Caged and Uncaged Ruthenium(II)-Polypyridine Complexes. Comparative Study of the Photochemical, Photophysical, and Electrochemical Properties

Article abstract of DOI:10.1021/ja00195a021

The absorption spectra, emission spectra (from 90 to 350 K), luminescence lifetimes (from 90 to 350 K), luminescence quantum yields, luminescence quenching by dioxygen, photochemical behavior, and redox potentials of a caged (4) and a hemicaged (3) Ru(II)-polypyridine complex have been studied and compared with those of the parent compounds Ru(bpy)3(2+) (1) and Ru(5,5'-(EtO2C)2-bpy)3(2+) (2) (bpy = 2,2'-bipyridine).The absorption band in the visible and the emission band of 4 are quite close in energy to the corresponding bands of 1, whereas those of 3 and 2 are red shifted.The oxidation and reduction processes of 3 and 4 take place at more positive potentials than those of 1.A linear correlation between the spectroscopic and electrochemical energies is observed for the four complexes.The luminescence lifetimes of 2 (0.57 μs) and 3 (1.9 μs) are shorter than those of 1 (4.8 μs) and 4 (4.8 μs) in nitrile rigid matrix at 90 K and are much more affected by the melting of the matrix (110-150 K).For T>150 K (i.e., in fluid solution), the luminescence lifetimes of 2 and 3 (0.09 and 0.45 μs) do not change up to 350 K, in contrast with the well-known behavior of 1, where a radiationless activated process with high-frequency factor (A ca. 1E14 s^-1) and large activation energy (ΔE ca. 4000 cm^-1) reduces the excited-state lifetime to 0.80 μs at room temperature.The caged complex 4 exhibits a less important radiationless activated process (A ca. 1E10 s^-1, ΔE ca. 2700 cm^-1) and maintains a longer lifetime (1.7 μs) at room temperature.In CH2Cl2 solution containing 0.01 M Cl(1-), Ru(bpy)3(PF6)2 undergoes a photodecomposition reaction with Φ_p = 0.017, whereas the PF6(1-) salts of 2-4 are photoinert (Φ_p <= 1E-6 for 4).The rate constant for the dioxygen quenching of the luminescent excited state of 4 is ca. 5 times smaller than that of 1.A comparative discussion of the properties of the four complexes is presented.The cage complex 4 exhibits all the properties that make 1 a widely used photosensitizer, with the additional advantages of a longer excited-state lifetime at room temperature in fluid solution and a 1E4 times higher stability toward ligand photodissociation.