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 of 3.4–12.17 × 10⁻³, with the lowest for [Cr(55DMB)₂(bpy)]³⁺ and the 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 lifetimes were in the range of 79 to 136 μs. Oxygen quenching of the excited states of the studied complexes showed quenching rate constants in the range of 2.75–3.73 × 10⁷ M⁻¹ s⁻¹ in 1 M HCl aqueous solution, and singlet oxygen quantum yields in the range of 0.16 to 0.25 in D₂O. The calculated efficiency of singlet oxygen production was found to be in the range of 0.32 to 0.58. The oxidation potentials Cr(+3/+4) for the studied complexes were found to be in the range of 0.31 to 0.60 V vs. SCE in acetonitrile; accordingly, the free energy change for complete electron transfer ΔG_CET was found to be in the range of −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 the parameter p_CT, which was evaluated for the current series of complexes to be in the range of 0.57 to 0.72. The activation energies for the charge-transfer exciplexes were calculated and found to be in the range of 25.67 to 27.35 kJ mol⁻¹ using a reorganization energy, λ, for the studied series of chromium complexes of 128 kJ mol⁻¹, and the charge-transfer character of the exciplexes, δ, was found to be 47.9. Based on this charge transfer character, the individual reorganization energies, λ, for each complex were evaluated and correlated to the charge-transfer quenching rate constants.