Ligand Substitutional Effects on Molecular Dynamics and Broadband Near-White Emission in 2D 2‑Fluoroethylammonium Cadmium Chlorobromide

Fine-tuning the physicochemical properties of halide organic–inorganic hybrids has recently become crucial for materials design. In this article, structural, thermal, phonon, and optical effects of partial halide ligand replacement in two-dimensional cadmium chloride templated by 2-fluoroethylammonium cations are discussed. The substituted compound exhibits at 266.2/271.2 K a first-order phase transition from a disordered orthorhombic Cmce phase to an ordered monoclinic P21/c structure. The mechanism of this order–disorder transformation accompanied by unusually high-entropy changes involves displacements within the inorganic layers. Thermal studies revealed the presence of a second, more continuous transformation, overlapping with the main phase transition, which was attributed to changes within the hydrogen bonding network. Presented studies showed that partial chloride-to-bromide ligand replacement resulted in (i) stronger octahedral distortions and displacements of the inorganic layers, (ii) a decrease in transition temperature and the change in entropy, (iii) a reduction of negative thermal expansion effect and octahedral tilting, and (iv) a lowering of organic cation confinement in the chloride–cadmium subnetwork, which leads to weakening of the contribution of the second phase transformation. Optical studies showed that ligand substitution causes narrowing in bandgap energy from 5.12 to 4.28 eV and triggering of white to yellow-green emission thermally quenched above 240 K.