Improved singlet oxygen generation in Rhenium(I) complexes functionalized with a pyridinyl selenoether ligand

The synthesis, characterization, electrochemical and photophysical properties of three novel polypyridine rhenium(I) complexes coordinated to an organoselenide ligand, 4-(phenylseleno)-pyridine (PhSepy), and structurally related polypyridine ligands, fac-[Re(CO)3(NN)(PhSepy)]+ NN = 1,10-phenanthroline (phen), 4,7-diphenyl-1,10-phenanthroline (ph2phen) and pyrazino[2,3-f]-1,10-phenanthroline (dpq), are reported. In addition, their ability to act as a photosensitizer agent for the generation of singlet oxygen was investigated. Cyclic and differential pulse voltammetry experiments showed an overlap of the redox waves characteristic of the 4-(phenylseleno)-pyridine ligand and the Re(I) complex. This finding is consistent with a strong contribution of the pyridine-based ligand on the HOMO levels of the three investigated complexes, further supported by quantum mechanical calculations. Moreover, the lowest energy band observed in the absorption spectra of the complexes was also influenced by the organoselenide ligand, with a combination of the usual MLCTRe→NN transition with a ligand-to-ligand charge transfer (LLCT) one. The three complexes showed typical emission spectra for this class of compounds ascribed to 3MLCTRe→NN, with excellent quantum yields for the singlet oxygen generation (Φ = 0.65-070). Remarkably, these are significantly larger (15-29%) than those for structurally related complexes with non-functionalized pyridyl ligands, revealing a significant ability as a photosensitizer agent. Therefore, we envisage this work to be of interest to those engaged in the development of novel rhenium(I) complexes for optoelectronic applications.