Narrowband Emitting Tin Dion–Jacobson Perovskites through Systematic Spacer Cation Design to Suppress Metal Lone-Pair Anisotropy

Two-dimensional (2D) Dion-Jacobson (DJ) tin perovskites are promising lead-free semiconductors, yet the limited number of phases reported and their frequently broad and asymmetric photoluminescence (PL) have hindered mechanistic understanding and rational design of their optoelectronic properties. Here, we systematically study 13 DJ Pb/Sn iodide perovskite pairs with identical diammonium spacer cations, including four newly synthesized Sn phases, to seek structure-property trends. We show that narrow and symmetric PL of Pb perovskites alone does not predict similarly favorable emission from Sn analogues. Instead, Sn DJ perovskites with narrow and symmetric PL share a structural motif defined by crystallographically equivalent perovskite pockets and minimal metal off-centering distortion, which can be achieved by symmetric spacer cations. First-principles calculations together with symbolic regression quantitatively connect this motif to reduced anisotropy of the Sn2+ lone pair expression and smaller excited-state structural reorganization, consistent with suppressed vibronic broadening of PL peaks. These structural and electronic criteria guide the identification and synthesis of (4F-PDMA)SnI4 (4F-PDMA = 2,3,5,6-tetrafluoro-1,4-phenylenedimethanammonium) that exhibits anticipated structural features, favorable PL characteristics, and excellent stability. This work introduces a rational spacer cation design strategy to suppress metal lone-pair anisotropy and achieve tin DJ perovskites with narrow PL emission for future high-performance optoelectronic applications.