Core- versus End-Alkylation: Tailoring Solid-State Structures and Properties of Near-Infrared-Absorbing Organic Semiconductors Based on Naphthodithiophenediones

In the design of near-infrared (NIR)-absorbing organic semiconductors, not only chromophoric π-conjugated backbones that govern the molecular electronic structures but also solubilizing substituents, which can significantly affect the solid-state packing structures, are crucial factors in tailoring solid-state optical and electrical properties as well as solution processability. In this study, we systematically investigated the structures and properties of a series of core- and end-alkylated donor–acceptor–donor triads based on a naphthodithiophenedione acceptor to elucidate the impact of the alkylation site on their solid-state structures and properties. Although the alkylation site marginally affects their molecular electronic structures, all of the core-alkylated triads showed significant red shifts of the main absorption band from the solution to the solid state, in sharp contrast to the blue shifts observed for the end-alkylated triads. Single-crystal X-ray analysis and thin-film X-ray diffraction revealed that the contrasting solid-state optical properties are likely attributed to the distinctly different arrangements of the chromophores in the solid state, depending on the alkylation site. The end-alkylated triads tend to form interchromophore cofacial and/or side-by-side arrangements, which results in the blue shift of absorption bands, whereas the core-alkylated triads tend to avoid such interchromophore arrangement and rather promote an in-plane orientation of the transition electric dipole moments in a slip-stacking manner, which leads to the red shift of absorption bands. Furthermore, the alkyl groups on the core effectively reduce the core-to-core interaction, thus improving the solubility of materials without compromising carrier transport properties. These results provide valuable insights into molecular design for developing solution-processable NIR-absorbing organic semiconductors.