Tailoring the Superatomic Characteristics and Optical Behavior of Metal-Free Boron Clusters via Ligand Engineering

It is of great importance to understand how the number and type of ligands influence the properties of clusters through ligand engineering, as this knowledge is crucial for the rational design and optimization of functional materials. Herein, the geometrical structures, binding energies, and electronic properties of nonmetallic B_n (n = 20 and 40) clusters with CO, PEt₃, F, NO₂, and CN ligands are systematically explored based on density functional theory (DFT) calculations. Our findings demonstrate that the CO ligand acts as an electron donor when attached to these two boron clusters, in contrast to their role as electron acceptors in interactions with metal oxide and metal chalcogenide clusters. This emphasizes the necessity of considering the intrinsic properties of the host cluster when modifying with ligands. Moreover, it was observed that substituting PEt₃ with F, NO₂, or CN converted the B₂₀ cluster from an electron acceptor to an electron donor, thereby demonstrating the versatility in tuning the redox characteristics of boron clusters by selecting appropriate ligands. Intriguingly, the attachment of the PEt₃, F, NO₂, and CN ligands to B₂₀ can significantly modulate the electronic properties of B₂₀ to realize the formation of metal-free superalkali (B₂₀(PEt₃)_n, n = 3-5) and superhalogen (B₂₀F, B₂₀NO_2, and B₂₀CN) clusters. Furthermore, the structure, stability, and optical absorption of the charge transfer complex B₂₀(PEt₃)₃⁺B₂₀F were analyzed. This complex has been identified as an efficient material for harvesting visible light. Our findings provide insights into the effects of ligand variations on boron cluster functionalities, offering a new perspective for the design of advanced materials with tailored cluster properties through ligand engineering.