Clusterization-Induced Emission Shift in Copper(I) Thiones

Cluster-induced luminescence depicts a significant role
in light-emitting
applications. The reaction between 1-isopropyl-3-(pyridine-2-yl)-benzimidazol-2-thione
(L) and one equivalent of copper(I) iodide gave luminogen
copper cluster, [(L)Cu(μ2-I)]2 (1). Cluster 1 is further seeded with a tetranuclear copper cluster by
treating L with four equivalents of copper iodide to
isolate luminogen [(L)Cu(μ2-I)]2·[(μ2-L)Cu2(μ2-I)(μ3-I)]2 (2), with unique emissive properties. The cocluster
(2) consists of two distinct copper clusters [(L)Cu(μ2-I)]2 and
[(μ2-L)Cu2(μ2-I)(μ3-I)]2. A redshift of Δλem = 55 nm in crystalline state emission has been observed for cocluster 2 as compared to that of cluster 1. Such variation
in crystalline state emissions is due to the presence of different
noncovalent interactions due to crystal packing. The optical band
gaps of 2.40 eV (1) and 2.45 eV (2) have
been calculated using the Tauc plot. Furthermore, to investigate the
electronic properties of both clusters, DFT and TD-DFT calculations
were performed. The HOMO–LUMO energy gap of 1 (3.03
eV) is considerably higher than that of 2 (2.75 eV).
TD-DFT calculations revealed that the nature of emission in these
clusters is a combination of 3XLCT, 3MLCT, and 3ILCT transitions. Both clusters have been coated on the surface
of a commercially available 3 V LED. The emissions of the coated LEDs
are similar to those of crystalline samples of pure clusters.