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 (4T2g). 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 4T2g excited states narrows, demonstrating the continued influence of thermally activated BISC even at 77 K.