Femtosecond quantum control of molecular dynamics in the condensed phase

We review the progress in controlling quantum dynamical processes in the condensed phase with femtosecond laser pulses. Due to its high particle density the condensed phase has both high relevance and appeal for chemical synthesis. Thus, in recent years different methods have been developed to manipulate the dynamics of condensed-phase systems by changing one or multiple laser pulse parameters. Single-parameter control is often achieved by variation of the excitation pulse's wavelength, its linear chirp or its temporal subpulse separation in case of pulse sequences. Multiparameter control schemes are more flexible and provide a much larger parameter space for an optimal solution. This is realized in adaptive femtosecond quantum control, in which the optimal solution is iteratively obtained through the combination of an experimental feedback signal and an automated learning algorithm. Several experiments are presented that illustrate the different control concepts and highlight their broad applicability. These fascinating achievements show the continuous progress on the way towards the control of complex quantum reactions in the condensed phase.     

An automated approach for the parameterization of accurate intermolecular force‐fields: Pyridine as a case study

An automated protocol is proposed and validated, which integrates accurate quantum mechanical calculations with classical numerical simulations. Intermolecular force fields, (FF) suitable for molecular dynamics (MD) and Monte Carlo simulations, are parameterized through a novel iterative approach, fully based on quantum mechanical data, which has been automated and coded into the PICKY software, here presented. The whole procedure is tested and validated for pyridine, whose bulk phase, described through MD simulations performed with the specifically parameterized FF, is characterized by computing several of its thermodynamic, structural, and transport properties, comparing them with their experimental counterparts. © 2011 Wiley Periodicals, Inc.     

Theoretical Shaping of Femtosecond Laser Pulses for Molecular Photodissociation with Control Techniques Based on Ehrenfest′s Dynamics and Time‐Dependent Density Functional Theory

The combination of nonadiabatic Ehrenfest‐path molecular dynamics (EMD) based on time‐dependent density functional theory (TDDFT) and quantum optimal control formalism (QOCT) was used to optimize the shape of ultra‐short laser pulses to achieve photodissociation of a hydrogen molecule and the trihydrogen cation H₃⁺. This work completes a previous one [A. Castro, ChemPhysChem, 2013 , 14, 1488–1495], in which the same objective was achieved by demonstrating the combination of QOCT and TDDFT for many‐electron systems on static nuclear potentials. The optimization model, therefore, did not include the nuclear movement and the obtained dissociation mechanism could only be sequential: fast laser‐assisted electronic excitation to nonbonding states (during which the nuclei are considered to be static), followed by field‐free dissociation. Here, in contrast, the optimization was performed with the QOCT constructed on top of the full dynamic model comprised of both electrons and nuclei, as described within EMD based on TDDFT. This is the first numerical demonstration of an optimal control formalism for a hybrid quantum–classical model, that is, a molecular dynamics method.     

Precise Polymer Synthesis by Autonomous Self‐Optimizing Flow Reactors

A novel continuous flow system for automated high‐throughput screening, autonomous optimization, and enhanced process control of polymerizations was developed. The computer‐controlled platform comprises a flow reactor coupled to size exclusion chromatography (SEC). Molecular weight distributions are measured online and used by a machine‐learning algorithm to self‐optimize reactions towards a programmed molecular weight by dynamically varying reaction parameters (i.e. residence time, monomer concentration, and control agent/initiator concentration). The autonomous platform allows targeting of molecular weights in a reproducible manner with unprecedented accuracy (<2.5 % deviation from pre‐selected goal) for both thermal and light‐induced reactions. For the first time, polymers with predefined molecular weights can be custom made under optimal reaction conditions in an automated, high‐throughput flow synthesis approach with outstanding reproducibility.     

Automatic Mechanism and Kinetic Model Generation for Gas‐ and Solution‐Phase Processes: A Perspective on Best Practices,Recent Advances,and Future Challenges

Completely automated mechanism generation of detailed kinetic models is within reach in the coming decade. The recent developments in this field of chemical reaction engineering are anticipated to lead to some groundbreaking discoveries in the future, extending our fundamental understanding and resolving many of today's society problems such as energy production and conversion, emission reduction, greener chemical production processes, etc. In the present review, the focus is on the core of these automated mechanism generation for gas‐phase and solution‐phase processes that is on how the reaction kinetics and thermodynamic and transport properties of species are estimated and calculated starting from the fundamental elements of the software. With tasks such as the definition of reaction rules and reaction families, the unambiguous representation of species, and the choice of different termination criteria, generating a good reaction mechanism is still not as simple as pressing a “run” button. One of the main challenges that still needs to be overcome is how to deal with data scarcity and the combination with affordable computational chemistry calculations seems the logical step forward. The best practices are illustrated in a butane pyrolysis case study, which also exposes the challenges in the field of automatic kinetic model generation.     

Site‐Targeted Interfacial Solid‐Phase Chemistry: Surface Functionalization of Organic Monolayers via Chemical Transformations Locally Induced at the Boundary between Two Solids

Effective control of chemistry at interfaces is of fundamental importance for the advancement of methods of surface functionalization and patterning that are at the basis of many scientific and technological applications. A conceptually new type of interfacial chemical transformations has been discovered, confined to the contact surface between two solid materials, which may be induced by exposure to X‐rays, electrons or UV light, or by the application of electrical bias. One of the reacting solids is a removable thin film coating that acts as a reagent/catalyst in the chemical modification of the solid surface on which it is applied. Given the diversity of thin film coatings that may be used as solid reagents/catalysts and the lateral confinement options provided by the use of irradiation masks, conductive AFM probes or stamps, and electron beams in such solid‐phase reactions, this approach is suitable for precise targeting of different desired chemical modifications to predefined surface sites spanning the macro‐ to nanoscale.     

Full‐Spectrum Persistent Luminescence Tuning Using All‐Inorganic Perovskite Quantum Dots

Applications of persistent luminescence phosphors as night or dark‐light vision materials in many technological fields have fueled up a growing demand for rational control over the emission profiles of the phosphors. This, however, remains a daunting challenge. Now a unique strategy is reported to fine‐tune the persistent luminescence by using all‐inorganic CsPbX₃ (X=Cl, Br, and I) perovskite quantum dots (PeQDs) as efficient light‐conversion materials. Full‐spectrum persistent luminescence with wavelengths covering the entire visible spectral region is achieved through tailoring of the PeQD band gap, in parallel with narrow bandwidth of PeQDs and highly synchronized afterglow decay owing to the single energy storage source. These findings break through the limitations of traditional afterglow phosphors, thereby opening up opportunities for persistent luminescence materials for applications such as a white‐emitting persistent light source and dark‐light multicolor displays.     

Multi‐Light‐Responsive Quantum Dot Sensitized Hybrid Micromotors with Dual‐Mode Propulsion

CdS quantum dots/C₆₀ tubular micromotors with chemical/multi‐light‐controlled propulsion and “on‐the‐fly” acceleration capabilities are described. In situ growth of CdS quantum dots on the outer fullerene layer imparts this layer with light‐responsive properties in connection to inner Pt, Pd or MnO₂ layers. This is the first time that visible light is used to drive bubble‐propelled tubular micromotors. The micromotors exhibit a broad absorption range from 320 to 670 nm and can be wirelessly controlled by modulating light intensity and peroxide concentration. The built‐in accelerating optical system allows for the control of the velocity over the entire UV/Vis light spectra by modulating the catalyst surface chemistry. The light‐responsive properties have been also exploited to accelerate the chemical dealloying and propulsion of micromotors containing a Cu/Pd layer. Such dual operated hybrid micromotors hold considerable promise for designing smart micromachines for on‐demand operations, motion‐based sensing, and enhanced cargo transportation.     

Efficient Light‐Induced pKa Modulation Coupled to Base‐Catalyzed Photochromism

Photoswitchable acid–base pairs, whose pK_a values can be reversibly altered, are attractive molecular tools to control chemical and biological processes with light. A significant, light‐induced pK_a change of three units in aqueous medium has been realized for two thermally stable states, which can be interconverted using UV and green light. The light‐induced pK_a modulation is based on incorporating a 3‐H‐thiazol‐2‐one moiety into the framework of a diarylethene photoswitch, which loses the heteroaromatic stabilization of the negatively charged conjugate base upon photochemical ring closure, and hence becomes significantly less acidic. In addition, the efficiency of the photoreactions is drastically increased in the deprotonated state, giving rise to catalytically enhanced photochromism. It appears that protonation has a significant influence on the shape of the ground‐ and excited‐state potential energy surfaces, as indicated by quantum‐chemical calculations.     

Diaminodicyanoquinones: Fluorescent Dyes with High Dipole Moments and Electron‐Acceptor Properties

Fluorescent dyes are applied in various fields of research, including solar cells and light‐emitting devices, and as reporters for assays and bioimaging studies. Fluorescent dyes with an added high dipole moment pave the way to nonlinear optics and polarity sensitivity. Redox activity makes it possible to switch the molecule's photophysical properties. Diaminodicyanoquinone derivatives possess high dipole moments, yet only low fluorescence quantum yields, and have therefore been neglected as fluorescent dyes. Here we investigate the fluorescence properties of diaminodicyanoquinones using a combined theoretical and experimental approach and derive molecules with a fluorescence quantum yield exceeding 90 %. The diaminodicyanoquinone core moiety provides chemical versatility and can be integrated into novel molecular architectures with unique photophysical features.     

Modeling Kinetics and Structural Properties in High‐Pressure Fluid‐Phase Polymerization

The present contribution provides an overview of actual applications in modeling free‐radical polymerizations. Topics of interest are the simulation of pulsed laser polymerization experiments with subsequent analysis of the formed product by size exclusion chromatography (PLP‐SEC), single pulse laser experiments, experimental techniques for determining rate coefficients of elementary reactions that control polymer properties, and technical applications. Aspects being investigated are model validation and testing predictive potential in polymerization models using well‐defined experiments as well as developing and testing experimental strategies for deriving rate coefficients of elementary reactions that exist (especially when dealing with copolymerizations) within a network of complex coupled reactions. In any of these fields remarkable success in modeling can be achieved. This demonstrates the great potential that can grow from combining modern mathematical methods, computational power and detailed kinetic insights into the mechanism of polymerization. It is the wide scope of applications, e. g. ranging from modeling kinetics to the investigation of termination processes being dependent on the chain‐length of the macroradical (as an example of pure fundamental research) to modeling of technical reactors, that provides attractiveness and defines challenges. Especially, the success in transforming results directly from laboratory experiments into technical applications justifies laborious efforts in determining highly precise rate coefficients and proves the concept breaking down a complex process into elementary subparts. A necessary boundary condition for this is keeping in mind the demands along the whole scope of applications and avoiding simplifications that are only applicable for part of them. Although at a first glance this may appear to hinder fast progress in one discipline, it is the essential requirement for final success.     

Conformational studies of dammarane‐type triterpenoids using computational and NMR spectroscopic methods

Natural triterpenoids are of great interest to researchers of various fields as they possess diverse physicochemical and biological properties. In medicinal chemistry, detailed information about the chemical structures of bioactive triterpenoids often helps find new lead compounds. Herein, the low‐energy structures of (20S)‐protopanaxadiol and (20S)‐protopanaxatriol, the aglycones of various triterpenoid saponins found in Panax ginseng, and their (20R)‐epimers have been predicted by the geometry optimization of the conformers extracted from molecular dynamics simulations with the self‐consistent‐charge density functional tight‐binding method. By performing quantum mechanical calculations on the low‐energy conformers, we have estimated the NMR chemical shifts of the compounds, which display good agreement with the most recently reported experimental values within an expected range of errors. Our results indicate that theoretical estimation of the NMR parameters of a relatively large molecule with a molecular mass of 500 is feasible. Copyright © 2015 John Wiley & Sons, Ltd.     

Perspective on Mechanism Development and Structure‐Activity Relationships for Gas‐Phase Atmospheric Chemistry

This perspective gives our views on general aspects and future directions of gas‐phase atmospheric chemical kinetic mechanism development, emphasizing on the work needed for the sustainable development of chemically detailed mechanisms that reflect current kinetic, mechanistic, and theoretical knowledge. Current and future mechanism development efforts and research needs are discussed, including software‐aided autogeneration and maintenance of kinetic models as a future‐proof approach for atmospheric model development. There is an overarching need for the evaluation and extension of structure‐activity relationships (SARs) that predict the properties and reactions of the many multifunctionalized compounds in the atmosphere that are at the core of detailed mechanisms, but for which no direct chemical data are available. Here, we discuss the experimental and theoretical data needed to support the development of mechanisms and SARs, the types of SARs relevant to atmospheric chemistry, the current status and limitations of SARs for various types of atmospheric reactions, the status of thermochemical estimates needed for mechanism development, and our outlook for the future. The authors have recently formed a SAR evaluation working group to address these issues.     

Automated Solid‐Phase Synthesis of a β‐(1,3)‐Glucan Dodecasaccharide

β‐Glucans are a group of structurally heterogeneous polysaccharides found in bacteria, fungi, algae and plants. β‐(1,3)‐D ‐Glucans have been studied in most detail due to their impact on the immune system of vertebrates. The studies into the immunomodulatory properties of these glucans are typically carried out with isolates that contain a heterogeneous mixture of polysaccharides of different chain lengths and varying degrees of branching. In order to determine the structure–activity relationship of β‐(1,3)‐glucans, access to homogeneous, structurally‐defined samples of these oligosaccharides that are only available through chemical synthesis is required. The syntheses of β‐glucans reported to date rely on the classical solution‐phase approach. We describe the first automated solid‐phase synthesis of a β‐glucan oligosaccharide that was made possible by innovating and optimizing the linker and glycosylating agent combination. A β‐(1,3)‐glucan dodecasaccharide was assembled in 56 h in a stereoselective fashion with an average yield of 88 % per step. This automated approach provides means for the fast and efficient assembly of linker‐functionalized mono‐ to dodecasaccharide β‐(1,3)‐glucans required for biological studies.     

Time‐dependent wave packet approach to Rabi oscillation in strong laser field

The phenomenon of Rabi oscillation is obtained in the investigation of the NO multiphoton ionization (MPI) in femtosecond laser fields. The “split operator‐Fourier transform” method of wave packet propagation is used in the representation of Rabi oscillation of population in each electronic state of NO molecule. The origin of Rabi oscillation and the effect on multiphoton ionization is analyzed. The high‐frequency oscillation riding on the top of Rabi oscillation is attributed to the nonrotating wave component in strong laser fields. Also illustrated is that through adjusting the pump‐probe delay time and the laser intensity appropriately the control of the ionization yield can be realized. This idea may be important for the laser control of chemical reaction. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 30–36, 2003     

Laser Spectroscopic Study of Cold Gas‐Phase Host‐Guest Complexes of Crown Ethers

The structure, molecular recognition, and inclusion effect on the photophysics of guest species are investigated for neutral and ionic cold host‐guest complexes of crown ethers (CEs) in the gas phase. Here, the cold neutral host‐guest complexes are produced by a supersonic expansion technique and the cold ionic complexes are generated by the combination of electrospray ionization (ESI) and a cryogenically cooled ion trap. The host species are 3n‐crown‐n (3nCn; n = 4, 5, 6, 8) and (di)benzo‐3n‐crown‐n ((D)B3nCn; n = 4, 5, 6, 8). For neutral guests, we have chosen water and aromatic molecules, such as phenol and benzenediols, and as ionic species we have chosen alkali‐metal ions (M⁺). The electronic spectra and isomer‐specific vibrational spectra for the complexes are observed with various laser spectroscopic methods: laser‐induced fluorescence (LIF); ultraviolet‐ultraviolet hole‐burning (UV‐UV HB); and IR‐UV double resonance (IR‐UV DR) spectroscopy. The obtained spectra are analyzed with the aid of quantum chemical calculations. We will discuss how the host and guest species change their flexible structures for forming best‐fit stable complexes (induced fitting) and what kinds of interactions are operating for the stabilization of the complexes. For the alkali metal ion?CE complexes, we investigate the solvation effect by attaching water molecules. In addition to the ground‐state stabilization problem, we will show that the complexation leads to a drastic effect on the excited‐state electronic structure and dynamics of the guest species, which we call a “cage‐like effect”.     

Porphyrin/Ionic‐Liquid Co‐assembly Polymorphism Controlled by Liquid–Liquid Phase Separation

Understanding and controlling multicomponent co‐assembly is of primary importance in different fields, such as materials fabrication, pharmaceutical polymorphism, and supramolecular polymerization, but these aspects have been a long‐standing challenge. Herein, we discover that liquid–liquid phase separation (LLPS) into ion‐cluster‐rich and ion‐cluster‐poor liquid phases is the first step prior to co‐assembly nucleation based on a model system of water‐soluble porphyrin and ionic liquids. The LLPS‐formed droplets serve as the nucleation precursors, which determine the resulting structures and properties of co‐assemblies. Co‐assembly polymorphism and tunable supramolecular phase transition behaviors can be achieved by regulating the intermolecular interactions at the LLPS stage. These findings elucidate the key role of LLPS in multicomponent co‐assembly evolution and enable it to be an effective strategy to control co‐assembly polymorphism as well as supramolecular phase transitions.     

Formation of Gas‐Phase Formate in Thermal Reactions of Carbon Dioxide with Diatomic Iron Hydride Anions

The hydrogenation of carbon dioxide involves the activation of the thermodynamically very stable molecule CO₂ and formation of a C−H bond. Herein, we report that HCO₂⁻ and CO can be formed in the thermal reaction of CO₂ with a diatomic metal hydride species, FeH⁻. The FeH⁻ anions were produced by laser ablation, and the reaction with CO₂ was analyzed by mass spectrometry and quantum‐chemical calculations. Gas‐phase HCO₂⁻ was observed directly as a product, and its formation was predicted to proceed by facile hydride transfer. The mechanism of CO₂ hydrogenation in this gas‐phase study parallels similar behavior of a condensed‐phase iron catalyst.     

All‐Optical Integrated Logic Operations Based on Chemical Communication between Molecular Switches

Molecular logic gates process physical or chemical “inputs” to generate “outputs” based on a set of logical operators. We report the design and operation of a chemical ensemble in solution that behaves as integrated AND, OR, and XNOR gates with optical input and output signals. The ensemble is composed of a reversible merocyanine‐type photoacid and a ruthenium polypyridine complex that functions as a pH‐controlled three‐state luminescent switch. The light‐triggered release of protons from the photoacid is used to control the state of the transition‐metal complex. Therefore, the two molecular switching devices communicate with one another through the exchange of ionic signals. By means of such a double (optical–chemical–optical) signal‐transduction mechanism, inputs of violet light modulate a luminescence output in the red/far‐red region of the visible spectrum. Nondestructive reading is guaranteed because the green light used for excitation in the photoluminescence experiments does not affect the state of the gate. The reset is thermally driven and, thus, does not involve the addition of chemicals and accumulation of byproducts. Owing to its reversibility and stability, this molecular device can afford many cycles of digital operation.