Excited-State Structural Dynamics of the Cubane-Type Metal Cluster [Cu4I4(py)4] Explored by Time-Resolved X-Ray Liquidography

Cubane-type metal clusters respond uniquely to stimuli like light and electric potential, resulting in behaviors such as crystal-to-crystal phase transitions. While structural adaptability is known to be linked to these responses, direct experimental evidence for the associated structural changes has been missing. This study addresses this gap by examining the structural dynamics of the copper(I) iodide cubane (Cu4I4(py)4, py = pyridine) upon photoexcitation using time-resolved X-ray liquidography. The results reveal: 1) 100 picoseconds (ps) after excitation, two distinct excited states-the cluster-centered triplet (3CC) state and the (metal+halide)-to-ligand charge transfer triplet (3(M/X)LCT) state-are present; 2) the 3(M/X)LCT state decays with an apparent time constant of 1.21 ns, primarily transitioning to the 3CC state, with a small fraction undergoing decay to the ground state (GS); and 3) the 3CC state eventually returns to the GS. The molecular structures, provided for these states serve as benchmarks for theoretical studies. Importantly, the 3CC structure exhibits significant distortion in the Cu4I4 core and reduced symmetry, findings that are unanticipated by previous models. This comprehensive investigation deepens the understanding of the structural transformations occurring upon photoexcitation, with a potential impact on future applications of these compounds as versatile components in photosensitive metal-organic frameworks.