Pentasubstituted Phospholes with extended π-conjugated Arm – Synthesis, electrochemistry, spectra and quantum chemical calculations

λ3–1H-Phosphole is a unique heterocyclic compound characterized by low aromaticity with a limited number of cross-coupling reactions to allow side-chain modification. This severely limits the use of pentasubstituted phospholes in materials chemistry, including organic light-emitting diodes (OLEDs), organic solar cells (OSCs) or probes. Therefore, we developed a procedure for simple side-chain modification by Suzuki reactions. The synthesized pentasubstituted phospholes with a π-conjugated arm in position 2 of the phosphole ring were investigated electrochemically. In addition to the electrochemical study, we measured also the UV–VIS and fluorescence spectra for the prepared 1H-phospholes. The experimentally acquired data were correlated with quantum chemical calculations. The oxidation and reduction of all derivatives in acetonitrile at various electrodes is (quasi)reversible, the respective potentials as well as the extent of intramolecular electron communication and π-delocalization depend on the combination of electron withdrawing and donating substituents. The difference between the first oxidation and the first reduction potential (ΔE1) can be correlated with excitation energy.