Utilisation des méthodes de localisation multi-référence pour les systèmes quasi-dégénérés

La structure électronique et les propriétés de transfert de charge intramoléculaire de certains composés à valence mixte ont été étudiés au niveau ab-initio multi-référence, en utilisant des orbitales moléculaires canoniques et localisées. Les composés chimiques étudiés sont : un cation de spiro pi-sigma-pi : 5,5'(4H,4H')-spirobi-[cyclopenta[c]pyrrole]2,2',6,6'tetrahydro cation, ("Spiro" dans le texte suivant); une série des chaînes cationiques linéaires composées d'atomes de béryllium : BeN, (avec N = 6, ..., 12); et deux molécules bis-triarylamines : N,N,N',N'-Tetra(4-méthoxyphényl)-1,4phénylènediamine cation, et bis{4-[N,N-di(4-méthoxyphnyl)amino]phényl}butadiyne cation. Les modèles théoriques et les méthodes de calcul utilisées dans ce travail sont les suivants : CAS-SCF, CAS+S, CAS+SD (MRCI), et CAS+SD en utilisant des orbitales localisées. Des différents bases contractées ont été utilisés. Les surfaces d'énergie potentielles adiabatiques de l'état électronique fondamentale et les trois états excités les plus bas du cation de Spiro, ont été calculés, au sein d'un modèle à deux états, et un potentiel de double-puits a été obtenu pour l'état électronique fondamental. En suivant la localisation/délocalisation de charge dans le système p de valence du Spiro à travers la coordonnée de réaction du transfert de charge intramoléculaire, nous avons montré un faible couplage électronique entre les deux moitiés d'oxydoréduction de ce cation moléculaire. L'effet de la corrélation dynamique, en utilisant des orbitales localisées ou canoniques, a été jugé cruciale pour une description quantitative de la structure électronique et les autres paramètres importantes de transfert d'électron de ce système modèle à valence mixte. Les résultats des chaînes linéaires de béryllium montrent une évolution progressive de classe III (couplage électronique fort) envers classe II (couplage électronique faible) en fonction de nombre d'atomes de béryllium. En effet, dans les cas où (N > 10), les chaînes cationiques ont été trouvés se rapprocher de la classe I, où le couplage disparaît. Les paramètres de transfert d'électrons intramoléculaire Vab, Ea, et Eopt ont été calculés pour chaque chaîne atomique. Il à été montré que la baisse des valeurs de Vab avec l'augmentation de N suit une courbe exponentielle.The electronic structure and the intramolecular charge transfer properties of a selected number of mixed valence compounds were investigated at multireference ab-initio level, using both canonical and localized molecular orbitals. The chemical compounds studied are: a spiro p-s-p molecular cation: 5,5_(4H,4H_)- spirobi[cyclopenta[c]pyrrole]2,2',6,6'tetrahydro cation (the "Spiro" molecule in the following); a series of cationic linear chains composed of beryllium atoms: BeN, (with N = 6, ..., 12); and two bis-Triaryl amines molecules: namely N,N,N',N'-Tetra(4-methoxyphenyl)-1,4-phenylenediamine cation, and bis{4-[N,N-di(4-methoxyphenyl)amino]phenyl}butadiyne cation. The theoretical models and computational methods used in this work are: CAS-SCF, CAS+S, CAS+SD (MRCI), and CAS+SD using localized orbitals. Different basis sets contractions were used. For Spiro cation, The potential energy surfaces of the adiabatic ground and the lowest three excited electronic states have been computed, within a two-state model, and a double-well potential has been obtained for the ground electronic state. We have demonstrated a low coupling interaction between the two redox moieties of this molecular cation by following the charge localization/delocalization in the valence _ system through the reaction coordinate of the intramolecular charge transfer. The effect of dynamical correlation, using either localized or canonical orbitals, was found to be crucial for a quantitative description of the electronic structure and some important electron transfer parameters of this model mixed-valence system. The results of the linear beryllium chains show a consistent gradual shift between different classes of mixed-valence compounds as the number of beryllium atoms increases, from class-III strong coupling toward class-II valence trapped. Indeed, in the largest cases (N > 10), the cationic chains were found to be closer to class I, where the coupling vanishes. The intramolecular electron transfer parameters Vab, Ea, and Eopt were calculated for each atomic chain. It is shown that the decrease of Vab with increasing N follows an exponential pattern

Insights into the magnetism and phase transitions of organic radical-based materials

Organic radicals have been consistently regarded as promising building blocks for the next generation of applied materials. Multiple radical families have been developed and characterized in the last decades, fostered by the ever-growing capabilities of organic synthesis. Thiazyl-, spiro-biphenalenyl-, 1,2,4-benzotriazinyl-, and nitroxide-based radicals have furnished striking examples of metal-free switchable materials, whose phase transitions are accompanied by changes in the magnetic, optical and/or electrical response. While similar in origin, their actual mechanism, driving force(s), and spin state stabilities often depict a different landscape. Fundamental knowledge on such aspects, as well as on the underlying network of spin exchange couplings and non-covalent interactions (including pancake bonding), are key to understand their spin transition, and the tailored modification of their properties. These complex features cannot be extracted based solely on experimental input, but rely on a computational interpretation. In this Perspective article, we discuss the insight gained from computational modelling into the magnetism and phase transitions of organic radical-based materials. We focus on the key importance of dynamic effects due to the labile nature of π-stack interactions assembling those materials, the structural distortions driven by spin changes, the coupling between electronic structure and order-disorder transitions, and the dependence of spin correlation upon temperature. All these phenomena uncovered by simulations should assist in the rational design of new dynamic organic crystals

Characterization of metallic and insulating properties of low-dimensional systems

Dans cette thèse nous avons étudié des indicateurs visant à caractériser les propriétés métalliques ou isolantes de systèmes de basse dimensionnalité à partir de calculs théoriques basés sur la fonction d'onde. Ces systèmes sont intéressants car ils permettent une compréhension en profondeur des phénomènes physiques qui peuvent ensuite être extrapolés à des systèmes plus étendus. Afin de réaliser cette étude nous avons utilisé un nouvel outil basé sur la théorie de la conductivité de Kohn : le tenseur de délocalisation total ou total position spread-tensor (TPS). Ce tenseur est défini comme le second cumulant de l'opérateur position : ? = - 2. Divisé par le numéro des électrons, il diverge quand la fonction d'onde est fortement délocalisée (forte fluctuation de la position des électrons) et converge vers une valeur finie dans le cas contraire. Ainsi, la conductivité est relié à la délocalisation de la fonction d'onde. Dans ce travail, deux définitions du TPS ont été abordées : une quantité sommée sur le spin (spin-summed TPS, SS-TPS) d'une part, et une décomposition selon le spin (spin-partitioned TPS, SSP-TSP) d'autre part. Cette dernière s'est avérée être un outil très efficace pour l'étude de systèmes fortement corrélés. Au cours de la thèse, nous avons commencé par étudier plusieurs systèmes diatomiques présentant des liaisons de natures différentes à l'aide de calculs d'interaction de configurations totale (FCI). Le TPS présente alors un maximum dans une zone précédant la rupture de liaison avant de converger asymptotiquement vers les valeurs atomiques, comme la consistance de taille du tenseur le laissait présager. Dans le cas de systèmes pour lesquels l'état électronique présente un croisement évité, le TPS diverge, mettant ainsi en évidence la forte délocalisation de la fonction d'onde. Le SS-TPS est donc un indicateur de choix pour suivre la nature de la liaison chimique. Nous avons ensuite considéré des systèmes à valence mixte de type II pour lesquels l'état fondamental présente un double-puits de potentiel avec un croisement évité avec le premier état excité. Il est donc nécessaire ici d'utiliser un traitement multi-configurationnel. Deux systèmes modèles ont ainsi été étudiés : i) deux di- mères H2 en interaction faible au niveau FCI et ii) un composé du type spiro au niveau CAS-SCF (à l'aide d'un code que nous avons implémenté dans Molpro). Dans les deux cas, le TPS présentait un maximum très marqué dans la région du croisement évité, signature d'une forte mobilité électronique. Nous nous sommes également intéressés à trois types de chaines d'atomes d'hydrogène : i atomes équidistants ii) chaines dimérisées à longueur de liaison H2 fixée et iii) chaines dimérisées. Tant le SS-TPS que le SP-TPS montrent des comportements différents selon le type de chaine considérée. Les premières ont un caractère métallique et une délocalisation de spin prononcée dans le régime fortement corrélé. Les secondes sont de nature isolante avec une délocalisation limitée. Les chaines dimérisées, quant à elle, dissocient très rapidement vers un état isolant mais avec une forte délocalisation de spin. Ces chaines demi-remplies ont aussi été traitées à l'aide d'hamiltonien de Hubbard et de Heisenberg. Nous avons ainsi pu rationaliser le comportement des SS-TPS et SP-TPS en variant le rapport de l'intégrale de saut et de la répulsion électron- électron (-t/U) entre sites adjacents. Le caractère ferromagnétique/anti-ferromagnétique a également pu être suivi en modifiant la valeur de la constante de couplage J dans le cas fortement corrélé. Finalement, ces indicateurs ont été mis en oeuvre pour des polyacenes cycliques. Dans ce cas, le TPS a permis de comprendre la nature des fonctions d'onde de l'état fondamental obtenues au niveau CAS-SCF et NEVPT2.I carried out a theoretical study to characterize metallic and insulating properties of low-dimensional systems using wave function methods. Low-dimensional systems are particularly important because they allow an understanding that can be extrapolated to higher dimensional systems. We have employed a new tool based on the theory of conductivity of Kohn that we have named: total position-spread tensor (TPS). The TPS is defined as the second moment cumulant of the total position operator: ? = - 2 . The tensor divided by the number of electrons diverges when the wave function is delocalized (high fluctuation of electrons' positions), and it takes finite values for localized ones. In this way, the electrical conductivity is related to the proper delocalization of the wave function. In addition, the tensor can be divided in spin-summed (SS-TPS) and spin-partitioned tensors (SP-TPS). The latter one becomes a powerful tool to the study of strongly correlated systems. In this dissertation, we started to investigate at full configuration interaction (FCI) level diatomic molecules showing different types of bond. The TPS presented a marked maximum before the bond was broken and in the asymptotic limit one recovers the TPS values of isolated atoms (size consistency). For the case of diatomic systems showing avoided-crossing electronic states, the TPS diverges evidencing the high delocalization of the wave function. Therefore, the SS-TPS is capable of monitoring and characterizing molecular wave functions. We considered mixed-valence systems that are often distinguished by a double-well potential energy surface presenting an avoided-crossing. Thus, such a configuration possesses a strongly multireference nature involving at least two states of the same symmetry. Two different systems were investigated: i) two weakly interacting hydrogen dimers that were investigated at Full CI level, and ii) a spiro like molecule where the TPS tensor was evaluated in a CAS-SCF state-averaged wave function using our implementation of the SS- TPS formalism in MOLPRO. We found that the tensor's component in the direction of the electron transfer (ET) shows a marked maximum in the avoided-crossing region, evidencing the presence of a high electron mobility. The formalisms of the SS- and SP-TPS was applied to one dimensional systems composed by three types of half-filled hydrogen chains: i) equally-spaced chains, ii) fixed-bond dimerized chains, and iii) homothetic dimerized chains. Both the SS- and SP-TPS showed different signatures associated to the three types of systems. Equally-spaced chains have metallic wave functions and a high spin delocalization in the strongly correlated regime. In contrast, fixed-bond dimerized chains have an insulating character and a restricted spin delocalization. Finally, homothetic dimerized chains dissociate very quickly which renders them in the insulating state but with a high spin delocalization. We also studied half-filled chains by using the Hubbard and the Heisenberg Hamiltonians. On the one hand, we were able to depict the response of the SS- and SP-TPS by varying the ratio between the hopping and electron-electron repulsion (-t/U parameter) of topological connected sites. On the other hand, the ferromagnetic and anti-ferromagnetic character of the wave functions were evaluated by varying the coupling constant (J) in the strongly correlated systems. A theoretical study of closed polyacenes (PAH) structures was performed at CAS-SCF and NEVPT2 level. Our methodology for choosing the active space using the Hückel Hamiltonian was able to characterize the ground state of the systems that indeed fulfilled the Ovchinnikov rule. Finally, we applied the SS-TPS to understand the nature of the wave functions of these PAHs

Proceedings of the Thirteenth International Conference on Time-Resolved Vibrational Spectroscopy

The thirteenth meeting in a long-standing series of “Time-Resolved Vibrational Spectroscopy” (TRVS) conferences was held May 19th to 25th at the Kardinal Döpfner Haus in Freising, Germany, organized by the two Munich Universities - Ludwig-Maximilians-Universität and Technische Universität München. This international conference continues the illustrious tradition of the original in 1982, which took place in Lake Placid, NY. The series of meetings was initiated by leading, world-renowned experts in the field of ultrafast laser spectroscopy, and is still guided by its founder, Prof. George Atkinson (University of Arizona and Science and Technology Advisor to the Secretary of State). In its current format, the conference contributes to traditional areas of time resolved vibrational spectroscopies including infrared, Raman and related laser methods. It combines them with the most recent developments to gain new information for research and novel technical applications. The scientific program addressed basic science, applied research and advancing novel commercial applications. The thirteenth conference on Time Resolved Vibrational Spectroscopy promoted science in the areas of physics, chemistry and biology with a strong focus on biochemistry and material science. Vibrational spectra are molecule- and bond-specific. Thus, time-resolved vibrational studies provide detailed structural and kinetic information about primary dynamical processes on the picometer length scale. From this perspective, the goal of achieving a complete understanding of complex chemical and physical processes on the molecular level is well pursued by the recent progress in experimental and theoretical vibrational studies. These proceedings collect research papers presented at the TRVS XIII in Freising, German

Nonlinear Optical Responsive Molecular Switches

Nonlinear optical (NLO) materials have gained much attention during the last two decades owing to their potentiality in the field of optical data storage, optical information processing, optical switching, and telecommunication. NLO responsive macroscopic devices possess extensive applications in our day to day life. Such devices are considered as assemblies of several macroscopic components designed to achieve specific functions. The extension of this concept to the molecular level forms the basis of molecular devices. In this context, the design of NLO switches, that is, molecules characterized by their ability to alternate between two or more chemical forms displaying contrasts in one of their NLO properties, has motivated many experimental and theoretical works. Thus, this chapter focuses on the rational design of molecular NLO switches based on stimuli and materials with extensive examples reported in the literature. The factors affecting the efficiency of optical switches are discussed. The device fabrication of optical switches and their efficiency based on the optical switch, internal architecture, and substrate materials are described. In the end, applications of switches and future prospectus of designing new molecules with references are suitably discussed

Studies of DTF and TTFV-based donor-acceptor systems and redox-active polymer thin films

1,4-Dithiafulvene (DTF) is a five-member heterocycle that has been frequently used as a redox-active molecular building block in various organic electronic materials. The combination of two DTF groups via an exo-ring C=C bond leads to formation of well-known tetrathiafulvalene (TTF), which has been extensively studied since the first discovery of its metallic conductivity. Previous research has demonstrated that DTF and tetrathiafulvalene vinylogue (TTFV)-based conjugated molecules and polymers show favored intermolecular interactions (e.g., π–π stacking and chargetransfer interactions) with electron-deficient nitroaromatic compounds (NACs), owing to the electron-donating nature of DTF and TTFV groups. Such properties can be utilized in the design of chemical sensors for detection of NACs, which are an important class of pollutants in the environment. To further understand the interplay between NACs and DTF/TTFV-containing π-systems, a group donor–acceptor ensembles containing nitrophenyl-substituted DTF and TTFV moieties have been investigated in this thesis work. Detailed synthetic methods and structure-property relationships will be discussed in the first chapter. In particular, the structural, electronic, and electrochemical redox properties were systematically examined by Xray single crystallographic, UV-Vis absorption, and cyclic voltametric analyses, in conjunction with density functional theory (DFT) modeling. With the fundamental properties characterized and understood, a new type of TTFV-based redox-active polymer was next designed and prepared. In the second part of this project, a strategy of double-layer polymer film will be introduced. With this method, robust and redox-active TTFV polymer thin films could be efficiently generated on the surface of glassy carbon electrodes. These modified electrodes were found to show sensitive responses to various phenolic compounds at low concentrations (10−8 to 10−7M), suggesting promising application in rapid electrochemical sensing of phenol derivatives and related chemicals

Atomistic simulations of the reactive processes in the heme-containing proteins

Heme proteins have a great impact in the protein research. Due to the unique electronic properties of heme these proteins are abundant in nature and have a wide range of biological functions in most of the organisms from archea to eukaryotes. The ability of heme proteins to bind and release small molecules like CO, NO, O2 defines the variety of physiological functions and is related to the structural dynamic properties of the protein matrix surrounding heme. Cytochrome c oxidase (CcO) is a heme-containing protein, which performs oxygen reduction to water as a part of the membrane complex. Cytochrome c oxidase forms a stable complex with CO in the binuclear heme a3 - Cu(B) active site and is a model system to study ligand binding and release. The pump-probe experiments performed for the CcO-CO system reported the ultrafast dynamics of the CO transfer from the heme Fe to Cu(B) site. Molecular dynamics simulations are used to provide the dynamic structural information during the transfer with atomic resolution. The kinetics of the process determined from the MD simulations is a qualitative agreement with the timescales reported in the experimental studies. The simulations show that the transfer dynamics is ballistic. The doming of the heme Fe observed after the photoexcitation significantly affects the probability of the heme Fe rebinding. Myglobin (Mb) is an oxygen storage protein, the active site of which contains heme. It allows studying the impact of the structural changes on binding and release of small molecules. The Mb complex with NO was studied using MD simulations. The heme doming effect observed after photodissociation makes the heme Fe less accessible to NO and slows down the rebinding. The DFT parametrized 2A state predicts the existence of the Fe-ON minimum, which is not observable in the experiments, this might be explained by the effect of the 4A PES, that activated during the photoexcitation and that has a lower energy for the configurations corresponding to the Fe-ON minimum

Functional organic fluorophores based on 1,3-dithiafulvene and pyrene

This thesis demonstrates the design, synthesis, and application of fluorene and pyrene-based organic π-systems, in which redox-active 1,3-dithiafulvenyl (DTF) groups are incorporated. The detailed studies encompass three classes of novel redox-active organic fluorophores. The first project deals with fluorene-cored π- conjugated phenylene vinylene dendrimers with DTF end groups attached to the peripheral positions. The oxidative coupling reactivity of the DTF group allowed these dendrimers to undergo facile electropolymerization on conductive substrates. By means of multi-cycle cyclic voltammetric scans, redox-active microporous polymer thin films were successfully generated. The redox-active polymer prepared through a double-layer strategy, in which a polymer thin film prepared from a dithiafulvenylsubstituted phenylacetylene precursor was surface-modified with a fluorene-cored phenylene vinylene dendrimer carrying dithiafulvenyl end groups. The experimental results indicated that the polymers can act as highly sensitive and selective electrochemical sensors for 2,4,6-trinitrotoluene (TNT), one of the most important nitroaromatic explosives. In the second project, a group of DTF-functionalized pyrene derivatives was prepared and investigated. The redox activities and electrochemical properties of these compounds were examined by voltammetric analysis. Polymer films generated through electropolymerization of these redox-active pyrenes have been found to show responsiveness to nitromethane. In addition, one of the derivatives shows an unusually high efficiency in terms of hydrogen/deuterium exchange, which can be potentially used for trapping deuterium isotope. The third project focuses on a comparative study of a series of new Kregion functionalized pyrene derivatives. Three new K-region functionalized pyrene derivatives were synthesized using pyrene-4,5-dione as the starting material. The Xray single crystal structures of three compounds were determined and examined by the Hirshfeld surface analysis. The molecular geometries of these pyrene derivatives were found to strongly affect the intramolecular noncovalent interactions in the solid state. Moreover, the electronic and electrochemical redox properties of these pyrene derivatives were characterized. Among the three pyrene derivatives, a dithiafulvenyl substituted derivative presents an intriguing D{A conjugated system with a small bandgap (Eg) and amphoteric redox behavior, making this compound a potentially useful organic semiconductor. The findings disclosed in this work show promise for the development of pyrene-based organic solid materials for advanced optoelectronic applications. The fourth and last project investigates a new class of pyreno/phenanthroannulated 1,3-diaza-4-azulenones, which could be readily synthesized through a straightforward one-pot condensation approach. The molecular properties of these compounds were analyzed by single-crystal X-ray crystallography, UV-vis absorption, and fluorescence spectroscopy. Our results showed that the emission behavior of these compounds is highly sensitive to solvent polarity and acidity, rendering them novel fluorosolvatochromic and acidochromic dyes that can be potentially utilized in chemosensing and bioimaging applications

Advanced Materials and Technologies in Nanogenerators

This reprint discusses the various applications, new materials, and evolution in the field of nanogenerators. This lays the foundation for the popularization of their broad applications in energy science, environmental protection, wearable electronics, self-powered sensors, medical science, robotics, and artificial intelligence