Two donoracceptor-type (DA-type) molecules containing thiazole-fused benzothiadiazole (BT) as the acceptor moiety were designed and synthesized. The thiazole-fused BT skeleton enables fine-tuning of the solubility and the electronic structure as well as two-dimensional extension of the π-conjugation. The combination with common donor units such as diphenylaminothiophene or triphenylamine-substituted thiophene resulted in the formation of DAD-type molecules, which exhibited fluorescence in the NIR region.Keywords: Near-infrared (NIR) emission | Benzothiadiazole | PhotostabilityFluorophores with strong emission in the near-infrared (NIR) region have received attention as NIR organic light-emitting diodes (OLEDs), which find applications in night vision-readable displays and sensors, 1 as well as in fluorescent probes for bioimaging. 2 For the latter, an emission in the biological window (650900 nm) is advantageous, where the effect of autofluorescence and absorption of tissue is minimized. So far, a variety of organic dyes such as borondipyrromethene (BODIPY), 2a,3 squaraine, 4 or cyanine-based dyes, 5 as well as transition-metal complexes, 1 have been reported as NIR-emissive materials. In addition to conventional dyes, donor (D)acceptor (A)-type molecules have also attracted considerable interest, as their optical properties can be easily tuned by varying the D and A units.6 From a molecular design perspective and in the interest of improved stability under atmospheric conditions, lowering the LUMO energy is preferable to raising the HOMO level.7 Therefore, strong acceptor units are highly important for generating materials with stable NIR emission. 2,1,3-Benzothiadiazole (BT) is a widely used acceptor unit, whose strong electronaccepting ability arises from the presence of two highly polarized C=N double bonds.8 However, structurally and synthetically it is difficult to introduce substituents in conventional BTs in order to increase the solubility and/or to fine-tune the electron-accepting properties. Recently, we have developed a thiazole-fused BT as a new acceptor unit. In the compounds containing this unit, the solubility and electronic structure can be fine-tuned by introducing substituents at the 2-position of the fused thiazole ring, while the π-conjugation is extended two-dimensionally via the fused thiazole moiety.9 Herein, we report a new and more effective synthetic method for the construction of the thiazole-fused BT skeleton. The combination of this unit with common D units such as diphenylaminothiophene or triphenylamine-substituted thiophene afforded DAD-type molecules that exhibited emission in the NIR region (Figure 1a). Furthermore, we examined the effect of different electron-donating units on the electrochemical and photophysical properties of these compounds (Figure 1b). Previously, we reported the synthesis of thiazole-fused benzothiadiazole 3a with the 2-ethylhexyl group by the thermal radical cyclization of thioamide 5a at 140°C under microwave irradiation (Scheme 1).9 While 3a, containin...

2,1,3-Benzothiadiazole (BT) is a widely used electron-accepting unit in organic electronics including organic solar cells (OSCs). As modifications of BT skeleton, two types of azolefused BT units were designed and synthesized; thiazole-fused BT with an electronwithdrawing C=N bond and imidazole-fused BT with an electron-donating nitrogen atom as well as an electron-withdrawing C=N bond. Electrochemical measurements and theoretical calculations suggest that thiazole-fusion enhances the electron-accepting ability, whereas imidazole-fusion endows the BT skeleton with electron-donating ability while maintaining its electron-accepting ability. Moreover, in thiazole-fused BT units, the electronic structure could be further modulated by varying the oxidation state of the sulfur atom in methylthio group at the fused thiazole ring.