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 pi-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 F(4)TCNQ, 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 pi-faces of these ladder-type molecules are essential to facilitate the doping interaction with F(4)TCNQ, 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.