Fluorescent benzene-centered mono-, bis- and tris-triazapentadiene-boron complexes

A series of novel benzene centered mono-, bis- and tris-1,3,5-triazapentadiene ligands was synthesized and investigated with respect to their reactivity towards triphenylborane. The resulting blue-fluorescent boron complexes with a six-membered ring chelate structure show excellent thermal and chemical stability. All title compounds were completely characterized including X-ray diffraction studies for and . Whereas the absorption spectra of all three classes of compounds are similar, the fluorescence spectra show distinct differences. Thus, the emission spectra of show Stokes shifts of 4100-6700 cm(-1) with low quantum yields both in solution and in the solid state. However, the more bulky compounds show markedly larger molar extinction coefficients and smaller bathochromic shifts compared to . For all compounds, we observe significantly more intense red-shifted fluorescence in the solid state compared to that in dichloromethane solutions. For the interpretation of the absorption properties TD-DFT studies were performed based on DFT geometry optimizations.