Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses

Abstract

Ultrafast photoinduced electron transfer preceding energy equilibration still poses many experimental and conceptual challenges to the optimization of photoconversion since an atomic-scale description has so far been beyond reach. Here we combine femtosecond transient optical absorption spectroscopy with ultrafast X-ray emission spectroscopy and diffuse X-ray scattering at the SACLA facility to track the non-equilibrated electronic and structural dynamics within a bimetallic donor-acceptor complex that contains an optically dark centre. Exploiting the 100-fold increase in temporal resolution as compared with storage ring facilities, these measurements constitute the first X-ray-based visualization of a non-equilibrated intramolecular electron transfer process over large interatomic distances. Experimental and theoretical results establish that mediation through electronically excited molecular states is a key mechanistic feature. The present study demonstrates the extensive potential of femtosecond X-ray techniques as diagnostics of non-adiabatic electron transfer processes in synthetic and biological systems, and some directions for future studies, are outlined.

Figure 1. The [¹Ru^II=¹Co^III] complex.

(a) The molecular structure of the dyad studied in this work. The Ru and Co centres are held 13 Å apart by the tpphz rigid bridge. (b) Absorption and emission spectra of [¹Ru^II=] and [¹Ru^II=¹Co^III] in acetonitrile. The pump wavelength used for all the optical and X-ray experiments is indicated by the blue arrow.

Figure 2. Ultrafast optical absorption spectroscopy.

Clockwise: (a) Transient optical absorption spectra of [¹Ru^II=¹Co^III] excited at 400 nm as a function of pump-probe time delay. (b) Transient absorption spectra at three pump-probe time delays: 150 fs, and 2 and 25 ps. (c) The three decay-associated spectra DAS₁, DAS₂ and DAS₃ returned by the global analysis fitting procedure (GA-fit). (d) Kinetic traces over the first 25 ps at three different probe wavelengths: 460, 540 and 625 nm.

Figure 3. Experimental setup.

This optical pump-X-ray probe detection scheme combining XES and XDS on photoexcited species in solution was implemented at the SACLA XFEL facility.

Figure 4. Ultrafast X-ray emission spectroscopy.

(a) Co Kα₁ ΔS_XES(t) at 2.5 (red) and 20 ps (blue) pump-probe delay. The shaded areas indicate the uncertainty level. The dashed black curve is the simulated reference for a ¹Co^III(LS)→⁴Co^II(HS) conversion, scaled to the 20 ps trace. (b) Kinetic trace at 6.93 keV (red dots) and single-exponential fit with a 1.9 ps lifetime, broadened by a 520±410 fs XFEL IRF (blue line). The error bars indicate the s.e. of each data point. (c) Time evolution for the fractions of [²Ru^III(=·)¹Co^III (LS)] (red), [²Ru^III=²Co^II(LS)] (green) and [²Ru^III=⁴Co^II(HS)] (blue) as monitored by the combination of femtosecond TOAS and XES, where the initial fraction of [²Ru^III(=·)¹Co^III (LS)] was renormalized to 1.

Figure 5. Ultrafast X-ray diffuse scattering.

(a) Median filtered ΔS_XDS(Q,t). (b) Experimental (black dots) and fitted (purple line) ΔS_XDS(Q,25 ps). (c) Contributions from the solute (blue) and from the solvent (red).

Figure 6. Ultrafast XDS kinetics.

(a) γ_XDS(t) (blue dots) and γ_XES(t) (green dots) as a function of pump-probe time delay. The single-exponential fits of γ_XDS(t) is indicated by the black line. (b) ΔT(t) kinetics (red dots), with its single-exponential fit (black line). The error bars six indicate the s.d. of the data points.

Figure 7. Low-lying electronically excited MOs and frontier MOs.

These MOs have been obtained from DFT calculations for (a) the LS [¹Ru^II=¹Co^III] and (b) the HS [²Ru^III=⁴Co^II]_α and [²Ru^III=⁴Co^II]_β.

Figure 8. Non-equilibrated ET across the photoexcited [¹Ru^II=¹Co^III] dyad.

The schematic summarizes the fundamental timescales, as obtained from TOAS and combined XES–XDS at the SACLA XFEL facility.