Abstract
A series of Ru complex ions, [Ru(bpy)2X]+ (bpy = bipyridyl) were synthesized. This was with a view to studying their photophysical properties by experimental and computation methods. In particular the effect of pH, concentration, temperature and the nature of ligand X has been investigated. The X-ligand represents pyridine, pyrazine or pyrimidine derivatives of 1,2 and 1,3 diazolates, 1,2,3 and 1,2,4 triazolates and 1,2,3,4 tetrazoles. Four of the compounds were studied by experimental photophysical and computational methods, namely pyridine and pyrazine derivative of 1,2-diazolate and 1,3-diazolate. The selection of the ligands also demonstrates the effect of the nitrogen content and nitrogen position on the absorption and emission properties of the complexes, as well as on the pH optical response of the [Ru(bpy)2X]+/[Ru(bpy)2XH]2+ in the absorption spectra. The dual state emission behaviour has also been studied in detail to elucidate the unexpected switching of the most prominent low energy emission band. The absorption and emission properties of the deprotonated and protonated complex ions were characterised by UV-vis and photoluminescence spectroscopy, as well as by time-dependent density functional theory. The 1MLCT transition with d-πazo-π*bpy character exhibit a tunability range of ~17 nm from experimental results and estimated to be ~33 nm from calculations. The X ligand shows a relatively small change on the first absorption band. When the six-membered ring of the X ligand has a higher nitrogen content the absorption wavelengths for the (d-πazo)-π*bpy transition shifts to lower wavelengths. A blue shift is also observed when comparing the position of the N atoms at the 1, and 2 or 1 and 3 positions of the azolate on the X ligand. Modification of the nitrogen content and nitrogen position within the protonatable/deprotonatable ligands is an effective approache for modulation of the pH-optosensing properties of Ru-polypyridyl complexes. The dual emission is shown to arise from the presence of two excited states consisting of a triplet metal-to-ligand charge transfer state localized on a bipyridine ligand - 3MLCT (bpy), and either a state that is entirely localized on the X ligand or is partially delocalized also spanning part of the bipyridine ligands - 3MLCT(X). The choice of the X ligand modulated the emission of the 3MLCT(bpy) and 3MLCT(X) states between 743 - 865 nm and from 555 - 679 nm, respectively. A blue shift of the 3MLCT(bpy) emission on increasing the nitrogen content of the six-membered ring of the X ligand is observed while by contrast a redshift for the 3MLCT(X) emission. The energy difference between 3MLCT(bpy) and 3MLCT(X) emissions can be tuned from 84–310 nm.