Spectroscopic characterization of chloride and pseudohalide ruthenium(II) complexes with 4-(4-nitrobenzyl)pyridine

Chloride, isocyanate and isothiocyanate hydride carbonyl ruthenium(II) complexes of 4-(4-nitrobenzyl)pyridine were synthesized from the precursor complex RuHCl(CO)(PPh₃)₃] and characterized by IR, NMR, UV–Vis spectroscopy and X-ray crystallography. The electronic structures of the complexes were investigated by means of DFT calculations, based on their crystal structures. The spin-allowed singlet–singlet electronic transitions of the complexes were calculated by time-dependent DFT, and the UV–Vis spectra are discussed on this basis. The emission properties of the complexes were studied at ambient temperature, and the quantum yields of fluorescence, the lifetimes and nature of the excited states are discussed. The chloride and isothiocyanate complexes are practically nonemissive, with quantum yields under 0.01 %. Interpretation of spectra, supported by TD-DFT calculations, indicates that in this energy region, the transitions have MLCT character with admixture of LLCT (chloride and isothiocyanate complexes). The dominant LLCT character was visible in the case of the most emissive (isocyanate) complex. The low values of the lifetimes and quantum yields for these complexes indicate the influence of the metal center in the emission process.