Electronic-structure engineering in metal-organic framework photocatalysts: Ligand engineering, orbital...

Photocatalysis has been an encouraging solution to convert solar energy into chemical fuels and address the direst environmental problems but the visible-absorption and rapid discharge of the photogenerated charge carriers currently are likely to restrict its practice. Due to their tunable architectures and well-defined coordination environments, metal-organic frameworks (MOFs) have become a multifunctional platform to surmount these shortcomings. This review highlights a design framework based on the strategy to regulate the electronic structure of MOFs on the fundamental level; this is performed by means of the engineering of ligands, bonding-antibonding/orbital hybridization, and the incorporation of the donor-acceptor (D-A) unit. Ligand engineering provides π-conjugation extension and functional group modulation to increase light harvesting and efficient charge transfer between ligands and metals. Other techniques to maximize the metal-ligand electronic interactions are bonding-antibonding and orbital hybridization, which make it easier to stretch the charge separation and raise carrier lifetimes. Meanwhile, (D-A) architectures add inherent electronic asymmetry and internal electric fields, allowing directional movement of charges and hindering recombination. Combination of these strategies in a synergistic manner creates a single system of regulation of light absorption, charge dynamics and catalytic activity. Some of the most significant applications of this design philosophy are in the area of pollutant degradation, hydrogen evolution and CO₂ reduction, in which the desired electronic modulation can be directly translated into a high photocatalytic activity, selectivity and stability. Lastly, existing issues and future outlooks are presented to reveal the necessity to achieve a deeper mechanistic insight, enhance material strength, and scalable design solutions. The review offers an overall roadmap on the rational design of the next-generation MOF-based photocatalysts in sustainable energy and environmental applications.