The Influence of 1,2-Boron-Nitrogen Substitutions on the Properties of Polycyclic Aromatic Hydrocarbons

Abstract

The formal replacement of a pair of carbon atoms by boron and nitrogen in polycyclic aromatic hydrocarbons (PAHs) modulates properties like aromaticity, emission wavelengths and fluorescence quantum yields. In this dissertation, novel BN-PAHs were synthesized and compared with their carbon analogs using various analytical methods to quantify the influence of the heteroatoms. BN-PAHs are commonly prepared via borylative cyclization of aromatic amines. A novel approach was developed, involving transition metal-catalyzed electrophilic cyclization of BN-aromatic alkynes. The reaction's versatility was demonstrated by synthesizing planar as well as helical BN-polycycles. Remarkably, an enantiomerically pure BN-hexahelicene exhibited a highly increased circularly polarized luminescence (CPL) response, compared to the all-carbon compound. Along with its high fluorescence quantum yield, this highlights the future potential for 3D display technologies. Furthermore, a dithienyl-substituted pyrene and its BN-substituted counterpart were synthesized and compared regarding the formation of dimers in the excited state. Unlike its carbon analog, the BN-pyrene showed no evidence of excimer formation both in solution and in the solid state, owing to its different electronic structure. However, the BN-unit induced a strong bathochromic shift in emission. Moreover, the bidentate 8-hydroxy-BN-naphthalene and 8-hydroxyquinoline ligands were used to synthesize chelate complexes with aluminum. X-ray diffraction analysis and theoretical investigations revealed fundamentally different N‒Al bonding parameters caused by charge differences between BN- and CN-doped aromatic molecules. Overall, it was shown that BN-doping of PAHs can strongly alter their photophysical properties, while leaving the geometrical structures mostly unaffected.