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 6a-e was synthesized and investigated with respect to their reactivity towards triphenylborane. The resulting blue-fluorescent boron complexes 14a-e 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 14a-c and 14e. Whereas the absorption spectra of all three classes of compounds are similar, the fluorescence spectra show distinct differences. Thus, the emission spectra of 14a, b 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 14c-e show markedly larger molar extinction coefficients and smaller bathochromic shifts compared to 14a,b. 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.