Designing Organic Semiconductors with Ultrasmall Reorganization Energies: Insights from Molecular Symmetry, Aromaticity and Energy Gap
Weicong Huang, Wen Jun Xie, Huijuan Huang, Heng Zhang, Hongguang Liu
Abstract
An intuitive design strategy for organic semiconductors with ultrasmall reorganization energy (λ) is proposed. Learning from a total of 98 molecules condensed by benzene and/or thiophene rings, we find that linear compounds in D2h symmetry have the smallest λ in each of the three molecular categories (PAHs, thienothiophenes, benzothiophenes). 2D expanded analogues that contain these D2h building blocks also give unusually small λ (<100 meV). λ of 1D elongated polycyclics show an approximate linear correlation with the ring-averaged HOMA indices and the HOMO–LUMO gaps. Compared to the symmetry principle, the HOMA and energy gap, though much less intuitive to design a priori, provide additional quantitative guidelines to further optimize λ through substitutions, for example, when molecules have the same symmetries. Our results indicate that ring-fused π-conjugates that have narrower HOMO–LUMO gaps and are less aromatic are better candidates to achieve ultrasmall λ.