Solution processable dithioalkylated methylidenyl cyclopentadithiophene based quinoidal small molecules for n-type organic field-effect transistors

Abstract

A new series of cyclopentadithiophene (CDT)-based quinoidal semiconductors—CDTSQ-8 (1), CDTSQ-10 (2), CDTSQ-12 (3), and CDTSQ-14 (4)—with various dithioalkylated-methylidenyl side chains were designed and synthesized as n-type organic small molecules for solution-processable organic field-effect transistors (OFETs). The physical, electrochemical, and electrical properties of these new compounds were thoroughly investigated. Further, single-crystal structures of CDTSQ-10, CDTSQ-12, and CDTSQ-14 were obtained. Optimized geometries obtained from single-crystal X-ray diffraction revealed the planarity of the central core. The smaller dihedral angle between dithioalkyl methylene and the CDT core (4.8°), a planar molecular structure, short main-core stacking distance (3.43 Å), short intramolecular (S⋯H), and intermolecular (S⋯N, N⋯H, and N⋯N) distances of the CDTSQ-12 molecule suggest good conditions for the extended π-orbital interaction of the corresponding molecule, resulting in better device performance. The favorable molecular packing and low-lying LUMO energy level (−4.10 eV) suggest that CDTSQs could be electron-transporting semiconductors. Thin-film morphological analysis by grazing-incidence wide-angle X-ray scattering revealed that all CDTSQ molecules are stacked on the substrate in an edge-on fashion. OFETs based on solution-sheared CDTSQ-12 exhibited the highest electron mobility of 0.14 cm² V⁻¹ s⁻¹ with good ambient stability. The electron mobility of solution-processable CDTSQ is 14 times larger than that of a previously reported dilakylated-CDT-based CDTRQ (0.01 cm² V⁻¹ s⁻¹, vacuum-processed) quinoidal derivative. These results demonstrate that side-chain engineering can improve the device performance of solution-sheared CDTSQ organic semiconductors.