X-ray absorption spectroscopy is a powerful probe of molecular structure, but it has previously been too slow to track the earliest dynamics after photoexcitation. We investigated the ultrafast formation of the lowest quintet state of aqueous iron(II) tris(bipyridine) upon excitation of the singlet metal-to-ligand-charge-transfer (1MLCT) state by femtosecond optical pump/x-ray probe techniques based on x-ray absorption near-edge structure (XANES). By recording the intensity of a characteristic XANES feature as a function of laser pump/x-ray probe time delay, we find that the quintet state is populated in about 150 femtoseconds. The quintet state is further evidenced by its full XANES spectrum recorded at a 300-femtosecond time delay. These results resolve a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which we identify as a simple 1MLCT-->3MLCT-->5T cascade from the initially excited state. The time scale of the 3MLCT-->5T relaxation corresponds to the period of the iron-nitrogen stretch vibration.

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.[

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 the Supporting Information) leads to population of the lowest high-spin (HS) excited quintet 5 T 2 state by nonradiative relaxation. [2] We recently characterized the relaxation cascade from the initially excited 1 MLCT state to the lowest excited quintet state of aqueous [Fe II (bpy) 3 ] 2+ upon 400 nm excitation using ultrafast optical and X-ray spectroscopic techniques [3,4] and showed that it is a three-step 1 MLCT-3 MLCT-5 T 2 process that takes place in approximately 150 fs and bypasses the intermediate 1,3 LF states.Since the minima of the 3 MLCT and 5 T 2 states lie approximately 1.3 eV [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. [8] reported on a sub-picosecond IR study of [Fe II (btpa)] 2+ and [Fe II (b(bdpa)] 2+ in the 1000-1100 cm À1 region (b(btpa) = bis(N,N,N',N'-tetrakis(2-pyridilmethyl)-6,6'-bis(aminoethyl)-2,2'-bipyrdine); bdpa = N,N'bis(benzyl)-N,N'-bis(2pyridylmethyl)-6,6'-bis(aminoethyl)-2,2'-bipyrdine), and found relaxation times near 10 ps in both cases. The quintet states of Fe II complexes absorb in the UV region near 300 nm, [2,7,9] and herein we use visible pump / UV continuum (290-370 nm) probe spectroscopy with a resolution of approximately 130 fs to monitor the vibrational relaxation in the quintet state of aqueous [Fe II (bpy) 3 ] 2+ (bpy = 2,2'-bipyridine). We report on the observation of vibrational wave packets for the first time in the HS state of this class of complexes, and we also determine the vibrational relaxation times. Figure 1 shows UV transient absorption spectra for different time delays, which are extracted from the 2D time-wavelength plots recorded with the UV continuum probe (see Figure S2 in the Supporting Information). The positive region between 300 and approximately 330 nm corresponds to an excited-state absorption (ESA), while elsewhere in the spectrum a bleach signal is dominant, which ma...

We give an overview of our recent work on ultrafast dynamics of chemical (organic dyes, metal-complexes, colloidal quantum dots), and biological (retinal and haem proteins) systems in the liquid phase, studied with a variety of ultrafast optical techniques from the infrared to the ultraviolet.