Abstract
Ultrafast isomerization reactions underpin many processes in (bio)chemical systems and molecular materials. Understanding the coupled evolution of atomic and molecular structure during isomerization is paramount for control and rational design in molecular science. Here we report transient X-ray absorption studies of the photo-induced linkage isomerization of a Ru-based photochromic molecule. X-ray spectra reveal the spin and valence charge of the Ru atom and provide experimental evidence that metal-centered excited states mediate isomerization. Complementary X-ray spectra of the functional ligand S atoms probe the nuclear structural rearrangements, highlighting the formation of two metal-centered states with different metal-ligand bonding. These results address an essential open question regarding the relative roles of transient charge-transfer and metal-centered states in mediating photoisomerization. Global temporal and spectral data analysis combined with time-dependent density functional theory reveals a complex mechanism for photoisomerization with atomic details of the transient molecular and electronic structure not accessible by other means.
Highlights
Ultrafast isomerization reactions underpin many processes inchemical systems and molecular materials
[aRtut(hbepRy)u2(Lp3y-EeSdOge)](22+.8. 4DkeifVfe)reanntdiaal t the S K-edge spectra were measured as a function of time delay following optical excitation of the 1MLCT transition, with time-resolution determined by the 70 ps X-ray pulse duration
The S K-edge spectrum is comprised of transitions from the S 1s core level to antibonding orbitals of primarily sulfoxide ligand character, as illustrated in Supplementary Fig. 2
Summary
Ultrafast isomerization reactions underpin many processes in (bio)chemical systems and molecular materials. As a particular class of materials to which isomerization is often highly relevant, photochromic transition metal complexes have demonstrated photo-responsive electronic, magnetic, and structural properties[14,15,16,17,18] They provide an important framework to investigate the coupling of electronic and nuclear motion that underpins linkage isomerizations and more generally, reactions of molecular photoswitches, molecular machines, and photocatalysts. This was suggested as an efficient pathway to formation of the O-bonded ground state (1GO), as shown, possibly via a sideways-bonded 3MLCT intermediate (3MLCTSO) In this scenario, the Ru–S bond breaking and Ru–O bond making processes involve both the lowest triplet and singlet PESs. the transient optical spectroscopy that is the basis for the isomerization mechanisms described above provides only indirect information on transient valence charge and atomic structure and is blind to transitions involving optically dark states, including metal-centered excited states that may be of particular interest for this class of systems, as described below
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