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

Ultrafast electronic-vibrational relaxation upon excitation of the singlet charge-transfer b (1)A' state of [Re(L)(CO) 3(bpy)] ( n ) (L = Cl, Br, I, n = 0; L = 4-Et-pyridine, n = 1+) in acetonitrile was investigated using the femtosecond fluorescence up-conversion technique with polychromatic detection. In addition, energies, characters, and molecular structures of the emitting states were calculated by TD-DFT. The luminescence is characterized by a broad fluorescence band at very short times, and evolves to the steady-state phosphorescence spectrum from the a (3)A" state at longer times. The analysis of the data allows us to identify three spectral components. The first two are characterized by decay times tau 1 = 85-150 fs and tau 2 = 340-1200 fs, depending on L, and are identified as fluorescence from the initially excited singlet state and phosphorescence from a higher triplet state (b (3)A"), respectively. The third component corresponds to the long-lived phosphorescence from the lowest a (3)A" state. In addition, it is found that the fluorescence decay time (tau 1) corresponds to the intersystem crossing (ISC) time to the two emissive triplet states. tau 2 corresponds to internal conversion among triplet states. DFT results show that ISC involves electron exchange in orthogonal, largely Re-localized, molecular orbitals, whereby the total electron momentum is conserved. Surprisingly, the measured ISC rates scale inversely with the spin-orbit coupling constant of the ligand L, but we find a clear correlation between the ISC times and the vibrational periods of the Re-L mode, suggesting that the latter may mediate the ISC in a strongly nonadiabatic regime.

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.