Self-Assemblies of Isomeric Copper Iodide Trimers with Geometry-Dependent Photophysical Properties

Zero-dimensional (0D) metal halides have attracted great research interest for the past few years due to their fascinating photophysical properties and extraordinary structural diversity. However, controlled synthesis of these metal halides in a rational way remains a big challenge, and the basic relationship between their crystal structures and photophysical properties is still not well understood. In this work, two new 0D self-assemblies of isomeric copper iodide trimers with the same formula of (BTMA)3Cu3I6 (BTMA = benzyltrimethylammonium) but different crystal structures were synthesized. Interestingly, we found that the photophysical properties of these copper iodide isomers depend strongly on the geometries of their copper iodide trimers, i.e., one isomer with boat-shaped [Cu3I6]3– trimers (T1) exhibits yellow emission centered at 606 nm at room temperature, while the other isomer with crown-shaped [Cu3I6]3– trimers (T2) shows orange emission centered at 639 nm. Density functional theory (DFT) calculations revealed that both the organic cations and inorganic copper iodide trimers participate in the luminescence process of these copper iodide isomers, and a through-space charge transfer (TSCT) between the spatially separated organic ligands and inorganic [Cu3I6]3– trimers would occur during the optical emission/absorption, which is quite different from the luminescence mechanism proposed for the previously reported 0D ionic copper halides.