Dicyanovinyl-substituted indolo[3,2-b]indole derivatives: low-band-gap π-conjugated molecules for a single-component ambipolar organic field-effect transistor

Abstract

A series of low-band-gap π-conjugated molecules comprising N,N′-dihexylindolo[3,2-b]indole as an electron donor (D) and dicyanovinyl as an electron acceptor (A) with A–π–D–π–A architecture have been designed and synthesized to fabricate a single-component ambipolar organic field-effect transistor (OFET). Molecules with different π-bridging units (none, thiophene, and bithiophene) were synthesized and characterized to investigate their structure–property correlation. Via the cooperative effects of intramolecular charge transfer (ICT) interactions and extension of conjugation, the band gap of the newly synthesized molecules was reduced to 1.41 eV in the solution state. Among other compounds, 2H2TIDID-DCV (with a thiophene π-spacer) exhibited highly balanced ambipolar charge transport with hole and electron mobilities of 0.08 cm² V⁻¹ s⁻¹ and 0.09 cm² V⁻¹ s⁻¹, respectively, from a vacuum-deposited OFET device. Spin-coated OFET devices using OD2TIDID-DCV, in which the hexyl side chains of 2H2TIDID-DCV are replaced by 2-octyldodecyl groups, also exhibited an ambipolar charge-transporting nature (mobilities of 9.67 × 10⁻² cm² V⁻¹ s⁻¹ for holes and 3.43 × 10⁻³ cm² V⁻¹ s⁻¹ for electrons). Both 2H2TIDID-DCV and OD2TIDID-DCV exhibited favorable thin-film morphology for the formation of charge-transporting channels, and structural analyses of these films revealed the same molecular packing characteristics of a three-dimensional lamellar π-stacking structure.