Molecular Crystal Systems Research Articles

Preparation of conductive bimetallic phthalocyanine with acceptor and their electrocatalytic properties for CO2 reduction

Abstract In order to develop a catalyst for electrochemical CO2 reduction with high power efficiency, we prepared molecular crystals with 2 types of metal phthalocyanine. Charge transfer complexes with acceptor were selected as the molecular crystal system to reduce the electrical resistance. Various bimetallic phthalocyanine systems consisting of cobalt phthalocyanine and another metal phthalocyanine were obtained as highly conductive separated stacked charge transfer complexes with iodine as an acceptor. The obtained catalysts for CO2 reduction were evaluated using gas diffusion electrodes. The catalysts containing the bimetallic phthalocyanine system of cobalt and copper phthalocyanines showed higher CO2 reduction current and higher CO production, indicating that the CO2 reduction on cobalt phthalocyanine is enhanced by the H2 formation reaction on copper phthalocyanine.

Dual responsive behaviors in fluorescence of molecular crystals based on naphthalene pyridyl derivatives

Three compounds based on naphthalene and pyridine structures with different nitrogen positions (NP-2, NP-3, and NP-4) were synthesized. Except for displaying aggregation-induced emission (AIE) properties, the solution and crystals of these compounds show reversible fluorescence changes under alternative acid and base treatments. Experimental results, including X-ray diffraction, optical spectroscopy measurements, and cross-polarized microscope characterization, suggested that compounds might undergo reversible crystal-to-crystal transition upon external acid and base stimuli. It also revealed that the position of the heteroatom nitrogen in the pyridine unit significantly affected the intra- and intermolecular interactions, leading to restricted-intermolecular motions (RIMs) that caused the AIE effect. Besides, computational calculations indicated significant changes in molecular geometries and electronic interactions between the initial states and the protonated states. An information encryption system involving activation and operation codes was established based on the multiple stimuli-responsive properties. Our work not only presents a facile way to develop multiple responsive properties in molecular crystal systems but also explores their potential as highly emissive functional dyes for future applications.

Assessment of Approximations to the Embedding Potential in Frozen-Density Embedding Theory for the Calculation of Electric Field Gradients.

The approximations to the embedding potential in frozen-density embedding theory (FDET) have been assessed for the first time for the calculation of the electric field gradient (EFG) at a nucleus. FDET-based methods using a hierarchy of approximations are applied to evaluate the EFG at the nuclei of an HCl molecule in several noncovalently bound clusters chosen to represent potential liquid or molecular crystal systems. A detailed assessment of such approximations is made for the Hartree-Fock treatment of electron-electron correlation (both in FDET and in the reference calculations for the whole cluster). The emerging choice of the optimal set of approximations is reconfirmed in calculations in which electron-electron calculations are treated at the MP2 level. Our optimized protocol produces average errors in the complexation-induced EFG shift on the order of 25% relative to conventional quantum mechanical calculations for the whole cluster. This protocol is shown to be numerically robust and leads to enormous computational savings compared to a complete quantum mechanical treatment of the embedded species and its environment. For a cluster comprising a Na+ cation and up to 24 water molecules, the computation time is reduced by a factor of 30,000 at the expense of introducing an error in the environment-induced EFG shift of 22%.

A transferable quantum mechanical energy model for intermolecular interactions using a single empirical parameter.

The calculation of intermolecular interactions in molecular crystals using model energies provides a unified route to understanding the complex interplay of driving forces in crystallization, elastic properties and more. Presented here is a new single-parameter interaction energy model (CE-1p), extending the previous CrystalExplorer energy model and calibrated using density functional theory (DFT) calculations at the ωB97M-V/def2-QZVP level over 1157 intermolecular interactions from 147 crystal structures. The new model incorporates an improved treatment of dispersion interactions and polarizabilities using the exchange-hole dipole model (XDM), along with the use of effective core potentials (ECPs), facilitating application to molecules containing elements across the periodic table (from H to Rn). This new model is validated against high-level reference data with outstanding performance, comparable to state-of-the-art DFT methods for molecular crystal lattice energies over the X23 set (mean absolute deviation 3.6 kJ mol-1) and for intermolecular interactions in the S66x8 benchmark set (root mean-square deviation 3.3 kJ mol-1). The performance of this model is further examined compared to the GFN2-xTB tight-binding model, providing recommendations for the evaluation of intermolecular interactions in molecular crystal systems.

New Mechanism Insights on Ca-Ion Storage in Small Molecular Crystal

Rechargeable calcium-ion batteries have become intriguing alternatives for grid-scale energy storage devices. However, the strong electrostatic interaction caused by the intrinsic divalent nature of Ca2+ leads to sluggish ion diffusion kinetics, thus resulting in low capacity. Herein, a small molecular crystal organic material is employed as the electrode material for the aqueous calcium-ion batteries. This kind of organic material could not only bypass the sluggish ion diffusion through carbonyl enolization but also avoid electrode inevitable dissolution due to the intermolecular hydrogen bonding, and flexible structure. In addition, the robust crystal structure allows fast Ca-ion diffusion and storage. Electrochemical characterization including electrochemical quartz crystal microbalance combined with first-principles density calculations and in(ex)-situ spectroscopy methods reveals the whole Ca-ion storage mechanism at the atomistic and macroscopic levels. The reversible enolization redox process, diffusion path, and structure change of molecular crystals have been clearly clarified. Consequently, the materials yield a premier rate capability (70.2 mAh g−1 at 5 A g−1), and excellent cycling stability (retention of 80.3% over 1000 cycles at 2 A g−1). This study extends the boundary of molecular crystal systems for battery chemistry to construct high-performance multivalent ion batteries.

Recent advances of dynamic molecular crystals with light-triggered macro-movements

Dynamic molecular crystals showing light-triggered macro-movements have attracted great attention due to their unique ability for light–force conversion. These molecular crystals are driven remotely without any intermediary devices like wires and motors, which can transform light energy into mechanical work directly. However, the limited space restricts molecular rotation and motion in the crystalline state; thus, realizing macro-movements in molecular crystal systems is still a formidable challenge. In this review, we aim to focus on the underlying working mechanism of the photo-controllable macroscopic motion of molecular crystals with special focus on their practical applications. In detail, we discuss the basic principles and macroscopic photomechanical effects of these dynamic molecular crystals, including their deformation (i.e., bending, twisting, curling); complex motion (i.e., crawling, rotating, rolling); and disintegration (i.e., photosalient effect). Then, we introduce the most promising applications of photomechanical molecular crystals in the fields of all-optical devices, crystal actuators, and biomimetic artificial muscles. Therefore, this review will provide inspiration to develop state-of-the-art dynamic molecular crystals by bridging the disciplines of physics, chemistry, and engineering science.

On the calculation of the electrostatic potential, electric field and electric field gradient from the aspherical pseudoatom model. II. Evaluation of the properties in an infinite crystal.

The previously reported exact potential and multipole moment (EP/MM) method for fast and precise evaluation of the intermolecular electrostatic interaction energies in molecular crystals using the pseudoatom representation of the electron density [Nguyen, Macchi & Volkov (2020), Acta Cryst. A76, 630-651] has been extended to the calculation of the electrostatic potential (ESP), electric field (EF) and electric field gradient (EFG) in an infinite crystal. The presented approach combines an efficient Ewald-type summation (ES) of atomic multipoles up to the hexadecapolar level in direct and reciprocal spaces with corrections for (i) the net polarization of the sample (the `surface term') due to a net dipole moment of the crystallographic unit cell (if present) and (ii) the short-range electron-density penetration effects. The rederived and reported closed-form expressions for all terms in the ES algorithm have been augmented by the expressions for the surface term available in the literature [Stenhammar, Trulsson & Linse (2011), J. Chem. Phys. 134, 224104] and the exact potential expressions reported in a previous study [Volkov, King, Coppens & Farrugia (2006), Acta Cryst. A62, 400-408]. The resulting algorithm, coded using Fortran in the XDPROP module of the software package XD, was tested on several small molecular crystal systems (formamide, benzene, L-dopa, paracetamol, amino acids etc.) and compared with a series of EP/MM-based direct-space summations (DS) performed within a certain number of unit cells generated along both the positive and negative crystallographic directions. The EP/MM-based ES technique allows for a noticeably more precise determination of the EF and EFG and significantly better precision of the evaluated ESP when compared with the DS calculations, even when the latter include contributions from an array of symmetry-equivalent atoms generated within four additional unit cells along each crystallographic direction. In terms of computational performance, the ES/EP/MM method is significantly faster than the DS calculations performed within the extended unit-cell limits but trails the DS calculations within the reduced summation ranges. Nonetheless, the described EP/MM-based ES algorithm is superior to the direct-space summations as it does not require the user to monitor continuously the convergence of the evaluated properties as a function of the summation limits and offers a better precision-performance balance.

Time-Domain Terahertz Spectroscopy and Solid-State Density Functional Theory Analysis of p-Nitrophenol Polymorphs

Accurate interpretations of THz vibrational spectra using density functional theory (DFT) must account for the often significant thermal expansion exhibited by molecular crystal systems. In this study, the THz spectra of two p-nitrophenol polymorphs were obtained in the spectral bandwidth of 20–95 cm−1, and solid-state DFT was used to assign the observed absorptions to specific vibrational mode characters. Computational treatments of the crystal systems involved comparison of phonon frequencies produced from both fixed- and full-unit cell geometry optimizations using a variety of functional and basis set combinations. Arbitrary frequency scaling was applied to overcome the temperature dependence of unit cell volumes and associated phonon vibrational frequencies. Fully relaxed cell geometries producing contracted cell volumes resulted in a large overestimation of THz vibrational frequencies. Employing appropriate frequency scaling factors to the computed mode frequencies allowed for proper vibrational mode assignments and enhanced the quality of spectral simulations versus fixed-cell calculations. Optimal frequency scalars in the range of 0.675 to 0.827 were determined to best reproduce the THz spectra of the p-nitrophenol polymorphs. This treatment of calculated phonon modes offers the key advantage of eliminating the temperature correlation between the crystal structure data and the THz data acquisition.

Ultrafast Dynamical Study of Pyrene-N,N-dimethylaniline (PyDMA) as an Organic Molecular Diode in Solid State

Femtosecond optical pump-probe spectroscopy has been employed for studying the directly linked electron donor-acceptor system pyrene-N,N-dimethylaniline (PyDMA) in solid state. This DMA-pyrene derivative discussed is being applied as a molecular diode system switching on an ultrafast time scale. Our ultrafast solid-state studies reveal a complex photochemistry of this molecular crystal system. Strong couplings of the optically induced charge-transfer state with the radical ion pair state allow a femtosecond transition of the latter. One could see on the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) description that a pure optical transition switches the system from a conducting to a blocked system because the molecular orbitals (MOs) of DMA moiety lie in a node plane of the LUMO. Within 800 fs the system relaxes back to the ground state and/or forms a radical ion pair, which is the surprising result of our study; when the system was probed further, the system underwent vibrational cooling and enhanced population inversion of the radical ion pair.

Recoil Effects in Valence Band Photoemission of Organic Solids

Recoil effects in valence band X-ray photoelectron spectroscopy (XPS) are studied for both abb-trifluorostyrene and styrene molecular crystal systems. The gradual changes of XPS spectra excited by several photon energies are theoretically investigated within the tight-binding approximation and harmonic approximation of lattice vibrations and have been explained in terms of not only atomic mass but also atomic orbital (AO) population. The recoil effect of valence band photoemission strongly depends on the population and partial photoionization cross section (PICS) of AOs as well as the masses of composite atoms. In abb-trifluorostyrene F 2p dominant bands show the recoil shift close to free F atom recoil shift, and C 2s dominant bands show that to free C atom recoil shift, whereas the mixed bands of C and F give rise to the peak asymmetries due to their different recoil shifts. For these systems, hydrogen contribution is negligibly small which is in contrast to our previous results for the crystals composed of small organic molecules. We also discuss some potential uses of the recoil shifts for these systems.

Polaron stability in molecular crystals

A semi-empirical Peierls–Holstein model is applied to studies of the stability of polarons in two-dimensional molecular crystal systems. Calculations for a broad range of intra- and inter-molecular parameters within this model were performed in order to obtain detailed knowledge concerning the stability of the polaron solution with respect to a rigid lattice band solution. For realistic values of the parameters the polaron solution is stable with a polaron energy in the range 50–100 meV. A metastable polaron solution is also identified. The polarons that result from our model are highly localized and it is questionable if adiabatic polaron transport can occur in the system.

Calculation of thermodynamic quantities for carbon tetrachloride (CCl4) close to the III–IV phase transition

We calculate the thermal expansion αp, isothermal compressibility κT and the specific heat, Cp-Cv, as a function of pressure using the observed V-P data at room temperature close to the III–IV phase transition in CCl4. Calculated κT, αp and Cp-Cv decrease as the pressure increases from phase III to phase IV in this molecular crystal system, as expected. On the basis of our calculations of the κT, αp and Cp-Cv, the Pippard relations are established close to the III–IV phase transition in CCl4, which can be verified experimentally.

A refinement strategy for Monte Carlo modelling of diffuse scattering from molecular crystal systems

A new strategy for modelling diffuse scattering from molecular crystals using Monte Carlo simulation is described. The use of harmonic (Hooke's law) springs to representeffectiveintermolecular interactions is preserved, in order to minimize computer requirements, but use is now made of a simple empirical formula to specify spring constants and provide an objective means for limiting the number of springs needed to be used. The method has been tested on diffuse scattering data obtained for form I of paracetamol.

Vortex molecular crystal and vortex plastic crystal states in honeycomb and kagomé pinning arrays

Using numerical simulations, we investigate vortex configurations and pinning in superconductors with honeycomb and kagom\'e pinning arrays. We find that a variety of vortex crystal states can be stabilized at integer and fractional matching field densities. The honeycomb and kagom\'e pinning arrays produce considerably more pronounced commensuration peaks in the critical depinning force than triangular pinning arrays, and also cause additional peaks at noninteger matching fields where a portion of the vortices are located in the large interstitial regions of the pinning lattices. For the honeycomb pinning array, we find matching effects of equal strength at most fillings $B∕{B}_{\ensuremath{\phi}}=n∕2$ for $ng2$, where $n$ is an integer, in agreement with recent experiments. For kagom\'e pinning arrays, pronounced matching effects generally occur at $B∕{B}_{\ensuremath{\phi}}=n∕3$ for $ng3$, while for triangular pinning arrays pronounced matching effects are observed only at integer fillings $B∕{B}_{\ensuremath{\phi}}=n$. At the noninteger matching field peaks in the honeycomb and kagom\'e pinning arrays, the interstitial vortices are arranged in dimer, trimer, and higher order $n$-mer states that have an overall orientational order. We call these $n$-mer states ``vortex molecular crystals'' and ``vortex plastic crystals'' since they are similar to the states recently observed in colloidal molecular crystal systems. We argue that the vortex molecular crystals have properties in common with certain spin systems such as Ising and $n$-state Potts models. We show that kagom\'e and honeycomb pinning arrays can be useful for increasing the critical current above that of purely triangular pinning arrays.

Faceted crystal shape evolution during dissolution or growth

AbstractA model for the prediction of faceted crystal shape evolution during growth or dissolution is presented. An ab initio mechanistic model for the relative growth or dissolution rates is also described for organic molecular crystal systems. The shape evolution model proves that while growing crystals evolve toward a steady‐state shape, dissolving crystals evolve away from a steady‐state shape. Thus, crystals cannot achieve a steady‐state during dissolution. This methodology may be used to probe crystal shapes that are obtainable by growing and/or dissolving crystals, including pharmaceutical crystals, inorganic materials and geological minerals. The technique has been successfully applied to predict the shape evolution of succinic acid growing or dissolving in water. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Habit Modification of Asparagine Monohydrate Crystals by Growth in Hydrogel Media

Crystal growth of a model molecular crystal system, asparagine monohydrate (Asn·H2O), was explored in a variety of hydrogel media. Morphologies of crystals grown in agarose, carrageenan, and gelatin gels were compared with those of crystals grown from pure aqueous solutions, as well as those grown from solutions containing various concentrations of related monomers, ions, and cosolvents. The morphological differences were analyzed and interpreted on the basis of complementary interactions at various gel/crystal interfaces. In many cases, the gel polymers were found to have a greater impact on the crystal habit than when solution growth occurred in the presence of comparable concentrations of monomeric or ionic species. These studies clearly indicate that gel methods can provide (at least some systems) a viable alternative means to generate novel crystal morphologies of small molecules that may not be attainable under more conventional solution growth methods.

Controlling Molecular Crystal Polymorphism with Self-Assembled Monolayer Templates

The control of crystal polymorphism is a long-standing issue in solid-state chemistry, which has many practical implications for a variety of commercial applications. At least four different crystalline forms of 1,3-bis(m-nitrophenyl) urea (MNPU), a classic molecular crystal system, are known to crystallize from solution in various concomitant combinations. Herein we demonstrate that the introduction of gold-thiol self-assembled monolayers (SAMs) of substituted 4'-X-mercaptobiphenyls (X = H, I, and Br) into the crystallization solution can serve as an effective means to selectively template the nucleation and growth of alpha-, beta-, and gamma-MNPU phases, respectively. Polymorph control in the presence of SAM surfaces persists under a variety of solution conditions and consistently results in crystalline materials with high phase purity. The observed selectivity is rationalized on the basis of long-range two-dimensional geometric lattice matching and local complementary chemical interactions at the SAM/crystal interfaces.

Optical probes of proton tunneling in molecular solids

Low-temperature optical spectroscopy of guest-host molecular crystal systems provides a very sensitive tool to study proton tunneling processes. Examples include the light-induced creation and evolution of “proton defects”, translational tunneling along hydrogen bonds as well as rotational tunneling of methyl groups. Absorption spectra at 2 K of γ-picoline single crystals evolve over time scales of days due to the slow spin conversion of the methyl groups which leads to an ordering of the crystal.

Proton tunneling in molecular solids

Low-temperature optical spectroscopy of guest-host molecular crystal systems gives direct access to proton tunneling processes. Examples include the photo-induced creation and evolution of “proton defects”, as well as translational tunneling along hydrogen bonds and rotational tunneling of methyl groups. The first observation of the effect of rotational tunneling on conventional absorption spectra is reported for γ-picoline single crystals.

Calculations of the electronic structure of various aperiodic polymers by an elongation method

AbstractAn elongation method, which had already been proposed to calculate the electronic structure of aperiodic polymer efficiently, was applied to atactic polypropylene, with the approximation level of the extended Hückel method. The atomic populations thus calculated were found to be dependent sensitively on the tacticity of the polymer. Next, this elongation method was applied to the π‐electron stacking system composed of ethylene and butadiene, etc. Excellent agreement was found between the elongation method and the usual extended Hückel calculations. Thus, the elongation method can confidently be applied to π‐electron molecular crystal systems. Finally, the local density of states of polyacetylene with both cis–transoid and trans–cisoid structures was calculated. We found that the site with large local density of state in the valence band of a polymer chain gives large electron transfer to the site with large local density of state in the conduction band of another polymer chain. © 1994 John Wiley & Sons, Inc.