Potassium Induced Charge Transfer Effects in OPE-based Molecular Layers
Steven W. Robey, Gregory J. Dutton
The impact of potassium (K) addition on the electronic structure of oligo- (phenylene ethynylene) (OPE) and NO2 substituted OPE (NO2-OPE) self-assembled monolayers was examined using ultraviolet photoemission. Addition below 0.5 K/molecule produced rigid shifts of molecular levels by about 0.5 eV for both molecular monolayers, with larger shifts at lower K addition for NO2-OPE. Continued addition of K led to increased molecular level shifts for OPE while saturation of the effect was observed for NO2-OPE. For large incorporation of K, modifications of the molecular electronic structure- additional structure and broadening- became evident, again with distinct differences between OPE and NO2-OPE. These results highlight the influence of NO2 substitution on K induced charge transfer effects. Shifts of the molecular levels are discussed within the context of the electrostatic potential within the monolayer due to K counter ions, coupled with filling of molecular levels, and compared to recent theoretical work.