Showing papers in "ChemPhysChem in 2016"
TL;DR: A model spin‐vibronic Hamiltonian is used to reveal the crucial role of non‐Born‐Oppenheimer effects in determining k rISC and demonstrates that vibronic (nonadiabatic) coupling between the lowest local excitation triplet and lowest charge transfer triplet opens the possibility for significant second‐order coupling effects and increasesk rISC by about four orders of magnitude.
Abstract: Factors influencing the rate of reverse intersystem crossing (krISC ) in thermally activated delayed fluorescence (TADF) emitters are critical for improving the efficiency and performance of third-generation heavy-metal-free organic light-emitting diodes (OLEDs). However, present understanding of the TADF mechanism does not extend far beyond a thermal equilibrium between the lowest singlet and triplet states and consequently research has focused almost exclusively on the energy gap between these two states. Herein, we use a model spin-vibronic Hamiltonian to reveal the crucial role of non-Born-Oppenheimer effects in determining krISC . We demonstrate that vibronic (nonadiabatic) coupling between the lowest local excitation triplet (3 LE) and lowest charge transfer triplet (3 CT) opens the possibility for significant second-order coupling effects and increases krISC by about four orders of magnitude. Crucially, these simulations reveal the dynamical mechanism for highly efficient TADF and opens design routes that go beyond the Born-Oppenheimer approximation for the future development of high-performing systems.
489 citations
TL;DR: The historic overview and discusses the theoretical and experimental concepts behind FSRS, and emphasis is put on the current state-of-the-art experimental realization and several variations of FSRS that have been developed.
Abstract: Femtosecond stimulated Raman spectroscopy (FSRS) is an ultrafast nonlinear optical technique that provides vibrational structural information with high temporal (sub-50 fs) precision and high spectral (10 cm(-1) ) resolution. Since the first full demonstration of its capabilities ≈15 years ago, FSRS has evolved into a mature technique, giving deep insights into chemical and biochemical reaction dynamics that would be inaccessible with any other technique. It is now being routinely applied to virtually all possible photochemical reactions and systems spanning from single molecules in solution to thin films, bulk crystals and macromolecular proteins. This review starts with an historic overview and discusses the theoretical and experimental concepts behind this technology. Emphasis is put on the current state-of-the-art experimental realization and several variations of FSRS that have been developed. The unique capabilities of FSRS are illustrated through a comprehensive presentation of experiments to date followed by prospects.
204 citations
TL;DR: It is concluded that strongly coordinating Solvents will preferentially form species with a low number of iodide ions and less coordinative solvents generate high concentration of PbI6 (-) and proposed that all these plumbate ions may act as structural defects determining electronic properties of the photovoltaic films.
Abstract: We show the influence of species present in precursor solution during formation of lead halide perovskite materials on the structural defects of the films. The coordination of lead by competing solvent molecules and iodide ions dictate the type of complexes present in the films. Depending on the processing conditions all PbIS5+, PbI2S4, PbI3S3−, PbI4S22−, PbI5S23−, PbI64−and 1D (Pb2I4)n chains are observed by absorption measurements. Different parameters are studied such as polarity of the solvent, concentration of iodide ions, concentration of solvent molecules and temperature. It is concluded that strongly coordinating solvents will preferentially form species with a low number of iodide ions and less coordinative solvents generate high concentration of PbI6−. We furthermore propose that all these plumbate ions may act as structural defects determining electronic properties of the photovoltaic films.
144 citations
TL;DR: This contribution quantitatively discusses, based on examples, the errors that can be introduced by disregarding the exact solution based on Maxwell's equations and shows that these errors can easily exceed one order of magnitude.
Abstract: For spectroscopists, the (Bouguer-)Beer-Lambert law is unquestionably an essential principle, since it is inseparably linked with one of the most important quantities in spectroscopy, the absorbance. In spite of its importance, a quantitative discussion of the legitimacy of relating the transmittance, the quantity that is usually measured, to the absorbance by assuming a logarithmic relation between both quantities cannot be found in literature. In this contribution, we quantitatively discuss, based on examples, the errors that can be introduced by disregarding the exact solution based on Maxwell's equations and show that these errors can easily exceed one order of magnitude. We also re-derive the Beer-Lambert law, thereby providing guidance as how to convert transmittance into absorbance properly.
144 citations
TL;DR: Progress in the understanding of a new dynamic mode of symmetry breaking, based on chirality synchronisation of transiently chiral molecules in isotropic liquids and in bicontinuous cubic, columnar, smectic and nematic liquid crystalline phases is discussed.
Abstract: Spontaneous mirror symmetry breaking is an efficient way to obtain homogeneously chiral agents, pharmaceutical ingredients and materials. It is also in the focus of the discussion around the emergence of uniform chirality in biological systems. Tremendous progress has been made by symmetry breaking during crystallisation from supercooled melts or supersaturates solutions and by self-assembly on solid surfaces and in other highly ordered structures. However, recent observations of spontaneous mirror symmetry breaking in liquids and liquid crystals indicate that it is not limited to the well-ordered solid state. Herein, progress in the understanding of a new dynamic mode of symmetry breaking, based on chirality synchronisation of transiently chiral molecules in isotropic liquids and in bicontinuous cubic, columnar, smectic and nematic liquid crystalline phases is discussed. This process leads to spontaneous deracemisation in the liquid state under thermodynamic control, giving rise to long-term stable symmetry-broken fluids, even at high temperatures. These fluids form conglomerates that are capable of extraordinary strong chirality amplification, eventually leading to homochirality and providing a new view on the discussion of emergence of uniform chirality in prebiotic systems.
140 citations
TL;DR: The present Review investigates the processes and mechanisms that drive the chemical functionalization of graphene at the edges from theoretical and experimental perspectives, with a particular focus on the characterization tools available to investigate the chemistry at the edge.
Abstract: The selective functionalization of graphene edges is driven by the chemical reactivity of its carbon atoms. The chemical reactivity of an edge, as an interruption of the honeycomb lattice of graphene, differs from the relative inertness of the basal plane. In fact, the unsaturation of the pz orbitals and the break of the π conjugation on an edge increase the energy of the electrons at the edge sites, leading to specific chemical reactivity and electronic properties. Given the relevance of the chemistry at the edges in many aspects of graphene, the present Review investigates the processes and mechanisms that drive the chemical functionalization of graphene at the edges. Emphasis is given to the selective chemical functionalization of graphene edges from theoretical and experimental perspectives, with a particular focus on the characterization tools available to investigate the chemistry of graphene at the edge.
129 citations
TL;DR: It is shown that topotactic cation-exchange reactions are a promising alternative route to complex multinary Cu chalcogenide NCs and hetero-NCs, which are not attainable by conventional routes.
Abstract: Over the past few years, colloidal copper chalcogenide nanocrystals (NCs) have emerged as promising alternatives to conventional Cd and Pb chalcogenide NCs. Owing to their wide size, shape, and composition tunability, Cu chalcogenide NCs hold great promise for several applications, such as photovoltaics, lighting and displays, and biomedical imaging. They also offer characteristics that are unparalleled by Cd and Pb chalcogenide NCs, such as plasmonic properties. Moreover, colloidal Cu chalcogenide NCs have low toxicity, potentially lower costs, and excellent colloidal stability. This makes them attractive materials for the large-scale deployment of inexpensive, sustainable, and environmentally benign solution-processed devices. Nevertheless, the synthesis of colloidal Cu chalcogenide NCs, especially that of ternary and quaternary compositions, has yet to reach the same level of mastery as that available for the prototypical Cd chalcogenide based NCs. This review provides a concise overview of this rapidly advancing field, sketching the state of the art and highlighting the key challenges. We discuss recent developments in the synthesis of size-, shape-, and composition-controlled NCs of Cu chalcogenides, with emphasis in strategies to circumvent the limitations arising from the need to precisely balance the reactivities of multiple precursors in synthesizing ternary and quaternary compositions. In this respect, we show that topotactic cation-exchange reactions are a promising alternative route to complex multinary Cu chalcogenide NCs and hetero-NCs, which are not attainable by conventional routes. The properties and potential applications of Cu chalcogenide NCs and hetero-NCs are also addressed.
127 citations
TL;DR: It is shown by using the constraints of microscopic reversibility that the power stroke model is incorrect as an explanation of how chemical energy is used by a molecular machine to do mechanical work.
Abstract: The concept of a "power stroke"-a free-energy releasing conformational change-appears in almost every textbook that deals with the molecular details of muscle, the flagellar rotor, and many other biomolecular machines. Here, it is shown by using the constraints of microscopic reversibility that the power stroke model is incorrect as an explanation of how chemical energy is used by a molecular machine to do mechanical work. Instead, chemically driven molecular machines operating under thermodynamic constraints imposed by the reactant and product concentrations in the bulk function as information ratchets in which the directionality and stopping torque or stopping force are controlled entirely by the gating of the chemical reaction that provides the fuel for the machine. The gating of the chemical free energy occurs through chemical state dependent conformational changes of the molecular machine that, in turn, are capable of generating directional mechanical motions. In strong contrast to this general conclusion for molecular machines driven by catalysis of a chemical reaction, a power stroke may be (and often is) an essential component for a molecular machine driven by external modulation of pH or redox potential or by light. This difference between optical and chemical driving properties arises from the fundamental symmetry difference between the physics of optical processes, governed by the Bose-Einstein relations, and the constraints of microscopic reversibility for thermally activated processes.
117 citations
TL;DR: The results indicate the formation of polarons at low doping levels and the existence of bipolarons at high doping levels, both of which are indicated by the presented spectroscopic data.
Abstract: In this work, we focus on the formation of different kinds of charge carriers such as polarons and bipolarons upon p-type doping (oxidation) of the organic semiconductor poly(3- hexylthiophene-2,5-diyl) (P3HT). We elucidate the cyclic voltammogram during oxidation of this polymer and present spectroscopic changes upon doping in the UV/Vis/near-IR range as well as in the mid-IR range. In the low-oxidation regime, two absorption bands related to sub-gap transitions appear, one in the UV/Vis range and another one in the mid-IR range. The UV/Vis absorption gradually decreases upon further doping while the mid-IR absorption shifts to lower energy. Additionally, electron paramagnetic resonance (EPR) measurements are performed, showing an increase of the EPR signal up to a certain doping level, which significantly decreases upon further doping. Furthermore, the absorption spectra in the UV/Vis range are analyzed in relation to the morphology (crystalline vs. amorphous) by using theoretical models. Finally, the calculated charge carriers from cyclic voltammogram are linked together with optical transitions as well as with the EPR signals upon p-type doping. We stress that our results indicate the formation of polarons at low doping levels and the existence of bipolarons at high doping levels. The presented spectroscopic data are an experimental evidence of the formation of bipolarons in P3HT.
107 citations
TL;DR: This review summarizes recent advances in electrodeposition in ILs and focuses on its similarities and differences with that in aqueous solutions.
Abstract: Due to their attractive physico-chemical properties, ionic liquids (ILs) are increasingly used as deposition electrolytes. This review summarizes recent advances in electrodeposition in ILs and focuses on its similarities and differences with that in aqueous solutions. The electrodeposition in ILs is divided into direct and template-assisted deposition. We detail the direct deposition of metals, alloys and semiconductors in five types of ILs, including halometallate ILs, air- and water-stable ILs, deep eutectic solvents (DESs), ILs with metal-containing cations, and protic ILs. Template-assisted deposition of nanostructures and macroporous structures in ILs is also presented. The effects of modulating factors such as deposition conditions (current density, current density mode, deposition time, temperature) and electrolyte components (cation, anion, metal salts, additives, water content) on the morphology, compositions, microstructures and properties of the prepared materials are highlighted.
107 citations
TL;DR: This minireview traces the development of artificial molecular machines from their prototypes in the form of shuttles and switches to their emergence as motors and pumps where supplies of energy become a minimum requirement for them to function away from equilibrium.
Abstract: The past quarter of a century has witnessed an increasing engagement on the part of physicists and chemists in the design and synthesis of molecular machines de novo. This minireview traces the development of artificial molecular machines from their prototypes in the form of shuttles and switches to their emergence as motors and pumps where supplies of energy in the form of chemical fuel, electrochemical potential and light activation become a minimum requirement for them to function away from equilibrium. The challenge facing this rapidly growing community of scientists and engineers today is one of putting wholly synthetic molecules to work, both individually and as collections. Here, we highlight some of the recent conceptual and practical advances relating to the operation of wholly synthetic rotary and linear motors.
TL;DR: It is predicted that, by forming nanocomposites with another carbon nitride (C2 N), the energy efficiency of g-C3 N4 can be significantly improved and provide a potential route to achieve highly efficient metal-free photocatalysts for water splitting.
Abstract: Water-splitting photocatalysts with good energy efficiency are highly desirable, among which metal-free graphitic carbon nitride (g-C3N4) is considered to be very promising and has been intensively studied in recent years. However, its practical application is hindered by the relatively low efficiencies of visible-light absorption and electron–hole separation. Herein, based on first-principles calculations, it is predicted that, by forming nanocomposites with another carbon nitride (C2N), the energy efficiency of g-C3N4 can be significantly improved. On one hand, C2N has a wide, strong optical absorption in the visible-light region, which acts as a photosensitizer and enhances the photoabsorption efficiency of the composite photocatalyst. On the other hand, C2N forms a type II heterojunction with g-C3N4, which leads to efficient separation of photogenerated electron–hole pairs through the chemical potential difference between the two components. These results provide a potential route to achieve highly efficient metal-free photocatalysts for water splitting.
TL;DR: Direct spectroscopic evidence for hydrogen‐bonded clusters of like‐charged ions is reported for ionic liquids and DFT‐D3 calculations of the pure multiply charged clusters suggest that the attractive hydrogen bonds can compete with repulsive Coulomb forces.
Abstract: Direct spectroscopic evidence for hydrogen-bonded clusters of like-charged ions is reported for ionic liquids. The measured infrared O-H vibrational bands of the hydroxyethyl groups in the cations can be assigned to the dispersion-corrected DFT calculated frequencies of linear and cyclic clusters. Compensating the like-charge Coulomb repulsion, these cationic clusters can range up to cyclic tetramers resembling molecular clusters of water and alcohols. These ionic clusters are mainly present at low temperature and show strong cooperative effects in hydrogen bonding. DFT-D3 calculations of the pure multiply charged clusters suggest that the attractive hydrogen bonds can compete with repulsive Coulomb forces.
TL;DR: An experimentally feasible strategy for direct laser cooling of polyatomic molecules with six or more atoms relies on the attachment of a metal atom to a complex molecule, where it acts as an active photon cycling site.
Abstract: An experimentally feasible strategy for direct laser cooling of polyatomic molecules with six or more atoms is presented. Our approach relies on the attachment of a metal atom to a complex molecule, where it acts as an active photon cycling site. We describe a laser cooling scheme for alkaline earth monoalkoxide free radicals taking advantageof the phase space compression of a cryogenic buffer-gas beam. Possible applications are presented including laser cooling of chiral molecules and slowing of molecular beams using coherent photon processes.
TL;DR: Ab initio molecular dynamic simulations reveal significantly reduced ion charges in several choline-based deep eutectic solvents, which are cheap and eco-friendly alternatives to ionic liquids.
Abstract: Ab initio molecular dynamic simulations reveal significantly reduced ion charges in several choline-based deep eutectic solvents, which are cheap and eco-friendly alternatives to ionic liquids. Increasing hydrogen bond strength between the anion and the organic compound enhances charge spreading from the anion to the organic compound while the positive charge is stronger located at the cation. Nonetheless, the negative charge transferred from chloride to urea in choline chloride urea mixtures is negligible. Thus, it seems questionable if charge delocalization occurring through hydrogen bonding between the halide anion and the organic compound is responsible for the deep eutectic melting point.
TL;DR: The photoelectron angular distribution from randomly oriented fenchone molecules in the gas phase that result from ionization with circularly polarized femtosecond laser pulses is measured and quantification shows a linear dependence with respect to the ee values.
Abstract: Photoelectron circular dichroism (PECD) is experimentally investigated with chiral specimens with varying amounts of enantiomeric excess (ee). As a prototype, we measure and analyze the photoelectron angular distribution from randomly oriented fenchone molecules in the gas phase that result from ionization with circularly polarized femtosecond laser pulses. The quantification of these measurements shows a linear dependence with respect to the ee values. In addition, differences in the ee values (denoted as detection limit) of below one percent can be distinguished for nearly enantiopure samples, as well as for almost racemates. In combination with the use of a reference, the assignment of absolute ee values is possible. The present measurement time is a few minutes, but this could be reduced. This table-top laser-based approach should facilitate widespread implementation in chiral analysis.
TL;DR: Three major classes of nontoxic quantum dots composed of carbon, silicon and Group I-III-VI elements are selected and the myriad of synthetic strategies and surface modification methods to synthesize quantum dots compose of these material systems are discussed.
Abstract: Semiconductor nanocrystals, or quantum dots (QDs), are candidates for biological sensing, photovoltaics, and catalysis due to their unique photophysical properties. The most studied QDs are composed of heavy metals like cadmium and lead. However, this engenders concerns over heavy metal toxicity. To address this issue, numerous studies have explored the development of nontoxic (or more accurately less toxic) quantum dots. In this Review, we select three major classes of nontoxic quantum dots composed of carbon, silicon and Group I-III-VI elements and discuss the myriad of synthetic strategies and surface modification methods to synthesize quantum dots composed of these material systems.
TL;DR: The potential of utilizing chemi-/bioluminescence as excitation source in the PDT of cancer, besides combining new approaches to overcome the limitations of this mode of treatment, is demonstrated.
Abstract: Photodynamic therapy (PDT) of cancer is known for its limited number of side effects, and requires light, oxygen and photosensitizer. However, PDT is limited by poor penetration of light into deeply localized tissues, and the use of external light sources is required. Thus, researchers have been studying ways to improve the effectiveness of this phototherapy and expand it for the treatment of the deepest cancers, by using chemiluminescent or bioluminescent formulations to excite the photosensitizer by intracellular generation of light. The aim of this Minireview is to give a precis of the most important general chemi-/bioluminescence mechanisms and to analyze several studies that apply them for PDT. These studies have demonstrated the potential of utilizing chemi-/bioluminescence as excitation source in the PDT of cancer, besides combining new approaches to overcome the limitations of this mode of treatment.
TL;DR: This Communication presents a study on T49, a liquid-crystalline oligomer exhibiting the twist-bend nematic phase, which has a molecular length that is of comparable dimensions to the sub-10 nm pitch determined for CB9CB, and provides new insights into the differentiation between the nano- and macro-science for self-assembling supermolecular systems.
Abstract: The twist-bend nematic phase (NTB ) has been described as the structural link between the untilted uniaxial nematic phase (N) and the helical chiral nematic phase (N*). The NTB phase exhibits phenomena of fundamental importance to science, that is, 1) the spontaneous formation of a helical pitch on the nanometer scale in a fluid and 2) the spontaneous breaking of mirror symmetry, leading to the emergence of chiral domains in an achiral system. In this Communication, we present a study on T49 [bis(4-(9-(4-((4-cyanobenzoyl)oxy)phenyl)nonyl)phenyl) 4,4'-(nonane-1,9-diyl)dibenzoate], a liquid-crystalline oligomer exhibiting the twist-bend nematic phase, which has a molecular length that is of comparable dimensions to the sub-10 nm pitch determined for CB9CB, and provide new insights into the differentiation between the nano- and macro-science for self-assembling supermolecular systems.
TL;DR: A thorough thermodynamic analysis of the enthalpic and entropic contributions to the overall free energy of PNC formation derived from three independent methods demonstrates that solute clustering is driven by entropy.
Abstract: The understanding of the molecular mechanisms underlying the early stages of crystallisation is still incomplete. In the case of calcium carbonate, experimental and computational evidence suggests that phase separation relies on so-called pre-nucleation clusters (PNCs). A thorough thermodynamic analysis of the enthalpic and entropic contributions to the overall free energy of PNC formation derived from three independent methods demonstrates that solute clustering is driven by entropy. This can be quantitatively rationalised by the release of water molecules from ion hydration layers, explaining why ion association is not limited to simple ion pairing. The key role of water release in this process suggests that PNC formation should be a common phenomenon in aqueous solutions.
TL;DR: This Minireview focuses on the hemithioindigo photoswitch and its use for the reversible control of three-dimensional peptide structure and related biological functions and a diverse range of chromopeptides with photoswitchable β-hairpin structures based on azobenzenes, stilbenes, and hemithiaindigo are compared.
Abstract: This Minireview focuses on the hemithioindigo photoswitch and its use for the reversible control of three-dimensional peptide structure and related biological functions. Both the general design aspects and biophysical properties of various hemithioindigo-based chromopeptides are summarized. Hemithioindigo undergoes reversible Z→E photoisomerization after absorption of visible light. The unique ultrafast switching mechanism of hemithioindigo combines picosecond isomerization kinetics with strong double-bond torsion after light absorption, making it the ideal tool for instantaneous modulation of biological structure. Various inhibitors and model peptides based on hemithioindigo are described that can directly regulate biological signaling or allow the fastest events in peptide folding to be studied. Finally, a diverse range of chromopeptides with photoswitchable β-hairpin structures based on azobenzenes, stilbenes, and hemithioindigo are compared to emphasize the unique properties of hemithioindigo.
TL;DR: This review covers the principles and aspects of nanothermometer design driven by two emerging areas: single-cell thermogenesis and image guided thermal treatments, and highlights the current trends innanothermometry illustrated with recent representative examples.
Abstract: Measuring temperature in cells and tissues remotely, with sufficient sensitivity, and in real time presents a new paradigm in engineering, chemistry and biology. Traditional sensors, such as contact thermometers, thermocouples, and electrodes, are too large to measure the temperature with subcellular resolution and are too invasive to measure the temperature in deep tissue. The new challenge requires novel approaches in designing biocompatible temperature sensors-nanothermometers-and innovative techniques for their measurements. In the last two decades, a variety of nanothermometers whose response reflected the thermal environment within a physiological temperature range have been identified as potential sensors. This review covers the principles and aspects of nanothermometer design driven by two emerging areas: single-cell thermogenesis and image guided thermal treatments. The review highlights the current trends in nanothermometry illustrated with recent representative examples.
TL;DR: This is the first CB-based molecular shuttle with an RTP signal as the output address of its shuttling and conformation and the switching RTP emission of the molecular shuttle, via pH adjusting, can be visibly identified by the naked eye.
Abstract: A visible room-temperature phosphorescence (RTP) signal, generated by complexation of cururbit[7]uril (CB[7]) and bromo-substituted isoquinoline in aqueous solution, is employed to address the shuttling of a pH-controlling molecular shuttle fabricated by CB[7] and a phosphor 6-bromoisoquinoline derivative IQC[5]. The CB[7] host shuttles along the axial guest under acidic conditions, accompanied by a weak RTP emission signal, while deprotonation of the guest IQC[5] makes the CB[7] wheel locate on the phosphor group, leading to intense RTP emission. The switching RTP emission of the molecular shuttle, via pH adjusting, can be visibly identified by the naked eye. This is the first CB-based molecular shuttle with an RTP signal as the output address of its shuttling and conformation.
TL;DR: This Minireview will focus on the controlled release of pharmaceutically active agents in biomedical applications by describing nanomachines that are supported on mesoporous silica nanoparticles that have an array of open, readily accessible pores with an average width of a few nanometers.
Abstract: Many machines (including nanomachines) consist of a solid support with moving parts that can undergo large amplitude motion to carry out specific tasks. In this Minireview, we will describe nanomachines that are supported on mesoporous silica nanoparticles that are typically 50-100 nanometers in diameter and have an array of open, readily accessible pores with an average width of a few nanometers. For triggering a large amplitude motion of the moving parts, we will focus primarily on external stimuli such as heat or light. As for the specific task the machines are carrying out, this Minireview will focus on the controlled release of pharmaceutically active agents in biomedical applications. We will discuss examples of how nanomachines can be used for remotely controlled cargo release and how existing machines that were originally designed to respond to internal physiological stimuli could be reconfigured to respond to external stimuli instead.
TL;DR: A vibronic multilevel Redfield model is reported for excitonically coupled electronic two‐level systems with a few explicitly included vibrational modes and interacting with a phonon bath, which results in faster energy transfer and longer‐lived mixed coherences.
Abstract: Nuclear vibrations play a prominent role in the spectroscopy and dynamics of electronic systems. As recent experimental and theoretical studies suggest, this may be even more so when vibrational frequencies are resonant with transitions between the electronic states. Herein, a vibronic multilevel Redfield model is reported for excitonically coupled electronic two-level systems with a few explicitly included vibrational modes and interacting with a phonon bath. With numerical simulations the effects of the quantized vibrations on the dynamics of energy transfer and coherence in a model dimer are illustrated. The resonance between the vibrational frequency and energy gap between the sites leads to a large delocalization of vibronic states, which then results in faster energy transfer and longer-lived mixed coherences.
TL;DR: NBO analysis showed that orbital effects are modest contributors to the global stabilization of the pnicogen σ-hole bonded complexes studied, proving the favorable nature of this counterintuitive binding mode in the solid state.
Abstract: The ability of several pnicogen sp(3) derivatives ZF3 (Z=N, P, As, Sb) to interact with electron-rich entities by means of the opposite face to the lone pair (lp) is investigated at the RI-MP2/aug-cc-pVQZ level of theory. The strength of the interaction ranges from -1 to -87 kJ mol(-1) , proving its favorable nature, especially when the lp is coordinated to a metal center, whereby the strength of the interaction is significantly enhanced. NBO analysis showed that orbital effects are modest contributors to the global stabilization of the pnicogen σ-hole bonded complexes studied. Finally, a selection of Cambridge Structural Database examples are shown that demonstrate the impact of this counterintuitive binding mode in the solid state.
TL;DR: A new experimental approach to study ion-molecule reactions at low temperatures and it is demonstrated that the Rydberg electron acts as a spectator and does not affect the cross sections, which are found to closely follow a classical Langevin-capture model in the collision energy range investigated.
Abstract: Studies of ion-molecule reactions at low temperatures are difficult because stray electric fields in the reaction volume affect the kinetic energy of charged reaction partners. We describe a new experimental approach to study ion-molecule reactions at low temperatures and present, as example, a measurement of the H2++H2→H3++H reaction with the H2+ ion prepared in a single rovibrational state at collision energies in the range Ecol /kB =5-60 K. To reach such low-collision energies, we use a merged-beam approach and observe the reaction within the orbit of a Rydberg electron, which shields the ions from stray fields. The first beam is a supersonic beam of pure ground-state H2 molecules and the second is a supersonic beam of H2 molecules excited to Rydberg-Stark states of principal quantum number n selected in the range 20-40. Initially, the two beams propagate along axes separated by an angle of 10°. To merge the two beams, the Rydberg molecules in the latter beam are deflected using a surface-electrode Rydberg-Stark deflector. The collision energies of the merged beams are determined by measuring the velocity distributions of the two beams and they are adjusted by changing the temperature of the pulsed valve used to generate the ground-state H2 beam and by adapting the electric-potential functions applied to the electrodes of the deflector. The collision energy is varied down to below Ecol /kB =10 K, that is, below Ecol ≈1 meV, with an energy resolution of 100 μeV. We demonstrate that the Rydberg electron acts as a spectator and does not affect the cross sections, which are found to closely follow a classical Langevin-capture model in the collision energy range investigated. Because all neutral atoms and molecules can be excited to Rydberg states, this method of studying ion-molecule reactions is applicable to other reactions involving singly charged cations.
TL;DR: The mechanism of the photoisomerisation of azobenzene has been studied by means of multiconfigurational ab initio calculations and it is found that the state populated by the initial excitation is the S2 state, which decays very efficiently to the S1 (nπ*) state at a pedal-like non-rotated geometry.
Abstract: The mechanism of the photoisomerisation of azobenzene has been studied by means of multiconfigurational ab initio calculations. Our results show that it is necessary to account for the dynamic electron correlation in the location of the critical points (CASPT2 optimizations) to obtain a correct description of the topography of the potential energy surfaces of the low energy singlet excited states. By using this methodology, we have found that the state populated by the initial excitation is the S2 (ππ*) state, which decays very efficiently to the S1 (nπ*) state at a pedal-like non-rotated geometry. In the S1 state, relaxation leads to a rotated geometry where the system decays to the ground state, in which further relaxation can lead to either the trans or cis geometries. However, the S1 /S0 conical intersection seam also extends to planar geometries, so this reaction path is also accessible for rotation-constrained systems. Our results explain the experimental observations satisfactorily.
TL;DR: The chemistry of the synthesis of these materials, their photophysical processes and the expected origins of their red emission are summarized in this Minireview, and one particular case is elaborated on in detail.
Abstract: Light-emitting Mn-doped semiconductor nanocrystals have been extensively studied for the last three decades for their intense and stable Mn d–d emission. In principle, this emission should be fixed at 585 nm (yellow), but recent studies have shown that the emission can be widely tuned even to 650 nm (red). This is a spectacular achievement as this would make Mn-doped nanocrystals efficient and tunable light emitters. Keeping these developments in view, the chemistry of the synthesis of these materials, their photophysical processes and the expected origins of their red emission are summarized in this Minireview. All the related important studies from 1992 onwards are chronologically discussed, and one particular case is elaborated on in detail. As these materials are potentially important for biology, and photovoltaic, sensing and light-emitting devices, this Minireview is expected to help researchers investigating the chemistry, physics and applications of these materials.