Four linear π-conjugated systems with 1,3-diethyl-1,3,2-benzodiazaborolyl [C(6)H(4)(NEt)(2)B] as a π-donor at one end and dimesitylboryl (BMes(2)) as a π-acceptor at the other end were synthesized. These unusual push-pull systems contain phenylene (-1,4-C(6)H(4)-; 1), biphenylene (-4,4'-(1,1'-C(6)H(4))(2)-; 2), thiophene (-2,5-C(4)H(2)S-; 3), and dithiophene (-5,5'-(2,2'-C(4)H(2)S)(2)-; 4) as π-conjugated bridges and different types of three-coordinate boron moieties serving as both π-donor and π-acceptor. Molecular structures of 2, 3, and 4 were determined by single-crystal X-ray diffraction. Photophysical studies on these systems reveal blue-green fluorescence in all compounds. The Stokes shifts for 1, 2, and 3 are notably large at 7820-9760 cm(-1) in THF and 5430-6210 cm(-1) in cyclohexane, whereas the Stokes shift for 4 is significantly smaller at 5510 cm(-1) in THF and 2450 cm(-1) in cyclohexane. Calculations on model systems 1'-4' show the HOMO to be mainly diazaborolyl in character and the LUMO to be dominated by the empty p orbital at the boron atom of the BMes(2) group. However, there are considerable dithiophene bridge contributions to both orbitals in 4'. From the experimental data and MO calculations, the π-electron-donating strength of the 1,3-diethyl-1,3,2-benzodiazaborolyl group was found to lie between that of methoxy and dimethylamino groups. TD-DFT calculations on 1'-4', using B3LYP and CAM-B3LYP functionals, provide insight into the absorption and emission processes. B3LYP predicts that both the absorption and emission processes have strong charge-transfer character. CAM-B3LYP which, unlike B3LYP, contains the physics necessary to describe charge-transfer excitations, predicts only a limited amount of charge transfer upon absorption, but somewhat more upon emission. The excited-state (S(1)) geometries show the borolyl group to be significantly altered compared to the ground-state (S(0)) geometries. This borolyl group reorganization in the excited state is believed to be responsible for the large Stokes shifts in organic systems containing benzodiazaborolyl groups in these and related compounds.

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.

. (2012) 'Diazaborolyl-boryl push pull systems with ethynylene arylene bridges as`turn-on' uoride sensors. ', Dalton transactions., 41 (34). pp. 10328-10346. Further information on publisher's website:http://dx.doi.org/10.1039/c2dt30438dPublisher's copyright statement:Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. IntroductionConjugated organic molecules and polymers with three-coordinate boron units as building blocks have attracted considerable interest because of their linear and non-linear optical and electronic properties, which make them potentially useful in functional materials. 1 Three-coordinate boron generally behaves as a π-acceptor due to its vacant p-orbital, which stabilizes the LUMO of an adjacent conjugated π-electron system and thus lowers the HOMO-LUMO gap of these molecules. This field of research has been dominated by the use of the dimesitylboryl group (BMes 2 , Mes = 2,4,6-Me 3 C 6 H 2 ), in which the unsaturated boron centre is stabilized towards oxidation and hydrolysis by the steric shielding of the four orthomethyl groups. [2][3][4][5] The BMes 2 group is considered to have an acceptor strength between that of NO 2 -and CN-groups. 6,7 Such electron-deficient compounds are efficient electron-transporting and/or emitting layers in organic light emitting diodes (OLEDs). 5 Compounds with BMes 2 groups are often strongly coloured and/or luminescent, 8 which renders them useful as colorimetric or luminescent sensors for fluoride ions. [9][10][11][12][13]

A series of diphenylacetylenes with one 1,3,2-benzodiazaborolyl end group (BDB) and a second end group X (X = H, OMe, NMe(2), SMe, CN and BDB) were synthesized using established 1,3,2-benzodiazaborole methodologies. The 1,3,2-benzodiazaborolyldiphenylacetylenes with X = p-H (4), p-OMe (5), p-NMe(2) (6), p-SMe (7) and p-CN (8) end groups are functionalized with cyano groups at the central ring in an ortho-position to the triple bond. Molecular structures of 2, 3, 5, 6 and 7 were determined by X-ray diffraction. These borylated systems show intense blue luminescence in cyclohexane, toluene, chloroform, dichloromethane and tetrahydrofuran, whereas green luminescence was observed in acetonitrile solutions. Thereby Stokes shifts in the range 1700-8600 cm(-1) and quantum yields of 0.60-1.00 were observed in cyclohexane solutions. The absorption maxima (308-380 nm) are well reproduced by TD-DFT computations (B3LYP/G-311G(d,p)) and arise from strong HOMO-LUMO transitions. The LUMOs in all the molecules under study are mainly located on the diphenylacetylene bridge, while with the exception of the dimethylamino derivative 6, the HOMO is largely benzodiazaborolyl in character. Thus, the S1←S0 absorption bands are assigned to π(diazaborolyl)-π*(diphenylacetylene) transitions. In contrast to this, in compound 6 the HOMO is mainly represented by the terminal dimethylaminophenyl unit. While calculated ground state dipole moments μ(g) are small (1.1-7.5 D), experimentally determined changes of the dipole moments upon excitation are large (14.8-19.7 D) and reflect a significant charge transfer upon excitation. NLO activities of the rod-structured compounds 2, 4, 6 and 8 are indicated by calculated static first-order hyperpolarizabilities β up to 76.8 × 10(-30) esu.

Gas-phase He I photoelectron spectra of a series of 2-arylethynyl-1,3,2-benzodiazaboroles BEP-X [1 (X = H), 2 (X = Me), 3 (X = OMe), 4 (X = SMe), 5 (X = NMe2), 6 (X = Br)] have been recorded and assessed by density functional theory calculations. The first ionization energies of these benzodiazaboroles are in the order 5 (7.0 eV) < 3 (7.15 eV) < 2 (7.2 eV) < 1 = 4 = 6 (7.3 eV). As an important result of this study it was disclosed that in contrast to the remaining diazaboroles the frontier orbitals of the amino derivative 5 lack contributions from the diazaborole part of the molecule. In keeping with this, the dipole moment of the ground state (5.51 D) is only slightly changed, to 6.60 D, upon electronic excitation. This also rationalizes the low Stokes shift of 2600 cm−1 in comparison to the other diazaboroles in this study (5900−7300 cm−1).

Reactions of equimolar amounts of dicyclohexylborane (DCB) with a series of 2‐alkynyl‐1,3‐diethyl‐1,3,2‐benzodiazaboroles R–C≡C–B(NEt)2C6H4 (1: R = nBu, 2: R = tBu, 3: R = Ph, 4: R = p‐Me2NC6H4, 5: R = p‐MeOC6H4) regioselectively afforded the 1,1‐diborylalkenes (Z)‐R(H)C=C{(B{NEt}2C6H4)B(c‐C6H11)2} (7a–11a) as the result of a cis‐addition of the BH bond of the borane to the C≡C triple bond in compounds 1 to 5. In contrast to this, reaction of Me3Si–C≡C–B(NEt)2C6H4 (6) with HB(c‐C6H11)2 yielded a 2.5:1 mixture of the 1,2‐diborylated alkene (Z)‐Me3Si{(c‐C6H11)2B}C=CHB(NEt)2C6H4 (12b) and the 1,1‐regioisomer (Z)‐Me3Si(H)C=C{B(NEt)2C6H4}{B(c‐C6H11)2} (12a). These results were rationalized by the differing π‐acceptor qualities of the benzodiazaborolyl and trimethylsilyl substituents at the C≡C triple bond in compounds 1 to 6. The novel products 7–12 were characterized by elemental analysis, mass spectrometry and NMR spectroscopy (1H, 11B, and 13C NMR). The molecular structures of 9b and 12b were substantiated by single‐crystal X‐ray diffraction analysis.

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1,3,2-Diazaboroles 4 and 7 with extended π-electron systems were synthesised and characterised by single crystal X-ray diffraction and quantum-chemical calculations.