Finding intersections between electronic excited state potential energy surfaces with simultaneous ultrafast X-ray scattering and spectroscopy

Light-driven molecular reactions are dictated by the excited state potential energy landscape, depending critically on the location of conical intersections and intersystem crossing points between potential surfaces where non-adiabatic effects govern transition probabilities between distinct electronic states. While ultrafast studies have provided significant insight into electronic excited state reaction dynamics, experimental approaches for identifying and characterizing intersections and seams between electronic states remain highly system dependent. Here we show that for 3d transition metal systems simultaneously recorded X-ray diffuse scattering and X-ray emission spectroscopy at sub-70 femtosecond time-resolution provide a solid experimental foundation for determining the mechanistic details of excited state reactions. In modeling the mechanistic information retrieved from such experiments, it becomes possible to identify the dominant trajectory followed during the excited state cascade and to determine the relevant loci of intersections between states. We illustrate our approach by explicitly mapping parts of the potential energy landscape dictating the light driven low-to-high spin-state transition (spin crossover) of [Fe(2,2′-bipyridine)3]2+, where the strongly coupled nuclear and electronic dynamics have been a source of interest and controversy. We anticipate that simultaneous X-ray diffuse scattering and X-ray emission spectroscopy will provide a valuable approach for mapping the reactive trajectories of light-triggered molecular systems involving 3d transition metals

FeII Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-To-Ligand Charge-Transfer Excited-State Lifetime

The iron carbene complex [FeII(btz)3](PF6)2 (where btz = 3,3′-dimethyl-1,1′-bis(p-Tolyl)-4,4′-bis(1,2,3-Triazol-5-ylidene)) has been synthesized, isolated, and characterized as a low-spin ferrous complex. It exhibits strong metal-To-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3MLCT state with a 528 ps excited-state lifetime in CH3CN solution that is more than one order of magnitude longer compared with the MLCT lifetime of any previously reported FeII complex. The low potential of the [Fe(btz)3]3+/[Fe(btz)3]2+ redox couple makes the 3MLCT state of [FeII(btz)3]2+ a potent photoreductant that can be generated by light absorption throughout the visible spectrum. Taken together with our recent results on the [FeIII(btz)3]3+ form of this complex, these results show that the FeII and FeIII oxidation states of the same Fe(btz)3 complex feature long-lived MLCT and LMCT states, respectively, demonstrating the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of oxidizing and reducing conditions

Tracking the picosecond deactivation dynamics of a photoexcited iron carbene complex by time-resolved X-ray scattering

Recent years have seen the development of new iron-centered N-heterocyclic carbene (NHC) complexes for solar energy applications. Compared to typical ligand systems, the NHC ligands provide Fe complexes with longer-lived metal-to-ligand charge transfer (MLCT) states. This increased lifetime is ascribed to strong ligand field splitting provided by the NHC ligands that raises the energy levels of the metal centered (MC) states and therefore reduces the deactivation efficiency of MLCT states. Among currently known NHC systems, [Fe(btbip)2]2+ (btbip = 2,6-bis(3-tert-butyl-imidazol-1-ylidene)pyridine) is a unique complex as it exhibits a short-lived MC state with a lifetime on the scale of a few hundreds of picoseconds. Hence, this complex allows for a detailed investigation, using 100 ps X-ray pulses from a synchrotron, of strong ligand field effects on the intermediate MC state in an NHC complex. Here, we use time-resolved wide angle X-ray scattering (TRWAXS) aided by density functional theory (DFT) to investigate the molecular structure, energetics and lifetime of the high-energy MC state in the Fe-NHC complex [Fe(btbip)2]2+ after excitation to the MLCT manifold. We identify it as a 260 ps metal-centered quintet (5MC) state, and we refine the molecular structure of the excited-state complex verifying the DFT results. Using information about the hydrodynamic state of the solvent, we also determine, for the first time, the energy of the 5MC state as 0.75 ± 0.15 eV. Our results demonstrate that due to the increased ligand field strength caused by NHC ligands, upon transition from the ground state to the 5MC state, the metal to ligand bonds extend by unusually large values: by 0.29 Å in the axial and 0.21 Å in the equatorial direction. These results imply that the transition in the photochemical properties from typical Fe complexes to novel NHC compounds is manifested not only in the destabilization of the MC states, but also in structural distortion of these states

Tracking structural solvent reorganization and recombination dynamics following e-photoabstraction from aqueous I-with femtosecond x-ray spectroscopy and scattering

We present a sub-picosecond resolved investigation of the structural solvent reorganization and geminate recombination dynamics following 400 nm two-photon excitation and photodetachment of a valence p electron from the aqueous atomic solute, I-(aq). The measurements utilized time-resolved X-ray Absorption Near Edge Structure (TR-XANES) spectroscopy and X-ray Solution Scattering (TR-XSS) at the Linac Coherent Light Source x-ray free electron laser in a laser pump/x-ray probe experiment. The XANES measurements around the L1-edge of the generated nascent iodine atoms (I0) yield an average electron ejection distance from the iodine parent of 7.4 ± 1.5 Å with an excitation yield of about 1/3 of the 0.1M NaI aqueous solution. The kinetic traces of the XANES measurement are in agreement with a purely diffusion-driven geminate iodine-electron recombination model without the need for a long-lived (I0:e-) contact pair. Nonequilibrium classical molecular dynamics simulations indicate a delayed response of the caging H2O solvent shell and this is supported by the structural analysis of the XSS data: We identify a two-step process exhibiting a 0.1 ps delayed solvent shell reorganization time within the tight H-bond network and a 0.3 ps time constant for the mean iodine-oxygen distance changes. The results indicate that most of the reorganization can be explained classically by a transition from a hydrophilic cavity with a well-ordered first solvation shell (hydrogens pointing toward I-) to an expanded cavity around I0 with a more random orientation of the H2O molecules in a broadened first solvation shell

Solvent control of charge transfer excited state relaxation pathways in [Fe(2,2′-bipyridine)(CN)4]2-

The excited state dynamics of solvated [Fe(bpy)(CN)4]2-, where bpy = 2,2′-bipyridine, show significant sensitivity to the solvent Lewis acidity. Using a combination of optical absorption and X-ray emission transient spectroscopies, we have previously shown that the metal to ligand charge transfer (MLCT) excited state of [Fe(bpy)(CN)4]2- has a 19 picosecond lifetime and no discernable contribution from metal centered (MC) states in weak Lewis acid solvents, such as dimethyl sulfoxide and acetonitrile.1,2 In the present work, we use the same combination of spectroscopic techniques to measure the MLCT excited state relaxation dynamics of [Fe(bpy)(CN)4]2- in water, a strong Lewis acid solvent. The charge-transfer excited state is now found to decay in less than 100 femtoseconds, forming a quasi-stable metal centered excited state with a 13 picosecond lifetime. We find that this MC excited state has triplet (3MC) character, unlike other reported six-coordinate Fe(ii)-centered coordination compounds, which form MC quintet (5MC) states. The solvent dependent changes in excited state non-radiative relaxation for [Fe(bpy)(CN)4]2- allows us to infer the influence of the solvent on the electronic structure of the complex. Furthermore, the robust characterization of the dynamics and optical spectral signatures of the isolated 3MC intermediate provides a strong foundation for identifying 3MC intermediates in the electronic excited state relaxation mechanisms of similar Fe-centered systems being developed for solar applications

الطقوس وجبروت الرموز: قراءة في الوظائف والدلالات ضمن مجتمع متحوّل

تتناول هذه الدراسة بالتحليل القضايا المتعلقة بالتحولات الاجتماعية التي ترافقها تحولات على مستوى الرمز و الدلالات. و بعد التساؤل عن فحوى هذه العلاقة بين ما هو اجتماعي و ما هو رمزي، و بعد التنظير لقضايا الرمز والشعائر الطقسية تحاول هذه الدراسة الإجابة عن الكيفية التي يواجه بها المجتمع المحلي هذه التحولات. إذ أن الحياة الاجتماعية قد تعلمنت و حركة تقسيم العمل قد عرفت مداها واستقلت أنشطة الحياة المختلفة عن الحياة الدينية، و مع هذه التغيرات، انتقلت الأنشطة الرمزية و الطقوسية إلى مستوى آخر مختلف عما سبق. لكن تمسك بعض الفئات بهذه الطقوس و الشعائر يمنحها تمسكا قويا بالهوية، و يؤدي وظيفة عيادية تجعلهم يفلتون من الرقابة اليومية و من طغيان الثقافة، البرمجيات والأزرار.Cet article aborde les questions relatives aux mutations sociales accompagnées de mutations symboliques et signifiantes. En s’interrogeant sur la nature de la relation qui existe entre ce qui est social et ce qui symbolique, et après théorisation des questions du symbole et du rite, cette étude tente de dévoiler la réaction de la société face à ces mutations.La vie sociale étant mondialisée et le mouvement de répartition des tâches ayant atteint ses limites, les différentes activités de la vie demeurent aujourd’hui indépendantes de la vie religieuse. Car grâce à ce changement, les activités symboliques et rituelles sont passées d’un niveau à un autre niveau. Cependant, l‘attachement de certains groupes à ces rituels, les reliant fortement à l’identité, exerce une fonction clinique qui leur permet de fuir le contrôle quotidien et la tyrannie de la culture, les logiciels et les manipulations digitales.This article tackles questions relating to so social change accompanied by important symbolic mutations. By questioning the nature of the relationship existing between what is social and what is symbolic, and after theorizing questions of symbol and rites, this study tries to reveal social reaction faced with these mutations.Social life being globalized and the repartition of tasks having reached its limits, the different activities of daily life nowadays remain independent from religious life. Therefore thanks to this change, symbolic activities and rituals have passed from one level to another one. Nevertheless the attachment of some groups to these rituals, strongly linked to identity, exerts a clinical function which allows them to flee from daily control and cultural tyranny, from software and digital manipulation.Este artículo aborda cuestiones relativas a las mutaciones sociales acompañadas de mutaciones simbólicas y significantes. Al interrogarnos sobre la naturaleza de la relación que existe entre lo que es social y lo que es simbólico, y tras la teorización de las cuestiones del símbolo y del rito, este estudio intenta desvelar la reacción de la sociedad frente a dichas mutaciones.Siendo mundializada la vida social y el movimiento del repartimiento de las tareas habiendo alcanzado sus limites, las diferentes actividades de la vida resultan hoy día independientes de la vida religiosa. Pues, gracias a este cambio, las actividades simbólicas y rituales han pasado de un nivel a otro nivel. Sin embargo, el apego de ciertos grupos a estos rituales que les ligan fuertemente a la identidad, ejerce una función clínica que les permite huir del control cotidiano y de la tiranía de la cultura, los logiciales y las manipulaciones digitales