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.

It is known that excitation by visible light of the singlet metal-to-ligand charge-transfer ((1)MLCT) states of Fe(II) complexes leads to population of the lowest-lying high-spin quintet state ((5)T) with unity quantum yield. Here we investigate this so-called spin crossover (SCO) transition in aqueous iron(II)tris(bipyridine). We use pump-probe transient absorption spectroscopy with a high time resolution of <60 fs in the ultraviolet probe range, in which the (5)T state absorbs, and of <40 fs in the visible probe range, in which both the hot MLCT state and the (5)T state absorb. Our results show that the (5)T state is impulsively populated in less than 50 fs, which is the time we measured for the depopulation of the MCLT manifold. We propose that non-totally-symmetric modes mediate the process, possibly high-frequency modes of the bipyridine (bpy) ligand. These results show that even though the SCO process in Fe(II) complexes represents a strongly spin-forbidden (ΔS = 2) two-electron transition, spin flipping occurs at near subvibrational times and is intertwined with the electron and structural dynamics of the system.

The ultrafast relaxation of aqueous iron(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band (1MLCT) has been characterized by femtosecond fluorescence up-conversion and transient absorption (TA) studies. The fluorescence experiment shows a very short-lived broad 1MLCT emission band at approximately 600 nm, which decays in < or =20 fs, and a weak emission at approximately 660 nm, which we attribute to the 3MLCT, populated by intersystem crossing (ISC) from the 1MLCT state. The TA studies show a short-lived (<150 fs) excited-state absorption (ESA) below 400 nm, and a longer-lived one above 550 nm, along with the ground-state bleach (GSB). We identify the short-lived ESA as being due to the 3MLCT state. The long-lived ESA decay and the GSB recovery occur on the time scale of the lowest excited high-spin quintet state 5T2 lifetime. A singular value decomposition and a global analysis of the TA data, based on a sequential relaxation model, reveal three characteristic time scales: 120 fs, 960 fs, and 665 ps. The first is the decay of the 3MLCT, the second is identified as the population time of the 5T2 state, while the third is its decay time to the ground state. The anomalously high ISC rate is identical in [RuII(bpy)3]2+ and is therefore independent of the spin-orbit constant of the metal atom. To reconcile these rates with the regular quasi-harmonic vibrational progression of the 1MLCT absorption, we propose a simple model of avoided crossings between singlet and triplet potential curves, induced by the strong spin-orbit interaction. The subsequent relaxation steps down to the 5T2 state dissipate approximately 2000 cm-1/100 fs. This rate is discussed, and we conclude that it nevertheless can be described by the Fermi golden rule, despite its high value.

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.

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The excited-state dynamics of metal-polypyridine complexes are of great importance in applications as diverse as solarenergy conversion [1][2][3] and information storage [4] because they can be photo-and redox-triggered. Ruthenium trisbipyridine ([Ru(bpy) 3 ] 2+ ) is the prototype for this class of complexes, the study of which has formed the basis for most photochemical applications. These complexes exhibit transitions due to charge transfer between the metal-centered d orbital and the ligand p orbital, commonly known as metal-to-ligand charge transfer (MLCT). Femtosecond transient-absorption studies on [Ru(bpy) 3 ] 2+ have shown that upon excitation of the singlet 1 MLCT state (absorption maximum 450 nm), ultrafast intersystem crossing (ISC) occurs in < 100 fs, leading to the formation of the triplet 3 MLCT state with near-unity quantum yield. [5,6] From 300 fs onwards, the transient-absorption spectrum remains unchanged.[5] The 3 MLCT state decays radiatively to the ground state with a lifetime of % 600 ns in aqueous solution at room temperature. [2,3,7] However, the issue of energy disposal and vibrational relaxation within the complex is still a subject of debate. Indeed, a 400-nm excitation corresponds to an excess energy of % 8500 cm À1 above the vibrationally relaxed 3 MLCT state, [8] which would be dissipated in % 300 fs, according to the literature. [5,6] To address this issue, Bhasikuttan et al. [9] carried out a fluorescence-upconversion study at single wavelengths that correspond to those at which the 1 MLCT (500 and 575 nm) and the 3 MLCT (620 nm) emissions are expected. Their results were interpreted in terms of fast ISC to the 3 MLCT state followed by vibrational cooling on a timescale of 0.6 to 1 ps. The emission by the 3 MLCT state could not be observed in their experiment owing to its low radiative rate. However, single-wavelength detection does not produce a complete picture of the relaxation dynamics. Consequently, Browne et al.[10] implemented a picosecond broadband detection technique and observed an emission band centered at 520 nm, which they attributed to the 1 MLCT state. Unfortunately, they could not capture the details of the relaxation dynamics within the 3 MLCT state with the time resolution (%3 ps) used.Herein we report for the first time a polychromatic femtosecond fluorescence-upconversion experiment in the 440-690 nm range, with a resolution of 110 AE 10 fs to capture the early relaxation processes leading to the steady-state emission of the 3 MLCT state of [Ru(bpy) 3 ] 2+ . The experimental procedure and the data analysis are explained in reference [11] and in the Supporting Information. Figure 1 a shows a typical 2D spectrum obtained upon excitation at 400 nm (25 000 cm À1 ). The spot at % 21 600 cm À1is the Raman line of water. Although fluorescence in the 15 000-20 000-cm À1 region was present at t = 0, it was very short-lived, converging within 200 fs to a weak emission in the 16 000-17 500-cm À1 (575-680 nm) region. Spectra at fixed 2+ under excitation at 25 000 cm À1 ...

We use light to transform a colloidal solution of spherical silver nanoparticles into larger nanoparticles of a different shape. The particle size and shape can be controlled by choosing the wavelength(s) of light used to drive the photochemical growth. These findings suggest a new way of thinking about growth of nanoparticles in solution and show the exciting possibility that light can be used as a major control parameter in metallic nanoparticle growth reactions.

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.