Density Functional Theory Study Predicts Low Reorganization Energies for Azadipyrromethene-Based Metal Complexes

Small internal reorganization energy is desirable for high-performance optoelectronic materials, as it facilitates both charge separation and charge transport. However, only a handful of n-type electron accepting materials are known to have small reorganization energies. Here, DFT calculations were performed to predict the reorganization energy of azadipyrromethene-based dyes and their complexes. All compounds studied were most stable in their anionic state and had high electron affinity, indicating their potential as n-type material. The homoleptic zinc(II) complexes had significantly lower reorganization energies than either the free ligands or the BF2+ chelates. The low reorganization energies of the zinc(II) complexes are explained by the large and rigid pi conjugated system that extends across the two azadipyrromethene ligands via interligand pi-pi interactions. This work suggests that Zn(II) complexation is a novel strategy for obtaining materials that combine low internal reorganization energy with high electron affinity for the development of novel n-type optoelectronic materials.