HOMO-LUMO energy gaps of complexes of transition metals with single and multi-ring aromatics

Optical and electronic properties of aromatics and their clusters are critical to interpreting phenomena ranging from flame-formed nanoparticles to interstellar dust. These properties are also important to their applications as organic semiconductor materials. Aromatic hydrocarbons often form coordination bonds with metal atoms and cations, forming complexes with electronic properties that differ from their parent aromatics. Metal incorporation to aromatics allows access to a versatile range of optical and electronic properties via the modulation of their band gaps. Here, we investigate the binding of four fourth-row transition metals (titanium, chromium, iron, and nickel) with four aromatic molecules (benzene, naphthalene, pyrene, and coronene), and the HOMO-LUMO energy gap of the metal-aromatic complexes using density functional theory calculations. Neutral and cationic (1+ and 2+) complexes are studied at different geometrical and spin configurations, and their binding energies and HOMO-LUMO gaps are computed for the ground state. It is observed that binding with metals can reduce the HOMO-LUMO gap of the aromatics significantly, and the gap energy of the metal-aromatic complexes is closely correlated with their ionization energy. The number of possible transitions from occupied to unoccupied molecular orbitals was also calculated, showing similar spectral energy features for naphthalene, pyrene, and coronene with and without metal incorporation.