Film‐State Low‐lying LUMO‐Derived Electronic Structure of Fusion‐Atom Boron‐Doped Polycyclic Aromatic Hydrocarbons
Po‐Wei Tung, Mandala Anitha, Tung‐Te Tien, To‐Jen Chin, Kum‐Yi Cheng, Chih‐Hsun Lin, Hsiang‐Ling Chuang, Yi Rick Lin, Yi‐Hung Liu, Jeffrey M. Farrell, Chun‐hsien ChenABSTRACT
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.