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) 2 L](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'-di tert -butyl-2,2'-bipyridine (4,4'-d t bpy), 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 (CF 3 SO 3 ) 2 ](CF 3 SO 3 ) 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, E 0-0 , 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, k q , which were found to be in the range 3.26-5.27 × 10 7 M -1 s -1 . Singlet oxygen luminescence photosensitized by these complexes was observed in D 2 O, 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 2 E 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 2 E 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 p CT that was found to vary from 0.35 to 0.68 for this series of Cr(III) complexes.