Femtosecond Electron Diffraction Study of the Spin Crossover Dynamics of Single Crystal [Fe(PM-AzA)2](NCS)2 Yifeng Jiang, Lai Chung Liu, Henrike M. Müller-Werkmeister, Meng Gao, Cheng Lu, Dongfang Zhang, Eric Collet and R.J. Dwayne Miller Abstract The atomic motions involved in spin crossover photo-switching dynamics of single crystal [Fe(PM-AzA)2](NCS)2 are investigated by femtosecond electron diffraction (FED). The experiment was performed with an ultrabright femtosecond electron source using 8.5 × 104 electrons per pulse with 400 fs temporal instrument response function.
1 Introduction Spin crossover (SCO), a conversion from low spin (LS) ground state to high spin (HS) excited state (or visa versa) due to temperature change or light absorption [1, 2], has been extensively studied for its potential applications in optical memory and photo switchable devices [3]. Among the compounds with SCO dynamics, the group of ferrous Fe(II) metal compounds with an FeN6 coordination environment is the largest [4]. After the Fe(II) compounds absorb light, the LS 1A1(t62g) ground state changes to the HS 5T2(t42ge2g) excited state by 2 paired electrons in the t2g sublevel entering the eg sublevel as unpaired electrons. As the Fe is bonded to the ligand by N atoms, the Fe-N bond distance in the HS state is elongated by *0.2 Å
Y. Jiang D. Zhang R.J.D. Miller (&) Centre for Free Electron Laser Science, Department of Chemistry and Physics, Centre for Free Electron Laser Science, Max Planck Institute for the Structure and Dynamics of Matter, University of Hamburg, Notkestrasse 85, 22607 Hamburg, Germany e-mail:
[email protected] L.C. Liu H.M. Müller-Werkmeister M. Gao C. Lu R.J.D. Miller Departments of Chemistry and Physics, University of Toronto, 80 St. George Street, Toronto, ON M5S 3H6, Canada E. Collet Institut de Physique de Rennes, Campus de Beaulieu, University Rennes 1, UMR 6251 UR1-CNRS,Bat 11A Campus de Beaulieu, Rennes, France © Springer International Publishing Switzerland 2015 K. Yamanouchi et al. (eds.), Ultrafast Phenomena XIX, Springer Proceedings in Physics 162, DOI 10.1007/978-3-319-13242-6_69
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Fig. 1 For single crystal [Fe(PM-AzA)2](NCS)2, the Fe-N bond length of the low spin state is 2 Å and it increases 0.2 Å upon conversion to the high spin state [6]. The Fe atom is represented by the large ball
concomitant with the transfer of two electrons to the antibonding eg orbital and loss of π-backbonding from the t2g [5] orbital. Therefore, the molecular structure rearranges between LS and HS states as shown in Fig. 1. Cis-bis(thiocyanato)-bis(N-2′-pyridyl methylene)-4-(phenylazo) aniline iron(II) ([Fe(PM-AzA)2](NCS)2) is a model system for Fe(II) metal compounds. Single crystal [Fe(PM-AzA)2](NCS)2 has been studied by femtosecond optical pumpprobe reflectivity [6]. Figure 2 illustrates the spin crossover process of single crystal [Fe(PM-AzA)2](NCS)2, which has been identified as a two-step photo-switching process with a fast relaxation of short-lived intermediate states (INT) and a slow vibrational cooling of the photoinduced high spin state. Additionally, X-ray diffraction with 100 ps time resolution studies on the SCO compound [(TPA) Fe(III)(TCC)]PF6 indicate that molecules are locally photoswitched with structural reorganization at constant volume [7, 8]. However, the photo-induced SCO structural behavior of single crystal [Fe(PM-AzA)2](NCS)2 on the sub 10 ps time scale is still unknown. Here we use FED to study the structural dynamics of [Fe(PMAzA)2](NCS)2 spin crossover in single crystal form. Here we use a ultrabright FED source to closely track structural signatures upon photoconversion from the LS to the HS state.
Fig. 2 A simplified energy level diagram of single crystal [Fe(PM-AzA)2](NCS)2 has been deduced from femtosecond optical pumpprobe reflectivity studies [6]
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Fig. 3 a Diffraction pattern of LT state of [Fe(PM-AzA)2](NCS)2 was obtained at 160 K. The signal is an average over 10 electron pulses. b Diffraction pattern of HT state of [Fe(PM-AzA)2] (NCS)2 was obtained at 300 K. c Relative intensity change of selected diffraction spot (−1, 1, 0) and (−2, 2, 0) from a where clear ultrafast structural processes are resolved
2 Experimental Methods In this study, a 95 keV hybrid DC-RF electron source is employed to generate ultrabright femtosecond electron pulses as an ultrafast structural probe [9]. We generate 8.5 × 104 electrons per pulse with a spot size of 300 µm diameter and a repetition rate of 100 Hz. To investigate the SCO dynamics of [Fe(PM-AzA)2] (NCS)2, we employed 60 fs pump pulses centered at 800 nm. At the sample position, the pulse energy was 8 µJ per pulse and the laser beam diameter was 520 µm. The incident excitation fluence was 4.2 mJ/cm2.
3 Results and Discussion Figure 3 shows preliminary results of the FED study. The diffraction pattern of the LS state of [Fe(PM-AzA)2](NCS)2 is shown in Fig. 3a, b shows the diffraction pattern of the HS state. The important point is the high quality of the diffraction data for an organic system. We have studied several crystalline orientations and have conducted in parallel fs transient absorption studies of these same single crystals to provide complementary information on the excited state dynamics to enable separation of the electronic and nuclear contributions to the spin cross over dynamics. Figure 3c shows the picosecond structural dynamics out to 100 ps for two diffraction orders. The full structural dynamics reconstructed from these studies are outside the scope of this brief report and will be reported elsewhere. Acknowledgements We thank J.F. Létard for providing the samples. This work was supported by Max Planck Institute for the Structure and Dynamics of Matter, University of Hamburg, DESY, Marie Curie Actions, University of Toronto and the NSERC.
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