Chlorination: a general route toward electron transport in organic semiconductors

We show that adding chlorine atoms to conjugated cores is a general, effective route toward the design of n-type air-stable organic semiconductors. We find this to be true for acenes, phthalocyanines, and perylene tetracarboxylic diimide (PDI)-based molecules. This general finding opens new avenues in the design and synthesis of organic semiconductors. We compared a series of fluoro- and chloro-functionalized acenes, phthalocyanines, and PDI-based molecules. The acenes synthesized showed high and balanced ambipolar transport in the top-contact organic field effect transistor (OFET) geometry. The electron-withdrawing halogen groups lowered the LUMO and the charge injection barrier for electrons, such that electron and hole transport occurred simultaneously. If the chlorine added does not distort the planarity of the conjugated core, we found that the chloro-functionalized molecules tend to have a slightly smaller HOMO-LUMO gap and a lower LUMO level than the fluoro-containing molecules, both from calculations and cyclic voltammetry measurements in solution. This is most likely due to the fact that Cl contains empty 3d orbitals that can accept pi-electrons from the conjugated core, while F does not have energetically accessible empty orbitals for such delocalization.