Molecular mechanism of rigidity- and planarity-promoted, state-dependent doping of conjugated ladder-type molecules

Abstract

Determining the doping mechanism of organic semiconductors (OSCs) using neutral dopants, whether through the formation of an ion-pair (IP) or a charge transfer complex (CTC), remains an open challenge. It is hypothesized that the rigidity and planarity of the π-backbone of OSCs can significantly impact their doping mechanism and efficiency. In this work, a series of fused-ring ladder-type small molecules (B-ICz, Th-ICz, and BDT-ICz) are synthesized as models to investigate this hypothesis. Upon mixing with the electron-deficient dopant F₄TCNQ, each of these compounds shows dominant IP formation in solution whereas both IPs and CTCs are observed in the solid-state. Control experiments and computational investigations reveal that the extended, rigid, and coplanar π-faces of these ladder-type molecules are essential to facilitate the doping interaction with F₄TCNQ, for both IP and CTC mechanisms. This work provides principles for the future rational design of molecular structures of OSCs and dopants for electronic doping purposes.