Unveiling phase-dependent structure-property relationships in cyclohexaphosphane (P6H6) molecular blocks: A DFT and TD-DFT study of electronic structure, optical response, and intermolecular interactions.

Phosphorene-based nanomaterials have attracted considerable interest because of Physicochemical properties. However, the influence of local molecular geometry on the properties of finite phosphorene-related molecular motifs remains insufficiently explored. In this work, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations are performed for four isolated cyclohexaphosphane structures used as finite molecular motifs associated with the α, β, γ, and δ phosphorene configurations. This molecular-block representation is intended to compare local structure-property trends and does not imply quantitative equivalence with periodic phosphorene sheets. Optimized geometries, vibrational spectra, frontier molecular orbitals, density of states, electrostatic surface potential (ESP), and optical properties are analyzed. The calculations reveal clear phase-dependent electronic characteristics: the α- and β-motifs exhibit larger gaps of 5.34 and 4.89 eV, respectively, whereas the γ- and δ-motifs show smaller gaps of 4.14 and 4.11 eV, together with higher chemical softness, electrophilicity, and Electronic Interaction Index values. The optical calculations also show distinct responses, with absorption maxima at 357 and 339 nm for γ and δ, respectively, both in the near-UV region. The emission-related quantities and Stokes shifts obtained from the adopted triplet-relaxation protocol further indicate phase-dependent excited-state relaxation. ESP maps reveal heterogeneous electrostatic distributions that may be useful for guiding future explicit interaction studies. Overall, the calculations establish phase-dependent molecular structure-property relationships and provide a theoretical basis for subsequent studies of molecular sensing, adsorption, and optoelectronic behavior. Biomedical applications such as targeted delivery, toxicity assessment, and photothermal therapy are considered only as prospective directions requiring explicit intermolecular, environmental, and experimental validation.