pH-Sensitive Ru(II) bis(2,2':6'2"-terpyridine) complexes: a DFT/TDDFT investigation of their spectroscopic properties

We report a DFT/TDDFT study on the geometric, electronic and optical properties of [Ru{4'-(4-pyridyl)-2,2':6',2"-terpyridine}2]2+. Because of the presence of the basic nitrogen atom on the terminal pyridyl ligands, in solution the complex can exist in three different protonation states depending on the pH, each with markedly different photophysical properties. We investigated the effect of protonation of the terminal pyridine groups on the electronic and optical properties of this RuII complex. TDDFT calculations in vacuo and aqueous solution were performed, finding good agreement between the simulated and the experimental absorption spectra, and reproducing the experimentally observed absorption redshift upon pyridine protonation. The calculated excited-state data provide a rationale for the luminescence properties observed by varying the solution pH, in terms of the different energy separation between MLCT and MC excited states in the complexes.