The Role of Exciplex Charge-Transfer Character and Reorganization Energy in Steering Oxygen Quenching Pathways of Chromium(III) Excited States

Abstract

In this study we report the synthesis, characterization and photophysical properties of some [Cr(bpy)2L](OTf)3 complexes. The steady-state absorption and luminescence emission of these complexes were measured in 1 M HCl. The obtained luminescence quantum yields were found to be in the range 3.4 -12.17 × 10-3, being the lowest for [Cr(55DMB)2(bpy)]3+ and highest for [Cr(55DMB)2(44DMB)]3+. Measurements of the transient absorption spectra in 1 M HCl under degassed and oxygen-saturated conditions revealed no significant differences at any observed time delay. This consistency demonstrates the absence of any ion-separated species in the excited state. The excited state lifetime was in the range of 79 to 136 μs. Oxygen quenching of the excited states of the studied complexes shows a quenching rate constant in the range of 2.75 – 3.73 × 107 M-1 s-1 in 1M HCL aqueous solution and singlet oxygen quantum yield in the range 0.16 to 0.25 in D2O. The calculated efficiency of singlet oxygen production was found to be in the range 0.32 to 0.58. Determination of the oxidation potential Cr(+3/+4) for the studied complexes were found to be in the range 0.31 to 0.60 V vs SCE in acetonitrile and accordingly the free energy change for complete electron transfer ΔGCET, were found to be in the range -68.87 to -40.69 kJ mol-1. The mechanism of quenching is discussed based on a spin statistical model, which quantifies the balance between charge transfer and other deactivation pathways with a parameter pCT, which was evaluated for the current series of complexes to be in the range 0.57 to 0.72. The activation energy for the charge transfer exciplexes were calculated and found to be in the range 25.67 to 27.35 kJ mol-1 using a reorganization energy, λ, for the studied series of chromium complexes of 128 kJmol-1 and charge transfer character of the exciplexes, ẟ, were found to be 47.9. Based on this charge transfer character, individual reorganization energy, λ, for each complex was evaluated and correlated to the charge transfer quenching rate constant.