Wavelength Dependent Photofragmentation Patterns of Tris(2,2,6,6-tetramethyl-3,5-heptanedionato)Ln (III) (Ln = Eu, Tb, Gd) in a Molecular Beam

Abstract

Laser photoionization and ligand photodissociation in Ln(thd)(3) (Ln = Eu, Tb, Gd; thd = 2,2,6,6-tetramethyl-3,5-heptanedionato) are studied in a molecular beam via time-of-flight mass spectrometry. The fragmentation patterns are strongly wavelength dependent. With 355 nm excitation, the mass spectrum is dominated by Ln(2+), Ln(+), and LnO(+) fragments. The bare Ln ions are believed to arise from photoionization of neutral Ln atoms. The Ln atoms, in turn, are produced from the Ln(thd)(3) complex in a sequence of Ln reductions (through ligand-to-metal charge-transfer transitions), with each reduction being accompanied by the dissociation of a neutral ligand radical. In contrast, under visible-light (410-450 nm) excitation, a significant Ln(thd)(n)(+) signal is observed (where n = 2,3 for Ln = Tb,Gd and n = 1-3 for Ln = Eu). Thus, with visible excitation, photoionization of Ln(thd)(n) competes effectively with the Ln-reduction/ligand-dissociation sequence that leads to the dominant bare Ln-ion signal seen with 355 nm excitation. The fact that monoligated Ln(thd)(+) is observed only for Ln = Eu is interpreted in terms of the relative accessibility of an excited ligand-to-metal charge-transfer state from the ground electronic state of neutral Ln(thd).

Photofragmentation of Gas-Phase Lanthanide Cyclopentadienyl Complexes: Experimental and Time-Dependent Excited-State Molecular Dynamics

“…This is very similar to the mechanism proposed for several other organic lanthanide complexes, 10,20 including Ln(Cp) 3 above. In a competing mechanismwhere the ejected TMCp radical extracts one hydrogen from the ligand which remains bound, thus leaving as the closed-shell molecule, HTMCp.…”

“…10,20 The suggested pathways for the ligand cracking in the Ln(Cp) 2 complex, which lead ultimately to LnC 2 , LnC 3 , and LnC 4 , are shown in lines 2–4. The first step in Ln(Cp) 2 cracking is the elimination of stable, closed-shell acetylene (C 2 H 2 ) or propyne (C 3 H 4 ) species to produce Ln(Cp)(C 3 H 3 ) or Ln(Cp)(C 2 H) intermediates, respectively.…”

Photofragmentation Pathways for Gas-Phase Lanthanide Tris(isopropylcyclopentadienyl) Complexes

“…A weak signal for Ln­(iCp) + fragments is also observed in the mass spectra (vide infra). This suggests an intact ligand stripping reaction pathway of with neutral ligand-radical ejection, through laser excitation into the ligand-to-metal charge-transfer states, similar to the mechanism proposed for several other lanthanide complexes and as also illustrated in eq . − The binding energies for the three dissociation steps illustrated in eq are calculated as 3.8, 4.0, and 4.6 eV, respectively, for Ln = La.…”

“…There have been many studies on the photofragmentation pathways of gas-phase molecular compounds, − metal–organic complexes, − and clusters. , Recently, our group reported the unimolecular gas-phase photodissociation mechanisms of lanthanide tris­(η 5 -cyclopentadienyl) complexes, Ln­(Cp) 3 , and lanthanide tris­(tetramethylcyclopentadienyl) complexes, Ln­(TMCp) 3 , through both experimental photoionization time-of-flight mass spectrometry (PI-TOF-MS) and theoretical atomistic simulation of the fragmentation dynamics . It was hypothesized that two types of reaction pathways dominate in the photofragmentation of both Ln­(Cp) 3 and Ln­(TMCp) 3 precursors: intact ligand stripping, leading to the production of bare metal, and ligand cracking within the metal–ligand complex, giving rise to metal carbide and metal hydrocarbide compounds.…”

Photoinduced Charge Transfer versus Fragmentation Pathways in Lanthanum Cyclopentadienyl Complexes

“…Luminescence spectroscopy and photoionization time-of-flight (PI-TOF) mass spectrometry have been used to study the photofragmentation of gas-phase metal-containing molecules. − The corresponding computational analysis of these processes, however, is complicated and challenging. The laser field can induce both excitation and de-excitation processes between the electronic states of the target molecule.…”