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

Abstract

[Ru(bpy) 3 ] 2 + is well-known as a prototype for the Ru(II) complexes used in a wide variety of photofunctional materials. The triplet metal-centered ( 3 MC) state is important in this complex, since it dominates the phosphorescence lifetime and photoreaction processes. Despite this, the 3 MC state has not yet been observed by spectroscopic methods. In the present study, we demonstrated that time-resolved infrared vibrational spectroscopy enables observations of the 3 MC state. A vibrational band at 1599 cm À 1 was found to exhibit unique temporal behavior that differed from that of other bands assignable to the triplet metal-to-ligand charge-transfer ( 3 MLCT) state. This unique behavior was assessed under various experimental conditions and it was concluded that the band arises from the short-term population (~23 ps) of the 3 MC state during relaxation to the bottom of the 3 MLCT state. These results agree with [Fe(bpy) 3 ] 2 + spectra, which show that the 5 MC state is the most stable excited state.Many applications of Ru(II) polypyridyl complexes exist that utilize their photoexcited states, [1] such as photocatalysts, [1b, c] photovoltaic cells, [1d] organic photosyntheses, [1e] luminescence probes, [1f] and anti-cancer drugs. [1g] However, there is still a lack of consensus regarding the fundamental photoexcited processes in these compounds, despite the many efforts that have been made to study these processes using a variety of ultrafast spectroscopic techniques. [2] In particular, the triplet metal-centered state ( 3 MC), also termed the triplet ligand field state ( 3 LF) or, is elusive. The 3 MC is an electronic state that occurs when an excited electron enters the higher dorbitals split by the ligand field of the ruthenium ion. Because it is Laporte forbidden, the optical transition to this 3 MC state from the ground state (GS) is hardly observed. [1a] Various experimental results have been used to examine the 3 MC state. The fluorescence lifetime [1,3] and the ligand substitution photoreaction [1,4] of these Ru(II) complexes have been found to vary exponentially with temperature. Because the 3 MC state is involved in these processes, these results indicate that the 3 MC state is thermally accessible from the meta-stable triplet metal-to-ligand charge transfer state ( 3 MLCT). However, strictly speaking, these results show the existence of an activation barrier to such transitions and give no direct information regarding the 3 MC potential. If the metal centered (MC) state is the most stable excited state in a complex, it should be possible to observe it spectroscopically. This has been realized by modification of the ligands in [Ru(bpy) 3 ] 2 + (bpy = 2,2'-bipyridine), following which the transient ultra-violet and visible (UV/VIS) spectra of the 3 MC state were observed. [5] Fe(II) complexes have also allowed the direct observation of a MC state. Since Fe(II) has the same outer electron configuration as Ru(II), the electronic structures of Fe(II) complexes resemble those of Ru(II) complexes. ...

Trapping intermediate MLCT states in low-symmetry {Ru(bpy)} complexes

“…Of great importance would be the C–N stretchings of CN – and NCS – , as well as the C–C vibrations of the bpy ligand. 74 , 75 …”

Photochemical Processes in a Rhenium(I) Tricarbonyl N-Heterocyclic Carbene Complex Studied by Time-Resolved Measurements

“…TR-IR measurements were carried out by the pump-probe method using an experimental setup described in previous papers [22][23][24][25] . A Ti:sapphire regenerative amplifier (Spectra Physics…”

Structural deformation of a ring-shaped Re(I) diimine dinuclear complex in the excited state

“…In the case of the Re(I) diimine carbonyl complexes, many excited state characteristics have been clarified by observing the vibrations of CO ligandsby TR-IR [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33].Quantum chemical calculations, using a program packagesuch as Gaussian,are another important tool for the study of transition complexes in the excited state. Although the results of such calculations for the excited states of organic compounds are not especially reliable, it has recently been shown thatthe resultsobtained for thelowest triplet state (T1)are in good agreement with the TR-IR spectragenerated bythe meta-stable triplet statesof transition metal complexes [22,25,26,28,[34][35][36]. Considering that the energies and intensities of sharp vibrational bands are very sensitive to electronic and molecular structures, the reliability of the quantum calculations issufficiently highso as to allow a discussion of the properties of such metal complexes in the excited state.…”