Photophysics of Bis-bipyridyl Nitro Complexes of Ruthenium(II) with Pyridine Ligands: Substituent Effects

Emission, excitation spectra, quantum yields, and emission lifetimes are reported for the mixed ligand, bis-(2,2′-bipyridine)­ruthenium­(II) complexes cis-[Ru­(bpy)₂(L)­(NO₂)]⁺ with 4-substituted pyridines as ligands L [isonicotinamide (isnc), 4,4′-bipyridine (bipy), pyridine (py), 4-phenylpyridine (phpy), 1,2-bis­(4-pyridyl)­ethane (bpa), 4-picoline (mepy), 4-aminopyridine (apy), and 4-dimethylaminopyridine (ampy)] in EtOH–MeOH, 4:1 (V:V), at 77 K. Radiative, k_r, and nonradiative, k_nr, decay rate constants were determined for the series of complexes, and a linear dependence of ln k_nr on E₀₀, with E₀₀ as the 0–0 energy gap determined by emission spectral fitting, was obtained with a slope of −(3.35 ± 0.23) × 10^–4 cm^–1. A linear correlation of E₀₀ for cis-[Ru­(bpy)₂(L)­(NO₂)]⁺ complexes and Hammett substituent constant σ_p is found.