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 (<b>L</b>) and one equivalent of copper(I) iodide gave luminogen copper cluster, [(<b>L</b>)Cu(<i>μ</i>_<i>2</i>-I)]₂ (<b>1</b>). Cluster <b>1</b> is further seeded with a tetranuclear copper cluster by treating <b>L</b> with four equivalents of copper iodide to isolate luminogen [(<b>L</b>)Cu(<i>μ</i>₂<i>-</i>I)]₂·[(<i>μ</i>₂-<b>L</b>)Cu₂(<i>μ</i>₂-I)(<i>μ</i>₃-I)]₂ (<b>2</b>), with unique emissive properties. The cocluster (<b>2</b>) consists of two distinct copper clusters [(<b>L</b>)Cu(<i>μ</i>₂-I)]₂ and [(<i>μ</i>₂-<b>L</b>)Cu₂(<i>μ</i>₂-I)(<i>μ</i>₃-I)]₂. A redshift of Δλ_em = 55 nm in crystalline state emission has been observed for cocluster <b>2</b> as compared to that of cluster <b>1</b>. 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 (<b>1</b>) and 2.45 eV (<b>2</b>) 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 <b>1</b> (3.03 eV) is considerably higher than that of <b>2</b> (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.