Femtosecond X-ray Spectroscopy Directly Quantifies Transient Excited-State Mixed Valency

Abstract

Quantifying charge delocalization associated with short-lived photoexcited states of molecular complexes in solution remains experimentally challenging, requiring local element specific femtosecond experimental probes of time-evolving electron transfer. In this study, we quantify the evolving valence hole charge distribution in the photoexcited charge transfer state of a prototypical mixed valence bimetallic iron–ruthenium complex, [(CN)5FeIICNRuIII(NH3)5]−, in water by combining femtosecond X-ray spectroscopy measurements with time-dependent density functional theory calculations of the excited-state dynamics. We estimate the valence hole charge that accumulated at the Fe atom to be 0.6 ± 0.2, resulting from excited-state metal-to-metal charge transfer, on an ∼60 fs time scale. Our combined experimental and computational approach provides a spectroscopic ruler for quantifying excited-state valency in solvated complexes.

Density Functional Theory for Electronic Excited States

“…Because the initial state is non-stationary, this requires that the "field on" simulation be referenced to a time-evolving "field-off" simulation. 436 This approach has recently been applied to simulate emerging transient x-ray experiments, [471][472][473][474] carried out at free-electron laser facilities using x-ray pulses with femtosecond time resolution. As an example, it is possible to follow metal-to-metal CT dynamics in the mixed-valence [(CN) 5 Fe II CNRu III (NH 3 ) 5 ] − compound, which occur on a ∼ 60 fs timescale following excitation at 800 nm, using time-resolved x-ray emission spectroscopy at the iron K edge (2p 3/2 → 1s transition at 7,114 eV).…”

“…As an example, it is possible to follow metal-to-metal CT dynamics in the mixed-valence [(CN) 5 Fe II CNRu III (NH 3 ) 5 ] − compound, which occur on a ∼ 60 fs timescale following excitation at 800 nm, using time-resolved x-ray emission spectroscopy at the iron K edge (2p 3/2 → 1s transition at 7,114 eV). 474…”

Computational approaches for XANES, VtC-XES, and RIXS using linear-response time-dependent density functional theory based methods

“…[89][90][91] LR-TDDFT/TDA is sufficiently predictive and has been applied to a broad range of systems to compute K-edge, L-edge, and M-edge XANES [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45]84,[92][93][94][95][96][97][98][99][100][101][102][103] as well as transient Xray absorption spectroscopy. [104][105][106][107][108][109] In Fig. 2 and 3 we show the experimental and computed K-edge XANES and L 3 -edge XANES of model Fe and Ru complexes in different oxidation states.…”

The Excited‐State Creutz–Taube Ion

“…Additionally, time-resolved spectroscopic techniques have been applied to study GSMV systems upon excitation in their IVCT band. [41][42][43][44][45][46][47][48][49][50] However, the advent of robust and sensitive ultrafast transient absorption techniques with broad band detection (especially in the NIR) has also promoted investigations in a different type of MV systems, which are not MV systems in their ground states. In such photoinduced mixed valence (PIMV) systems, MV fragments are created only upon absorption of a photon, [51][52][53][54][55][56] and photoinduced intervalence charge transfer (PIIVCT) bands are observed only in the excited state.…”

“…This has been favored by the experimental accessibility of IVCT absorptions, on one hand, and ground state redox potentials, which are an additional key feature that allow to characterize electronic communication between the redox sites, [40] on the other hand. Additionally, time‐resolved spectroscopic techniques have been applied to study GSMV systems upon excitation in their IVCT band [41–50] . However, the advent of robust and sensitive ultrafast transient absorption techniques with broad band detection (especially in the NIR) has also promoted investigations in a different type of MV systems, which are not MV systems in their ground states.…”