Chemistry with F Centers: Reduction of Organic Molecules on the Defective Potassium Chloride(100) Surface

The local electronic
structure and the resulting reactivity of
crystalline surfaces are significantly influenced by the presence
of defect centers. The most prominent type of point defects of alkaline-earth
halides is the so-called F center, an isolated neutral halide vacancy
where the remaining electron is localized at the defect site. In an
earlier study [Chen, W.; Tegenkamp, C.; Pfnür, H.; Bredow,
T., Phys. Rev. B, 2009, 79, p. 235419.], it has been demonstrated that the defect electron
located in NaCl(100) surface F centers can be transferred to adsorbed
salicylic acid. In this study, we extend this study to the KCl(100)
surface and a wider variety of adsorbed organic molecules to reveal
the general trends for this charge transfer. The adsorbed molecules
comprise both aromatic annulenes and heteroaromatic systems as well
as substituted hydrocarbons. As expected, the tendency toward charge
transfer is in line with the molecular electron affinity (EA). However,
even adsorbates with a negative EA up to −1.76 eV are reduced
due to the strong electrostatic interaction with the defective surface.
In particular, the lowest unoccupied orbitals of the adsorbed molecules
are lowered near the F center so that electron transfer from the surface
defect is facilitated. In some cases, partial electron transfer and
mixing of molecular and defect orbitals are observed. In halogenated
molecules like dichloromethane, 2,3,7,8-tetrachlorodibenzodioxin,
or chlorobenzene, halogen bond dissociation and a subsequent healing
of the surface defect occur.