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R-K. Femtosecond spectroscopy and
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The quintet state of [Fe(terpy)2]2+ has been prepared with 10 ps-long 532 nm laser pulses in aqueous solution, and the changes of its molecular and electronic structure were characterized at 80 ps delay after light excitation with synchrotron probes. High-quality X-ray absorption, nonresonant and resonant X-ray emission spectra as well as X-ray diffuse scattering data (obtained at APS, Argonne) clearly reflect the formation of the quintet state.
Besides, extended X-ray absorption fine structure spectroscopy resolves the Fe-N bond-length variations with extraordinary bond-bond-length accuracy in time-resolved experiments. By ab initio calculations we have determined why one configurational coordinate is insufficient for description of the low-spin (LS) - high-spin (HS) transitions. We have identified the electronic structure origin of the differences between the two possible quintet modes, and unambiguously characterized the formed quintet state as 5E. This exhaustive study also demonstrates the high performance of the X-ray spectroscopic and scattering techniques available in studying transient states on ultrafast time scales (Figure 1).
Figure 1. Visual summary of our published paper providing a detailed characterization of the [Fe(terpy)2]2+ system.
(The full article can be accessed at http://dx.doi.org/10.1021/acs.jpcc.5b00557.)
Light-harvesting systems. — In our most remarkable experimental result published this year we successfully took advantage of the shorter-than-ps time resolution of the X-ray free electron lasers (XFEL), the element specificity as well as the spin momentum sensitivity of X-ray emission spectroscopy (XES), to study a model system of light-harvesting molecules. The efficiency of the widely spread multi-component photocatalytic systems is reduced by several diffusion controlled steps and cross section factors during their operation. In order to get rid of these disadvantages, it is possible to integrate both the photosensitive and the catalytic function into a single molecule. During the excitation of such a molecular system containing Ru and Co metallic centers, an electron, stemming from the absorbing Ru ion, is transported through the bridging ligand to the Co center and activates that. However, only the absorption bands belonging to the Ru ion and the electron on the bridging ligand can be identified by ultrafast optical spectroscopy, as neither the ground, nor the excited state of the Co ion has absorption in the visible range. However, with the help of different X-ray techniques the electron and structural dynamics of the Co can be investigated selectively, thus the details of the mechanism related to the Co centre can be revealed in XFEL studies. Figure 2 shows the molecule investigated at the SACLA hard X-ray free-electron laser and the steps of the light-induced transformation. X-ray emission spectroscopy revealed that the Co ion undergoes a consecutive charge and spin state change. The Co–N antibonding molecular orbitals are also populated with the spin state change, which causes a 0.2 Å bond length elongation,
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determined by X-ray diffuse scattering (XDS). This method also showed that the thermalization of the excited molecule happens with a 12 ps characteristic time. The details of the mechanism are listed on Figure 2.
Figure 2. The mechanism of the light-excitation triggered electron transfer process in the investigated photocatalytic model system, with the relevant characteristic times.
The abbreviations of the techniques revealing the sequential steps are what follows.
TOAS: transient optical absorption spectroscopy, XES: X-ray emission spectroscopy, XDS: X-ray diffuse scattering. (The article can be reached at the URL:
http://dx.doi.org/10.1038/ncomms7359.) Valence-to-core X-ray emission spectroscopy as an ultrafast probe. — In the valence-to-core (vtc) X-ray emission, the electron hole created by the ionization of a core electron is filled by a valence electron. The final state of the process is equivalent to that in photoelectron spectroscopy; therefore, the chemical sensitivity of vtc XES is the highest among the XES techniques. It would be advantageous to apply this method as a probe during ultrafast experiments because of this sensitivity combined with the large penetration and element selectivity of the hard X-rays. However, the cross section for this emission process is quite low, three orders of magnitude lower than for the Kα emission.
The potential of the usage and development of vtc XES as an ultrafast probe were investigated in a photoionization process. The [Fe(II)(CN)6]4– complex emits an electron as an effect of UV radiation, which process is used to produce solvated electrons frequently. Based on our density function theory (DFT) calculations this phenomena can be monitored via vtc XES, since the energy of the molecular orbitals shifts significantly with the change of the charge. The [Fe(III)(CN)6]3– complex, the product of the ionization, is stable and can be prepared chemically, thus the expected spectral changes can be verified by static measurements. On the upper part of Figure 3 the static spectra of the above mentioned complexes are shown with blue and red line, respectively, while the difference of them is drawn with a green line (of which thickness corresponds to the experimental uncertainty). The lower part of the figure demonstrates the result of the DFT calculation, and for the Fe(II) complex it is also marked at the main transitions from which molecular orbital the electron comes from when the 1s electron hole is filled. During the first time-dependent experiment at the APS synchrotron the data acquisition was limited to only one delay time and four equidistant energy points due to the limited machine time (8 hours). The result of this experiment is shown on the upper part of Figure 3 with red dots. As it is clearly seen, the experimental transient matches the calculated one quite well. Although the intensity of the X-rays and the frequency of the data acquisition was not optimal during this experiment, the feasibility of vtc XES as a probe were successfully demonstrated, and the required experimental conditions were determined. This technique can prove particularly fruitful at the X-ray free-electron lasers and synchrotron
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beamlines with pink beam. Based on this experiment, and with the help of quantum chemical calculations, the feasibility of a specific experiment can be predicted with confidence.
Figure 3. The valence to core X-ray emission spectra of the [Fe(II)(CN)6]4– (blue) and [Fe(III)(CN)6]3– (red) complexes, and their difference (green). The experimental spectra are shown in the upper, while the DFT-calculated ones in the lower part of the figure. For [Fe(II)(CN)6]4–, the molecular orbitals giving rise to the transitions are also plotted. The red dots stand for the difference of the laser excited and the ground state (excitation wavelength is 266 nm, and the delay time was 120 ps after the excitation).
The paper can be downloaded from http://dx.doi.org/10.1021/jp511838q.
Grants
ERC Starting Grant, ERC-StG 259709, X-cited! : Electronic transitions and bistability: states, switches, transitions and dynamics studied with high-resolution X-ray spectroscopy (G.
Vankó, 2010–2015)
“Momentum” Program of the HAS (G. Vankó, 2013 – 2018)
International cooperation
Main cooperations: Prof. C. Bressler (Hamburg), Prof. F. M. F. de Groot (Utrecht), Dr. Kelly Gaffney (SLAC), Dr. A. Juhin (Paris), Dr. K. Knízek (Prague), Prof. M. M. Nielsen (Copenhagen), Prof. F. Renz (Hannover), Dr. S. H. Southworth (Argonne), Prof. V. Sundström (Lund), Dr. Jakub Szlachetko and Dr. Christopher Milne (SwissFEL, PSI, Switzerland)
Publications
Articles
1. Canton SE et al. incl. Vankó G, Németh Z, Pápai M (33 authors): Visualizing the Nonequilibrium Dynamics of Photoinduced Intramolecular Electron Transfer with Femtosecond X-ray Pulses. NAT COMMUN 6: Paper 6359. 9 p. (2015)
2. Canton SE, Zhang X, Liu Y, Zhang J, Pápai M, Corani A, Smeigh AL, Smolentsev G, Attenkofer K, Jennings G, Kurtz CA, Li F, Harlang T, Vithanage D, Chabera P, Bordage A, Sun L, Ott S, Wärnmark K, Sundström V: Watching the dynamics of electrons and atoms at work in solar energy conversion. FARADAY DISCUSS 185: pp. 51-68. (2015)
3. March AM, Assefa TA, Bressler C, Doumy G, Galler A, Gawelda W, Kanter EP, Németh Z, Pápai M, Southworth SH, Young L, Vankó G: Feasibility of Valence-to-Core X-ray Emission Spectroscopy for Tracking Transient Species. J PHYS CHEM C 119:(26) pp. 14571-14578.
(2015)
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4. Spiering H, Ksenofontov V, Leupold O, Kusz J, Deák L, Németh Z, Bogdán C, Bottyán L, Nagy DL: Line shape of 57Co sources exhibiting self absorption. HYPERFINE INTERACT 237:
Paper 58. 9 p. (2015)
5. Vankó G, et al. incl. Pápai M, Németh Z, Rozsályi E (29 authors): Detailed Characterization of a Nanosecond-lived Excited State: X-Ray and Theoretical Investigation of the Quintet State in Photoexcited [Fe(terpy)2]2+. J PHYS CHEM C 119: pp. 5888-5902. (2015)
See also: R-M.5
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