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This first introduction to the rapidly growing field of molecular magnetism is written with Masters and PhD students in mind, while postdocs and other newcomers will also find it an extremely useful guide. Adopting a clear didactic approach, the authors cover the fundamental concepts, providing many examples and give an overview of the most important techniques and key applications. Although the focus is one lanthanide ions, thus reflecting the current research in the field, the principles and the methods equally apply to other systems. The result is an excellent textbook from both a scientific and pedagogic point of view.

The Debye–Waller factor, introduced a century ago, remains a fundamental component in the refinement of crystal structures against X-ray, neutron and electron diffraction data. This review marks its centenary by exploring its applications in small-molecule crystallography. We provide a historical overview of the development of the Debye–Waller factor and its foundations in lattice dynamics. The review discusses the practical use of anisotropic displacement parameters and their role in accurate structure determination. We also address the challenges and advancements in modelling thermal motion and disorder, the role of multi-temperature measurements and modern computational approaches.

In this review, we present a survey on the use of molecular nonlinear crystals in the context of terahertz (THz) photonics. The fundamentals of nonlinear optics for converting optical and infrared radiation into THz radiation with the basic theory of femtosecond optical rectification and difference frequency generation are described. Various types of phase-matching conditions that can be achieved in molecular crystals are discussed. It is shown that one of the unique features of molecular crystals is the ability to generate tunable narrowband terahertz radiation using femtosecond lasers. We also provide a detailed description of the most commonly used and promising molecular crystals such as DAST, DSTMS, OH1, HMQ-TMS, DCMBI, and GUHP. This review also presents a description of recent publications which show the prospects of using molecular nonlinear optical crystals in THz photonics.

Dynamic molecular crystals have recently received ample attention as an emerging class of energy-transducing materials, yet have fallen short of developing into fully realized actuators. Through the trans–cis surface isomerization of three crystalline azobenzene materials, here, we set out to extensively characterize the light-to-work energy conversion of photoinduced bending in molecular crystals. We distinguish the azobenzene single crystals from commonly used actuators through quantitative performance evaluation and specific performance indices. Bending molecular crystals have an operating range comparable to that of microactuators such as microelectromechanical systems and a work-generating capacity and dynamic performance that qualifies them to substitute micromotor drivers in mechanical positioning and microgripping tasks. Finite element modeling, applied to determine the surface photoisomerization parameters, allowed for predicting and optimizing the mechanical response of these materials. Utilizing mechanical characterization and numerical simulation tools proves essential in accelerating the introduction of dynamic molecular crystals into soft microrobotics applications.

Vibrational properties of molecular crystals are constantly used as structural fingerprints, in order to identify both the chemical nature and the structural arrangement of molecules. The simulation of these properties is typically very costly, especially when dealing with response properties of materials to e.g. electric fields, which require a good description of the perturbed electronic density. In this work, we use Gaussian process regression (GPR) to predict the static polarizability and dielectric susceptibility of molecules and molecular crystals. We combine this framework with ab initio molecular dynamics to predict their anharmonic vibrational Raman spectra. We stress the importance of data representation, symmetry, and locality, by comparing the performance of different flavors of GPR. In particular, we show the advantages of using a recently developed symmetry-adapted version of GPR. As an examplary application, we choose Paracetamol as an isolated molecule and in different crystal forms. We obtain accurate vibrational Raman spectra in all cases with fewer than 1000 training points, and obtain improvements when using a GPR trained on the molecular monomer as a baseline for the crystal GPR models. Finally, we show that our methodology is transferable across polymorphic forms: we can train the model on data for one crystal structure, and still be able to accurately predict the spectrum for a second polymorph. This procedure provides an independent route to access electronic structure properties when performing force-evaluations on empirical force-fields or machine-learned potential energy surfaces.

Since the synthesis of the first krypton compound, several other Kr-bearing connections have been obtained. However, in all of them krypton adopts the +2 oxidation state, in contrast to xenon which forms numerous compounds with an oxidation state as high as +8. Motivated by the possibility of thermodynamic stabilization of exotic compounds with the use of high pressure (exceeding 1 GPa = 10 kbar), we present here theoretical investigations into the chemistry of krypton and fluorine at such large compression. In particular we focus on krypton tetrafluoride, KrF4, a molecular crystal in which krypton forms short covalent bonds with neighboring fluorine atoms thus adopting the +4 oxidation state. We find that this hitherto unknown compound can be stabilized at pressures below 50 GPa. Our results indicate also that, at larger compressions, a multitude of other KrmFn fluorides should be stable, among them KrF which exhibits covalent Kr–Kr bonds. Our results set the stage for future high-pressure synthesis of novel krypton compounds.

Porous crystals made to order Controlling and predicting the structural properties of porous molecular crystals would have important implications in gas adsorption, separation and catalysis applications, but remain an unmet goal. This paper introduces a new concept of modular assembly at the molecular level for the formation of porous crystalline solids. Different large chiral molecules with intrinsic nanosize pores, or porous modules, self-assemble through chiral recognition during co-crystallization to produce solid porous frameworks. The three-dimensional structure of the final material can be predicted theoretically. The paper explores four different, albeit analogous, porous modules, which form four different porous solids. Nanoporous molecular frameworks 1 , 2 , 3 , 4 , 5 , 6 , 7 are important in applications such as separation, storage and catalysis. Empirical rules exist for their assembly but it is still challenging to place and segregate functionality in three-dimensional porous solids in a predictable way. Indeed, recent studies of mixed crystalline frameworks suggest a preference for the statistical distribution of functionalities throughout the pores 7 rather than, for example, the functional group localization found in the reactive sites of enzymes 8 . This is a potential limitation for ‘one-pot’ chemical syntheses of porous frameworks from simple starting materials. An alternative strategy is to prepare porous solids from synthetically preorganized molecular pores 9 , 10 , 11 , 12 , 13 , 14 , 15 . In principle, functional organic pore modules could be covalently prefabricated and then assembled to produce materials with specific properties. However, this vision of mix-and-match assembly is far from being realized, not least because of the challenge in reliably predicting three-dimensional structures for molecular crystals, which lack the strong directional bonding found in networks. Here we show that highly porous crystalline solids can be produced by mixing different organic cage modules that self-assemble by means of chiral recognition. The structures of the resulting materials can be predicted computationally 16 , 17 , allowing in silico materials design strategies 18 . The constituent pore modules are synthesized in high yields on gram scales in a one-step reaction. Assembly of the porous co-crystals is as simple as combining the modules in solution and removing the solvent. In some cases, the chiral recognition between modules can be exploited to produce porous organic nanoparticles. We show that the method is valid for four different cage modules and can in principle be generalized in a computationally predictable manner based on a lock-and-key assembly between modules.

Cytoskeletal active nematics exhibit striking nonequilibrium dynamics that are powered by energy-consuming molecular motors. To gain insight into the structure and mechanics of these materials, we design programmable clusters in which kinesin motors are linked by a double-stranded DNA linker. The efficiency by which DNA-based clusters power active nematics depends on both the stepping dynamics of the kinesin motors and the chemical structure of the polymeric linker. Fluorescence anisotropy measurements reveal that the motor clusters, like filamentous microtubules, exhibit local nematic order. The properties of the DNA linker enable the design of force-sensing clusters. When the load across the linker exceeds a critical threshold, the clusters fall apart, ceasing to generate active stresses and slowing the system dynamics. Fluorescence readout reveals the fraction of bound clusters that generate interfilament sliding. In turn, this yields the average load experienced by the kinesin motors as they step along the microtubules. DNA-motor clusters provide a foundation for understanding the molecular mechanism by which nanoscale molecular motors collectively generate mesoscopic active stresses, which in turn power macroscale nonequilibrium dynamics of active nematics.

The luminescence color of molecule‐based photoactive materials is the key to the applications in lighting and optical communication. Realizing continuous regulation of emission color in molecular systems is highly desirable but still remains a challenge due to the individual emission band of purely organic molecules. Herein, a novel alloy strategy based on molecular co‐crystals is reported. By adjusting the molar ratio of pyrene (Py) and fluorathene (Flu), three types of molecular co‐crystal alloys (MCAs) assemblies are prepared involving Py‐Flu‐OFN‐x%, Py‐Flu‐TFP‐x%, Py‐Flu‐TCNB‐x%. Multiple energy level structure and Förster resonance energy transfer (FRET) process endow materials with tunable full‐spectra emission color in visible region. Impressively, these MCAs and co‐crystals can be successfully applied to low optical loss waveguide and optical logic gate by virtue of all‐color luminescence from blue across green to red, together with smooth surface of one‐dimensional microrods, which show promising applications as continuous light emitters for advance photonics applications. By using a molecular organic alloys (MCAs) strategy, several MCAs are prepared through tuning the stoichiometric ratio of pyrene and fluoranthene. Multiple energy level structure and Förster Resonance Energy Transfer (FRET) endow materials with tunable emission. Owing to smooth morphology and weak self‐absorption, prepared co‐crystals and molecular organic alloys are capable of low‐loss optical waveguide and optical logic gate.

Ferroelectricity goes organic Ferroelectric compounds have a range of properties useful in practical applications, including polarity reversal in electric fields, temperature sensitivity and the ability to convert mechanical stress into electricity. It is generally assumed that ferroelectricity is rare and mostly poor in organic compounds, but Horiuchi et al . now report the discovery of above-room-temperature ferroelectricity with relatively high polarization in the organic crystal croconic acid, a component of black dyes. These properties are not readily apparent in the crystalline form of this simple molecule, but they emerge on application of a modest electric field that induces a molecular-topological keto-enol conversion. This finding raises the prospect that organic ferroelectrics might be much more abundant that previously thought. Ferroelectrics are electro-active materials that can store and switch their polarity, sense temperature changes, interchange electric and mechanical functions, and manipulate light. Subtle changes in the topology of certain chemical bonds have long been identified as a possible route for achieving ferroelectricity in organic molecular crystals. Ferroelectricity above room temperature is now demonstrated by applying an electric field to coherently align the molecular polarities in crystalline croconic acid. Ferroelectrics are electro-active materials that can store and switch their polarity (ferroelectricity), sense temperature changes (pyroelectricity), interchange electric and mechanical functions (piezoelectricity), and manipulate light (through optical nonlinearities and the electro-optic effect): all of these functions have practical applications. Topological switching of π-conjugation in organic molecules, such as the keto-enol transformation, has long been anticipated as a means of realizing these phenomena in molecular assemblies and crystals 1 . Croconic acid, an ingredient of black dyes 2 , was recently found to have a hydrogen-bonded polar structure in a crystalline state 3 . Here we demonstrate that application of an electric field can coherently align the molecular polarities in crystalline croconic acid, as indicated by an increase of optical second harmonic generation, and produce a well-defined polarization hysteresis at room temperature. To make this simple pentagonal molecule ferroelectric, we switched the π-bond topology using synchronized proton transfer instead of rigid-body rotation. Of the organic ferroelectrics, this molecular crystal exhibits the highest spontaneous polarization (∼20 μC cm -2 ) in spite of its small molecular size, which is in accord with first-principles electronic-structure calculations. Such high polarization, which persists up to 400 K, may find application in active capacitor and nonlinear optics elements in future organic electronics.