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A: Spectroscopy, Molecular Structure, and Quantum Chemistry
Ultrafast Electronic Relaxation in Metalloporphyrins Probed by Broad-Band Fluorescence Up-Conversion Olivier Bräm, Andrea Cannizzo, and Majed Chergui J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b00007 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019
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Ultrafast Broad-band Fluorescence Up-conversion Study of the Electronic Relaxation of Metalloporphyrins Olivier Bräm, Andrea Cannizzo,† Majed Chergui* Laboratoire de Spectroscopie Ultrarapide, ISIC and Lausanne Centre for Ultrafast Science (LACUS), Ecole Polytechnique Fédérale de Lausanne (EPFL) CH-1015 Lausanne, Switzerland
[email protected] Abstract 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, 18octaethylporphin (OEP), with open 3d shell metals. We also revisit the cases of the closedshell 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 BQ 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 for example 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.
†
Now at the Institute of Applied Physics, University of Bern, Silderstr. 5, CH-3012 Bern, Switzerland
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I. Introduction Porphyrins are the focus of intense interest due to their widespread occurrence in Nature. They are the bioactive centre of heme proteins and as such, they play a crucial role in respiration, biological signalling and neurotransmission. They are also the key players in the photosynthetic system, either as photoreceptors or intermediates in energy/charge transport. Because of their remarkable electronic properties that can be engineered by chemical synthesis, they are potential candidates for functional and scalable devices in artificial photosynthesis1-2 and in photovoltaics based on dye sensitized solar cells,3-5 in photodynamic therapy,6-8 oxygen sensing9-11 and molecular photonics and electronics.12-15 From a fundamental point of view, porphyrins are exquisite objects whose photophysical properties can be tuned quite dramatically by substitution of the central metal atom and/or the side groups that also alter the planarity of the system.16-19 All these applications call for a detailed understanding of their ultrafast photophysics. Porphyrins are characterised by dipole-forbidden but vibronically-induced bands in the green to red region of the spectrum, the so-called Q-bands, and by intense bands around 400 nm, the so-called Soret (B) bands. In the literature, the former is associated with the Q state, while the latter with the B state. However, in view of a more complex electronic structure, as revealed by calculations,20-22 we refrain from using this classification and will rather use Q-state and Bstate. Because of the high absorption coefficient of the latter, they are often the doorway states for photoinduced processes. In this contribution, we focus on the ultrafast relaxation processes occurring upon Soret band excitation. In this respect, closed shell meso-substituted Zn-tetraphenylporphyrins were investigated by transient absorption (TA) spectroscopy and fluorescence up-conversion spectroscopy with monochromatic detection.23-28 The general picture that emerged from these studies is that after a polarization-independent 60-90 fs rise of the Soret emission, assigned to vibrational
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The Journal of Physical Chemistry
relaxation within the B-state, a biphasic internal conversion (IC) to Q-state was reported with time constants of 200-400 fs and 1-2 ps, followed by vibrational relaxation on a timescale of tens of ps.23, 26 This behaviour was attributed to the presence of two independent and at most, weakly coupled states in the Soret region, designated B and B’ (hereafter, we will use B and B’),23, 29 which undergo IC to Q on different timescales.23, 27 The existence of two B states was suggested in TDDFT calculations by Baerends et al,30 and experimentally confirmed for free base H2-TTP and ZnTPP porphyrins by Schalk et al,29 using visible pump/near-IR probe studies. Tripathy et al. further investigated closed-shell Mg and Zn porphyrins and reported B-Q decays times of the order of 1.5 to 3 ps for both with some degree of solvent dependence.25, 31 They also investigated the case of the open-shell Cd-TPP system and found a much faster decay (typ. 170-200 fs) of the B state. They concluded that no general explanation could be applied to all the studied compounds, but some trends are observed, like the dependence of the B-Q coupling strength on the β-alkyl substitution or on the non-planar distortion of the macrocycle. Kim and co-workers26 and Liu et al32 investigated the case of zinc (II) octaethylporphyrin (ZnOEP) upon Soret band excitation and reported a decay of the B state in
