Nuclear dynamic effects on electronic properties of functionalized diamondoids

Abstract

By combining time-dependent density functional theory (TD-DFT) and Franck-Condon (FC) approximation, the effects of nuclear dynamic on the optical gap (OG) of the smallest diamondoids (adamantane and diamantane) when functionalized with three different chemical groups or atom (hydroxyl, amino, fluorine) are studied. The effects of structural relaxation and quantum zero-point on excitation energy are considered. We found that vibrationally-resolved electronic spectra and the amount of the OG renormalizations are modified in small diamondoids by the chemistry of the surface. The lowest unoccupied molecular orbitals (LUMO) do not show significant couplings to vibrations. Functional groups (–OH or NH2) and the fluorine atom show different coupling features. For small diamondoids, HOMO orbitals are dominantly characterized by nonbonding pairs of OH or NH2 groups, which suppress the influence of coupling the cage electrons to nuclear motion through the electronic transition. Destroying the coupling of electrons to nuclear motion is more significant in functionalization with OH group. In contrast, at least for low fluorine coverage, the HOMO orbital is localized inside the cages. Consequently, nuclear dynamics follows the same feature as for pristine diamondoids in fluorinated diamondoids.