From Dopant to Host: Solution Synthesis and Light‐Emitting Applications of Organic‐Inorganic Lanthanide‐Based Metal Halides

The rich and unique energy level structure arising from 4f n inner shell configuration of trivalent lanthanide ions (Ln 3+ ) renders them highly attractive for light‐emitting applications. Currently, research primarily focuses on Ln 3+ doping in either traditional garnets or the recently developed perovskite phosphors. However, there have been few reports on stable phosphors crystallized with pure lanthanide elements. Herein, a universal solution‐based route to eight Ln 3+ ‐based metal halides from the organic‐inorganic A 4 LnX 7 family is described, where A + = 4,4‐difluoropiperidinium (DFPD + ), Ln 3+ = Nd 3+ , Eu 3+ , Ho 3+ , Sm 3+ , Tm 3+ , Tb 3+ , Yb 3+ , Er 3+ , and X − = Cl − , Br − . Visible photoluminescence (PL) is observed from Tb 3+ ‐, Eu 3+ ‐, Ho 3+ ‐, and Sm 3+ ‐based compounds with Tb and Eu compositions exhibiting high PL quantum yields of 90–100%; Nd 3+ ‐, Tm 3+ ‐, Yb 3+ ‐, and Er 3+ ‐based crystals show fascinating near‐infrared emission. Light‐emitting diodes (LEDs) fabricated with (DFPD) 4 TbCl 7 yield characteristic emission of Tb 3+ , representing the first demonstration of electroluminescence from these organic‐inorganic Ln 3+ ‐based metal halides. Moreover, these materials exhibit distinct excitation wavelength‐dependent emission after alloying with different Ln 3+ ions, making them interesting for multicolor display and multilevel information encryption applications. It is foreseen that this study will open up the way to a possible design of robust optoelectronic devices based on lanthanide metal halides.