Solvatochromism and fluoride sensing of thienyl-containing benzodiazaboroles

Abstract

Static and time-resolved fluorescence studies were carried out to investigate the photophysical properties and fluoride sensing abilities of highly fluorescent thienyl-containing 1,3-diethyl-1,3,2-benzodiazaboroles. Absorption and fluorescence spectra were measured in various solvents, showing the fluorophores to emit in the visible wavelength region with colors varying from blue to orange and quantum yields ranging between 0.21 and 1. Measured Stokes shifts of 2898 cm(-1) to 9308 cm(-1) were used to calculate the difference between excited- and ground-state dipole moments of the fluorophores. Values up to 18.8 D are of the same magnitude as for designed polarity probes such as PRODAN, supporting the idea of internal charge transfer transitions. Quenching studies with pyridine observing static and time-resolved fluorescence revealed a purely dynamic quenching mechanism and low Lewis acidity of the boron within the benzodiazaborolyl moiety compared to other triarylboranes. In contrast to this, quenching with fluoride was shown to stem from adduct formation. Reversible complexation of fluoride follows a simple mechanism for multi-functionalized benzodiazaboroles 2b and 2c, while those containing only one benzodiazaborole moiety (1 and 2a) show a more complicated behaviour, which might be explained by aggregation. Combining a benzodiazaborole group and a dimesitylborane function results in spectrally switchable fluoride sensors 3a and 3b, since the two boron sides can be deactivated for fluorescence in a stepwise manner.

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

“…Fluoride anion additions of diazaboroles III, IV, VI, XI-XIV have been explored by spectroscopic means but no salts were isolated. 24,25 Detailed fluoride additions have been carried out on diboron compounds (Chart II, XV-XIX) with various bridges between the two BMes 2 groups elsewhere. 4j These are looked at here for comparison with the bisdiazaborole 20 to estimate the affinity strengths of the second BMes 2 group towards the fluoride anion.…”

“…4j These are looked at here for comparison with the bisdiazaborole 20 to estimate the affinity strengths of the second BMes 2 group towards the fluoride anion. XI [24] XII [24,25] XIII [24] XIV [25] XV [5j]…”

“…For synthetic reasons the 1,3-diethyl-1,3,2-benzodiazaborole unit is the most frequently employed representative, and compounds containing this group as a substituent are moderately air-stable. 16,[22][23][24][25][26][27] As the BMes 2 group is known as an effective acceptor (A) and the benzodiazaborolyl unit has been suggested to be a π-donor (D) 27 the novel "push-pull"-systems [D-bridge-A] with 1,4-phenylene-, 4,4'-biphenylene-, 2,5-thiophene-and 5,5'-dithiophene-scaffolds (I-IV, Chart I) have been investigated recently. 28 Photophysical studies on these compounds reveal blue-green fluorescence and Stokes shifts for the first three representatives of 7820-9760 cm -1 in THF, whereas the Stokes shift for the last compound is significantly smaller (5510 cm -1 in THF).…”

The role of 2,3-dihydro-1-H-1,3,2-diazaboroles in luminescent molecules

“…Syntheses of 73, 74, 76 and 77 [55]. [52]. This shows that despite the fact that the benzodiazaborolyl functions as a -donor, acceptor properties are still present.…”

“…For the equilibrium constant Table 14 Reduction potentials from cyclic voltammetry data of 103, 104 and 1,2-Ph2-1,2-C2B10H10 with a glassy working electrode in acetonitrile [a] or a platinum electrode in dichloromethane [b] versus Fc + /Fc-standard. of adduct formation between 21 and TBAF in CDCl 3 at 20 • C a value of 5.2 × 10 −4 was estimated by 19 F NMR spectroscopy [32,52].…”