Design, Synthesis And Properties Of Novel Benzothiazole Organoboron

Boron dipyrromethenes(BODIPYs) are an outstanding class of organic dye molecules that have been wildly applied in the field of fluorescent sensing/imaging, biological labelling, and dye-sensitized solar cells. The success of BODIPYs in these applications is attributed to their excellent properties(e.g. high molar absorption coefficients and fluorescence quantum yields, and tunable emission from visible light to near infrare) in their diluted solution state. However, the solid-state emission properties of most BODIPY derivatives have been found to be poor. The non-emissive property in solid state is mainly due to the flat and symmetrical ?-conjugated systems, which facilitates tight packing in most cases, and thus significant luminescence quenching via ?–? interactions. Their small Stokes shifts also lead to self-quenching through energy transfer. These have greatly limited the practical applications of BODIPYs in the field of material science.In this paper, we developed the synthesis and luminescence properties of benzothiazole-enolate-ligandbased boron complexes with asymmetrical and propeller shaped structures showed interesting fluorescence properties such as large Stokes shift, high solid-state emission and AIE effect, which promoted the practical applications of BODIPYs in the field of material science. Photophysical properties of the dyes in solution and solid-state were investigated systematically via spectroscopic and theoretical methods. The main results are as follows:1. The synthesis of boron-benzothiazole-pyrimidine dyes, a novel type of BODIPY analogue, involves a facile and scalable two-step reaction. Broad envelopes of intense vibrational bands and large Stokes shift(660-2120 cm–1) are observed for the main spectral bands in the absorption and emission spectra. High fluorescence quantum yields(0.27-0.79) are obtained in Hexane, with significant intensity also observed in the solid-state(?fmax = 0.14). Current results indicate that N^N-bidentate ligand-based BODIPYs provides a valuable approach for developing solid-state emissive BODIPYs. Besides, compounds 1, 2 and 9 were developed as new fluorescent sensors for Cys. The sensors exhibit highly selective “turn-on” response to cysteine over Hcy, GSH and other amino acids in aqueous solution at physiological pH. The compound 9 was successfully applied in living cells, indicating that these sensors hold great potential for biological applications.2. By changing pyrimidine unit into a benzene ring to rotate freely, a new type of N^N-bidentate ligand-based BODIPY analogs 10-15 with aggregation-induced emission(AIE) are synthesized through. These BODIPY analogs show broad envelopes of intense vibrational bands and large Stokes shift(3590-7400 cm–1) in the absorption and emission spectra with moderate fluorescence quantum yields. In comparison to those of classical BODIPYs, significant fluorescence intensity(?fmax = 0.10) were observed for these analogs in solid-state. Moreover, AIE effect is also observed for these analogs in the THF/water mixture. Although almost completely overlaps bulk form like traditional symmetrical structure BODIPY molecule was not observed through the introduction of benzene ring increasing intermolecular distance, the compounds 10–15 still existed partial overlaps within the crystals, and thus, leading these compounds were not with significant intensity in the solid-state. The currently proposed N^N-bidentate ligand and AIE are a reliable strategy to construct BODIPYs showed interesting fluorescence properties such as large Stokes shift and high solid-state emission.3. A new series of BF2 luminophores with propeller-shape structures were developed by reacting benzothiazole-enamide/enolate-based desymmetrized N,N-bidentate ligands with boron trifluoride etherate or triphenyl borane. Broad envelopes of intense vibrational bands were observed for the main spectral bands in the absorption and emission spectra. Due to the intramolecular rotation induced nonradiative process, the complexes exhibited AIE characteristics and fluorescence efficiently in the aggregate state(?fmax = 0.43). Furthermore, compounds 17, 19 and 22–25 were capable of sensing of acidic gases by reversible changes of emission, which may potentially serve as the solid-state luminescent sensors for acidic vapors. The currently proposed desymmetrized N,N-bidentate-ligand-based boron complexes having a propeller-shaped structure a reliable strategy to construct BODIPYs showed intense solid-state emission.