Impact of Alkali Metal Coordination on the Photoluminescence Quantum Efficiency Trend in Hybrid Supramolecular Trimetallic Halides

Hybrid supramolecular trimetallic halides with a double-shell clathrate structure belong to a type of functional material with complex compositions and wide tunability. The double-shell structure is generally composed of a crown ether molecule sandwiched between different metal halide units. Here, we synthesize six new hybrid trimetallic halides and their solid solutions with the general formula [(A18C6)₃MX₄][BX₄] (where A = K⁺, Rb⁺, Cs⁺; M = Mn²⁺ or Mn_1–xZn_x²⁺; B = In³⁺; X = Cl^– or Br^–), abbreviated as A-M-X. Through the structural analysis of the crystal structure and characterizations of the optical properties of these materials, we have found a direct correlation between the photoluminescence quantum yield (PLQY) and the suitability of the coordination between the A-site metal and the crown ether. Specifically, as the radius of the alkali metal increases (from K to Rb), the PLQY decreases due to poorer suitability. The suitability of the coordination has been analyzed in detail by quantifying the structural parameters (i.e., A–O bond distance, O–A–O bond angle, and distance from A metal to the crown ether center). This trend has been further confirmed via two independent systems using solid solutions through M-site alloying and comparing the chloride and bromide analogs. Surprisingly, a selective coordination mode has surfaced when alloying on the A-site, where the larger alkali metal preferentially coordinates with the connected crown ether instead of the isolated one. This effective approach leads to a wide range of tunability of the PLQY from 21.7 to 95.2% within the total 12 materials. Through further analysis, we have found that the substitution of the A-site metal from K to Cs increases the Mn–X distance, which weakens the crystal field strength of Mn and aligns with the observed trend in PLQY. Our work has further expanded the material space of hybrid trimetallic double-shell clathrates and provides determining insights into controlling the structure and PLQY.