Quinoid‐Aromatic Resonance for Very Small Optical Energy Gaps in Small‐Molecule Organic Semiconductors: A Naphthodithiophenedione‐oligothiophene Triad System
Kohsuke Kawabata, Kazuo Takimiya
Abstract
Abstract 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 naphtho[1,2‐ 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 cm 2 V −1 s −1 , respectively, in field‐effect transistors.