Introduction of Phosphorus Hexacycles into Aromatic Structures

Organophosphorus π-conjugated systems have become the subject of intensive research in recent years, mainly due to their use in materials chemistry (OLEDs, OFETs, nonlinear optical devices, photovoltaic cells, and various sensor devices). A special place in this group of substances is occupied by phosphorus hexacycles, in which the phosphorus atom often exhibits a special hyperconjugation effect, which distinguishes these compounds from other π-conjugated organophosphorus molecules. The phosphorus hexacycle in polyaromatics increases their solubility in common organic solvents, increases photostability, provides ambipolar redox properties, and often improves electroluminescence both in solution and in the solid state. The presence of the phosphorus atom then facilitates further derivatization (oxidation (O, S, Se), borylation, alkylation, formation of metal complexes), thereby effectively changing some key properties of the target molecules (HOMO/LUMO difference, charge transfer rate, triplet-triplet spin interaction, absorption and luminescence properties), thereby adapting them to the intended use. Consequently, phosphorus hexacycles have already been used as biomarkers for cancer treatment, sensors, catalysts, or in material chemistry (OLED, EFD).

Although considerable progress has been made recently in the synthesis of phosphorus hexacycles, the preparation of phosphanaphthalenes and other polyaromatic compounds containing a phosphine ring is rare and often faces various limitations such as the use of complex synthetic procedures or a narrow group of similar derivatives that can be obtained in this way. In the group Dr. J. Storch has recently developed a robust and efficient synthesis by which the phosphinine core can be introduced into aromatic compounds with relative ease. Therefore, several phosphanaphthalene derivatives were prepared, followed by several larger π-systems containing phosphine. During this study, an unexpected reactivity leading to a hitherto unknown group of substances – azaphosphorines – was also discovered.