Caged and uncaged ruthenium(II)-polypyridine complexes. Comparative study of the photochemical, photophysical, and electrochemical properties

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){sub 3}{sup 2+} (1) and Ru(5,5{prime}-(EtO{sub 2}C){sub 2}-bpy){sub 3}{sup 2+} (2) (bpy = 2,2{prime}-bipyridine). A linear correlation between the spectroscopic and electrochemical energies is observed for the 4 complexes. The luminescence lifetimes of 2 (0.57 {mu}s) and 3 (1.9 {mu}s) are shorter than those of 1 (4.8 {mu}s) and 4 (4.8 {mu}s) 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 {mu}s) 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 {approximately} 10{sup 14} s{sup {minus}1}) and large activation energy ({Delta}E {approximately} 4,000 cm{sup {minus}1}) reduces the excited-state lifetime to 0.80 {mu}s at room temperature. The caged complex 4 exhibits a less important radiationless activatedmore » process (A {approximately} 10{sup 10} s{sup {minus}1}, {Delta}E {approximately} 2,700 cm{sup {minus}1}) and maintains a longer lifetime (1.7 {mu}) at room temperature. In CH{sub 2}Cl{sub 2} solution containing 0.01 M Cl{sup {minus}}, Ru(bpy){sub 3}(PF{sub 6}){sub 2} undergoes a photodecomposition reaction with {Phi}{sub p} = 0.017, whereas the PF{sub 6}{sup {minus}} salts of 2-4 are photoinert ({Phi}{sub p} {<=} 10{sup {minus}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.« less