Theoretical study on molecular charge populations of 1D π-stacked multimers in neutral and electron oxidation states

Abstract

Abstract We theoretically investigated molecular charge populations of 1D π-stacked multimers consisting of π-conjugated molecules in the neutral and electron oxidation states based on the valence-bond (VB) theory. Qualitative analysis for a π-stacked trimer model based on the VB mixing diagram suggested that the inner monomer site tends to be more positively charged than the outer sites in the monocationic π-stacked trimer. Spatial expansion of each molecular site orbital toward the stacking direction is predicted to enhance the difference of positive charge populations between the inner and outer monomers. In contrast, an opposite tendency for the site charges was expected in the dicationic π-stacked trimer, primarily due to the hole–hole Coulomb repulsions. To generalize the results of the trimer to π-stacked N-mers, 1D N-site VB configuration interaction models were constructed considering the orbital expansion effects between the sites. We examined how the number of monomers (N), stacking distance (R), and characteristic orbital exponent for the monomers (ζ) affected the molecular charge populations in the monocationic and dicationic π-stacked N-mers through the parameters χij characterizing the orbital expansion effect. The results are expected to help establish design strategies for novel electronic functional materials based on discrete stacks of π-conjugated molecules.