Modulating the band gap of a pyrazinoquinoxaline-based metal–organic framework through orbital hybridization for enhanced visible light-driven CN bond construction

Abstract

Massive efforts have been devoted to developing photocatalysts based on metal–organic frameworks (MOFs), which are expected to play a significant role in the fields of energy conversion and environmental remediation. Nevertheless, further research is still required to better design the photoactive ligands and select suitable metal nodes to optimize the band gap and electron transfer pathways of MOFs in order to improve their photocatalytic efficiency. To this end, pyrazinoquinoxaline tetracarboxylic acid with a highly conjugated and electron-rich structure was synthesized to function as a photoactive ligand. However, the large energy difference between the metal nodes and ligands requires strong energy excitation, which hinders visible light-driven photocatalytic reactions from occurring. To identify the influence of energy-level matching and orbital hybridization between the metal nodes and ligands on the photocatalytic performance, an indium-based MOF was consequently prepared. In this way, efficient orbital hybridization between the metal nodes and photoactive ligands was established, and hence, the metal-to-ligand charge-transfer (MLCT) process could be triggered with visible light irradiation. Here, the combination of colorless In³⁺ ions and brown ligands gave rise to dark red crystalline materials, which effectively promoted visible light-energy utilization. As a result, the In-MOF exhibited a wide visible light absorption range with a relatively narrow band gap (1.474 eV), and it could facilitate two reactions under visible light irradiation concerning the construction of CN bonds, which is significant in the fine chemicals and pharmaceuticals industries. In-depth theoretical calculations were conducted to clarify the mechanism of the reactions and the catalytic activity of In-MOF. It turns out that two approaches of charge transfer, namely MLCT and intra-ligand charge-transfer (ILCT), and the interaction between the host and intermediates played a crucial role in these transformations. In this way, this study has established a template strategy for modulating the orbital hybridization between the components to control the band structure of MOFs.