The role of conformational dynamics in ground and excitedruthenium(II) polypyridyl complexes
2014
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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