Energy and electron transfer reactions of rigidly bridged multinuclear metal complexes
Description
Chapter 1 provides a general intraduction and a theoretical background to the work described in this dissertation. Key features of natural photosynthesis are discussed along with reasons for research interests in artificial photosynthetic systems. Theories and mechanisms used to explain energy and electron transfer reactions are reviewed. A review of studies of electron transfer in organic compounds and inorganic metal complexes is also provided Chapter 2 gives a description of the various synthetic strategies investigated for the synthesis of rigidly bridged bis-bipyridine compounds. The first attempt using the inverse electron demand Diels-Alder strategy was unsuccessful. A later attempt using Wittig and Diels-Alder reactions was unsatisfactory because of low product yields and many reaction steps. However, this strategy provided the bridging ligands, bbdb and bchb that had interesting spectroscopic properties, in addition to bphb which was rigid and possessed desired luminescence properties. (bbdb = 1,4-bis(4$\sp\prime$-methyl-2,2$\sp\prime$-bipyrid-4-yl)buta-1,3 -diene, bchb = 1,4-bis(4$\sp\prime$-methyl-2,2$\sp \prime$-bipyrid-4-yl)2-cyclohexenyl-5,6-dicarboxy-ethyl ester and bphb = 1,4-bis(4$\sp\prime$-methyl-2,2$\sp \prime$-bipyrid-4-yl)benzene). A more satisfactory synthetic scheme involves the use of the Hantzsch/Krohnke reaction to couple 1,4-bis(pentenal)benzene or 1,4-bis(3-carboxy-3-oxo-prop-1-en-yl)benzene with 2-acetyl- pyridine-pyridinium salt in the presence of ammonium acetate Chapter 3 examines the effect of unsaturation on the excited state properties of Ru(II) diimine compounds. The complexes $\{$ ((dmb)$\sb2$Ru) $\sb2$(b-b)$\}$(PF$\sb6$)$\sb4$ and ((dmb)$\sb2$Ru(b-b)) (PF$\sb6)\sb2$ where b-b=bbdb, bchb and bphb) were prepared. Their redox and spectroscopic properties are discussed in terms of the relative energy of the $\sp3$IL state to that of the $\sp3$MLCT state. Based on transient absorption spectroscopy and triplet state quenching the $\sp3$IL state energies of the bbdb and bphb bridging ligands were determined. A good correlation was obtained between the excited state energy of the bridging ligand and that of the related hydrocarbon The syntheses and spectroscopic properties of Re(I) diimine complexes of bbbd and bphb, $\{$ ((CO)$\sb3$(CH$\sb3$CN)Re) $\sb2$(bbdb)$\}\sp{2+}$(PF$\sb6$)$\sb2$ and $\{$ ((CO)$\sb3$(CH$\sb3$CN)Re) $\sb2$(bphb)$\}\sp{2+}$(PF$\sb 6$)$\sb2,$ respectively are provided in chapter 4. The presence of room temperature phosphorescence in these complexes were investigated to enable establishment of criteria for observing this phenomenon in other compounds. The bphb complex showed expected phosphorescence at 77 K using time-resolved emission spectroscopy but the bphb ligand did not (in the presence of 10% EtI). The bbdb bridging ligand and complex appear to show weakly structured phosphorescence that could be assigned to the $\sp3$IL state Chapter 5 describes the investigation of energy transfer in unsymmetrical donor-acceptor complexes. The complexes (CO)$\sb3$(L)Re(bphb)Ru(bpy)$\sb2,$ (CO)$\sb3$(L)Re(bphb)Ru(tpy)CN, (CO)$\sb3$LRe(bphb)Ru(decb)$\sb2$ and (bpy)$\sb2$Ru(bphb)Ru(tpy)CN. (L = CH$\sb3$CN, py = pyridine and NMI = N-methyl-imidazole, tpy = terpyridine and decb = 4,4$\sp\prime$-diethyl-carboxy-2,2$\sp\prime$-bipyridine) were synthesized. Thier redox and luminescence properties are reported. However for most of these complexes the emission lifetime could not be measured because they were in the picosecond domain. Assignment of energy transfer as the main process of donor emission quenching is based on the exergonicity ($\Delta$G $<$ 0) of the process. The mechanism of energy transfer is discussed in terms of Forster (coulombic) and Dexter (exchange) transfer mechanisms. (Abstract shortened by UMI.)