Femtosecond X-Ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated[Co(terpy)2]

Abstract

We study the structural dynamics of photoexcited ½CoðterpyÞ 2 2þ in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ∼7 ps. DOI: 10.1103/PhysRevLett.117.013002 Several Co(II) compounds are known to transition between their low spin (LS) and high spin (HS) electronic states [1][2][3]. Such transitions can be induced by temperature increase, excitation by light, or high magnetic fields [4], and they are accompanied by distinct changes in magnetic and structural properties that may be exploited in the design of display and memory devices [5,6] and in single-molecule spintronic applications [7]. The realization of exploitable spin-state transitions (SSTs) in Co(II) compounds is more challenging than in the corresponding Fe(II) complexes, which have been investigated in great detail during the last decades [8][9][10][11][12][13][14][15]. These challenges stem from the partial occupation of the antibonding e à g orbitals in the ground state, which leads to smaller structural changes arising from the SST phenomenon; the corresponding smaller energy barriers between the potential surfaces of the HS and LS Co(II) states result in faster dynamics [1], as well as a high sensitivity to the crystalline environment or to the solvent properties [2]. The key structural parameters for the SSTs are the Co-N bond lengths [8], but the time scales and the dynamics of the LS-HS transitions have remained unclear for Co compounds. Time-resolved x-ray scattering can be used to monitor such structural changes and dynamics if the time resolution of the experiment is sufficiently high. X-ray free electron lasers (XFELs) provide ultrashort (∼30 fs) x-ray pulses and high flux allowing the nuclear dynamics following photoexcitation to be recorded at the required femtosecond time scales [16,17]. Here, we report, for the first time, direct measurements of the excited-state structure and the ultrafast structural dynamics of a solvated Co(II) complex upon a photoinduced SST. Figure 1 shows the molecular structure of ½CoðterpyÞ 2 2þ ðterpy ¼ 2; 2 0 ∶6 0 ; 2 00 − terpyridineÞ. In this six-coordinated complex, the d 7 Co center can be either a LS doublet state or a HS quartet state [2,18]. In solid-state samples, the relative populations of both spin states depend strongly on the In this work, we utilized x-ray diffuse scattering (XDS) laser pump-x-ray probe experiments to study the formation, structure, and decay of...

Perspective: Opportunities for ultrafast science at SwissFEL

“…Fs XRS was recently realised by Biasin et al 150 who probed the structural dynamics upon photoinduced SCO in aqueous [Co II (terpy) 2 ]. Their analysis showed that photoexcitation leads to impulsive elongation of the Co-N bonds, giving rise to coherent Co-N bond length oscillations.…”

Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering

“…A powerful method to disentangle the electronic and nuclear motions of 3d transition metal complexes during ultrafast nonadiabatic processes is time-resolved 1s-2p (Ka) and 1s-3p (Kβ) X-ray Emission Spectroscopy (XES) [10][11][12][13][14] combined with Wide Angle X-ray Scattering (WAXS) [15][16][17][18][19][20][21][22][23]. These experiments have been utilized to assign the electronic and structural motions during excited state dynamics [24][25][26] and to project the locations of conical intersections between excited state potential energy surfaces onto critical structural coordinates [27].…”

“…Coherent nuclear dynamics are ubiquitous in photoexcited systems. Oscillatory wavepacket motions have been clearly measured in time-resolved WAXS [19,27,32], however the effects of purely nuclear dynamics in ultrafast Kα/Kβ XES has not been observed [10][11][12][13][14]. This contrasts with other X-ray spectroscopic methods that directly involve valence orbitals, such as X-ray absorption [33][34][35][36][37][38] and resonant inelastic X-ray scattering [39][40][41], which are sensitive to both electronic and nuclear structure.…”

Jitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser

“…, clusters and nanoparticles (Krikunova et al 2012a, 2012b, Clark et al 2015, Ferguson et al 2016, Flückiger et al 2016, Gorkhover et al 2016, liquids (Hallmann et al 2010, Wernet et al 2015, Biasin et al 2016, solids (Rajkovic et al 2010, Dell'Angela et al 2013, Siefermann et al 2014, Gleason et al 2015, Öström et al 2015, Rettig et al 2016, and even biological systems such as photoactive proteins (Aquila et al 2012, Tenboer et al 2014, Barends et al 2015, Pande et al 2016. Many of these time-resolved experiments require precise synchronization of an optical femtosecond laser with the FEL.…”