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−1PbnBr3n+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)­PbBr4 and (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−1PbnBr3n+1 all 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)­PbBr4 and (BA)­(Tz)­PbBr4, their band gaps are in between
the parent n = 1 compounds [(BA)2PbBr4 or (PA)2PbBr4, and (Tz)2PbBr4]. Density functional theory calculations suggest
that the band structures of (PA)­(Tz)­PbBr4 and (BA)­(Tz)­PbBr4 combine 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.