The light-induced ultrafast spin and structure changes upon excitation of the singlet metal-toligand-charge-transfer (1 MLCT) state of Fe(II)-polypyridine complexes are investigated in detail in the case of aqueous iron(II)-tris-bipyridine ([Fe II (bpy) 3 ] 2+) by a combination of ultrafast optical and X-ray spectroscopies. Polychromatic femtosecond fluorescence upconversion, transient absorption studies in the 290-600 nm region and femtosecond X-ray absorption spectroscopy allow us to retrieve the entire photocycle upon excitation of the 1 MLCT state from the singlet low spin ground state (1 GS) as the following sequence: 1,3 MLCT→ 5 T→ 1 GS, which does not involve intermediate singlet and triplet ligand field states. The population time of the HS state is found to be~150 fs, leaving it in a vibrationally hot state that relaxes in 2-3 ps, before decaying to the ground state in 650 ps. We also determine the structure of the high-spin quintet excited state by picosecond X-ray absorption spectroscopy at the K edge of Fe. We argue that given the many common electronic (ordering of electronic states) and structural (Fe-N bond elongation in the high spin state, Fe-N mode frequencies, etc.) similarities between all Fe(II)-polypyridine complexes, the results on the electronic relaxation processes reported in the case of [Fe II (bpy) 3 ] 2+ are of general validity to the entire family of Fe(II)-polypyridine complexes.

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.

Iron(II)-based molecular complexes have been a subject of intense study since the discovery of light-induced excitedstate spin trapping (LIESST). [1] In this process, excitation of the singlet low-spin (LS) ground state with UV or visible light to the metal-to-ligand charge-transfer states ( 1,3 MLCT) or to the lower-lying singlet and triplet ligand-field ( 1,3 LF, also called metal-centered) states (see Figure S1 in 2+ upon 400 nm excitation using ultrafast optical and X-ray spectroscopic techniques [3,4] and showed that it is a three- [3,5] apart, the above mechanism implies that the energy difference is stored as vibrational energy in the 5 T 2 state. Given that a large number of Fe II complexes can undergo LS-to-HS transitions (and vice versa) under the effect of temperature, [6] the issue of vibrational energy storage and relaxation in the quintet state is important. It was recently addressed by McCusker and co-workers using femtosecond stimulated Raman scattering of [Fe II (tren(py) 3 )] 2+ in acetonitrile (tren(py) 3 = tris(2-pyridylmethyliminoethyl)amine), [7] who reported a bimodal time evolution of the high-frequency CÀN stretching mode with time constants of (190 AE 50) fs and (10 AE 3) ps. [7] The latter was attributed to vibrational cooling, while the former was associated with the structural change from LS to HS. Wolf et al.[

Tryptophan is commonly used to study protein structure and dynamics, such as protein folding, as a donor in fluorescence resonant energy transfer (FRET) studies. By using ultra-broadband ultrafast two-dimensional (2D) spectroscopy in the ultraviolet (UV) and transient absorption in the visible range, we have disentangled the excited state decay pathways of the tryptophan amino acid residues in ferric myoglobins (MbCN and metMb). Whereas the more distant tryptophan (Trp(7)) relaxes by energy transfer to the heme, Trp(14) excitation predominantly decays by electron transfer to the heme. The excited Trp(14)→heme electron transfer occurs in <40 picoseconds with a quantum yield of more than 60%, over an edge-to-edge distance below ~10 angstroms, outcompeting the FRET process. Our results raise the question of whether such electron transfer pathways occur in a larger class of proteins.

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