UV/Vis Spectroscopy of Copper Formate Clusters: Insight into Metal-Ligand Photochemistry.

The electronic structure and photochemistry of copper formate clusters, CuI 2 (HCO2 )3 - and CuII n (HCO2 )2n+1 - , n≤8, are investigated in the gas phase by using UV/Vis spectroscopy in combination with quantum chemical calculations. A clear difference in the spectra of clusters with CuI and CuII copper ions is observed. For the CuI species, transitions between copper d and s/p orbitals are recorded. For stoichiometric CuII formate clusters, the spectra are dominated by copper d-d transitions and charge-transfer excitations from formate to the vacant copper d orbital. Calculations reveal the existence of several energetically low-lying isomers, and the energetic position of the electronic transitions depends strongly on the specific isomer. The oxidation state of the copper centers governs the photochemistry. In CuII (HCO2 )3 - , fast internal conversion into the electronic ground state is observed, leading to statistical dissociation; for charge-transfer excitations, specific excited-state reaction channels are observed in addition, such as formyloxyl radical loss. In CuI 2 (HCO2 )3 - , the system relaxes to a local minimum on an excited-state potential-energy surface and might undergo fluorescence or reach a conical intersection to the ground state; in both cases, this provides substantial energy for statistical decomposition. Alternatively, a CuII (HCO2 )3 Cu0- biradical structure is formed in the excited state, which gives rise to the photochemical loss of a neutral copper atom.