Time-resolved IR spectra indicated fast electron transfer from the reduced photosensitizer to the catalyst.

[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. ...

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.

We carried out time-resolved infrared (TR-IR) and emission lifetime measurements on a Re(I) carbonyl complex having an N-heterocyclic carbene ligand, namely, fac-[Re(CO)(PyImPh)Br], under photochemically reactive (in solution in acetonitrile) and nonreactive (in solution in dichloromethane) conditions to investigate the mechanism of photochemical ligand substitution reactions. The TR-IR measurements revealed that no reaction occurs on a picosecond time scale and the cationic product, namely, fac-[Re(CO)(PyImPh)(MeCN)], is produced on a nanosecond time scale only in solution in acetonitrile, which indicates that the reaction proceeds thermally from the excited state. Because no other products were observed by TR-IR, we concluded that this cationic product is an intermediate species for further reactions. The measurements of the temperature-dependent emission lifetime and analysis using transition-state theory revealed that the photochemical substitution reaction proceeds from a metal-to-ligand charge transfer excited state, the structure of which allows the potential coordination of a solvent molecule. Thus, the coordinating capacity of the solvent determines whether the reaction proceeds or not. This mechanism is different from those of photochemical reactions of other types of Re(I) carbonyl complexes owing to the unique characteristics of the carbene ligand.

We constructed an instrument for time-resolved photoemission electron microscopy (TR-PEEM) utilizing femtosecond (fs) laser pulses to visualize the dynamics of photogenerated electrons in semiconductors on ultrasmall and ultrafast scales. The spatial distribution of the excited electrons and their relaxation and/or recombination processes were imaged by the proposed TR-PEEM method with a spatial resolution about 100 nm and an ultrafast temporal resolution defined by the cross-correlation of the fs laser pulses (240 fs). A direct observation of the dynamical behavior of electrons on higher resistivity samples, such as semiconductors, by TR-PEEM has still been facing difficulties because of space and/or sample charging effects originating from the high photon flux of the ultrashort pulsed laser utilized for the photoemission process. Here, a regenerative amplified fs laser with a widely tunable repetition rate has been utilized, and with careful optimization of laser parameters, such as fluence and repetition rate, and consideration for carrier lifetimes, the electron dynamics in semiconductors were visualized. For demonstrating our newly developed TR-PEEM method, the photogenerated carrier lifetimes around a nanoscale defect on a GaAs surface were observed. The obtained lifetimes were on a sub-picosecond time scale, which is much shorter than the lifetimes of carriers observed in the non-defective surrounding regions. Our findings are consistent with the fact that structural defects induce mid-gap states in the forbidden band, and that the electrons captured in these states promptly relax into the ground state.

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