Thienyl substituted 2H-1,4,2-diazaphosphole complexes 3a,b were prepared via highly selective ring-expansion reactions of 2H-azaphosphirene complex 1 and nitriles with our new synthetic protocol using triflic acid and NEt3. The single-crystal X-ray structures of 3a,b show that the 3,5-substituents adopt a coplanar arrangement with the diazaphosphole ring resulting in extended π-conjugation, thus giving rise to absorptions at long wavelengths in their UV/Vis spectra. On the basis of Time-Dependent DFT (TD-DFT) calculations the longest-wavelength absorption could be assigned to a metal–ligand charge transfer (MLCT) process and another low-energy band was interpreted as a superposition of π–π* and n–π* transitions. Protonation of the ring nitrogen yields a pronounced bathochromic shift of all bands along with an increase in their intensity. These effects can be explained by the different extent to which the orbital energies are affected by protonation.
