Lead Halide Hybrids Templated by Two Coordinating Ligands for Enhanced and Stable Self-Trapped Emission

Abstract

Lead halide hybrids templated by coordinating ligands are a class of ultrastable broadband self-trapped emitters that overcome the stability problems of conventional ionically bound halide hybrids. However, enhancing their photoluminescence (PL) performances by crystal engineering remains a huge challenge. Herein, for the first time, we have successfully employed the synthetic strategy of two coordinating ligands to synthesize a series of layered lead halide coordination polymers, [Pb₆X₁₀]²⁺(chdc^2-)(2,2'-bpy)₂ (X = Cl/Br, chdc = trans-1,4-cyclohexanedicarboxylate), which involves chdc as a pillaring strut and 2,2'-bpy as a chelating ligand. The introduction of a chelating ligand (2,2'-bpy) enables stronger lattice distortion of lead halide layers and enhances UV-light absorption and ligand-to-metal charge transfer (LMCT) process, thereby achieving a substantial improvement of photoluminescence quantum yields (PLQYs) over the control layered materials templated by a single chdc ligand. This class of lead halide hybrids templated by two coordinating ligands exhibit chemical "inertness" after being subjected to various chemical conditions for 48 h, maintaining stable and efficient broadband emission. Density functional theory calculations and femtosecond transient absorption spectra (fs-TA) demonstrate that the broadband emission originates from self-trapped excitons, which are more populated with the LMCT contribution from 2,2'-bpy. This study shows a rational strategy at the molecular level to modulate the photophysical properties of chemically robust lead halide coordination polymers.