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 [( A 18C6) 3 MX 4 ][ BX 4 ] (where A = K +, Rb +, Cs +; M = Mn 2+ or Mn 1– x Zn x 2+; B = In 3+; 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.