Coordination and Electronic Structure of Ruthenium (II)-tris-2, 2

Article pubs.acs.org/JPCC

Coordination and Electronic Structure of Ruthenium(II)-tris-2,2′bipyridine in the Triplet Metal-to-Ligand Charge-Transfer Excited State Observed by Picosecond Time-Resolved Ru K-Edge XAFS Tokushi Sato,*,† Shunsuke Nozawa,† Ayana Tomita,† Manabu Hoshino,‡ Shin-ya Koshihara,‡ Hiroshi Fujii,§ and Shin-ichi Adachi†,⊥ †

Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan ‡ Department of Chemistry and Materials Science, Tokyo Institute of Technology, CREST (JST), Meguro-ku, To-kyo152-8551, Japan § Institute for Molecular Science and Okazaki Institute for Integrative Bioscience, National Institutes of Natural Sciences, Myodaiji, Okazaki, Aichi 444-8787, Japan ⊥ PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ABSTRACT: Time-resolved X-ray absorption spectra of photoexcited ruthenium(II)-tris-2,2′bipyridine ([RuII(bpy)3]2+) in the triplet metal-to-ligand charge transfer (3MLCT) state are measured and analyzed to investigate transient structural changes directly related to the photophysical properties of the complex. The results from visible (400 nm) and UV (267 nm) excitation indicate that electrostatic interaction between the oxidized Ru atom and the reduced bipyridine ligand is the dominant factor affecting the Ru−N bond contraction. This thus leads to two groups of Ru ligand distances, one exhibiting the ground-state Ru−N distance and another yielding a slightly decreased Ru−N distance due to the localized MLCT excited state. The EXAFS analysis of the photoexcited complex was analyzed toward one single Ru−N distance, yielding a contraction of 0.04 (0.01) Å with an increased DW factor (corresponding to a 0.05 Å mean increase).

1. INTRODUCTION Ruthenium(II)-tris-2,2′-bipyridine ([RuII(bpy)3]2+) has been extensively studied as an efficient photosensitizer for solar energy conversion.1,2 The wide absorption band of the Ru complex spans the visible and UV regions, which is particularly important to utilize the spectral range of the sun. The absorption spectrum consists of metal-to-ligand charge transfer (MLCT), metal-centered (MC), and ligand-centered (LC) excitations.1,2 The characterization of the long-lived triplet state (3MLCT) generated after excitations at various wavelengths provides fundamental information to investigate the solar energy conversion by [RuII(bpy)3]2+. The molecular structure of the excited state depends on the energy level of intramolecular charge transfer, because the 3 MLCT state is described as having one formally reduced bipyridine ligand and two neutral ligands.1−3 A detailed structural characterization of the excited state of [RuII(bpy)3]2+ with atomic resolution is critically important to understand the efficient photosensitizing reactions of the complex. However, studies of charge transfer in the excited state have largely been performed using conventional optical spectroscopy techniques that indirectly provide information related to the molecular structure.4−10 Time-resolved measurements of excited states in solution, such as time-resolved X-ray absorption fine structure (TRXAFS) and time-resolved liquidography, have been developed over the past 10 years and are becoming powerful methods to © 2012 American Chemical Society

investigate the photochemical reactions of metal complexes.11−31 Between these techniques, XAFS not only reveals the local structure of molecules via extended X-ray absorption fine structure (EXAFS) but also reveals the energy level of molecular orbitals via X-ray absorption near-edge structure (XANES). TR-XAFS study of [RuII(bpy)3]2+ at the LIII absorption edge has revealed unique information about the molecular and electronic structure of the 3MLCT state, and the contraction of Ru−N bond length by 0.03 Å was reported in the 3MLCT state.11−16 We performed a time-resolved EXAFS study of [RuII(bpy)3]2+ at the Ru K absorption edge to improve the spatial resolution up to 0.01 Å. We present a structural analysis of the 3MLCT excited state of [RuII(bpy)3]2+ using visible (400 nm) and UV (267 nm) excitation.

2. EXPERIMENTAL METHODS In this TR-XAFS study, all measurements were performed using the fluorescence XAFS method on the undulator beamline, NW14A, at the PF-AR.32,33 X-ray pulses at 794 kHz were monochromatized using a Si(111) monochromator. The X-ray beam was focused on an size of 340 (H) × 214 (V) μm at the sample position using a Rh-coated, bent cylindrical Received: April 20, 2012 Revised: June 1, 2012 Published: June 28, 2012 14232

dx.doi.org/10.1021/jp3038285 | J. Phys. Chem. C 2012, 116, 14232−14236

The Journal of Physical Chemistry C

Article

analysis using the Ru−N and Ru−C shells was performed in k space after Fourier filtering of the peaks of interest and performance of an inverse Fourier transformation. The scattering amplitudes and phase shifts were derived using FEFF 8.20 code for the crystal structure of [RuII(bpy)3](PF6)3.35 An R-range filter of 1−4.5 Å was employed in all of the spectra. Throughout the curve-fitting analysis, the coordination numbers (N) of the Ru−N and the Ru−C shells were held constant at 6.0, whereas the fitting parameters ΔE0, excitation yield (τ), and Ru−N bond distances were varied to minimize the rχ2, defined by the following equation

mirror. A regenerative amplified Ti:sapphire laser operating at a frequency of 945 Hz was used as the pump source. The laser wavelength was alternated between 400 and 267 nm using a barium borate (BBO) crystal and focused using a lens at the sample position with sizes of 430 (H) × 340 (V) and 420 (H) × 340 (V) μm for 400 and 267 nm, respectively. The power outputs of the laser on the samples were 0.2 (400 nm) and 0.04 mJ (267 nm). The pump and probe measurements were performed by detecting the fluorescence X-ray signals prior to and after the laser pulse using gated integrators synchronized with the laser pulse (945 Hz). The output voltage from the gated integrators was converted into a frequency signal using a voltage-tofrequency (V−F) converter. The output pulses from the V−F converter were counted with a 100 MHz counter and were used to obtain the X-ray absorption spectrum. [RuII(bpy)3]2+ was prepared using a published method.34 The sample was dissolved in water to a concentration of 10 mM. The aqueous sample was circulated using a magnetic gear pump to reduce radiation damage by the laser and was shaped to form a stable 50 μm thick jet using a metal nozzle.

r

χ 2 (i , F(τ ), ΔE0) 1 = (N − 1)

⎛ x /F(τ ) − ESi (ΔR , ΔE ) ⎞2 j i 0 ⎟⎟ ∑ ⎜⎜ j τ x / F ( ) Δ j ⎠ j=1 ⎝ N

(2)

with a selected range of N data points. xj is the experimental data point j from the time-resolved measurement, Δxj is the experimental error, F(τ) is the excited population, and ESij (i denotes the spectrum derived from the structural model with an Ri distance between Ru and N atoms) is the simulated value for a given new Ru-bpy distance change ΔRi calculated after including the energy shift ΔE0 for excited-state spectra. This strategy follows in general the recipe delivered in ref 23; however, here we only changed Ru−N distance and not that of the entire bpy ligand molecular structure. Our procedure is nevertheless somewhat justified, as the Ru−N distance changes are fairly small and thus do not violate the bpy structure integrity too much. The limited useful k-range in this fitting procedure cannot distinguish between different groups of bpy ligands, which may exhibit two different distances due to the localized nature of the localized MLCT excited electron on the ligands. The expected different bond lengths are rather reflected in the increased Debye−Waller factor extracted for the Ru−N distance, which hints toward this expected distortion. The rχ2 value was minimal at an excitation yield of 3% as presented in Figure 2. This value is somewhat 10−20 times lower than expected, if calculated from absorption cross section, sample thickness, and laser-illuminated area, which would yield around 50%. The precision of Ru−N bond distance of 3MLCT state was estimated by the confidence level of 90%, as presented in Figure 3. This result shows that the error of Ru−N bond length is ±0.01 Å.

3. RESULTS AND DISCUSSION The EXAFS spectra k2 × χ(k) of the ground state and transient excited state at τ = 50 ps after 400 nm excitation are presented in Figure 1. The k-ranges used for the Fourier transformation

Figure 1. EXAFS spectra for [RuII(bpy)3]2+. The lines correspond to the fits of the data using FEFF 8.20 code.

were 2−10 Å−1.The 3MLCT EXAFS spectrum was obtained using the following equation χ (k)ES = Δχ (k , τ )/F(τ ) + χ (k)gs

(1)

where F(τ) is the excited population of the delay time at τ = 50 ps. χ(k)gs is the absorption spectrum of the ground state, and Δχ(k,τ) is the difference transient spectrum at τ. A curve-fitting

Figure 2. Least-square values with their associated excitation yield indicated in percent. 14233



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Figure 3. Square residuals between the experimental data and 3MLCT simulation at the excitation yield of 3% by changing Ru−N distance in steps of 0.0025 Å. Dashed line indicates rχ2 value of 0.77 that corresponds to confidence level of 90%.

The Ru−N bond lengths and Debye−Waller factors of the ground and excited states are presented in Table 1. TR-EXAFS analysis at the Ru K-edge revealed a 0.04 Å contraction in the Ru−N bond lengths in the 3MLCT state compared with the ground state.3 Figure 4. Ground-state Ru K-edge XANES spectrum of [RuII(bpy)3]2+ (black) and the transient difference spectra with 400 nm excitation (red) and 267 nm excitation (blue) at 50 ps. The dotted line indicates zero difference.

Table 1. Structural Parameters Obtained from the EXAFS Analysis ground state MLCT state

3

shell

R (Å)

σ2 (Å2)

Ru−N Ru−N

2.06 (1) 2.02 (1)

0.004 (1) 0.014 (1)

Previous laser pump−probe experiments have reported the localization of electrons and electron hopping to other ligands on subpicosecond time scales in the 3MLCT state.1,2,4−10 In this study, EXAFS analysis showed the Debye−Waller factor slightly increased in the 3MLCT state compared with the ground state, as shown in Table 1. The increase in the Debye− Waller factor suggests that a structurally distorted state exists in the 3MLCT state with a shorter lifetime than the X-ray pulse duration of 60 ps. The solid black line in Figure 4 shows the XANES spectrum of the ground state. The difference spectra between the ground and transient states at 50 ps after photoexcitation at 400 and 267 nm are presented in the bottom of Figure 4 by filled red circles and empty blue circles, respectively. Both difference spectra show that the edge shift to higher energy relative to the ground state. These edge shifts are due to the change of the oxidation state from Ru(II) to Ru(III). The edge-shift energy was 3.0 eV estimated by the peak in the first derivative. A time course of the intensity at 22125 eV (black arrow in Figure.4) immediately after 400 nm excitation is shown in the left panel of Figure 5. The photoinduced conversion from the ground state to the 3MLCT state via the 1MLCT state occurs in the subpicosecond time domain,4,5 and the convolution was approximated using a Gaussian error step function at time zero. The right panel of Figure 5 shows the decay in the intensity at 22125 eV. Fitting the line with a single exponential function resulted in a decay time of 150 ns, consistent with the lifetime of the 3MLCT state with air-saturated water.1,2,36,37 In Figure 6, the UV excitation of [RuII(bpy)3]2+ at 267 nm indicates a transient variation at 22125 eV with a lifetime identical to that in the 3MLCT state. This result indicates that the initial excited

Figure 5. Time course of the intensity at 22125 eV after excitation at 400 nm. The dotted line indicates zero level.

state formed at 267 nm is mainly converted to 3MLCT state on the subnanosecond time scale. The excitation yield with 267 nm was estimated to be 0.75% by the excitation yield of 400 nm excitation and the scale factor of XANES region results, which is a factor of four less than that at 400 nm. According to the UV−vis absorption spectrum, this ratio should amount to 3:1, which is satisfactory due to the fact that the UV absorption cross section at 267 nm rides on a steep slope. This photochemical property is particularly important for an efficient photosensitizer with a wide absorption band. Previous X-ray crystal structure analysis of [RuII(bpy)3]2+ and [RuIII(bpy)3]3+ revealed that the Ru−N distances of the two complexes are nearly identical (Ru(II) = 2.053(2) Å, Ru(III) = 2.057(3) Å).35 This result indicates that the oxidized 14234



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Figure 6. Time course of the intensity at 22125 eV with UV (267 nm) excitation. The decay time obtained by fitting was 155 ± 110 ns.

Ru atom itself is not the factor to move the bpy ligand inward. However, in the 3MLCT state, there is the additional factor of the electrostatic attraction between Ru(III) and bpy− due to the transfer of electron from the Ru atom to the bpy ligand. Therefore, the 3MLCT state has a shorter Ru−N bond distance than the ground state.

4. CONCLUSIONS TR-XAFS analysis at the K-edge of Ru in the ground and excited states of [RuII(bpy)3]2+ in aqueous solution between 50 ps and hundreds of nanoseconds after photoexcitation with ultrashort 400 and 267 nm laser pulses has been reported. Analysis of the TR-EXAFS modulations revealed contractions of the Ru−N bond distances of 0.04 Å. These contractions were discussed in terms of electrostatic interactions between the oxidized Ru atom and the reduced bpy ligands. An increase in Debye−Waller factor suggests that the local structural distortion exists in the 3MLCT state. The results under different excitations indicated that the initial excited state is mainly converted to 3MLCT state with a wide absorption band. Our results provide a better understanding of photosensitizing reactions by providing structural information for the theoretical calculation of the reorganization energy of electron-transfer reactions beginning with the 3MLCT state and will be useful for designing artificial photochemical systems.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-(29) 879-6185. Fax: +81-(29) 879-6187. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by PRESTO/JST (to S.A.) and approved by the Photon Factory Program Advisory Committee (PF-PAC No. 2009S2-001 and 2009G693). This work was partially supported by grants from Basic Science Research Project from Sumitomo Foundation (to S.N.).

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REFERENCES

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Coordination and Electronic Structure of Ruthenium (II)-tris-2, 2

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