Direct observation of structural dynamics upon photo-excitation in a spin crossover crystal with femtosecond electron diffraction

Yifeng Jiang,Dongfang Zhang,Eric Collet,Lai Chung Liu,R J Dwayne Miller,Ryan L Field,Cheng Lu,Antoine Sarracini,Henrike M Müller-Werkmeister,Gustavo Moriena,G Cerullo,F Kärtner,R Li,J Ogilvie,M Khalil

doi:10.1051/epjconf/201920507005

Abstract

Photoinduced spin transitions are studied by femtosecond electron diffraction to understand ultrafast structural dynamics associated with intersystem crossing. The results indicate the structural reorganization occurs within 2.3 ps, as the metal-ligand bond distribution narrows during intramolecular vibrational energy redistribution.

Highlights

Photoinduced spin crossover (SCO) dynamics undergo extensive electronic spin transitions and distortions of the molecular structure with unit quantum yield on femtosecond timescales
The results indicate the structural reorganization occurs within 2.3 ps, as the metal-ligand bond distribution narrows during intramolecular vibrational energy redistribution
The spin transition occurs through a series of radiationless processes that coupled the states of different spin multiplicity: typically, the initial singlet state is excited by visible light to a singlet metal-to-ligand charge transfer (1MLCT) state; a1n intersystem spin transition occurs to a higher spin state followed by a relaxation cascade from the initial Franck-Condon modes connecting the two surfaces to potential minimum of the HS surface

Summary

Introduction

Photoinduced spin crossover (SCO) dynamics undergo extensive electronic spin transitions and distortions of the molecular structure with unit quantum yield on femtosecond timescales. These SCO complexes can trigger spin crossover (SCO), a reversible electronic transition between a low-spin (LS) to a high-spin (HS) state that can be induced by applied pressure, temperature, magnetic fields, or light irradiation [1,2]. The photoinduced SCO process in single crystalline [Fe(PM–AzA)2](NCS) has been identified by femtosecond optical pump-probe reflectivity studies and XANES [3,4] as a two-step ISC process with a fast (< 150fs) relaxation of short-lived intermediate states (INT) followed by vibrational cooling of the subsequent HS state within 1.5-2 ps. The precise nature of the structural dynamics remains elusive, especially in the solid state

Experimental Setup

Result and Discussion