Structural dynamics upon photoexcitation-induced charge transfer in a dicopper(i)–disulfide complex

Abstract

The structural dynamics of charge-transfer states of nitrogen-ligated copper complexes has been extensively investigated in recent years following the development of pump-probe X-ray techniques. In this study we extend this approach towards copper complexes with sulfur coordination and investigate the influence of charge transfer states on the structure of a dicopper(i) complex with coordination by bridging disulfide ligands and additionally tetramethylguanidine units [Cu(NSSN)]. In order to directly observe and refine the photoinduced structural changes in the solvated complex we applied picosecond pump-probe X-ray absorption spectroscopy (XAS) and wide-angle X-ray scattering (WAXS). Additionally, the ultrafast evolution of the electronic excited states was monitored by femtosecond transient absorption spectroscopy in the UV-Vis probe range. DFT calculations were used to predict molecular geometries and electronic structures of the ground and metal-to-ligand charge transfer states with singlet and triplet spin multiplicities, i.e. S, MLCT andMLCT, respectively. Combining these techniques we elucidate the electronic and structural dynamics of the solvated complex upon photoexcitation to the MLCT states. In particular, femtosecond optical transient spectroscopy reveals three distinct timescales of 650 fs, 10 ps and >100 ps, which were assigned as internal conversion to the ground state (S → S), intersystem crossing MLCT →MLCT, and subsequent relaxation of the triplet to the ground state, respectively. Experimental data collected using both X-ray techniques are in agreement with the DFT-predicted structure for the triplet state, where coordination bond lengths change and one of the S-S bridges is cleaved, causing the movement of two halves of the molecule relative to each other. Extended X-ray absorption fine structure spectroscopy resolves changes in Cu-ligand bond lengths with precision on the order of 0.01 Å, whereas WAXS is sensitive to changes in the global shape related to relative movement of parts of the molecule. The results presented herein widen the knowledge on the electronic and structural dynamics of photoexcited copper-sulfur complexes and demonstrate the potential of combining the pump-probe X-ray absorption and scattering for studies on photoinduced structural dynamics in copper-based coordination complexes.

Reaction of N-Acetylcysteine with Cu2+: Appearance of Intermediates with High Free Radical Scavenging Activity: Implications for Anti-/Pro-Oxidant Properties of Thiols

“…In particular, the coordination of the second disulfide in the IM2 suppresses the rotation of Cu atoms around the SS bond and fixes them in a position suitable for trapping the dioxygen molecule. As the Cu–Cu distance (3.9 Å) in dicopper–disulfide complexes [ 38 , 79 , 95 ] is close to that (3.6 Å) in oxy-hemocyanin [ 96 ], the same side-on mode for dioxygen binding is expected. Coordination of the four carboxylate anions of the two diNACs with Cu 2+ ions effectively compensates for the positive charge generated at the catalytic center upon the dioxygen molecule binding.…”

“…The degradation of the Cu(I)–disulfide complexes is O 2 -dependent ( Figure 4 b) and could be aided by a UV photodissociation of the disulfide bonds [ 104 ], in agreement with the observed effect of light on the decay rate of IM1 ( Figure S1 ). Importantly, a cleavage of one of the disulfide bridges in a Cu I 2 (RSSR) 2 complex upon photoexcitation-induced CT has been recently described [ 95 ].…”

First experiments with a water-jet plasma X-ray source driven by the novel high-power–high-repetition rate L1 Allegra laser at ELI Beamlines

“…The HHG source serves two stations for XUV science: 'MAC' for atomic, molecular and optical science and coherent diffractive imaging (Klimesova ´et al, 2021), and 'ELIps' for XUV materials science applications (Espinoza et al, 2020). Complementary methods for ultrafast optical spectroscopy are also available (using support lasers): transient optical absorption (Naumova et al, 2018), stimulated Raman scattering (Andrikopoulos et al, 2020), time-resolved spectroscopic ellipsometry (Richter et al, 2021) and IR (1D and 2D) spectroscopy.…”

“…visible, terahertz and X-ray, using the same pump. The source described here is installed within a modular X-ray spectroscopy end-station, potentially motivating the use of multiple complementary methods for a comprehensive study [see De Roche et al (2003), Naumova et al (2018), Dicke et al (2018), Kunnus et al (2020) and Kjaer et al (2019) for examples].…”

Ultrafast X-ray Photochemistry at European XFEL: Capabilities of the Femtosecond X-ray Experiments (FXE) Instrument

“…This allowed for comparison of both complementary structural tools on the same time-resolved experiment, where van der Veen et al [6] analyzed the recorded time-resolved EXAFS of a photoexcited Pt-POP compound, while M. Christensen et al [12] analyzed the pump-probe WAXS signals from the same molecule, yielding nearly identical results. In view of the difficulty to model the excited state structures in an unambiguous fashion, it is highly attractive to employ complementary structural tools for the same sample and experiment [10,16,17]. This also marks the strategy of the FXE instrument design at European XFEL: to enable complementary structural dynamics tools quasi-simultaneously.…”