Relaxation dynamics of chromium(III)-oxalate complexes upon excitation in the UV

Relaxation dynamics of chromium(III) oxalate coordination complexes were investigated with the aid of time-resolved infrared (TRIR) and low-temperature emission spectroscopy. Interpretation of these results was aided by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. Time resolved infrared (TRIR) spectroscopy following excitation at 267 nm was used to probe the C-O and CO vibrational modes in a series of Cr(III)oxalate complexes. Lifetimes of 10-15 ps were obtained and assigned to the ground-state recovery from the lowest quartet excited state (⁴T_2g). A long-lived nanosecond component was also observed for [Cr(III)(oxalate)(bpy)₂]⁺, and attributed to population of the lowest doublet state. Vibrational cooling with a lifetime of ∼4 ps was proposed to suppress back inter-system crossing (BISC). In the other chromium complexes, more efficient BISC likely prevents doublet-state accumulation. Low-temperature emission measurements revealed phosphorescence which was absent at room temperature. These emissions exhibited decreasing lifetimes as the energy gap between the ²E and ⁴T_2g excited states narrows, demonstrating the continued influence of thermally activated BISC even at 77 K.