Wavelength Dependence of Photooxidation vs Photofragmentation of Chromocene

Abstract

Photooxidation and metal-ligand photolysis reactions of bis(cyclopentadienyl)chromium, chromocene, in the range 24 390-15 630 cm -1 are studied in the gas phase by using time-of-flight mass spectroscopic detection. Photooxidation of the intact chromocene molecule unexpectedly dominates in the range 23 530-24 000 cm -1 . The relative importance of photooxidation compared to photofragmentation is strongly wavelength dependent. A prominent species at all wavelengths is the chromium ion, but in a wavelength region corresponding to the lowest energy ligand to metal charge transfer excited electronic state absorption, the strongest peak is from the chromocene ion. The excitation spectra are reported for three selected species: chromocene ion, mono-(cyclopentadienyl)chromium ion, and the chromium ion. The spectrum obtained by monitoring the metal ion contains sharp peaks that are assigned to neutral chromium atom resonances. Sharp losses of intensities in the molecular ion spectra are observed at these wavelengths. The wavelength dependencies of the photoreactions are interpreted and explained in terms of the identity of the initially populated excited electronic state and the ionization energy of the molecule. When the initially populated excited electronic state is the ligand to metal charge transfer state, the first photon causes minimal bond weakening and the second photon excites the intact chromocene above the ionization energy, resulting in efficient ionization of the parent molecule. When the initially populated state is a lower energy ligand field excited electronic state, bond weakening occurs and absorption of a second photon results in significant photodissociation producing intense fragment peaks dominated by the metal ion peak.

High-Resolution Mass-Analyzed Threshold Ionization Study of Deuterated Derivatives of Bis(η6-benzene)chromium

“…Only for chromocene has a low-resolution molecular REMPI spectrum containing one broad structureless band been obtained. 24 On the other hand, recently we have demonstrated that ferrocene, 25,26 nickelocene, 27 and 1 28 can be efficiently ionized with nanosecond laser pulses when the transition from the metallocalized 3d z 2 orbital to the lowest Rydberg p x,y level is used as the first step of the multiphoton excitation. The REMPI spectrum of 1 28 revealed a structured Rydberg transition with wellresolved vibronic components.…”

“…As a result of MPD, the REMPI spectra of sandwich compounds revealed peaks corresponding to the electronic transitions in the metal atoms. Only for chromocene has a low-resolution molecular REMPI spectrum containing one broad structureless band been obtained …”

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

“…For example, Cr(Cp)(C 3 H 3 ) and Cr(C 3 H 3 ) have been reported as stable fragments in the gas-phase laser photochemistry of chromocene. 27 Also, a stable YC 3 H 3 product has been observed by Davis et al 28 in the reaction of yttrium with 2-butyne. The formation and stability of small lanthanide carbides or metallocarbon clusters (LnC n ) have also been reported.…”

“…Structures similar to Ln­(Cp)­(C 3 H 3 ) and Ln­(C 3 H 3 ) have been identified in other work. For example, Cr­(Cp)­(C 3 H 3 ) and Cr­(C 3 H 3 ) have been reported as stable fragments in the gas-phase laser photochemistry of chromocene . Also, a stable YC 3 H 3 product has been observed by Davis et al in the reaction of yttrium with 2-butyne.…”

5. Structural and electronic characterization of nanosized inorganic materials by X-ray absorption spectroscopies

“…Only the lowest frequency component (21,000 cm −1 ) has been definitely assigned in literature in terms of a d -d transition ( 3 E 2g → 3 E 1g , 3 E 2g , 3 A 1g , 3 A 2g ). Based on this, band I is due to an electronic transition from the HOMO to the LUMO [87][88][89]. The d -d character of transition I is easily explained by considering that both the HOMO and the LUMO are primarily metal in character (d xy,x 2 −y 2 for the HOMO and dxz,yz for the LUMO) [90].…”