Diazaborolyl-boryl push–pull systems with ethynylene–arylene bridges as ‘turn-on’ fluoride sensors
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 (Φfl) of 0.99, 0.44 and 0.94, respectively, but weak blue-green luminescence in tetrahydrofuran (Φfl = 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.