapped Excitons in a New Family of White-Light-Emitting Zero-Dimensional Compounds

As illumination is a fundamental human need, the exploration of illumination sources possessing high efficiency and broadband white-light emission is highly desirable. Zero-dimensional (0D) metal halide compounds are promising candidates, and some lead-free antimony-containing compounds exhibit bimodal white-light emissions. However, their origins are still unclear. To solve this issue, we designed and prepared a new family of 0D metal halide compounds consisting of [M(18-crown-6)]+ (M = NH4, Rb) and SbX52– (X = Cl, Br) units. We found that the emission profiles of 0D compounds are distinct to and well separated from that of 18-crown-6 ether, excluding the intraligand charge transfer mechanism proposed in several reports. Femtosecond transient absorption data and the compositional dependence of photophysical properties imply that bimodal white-light emission is induced by both singlet state and triplet state of the self-trapped excitons (1STE and 3STE) coupled to metal halides. These 0D compounds are also very efficient emitters, with a white-light photoluminescence quantum yield as high as 54%.