On the Aromatic Stabilization of Fused Polycyclic Aromatic Hydrocarbons

The thermodynamic properties and band gap energies were evaluated for six ortho- and peri-fused polycyclic aromatic hydrocarbons (PAHs): triphenylene; benzo[a]pyrene; benzo[e]pyrene; perylene; benzo[ghi]perylene; coronene. The standard molar enthalpies of formation in the crystalline state and the standard molar enthalpies of sublimation were measured by high precision combustion calorimetry and Knudsen effusion methodology, respectively. The combination of the molar enthalpies of formation in the crystalline state with the respective enthalpies of sublimation was used to evaluate the energetics of the progressive peri-fusion of the aromatic moieties from triphenylene to coronene aiming to investigate the hypothetical superaromaticity character of coronene. The linear trend of the enthalpy of formation in crystalline and gaseous phases in the series (from benzo[e]pyrene to coronene) is an irrefutable indication of a non-superaromaticity character of coronene. High accurate thermodynamic properties of sublimation (volatility, enthalpy, and entropy of sublimation) were derived by the measurement of vapor pressures as a function of temperature, using a Knudsen/quartz crystal effusion methodology. Furthermore, the π-electronic conjugation of these compounds was explored by evaluation of the optical band gaps along with this series of compounds. The morphology of perylene, benzo[ghi]perylene, and coronene thin films, deposited by physical vapor deposition onto transparent conductive oxide substrates (ITO and FTO), was used to analyze the nucleation and growth mechanisms. The morphologies observed were found to be related to the cohesive energy and entropy of the bulk.