Photochemistry and photophysics of mononuclear and covalently linked polynuclear ruthenium (II) bipyridyl complexes
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Description
The preparation and photophysical characterization of a series of tris(polypyridine) ruthenium (II) complexes of the formula (Ru(dmb)$\sb{\rm n}$(decb)$\sb{3-\rm n}$) $\sp{2+}$, where n is an integer between 0 and 3, dmb is 4,4$\sp\prime$-dimethyl-2,2$\sp\prime$-bipyridine, and decb is 4,4$\sp\prime$-bis(carboxyethyl)-2,2$\sp\prime$-bipyridine is described. Absorption and emission energies decrease in the order (Ru(dmb)$\sb3$) $\sp{2+}$ $>$ (Ru(decb)$\sb3$) $\sp{2+}$ $>$ (Ru(decb)$\sb2$dmb) $\sp{2+}$ $>$ (Ru(dmb)$\sb2$decb) $\sp{2+}$ and are linearly related to $\Delta$E$\sp\circ$ (difference between the first oxidation and reduction potential of each complex). Temperature dependent quantum yields and luminescence lifetimes yield activation barriers, $\Delta$E$\sp\prime$, for non-radiative decay from the $\sp3$MLCT state representing the thermal barrier to population of a metal centered excited state ($\sp3$MC). Photolysis in the presence of Cl$\sp-$ results in the photosubstitution reaction giving cis-chloro complexes. These photochemical and photophysical results are discussed in terms of the relationship and energetic separation between the $\sp3$MLCT and $\sp3$MC states The synthesis of a covalently linking bridging bipyridyl ligand 1,4-bis (2-(4$\sp\prime$-methyl-2,2$\sp\prime$-bipyridyl-4-yl)ethyl) benzene, b-b, as well as a series of multinuclear complexes prepared using this ligand, is presented. Employing (Ru(dmb)$\sb2$b-b) $\sp{2+}$ the complexes (Ru(dmb)$\sb2$b-bRu(dmb)$\sb2$) $\sp{4+}$, ((Ru(dmb)$\sb2$b-b)$\sb3$Ru) $\sp{8+}$, (Ru(dmb)$\sb2$b-bRu(decb)$\sb2$) $\sp{4+}$, ((Ru(decb)$\sb2$b-b)$\sb3$Ru) $\sp{8+}$, (Ru(dmb)$\sb2$b-bRu(bpyrm)$\sb2$) $\sp{4+}$, (Ru(dmb)$\sb2$b-bOs(dmb)$\sb2$) $\sp{4+}$, and (Ru(dmb)$\sb2$b-bRe(CO)$\sb3$(CH$\sb3$CN)) $\sp{3+}$ are prepared where bpyrm is 2,2$\sp\prime$-bipyrimidine. Spectroscopic, electrochemical, and photophysical properties of these complexes are examined. Self-quenching of the luminescence of ((Ru(dmb)$\sb2$b-b)$\sb3$Ru) $\sp{8+}$ is observed under low intensity excitation. Intramolecular quenching of the emission of the donor chromophore for all the mixed ligand and mixed metal complexes is observed. Through excitation spectra and luminescence studies, energy transfer is definitively established as the principal mode of quenching, except for (Ru(dmb)$\sb2$b-bRu(bpyrm)$\sb2$) $\sp{4+}$, where the quenching mechanism is unclear The electron transfer quenching of the symmetrical ruthenium (II) clusters by TMPD and use of the reduced ruthenium formed to reduce vicinal dihaloalkanes is examined. Back electron transfer is shown to be dependent on the charge and size of the complexes at zero ionic strength. Ionic strength effects on the ion recombination are also discussed. Photolysis of the complexes with N,N,N$\sp\prime$,N$\sp\prime$-tetramethylphenylenediamine (TMPD) and 1,2-dibromostilbene (DBS) yields the irreversible redox product trans-stilbene and accumulation of the TMPD cation radical. The possible mechanistic steps are also outlined and examined