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 present a comparative study of the ultrafast photophysics of all-trans retinal in the protonated Schiff base form in solvents with different polarities and viscosities. Steady-state spectra of retinal in the protonated Schiff base form show large absorption-emission Stokes shifts (6500-8100 cm(-1)) for both polar and nonpolar solvents. Using a broadband fluorescence up-conversion experiment, the relaxation kinetics of fluorescence is investigated with 120 fs time resolution. The time-zero spectra already exhibit a Stokes-shift of approximately 6000 cm(-1), indicating depopulation of the Franck-Condon region in < or =100 fs. We attribute it to relaxation along skeletal stretching. A dramatic spectral narrowing is observed on a 150 fs timescale, which we assign to relaxation from the S(2) to the S(1) state. Along with the direct excitation of S(1), this relaxation populates different quasistationary states in S(1), as suggested from the existence of three distinct fluorescence decay times with different decay associated spectra. A 0.5-0.65 ps decay component is observed, which may reflect the direct repopulation of the ground state, in line with the small isomerization yield in solvents. Two longer decay components are observed and are attributed to torsional motion leading to photo-isomerization. The various decay channels show little or no dependence with respect to the viscosity or dielectric constant of the solvents. This suggests that in the protein, the bond selectivity of isomerization is mainly governed by steric effects.

We present steady-state and broadband femtosecond fluorescence spectra of the protonated Schiff base of retinal in various protic and aprotic solvents, as a function of the excitation wavelength. A detailed spectral decomposition of the time-resolved fluorescence spectra allows us to isolate three spectral components: (i) the vibrationally relaxed S(1) fluorescence, (ii) a vibrationally hot S(1) fluorescence, and (iii) a higher-lying emission that undergoes spectral evolution on a time scale of 300-400 fs, which we assign to S(2) fluorescence. The vibrationally "cold" S(1) fluorescence exhibits three decay components upon 400 nm excitation (except in acetonitrile, which has two), but only two of them upon 570 nm excitation. These components clearly demonstrate the heterogeneity of the S(1) state in the sense that emission stems from several shallow potential surface minima. We discuss a connection between these decay channels and reactive and nonreactive excited-state paths on the basis of their solvent-dependent population and of previous high-performance liquid chromatography studies. There is no clear trend of the fluorescence decay times with solvent properties. Rather, a solvent effect manifests itself in acetonitrile, in that the number of fluorescence decay channels is smaller and that the quenching of the hot fluorescence seems more efficient. This effect has to do with the population pathways leading to the fluorescent states. These observations stress the heterogeneity of excited retinal Schiff base, influencing the decay but also the population channels. They also reinforce the claim that steric effects play an important role in the dynamics of the protein.