Development of metal-metal quadruply bonded complexes with pi-extended structures: Synthesis, characterization, photophysical properties and applications as molecular synthons

In this thesis, a series of new homoleptic and bis-bis M2 quadruply bonded complexes bearing peripheral functionalities on pi-conjugated ligands have been prepared. The photophysical properties have been studied by steady state and time resolved techniques and the peripheral functionalities used in reactions to generate extended structures. The electronic absorption and emission spectra of the compounds have been shown to be metal (Mo or W) and ligand dependent. Intense metal to ligand charge transfer absorption bands are present in all species and can be used as an indicator of reaction progress. The Mo2 compounds are dual emission and show photoluminescence from their 1MLCT and 3deltadelta* states. The W2 compounds show only fluorescence from the 1MLCT state. Further photophysical studies using ns-TA and fs-TA reveal the presence of relatively long-lived 1MLCT states with lifetimes of 0.5 -- 10 ps. For W2, the T1 states are 3MLCT and have lifetimes of 1 -- 100 ns. For Mo2, the T1 states are 3deltadelta with lifetimes of 50 -- 100 mus.;The Mo2 compounds have been shown to successfully undergo Heck, Sonogashira, Stille, and dehydrohalogenation reactions to prepared extended molecules and polymers. Several functionalities have been screened including alkynes, alkenes, and aryl halides and the products characterized by several techniques including X-Ray crystallography.;Electronic structure calculations have been carried out on model complexes using density functional theory and time-dependent density functional theory to correlate the electronic absorption and emission data. These aided in the assignment of transitions and the lowest energy excited states. Calculations on the model complex anions were used in the characterization of the MLCT excited states in the TRIR experiments.;Additionally, Time resolved infrared spectroscopy was successfully employed on the complexes bearing -C≡C- and -C≡N functionalities to determine excited state electron distribution. For the bis-bis complexes, the 1MLCT excited states for Mo2 compounds and 1MLCT and 3MLCT excited states for W2 compounds were found to be delocalized over the two trans- pi-conjugated ligands. For the homoleptic compounds in Chapter 6 bearing four pi-conjugated ligands, the 1MLCT starts were found to be delocalized principally over two trans- ligands similar to the bis-bis complexes, and not over all four.;In another study, the electronic structures of oxalate bridged M 2 linear oligomers have been studied by DFT and TD-DFT. The calculations predict the development of bandlike structures in the delta and pi* orbitals with band widths of ∼ 1.0 eV. This produces a significant redshift of the calculated 1MLCT transitions as the chain length is extended. Efforts to synthesize these compounds were unsuccessful.