Materials and Methods Figs. S1 to S3 Table S1 References S1 SUPPORTING MATERIAL Materials and Methods Preparation of Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu I Mutant azurins were expressed and Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu I was prepared using previously published protocols (S1,S2). Crystal Structure of Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu II Crystals of Re(4,7-dimethyl-1,10-phenanthroline)(CO) 3 (H 124){T 124 H|K 122 W|H 83 Q}(Cu II)azurin (Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu II ; space group I222, cell dimensions 63.22 × 69.08 × 68.94 Å 3 ; α = β = γ = 90.00°, one molecule per asymmetric unit) grew from 4 μL drops made from equal volumes of 30 mg/mL Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu II in 25 mM HEPES pH 7.5 and reservoir by vapor diffusion. The drops were equilibrated against 500 μL of reservoir

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Femto-to picosecond excited-state dynamics of the complexes [Re(L)(CO) 3 (N,N)] n (N,N = bpy, phen, 4,7dimethyl-phen (dmp); L = Cl, n = 0; L = imidazole, n = 1þ) were investigated using fluorescence up-conversion, transient absorption in the 650-285 nm range (using broad-band UV probe pulses around 300 nm) and picosecond time-resolved IR (TRIR) spectroscopy in the region of CO stretching vibrations. Optically populated singlet charge-transfer (CT) state(s) undergo femtosecond intersystem crossing to at least two hot triplet states with a rate that is faster in Cl (∼100 fs) -1 than in imidazole (∼150 fs) -1 complexes but essentially independent of the N,N ligand. TRIR spectra indicate the presence of two long-lived triplet states that are populated simultaneously and equilibrate in a few picoseconds. The minor state accounts for less than 20% of the relaxed excited population. UV-vis transient spectra were assigned using open-shell time-dependent density functional theory calculations on the lowest triplet CT state. Visible excited-state absorption originates mostly from mixed L;N,N •f Re II ligand-to-metal CT transitions. Excited bpy complexes show the characteristic sharp near-UV band (Cl, 373 nm; imH, 365 nm) due to two predominantly ππ*(bpy •-) transitions. For phen and dmp, the UV excited-state absorption occurs at ∼305 nm, originating from a series of mixed ππ* and Re f CO;N,N •-MLCT transitions. UV-vis transient absorption features exhibit small intensity-and band-shape changes occurring with several lifetimes in the 1-5 ps range, while TRIR bands show small intensity changes (e5 ps) and shifts (∼1 and 6-10 ps) to higher wavenumbers. These spectral changes are attributable to convoluted electronic and vibrational relaxation steps and equilibration between the two lowest triplets. Still slower changes (g15 ps), manifested mostly by the excited-state UV band, probably involve local-solvent restructuring. Implications of the observed excited-state behavior for the development and use of Re-based sensitizers and probes are discussed.

Abstract:The ultrafast vibrational-electronic relaxation upon excitation into the singlet 1 A 2u (dσ*fpσ) excited state of the d 8 -d 8 binuclear complex [Pt 2 (P 2 O 5 H 2 ) 4 ] 4-has been investigated in different solvents by femtosecond polychromatic fluorescence up-conversion and femtosecond broadband transient absorption (TA) spectroscopy. Both sets of data exhibit clear signatures of vibrational relaxation and wave packet oscillations of the Pt-Pt stretch vibration in the 1 A 2u state with a period of 224 fs, that decay on a 1-2 ps time scale, and of intersystem crossing (ISC) into the 3 A 2u state. The vibrational relaxation and ISC times exhibit a pronounced solvent dependence. We also extract from the TA measurements the spectral distribution of the wave packet at a given delay time, which reflects the distribution of Pt-Pt bond distances as a function of time, i.e., the structural dynamics of the system. We clearly establish the vibrational relaxation and coherence decay processes, and we demonstrate that PtPOP represents a clear example of a harmonic oscillator that does not comply with the optical Bloch description due to very efficient coherence transfer between vibronic levels. We conclude that a direct Pt-solvent energy dissipation channel accounts for the vibrational cooling in the singlet state. ISC from the 1 A 2u to the 3 A 2u state is induced by spin-vibronic coupling with a higher-lying triplet state and/or (transient) symmetry breaking in the 1 A 2u excited state. The particular structure, energetics, and symmetry of the molecule play a decisive role in determining the relatively slow rate of ISC, despite the large spin-orbit coupling strength of the Pt atoms.