Picosecond Photolysis of Axial Ligands on Cobalt(II) and Cobalt(III) Porphyrins

Article abstract of DOI:10.1021/ja00334a032

Excited-state photoprocesses of cobalt(II) octaethylporphyrin (OEP), cobalt(II) tetraphenylporphyrin (TPP), Co^III(OEP)(CN), and Co^II(OEP)(CN)(γ-picoline) in noncomplexing solvents and in the presence of coordinating species Me2SO and piperidine have been studied.The transient absorption spectra suggest that the excited-state relaxation of cobalt(II) porphyrins proceeds via the tripdoublet, ²T(?,?*), and the ²(?, d_z2) charge-transef (CT) states and that rapid deactivation of the latter state competes favorably with axial ligand release, accounting for the low yield for photodissociation.When five-coordinate Co^III(OEP)(CN) is excited, the transient spectra suggest that the initial (?,?*) and (?,d) CT excited states decay to ³(d_?, d_z2), followed by ligand loss to produce four-coordinate III(OEP)>⁺.However, when six-coordinate III(OEP)L2>⁺ systems are excited, the transient spectra suggest that both five-coordinate III(OEP)L>⁺ and four-coordinate III(OEP)>⁺ are produced, both presumably from ³(d_?, d_z2), which is not observed.In 1-4 M ligating solvents III(OEP)L>⁺ rebinds a ligand in 250 ps to 1.4 ns, whereas III(OEP)>⁺ is stable for >/=1 ns.This difference in stablity is attributed to the five-coordinate species having a singlet ground state, while the four-coordinate species is a triplet ligand-field (d, d) state.