Support effects on conical intersections of Jahn-Teller fluxional metal clusters on the sub-nanoscale

The concept of fluxionality has been invoked to explain the enhanced catalytic properties of atomically precise metal clusters of subnanometer size. Cu₃ isolated in the gas phase is a classical fluxional metal cluster where a conical intersection leads to a Jahn-Teller (JT) distortion resulting in a potential energy landscape with close-lying multiminima and, ultimately, fluxional behavior. In spite of the role of conical intersections in the (photo)stability and (photo)catalytic properties of surface-supported atomic metal clusters, they are largely unexplored. In this work, by applying a high-level multi-reference ab initio method aided with dispersion corrections, we analyze support effects on the conical intersection of Cu₃ considering benzene as a model support molecule. We verify that the region around the conical intersection and the associated Jahn-Teller (JT) distortion is very slightly perturbed by the support when the Cu₃ cluster approaches it in a parallel orientation: Two electronic states remain degenerate for a structure with a C₃ symmetry consistent with the D_3h symmetry of unsupported Cu₃ at the conical intersection. It extends over a one-dimensional seam that characterizes a physisorption minimum of the Cu₃-benzene complex. The fluxionality of the Cu₃ cluster, reflected in large fluctuations of relaxed Cu-Cu distances as function of the active JT mode, is kept unperturbed upon complexation with benzene as well. In stark contrast, for the energetically favored perpendicular orientation of the Cu₃ plane to the benzene ring plane, the conical intersection (CI) is located ~1.5 eV above the chemisorption minimum, with the fluxionality being kept at the CI's nearby and lost at the chemisorption well. The first excited state at the perpendicular orientation has a deep well, being energetically closer to the CI. The transition dipole moment between ground and excited states has a significant magnitude, suggesting that the excited state can be observed through direct photo-excitation from the ground state. Besides demonstrating that the identity of an isolated Jahn-Teller metal cluster can be preserved against support effects at a physisorption state and lifted out at a chemisorption state, our results indicate that a correlation exists between conical intersection topography and fluxionality in the metal cluster's Cu-Cu motifs.