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An Efficient Indirect Mechanism for the Ultrafast Intersytem Crossing in Copper Porphyrins After S Excitation 2
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Minh-Huong Ha-Thi, Niloufar Shafizadeh, Lionel Poisson, and Benoit Soep J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp4008015 • Publication Date (Web): 15 May 2013 Downloaded from http://pubs.acs.org on May 23, 2013
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The Journal of Physical Chemistry
An Efficient Indirect Mechanism for the Ultrafast Intersystem Crossing in Copper Porphyrins. Minh-Huong Ha-Thi 1, Niloufar Shafizadeh* 1, Lionel Poisson2, Benoit Soep2 1
Institut des Sciences Moléculaires d'Orsay UMR 8214, CNRS Université de Paris-Sud, Bâtiment 210, 91405 Orsay, Cedex, France.
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Laboratoire Francis Perrin CEA/DSM/IRAMIS/SPAM – CNRS URA 2453, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France. Abstract The ultrafast dynamics of copper tetraphenylporphyrin (CuTPP), copper octaethylporphyrin (CuOEP) and of the free base tetraphenylporphyrin (H2TPP), excited in the S2 state have been
investigated in the gas phase by femtosecond pump/probe experiments. The porphyrins were excited in the Soret band at 400 nm. Strikingly, the S2-S1 internal conversion in H2TPP is very rapid (110 fs), as compared to that of ZnTPP (600 fs), previously observed. In turn, CuTPP and CuOEP, excited in S2, follow an efficient and different relaxation pathway from that of other open-shell metalloporphyrins. These two molecules exhibit a sequential four-step decay ending on a slow evolution in the nanosecond range 2S2→2CT→2T→2Ground State. This latter evolution is linked to the formation of the 2T, Tripdoublet state in CuTPP, observed in the condensed phase. It is shown that an intermediate charge transfer state plays a crucial role in linking the porphyrin centered 1ππ* and 3ππ* configurations. A simple model is presented that allows a rapid evolution between these two configurations, via coupling of the porphyrin π system with the free d electron on the copper. The mechanism obviates the need for the spin orbit coupling within the porphyrin. The result is that these copper porphyrins can exhibit an ultrafast apparent intersystem crossing, unprecedented for organic molecules.
Keywords: Femtosecond Dynamics, Electronic Relaxation, Biomimetic Molecules, Gas phase, Copper Porphyrins, Charge Transfer, Intersystem Crossing *Author for correspondence
[email protected] Tel 33 1 69 15 75 02
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Introduction Metalloporphyrins are ubiquitous molecules in our world. This spans the geological world with etioporphyrins, tracers of the crude oil history1 up to biology with hemoproteins, the vectors of small molecules within living systems. They serve as well as agents to transport molecules (hemoproteins) or electrons (chlorophylls). They are also important as photochemical agents with chlorophyll antennae. Also, synthetic porphyrins are used in the phototherapy of cancer 2 or in solar cells 3, through their unique photophysical properties. This pervasiveness and these properties arise from the special structure of metalloporphyrins. Metalloporphyrins are formed by the inclusion of a metal closely bound in the pocket of a stable π conjugated molecule. This matched interaction leads to unique stereodynamic properties allowing in hemoproteins, the reversible attachment of small molecules. The properties and thus the functions of the porphyrins depend specifically on the symbiotic metal nested at their centre. The electronic absorption of metalloporphyrins is always dominated by two types of bands, the Q bands (S1-S0) at 530 nm and the Soret band (S2 –S0) at 400 nm, both of which involve π,π* excitation of the macrocycle. These bands are located in the same energy range for the free base porphyrin and for metalloporphyrins, thus the absorption properties of the metal porphyrin system depends solely on the π system of the porphyrin chromophore. On the other hand, the relaxation properties of metalloporphyrins are determined by the nature of the caged metal. For a light Mg atom, the fluorescence quantum yield is the same as for the metal free porphyrin (H2TPP), when this metal is a transition element, the electronically excited metalloporphyrins return to the ground state within picoseconds, dissipating the electronic energy in vibrations of the porphyrin. This arises from the fact that a wealth of new background levels (see figure 1) are added by the presence of the half filled d shell in the transition metal in the ligand field of the porphyrin. These levels can form a variety of d excited states, labeled πd* or generate charge transfer states from the porphyrin to the metal d orbitals or vice versa. It has been shown in the and gas phase
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that these states are the
doorway to the electronic relaxation to the porphyrin ground state. In particular porphyrin to metal CT states are rapidly populated in the gas phase after S2 excitation of the metalloporphyrin. Copper II (3d94s0) is a specific metal since, in difference to other first row transition metal porphyrins, the electronic energy of CuTPP after a similar ultrafast decay is observed to end up on the lowest ππ* T1 state5,6,7. This is of great importance since, by an allowed energy
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transfer mechanism; it can activate a known scavenger in biochemistry, singlet oxygen. This is why we have explored here the time evolution of copper porphyrins, in the gas phase. Since the early works of Gouterman et al.6 on the luminescence of Cu porphyrins in the condensed phase, much has been accomplished in terms of excited state dynamics. Gouterman has also laid the theoretical basis for the understanding of the electronic properties of porphyrins: a Hückel orbital model on the conjugated macrocycle. It describes the HOMO’s in terms of two delocalised orbitals, one centered on the inner porphyrin ring, the other completely delocalised. The two optical transitions from these two HOMO’s lead to a doubly degenerate LUMO
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and to S1 and S2. When locking the metal spin for copper porphyrins with the π
system, this has resulted in the classification in singdoublet 2S, tripdoublet 2T and tripquartet 4
T states9, see figure 1. The doublet and quartet superscripts refer to the total porphyrin+metal
spin, while the S and T refer to the local multiplicity of the porphyrin cycle. Ake et al. 9, Holten 10 et al., Koyabashi11 et al. have shown that a variety of substituted copper porphyrins have analogous behaviours at early times, where the ππ* triplet state T (2T or 4T) is rapidly and efficiently produced within t
