The role of conformational dynamics in ground and excited ruthenium(II) polypyridyl complexes

Rahman, Mohammad Aminur (2014) The role of conformational dynamics in ground and excited ruthenium(II) polypyridyl complexes. Masters thesis, Memorial University of Newfoundland.

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Abstract

Initially, the fluxional solution dynamics of the terpyridine (tpy) ligand in [Ru(bpy-d₈)₂(h²-tpy)]²⁺ were studied by proton NMR spectroscopy, prior to a study of the excited state dynamics. Variable temperature 1H NMR data reveals that the pendant pyridine ring of the tpy ligand wobbles or rotates rather than undergoing an exchange or tick-tock twist mechanism. Then, the excited state dynamics of [Ru(bpy)₂(h²-tpy)]²⁺ were studied. Protonation of the pendant pyridine ring of [Ru(bpy)₂(h²-tpy)]²⁺ demonstrates that intra ligand charge transfer (ILCT) occurs between the pendant pyridine and adjacent bpy ligand in the [Ru(bpy)₂(h²-tpy)]²⁺ complex. Thus, addition of trifluroacetic acid (TFA) results in slight spectral changes which have been assigned to the protonation of the pendant pyridine of [Ru(bpy-d8)₂(η²-tpy)]²⁺. The protonation of the pendant pyridine increases the lifetime from 0.082 ns to 2.6 ns. Mulliken-Hush analysis of the ILCT of [Ru(bpy)₂(η²-tpy)]²⁺ has been done and compared with that of [Ru(bpy)3]²⁺. The value for the electronic coupling element (HDA) for [Ru(bpy)₂(η²-tpyH⁺)]³⁺ suggests that the electronic coupling between bpy and the pendant pyridne is quite strong. A preliminary study on photoinduced ligand loss from [Ru(bpy)₂(py)²]²⁺ has been made. The photolysis of cis-[Ru(bpy)₂(py)²]²⁺ in the presence of trifluoroacetic acid, using broad band, white light excitation, leads to a shift in lmax from 458 nm to 472 nm, consistent with a substitution reaction of the pyridine by THF. A flash photolysis study was also performed to investigate the excited state changes of [Ru(bpy)₂(py)²]²⁺ in THF.

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