Lowering the dimensionality of cationic lead bromide units in robust coordination polymers for enhanced self-trapped broadband emission

Abstract

Organolead halide hybrids are a potential class of intrinsic self-trapped photoemitters but susceptible to degradation and fluorescence quenching in moisture, owing to the intrinsic instability of ionic structures. Our group focuses on the synthesis of organolead halide-based coordination polymers demonstrating high structural stability, but very few of them show high quantum efficiency due to limited structural strain. Herein, we introduce the crystal engineering approach to reduce the dimensions of Pb–Br connectivity by tuning the synthetic conditions. First, a layered organolead bromide coordination polymer, [Pb₃Br₃³⁺][imdc³⁻] (H₃imdc = 4,5-imidazoledicarboxylic acid), is synthesized and consists of two-dimensional (2D) [Pb₂Br₃]⁺ layers. The increasing incorporation of imdc³⁻ results in the isolation of inorganic species to form rare zero-dimensional (0D) cationic bromoplumbate nodes, i.e. [Pb₂Br]³⁺ clusters. Both materials exhibit broadband emission covering the visible light spectrum, but the low-dimensional [Pb₂Br]³⁺ clusters exhibit a five-fold enhancement in photoluminescence quantum yields (PLQYs). Despite coordinatively bridged by organocarboxylate linkers, 0D [Pb₂Br]³⁺ units occupy stronger charge localization and higher tendency to form self-trapped electron/hole polarons, based on the density functional theory calculations and a variety of photophysical studies. This work provides a promising structural modulation strategy to rationally optimize the photoluminescence performance of robust organolead halide coordination polymers.