Synthesis, Photophysical properties and OFET application of thienothiophene and benzothiadiazole based Donor-π-Acceptor-π (D-π -A-π) type conjugated polymers

Novel conjugated donor- π -acceptor- π (D- π -A- π) type polymers (P1-P3), possessing thieno[3,2-b]thiophene (TT) with different aromatic substituents as donors, benzo[1–3]thiadiazole (BT) as an acceptor and thiophene as a π -linker, were designed and synthesized via palladium-catalyzed Stille coupling reaction. Their electronic, optical and thermal properties were investigated using UV–vis and fluorescence spectroscopies, cylic voltammetry, and thermal gravimetric analysis. Photophysical characterizations of these novel polymers showed a remarkable mega Stokes shift, reaching up to 130 nm and optic/electronic band gaps between 1.70 and 2.00 eV, as well as good thermal stability of degradation temperature at around 260 °C. To study the effect of electron donating and withdrawing groups on the electronic properties of the π -extended polymers, their organic field-effect transistors (OFETs) were fabricated and charge transport characteristics were investigated. While all three polymers showed a p-type field-effect behaviour, dimethylamine substituted P3 exhibited the highest average saturated hole mobility, μ_sat, 0.04 cm² V⁻¹ s⁻¹, on/off current ratio, I_on/I_off = 3.0 × 10³, and the smallest subthreshold swing, SS, 250 mV dec^-1, outperforming similar p-type D- π -A- π semiconducting polymers reported in the literature. The results presented in this work corroborate that the three novel TT-BT polymers have promising potential for electronic and optoelectronic applications, in particular where the tunability of field-effect behaviour is essential for performance.