Interfacial Polarization and Charge Transfer in Al₁₃ Superatom-Organic Interfaces: Role of Frontier Orbital Overlap

Superatoms (SAs) have emerged as atomically precise building blocks for functional materials; however, their interfacial electronic interactions with organic substrates remain poorly understood, particularly in the absence of ligand protection. While most reported SAs are stabilized by ligands, ligand-free SAs provide a unique platform to probe intrinsic interfacial behavior. In particular, X-ray photoelectron spectroscopy (XPS) core-level shifts at SA-organic interfaces are interpreted as signatures of charge transfer, although the contributions of interfacial polarization remain unclear. Here, we present a comparative density functional theory (DFT) study of a ligand-free Al₁₃ SA adsorbed on two π-conjugated substrates: hexa-peri-benzocoronene (HBC) and C₆₀. Despite their comparable sizes and extended π systems, these substrates exhibit fundamentally distinct molecular interaction regimes. For Al₁₃-HBC, minimal charge transfer from HBC to Al₁₃ occurs and the interaction is dominated by dispersion and polarization, yet computed core-level shifts are consistent with an electron-enriched SA. In contrast, Al₁₃-C₆₀ shows substantial electron transfer from Al₁₃ to C₆₀, accompanied by covalent bond formation and pronounced electronic reconstruction. Real-space, dipole moment, and charge-partitioning analyses collectively reveal that core-level shifts can arise predominantly from interfacial polarization, even in the absence of substantial integer electron transfer. Projected density of states analysis further shows that charge transfer is enabled when favorable energy alignment is coupled with symmetry-allowed frontier orbital overlap. Environmental polarization and peripheral chemical substitution of HBC induce only minor changes, supporting that polarization alone is insufficient to drive electron transfer. These results establish that both energetic and symmetry constraints determine charge transfer at SA-organic interfaces, and provide design principles for superatomic hybrid materials.