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(3n-1) were synthesised and characterised for the representatives with n = 1, 2, and 3 [5-AVA = 5-amino valerian acid cation, HOOC(CH2)(4)NH3+]. In addition, we have investigated the chloride compound 5-AVA(2)PbCl(4) and the binary compounds 5-AVAX (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 PbBr6 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 PbBr6 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.