Ultrafast, polarization-selective time-resolved X-ray absorption near-edge structure (XANES) was used to characterize the photochemistry of vitamin B, cyanocobalamin (CNCbl), in solution. Cobalamins are important biological cofactors involved in methyl transfer, radical rearrangement, and light-activated gene regulation, while also holding promise as light-activated agents for spatiotemporal controlled delivery of therapeutics. We introduce polarized femtosecond XANES, combined with UV-visible spectroscopy, to reveal sequential structural evolution of CNCbl in the excited electronic state. Femtosecond polarized XANES provides the crucial structural dynamics link between computed potential energy surfaces and optical transient absorption spectroscopy. Polarization selectivity can be used to uniquely identify electronic contributions and structural changes, even in isotropic samples when well-defined electronic transitions are excited. Our XANES measurements reveal that the structural changes upon photoexcitation occur mainly in the axial direction, where elongation of the axial Co-CN bond and Co-N bond on a 110 fs time scale is followed by corrin ring relaxation on a 260 fs time scale. These observations expose features of the potential energy surfaces controlling cobalamin reactivity and deactivation.

Rhenium catalysts have shown promise to promote carbon neutrality by reducing a prominent greenhouse gas, CO2, to CO and other starting materials. Much research has focused on identifying intermediates in the photocatalysis mechanism as well as time scales of relevant ultrafast processes. Recent studies have implemented multidimensional spectroscopies to characterize the catalyst's ultrafast dynamics as it undergoes the many steps of its photocycle. Two-dimensional infrared (2D-IR) spectroscopy is a powerful method to obtain molecular structure information while extracting time scales of dynamical processes with ultrafast resolution. Many observables result from 2D-IR experiments including vibrational lifetimes, intramolecular redistribution time scales, and, unique to 2D-IR, spectral diffusion, which is highly sensitive to solute-solvent interactions and motional dynamics. Spectral diffusion, a measure of how long a vibrational mode takes to sample its frequency space due to multiple solvent configurations, has various contributing factors. Properties of the solvent, the solute's structural flexibility, and electronic properties, as well as interactions between the solvent and solute, complicate identifying the origin of the spectral diffusion. With carefully chosen experiments, however, the source of the spectral diffusion can be unveiled. Within the context of a considerable body of previous work, here we discuss the spectral diffusion of several rhenium catalysts at multiple stages in the catalysis. These studies were performed in multiple polar liquids to aid in discovering the contributions of the solvent. We also performed electronic ground state 2D-IR and electronic excited state transient-2D-IR experiments to observe how spectral diffusion changes upon electronic excitation. Our results indicate that with the original Lehn catalyst in THF, relative to the ground state, the spectral diffusion slows by a factor of 3 in the equilibrated triplet metal-to-ligand charge transfer state. We attribute this slowdown to a decrease in dielectric friction as well as an increase in molecular flexibility. It is possible to partially simulate the charge transfer by altering the electron density moderately by adding electron donating or withdrawing substituents symmetrically to the bipyridine ligand. We find that unlike the significant electronic structure change induced by MLCT, such small substituent effects do not influence the spectral diffusion. A solvent study in THF, DMSO, and CH3CN found there to be an explicit solvent dependence that we can correlate to the solvent donicity, which is a measure of its nucleophilicity. Future studies focused on the solvent effects on spectral diffusion in the crucial photoinitiated state can illuminate the role the solvent plays in the catalysis.

Polarized ultrafast time-resolved X-ray absorption near edge structure (XANES) allows characterization of excited state dynamics following excitation. Excitation of vitamin B, cyanocobalamin (CNCbl), in the αβ-band at 550 nm and the γ-band at 365 nm was used to uniquely resolve axial and equatorial contributions to the excited state dynamics. The structural evolution of the excited molecule is best described by a coherent ballistic trajectory on the excited state potential energy surface. Prompt expansion of the Co cavity by ca. 0.03 Å is followed by significant elongation of the axial bonds (>0.25 Å) over the first 190 fs. Subsequent contraction of the Co cavity in both axial and equatorial directions results in the relaxed S excited state structure within 500 fs of excitation.

The spectral dynamics of a series of rhenium photocatalysts, fac-Re(4,4'-R2-bpy)(CO)3Cl, where R = H, methyl, t-butyl, and carboxylic acid, as well as Re(1,10-phenanthroline)(CO)3Cl were observed in multiple aprotic solvents using two-dimensional infrared spectroscopy (2DIR). The carbonyl vibrational stretching frequencies showed slight variations due to the electron-donating or -withdrawing nature of the substituents on the bipyridine. The different substituents had minimal to no influence on the spectral diffusion time scales of the compounds within a particular solvent, but among the three different solvents investigated (DMSO, THF, and CH3CN), we find the spectral diffusion times to correlate with the solvent's donor number (DN). Because the donicity is a measure the Lewis basicity of the solvent, these findings may help establish a more complete dynamical picture of the photocatalysis, where the first chemical step following optical excitation is electron transfer from a sacrificial donor to the rhenium complex.

A detailed understanding of photocatalyzed reaction dynamics requires a sensitive means of investigating the transient catalytically active species. Ideally, the method should be able to compare the electronically excited photocatalyst directly to the ground state species. We use equilibrium and transient two-dimensional infrared (2DIR and t-2DIR) spectroscopy to study the ground and excited state spectral dynamics of [Re(CO)3(bpy)Cl] in tetrahydrofuran (THF). We leverage the long-lived triplet excited state of the molecule to re-establish an equilibrated state relative to intersystem crossing dynamics and external solvent fluctuations, allowing access to the dynamics experienced by the excited state photocatalyst. The decay of frequency correlations within the excited triplet state species differs significantly from the ground state (slower by a factor of 3), indicating that the electronic excitation and subsequent metal-to-ligand charge transfer and associated structural changes are sufficient to perturb the spectral dynamics as sensed by the carbonyl ligands. In addition, we observe a 2-fold slowdown in ground state spectral dynamics around the in-phase symmetric vibrational mode compared to the two lower frequency, out-of-phase symmetric and asymmetric modes. Following electronic absorption and metal-to-ligand charge transfer the symmetry of the vibrational modes are disrupted, and all vibrational modes experience inhomogeneous broadening and spectral diffusion. The qualitative change in broadening mechanisms arises from the charge redistribution, indicating that direct comparisons of vibrational spectral dynamics on different electronic states-reported here for the first time-can be highly sensitive indicators of changes in electronic structure and in the concomitant solvation dynamics that underlie the microscopic details of charge transfer reactions.