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

Vacancy-ordered double perovskites (VODPs) of the A₂B□X₆ type (□ = ordered B-site vacancy) incorporating stereochemically active ns² metals are promising lead-free self-trapped exciton (STE) emitters. However, efficient and stable iodide VODPs with ns² metals as the lattice centers remain rare. Here, we develop N-alkylated 1,4-diazabicyclo[2.2.2]octane (dabco)-derived diammonium cations Rdabco²⁺ (R = Me, Et, or Pr; mono-alkylated or N,N'-dialkylated), to access iodide VODPs (Rdabco)₂B□I₆ 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 [BI₆]^{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 [GeI₆]^{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.