First-principles molecular-level study of phosphonate-induced electronic modulation and visible-light response in a tungsten–oxo cluster

Understanding how local coordination environments influence the electronic structure of tungsten-based hybrid systems is essential for the rational design of visible-light-responsive functional materials. In this work, we present a first-principles investigation of the electronic and optical properties of a phosphonate-coordinated tungsten-oxo model using a finite-cluster approach. A minimal W-phosphonate unit was adopted to capture the essential local features of the W-O-P motif while remaining computationally tractable for hybrid-functional and excited-state calculations. Ground-state calculations at the PBE level show that the occupied frontier states are predominantly ligand-centered, mainly derived from O 2p orbitals, whereas the low-lying unoccupied states are largely localized on W 5d orbitals, indicating a ligand-to-metal charge-transfer (LMCT)-like character. Hybrid HSE06 calculations yield a frontier energy separation of 2.44 eV and enable vacuum-referenced alignment of the frontier levels relative to the H⁺/H₂ and O₂/H₂O redox potentials. The resulting level positions provide qualitative redox-relevant molecular descriptors and suggest frontier-level arrangements potentially relevant to photoinduced charge-transfer processes. However, these descriptors should not be interpreted as direct evidence of photocatalytic HER or OER activity. Time-dependent DFT calculations using the CAM-B3LYP range-separated hybrid functional further reveal pronounced visible-light absorption, with an intense band centered at approximately 545 nm assigned to LMCT-like excitations. Overall, these results indicate that phosphonate coordination can modulate both the frontier electronic structure and the optical response of tungsten-oxo motifs, providing molecular-level insight into ligand-induced electronic modulation in phosphonate-functionalized tungsten-based materials.