Film-State Low-lying LUMO-Derived Electronic Structure of Fusion-Atom Boron-Doped Polycyclic Aromatic Hydrocarbons

Low-lying LUMO levels are essential for n-type organic semiconductors, yet for boron-doped polycyclic aromatic hydrocarbons (B-PAHs), LUMO energetics have largely been inferred from solution redox estimates of individual molecules, leaving device-relevant film-state electronic structure largely unmeasured. Here, B-PAH 1, a planar, sterically unhindered ring-fusion B-PAH with two covalently embedded boron atoms whose empty p_z orbitals stabilize unoccupied states, and its all-carbon analogue PAH 2 provide a matched platform to connect molecular design, assembly, and frontier-level alignment on highly oriented pyrolytic graphite. B-PAH 1 forms ordered monolayers, electric-pulse-induced bilayers, and vapor-deposited terraced films. Spectroscopy shows that boron incorporation stabilizes the unoccupied band edge, whose energetic position varies across monolayer, bilayer, and thin-film environments, with the thin-film LUMO-derived onset of B-PAH 1 reaching -4.04 eV versus vacuum, 1.27 eV lower than that of PAH 2. Unexpectedly, despite its deeper unoccupied states, B-PAH 1 exhibits a 0.20 eV smaller effective work function than PAH 2, revealing distinct film-surface energetics whose microscopic origin remains unresolved. These results establish planar B-PAHs as experimentally accessible low-LUMO molecular films with tunable surface energetics.