From Byproducts to Efficient Fluorophores: A Route to the Synthesis of Fluorubines

The reaction of bis(imidoyl) chlorides of oxalic acid with monoalkyl hydrazines leads to substituted Delta(2)-1,2-diazetines, which are versatile building blocks for ring-transformation reactions. One remarkable product originating from side reactions featured by a strong orange/red fluorescence was confirmed as a novel fluorubine derivative. In continuing our studies to substituted oligoazaacenes, we developed several synthetic entries to build up novel fluorubine derivatives, in which particularly aminobridged bis(quinoxaline)s are the key products for cationic hexaazapentacenes. We would like to discuss the possible formation pathways of these fluorubine derivatives, which exhibit interesting photophysical and chemical properties. The structures of all new derivatives were confirmed by common analytical methods (NMR spectroscopy, CV, UV/Vis, mass spectrometry, elemental analysis, and X-ray structural analysis) and will be discussed on selected examples in more detail.