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

Abstract

New 2-arylalkynyl benzo-1,3,2-diazaboroles, 2-(4′-XC₆H₄CC)-1,3-Et₂-1,3,2-N₂BC₆H₄ (X =Me 2; MeO 3; MeS 4; Me₂N 5), were prepared from B-bromodiazaborole, 2-Br-1,3-Et₂-1,3,2-N₂BC₆H₄, with the appropriate lithiated arylacetylene, ArCCLi. Molecular structures of 2, 3 and 5 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 2–5. The experimental findings were complemented by DFT and TD-DFT calculations. The Stokes shift of only 2600 cm⁻¹ for 5, compared to Stokes shifts in the range of 5900–7300 cm⁻¹ for 1–4, is partly explained by the different electronic structures found in 5 compared to 1–4 (X = H). The HOMO is mainly located on the aryl group in 5 and on the diazaborolyl group in 1–4 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-Me₃C₆H₂), in which the boron serves as a π-acceptor, the 10-π electron benzodiazaborole moiety appears to function as a π-donor moiety.