Molecular Design\nof n‑Type Organic Semiconductors\nwith Ultralow Electron Reorganization Energies

Bottom-up design of n-type organic semiconductors with\nsalient\nelectron-transport properties is of fundamental importance. Here,\nwith the aid of density functional theory, we demonstrate that condensing\nodd-membered carbon rings (5MR/7MR) to small polycyclic aromatic hydrocarbons\n(PAHs) can endow molecules with ultralow electron reorganization energies\n(λ < 100 meV). Studying 60 molecules, we find that introducing\npolycyclic fragments with built-in 5MR and 7MR to linear PAHs at the\nC<sub>α,β</sub> positions or to nonlinear PAHs in D<sub>2h</sub> symmetry constitutes molecules with λ as low as 65\nmeV. A joint ACID and NICS analysis proves that the stronger the molecular\naromaticity, the lower the λ will be. This is contrary to what\nhas been previously found for p-type molecules. Furthermore, we propose\n“acupoints” on molecules for N-doping and cyanation,\nwhich can be used to precisely locate the substitution sites to reduce\nthe LUMO energies (in favor of electron injection and air stability)\nof low-λ molecules while it does not elevate λ. These\nfindings would help to reduce the synthetic blindness/cost and contribute\nto the bottom-up design of n-type small-molecule organic semiconductors.