Excited state properties of Rh(phi)_2(phen)^(3+) and related complexes: a strong photooxidant

Abstract

The excited state properties and reactivity of Rh(phi)_2(phen)^(3+) (phi = 9,10-phenanthrenequinone diimine, phen = 1,10-phenanthroline) were investigated and compared to those of related rhodium(III) and zinc(II) complexes. At 77 K Rh(phi)_2(phen)^(3+) exhibits LC^3ππ∗ dual emission from both phi and phen ligands, whose energies are 2.8 eV and 3.0 eV, respectively, with a biexponential decay that can be fit to 5 ns (98%) and 60 ns (2%) lifetime components. A longer-lived transient is observed by transient absorption both at 77 K and 298 K, which decays monoexponentially with a time constant of 900 ns and 235 ns at each temperature, respectively, in ethanol. The transient can be attained with visible excitation (420 nm to 532 nm) and it is assigned to be intra-ligand charge transfer (ILCT) in nature. It was determined from transient absorption experiments that the ILCT excited state of Rh(phi)_2(phen)^(3+) lies approximately 2.0 eV above the ground state and that it is a powerful oxidizing agent with E_(12)([Rh]^(3+∗/2+))≈2.0V versus NHE. The transient absorption experiments with electron donors reveal that the one-electron reduced Rh(phi)_2(phen)^(2+) absorbs at 540 nm with ε ≈ 3300 M^(−1)cm^(−1).