Characterization of the excited states of distal- and proximal-[Ru(tpy)(pynp)OH₂]²⁺ in aqueous solution using time-resolved infrared spectroscopy

The quasi-stable excited states of distal- and proximal-[Ru(tpy)(pynp)OH₂]²⁺ (tpy = 2,2′,6′,2″-terpyridine, pynp = 2-(2-pyridyl)-1,8-naphthyridine), which exhibits unique irreversible photoisomerization, were assessed in aqueous solution using time-resolved infrared vibrational spectroscopy and quantum chemical calculations. Transient vibrational spectra from 1300 to 1650 cm^-1 were acquired 120 and 2 ps after photoexcitation with a 400 nm pulse for the distal- and proximal-isomers, respectively. The calculated vibrational spectra assuming the lowest triplet states were in good agreement with the experimentally observed spectra, and the vibrational bands are assigned to normal vibrational modes obtained from calculations. The lifetimes of the excited state were determined to be 9.7 ns and 6.4 ps for the distal- and proximal-isomers, respectively. Based on these results, we determined various characteristic features of these excited states. Despite the large difference in lifetime, both isomers were assigned to the triplet metal-to-ligand charge transfer (³MLCT) state and there was almost no difference in the positions of the atoms directly coordinated to the Ru atom. The dissimilarity between the isomers is that the pynp ligands are significantly distorted in the case of the distal-isomer while there is hydrogen bonding between the pynp and OH₂ ligands in the proximal-isomer. The strong interaction between the low lying ³MLCT state and the ground state presumably causes the very short lifetime and the forbidden photoisomerization of the proximal-isomer, whereas photoisomerization occurs via the thermally populated metal centered (³MC) state from the high-lying ³MLCT state in the case of the distal-isomer.