Molecular Orientation Dependence of the Ionization Energy of Pentacene in Thin Films

An isolated individual molecule clearly has only one ionization energy. For ordered molecular assemblies, however, multiple values have been found depending on the orientation of the molecules relative to a supporting substrate. This intriguing observation is rationalized here for the prototypical molecule pentacene, in terms of intrinsic "surface dipoles" built into the molecules, which collectively give rise to the orientation dependence of the molecular ionization energy.