Synthese, Struktur, Elektrochemie und optische Eigenschaften von alkinylfunktionalisierten 1,3,2-Diazaborolen und 1,3,2-Diazaborolidinen

A carbon-functionality-appended diborylacetylene available for a component of organic synthesis and OLEDs

We present new 1,2-diborylacetylene derivatives with planar 9-oxa-10-boraanthracene termini, which display excellent stability to allow usual handling and even thermal evaporation for the preparation of thin films for OLEDs, and also undergo typical reactions of alkynes such as the Diels-Alder reaction.

Syntheses and structures of benzo-bis(1,3,2-diazaboroles) and acenaphtho-1,3,2-diazaboroles

Colorless crystalline 2,6-dibromo-4,8-dimethyl-1,3,5,7-tetraphenylbenzobis(diazaborole) 4 resulted from the cyclocondensation of 3,6-dimethyl-1,2,4,5-tetraphenylaminobenzene 3d with two equivalents of boron tribromide in the presence of calcium hydride. Synthesis of the dark-red crystalline 2-bromo-N,N'-bis(diisopropylphenyl)acenaphtho-1,3,2-diazaborole 7 was effected by the cyclocondensation of 1,2-bis(N-2',6'-diisopropylphenylimino)acenaphthene (5) and boron tribromide with subsequent sodium amalgam reduction of the initially formed burgundy red diazaborolium salt 6. Compounds 4, 6 and 7 are characterised by elemental analyses, ¹H, ¹¹B and ¹³C NMR spectroscopy, as well as by single X-ray diffraction studies. The electronic structures of 4, 6 and 7 are subject to DFT calculations.

C,C'-bis(benzodiazaborolyl)dicarba-closo-dodecaboranes: synthesis, structures, photophysics and electrochemistry

Six new C,C'-bis(benzodiazaborolyl)dicarba-closo-dodecaboranes, 1,A-R2-1,A-C2B10H10, where R represents the group 2-(1,3-Et2-1,3,2-N2BC6H4) or 2-(1,3-Ph2-1,3,2-N2BC6H4) and A is 2, 7 or 12, were synthesized from o-, m-, and p-dicarbadodecaboranes (carboranes) by lithiation and subsequent treatment with the respective 2-bromo-1,3,2-benzodiazaboroles. UV-visible and fluorescence spectra of all carboranes display low energy charge transfer emissions. While such emissions with Stokes shifts between 17,330 and 21,290 cm(-1) are typical for C,C'-bis(aryl)-ortho-carboranes, the observed low-energy emissions with Stokes shifts between 8320 and 15,170 cm(-1) for the meta- and para-isomers are unusual as high-energy emissions are typical for meta- and para-dicarbadodecaboranes. Fluorescence quantum yields (φF) for the novel 1,7- and 1,12-bis(benzodiazaborolyl)-carboranes depend on the substituents at the nitrogen atoms of the heterocycle. Thus, the para-carborane with N-ethyl substituents 1,12-(1',3'-Et2-1',3',2'-N2BC6H4)2-1,12-C2B10H10 has a φF value of 41% in cyclohexane solution and only of 9% in the solid state, whereas the analogous 1,12-(1',3'-Ph2-1',3',2'-N2BC6H4)2-1,12-C2B10H10 shows quantum yields of 3% in cyclohexane solution and 72% in the solid state. X-ray crystallographic, computational and cyclic voltammetry studies for these carboranes are also presented.

Luminescence properties of C-diazaborolyl-ortho-carboranes as donor-acceptor systems

Seven derivatives of 1,2-dicarbadodecaborane (ortho-carborane, 1,2-C(2)B(10)H(12)) with a 1,3-diethyl- or 1,3-diphenyl-1,3,2-benzodiazaborolyl group on one cage carbon atom were synthesized and structurally characterized. Six of these compounds showed remarkable low-energy fluorescence emissions with large Stokes shifts of 15100-20260 cm(-1) and quantum yields (Φ(F)) of up to 65% in the solid state. The low-energy fluorescence emission, which was assigned to a charge-transfer (CT) transition between the cage and the heterocyclic unit, depended on the orientation (torsion angle, ψ) of the diazaborolyl group with respect to the cage C-C bond. In cyclohexane, two compounds exhibited very weak dual fluorescence emissions with Stokes shifts of 15660-18090 cm(-1) for the CT bands and 1960-5540 cm(-1) for the high-energy bands, which were assigned to local transitions within the benzodiazaborole units (local excitation, LE), whereas four compounds showed only CT bands with Φ(F) values between 8-32%. Two distinct excited singlet-state (S(1)) geometries, denoted S(1)(LE) and S(1)(CT), were observed computationally for the benzodiazaborolyl-ortho-carboranes, the population of which depended on their orientation (ψ). TD-DFT calculations on these excited state geometries were in accord with their CT and LE emissions. These C-diazaborolyl-ortho-carboranes were viewed as donor-acceptor systems with the diazaborolyl group as the donor and the ortho-carboranyl group as the acceptor.

Luminescent Diazaborolyl-Functionalized Polystyrene

We present two different procedures for the synthesis of poly[4-(1',3'-diethyl-1',3',2'-benzodiazaborolyl)styrene] (3a) and poly[4-(1',3'-diphenyl-1',3',2'-benzodiazaborolyl)styrene] (3b). The new polymers were fully characterized by GPC, multinuclear NMR, and elemental analysis. The thermal properties and stability were studied by DSC and TGA, and the optical characteristics were examined by absorption and time-resolved fluorescence spectroscopy. Remarkably high quantum yields of up to 77% were measured. In comparison to molecular species we found significantly shorter lifetimes, likely as a result of incorporation of the chromophores into the polymer structure.

Synthetic, structural, photophysical and computational studies on 2-arylethynyl-1,3,2-diazaboroles

New 2-arylalkynyl benzo-1,3,2-diazaboroles, 2-(4'-XC(6)H(4)C[triple bond, length as m-dash]C)-1,3-Et(2)-1,3,2-N(2)BC(6)H(4) (X =Me ; MeO ; MeS ; Me(2)N ), were prepared from B-bromodiazaborole, 2-Br-1,3-Et(2)-1,3,2-N(2)BC(6)H(4), with the appropriate lithiated arylacetylene, ArC[triple bond, length as m-dash]CLi. Molecular structures of , and were determined by X-ray diffraction studies. UV-vis and luminescence spectroscopic studies on these diazaboroles reveal intense blue/violet fluorescence with very large quantum yields of 0.89-0.99 for . The experimental findings were complemented by DFT and TD-DFT calculations. The Stokes shift of only 2600 cm(-1) for , compared to Stokes shifts in the range of 5900-7300 cm(-1) for , is partly explained by the different electronic structures found in compared to (X = H). The HOMO is mainly located on the aryl group in and on the diazaborolyl group in whereas the LUMOs are largely aryl in character for all compounds. Thus, in contrast to other conjugated systems containing three-coordinate boron centers such as B(Mes)(2), (Mes = 2,4,6-Me(3)C(6)H(2)), in which the boron serves as a pi-acceptor, the 10-pi electron benzodiazaborole moiety appears to function as a pi-donor moiety.

B-N as a C-C substitute in aromatic systems

The substitution of isoelectronic B–N units for C–C units in aromatic hydrocarbons produces novel heterocycles with structural similarities to the all-carbon frameworks, but with fundamentally altered electronic properties and chemistry. Since the pioneering work of Dewar some 50 years ago, the relationship between B–N and C–C and the wealth of parent all-carbon aromatics has captured the imagination of organic, inorganic, materials, and computational chemists alike, particularly in recent years. New applications in biological chemistry, new materials, and novel ligands for transition-metal complexes have emerged from these studies. This review is aimed at surveying activity in the area in the past couple of decades. Its organization is based on ring size and type of the all-carbon or heterocyclic subunit that the B–N analog is derived from. Structural aspects pertaining to the retention of aromaticity are emphasized, along with delineation of significant differences in physical properties of the B–N compound as compared to the C–C parent.Key words: boron-nitrogen heterocycles, aromaticity, organic materials, main-group chemistry.