Benzothiadiazole-dithienopyrrole donor-acceptor-donor and acceptor-donor-acceptor triads: Synthesis and optical, electrochemical, and charge-transport properties

2,2′-(Benzo[c][1,2,5]thiadiazol-4,7-diyl)-4,4′-dialkyl-bis(4H- dithieno[3,2-b:2′,3′-d]pyrrole) (DTP-BTD-DTP) donor-acceptor-donor (D-A-D) and 4-alkyl-2,6-bis(benzo[c][1,2,5]thiadiazol-4-yl)-4H-dithieno[3,2-b: 2′,3′-d]pyrrole (BTD-DTP-BTD) acceptor-donor-acceptor (A-D-A) triads, with or without additional alkylation in the DTP 6- or BTD 7-positions, respectively, have been synthesized using Stille coupling reactions, characterized using UV-vis absorption spectroscopy and electrochemistry, modeled using density functional theory calculations, and used as charge-transport materials in field-effect transistors. The choice of alkyl substitution pattern has only minor effects on the optical and redox behavior but can be used to modify the thermal properties and solubility of these compounds. The D-A-D and A-D-A triads show long-wavelength absorption maxima at 566-588 and 517-521 nm, respectively, in solution. These transitions are attributed to excitation from a delocalized HOMO to a BTD-localized LUMO and, accordingly, are bathochromically shifted from those of analogous compounds in which the BDT moieties are replaced by benzene rings (393-417 nm). The triads are oxidized at potentials of +0.01 to +0.37 V vs ferrocenium/ferrocene and are reduced at potentials of -1.95 to -1.74 V, with the D-A-D species being both the most easily oxidized and most easily reduced. Field-effect transistors based on solution-processed films of some of the triads showed p-channel behavior; the highest average hole mobility value measured was 5.9 × 10^-3 cm² V ^-1 s^-1 for 2,2′-(benzo[c][1,2,5]thiadiazol-4,7-diyl) -4,4′-di-n-dodecyl-bis(4H-dithieno[3,2-b:2′,3′-d]pyrrole) and was accompanied by an on/off current ratio of ca. 10³.