Ultrafast spin crossover photochemical mechanism in [Fe(2,2\u27−bipyridine)3]2+ revealed by quantum dynamics

The role of triplet intermediates in the photoinduced spin crossover reaction of [FeII(2,2\u27-bipyridine)3]2+ is still under debate. Employing quantum dynamics, we show that the metal-centered (MC) triplets are responsible for the transfer to the quintet high-spin state. This photochemical pathway is made possible thanks to bipyridine stretching vibrations, facilitating the transfer between the initial metal-ligand charge transfer (MLCT) states to the MC triplets. These results show the central role of ligands in modulating the excited state spectrum and the photochemical mechanism, opening the route for increased metal-centered lifetime that increases the effciency of iron-based photocatalysts

The (<i>E</i> + <i>A</i>) × (<i>e</i> + <i>a</i>) Jahn–Teller and Pseudo-Jahn–Teller Hamiltonian Including Spin–Orbit Coupling for Trigonal Systems

The Hamiltonian describing <i>E</i> × <i>e</i> Jahn–Teller (JT) coupling and (<i>E</i> + <i>A</i>) × (<i>e</i> + <i>a</i>) pseudo-JT (PJT) coupling is developed beyond the standard JT theory for the example of XY₃ systems, taking the bending modes of <i>a</i> and <i>e</i> symmetry into account. For the electrostatic (spin-free) Hamiltonian, the conventional Taylor expansion up to second order in symmetry-adapted displacements is replaced by an expansion in invariant polynomials up to arbitrarily high orders. The relevance of a systematic high-order expansion in the three large-amplitude bending modes is illustrated by the construction of an eighth-order three-sheeted three-dimensional <i>ab initio</i> potential-energy surface for PH₃⁺. The theory of spin–orbit coupling in trigonal JT/PJT systems is extended beyond the standard model of JT theory by an expansion of the microscopic Breit–Pauli operator up to second order in symmetry-adapted vibrational coordinates. It is shown that a linear <i>E</i> × <i>e</i> JT effect of relativistic origin exists in <i>C</i>_3<i>v</i> systems which vanishes at the planar (<i>D</i>_3<i>h</i>) geometry. The linear relativistic ²<i>E</i> – ²<i>A</i> PJT coupling, on the other hand, persists at the planar geometry

Ultrafast nuclear dynamics of the acetylene cation C2H2+ and its impact on the infrared probe pulse induced C–H bond breaking efficiency

The ultrafast nuclear dynamics of the acetylene cation C2H2+ following photoionization of the neutral molecule is investigated using an extreme-ultraviolet pump/infrared probe setup. The observed modulation of the C2H+ fragment ion yield with pump–probe delay is related to structural changes induced by the extreme-ultraviolet pump pulse taking place on the femtosecond timescale. High-level simulations suggest that the trans-bending and C–C bond stretching motion of the C2H2+ cation govern the observed interaction with the infrared pulse. Depending on the molecular configuration at arrival of the infrared pulse, it either transfers population to higher-lying states or to the C2H2+ ground state, thereby enhancing or lowering the C2H+ yield. Our ultrafast pump–probe scheme can thus be used to track excited state nuclear dynamics with a resolution of a few femtoseconds, leading the way to studying fast dynamics also in larger hydrocarbon molecules.ISSN:1463-9084ISSN:1463-907

La vie de Marianne, ou Les avantures de Mme la comtesse de ***. 1e partie / , par M. de Marivaux. Première partie

[La vie de Marianne (français)

Influence of pump laser fluence on ultrafast structural changes in myoglobin

High-intensity femtosecond pulses from an X-ray free-electron laser enable pump probe experiments for investigating electronic and nuclear changes during light-induced reactions. On time scales ranging from femtoseconds to milliseconds and for a variety of biological systems, time-resolved serial femtosecond crystallography (TR-SFX) has provided detailed structural data for light-induced isomerization, breakage or formation of chemical bonds and electron transfer 1 . However, all ultra-fast TR-SFX studies to date have employed such high pump laser energies that several photons were nominally absorbed per chromophore 2-14 . As multiphoton absorption may force the protein response into nonphysiological pathways, it is of great concern 15 whether this experimental approach 16 allows valid inferences to be drawn vis-à-vis biologically relevant single-photon-induced reactions 17 . Here we describe ultrafast pump-probe SFX experiments on photodissociation of carboxymyoglobin, showing that different pump laser fluences yield markedly different results. In particular, the dynamics of structural changes and observed indicators of the mechanistically important coherent oscillations of the Fe-CO bond distance (predicted by recent quantum wavepacket dynamics 15 ) are seen to depend strongly on pump laser energy. Our results confirm both the feasibility and necessity of performing TR-SFX pump probe experiments in the linear photoexcitation regime. We consider this to be a starting point for reassessing design and interpretation of ultrafast TR-SFX pump probe experiments 16 such that biologically relevant insight emerges

Influence of pump laser fluence on ultrafast myoglobin structural dynamics

International audienceHigh-intensity femtosecond pulses from an X-ray free-electron laser enable pump–probe experiments for the investigation of electronic and nuclear changes during light-induced reactions. On timescales ranging from femtoseconds to milliseconds and for a variety of biological systems, time-resolved serial femtosecond crystallography (TR-SFX) has provided detailed structural data for light-induced isomerization, breakage or formation of chemical bonds and electron transfer 1,2 . However, all ultrafast TR-SFX studies to date have employed such high pump laser energies that nominally several photons were absorbed per chromophore 3–17 . As multiphoton absorption may force the protein response into non-physiological pathways, it is of great concern 18,19 whether this experimental approach 20 allows valid conclusions to be drawn vis-à-vis biologically relevant single-photon-induced reactions 18,19 . Here we describe ultrafast pump–probe SFX experiments on the photodissociation of carboxymyoglobin, showing that different pump laser fluences yield markedly different results. In particular, the dynamics of structural changes and observed indicators of the mechanistically important coherent oscillations of the Fe–CO bond distance (predicted by recent quantum wavepacket dynamics 21 ) are seen to depend strongly on pump laser energy, in line with quantum chemical analysis. Our results confirm both the feasibility and necessity of performing ultrafast TR-SFX pump–probe experiments in the linear photoexcitation regime. We consider this to be a starting point for reassessing both the design and the interpretation of ultrafast TR-SFX pump–probe experiments 20 such that mechanistically relevant insight emerges