LSU Historical Dissertations and Theses

1988

Dissertation

Degree Name

Doctor of Philosophy (PhD)

William R. Cherry

Abstract

It is shown that the charge of the photodonor in electron-transfer reactions is an additional variable that should be considered in strategies to inhibit energy-wasting back electron-transfer reactions. In particular, when microheterogeneous media are used in water-splitting schemes and the photodonor charge is adjusted to take advantage of the electrostatics of a micellar potential surface, the back electron transfer is reduced. It is known that a 'chemical soup' consisting of the photodonor tris(2,2$\sp\prime$-bipyridine)-ruthenium(II) (Ru(bpy)$\sb3\sp{2+}$), the electron relay methyl viologen (MV$\sp{2+}$), and a sacrificial donor like triethanolamine (TEOA) can generate dihydrogen from water in the presence of appropriate catalysts. Back electron transfer is inhibited by attaching a long chain to the viologen to make it lymphophilic and partitioning the relay into a micellar phase. Inhibition of back electron transfer in this system is improved by an order of magnitude by using the neutral photodonor bis(2,2$\sp\prime$-bipyridine)-4,4$\sp\prime$-dicarboxy-2,2$\sp\prime$-bipyridine-ruthenium (Ru(bpy)$\sb2$dcb). The photophysics of Ru(bpy)$\sb2$dcb and Ru(bpy)$\sb2$dcbH$\sb2\sp{2+}$ are compared to that of Ru(bpy)$\sb3\sp{2+}$. It is shown that the triplet metal-to-ligand charge transfer state ($\sp3$MLCT) is stabilized for these complexes and thus the activation energy, $\Delta$E, for the thermal population of the ligand field state (LF) is increased. Population of the LF state can result in the photodegradation of photocatalysts. The 4,5-diazafluorene ligand was used to investigate the nature of photosubstitution for polypyridine ruthenium complexes. Diazafluorene is shown to lower LF state(s) without perturbing MLCT states. The photophysics for the series, Ru(bpy)$\sb{\rm 3-n}$diaz$\sb{\rm n}\sp{2+}$ (n = 0,1,2,3), is examined. Each substitution of a diazafluorene for bipyridine is shown to progressively lower the LF state. Surprisingly, the photoanation of Ru(bpy)$\sb2$diaz$\sp{2+}$ in acetonitrile has little temperature dependence in contrast to that of Ru(bpy)$\sb3\sp{2+}$. The photoanation of the Ru(bpy)$\sb{\rm 3-n}$diaz$\sb{\rm n}\sp{2+}$ (n = 0,1,2,3) series in dichloromethane is studied. This study directly confirms Meyer's chelate model of photosubstitution for polypyridine complexes. An intermediate that has a monodentate diazafluorene, ((diaz)$\sb2$Ru(Cl)diaz) $\sp+$, is identified spectrally, and its thermal ring closure regenerating Ru(diaz)$\sb3\sp{2+}$ is observed and has an activation energy of 15.7 kcal mol$\sp{-1}$.

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