Isopropylammonium as a Dual-Role Cation: Spacer and Perovskitizer in Ruddlesden-Popper Lead Bromides.

Layered hybrid perovskites are attractive optoelectronic materials for a wide range of applications. This study reports three layered Ruddlesden-Popper (RP) bromides comprising small isopropylammonium (IPA+) cations: IPA2MA2Pb3Br10, IPA2DMAPb2Br7, and IPA2(IPA0.77MHy0.23)Pb2Br7 (MA+ = methylammonium, DMA+ = dimethylammonium, MHy+ = methylhydrazinium). Crucially, the IPA2(IPA0.77MHy0.23)Pb2Br7 compound represents a unique case where IPA+ cations occupy the perovskite cavities, thus acting as a perovskitizer, a dual role previously unobserved for this cation in lead halide perovskites. All compounds undergo structural phase transitions, the mechanisms of which are analyzed based on X-ray diffraction, Raman spectroscopy, and dielectric data. Nonlinear optical (NLO) and dielectric studies demonstrate that IPA2MA2Pb3Br10 exhibits second-harmonic generation (SHG) at low temperatures and bistable dielectric switching. Photoluminescence (PL) studies reveal that IPA2MA2Pb3Br10 displays narrowband purplish-blue emission attributed to free excitons (FEs), whereas the other compounds exhibit broadband PL attributed to self-trapped excitons (STEs). The broadband emission shows remarkable thermochromism, transitioning from yellow (orange) at 80 K to blue (white) at high temperatures for IPA2DMAPb2Br7 and IPA2(IPA0.77MHy0.23)Pb2Br7, respectively. These results exemplify the unusual ability of IPA+ to function simultaneously as a spacer and a perovskitizer, highlighting the critical role of A-site cations in modulating the structural, electric, and optical properties of multilayered perovskites.