Crystalline Molecular Solids Research Articles

Unraveling the Complex Solid-State Phase Transition Behavior of 1-Iodoadamantane, a Material for Which Ostensibly Identical Crystals Undergo Different Transformation Pathways.

Phase transitions in crystalline molecular solids have important implications in the fundamental understanding of materials properties and in the development of materials applications. Herein, we report the solid-state phase transition behavior of 1-iodoadamantane (1-IA) investigated using a multi-technique strategy [synchrotron powder X-ray diffraction (XRD), single-crystal XRD, solid-state NMR, and differential scanning calorimetry (DSC)], which reveals complex phase transition behavior on cooling from ambient temperature to ca. 123 K and on subsequent heating to the melting temperature (348 K). Starting from the known phase of 1-IA at ambient temperature (phase A), three low-temperature phases are identified (phases B, C, and D); the crystal structures of phases B and C are reported, together with a re-determination of the structure of phase A. Remarkably, single-crystal XRD shows that some individual crystals of phase A transform to phase B, while other crystals of phase A transform instead to phase C. Results (from powder XRD and DSC) on cooling a powder sample of phase A are fully consistent with this behavior while also revealing an additional transformation pathway from phase A to phase D. Thus, on cooling, a powder sample of phase A transforms partially to phase C (at 229 K), partially to phase D (at 226 K) and partially to phase B (at 211 K). During the cooling process, each of the phases B, C, and D is formed directly from phase A, and no transformations are observed between phases B, C, and D. On heating the resulting triphasic powder sample of phases B, C, and D from 123 K, phase B transforms to phase D (at 211 K), followed by the transformation of phase D to phase C (at 255 K), and finally, phase C transforms to phase A (at 284 K). From these observations, it is apparent that different crystals of phase A, which are ostensibly identical at the level of information revealed by XRD, must actually differ in other aspects that significantly influence their low-temperature phase transition pathways. This unusual behavior will stimulate future studies to gain deeper insights into the specific properties that control the phase transition pathways in individual crystals of this material.

Enantiotropy of Simvastatin as a Result of Weakened Interactions in the Crystal Lattice: Entropy-Driven Double Transitions and the Transient Modulated Phase as Seen by Solid-State NMR Spectroscopy.

In crystalline molecular solids, in the absence of strong intermolecular interactions, entropy-driven processes play a key role in the formation of dynamically modulated transient phases. Specifically, in crystalline simvastatin, the observed fully reversible enantiotropic behavior is associated with multiple order–disorder transitions: upon cooling, the dynamically disordered high-temperature polymorphic Form I is transformed to the completely ordered low-temperature polymorphic Form III via the intermediate (transient) modulated phase II. This behavior is associated with a significant reduction in the kinetic energy of the rotating and flipping ester substituents, as well as a decrease in structural ordering into two distinct positions. In transient phase II, the conventional three-dimensional structure is modulated by periodic distortions caused by cooperative conformation exchange of the ester substituent between the two states, which is enabled by weakened hydrogen bonding. Based on solid-state NMR data analysis, the mechanism of the enantiotropic phase transition and the presence of the transient modulated phase are documented.

Photochemical Control of the Mechanical and Adhesive Properties of Crystalline Molecular Solids

Photochemical Control of the Mechanical and Adhesive Properties of Crystalline Molecular Solids

Luminescence in Crystalline Organic Materials: From Molecules to Molecular Solids

AbstractLuminescent small, all‐organic molecules are of tremendous interest in materials and life science applications. Nevertheless, targeted design requires a basic understanding of the excited state deactivation pathways of the molecules themselves, and the modulations of the processes that occur in the solid state. This particularly concerns crystalline molecular solids, as here not only solid‐state rigidification contributes to these modulations, but specific intermolecular interactions as well. Starting from the molecular properties, this work carefully disentangles all intramolecular and intermolecular factors to the radiative and nonradiative processes in crystalline all‐organic molecular solids to provide guidelines for targeted molecular materials design.

Role of imidazole edge to edge supramolecular interaction in the crystal packing of Cu(II)(SCN−)2(imidazole)2 complex: A novel variety of supramolecular interaction revealed by CCDC database analysis and explored through DFT computational studies

Crystalline molecular solids are self-assembled entities in which molecular components are packed by various intermolecular forces. A novel edge to edge supramolecular interaction between the imidazole molecules has been observed in the crystal packing of the coordination complexes Cu(II)(SCN−)2(imidazole)2. In this edge to edge interaction, one of the C-N bonds of an imidazole molecule exhibit non-bonded interaction with the complementary C-N bond of another imidazole molecule with no facial overlap between the imidazole molecules. This edge-to-edge interaction of imidazole molecules along with NH···S, CH···S interaction governs the packing of molecular complexes in the solid state. Basis set superposition error corrected interaction energy of this edge to edge supramolecular interaction of imidazole has been estimated to be -2.59 kcal/mole. The electronic nature of this novel intermolecular interaction has been computed through DFT methodology. A CCDC database search has revealed that, a sizable number of crystals containing imidazole moiety shows this type of edge to edge supramolecular interaction. Hirshfeld surface analysis shows that this edge to edge interaction play cooperative role along with other intermolecular forces in the crystal packing of the present complex.

Concurring Chalcogen- and Halogen-Bonding Interactions in Supramolecular Polymers for Crystal Engineering Applications.

The engineering of crystalline molecular solids through the simultaneous combination of distinctive non-covalent interactions is an important field of research, as it could allow chemist to prepare materials depicting multi-responsive properties. It is in this context that, pushed by a will to expand the chemical space of chalcogen-bonding interactions, a concept is put forward for which chalcogen- and halogen-bonding interactions can be used simultaneously to engineer multicomponent co-crystals. Through the rational design of crystallizable molecules, chalcogenazolo pyridine scaffold (CGP) modules were prepared that, bearing either a halogen-bond acceptor or donor at the 2-position, can interact with suitable complementary molecular modules undergoing formation of supramolecular polymers at the solid state. The recognition reliability of the CGP moiety to form chalcogen-bonded dimers allows the formation of heteromolecular supramolecular polymers through halogen-bonding interactions, as confirmed by single-crystal X-ray diffraction analysis.

On the origin of exothermic DSC peak prior to glycine-nitrate gel combustion during ThO2 synthesis

Nature of interaction between Th4+, glycine, and nitrate ions and associated thermo chemical-changes prior to glycine-nitrate gel combustion for the synthesis of ThO2 powder have been investigated by DSC and XRD techniques, combined with FT-IR and Raman spectroscopic measurements. Combustion of dried gel is preceded by a distinct exothermic process; origin of which has been probed by systematic analysis of DSC thermograms. Mono- and multi-nuclear complexes of thorium with glycine exist in the gel before combustion and their relative concentrations depend on the starting stoichiometry, heat treatment temperature and extent of water content. Depending on the rate at which water gets removed from the gel, complexes mentioned above undergo rapid or slow aggregation to form amorphous or crystalline molecular solids. Observed additional exothermic peak in DSC thermogram prior to combustion of the gel confirms the formation of such molecular solids.

Calculation of excess free energy of molecular solids comprised of flexible molecules using Einstein molecule method

Knowledge of free energies of crystalline solid phases is pivotal in determination of phase equilibria; however, their computations using molecular simulations are quite challenging. In this paper, we apply the Einstein molecule method to compute the excess free energies of crystalline molecular solids comprising of molecules described by flexible models. This work can be viewed as an extension of the method described in Aragones et al. [J Chem Phys. 2013;139:a034104]. In this article, we present our calculations of the free energies of ice-Ih described by q-TIP4P/F model and Orcinol Form I described by the OPLS-AA forcefield. The free energies computed for ice-Ih are in agreement with previous reported results, and those of Orcinol are shown to be thermodynamically consistent. We demonstrate that the efficiency of our calculations compares favourably with existing methods. The method described here can be easily implemented in popular publicly available molecular dynamics packages without any modification to their existing source codes.

Aromatic C-Nitroso Compounds and Their Dimers: A Model for Probing the Reaction Mechanisms in Crystalline Molecular Solids

This review is focused on the dimerization and dissociation of aromatic C-nitroso compounds and their dimers, the reactions that could be used as a convenient model for studying the thermal organic solid-state reaction mechanisms. This molecular model is simple because it includes formation or breaking of only one covalent bond between two nitrogen atoms. The crystalline molecular solids of nitroso dimers (azodioxides) dissociate by photolysis under the cryogenic conditions, and re-dimerize by slow warming. The thermal re-dimerization reaction is examined under the different topotactic conditions in crystals: disordering, surface defects, and phase transformations. Depending on the conditions, and on the molecular structure, aromatic C-nitroso compounds can associate to form one-dimensional polymeric structures and are able to self-assemble on gold surfaces.

Synthesis and crystal structure of N,N-dimethylformamide solvate of thiocyanuric acid

Considerable attention has been paid recently to crystal engineering; which involves the design and preparation of new crystalline molecular solids with desired properties [1–4]. Crystalline materials with specific properties find applications in petrochemical industry for separation and purification. Moreover, crystal engineering provides products designed for manufacturing catalysts and high-valued chemicals for specific purposes. Recently, crystalline materials find application in pharmaceutical, food and microelectronic industries [5]. The main two strategies that are used for crystal engineering are based on hydrogen bonding and coordination bonding [6]. Since the hydrogen bonding is usually stronger and more directional than the other methods, more new crystal materials have been prepared based on this method. We are here able to design crystalline materials based on hydrogen bonding and study their solid state structures. N, N-dimethylformamide (DMF)-solvate of thiocyanuric acid (TCUA) and dimethyl sulfoxide (DMSO)-solvate of thiocyanuric acid (TCUA) were successfully prepared at room temperature in the presence of aqueous solution of sodium nitrate (NaNO3). To the best of our knowledge, this study presents the easy, modest, and rapid method to prepare co-crystal formation based on thiocyanuric acid (TCUA) and solvent-containing hydrogen bonding functionality. In this paper, we present the most effective method to synthesize the co-crystals of (TCUA), and as evidence, the crystal structure of (TCUA) in DMF is fully studied and presented in this paper. The N,N-dimethylformamide (DMF)-solvate of thiocyanuric acid (TCUA) was successfully prepared at room temperature, and was characterized spectroscopically by nuclear magnetic resonance (NMR) and single-crystal X-ray diffraction (SXRD). The asymmetric unit of the title compound contains one molecule of thiocyanuric acid (TCUA) features an almost planar six-membered ring having exocyclic C-S thione double bonds and one molecule of N,N-dimethylformamide (DMF). It was crystallized in the monoclinic, P21/c with unit cell parameters of a = 9.6255 (4) Å, b = 12.6864 (5) Å, c = 9.1367 (4) Å, β = 90.095 (2)°, V = 1115.71 (8) Å3, Z = 4. The structure is composed of 1-D TCUA ribbons formed via N–H–S hydrogen bonds. The ribbons are separated by DMF molecules, which are bridged to the ribbons by N–H–O hydrogen bonds. The ribbons and their DMF molecules form 2-D sheets which are in turn π-stacked to build up a layered, 3-D structure. The proton and carbon-13 NMR studies confirmed the formation of such solvate between DMF and TCUA.

Defects as Color Centers: The Apparent Color of Metal-Organic Frameworks Containing Cu2+-Based Paddle-Wheel Units.

As in the case of other semiconducting materials, optical and electronic properties of metal-organic frameworks (MOFs) depend critically on defect densities and defect types. We demonstrate here that, in addition to the influence of imperfections on MOF chemical properties like guest binding energies and catalytic activity, the optical properties of these crystalline molecular solids also crucially depend on deviations from the perfect crystalline structure. By recording UV-vis absorption spectra for MOF thin films of particularly high quality, we demonstrate that low-defect samples of an important MOF, HKUST-1, are virtually colorless. Electronic structure calculations of the excited states by employing complete active space self-consistent field (CASSCF) calculations show that the d-d excitations in defects result in the typical green color of the MOF material synthesized by conventional methods.

Optical and Electrochemical Band Gaps in Mono-, Oligo-, and Polymeric Systems: A Critical Reassessment1

The concept of a band gap was originally developed to describe periodic, perfect single-crystal materials in solid-state physics and subsequently applied to crystalline and amorphous monomeric, oligomeric, and polymeric solid materials. Furthermore, applications in organic electronics and electrochemistry have been discussed and studied with a focus on intrinsically conducting polymer ICPs and crystalline molecular semiconductors. A summary of this background and a molecular system description (of both single molecules and crystalline molecular solids) are included. This is followed by an inspection of band gap descriptions for oligomeric and polymeric systems. In particular, apparent discrepancies between band gap energies for intrinsically conducting polymers and organic semiconductors obtained using optical spectroscopy and electrochemical measurements have puzzled researchers for years. These contradictory data have, at times, been reported without any explanation or reconciliation. Similar disappointing results have been reported for molecular systems when a band model for the HOMO–LUMO gap was considered. In contrast, data pertaining to monomeric, oligomeric, and polymeric systems confirm that either positive or no significant correlations exist.

Terahertz Vibrations of Crystalline Acyclic and Cyclic Diglycine: Benchmarks for London Force Correction Models

Terahertz spectroscopy provides direct information concerning weak intermolecular forces in crystalline molecular solids and therefore acts as an excellent method for calibrating and evaluating computational models for noncovalent interactions. In this study, the low-frequency vibrations of two dipeptides were compared, acyclic diglycine and cyclic diglycine, as benchmark systems for gauging the performance of semiempirical London force correction approaches. The diglycine samples were investigated using pulsed terahertz spectroscopy from 10 to 100 cm(-1) and then analyzed using solid-state density functional theory (DFT) augmented with existing London force corrections, as well as a new parametrization (DFT-DX) based on known experimental values. The two diglycine molecules provide a useful test for the applied models given their similarities, but more importantly the differences in the intermolecular forces displayed by each. It was found that all of the considered London force correction models were able to generate diglycine crystal structures of similar accuracy, but considerable variation occurred in their abilities to predict terahertz frequency vibrations. The DFT-DX parametrization was particularly successful in this investigation and shows promise for the improved analysis of low-frequency spectra.

Metal–Organic Frameworks for Light Harvesting and Photocatalysis

Metal–organic frameworks (MOFs), a new class of crystalline molecular solids built from linking organic ligands with metal or metal-cluster connecting points, have recently emerged as a versatile platform for developing single-site solid catalysts. MOFs have been used to drive a range of reactions, including Lewis acid/base catalyzed reactions, redox reactions, asymmetric reactions, and photocatalysis. MOF catalysts are easily separated from the reaction mixtures for reuse, and yet their molecular nature introduces unprecedented chemical diversity and tunability to drive a large scope of catalytic reactions. This Perspective aims to summarize recent progress on light harvesting and photocatalysis with MOFs. The charge-separated excited states of the chromophoric building blocks created upon photon excitation can migrate over long distances to be harvested as redox equivalents at the MOF/liquid interfaces via electron transfer reactions or can directly activate the substrates that have diffused into the MO...

Ultrafast Dynamics of Frenkel Excitons in Tetracene and Rubrene Single Crystals

Characteristics of thermally driven environmental motion (i.e., spectral densities) similarly control energy and charge-transport processes in self-assembled molecular aggregates, crystalline molecular solids, and photosynthetic antennae. A true microscopic understanding of these transport processes necessarily involves decomposing the spectral density of thermal noise into contributions from specific nuclear motions (i.e., modes). To this end, molecular solids may serve as excellent model systems due to their known crystal structures and well-defined intermolecular modes. Here, the electronic relaxation dynamics of tetracene (Tc) and rubrene (Rb) single crystals are investigated using a variety of nonlinear optical spectroscopies in conjunction with a Frenkel exciton model. Parameterization of the model is achieved by comparing simulated optical signals with those measured in experiments. In addition, electronic population transfer rates are computed with this same set of parameters using a modified Redfield theory. An important aspect of the model is its use of femtosecond stimulated Raman spectroscopies to obtain nuclear mode-specific spectral densities (i.e., polarizability spectral densities). Attainment of the spectral densities facilitates the interpretation of electronic spectroscopies, which are sensitive to both exciton delocalization and population transfer kinetics. One important prediction of the model, which is based on the comparison of low-temperature linear absorption spectra and model calculations, is that the exciton sizes for both Tc and Rb are approximately 18 molecules at 200 and 78 K, respectively. In addition, transient grating experiments detect sub-100 fs intraband population transfer processes in both crystals. Photon echo experiments and model calculations further support the assignment of these dynamics to electronic population transfer. The role of spatial correlations in the spectral densities at different molecular sites is also investigated. Calculations predict a remarkable behavior in which variation in the amount of spatial correlation for just a single mode in the spectral density, among many, causes the electronic relaxation rates to vary over an order of magnitude. This finding holds for both crystals and will likely generalize to molecular aggregates and photosynthetic antennae, thereby contributing to a microscopic understanding of phenomena that originate in spatially correlated fluctuations (e.g., coherent energy transfer).

Anisotropy of Atomic Van der Waals Radii in the Gas-Phase and Condensed Molecules\sp{1}

The anisotropic components of van der Waals radii for 5b, 6b and 7b Group elements were determined from the structures of gaseous van der Waals complexes and of crystalline molecular solids. The transition from the gaseous to the solid state reduces the anisotropy of a van der Waals atomic shape. This anisotropy is largely responsible for the changes of intermolecular distances, which are often misinterpreted as an effect of hydrogen bonds.

Aromatic ring dynamics in crystalline molecular solids by one‐ and two‐dimensional exchange spectroscopy

AbstractVariable‐temperature phase‐sensitive cross‐polarization/magic angle spinning magnetization transfer experiments in one and two dimensions, in conjunction with chemical shift line‐shape analyses, have been used in the quantitative characterization of slow aromatic ring dynamics in crystalline 4‐nitro‐1‐methoxybenzene and in crystalline penicillin G phenyl ester. The results obtained indicate the value of such approaches for investigating specific motional events in molecular solids.

Positronium in molecular substances

The progress of research into the behavior of the positronium in molecular substances in the last several years is reviewed. The Ps atom can be treated as a solute in a liquid. The repulsive exchange force is the dominant force which reacts against the van der Waals attractive force. One of the most useful general relationships found is the one between the pick-off quenching rate and the surface tension of the liquid. The pick-off quenching rate is found, in general, to be an additive property of the functional groups of a molecule. The quenching of theo-Ps atom by an active quenching agent in an inert solvent has been found to be primarily diffusion controlled. Some deviations from the simple Arrhenius equation have been found. For gases, more theoretical work on the collision problems of Ps with atoms or molecules is needed. For amorphous molecular solids, it is likely that the property of pick-off quenching ofo-Ps is similar to that in liquids. However, in crystalline molecular solids, the situation is quite different. A few definite cases have been found where a Ps atom does not form or exist. Further work in this area should be very interesting. The foregoing leads us to believe that positronium or positron annihilation can be useful in the study of fundamental properties of molecular liquids and solids.