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 OsIIP(CO)(L) and OsIIP(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 OsII porphyrins can be contrasted with those found previously for the analogous RuII 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.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry