Sequential Activation of Molecular Breathing and Bending during Spin-Crossover Photoswitching Revealed by Femtosecond Optical and X-Ray Absorption Spectroscopy

We study the basic mechanisms allowing light to photoswitch at the molecular scale a spin-crossover material from a low- to a high-spin state. Combined femtosecond x-ray absorption performed at LCLS X-FEL and optical spectroscopy reveal that the structural stabilization of the photoinduced high-spin state results from a two step structural trapping. Molecular breathing vibrations are first activated and rapidly damped as part of the energy is sequentially transferred to molecular bending vibrations. During the photoswitching, the system follows a curved trajectory on the potential energy surface.

Figure 1

(a) LS (140 K, blue) and HS (200 K, red) structures of [$\mathrm{Fe}{(\text{phen})}_2{(\mathrm{NCS})}_2$] ($\text{phen}=1$, 10-phenanthroline). The central Fe atom is bounded by N to the phenantroline and NCS groups. Green arrows represent the breathing mode and purple ones the bending mode. (b) XANES spectra and difference $\mathrm{\Delta }\mathrm{XANES}$ measured between the LS and HS states.

Figure 2

Kinetic traces of XANES at 7125 and 7148 eV, fit with a single-exponential function of the rising edge [${\tau }_{\mathrm{Fe}-\mathrm{N}}=170$ (10) fs] represented by red solid lines (a). Kinetic traces of OR and OT at 760 nm (b), OR at 900 and 550 nm and OT at 900 nm (c) and OT 850 nm (d). Fits by a single-exponential function describing the fast change plus a fifth order polynomial are represented by red solid lines.

Figure 3

(a)–(c) Oscillating component of OR at 760, OR at 900, and OT at 850 nm. (d)–(f) Time dependent FFT of the experimental data, showing the activation of the breathing mode and the delayed activation of the bending mode. Combined fits of OR at 900 nm and OT at 850 nm (red line) by the coupled oscillator model, which show the contribution of the bending ($\mathrm{\Sigma }$) mode (b) and the superposition of bending ($\mathrm{\Sigma }$) and breathing ($D$) modes. (c) The time course of the oscillating component $D_{\text{osc}}$ (300 fs) and ${\mathrm{\Sigma }}_{\text{osc}}$ (390 fs) obtained by the fit in (b) and (c) are displayed in (g) and the average evolutions of $D_{\text{mean}}$ and ${\mathrm{\Sigma }}_{\text{mean}}$ obtained by the fit in Fig. 3 are displayed in (h).

Figure 4

(a) Schematic representation of the elongation and damping of the breathing mode along the $D$ coordinate. (b) Classical trajectory in the ($D$, $\mathrm{\Sigma }$) space. Molecules in the LS (blue) potential reach the ${}^1\mathrm{MLCT}$ state by light excitation. Fast ISC, through possible INT states, drives $D$ elongation during step 1 with the generation and damping of breathing phonon, followed by activation of additional bending phonons such as distortion $\mathrm{\Sigma }$, during step 2. The sequence is sketched at the bottom.