Core- versus\nEnd-Alkylation: Tailoring Solid-State\nStructures and Properties of Near-Infrared-Absorbing Organic Semiconductors\nBased on Naphthodithiophenediones

In the design of\nnear-infrared (NIR)-absorbing organic semiconductors,\nnot only chromophoric π-conjugated backbones that govern the\nmolecular electronic structures but also solubilizing substituents,\nwhich can significantly affect the solid-state packing structures,\nare crucial factors in tailoring solid-state optical and electrical\nproperties as well as solution processability. In this study, we systematically\ninvestigated the structures and properties of a series of core- and\nend-alkylated donor–acceptor–donor triads based on a\nnaphthodithiophenedione acceptor to elucidate the impact of the alkylation\nsite on their solid-state structures and properties. Although the\nalkylation site marginally affects their molecular electronic structures,\nall of the core-alkylated triads showed significant red shifts of\nthe main absorption band from the solution to the solid state, in\nsharp contrast to the blue shifts observed for the end-alkylated triads.\nSingle-crystal X-ray analysis and thin-film X-ray diffraction revealed\nthat the contrasting solid-state optical properties are likely attributed\nto the distinctly different arrangements of the chromophores in the\nsolid state, depending on the alkylation site. The end-alkylated triads\ntend to form interchromophore cofacial and/or side-by-side arrangements,\nwhich results in the blue shift of absorption bands, whereas the core-alkylated\ntriads tend to avoid such interchromophore arrangement and rather\npromote an in-plane orientation of the transition electric dipole\nmoments in a slip-stacking manner, which leads to the red shift of\nabsorption bands. Furthermore, the alkyl groups on the core effectively\nreduce the core-to-core interaction, thus improving the solubility\nof materials without compromising carrier transport properties. These\nresults provide valuable insights into molecular design for developing\nsolution-processable NIR-absorbing organic semiconductors.