1D-iodoplumbates(II) of cyclic aliphatic diammoniums: Role of organic cation in the supramolecular organization and its influence on the anionic substructure and luminescent properties

Abstract

Low-dimensional metal halides emerge as materials for optoelectronics beyond perovskite photovoltaic applications. In a search for physical grounds of optical halometallate properties differences, we have investigated two homologous dications of cyclic aliphatic amines that are homopiperazine (Hpipe, C₅H₁₂N₂, 1,4-diazocycloheptan) and piperazine (Pipe, C₄H₁₀N₂, 1,4-diazocyclohexan) as structure-directing agents. Mixing of solutions of respective cations and PbI₂ in concentrated hydroiodic acid results in obtaining new compound (HpipeH₂){PbI₅}(I)·H₂O (1) and (PipeH₂){PbI₅}(I)·H₂O (2), the latter being described in the literature. 1 and 2 demonstrate analogous yet non-isomorphous crystal structures, they share an identical anionic substructure, composed by a zigzag chain-like cis-{PbI₅}^3– 1D-anion and an isolated I⁻ ion, as well as the same packing of all structural fragments. This rare situation among low-dimensional hybrid metal halide structures provides a unique platform for disentangling the effects of closely related cations on the supramolecular organization, more importantly, on the geometry of the halometallate anion without packing nuances, and, consequently, on optical properties, especially luminescence. A minor distinction in the cation's size and topology leads to dissimilar hydrogen bond systems and, most critically, distortion degrees of the iodoplumbate anion. Both compounds exhibit dual-band photoluminescence consisting of violet free exciton (FE) and yellow/orange (1/2) self-trapped exciton (STE) emission. The greater distortion of halometallate anion in 1 results in a larger Huang–Rhys factor, which governs the characteristics of the STE band and influences the thermal quenching efficiency. Vibrational spectra and thermal stability are also analyzed and correlated with structural features in this paper.