Experimental and Theoretical Studies on Organic D‐π‐A Systems Containing Three‐Coordinate Boron Moieties as both π‐Donor and π‐Acceptor

Abstract

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.

Oligo(p‐phenyleneethynylene) (OPE) Molecular Wires: Synthesis and Length Dependence of Photoinduced Charge Transfer in OPEs with Triarylamine and Diaryloxadiazole End Groups

“…The hybrid functional CAM-B3LYP is used to compute charge transfer transitions as it has the necessary physics to model charge contributions more correctly than the widely used B3LYP functional. [37] TD-DFT data are summarised in Table 4 The computed absorption maxima for 2a and 6a with similar molecular lengths agree with those observed experimentally for 2 and 6 in non-polar media. Such similarities also apply to the longer molecules, 3a, 7a, 3 and 7.…”

On the ambiguity of 1,3,2-benzodiazaboroles as donor/acceptor functionalities in luminescent molecules

“…20 The data summarised in Table 3 show very good agreement between the observed absorption maxima of the strong bands and the computed maxima with predicted strong bands. However, bands with lower oscillator strengths are predicted at lower energies for 6, 7, 9 and 10.…”

“…19 It was experimentally confirmed that the π-donor capacity of the 1,3-diethyl-1,3,2-benzodiazaborolyl group towards the BMes 2 -acceptor is between those of 3 methoxy-and dimethylamino-groups. 20 Benzodiazaboroles are also efficient donors when linked to the carbon atoms of carborane clusters. 25,26 In a recent paper, we reported on a series of carbazolylphenyl-and carbazolylthienylderivatives of benzodiazaboroles, 1-4, (Chart 1) where both N-atoms were substituted by electron-withdrawing pentafluorophenyl or tetrafluoropyridyl groups.…”

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

“…Titration experiments were performed with a commercially available tetrabutylammonium fluoride solution (1 M in THF). 28 NMR spectra were recorded at room temperature with a Bruker AM Avance DRX-500 spectrometer { 1 H, 11 B, 13 C} by using TMS and BF 3 ·OEt 2 as external standards. Mass spectra were taken with a VG autospec sector field mass spectrometer (Micromass).…”

“…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). For comparison with closely related derivatives 3 and V-X, the cyano compound 5 was synthesized here by the lithiation of 4-cyanophenylacetylene 4 31 in THF and the subsequent treatment of the organolithium species with an equimolar amount of 2-bromo-1,3,2-benzodiazaborole 1.…”