Methylthio side-chain modified quinoidal benzo-[1,2-b:4,5-b′] dithiophene derivatives for high-performance ambipolar organic field-effect transistors

Quinoidal small molecule semiconductors hold huge potential in ambipolar organic field-effect transistors (OFETs) and organic spintronic devices. Here, two quinoidal molecules with methylthio side chains were synthesized to develop molecular semiconductors with high ambipolar mobility, designated QBDTS and QTBDTS. The theoretical calculation results reveal that QBDTS has a closed-shell structure while QTBDTS showed an open-shell structure with a biradical character (y(0)) of 0.46 and its magnetic properties were further investigated using electron paramagnetic resonance (EPR) and superconducting quantum interference device (SQUID) methods. The methyl side chains showed a large impact on the molecular orbital levels. The HOMO/LUMO levels of QBDTS and QTBDTS were measured to be -5.66/-4.56 and -5.27/-4.48 eV, respectively, which are favorable for ambipolar charge transport in OFETs. Importantly, the spin-coated QBDTS displayed hole and electron mobilities of 0.01 and 0.5 cm(2) V-1 s(-1) while QTBDTS showed a record high hole mobility of 1.8 cm(2) V-1 s(-1) and electron mobility of 0.3 cm(2) V-1 s(-1). Moreover, comparative studies of the thin film morphologies also manifested the beneficial influence of methyl side chains on film crystallinity and molecule orientation. These results strongly proved that methyl side chain engineering can be a simple but efficient strategy for modulating electronic properties and molecular stacking behaviors. This work also represents a big advancement for quinoidal molecular semiconductors in ambipolar OFET applications.