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.