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.