PUB - Publikationen an der Universität Bielefeld

A three coordinate boron atom is generally functioning as a Pi-acceptor in organic molecules with extended Pi-conjugation, which is due to its vacant pz-orbital. In contrast to this, benzodiazaboroles are Pi-donors. According it is challenging to design push-pull molecules in which the benzodiazaborole unit is forced into the role of an acceptor. In the first part of this work we tried to connect a powerful Pi-donor at the end of an organic scaffold opposite to the benzodiazaborole function. For this purpose we selected carbazolyl units and investigated the structural and photophysical properties of such arrangements.Thereby, we learned that in all of these compounds absorptions and emissions only occur in the carbazole part of the system without any participation of the benzodiazaborole. For comparison, studies of molecules with a dimesitylboryl group in place of the benzodiazaborole ring displayed the expected optical behavior of push pull systems. To switch the character of benzodiazaboroles we introduced powerful electron-accepting substituents at both nitrogen atoms of the B/N-heterocycle and obtained the novel 1,3-bis(perfluoroaryl)-1,3,2-benzodiazaboroles. The precursors, 2-bromo-1,3-bis(perfluoroaryl)-benzodiazaboroles 30-32, showed a significantly different reactivity than their non fluorinated analogues. These molecules cleaved the C-O bond of ethers with the formation 2 alkoxybenzodiazaboroles. The syntheses of 2-aryl-1,3-benzodiazaboroles 46-51 were realized by the treatment of trimethylsilylarenes with boron tribromide. The resulting organodibromoboranes were first converted into their less reactive triphenylphosphane adducts before cyclocondensations with the o-phenylendiamines 27-29 in hot toluene and in the presence of 2,2,6,6-tetramethylpiperidine as a base were performed. The photophysical data of the 2-aryl-1,3-benzodiazaboroles 46-51 were characterized by huge Stokes shifts which implies a geometrical reorganization of the molecules in the excited state. After the reorganization a charge transfer from the B/N heterocycle to the perfluoraryl substituents must have occurred. In the next step the syntheses of push pull molecules with the new benzodiazaborole units on the opposite end of the carbazolyl systems were envisaged The novel benzodiazaborole compounds 61-64 show intense fluorescence with a pronounced solvatochromism of their emission maxima. These findings underline the Pi-acceptor character of the novel 1,3-bis(perfluoroaryl)-1,3,2-benzodiazaborolyl fragments. In the last section two benzodiazaborole units with different substituents at the nitrogen atoms of the B/N heterocycle were attached to a Pi-electron conveying spacer. The resulting products 69-73 are the first push pull systems with two different benzodiazaborole functionalities in the molecule.