Comparative Study of Emission Quenching of Tris(${\alpha},{\alpha}'$-diimine)-Ruthenium(II) Complexes in Homogeneous and Sodium Dodecyl Sulfate Micellar Solutions

Emission quenching of photoexcited tris(${\alpha},{\alpha} '$-diimine)-ruthenium(II) complex cations, $RuL_3^{2+}$ (L: 2,2'-bipyridine, 4,4'-dimethyl-2,2'-bipyridine; 4,4'-diphenyl-2,2'-bipyridine; 1,10-phenanthroline; 5-methyl-1,10-phenanthroline; 5,6-dimethyl-1,10-phenanthroline or 4,7-diphenyl-1,10-phenanthroline) by $Cu^{2+}$, dimethylviologen $(MV^{2+})$, nitrobenzene (NB), and oxygen was studied in aqueous homogeneous and sodium dodecyl sulfate (SDS) micellar solutions. The apparent bimolecular quenching rate constants $k_q$ were determined from the quenching data and life-times of $^{\ast}RuL_3^{2+}$. In homogeneous media, the quenching rate was considerably slower than that for the diffusion-controlled reaction. The decreasing order of quenching activity of quenchers was $NB>O_2>MV^{2+}>Cu^{2+}$. The rate with $Cu^{2+}$ was faster as the reducing power of $^{\ast}RuL_3^{2+}$ is greater. On the other hand, the rates with NB and $O_2$ were faster as the ligand is more hydrophobic. This was attributed to the stabilization of encounter pair by van der Waals force. The presence of SDS enhanced the rate of quenching reactions with $Cu^{2+}$ and $MV^{2+}$, whereas it attenuated the quenching activity of NB and $O_2$ toward $RuL_3^{2+}$. The binding affinity of quenchers to SDS micelle and binding sites of the quenchers and $RuL_3^{2+}$ in micelle appear to be important factors controlling the micellar effect on the quenching reactions.