Collective Electrostatics vs through-Space Interactions: Electronic Properties of Molecules with Multiple Polar Substituents

Collective electrostatic has been identified as the single most important factor determining the electronic structure and (electronic) functionality of heterogeneous interfaces. It changes spectroscopically determined quantities like electron binding energies and core-level shifts. Additionally, it results in massive changes of surface potentials and injection barriers in conventional and molecular electronic devices and shifts the electrostatic potential within the channels of porous materials. Collective electrostatics is triggered by the superposition of the electric fields of dipoles, which are arranged in a (semi)-periodic fashion. This raises the questions, which role it plays in individual molecules comprising multiple polar substituents and how collective electrostatics is related to the widely discussed through-space interactions between molecular backbones and polar substituents. Thus, the current manuscript will specifically address the question, whether through-space interactions can be regarded as yet another manifestation of collective electrostatics. To that aim, first a model-system is designed in which through-bond interactions with substituents are essentially eliminated. Subsequently, the localization of the encountered frontier orbitals and charging induced polarization effects are studied. Additionally, the evolution of ionization energies, electron affinities and the local distribution of the potential shifts with the number of polar substituents are analyzed. The data as a whole suggest that through space interactions can massively change the electronic properties of molecules due to the combined electric field of the polar substituents; still, distinct deviations from the typical characteristics of systems dominated by collective electrostatics are observed. This shows that in molecules one is rather in the realm of cumulative local-field electrostatic effects.