Detailed Characterization of a Nanosecond-Lived Excited State: X-ray and Theoretical Investigation of the Quintet State in Photoexcited [Fe(terpy)₂]²⁺

Abstract

Theoretical predictions show that depending on the populations of the Fe 3dxy, 3dxz, and 3dyz orbitals two possible quintet states can exist for the high-spin state of the photoswitchable model system [Fe(terpy)2]2+. The differences in the structure and molecular properties of these 5B2 and 5E quintets are very small and pose a substantial challenge for experiments to resolve them. Yet for a better understanding of the physics of this system, which can lead to the design of novel molecules with enhanced photoswitching performance, it is vital to determine which high-spin state is reached in the transitions that follow the light excitation. The quintet state can be prepared with a short laser pulse and can be studied with cutting-edge time-resolved X-ray techniques. Here we report on the application of an extended set of X-ray spectroscopy and scattering techniques applied to investigate the quintet state of [Fe(terpy)2]2+ 80 ps after light excitation. High-quality X-ray absorption, nonresonant emission, and resonant emission spectra as well as X-ray diffuse scattering data clearly reflect the formation of the high-spin state of the [Fe(terpy)2]2+ molecule; moreover, extended X-ray absorption fine structure spectroscopy resolves the Fe–ligand bond-length variations with unprecedented bond-length accuracy in time-resolved experiments. With ab initio calculations we determine why, in contrast to most related systems, one configurational mode is insufficient for the description of the low-spin (LS)–high-spin (HS) transition. We identify the electronic structure origin of the differences between the two possible quintet modes, and finally, we unambiguously identify the formed quintet state as 5E, in agreement with our theoretical expectations.

X-ray Absorption and Emission Spectroscopic Studies of [L2Fe2S2]n Model Complexes: Implications for the Experimental Evaluation of Redox States in Iron–Sulfur Clusters

“…This is consistent with recent studies by Pollock et al 75 and serves as a cautionary note against using Kβ mainlines as an isolated probe of spin state. 76,77 …”

Probing Transient Valence Orbital Changes with Picosecond Valence-to-Core X-ray Emission Spectroscopy

“…16,17 The HS state decays nonradiatively back to the LS ground state with a lifetime of 2.6 ns in room-temperature aqueous solution. 18 …”

Structural Dynamics of Spin Crossover in Iron(II) Complexes with Extended-Tripod Ligands

“…Tuning of the reaction coordinate in this context therefore necessarily means to add an anisotropic component to the SCO induced molecular motion. Besides the coupled radial and angular motion in the hallmark system [Fe(tpy) 2 ] 2+ [11,[66][67][68][69], which has profound consequences for the SCO dynamics, only very few other systems have been reported to couple the breathing motion to other vibration or torsion modes. Based on a XRD-calibrated DFT study of iron(II) complexes, we have recently suggested that the coordinate of SCO is heavily affected by the nature of the ligand.…”