Designing Organic Semiconductors with Ultrasmall Reorganization Energies: Insights from Molecular Symmetry, Aromaticity and Energy Gap.

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.