Patricia M. Bradley

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

Abstract 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.

Observation of the photogenerated CO-loss intermediate from [CpFe(CO)]2(μ-CO)(μ-CHCH3) via time-resolved IR spectroscopy

Nanosecond time-resolved IR (TRIR) measurements were performed for both [CpFe(CO)(μ-CO)]2 (1) and [CpFe(CO)]2(μ-CO)(μ-CHCH3) (4). The results obtained for 1 are consistent with those previously reported. The TRIR spectrum of 4 in cyclohexane at room temperature (λexc=355 nm) both under argon and in CO atmosphere is consistent with the formation of the triply-bridged intermediate [CpFe]2(μ-CO)2(μ-CHCH3) (5) that results from loss of CO. The IR absorption peaks observed at 1840 and 1808 cm−1 in the TRIR spectrum of 5 are similar to those previously reported in matrix isolation experiments and the lifetimes are consistent with published time-resolved data in the visible region. In addition, DFT calculations result in the calculated frequencies of 1842 and 1810 cm−1 for the predicted 3B2 ground state of the structurally related [CpFe]2(μ-CO)2(μ-CH2) complex.