Two-Dimensional Lead Iodide Perovskites with Extremely Reduced Dielectric Confinement: Embedded Self-Erasing Second-Harmonic Generation Switching, Thermochromism, and Photoluminescence

Ethanolammonium lead iodide (EOA2PbI4) and propanolammonium lead iodide (AP2PbI4) are two-dimensional hybrid organic–inorganic perovskites with reduced dielectric confinement and exciton binding energy. These compounds exist in three polymorphic forms, two of which are metastable, each with distinct dielectric constants, nonlinear optical (NLO) properties, and photoluminescence characteristics. The initial red phases I are isostructural, displaying monoclinic P21/c symmetry and narrow-line green PL, stable up to approximately 369 K for EOA2PbI4 and 319 K for AP2PbI4. Beyond these temperatures, they undergo a reconstructive first-order phase transition to high-temperature phase II, characterized by unlocked organic cation motion, as indicated by Raman and dielectric data. This transition results in a color change to orange (yellow) for EOA2PbI4 (AP2PbI4) due to a blue shift in the band gap and excitonic absorption, more pronounced in AP2PbI4. The high-temperature phase II is prone to supercooling to a metastable regime, where it either reverts to phase I or transforms to a new low-temperature phase III with ordered organic cations and a slightly more distorted inorganic substructure. In phase III, photoluminescence is red with a single broad peak, large Stokes shift, and electron–phonon coupling, particularly pronounced in AP2PbI4 (>1 eV), indicating significant inorganic substructure deformation. Phase III of AP2PbI4 lacks second-harmonic generation (SHG) activity, while the SHG response is observed in phase III of EOA2PbI4. The metastability of acentric phase III and supercooled centric phase II of EOA2PbI4 is harnessed to introduce a new type of NLO functionality, namely, embedded self-erasing SHG switching. The “embedded” scheme requires thermodynamically unstable crystal phases, one of which is noncentrosymmetric, and necessitates thermal activation, such as annealing followed by cooling. This unconventional route of triggering NLO responses, along with spontaneous reversion to the centrosymmetric phase, enables self-erasing switching between the SHG-on and SHG-off states.