Ordered Mixed-Spacer 2D Bromide Perovskites and the Dual Role of 1,2,4-Triazolium Cation

Two-dimensional (2D) halide perovskites exhibit extraordinary stability and structural diversity because they can incorporate different organic cations (both cage A-site cation and the spacers). Here, we report on the behavior of Tz = 1,2,4-triazolium as an A-site cation as well as a spacer cation in new perovskite compounds. We describe the synthesis and structure-property relationships of a new family of hydrogen-bonding stabilized 2D perovskites (IPA)₂(Tz)_n-1Pb_nBr_3n+1(n = 1-3) (IPA = isopropylammonium, Tz = 1,2,4-triazolium), which represents the rare example of 2D perovskites incorporating the large A-site cation (Tz) that can form both the n = 2 and 3 phases. However, excess of Tz cations can split the n = 2 layers to act as a spacer and combine with another spacer PA (PA = propylammonium) or BA (BA = butylammonium) to form n = 1 structures (PA)(Tz)PbBr₄and (BA)(Tz)PbBr₄. These ordered mixed-spacer perovskites with different interlayer distances are unusual because the different spacers tend to stay in the same interlayer. (IPA)₂(Tz)_n-1Pb_nBr_3n+1all exhibit broad photoluminescence emission but for different reasons. For the n = 1 phase, the large octahedral tilting (small Pb-Br-Pb angle) gives rise to the broad emission, while for the n = 2 and 3 phases, the individual octahedral distortion (large distortion index) plays a more important role. For the mixed-spacer (PA)(Tz)PbBr₄and (BA)(Tz)PbBr₄, their band gaps are in between the parent n = 1 compounds [(BA)₂PbBr₄or (PA)₂PbBr₄, and (Tz)₂PbBr₄]. Density functional theory calculations suggest that the band structures of (PA)(Tz)PbBr₄and (BA)(Tz)PbBr₄combine features from both parent compounds yet differ from a simple superposition. This work demonstrates the dual role of Tz molecules as the A-site cation in multilayer 2D perovskites and as the spacer in ordered mixed-spacer 2D phases.