N-Aryl- and N-Thienylcarbazoles with Dimesitylboryl and 1,3,2-Benzodiazaborolyl Functions

The reaction of the N‐lithiated 3,6‐di‐tert‐butylcarbazole with fluorodimesitylborane afforded the N‐carbazolyl‐functionalized dimesitylborane 1 as a colorless solid in 70 % yield. [4‐(3,6‐Di‐tert‐butylcarbazol‐9‐yl)phenyl]dimesitylborane (2) was synthesized in 59 % yield by the lithiation of N‐(4‐bromophenyl)‐3,6‐di‐tert‐butylcarbazole and the subsequent treatment of the organolithium compound with fluorodimesitylborane. Synthesis of yellow crystalline [5‐(carbazol‐9‐yl)‐2‐thienyl]dimesitylborane 3 was effected in 66 % yield by the lithiation of 3,6‐di‐tert‐butyl‐N‐(2‐thienyl)carbazole and the subsequent reaction with fluorodimesitylborane. Coupling of N‐(4‐bromophenyl)‐3,6‐di‐tert‐butylcarbazole and 2‐bromo‐1,3‐diethyl‐1,3,2‐benzodiazaborole with magnesium metal in boiling THF in the presence of lithium chloride led to the formation of the functionalized benzodiazaborole 5 as a colorless solid in 68 % yield. Compounds 1–3 and 5 were characterized by elemental analyses, IR and NMR spectroscopy (1H, 11B, 13C) and mass spectrometry. The molecular structures of 1 and 3 were elucidated by X‐ray diffraction analyses. The borylated systems show intense blue luminescence. The spectroscopic results were reproduced by TD‐DFT calculations at the [B3LYP/6‐311G(d,p)] level of theory. Thus, it was discovered that the LUMOs of 1–3 are located on the vacant 2pz orbital of the boron atom with contributions of the π* orbital of the phenyl (in 2) or thiophene (in 3) unit, whereas the HOMOs are mainly represented by the carbazolyl unit. The study of N‐(dimesitylboryl)carbazole 1 and N‐phenyl‐ and N‐thienylcarbazoles with dimesitylboryl groups 2 and 3 or 1,3,2‐benzodiazaborolyl units 4–6 shows that the benzodiazaborolyl group does not contribute to the frontier orbitals in 4–6. The LUMOs of 1–3 are mainly represented by the 2pz orbitals of the Mes2B unit, whereas the HOMO is located on the carbazole part of the molecules.