Broadband ultrafast transient absorption of iron (III) tetraphenylporphyrin chloride in the condensed phase

Ultrafast transient absorption of iron (III) tetraphenylporphyrin chloride [Fe(III)TPPCl] is used to probe the excited state relaxation in room temperature solution. The ground state was excited either at 400 nm near the Soret resonance or at 520 nm in the Q resonance. Relaxation from the lowest lying porphyrin singlet (π, π∗) manifold results in formation of a LMCT (π, dπ) state on a time scale of 0.4-0.6 ps followed by a metal-to-ligand charge transfer transition from the iron to the porphyrin on a time scale of 1.8-2.3 ps, producing an excited 4T intermediate spin state of Fe(III)TPPCl. The system relaxes back to the high spin 6A1 ground state on a time scale of 13-18 ps. The relatively slow electronic relaxation of the more symmetric Fe(III)TPPCl compound is contrasted with the much faster subpicosecond ground state recovery observed for less symmetric biologically active heme systems at room temperature.

Highlights► Nonradiative relaxation of Fe(III)TPP chloride is complete within picoseconds. ► Similar pathways are observed following S2 and S1 excitation. ► The intermediate spin 4T iron state is formed before ground state recovery. ► Ground state recovery occurs within 13-18 ps at room temperature.