Near-infrared Emissive Donor–Acceptor-type Molecules Containing Thiazole-fused Benzothiadiazole as an Electron-acceptor Moiety

Abstract

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...

Fused Bis-Benzothiadiazoles as Electron Acceptors

“…Electron-deficient (hetero)­aromatic moieties are key components of n-type organic semiconducting materials. − Various electron acceptor moieties such as dicyanovinylene, − diketopyrrolopyrrole (DPP), , [1,2,5]-thiadia-zolo­[3,4-g]­quinoxaline (TQ), − benzothiadiazole (BT), perylene diimide (PDI), − naphthalene diimide (NDI), and isoindigo (IID) have already been developed. Among them, BT is a particularly popular case and has been introduced into oligomers and polymers for use in (opto)­electronic devices. ,− Moreover, some other BT derivatives, such as thiazole-fused BTs, , thiadiazolopyridine, , and thiadiazoloquinoxaline have also been synthesized, which largely enriched the BT-based electron acceptors.…”

“…Among them, BT is a particularly popular case and has been introduced into oligomers and polymers for use in (opto)electronic devices. 16,22−34 Moreover, some other BT derivatives, such as thiazole-fused BTs, 35,36 thiadiazolopyridine, 37,38 and thiadiazoloquinoxaline 39 have also been synthesized, which largely enriched the BT-based electron acceptors.…”

Moving Beyond Boron-Based Substituents To Achieve Phosphorescence in Tellurophenes

“…Given the widespread use of the triphenylamine (TPA) substituent as an electron donor and two-photon absorbing unit in optoelectronics, − we decided to investigate its influence on photophysical properties by replacing the BPin substituents in B–Te–6–B with TPA groups. Added motivation stemmed from the possible use of these species as hole transporting/electron-blocking layers in perovskite solar cells. − The synthesis of the triphenylamine (TPA) derivative TPA–Te–6–TPA proceeded via a similar zirconium-mediated alkyne coupling protocol (Scheme ) as previously reported. − ,− This reaction yields the five-membered ZrC 4 zirconacycle TPA–ZrCp 2 –6–TPA as an intermediate species, which can be directly converted in situ into the target tellurophene via Zr/Te transmetalation (Scheme ) with the Te­(II) source Bipy·TeCl 2 (Bipy = 2,2′-bipyridine).…”

Synthesis of Azole-fused Benzothiadiazoles as Key Units for Functional π-Conjugated Compounds

“…We have previously reported the construction of thiazole-fused BT skeleton by thermal [11] or oxidative [12] cyclization of thioamide. In those cases, aryl groups have to be introduced at 4,7-position of BT before the cyclization.…”

“…1c, d). We anticipated that thiazole-fusion enables the further lowering of LUMO energy level by the existence of the electron-withdrawing imine moiety [11,12] (Fig. 2).…”