Infrared Vibrational Spectroscopy of [Ru(bpy)₂(bpm)]²⁺ and [Ru(bpy)₃]²⁺ in the Excited Triplet State

Abstract

This work involved a detailed investigation into the infrared vibrational spectra of ruthenium polypyridyl complexes, specifically heteroleptic [Ru(bpy)2(bpm)](2+) (bpy = 2,2'-bipyridine and bpm = 2,2'-bipyrimidine) and homoleptic [Ru(bpy)3](2+), in the excited triplet state. Transient spectra were acquired 500 ps after photoexcitation, corresponding to the vibrational ground state of the excited triplet state, using time-resolved infrared spectroscopy. We assigned the observed bands to specific ligands in [Ru(bpy)2(bpm)](2+) based on the results of deuterium substitution and identified the corresponding normal vibrational modes using quantum-chemical calculations. Through this process, the more complex vibrational bands of [Ru(bpy)3](2+) were assigned to normal vibrational modes. The results are in good agreement with the model in which excited electrons are localized on a single ligand. We also found that the vibrational bands of both complexes associated with the ligands on which electrons are little localized appear at approximately 1317 and 1608 cm(-1). These assignments should allow the study of the reaction dynamics of various photofunctional systems including ruthenium polypyridyl complexes.

Direct Observation of the Triplet Metal-Centered State in [Ru(bpy)3]2+Using Time-Resolved Infrared Spectroscopy

“…The intensity of the higher wavenumber band at 1608 cm −1 gradually increases, with a time constant of 18 ps, although the spectral shape and position remain relatively unchanged up to 50 ps (black triangles in Figures (c) and (d), respectively). This temporal behavior is the same as that of the other bands reported in our previous paper . In addition, this behavior is in good agreement with data obtained from time‐resolved Raman spectroscopy .…”

“…We discuss here the reason why the 3 MC vibrational mode is observed only in the neighborhood of 1600 cm −1 . According to our previous study, the vibrational modes discussed herein belong to the bpy ligands on which the excited electron is only marginally localized, and consist primarily of C=C and C=N stretching vibrations of the bpy rings . Likely because the strengths of C=C and C=N double bonds are considerably affected by the surrounding charge distribution, these modes are not only sensitive to differences in charge (Ru(II) vs. Ru(III)) but also to the excitation state of the central metal (Ru(II) vs. Ru(II)*).…”

“…The details of this method are provided in the Supporting Information. In our previous study, we obtained difference absorbance (ΔAbs) spectra over the range of 1000–1700 cm −1 for a 1 mM CH 3 CN solution of [Ru(bpy) 3 ](PF 6 ) 2 at the 500 ps mark following photoexcitation with a 400 nm pulse, and found that the majority of the transient absorption bands could be assigned to vibrational transitions within the 3 MLCT state of [Ru(bpy) 3 ] 2+ . We also determined that only one band (at approximately 1600 cm −1 ) exhibited different temporal behavior, thus advancing the theory that this band could be assigned to another potential.…”

“…TR‐IR spectra were acquired using a pump–probe method in conjunction with a femtosecond Ti:sapphire chirped pulse amplifier (CPA) and a multichannel IR detector array . The details of this method are provided in the Supporting Information.…”

“…Next, we studied the effect of substituting one ligand for another, using [Ru(bpy) 2 (bpm)](PF 6 ) 2 and [Ru(bpy) 2 (bpm‐Me 2 )](PF 6 ) 2 . Although the excited electron was found to be more highly localized on the substituted ligand, the temporal behavior of the corresponding ΔAbs spectra was unaffected (Figure S4). These results imply that the temporal behavior of the 1599 cm −1 band is not tied to charge localization on a single ligand in the 3 MLCT state, but rather to the excited state of the central metal (that is, the 3 MC state).…”