Pechmann Dye-Based Molecules Containing Fluorobenzene Moieties for Ambipolar Organic Semiconductors

Two hybrid A₂-D-A₁-D-A₂ (A=acceptor, D=donor) molecules, that is, (E)-5,5′-bis[5-(4-fluorophenyl)thiophen-2-yl]-1,1′-dioctyl-[3,3′-bipyrrolylidene]-2,2′(1H,1′H)-dione (FBBPD) and (E)-1,1′-dioctyl-5,5′-bis[5-(3,4,5-trifluorophenyl)thiophen-2-yl]-[3,3′-bipyrrolylidene]-2,2′(1H,1′H)-dione (TFBBPD), were synthesized and characterized. Each of these compounds contain the Pechmann dye derivative (E)-[3,3′-bipyrrolylidene]-2,2′(1H,1′H)-dione (BPD) as the electron-accepting core, which is flanked by an electron-donor moiety (i.e., thiophene) and other electron acceptors (i.e., 4-fluorobenzene or 3,4,5-trifluorobenzene). Organic field-effect transistor (OFET) results of FBBPD show ambipolar properties with electron (μ_e ^max) and hole mobilities (μ_h ^max) up to 0.20 and 0.023 cm² V⁻¹ s⁻¹, respectively. In comparison, the electron and hole mobilities of the OFET based on TFBBPD, which contains a more electron-accepting moiety, are higher (μ_e ^max=0.58 cm² V⁻¹ s⁻¹ and μ_h ^max=0.054 cm² V⁻¹ s⁻¹). X-ray diffraction and atomic force microscopy studies were carried out to investigate the variations in the mobilities. Overall, these studies show the potential of BPD in hybrid D-A molecules to act as the electron-acceptor moiety in organic semiconducting studies.