An Important Key to Design Molecules with Small Internal Reorganization Energy: Strong Nonbonding Character in Frontier Orbitals

Abstract

For an electron to move between molecules (reactants), structural reorganizations of the reactants and their surrounding molecules are needed. The energy cost of the reorganizations, which is determined by structural and electronic features of molecules involved, contributes to the energy barrier of an electron transfer reaction. Finding the factors affecting the energy cost is of fundamental and technological importance. It is believed that extended π-conjugation and a rigid molecular framework are beneficial for minimizing the energy cost. We prove with phenalenyl and phthalocyanine derivatives that the extent of local nonbonding character in frontier molecular orbitals is in fact more crucial than extended π-conjugation; unprecedented small energy cost for reorganization has been found with the help of the nonbonding character. This finding provides a much better understanding of the literature data, as well as a new focus of the molecular design of cutting-edge organic electronics materials.