Yeseul Han

Radical-arene contact complexes have long been proposed as key intermediates for selectivity control in radical reactions, but their solution-phase structures and dynamics underlying this role have remained experimentally inaccessible. Here, we use time-resolved X-ray solution scattering to directly observe transient I•–arene contact complexes in benzene and mesitylene. Our results show that I•–benzene adopts an η1/η2-like off-center geometry rather than an η6 π-complex. In contrast, I•–mesitylene exhibits a localized η1 geometry directed toward a CH site. DFT calculations support the relative energetic preferences of these contact motifs, and AIMD simulations reveal rapid dynamic averaging among closely related off-center configurations in benzene but a much narrower orientational range in mesitylene. These findings resolve the detailed structures, interaction motifs, and dynamics of I•–arene contact complexes and show that aromatic substitution substantially reshapes each of these features. Taken together, these results provide a structural and dynamic framework for understanding selectivity in radical reactions.

Nickel–iridium metallophotoredox catalysis has emerged as an effective strategy for promoting C–C and C–heteroatom bond-forming reactions, overcoming the often sluggish reductive elimination from Ni(II) intermediates, thereby expanding the synthetic utility of nickel catalysis. Despite extensive study, however, its precise mechanism has remained largely unresolved because of the experimental difficulty of disentangling the two catalytic centers and the complex electronic structure of nickel. Here, we directly probe the ultrafast electronic dynamics of both metal centers using element-specific femtosecond time-resolved X-ray absorption spectroscopy at an X-ray free-electron laser (XFEL). We identify a transient higher-energy triplet metal-to-ligand charge-transfer (3MLCT) state of nickel that appears exclusively in the presence of the iridium photocatalyst, providing evidence for the energy transfer within a preassociated Ni–Ir complex. Comparison with quantum calculations further shows that this state promotes access to a repulsive Ni–C σ–σ* state, thereby suggesting a possible excited-state pathway relevant to reductive elimination. These findings reveal how the iridium photocatalyst activates the nickel catalytic cycle and highlight selective population of optically dark yet reactive excited states as a general strategy for enabling otherwise inaccessible reaction pathways.

The search for the liquid-liquid critical point in supercooled water is challenging owing to rapid crystallization. We studied supercooled water at timescales before ice formation by heating high- and low-density amorphous ices using infrared ultrafast laser pulses, followed by x-ray scattering. By varying the pump laser fluence, we accessed liquid states straddling the predicted critical point. We observed a crossover from a discontinuous to a continuous transition at which broad and slow structural variations occurred, consistent with critical fluctuations and slowing down. We also observed a rapid increase in the heat capacity indicating a critical divergence at 210 ± 8 K coincident with enhanced density fluctuations. These results suggest that our experiments have directly probed the vicinity of a critical point in supercooled water.