Time-resolved X-ray and Optical Studies of Metalloproteins and Iridium Complexes

A wide variety of physicochemical processes at a molecular level, in particular charge or energy transfer, electronic and vibrational relaxation, are at the origin of biological functionality of proteins and organometallic compounds. The work reported in this thesis is devoted to the study of these molecular dynamics in selected organometallic systems. In particular, it focuses on the role of these dynamics in the ultrafast photophysics and photochemistry of Myoglobin and iridium complexes having a transition metal active center of d6 configuration. The nature of ligand recombination to the active centre has a strong impact on the reactivity and activation of heme proteins. Using picosecond (ps) Fe K-edge X-ray absorption spectroscopy (XAS) we probed the NO-heme recombination with direct sensitivity to the Fe-NO binding in physiological solution. The transient XAS at 70 ps and 300 ps are identical, but they deviate from the difference between the static spectra of deoxyMb and nitrosyl myoglobin demonstrating the formation of an intermediate species, supposedly the 6-coordinated domed form that is populated on a time scale of ~200 ps and relaxes in ~30 ps. A broadband femtosecond (fs) luminescence study of Ir(ppy)3 (Ir1), Ir(ppy)2(pic) (Ir2), [Ir(ppy)2(bpy)]+ (Ir3) and Ir(ppz)3 (Ir4) provides the first extensive picture of their ligand dependence. After excitation to a ligand-centered state (266 nm), we directly clock the relaxation cascade of Ir1 leading to the lowest metal-to-ligand charge transfer (3MLCT) state. The cascade proceeds within ≀ 10 fs, being faster than some of the high-frequency modes of the system. In Ir4 the 3MLCT state decays (530 fs) by non-radiative channels. For Ir1-3, the early emission (<1ps) upon 400 nm excitation is dominated by an intermediate 3MLCT state on the ppy moiety. At t>100s ps the emission evolves to the steady-state showing evidence of dual emission for Ir1,2 due to a double-well minimum of the lowest 3MLCT state. For Ir3, the final emission stems from a ppy to bpy inter-ligand CT state. The studies of the electronic and geometric structure of Ir1-4 by Ir L3 XAS in solution reveals overall identical above-edge multiple scattering resonances and EXAFS indicating similar bond lengths and angles. Simulation of the XAS using TDDFT and FEFF9 shows good overall agreement with the experiment. The near-edge region of the spectra shows distinct differences among the studied complexes. Ir4 has stronger WL intensity than Ir1, indicating weaker π-back donation. The XAS spectra of Ir2,3 indicate larger 5d-electron density on the iridium. The XAS spectra of Ir1 and Ir4 show weak pre-edge resonance due to metal (2p3/2)-to-ligand charge transfer excitations. The absence of the pre-edge in Ir2,3 is due to the reduced covalency of the bonds. However, the existence of such transitions remains open for debate since Ir+3 has only occupied t2g orbitals in the low spin configuration. Using ps XAS, we have successfully captured the transient spectrum at the Ir L3-edge of both Ir1 and Ir4 probed 150 ps after laser excitation in solution. Upon excitation of a MLCT band (355 nm), we observed oxidation state change of Ir from +3 to +4. In DCM, the transient spectra are almost identical for both Ir1,4. The only significant changes in the transient spectra are those observed around the WL. The calculations reproduce the blue shift of the absorption spectrum due to the reduced electron density on the Ir, consistent with the MLCT excitation.

Chergui, Majed
Lausanne, EPFL
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis6797-9

 Record created 2015-11-09, last modified 2018-03-17

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