Ultrafast Relaxation Dynamics of Photoexcited Heme Model Compounds: Observation of Multiple Electronic Spin States and Vibrational Cooling

Primary processes of the archetypal model complex azido(porphinato)iron(III) from ultrafast vibrational-electronic spectroscopy

Azidoiron complexes serve as valuable photochemical precursors for catalytically active species containing high-valent iron. In bioinorganic chemistry, azido(tetraphenylporphinato)iron(III), i.e., [FeIII(tpp)(N3)] with tpp = 5, 10, 15, 20-tetraphenylporphyrin-21, 23-diido, constitutes the archetypal model system that was used to access for the first time the terminal nitridoiron core, FeV ≡ N, in the biomimetic redox-non-innocent ligand environment. So far, the light-induced dynamics leading to the oxidation of the metal and the release of dinitrogen from the N3-ligand have only been studied for precursors featuring redox-innocent auxiliary ligands that simplify the electronic structure change accompanying the photo-transformation. Here, we monitored the primary events of the above paradigmatic complex, following its optical excitation in the ultraviolet-to-visible spectral range using femtosecond spectroscopy with probing in both the UV-vis and mid-infrared regions. Following ultrafast Soret-excitation at 400 nm, the complex relaxes to the lowest excited sextet state by a first internal conversion in less than 200 fs. The excited state then undergoes vibrational relaxation on a time scale of roughly 2 ps before internally converting yet again to recover the sextet electronic ground state within 19.5 ps. Spectroscopic evidence is obtained neither for a transient occupation of the energetically lowest metal-centered state, 41A1, nor for vibrational relaxation in the ground-state. The primary processes seen here are thus in contrast to those previously derived from ultrafast UV-pump/vis-probe and UV-pump/XANES-probe spectroscopies for the halide congener [FeIII(tpp)(Cl)]. Any photochemical transformation of the complex arises from two-photon-induced dynamics.

Ultrafast photochemistry of free-base porphyrin: a theoretical investigation of B → Q internal conversion mediated by dark states

We examine the mechanism of ultrafast internal conversion between the B band (Soret band) and the Q band in porphine (H2P), the prototypical free-base porphyrin, using electronic structure studies and on-the-fly surface-hopping nonadiabatic dynamics. Our study highlights the crucial role of dark states within the N band which are found to mediate B/Q state transfer, necessitating a treatment beyond Gouterman's classic four-orbital model. The sequential B → N → Q pathway dominates largely over the direct B → Q pathway which is found to be energetically unfavorable. Potential energy surface cuts and conical intersections between excited states are determined by TDDFT and validated by CASSCF/CASPT2 and XMCQDPT2 calculations. Both the static analysis and on-the-fly surface-hopping calculations suggest a pathway which involves minor structural deformations via in-plane vibrations. The B → N conversion is a barrierless adiabatic process occurring within ∼20 fs, while the subsequent N → Q conversion occurs via a conical intersection within ∼100 fs, in agreement with time-resolved experiments for porphine and related free base porphyrins. Furthermore, evidence for both sequential and direct transfer to the Qx and Qy states is obtained.