This work presents an interpretation of the origin of changes in absorption spectra upon one-electron oxidation and reduction of two ruthenium polypyridyl complexes based on a combination of UV-Vis spectroelectrochemical experiments and theoretical calculations using the Gaussian 09 program. A bis-chelating ligand containing a p-bromobenzoylthiourea unit connected to 1,10-phenanthroline (phen-p-BrBT) has been prepared. Complexation of phen-p-BrBT to ruthenium bis-diimine centres, Ru(N-N)2 [N-N = 2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen)], affords octahedral Ru(ii) tris-diimine complexes that are synthesised and structurally characterised. The two complexes exhibit similar MLCT bands and electronic energy levels owing to the similar electronic structures of the bpy and phen ligands. However, [Ru(phen)2(phen-p-BrBT)]2+ exhibits a slightly broader visible region MLCT (metal-to-ligand-charge transfer) band than [Ru(bpy)2(phen-p-BrBT)]2+ as expected from a slightly more delocalised π-electron system in the phen diimine ligands. In addition, the π → π* absorption in the UV is blue-shifted for [Ru(phen)2(phen-p-BrBT)]2+ relative to that for [Ru(bpy)2(phen-p-BrBT)]2+, because of greater stabilisation of the bpy HOMO relative to that of phen. The extra C-C bond in phen produces greater delocalisation of electron density leading to a blue-shift in the π → π* transition. The MLCT band is blue-shifted and diminished in intensity upon oxidation due to stabilisation of the Ru d-orbitals by removal of one electron. A new broad absorption band appears in the UV region upon reduction. The new transition is attributed to a blue-shift of the first MLCT transition for [Ru(bpy)2(phen-p-BrBT)]2+ and a red-shift of the second MLCT transition for [Ru(phen)2(phen-p-BrBT)]2+. The new transitions originate from destabilisation or stabilisation of the ligand LUMO orbitals relative to the Ru d-orbitals. A red-shift of the UV band in the initial complex also contributes to the new band produced upon reduction of [Ru(bpy)2(phen-p-BrBT)]2+. The new band does not involve an n(C[double bond, length as m-dash]S) → π* transition. Although both complexes show subtle differences in behaviour, their spectral changes are distinct, and the origin of changes in their absorption spectra upon oxidation and reduction is successfully interpreted.
An absorbance-based sensor employing ruthenium bipyridyl with a phenanthroline-fused benzoylthiourea moiety formulated as [Ru(ii)(bpy)2(phen-nBT)](PF6)2 {bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, nBT = n-benzoylthiourea} has been synthesized and characterized by elemental analyses, mass spectrometry, and infrared, ultraviolet-visible, luminescence and nuclear magnetic resonance spectroscopy. The changes in the intensity of absorption and emission of the complex induced by functionalization of the benzoylthiourea ligands with amino and carbonyl in their protonated and deprotonated forms were studied experimentally. The absorption and emission properties of the complex exhibit a strong dependence on the pH (1-11) of the aqueous medium. This work highlights the pH-sensitivity augmentation of the absorption band by elongating the conjugation length in the structure of the ruthenium bipyridine complex. The principle of this work was to design the title compound to be capable of enhancing the differences in the absorption sensitivity responses towards pH between the protonated and deprotonated complexes in the absorption measurement. Along with significant and noticeable changes in the absorption spectra, subsequent theoretical investigations specifically on the electronic and absorbance properties of the title compound were carried out in this study. Protonation of the molecule significantly stabilized the lowest-unoccupied molecular orbital (LUMO), whereas the highest-occupied molecular orbital (HOMO) is greatly destabilized upon deprotonation. A time-dependent density functional theory (TDDFT) calculation in the linear-response (-LR) regime was performed to clarify the origin of the experimentally observed linear dependence of absorption intensity upon pH (1-11). The MLCT band exhibits hyperchromic shift at low pH as indicated by the large transition dipole moment and a wider distribution of the response charge of the molecule, which is induced by the stabilization of the electrostatic potential at the carbonyl moiety by protonation. This study provides the possibility of employing theoretical information to gain insight into the origin of the optical absorption obtained experimentally. The ruthenium complex was designed with an elongated ligand conjugation length and exhibited a tremendously large change in the absorption intensity of the protonated and deprotonated forms, which therefore demonstrates its feasibility as an indicator molecule especially for absorbance measurements.