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

Abstract

The electronic structure and photochemistry of copper formate clusters, Cu I 2 (HCO 2 ) 3 − and Cu II n (HCO 2 ) 2 n +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 Cu I and Cu II copper ions is observed. For the Cu I species, transitions between copper d and s/p orbitals are recorded. For stoichiometric Cu II 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 Cu II (HCO 2 ) 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 Cu I 2 (HCO 2 ) 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 Cu II (HCO 2 ) 3 Cu 0− biradical structure is formed in the excited state, which gives rise to the photochemical loss of a neutral copper atom.

Spectroscopy and photochemistry of copper nitrate clusters

“…Furthermore, UV/vis photodissociation of copper(II) formate provided detailed insights into its photochemistry. 66 Internal conversion dominates after exciting 3d-3d excitations in the visible region and Ligand-to-Metal Charge Transfer (LMCT) excitations in the UV, which additionally allowed the photochemical loss of the neutral ligand. 66 In contrast, in a previous UV/vis study of Cu(II)(NO 3 ) 3 À the 3d-3d excitations expected for the [Ar]3d 9 configuration of copper(II) have not been reported.…”

“…66 Internal conversion dominates after exciting 3d-3d excitations in the visible region and Ligand-to-Metal Charge Transfer (LMCT) excitations in the UV, which additionally allowed the photochemical loss of the neutral ligand. 66 In contrast, in a previous UV/vis study of Cu(II)(NO 3 ) 3 À the 3d-3d excitations expected for the [Ar]3d 9 configuration of copper(II) have not been reported. 67 Two bands in the UV were observed in this study, along with a single decomposition fragment, Cu(I)(NO 3 ) 2 À .…”

“…The photodissociation spectra are corrected for fragmentation caused by black body infrared radiative dissociation (BIRD) [69][70][71][72][73][74] and collisions with the background gas, while the photodissociation cross sections are calculated as described in detail before. 66,75 Hereby, the beam profile is estimated as Gaussian beam and a correction factor is applied to the measured laser power after each mass spectrum to account for losses in beam steering optics and windows. An additional correction factor is applied to compensate for the beam walk-off upon switching between the OPO regions, signal to idler at 710 nm, sum frequency generation to signal at 410 nm, and second harmonic generation to sum frequency generation at 296 nm, as described before.…”

“…An additional correction factor is applied to compensate for the beam walk-off upon switching between the OPO regions, signal to idler at 710 nm, sum frequency generation to signal at 410 nm, and second harmonic generation to sum frequency generation at 296 nm, as described before. 66 For the ground-state calculations of copper nitrate clusters, density functional theory (DFT) within the B3LYP/def2TZVP level is used for geometry optimization, based on extensive benchmarking with the previously studied copper salt, copper formate. [63][64][65][66] The good performance of the B3LYP method was independently assessed in the decomposition of copper nitrate.…”

“…66 For the ground-state calculations of copper nitrate clusters, density functional theory (DFT) within the B3LYP/def2TZVP level is used for geometry optimization, based on extensive benchmarking with the previously studied copper salt, copper formate. [63][64][65][66] The good performance of the B3LYP method was independently assessed in the decomposition of copper nitrate. 68 .…”