Excitation wavelength dependent O2 release from copper(II)-superoxide compounds: laser flash-photolysis experiments and theoretical studies

Abstract

Irradiation of the copper(II)-superoxide synthetic complexes [(TMG3tren)Cu(II)(O2)](+) (1) and [(PV-TMPA)Cu(II)(O2)](+) (2) with visible light resulted in direct photogeneration of O2 gas at low temperature (from -40 °C to -70 °C for 1 and from -125 to -135 °C for 2) in 2-methyltetrahydrofuran (MeTHF) solvent. The yield of O2 release was wavelength dependent: λexc = 436 nm, ϕ = 0.29 (for 1), ϕ = 0.11 (for 2), and λexc = 683 nm, ϕ = 0.035 (for 1), ϕ = 0.078 (for 2), which was followed by fast O2-recombination with [(TMG3tren)Cu(I)](+) (3) and [(PV-TMPA)Cu(I)](+) (4). Enthalpic barriers for O2 rebinding to the copper(I) center (∼10 kJ mol(-1)) and for O2 dissociation from the superoxide compound 1 (45 kJ mol(-1)) were determined. TD-DFT studies, carried out for 1, support the experimental results confirming the dissociative character of the excited states formed upon blue- or red-light laser excitation.

Figure 1

(A) Absorption spectrum of 1 (red line) obtained from oxygenation of 3 (black line) at 218 K in MeTHF. (B) Transient absorption difference spectra collected at the indicated delay times after 436 nm laser excitation (15 mJ/pulse, 8–10 ns fwhm) of 1 in MeTHF at 218 K. Overlaid in red on the experimental data is a simulated spectrum (Abs(3) - Abs(1) ).

Figure 2

TD-DFT calculated excited state potential energy surfaces (PESs) as a function of copper-oxygen bond distance.

Figure 3

TD-DFT calculated energy and shape of the beta HOMO and beta LUMO orbitals as a function of copper-oxygen bond distance.

Scheme 1

Chart 1