Detailed Characterization of a Nanosecond-Lived Excited State: X-ray and Theoretical Investigation of the Quintet State in Photoexcited Fe(terpy)22+

摘要

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 scatteringdata clearly reflect the formation of the high-spin state of the [Fe(terpy)2]²⁺ molecule; moreover, extended X-ray absorptionfine structure spectroscopy resolves the Fe–ligand bond-lengthvariations with unprecedented bond-length accuracy in time-resolvedexperiments. With ab initio calculations we determinewhy, in contrast to most related systems, one configurational modeis insufficient for the description of the low-spin (LS)–high-spin(HS) transition. We identify the electronic structure origin of thedifferences between the two possible quintet modes, and finally, weunambiguously identify the formed quintet state as ⁵E,in agreement with our theoretical expectations.