Excited State Properties of Rh₂(O₂CCH₃)₄: Solution Photochemistry and Photoinitiated DNA Cleavage

The photophysical properties of Rh2(O2CCH3)4(L)2 (L = CH3OH, THF = tetra-hydrofuran, PPh3 = triphenylphosphine, py = pyridine) were explored upon excitation with visible light. All the complexes exhibit a long-lived transient absorption signal (τ = 3.5–5.0 μs) assigned as an electronic excited state of each molecule. An optical transition at −760 nm is observed in the spectra of the transients, whose position is relatively independent of axial ligand. No emission from the Rh2(O2CCH3)4(L)2 (L = CH3OH, THF, PPh3, py) systems was observed at room temperature or at 77 K, but energy transfer from excited Rh2(O2CCH3)4(PPh3)2 to tetracene and perylene takes place to form the 3ππ∗ excited state of each acceptor. Electron transfer from ∗Rh2(O2CCH3)4(PPh3)2 to dimethyl viologen (MV2+) and chloro-p-benzoquinone (Cl-BQ) takes place with quenching rate constants (kq) of 8.0 × 106 M−1s−1 and 1.2 × 106 M−1s−1 in methanol, respectively. A kq value of 2 × 108 M−1s−1 was measured for the quenching of the excited state of Rh2(O2CCH3)4(PPh3)2 by O2 in methanol. The results of the energy and electron transfer experiments are consistent with the production of an excited state of Rh2(O2CCH3)4(PPh3)2 with energy, E00, between 1.34 eV and 1.77 eV. The excited state of Rh2(O2CCH3)4 is not able to undergo hydrogen abstraction chemistry. However, the photoproduced one-electron oxidized complex, Rh2(O2CCH3)4 +, is able to convert isopropanol to acetone and to efficiently cleave DNA with λirr≥ 610 nm.