Transformation of the Phosphorus Atom in Hexacyclic Polyaromates and Its Impact on Physicochemical Properties

A series of eight phenanthrene-based phosphorus heterocycles were prepared to evaluate the effect of phosphorus-center modification on the optical and electrochemical properties of π-conjugated systems. A versatile synthetic route provided access to λ³-, λ⁴-, and λ⁵-phosphorus derivatives, including phosphinine, phosphine oxide, sulfide, selenide, phosphinic acid, amide, and phosphonium species. The resulting compounds were investigated using DFT calculations in combination with absorption and emission spectroscopy as well as cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The data show that phosphinic acid derivatives exhibit only minor changes in optical and redox properties, whereas chalcogen exchange (O, S, and Se) has a more pronounced effect on the optical behavior. All compounds display irreversible oxidation waves above 0 V. Comparison with carbo and aza analogues revealed that the incorporation of phosphorus into the phenanthrene scaffold leads to a bathochromic shift in both absorption and emission. In contrast, the first reduction potentials (E_red1) remain comparable across the series. Overall, these results provide a systematic overview of the influence of the phosphorus-center modification on the electronic properties of phenanthrene-based systems.