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Ultrafast Broadband Fluorescence Up-conversion Study of the Electronic Relaxation of Metalloporphyrins.

We present a systematic study of the ultrafast fluorescence with broadband detection and ∼110 fs resolution of 5,10,15,20-tetraphenylporphin (TPP) and 2,3,7,8,12,13,17,18-octaethylporphin (OEP), with open 3d-shell metals. We also revisit the cases of the closed-shell ZnTPP and ZnOEP systems. We find that in all cases, the relaxation from the Soret (B)-state to the Q-states (S1) occurs on ultrafast time scales of <50-100 fs, regardless of the metal, its oxidation state, and the peripheral groups of the macrocycle. The analogy with free base TPP and OEP leads us to conclude that the B-Q relaxation involves only the porphyrin states. ZnTPP is an outlier compared to the entire set of investigated systems, in the sense that the B-Q relaxation is significantly slowed down and is multiexponential. We argue that because of a lower density of states in the region of the Soret band, compared to ZnOEP, the relaxation time becomes much longer. Finally, the role of metal orbitals is apparent in the relaxation of the Q-state, which is found to be much faster in the case of open-shell metals compared to closed-shell ones, hinting to an electron transfer from the porphyrin to the metal.