Highly Fluorinated Dithieno[3,2-b:2′,3′-d]phospholes with Stabilized LUMO Levels

A series of dithieno[3,2-b:2',3'-d]phospholes with a perfluorophenyl substituent at the phosphorus center has been synthesized. Further cross-coupling reactions with additional fluorinated phenyl groups afforded extended conjugated materials with different substitution patterns. Investigation of the optoelectronic properties of the materials revealed smaller highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps for the extended materials that were also confirmed by theoretical density functional theory (DFT) calculations. These further indicated that the LUMO levels experience a more significant drop in energy than the respective HOMO levels. This fact can be attributed to the strongly electron-withdrawing nature of the fluoroaryls. In addition, the absolute energies of the frontier orbitals depend on the position of the fluoro substituents in the terminal aryl groups, with meta-positioned fluoro atoms having the strongest effect. Electrochemical investigations as well as theoretical calculations toward the electron affinity of the extended, highly fluorinated dithienophospholes revealed promising features that may prove useful for application as n-type semiconductors in corresponding organic field-effect transistors (OFETs).