Excited state dynamics of Rh(II) tetramesityl porphyrin monomer from nanosecond transient absorption and emission spectroscopy

The excited states of [tetrakis(2,4,6-trimethylphenyl)porphyrinato]rhodium(II), Rh(II)TMP, have been studied using nanosecond transient absorption spectroscopy, emission spectroscopy, and electrochemistry. Rh(II)TMP has a long-lived excited state, with a 460 nm absorption band, that decays with a time constant of 180 +/- 22 ns, in benzene and 205 +/- 28 ns in 1,3-difluorobenzene. No transient absorption features are seen between 600 and 700 nm, but weak phosphorescence is detected at 743 nm at 77 K. Thus, the excited state has the characteristics of a triplet (pi,pi*) state, although a low lying, charge transfer state had been expected from iterative extended Huckel calculations. The phosphorescence is biphasic, and highly temperature dependent, consistent with a trip-quartet [T-4] lowest excited state, lying not far below a trip-doublet [T-2] state. Electrochemistry establishes that the Rh(II)/(I) reduction potential is more negative than -1.5 V versus SSCE while the porphyrin cation reduction potential is +0.55 V. These potentials place the lowest LMCT state at >2.05 V, well above the triplet states. In addition to elucidating the photophysics of a novel type of metalloporphyrin, these results have significant bearing on the feasibility of using Rh(II) porphyrins in photoinduced catalytic systems.