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.