Perovskite‐Related 2D Compounds in the System 5‐Amino Valerian Acid Cation/MA/Pb/ X ( X = Cl, Br) – Synthesis, Crystal Structures, and Optical Properties

Perovskite‐related 2D‐compounds of the series 5‐AVA 2 MA n –1 Pb n Br 3 n –1 were synthesised and characterised for the representatives with n = 1, 2, and 3 [5‐AVA = 5‐amino valerian acid cation, HOOC(CH 2 ) 4 NH 3 + ]. In addition, we have investigated the chloride compound 5‐AVA 2 PbCl 4 and the binary compounds 5‐AVA X ( X = Cl, Br). The crystal structures of the 2D compounds represent well‐known cut‐outs of the cubic 3D perovskites with layers of corner‐sharing PbBr 6 octahedrons in (100) orientation. According to the value of n the layer thickness comprises one, two or three layers. Similar to most of the perovskite‐related representatives the PbBr 6 octahedrons are rotated, but with slight differences. We have analysed the rotation pattern with respect to the different aspects of hydrogen bonding and orientation of the organic cations 5‐AVA + and MA + . Measurements of the optical properties reveal a decrease of the band gap from 3.02 eV ( n = 1) over 2.80 eV ( n = 2) to 2.70 eV ( n = 3). For 5‐AVA 2 PbBr 4 we observed strong blue fluorescence (413 nm) with a remarkably small Strokes shift (47 nm). All investigations were influenced by the reduced stability of the representatives with n = 2 and n = 3 with respect to the degradation into the simpler compounds 5‐AVA 2 PbBr 4 and MAPbBr 3 . This is in contrast to similar 2D‐iodides, where many representatives are known and n can reach values up to 6 and 7. Our series with n = 1, 2, and 3 represent the third example for a series of compounds with n = 1–3.