Molecular Orbital-Based Design of π-Conjugated Organic Materials with Small Internal Reorganization Energy: Generation of Nonbonding Character in Frontier Orbitals

One of the key parameters determining the rate of electron transfer is reorganization energy, an energy associated with geometry change during electron/hole transfer between molecules. To achieve efficient electron transfer, molecules with small reorganization energy are pursued, but the design guidelines remain elusive. It has been shown that a it-conjugated organic molecule with strong local nonbonding character in frontier orbitals may have small internal reorganization energy (lambda). To explore how one can introduce such character in frontier orbitals so as to design high-performance materials, in this study we employed fragment molecular orbital analysis and pairing theorem to understand why the frontier orbital of phenalenyl radical had perfect local nonbonding character. The principles learned from phenalenyl radical lead to the design of various closed-shell pi-conjugated skeletons with small lambda. Functionalization of these skeletons afforded potential n-type materials with small lambda (<100 meV) and large electron affinity. Overall, the present work showed that one can design molecules with desired lambda by assembling common pi-conjugated building blocks in a rational way directed by simple qualitative molecular orbital theory.