Effect of deprotonation on absorption and emission spectra of Ru(II)-bpy complexes functionalized with carboxyl groups

Changes in the ground and excited state electronic structure of the [Ru(bpy)(3)](2+) (bpy = 2,2'-bipyridine) complex induced by functionalization of bpy ligands with carboxyl and methyl groups in their protonated and deprotonated forms are studied experimentally using absorption and emission spectroscopy and theoretically using density functional theory (DFT) and time dependent DFT (TDDFT). The introduction of the carboxyl groups shifts the metal-to-ligand-charge-transfer (MLCT) absorption and emission bands to lower energies in functionalized complexes. Our calculations show that this red-shift is due to the stabilization of the lowest unoccupied orbitals localized on the substituted ligands, while the energies of the highest occupied orbitals localized on the Ru-center are not significantly affected. Consistent with previously observed trends in optical spectra of related Ru(II) complexes, deprotonation of the carboxyl groups results in a blue shift in the absorption and phosphorescence spectra. The effect originates from interplay of positive and negative solvatochromism in the protonated and deprotonated complexes, respectively. This results in more delocalized character of the electron transition orbitals in the deprotonated species and a strong destabilization of the three lowest unoccupied orbitals localized on the substituted and unsubstituted ligands, all of which contribute to the lowest-energy optical transitions. We also found that owing to the complexity of the excited state potential energy surfaces, the calculated lowest triplet excited state can be either weakly optically allowed (MLCT)-M-3 or optically forbidden Ru (3)d-d transition depending on the initial wavefunction guess used in TDDFT calculations.