Theoretical studies of the electronic properties of confined aromatic molecules in support of electronic confinement effect

Theoretical studies of the electronic properties of three confined aromatic molecules—benzene, naphthalene and anthracene—have been presented in support of the electronic confinement effect. The confined space of the cavities has been modeled using a mica sheet with the molecule–surface distance in the range of 1.5–4.0 A. Evidence of the confinement has been revealed by semiempirical calculations, which are theoretically interpreted by means of the Huckel molecular orbital theory. It has been found that the HOMO has been predicted to be more sensitive to the confinement than the LUMO and the overall effect is a reduction on the band gap of the frontier molecular orbitals when the molecule–surface distance is less than ca. 2.5 A. The variations of the frontier orbital energies and band gaps are correlated with the increase of both Coulomb integral, α, and resonance integral, β. The order of magnitude of the energy increment of Δα and Δβ values is evaluated from data of the above semiempirical calculations. It is also found that the confinement effect is associated with the conjugated system of the aromatic molecules. The theoretical evaluations here prove that confining organic molecules in the cavities is sufficient to alter their electronic properties as a consequence of changes in the molecular orbital energies and band gaps.