Highly Emissive and Stable Ge(II)‐ and Sn(II)‐Based Vacancy‐Ordered Iodide Double Perovskites

ABSTRACT Vacancy‐ordered double perovskites (VODPs) of the A 2 B □ X 6 type (□ = ordered B ‐site vacancy) incorporating stereochemically active n s 2 metals are promising lead‐free self‐trapped exciton (STE) emitters. However, efficient and stable iodide VODPs with n s 2 metals as the lattice centers remain rare. Here, we develop N ‐alkylated 1,4‐diazabicyclo[2.2.2]octane (dabco)‐derived diammonium cations Rdabco 2+ (R = Me, Et, or Pr; mono‐alkylated or N , N ′‐dialkylated), to access iodide VODPs (Rdabco) 2 B □I 6 with B = Ge(II) or Sn(II). Varying the R group and alkylation across six cations effectively tunes the steric hindrance and lattice strain, enhancing oxidation tolerance and yielding zero‐dimensional lattices of isolated [ B I 6 ] 4 − octahedra. The materials exhibit bright room‐temperature STE emission with photoluminescence quantum yields (PLQYs) up to 35.0% for Ge and 36.8% for Sn. Notably, the highest PLQY of the Ge VODP is approximately seven times that of the highest reported Ge(II) iodide compounds. Structure–photophysics correlations reveal distinct determinants: the Ge PLQYs correlate with [GeI 6 ] 4 − octahedral distortion, whereas the Sn PLQYs reflect a more complex balance between radiative and nonradiative relaxation channels rather than a single distortion metric. These materials exhibit high ambient stability up to two months whereas the 3D perovskite analogs decompose within three days.