New donor-acceptor-donor molecules with Pechmann dye as the core moiety for solution-processed good-performance organic field-effect transistors

In this paper, we report the synthesis and characterization of two new D-A-D molecules (E)-5,5′-bis(5-(benzo[b]thiophen-2-yl)thiophen-2-yl)-1, 1′-bis(2-ethyl- hexyl)-[3,3′-bipyrrolylidene]-2,2′(1H, 1′H)-dione (BTBPD) and (E)-5,5′-bis- (5-(benzo[b]furan-2-yl) thiophen-2-yl)-1,1′-bis(2-ethylhexyl)-[3,3′-bipyrrolylidene]-2, 2′(1H,1′H)-dione (BFBPD). They entail bipyrrolylidene-2,2′(1H, 1′H)-dione (BPD, known as Pechmann dye) as the electron-accepting core that is flanked by two benzo[b]thiophene moieties and two benzo[b]furan moieties, respectively. Crystal structures of BTBPD and BFBPD provide solid evidence for the intermolecular donor-acceptor (D-A) interactions, which are favorable for improving charge transport performance. Organic field-effect transistors (OFETs) were prepared based on thin films of BTBPD and BFBPD through solution-processed technique. OFETs of BTBPD exhibit relatively high hole mobility up to 1.4 cm² V^-1 s^-1 with high on/off ratio up to 10⁶. In comparison, the hole mobility of OFETs with BFBPD (0.14 cm² V^-1 s^-1) is relatively low, because of the poor thin-film morphology and low molecular ordering, even after annealing. Thin-film morphological and XRD studies were carried out to understand the variation of hole mobilities after annealing at different temperatures. The present studies clearly demonstrate the potentials of BPD that is planar and polar as the electron-acceptor moiety to build D-A molecules for organic semiconductors with good performance.