Reassessment of the Electronic Structure of Cr(VI) Sites Supported on Amorphous Silica and Implications for Cr Coordination Number

The electronic structure of isolated Cr­(VI) sites supported on silica was reinvestigated using multiple, complementary electronic spectroscopies applied to transparent xerogel monoliths. The absorption spectrum exhibits three previously reported peaks, at 22 800, 29 100, and 41 500 cm^–1, as well as a previously unresolved band at ca. 36 900 cm^–1. The emission is a long-lived red luminescence with λ_max = 13 600 cm^–1, emanating from the lowest excited state. Assignment of the excited states was facilitated using time-dependent density functional theory (TD-DFT) calculations performed on cluster models. All of the observed electronic transitions and their energies are accounted for by dioxoCr­(VI) sites. The lowest energy observed excitation at 22 800 cm^–1 populates a singlet excited state, while the emitting state is the corresponding triplet state, accessed by intersystem crossing from the singlet state. Spectroscopic bands observed at 29 100, 36 900, and 41 500 cm^–1 were assigned, based on the TD-DFT calculation, to spin-allowed transitions that are consistent with emission polarization anisotropy measurements. Small variations in site symmetry at Cr result principally in inhomogeneous broadening of the spectral bands, as well as a red-edge effect in the photoemission spectrum. There is no evidence for a significant contribution from five-coordinate mono-oxoCr­(VI) sites.