Investigation of the Lowest Electronic States of Osmium(II) Tetratolylporphyrins: Photophysics of Metalloporphyrin (d,π*) Charge Transfer States

The nature of the lowest excited states of a series of osmium(II) porphyrins has been investigated using static and time-resolved emission spectroscopy and ultrafast transient absorption measurements. It is found that a metalto- ring³(d,π*) charge transfer excited state is the lowest excited state of the Os^IIP(CO)(L) and Os^IIP(L)₂ complexes, where the porphyrin macrocycle (P) is either tetratolyl- or octaethylporphyrin and the axial ligand L is a σ-donor such as pyridine. Previous studies on OsTTP(CO)(py) had led to the assignment of the lowest excited state as the ³(π,π*) state of the porphyrin ring. The results on the Os^II porphyrins can be contrasted with those found previously for the analogous Ru^II porphyrins, in which the lowest excited state was found to switch from 3(π,π*) in the RuP(CO)(L) complexes to ³(d,π*) in the RuP(L)₂ compounds. The studies further establish the electronic origin of the red-region features in the absorption spectra of metalloporphyrin 3(π,π*) and 3(d,π*) excited states. The combined results demonstrate the critical role played by π-accepting and σ-donating axial ligands in controlling the nature of the lowest excited states, the electronic properties, and excited state dynamics of the biologically-relevant d⁶ metalloporphyrins.