Design and synthesis of stable indigo polymer semiconductors for organic field-effect transistors with high fluoride sensitivity and selectivity

We report the design and synthesis of two novel indigo donor–acceptor (D–A) polymers, PIDG-T-C20 and PIDG-BT-C20, comprising an indigo moiety that has intramolecular hydrogen-bonds as the acceptor building block and thiophene (T) and bithiophene (BT) as the donor building block, respectively. PIDG-T-C20 and PIDG-BT-C20 exhibited characteristic p-type semiconductor performance, achieving hole mobilities of up to 0.016 and 0.028 cm(2) V(−1) s(−1), respectively, which are highest values reported for indigo-based polymers. The better performing PIDG-BT-C20 was used for the fabrication of water-gated organic field-effect transistors (WGOFETs), which showed excellent stability at ambient conditions. The PIDG-BT-C20-based WGOFETs exhibited rapid response when fluoride ions were introduced to the water gate dielectric, achieving a limit of detection (LOD) of 0.40 mM. On the other hand, the devices showed much lower sensitivities towards other halide ions with the order of relative response: F(−) ≫ Cl(−) > Br(−) > I(−). The high sensitivity and selectivity of PIDG-BT-C20 to fluoride over other halides is considered to be realized through the strong interaction of the hydrogen atoms of the N–H groups in the indigo unit with fluoride ions, which alters the intramolecular hydrogen-bonding arrangement, the electronic structures, and thus the charge transport properties of the polymer.