Mechanism of Oxygen Quenching of the Excited States of Heteroleptic Chromium(III) Phenanthroline Derivatives

In this study, we report the synthesis and characterization of some heteroleptic Cr(III) complexes of the form [Cr(Phen)2L](OTf)3, where Phen = 1,10-phenanthroline and L is either 2,2′-bipyridine (bpy) or its derivatives, such as 4,4′-dimethyl-2,2′-bipyridine (4,4′-DMB), 4,4′-dimethoxy-2,2′-bipyridine (4,4′-DMOB), 4,4′-ditert-butyl-2,2′-bipyridine (4,4′-dtbpy), 5,5′-dimethyl-2,2′-bipyridine (5,5′-DMB), 4,4′-dimethoxycarbonyl-2,2′-bipyridine (4,4′-dmcbpy) or 1,10-phenanthroline derivatives, such as 5-methyl-1,10-phenanthroline (5-Me-Phen) and 4,7-dimethyl-1,10-phenanthroline (4,7-DMP). Heteroleptic complexes were prepared in two stages via the intermediate [Cr(Phen)2(CF3SO3)2](CF3SO3) and five examples have been crystallographically characterized. Steady-state absorption and luminescence emission characteristics of these complexes were measured in 1 M HCl solutions. The luminescence quantum yield of these complexes was found to be the lowest for [Cr(Phen)2(4,4′-dmcbpy)](OTf)3 and the highest for [Cr(Phen)2(4,4′-DMB)](OTf)3 with values of 0.31 × 10–2 and 1.48 × 10–2, respectively. The calculated excited state energy, E0–0, was found to vary within the narrow range of 163.1–165.0 kJ mol–1 across the series. Transient absorption spectra in degassed, air-equilibrated, and oxygen-saturated 1 M HCl aqueous solutions were also measured at different time decays and demonstrated no significant differences, indicating the absence of any ion-separated species in the excited state. Excited-state decay traces at the wavelength of maximum absorption were used to calculate oxygen quenching rate constants, kq, which were found to be in the range 3.26–5.27 × 107 M–1 s–1. Singlet oxygen luminescence photosensitized by these complexes was observed in D2O, and its luminescence intensity at 1270 nm was used for the determination of singlet oxygen quantum yields for these complexes, which were in the range of 0.20–0.44, while the fraction of the excited 2E state quenched by oxygen was in the range of 0.22–0.68, and the efficiency of singlet oxygen production was in the range of 0.44–0.90. The mechanism by which the excited 2E state is quenched by oxygen is explained by a spin statistical model that predicts the balance between charge transfer and noncharge transfer deactivation pathways, which was represented by the parameter pCT that was found to vary from 0.35 to 0.68 for this series of Cr(III) complexes.