Quinoid-Aromatic Resonance for Very Small Optical Energy Gaps in Small-Molecule Organic Semiconductors: A Naphthodithiophenedione-oligothiophene Triad System

Organic semiconductors with very small optical energy gaps have attracted a lot of attention for near-infrared-active optoelectronic applications. Herein, we present a series of donor-acceptor-donor (D?A?D) organic semiconductors consisting of a highly electron-deficient naphtho1,2-b:5,6-b′]dithiophene-2,7-dione quinoidal acceptor and oligothiophene donors that show very small optical energy gaps of down to 0.72 eV in the solid state. Investigation of the physicochemical properties of the D?A?D molecules as well as theoretical calculations of their electronic structures revealed an efficient intramolecular interaction between the quinoidal acceptor and the aromatic oligothiophene donors in the D?A?D molecules; this significantly enhances the backbone resonance and thus reduces the bond length alternation along the π-conjugated backbones. Despite the very small optical energy gaps, the D?A?D molecules have low-lying frontier orbital energy levels that give rise to air-stable ambipolar carrier transport properties with hole and electron mobilities of up to 0.026 and 0.043 cm² V^?1 s^?1, respectively, in field-effect transistors.