Quinoidal Acenedichalcogenophenediones for Near-Infrared-Absorbing Organic Semiconductors: Effects of Chalcogen Atom Substitution on the Physicochemical and Carrier Transport Properties
Kohsuke Kawabata, Kazuo Takimiya
Abstract
To develop near-infrared (NIR) absorbing organic semiconductors via the donor–acceptor approach, a highly electron-deficient acceptor unit that lowers the LUMO energy level is important to reduce the optical energy gap while keeping its low-lying frontier orbital energy levels. Here, we synthesized and investigated donor–acceptor–donor (D-A-D) triad molecules incorporating two isoelectronic series of benzo[1,2- b:4,5- b ′]dichalcogenophene-2,6-diones and naphtho[1,2- b:5,6- b ′]dichalcogenophene-2,7-diones having different chalcogen atoms, namely, oxygen, sulfur, and selenium atoms. Optical and electrochemical measurements revealed that the triad molecules have low-lying HOMO and LUMO energy levels below −5.0 and −4.0 eV, respectively, with small optical energy gaps of down to 0.76 eV. The key structural feature for the small optical energy gaps is the carbonyl-terminated p -quinodimethane and 2,6-naphthoquinodimethane skeletons in the acenedichalcogenophenediones, which facilitate intramolecular charge transfer from the donor to acceptor units regardless of the chalcogen atoms. On the other hand, the field-effect hole and electron mobilities of the thin-film transistor devices based on the oxygen analogues (∼10 –1 cm 2 V –1 s –1 ) were one order of magnitude higher than those of the sulfur and selenium analogues (∼10 –2 cm 2 V –1 s –1 ). Systematic investigation of the crystal structures, thin-film microstructures, and melting points as well as theoretical calculations revealed that the oxygen analogues have significantly high coplanarity and rigidity of the D-A-D π-conjugated backbone resulting in the low structural and/or energetic disorder in the solid state, which is a reason for the superior carrier transport properties to those of the sulfur and selenium analogues. These molecular insights are helpful for the development of superior donor–acceptor NIR-absorbing organic semiconductors.