Diazaborolyl-boryl push–pull systems with ethynylene–arylene bridges as ‘turn-on’ fluoride sensors

Abstract

Two linear π-conjugated systems with 1,3-diethyl-1,3,2-benzodiazaborolyl [C₆H₄(NEt)₂B–] as a donor group and dimesitylboryl (–BMes₂) as acceptor were synthesised with –ethynylene–phenylene- (–CC-1,4-C₆H₄–, 3) and –ethynylene–thiophene– (–CC-2,5-C₄H₂S– 12) bridges between the boron atoms. An assembly (20) consisting of two diazaborolyl-ethynylene–phenylene-boryl units, [C₆H₄(NCy)(N′)B–CC-1,4-C₆H₄-BMes₂] joined via a 1,4-phenylene unit at the nitrogen atoms (N′) of the diazaborolyl units was also synthesised. The three push–pull systems, 3, 12 and 20, form salts on fluoride addition with the BMes₂ groups converted into (BMes₂F)⁻ anions. The molecular structures of 3, 12 and (NBu₄)(12·F) were elucidated by X-ray diffraction analyses. The borylated systems 3, 12 and 20 show intense blue luminescence in cyclohexane with quantum yields (Φ_fl) of 0.99, 0.44 and 0.94, respectively, but weak blue-green luminescence in tetrahydrofuran (Φ_fl = 0.02–0.05). The charge transfer nature of these transitions is supported by TD-DFT computations with the CAM-B3LYP functional. Addition of tetrabutylammonium fluoride to tetrahydrofuran solutions of 3 and 20 resulted in strong violet-blue luminescence with emission intensities up to 46 times more than the emission intensities observed prior to fluoride addition. Compounds 3 and 20 are demonstrated here as remarkable ‘turn-on’ fluoride sensors in tetrahydrofuran solutions.