Divalent Methylhydrazinium─An Ultrasmall Organic Cation for Construction of Hybrid Perovskites

Hybrid lead halide perovskites exhibit remarkable optoelectronic properties that can be tailored by incorporating diverse organic cations. Monoprotonated methylhydrazine (MHy⁺) has emerged as an effective cation for constructing perovskites with unique properties due to its intrinsic ability to form Pb–N coordination bonds, small size, and high dielectric permittivity. In this work, we enter the uncharted territory of hybrid perovskites comprising ultrasmall diprotonated amines. We show that methylhydrazine can exist in a diprotonated form (MHy²⁺) in hybrid perovskites, enabling the design of compounds with enhanced nonlinear optical responses, unconventional crystal architectures, and tunable emission properties. Using strongly acidic conditions at elevated temperatures, we obtained three distinct MHy²⁺-based lead halides: polar MHy^IIPb₂Cl₆·2H₂O (P2₁) and MHy₂^IIMHy^IPb₂Br₈Br (Pna2₁), as well as centrosymmetric GAMHy^IIPbBr₅ (P2₁/m), where GA⁺ stands for guanidinium. Structural study reveals that MHy²⁺ drives unprecedented architectures, including Br^–/Pb²⁺-deficient trilayered Dion-Jacobson phase in MHy₂^IIMHy^IPb₂Br₈Br with mixed mono/diprotonated cation coexistence. MHy^IIPb₂Cl₆·2H₂O, a compound which involves only MHy²⁺ as an organic constituent, exhibits a warm white emission (CIE 0.32, 0.38 at 80 K) through dual bound/self-trapped exciton recombination and demonstrates a second-harmonic generation (SHG) efficiency of 0.51 × KDP, nearly three times higher than MHy⁺-based analogues. MHy₂^IIMHy^IPb₂Br₈Br shows a narrow purplish-blue emission (fwhm of 8 nm) and SHG activity comparable to those of extant 2D MHy⁺ perovskites. This discovery expands the toolkit for a hybrid perovskite design, establishing methylhydrazine as an ultrasmall amine capable of incorporating two different protonation states and opening avenues for developing advanced optoelectronic materials with enhanced nonlinear optical properties and tunable emissions.