A Non-π-Conjugated Molecular Crystal with Balanced Second-Harmonic Generation, Bandgap, and Birefringence.

Traditional nonlinear optical (NLO) crystals are exclusively limited to ionic crystals with π-conjugated groups and it is a great challenge to achieve a subtle balance between second-harmonic generation, bandgap, and birefringence for them, especially in the deep-UV spectrum region (Eg  > 6.20 eV). Herein, a non-π-conjugated molecular crystal, NH3 BH3 , which realizes such balance with a large second-harmonic generation response (2.0 × KH2 PO4 at 1064 nm, and 0.45 × ß-BaB2 O4 at 532 nm), deep-UV transparency (Eg > 6.53 eV), and moderate birefringence (Δn = 0.056@550 nm) is reported. As a result, NH3 BH3 exhibits a large quality factor of 0.32, which is evidently larger than those of non-π-conjugated sulfate and phosphate ionic crystals. Using an unpolished NH3 BH3 crystal, effective second-harmonic generation outputs are observed at different wavelengths. These attributes indicate that NH3 BH3 is a promising candidate for deep-UV NLO applications. This work opens up a new door for developing high-performance deep-UV NLO crystals.

Halogen Impact on the Supramolecular Organization of Chiral Phthalimide EmittersDisplaying Room Temperature Phosphorescence.

Room temperature phosphorescence from organic materials has attracted an increasing attention in the recent years due to their potential application in various advancing technologies, notably in bioimaging and displays. In this context, heavy atoms such as halogen ones revealed useful tools to enhance the spin-orbit coupling (SOC) of molecular organic phosphors. However, the effect of halogen at the supramolecular level remains less understood, especially in the field of molecular crystals where additional factors can impact the phosphorescence emission. Here, we investigate external effect of halogens on the phosphorescence of chiral phthalimides molecular crystals. The results show that changing the nature of the halogen atom onto the phthalimide core leads to an evolution of the photophysical properties of the materials which does not necessarily follow the classical trend imposed by the expected internal heavy atom effect. Beyond this aspect, we showed that the halogen atom has a profound impact on the packing between the chromophores at the supramolecular level which is of paramount importance towards the optical properties (PLQY and lifetimes) of the different phosphors examined.

Performance of molecular crystals in conversion of light to mechanical work.

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.

Strategies to Diversification of the Mechanical Properties of Organic Crystals.

Structurally ordered soft materials that respond to complementary stimuli are susceptible to control over their spatial and temporal morphostructural configurations by intersectional or combined effects such as gating, feedback, shape-memory, or programming. In the absence of general and robust design and prediction strategies for their mechanical properties, at present, combined chemical and crystal engineering approaches could provide useful guidelines to identify effectors that determine both the magnitude and time of their response. Here, we capitalize on the purported ability of soft intermolecular interactions to instigate mechanical compliance by using halogenation to elicit both mechanical and photochemical activity of organic crystals. Starting from (E)-1,4-diphenylbut-2-ene-1,4-dione, whose crystals are brittle and photoinert, we use double and quadruple halogenation to introduce halogen-bonded planes that become interfaces for molecular gliding, rendering the material mechanically and photochemically plastic. Fluorination diversifies the mechanical effects further, and crystals of the tetrafluoro derivative are not only elastic but also motile, displaying the rare photosalient effect.

Maneuverability and Processability of Molecular Crystals.

Crystal adaptronics, a burgeoning field at the intersection of materials science and engineering, focuses on harnessing the unique properties of organic molecular crystals to achieve unprecedented levels of maneuverability and processability in various applications. Increasingly, ordered stacks of crystalline materials are being endowed with fascinating mechanical compliance changes in response to external environments. Understanding how these crystals can be manipulated and tailored for specific functions has become paramount in the pursuit of advanced materials with customizable properties. Simultaneously, the processability of organic molecular crystals plays a pivotal role in shaping their utility in real-world applications. From growth methodologies to fabrication techniques, the ability to precisely machine these crystals opens new avenues for engineering materials with enhanced functionality. These processing methods enhance the versatility of organic crystals, allowing their integration into various devices and technologies, and further expanding the potential applications. This review aims to provide a concise overview of the current landscape in the study of dynamic organic molecular crystals, with an emphasis on the interconnected themes of operability and processability.

Tandem Solid-Solution Phase Post-Synthetic Modification of Porous Molecular Crystals for In-Situ Generation of Fluorophores.

Conventional synthetic methods of organic luminescent molecules often involve labor-intensive solution-phase organic synthesis, which violate the principles of atom-economic transformation. Post-synthetic modification (PSM) offers a promising alternative, allowing direct transformation from one fluorophore to another. Although PSM is commonly implemented in extended frameworks, its application in porous molecular crystals remains challenging. Herein, we focus on utilizing porous molecular crystals, specifically tetraphenylethylene-cored frameworks, as versatile platforms for tandem PSM reactions to customize organic fluorophores. The tailored skeleton design ensures both the formation of porous structures and the occurrence of tandem solid-solution phase reactions while maintaining the solid state of reactants and products in each step. The inherent non-covalent bonding nature of the frameworks facilitates processing and characterization, offering unparalleled advantages for porous networks. The accompanying solid-state fluorescence transition from green to blue and then to green (or yellow) enables real-time monitoring of tandem reactions and provides intuitive mechanistic insights. This phenomenon is exploited for the facile construction of a dynamic information encryption system using fluorescent quick response codes.

Conformation-Confined Organic Butterfly-Molecule with High Photoluminescence Efficiency, Deep-Blue Amplified Spontaneous Emission, and Unique Piezochromic Luminescence.

Organic fluorophores with tunable π-conjugated paths have attracted considerable attention owing to their diverse properties and promising applications. Herein, we present a tailored butterfly like molecule, 2,2'-(2,5-bis (2,2-diphenylvinyl)-1,4-phenylene)dinaphtha-lene (BDVPN), which exhibits diverse photophysical features in its two polymorphs. The BP phase crystal, with its "aligned wings" conformation, possesses emissive characteristics that are nearly identical to those in dilute solutions. In contrast, the BN phase crystal, which adopts an "orthogonal wings" conformation, exhibits an unusual hypsochromic-shifted emission compared to its dilute solution counterparts. This intriguing hypsochromic-shifted emission originates from the reduction in the effective conjugated length of the molecular skeleton. Notably, BN phase crystals also exhibit exceptional optical performance, featuring high-efficiency emission (76.6%), low-loss optical waveguides (0.571 dB mm-1), deep-blue amplified spontaneous emission (ASE) with a narrow full width at half maximum (FWHM: 6.4 nm), and a unique 200 nm bathochromic shift of piezochromic luminescence.

Functional Molecular Crystals from the Arylation of a Halogenoplatinum Complex: Stimuli Responsiveness, Comproportionation, and π-Bridged Dimerization.

A chloroplatinum complex was arylated to obtain stimuli-responsive molecular crystals. The resulting arylplatinum complex showed polymorph-dependent emission, mechano- and thermochromic luminescence as well as comproportionation and π-bridged dimerization. Simple mixing of structurally similar arylplatinum complexes at room temperature resulted in the transfer of their aryl groups (comproportionation), which allowed their mechanochromic profiles to be tuned. We also found that recrystallization of the complex afforded a dimerized product in which two platinum ions are bridged by aryl groups resulting in a very short (3.0466(10) Å) Pt-Pt distance.

How Accurate Can Crystal Structure Predictions Be for High-Energy Molecular Crystals?

Molecular crystals have shallow potential energy landscapes, with multiple local minima separated by very small differences in total energy. Predicting molecular packing and molecular conformation in the crystal generally requires ab initio methods of high accuracy, especially when polymorphs are involved. We used dispersion-corrected density functional theory (DFT-D) to assess the capabilities of an evolutionary algorithm (EA) for the crystal structure prediction (CSP) of well-known but challenging high-energy molecular crystals (HMX, RDX, CL-20, and FOX-7). While providing the EA with the experimental conformation of the molecule quickly re-discovers the experimental packing, it is more realistic to start instead from a naïve, flat, or neutral initial conformation, which reflects the limited experimental knowledge we generally have in the computational design of molecular crystals. By doing so, and using fully flexible molecules in fully variable unit cells, we show that the experimental structures can be predicted in fewer than 20 generations. Nonetheless, one must be aware that some molecular crystals have naturally hindered evolutions, requiring as many attempts as there are space groups of interest to predict their structures, and some may require the accuracy of all-electron calculations to discriminate between closely ranked structures. To save resources in this computationally demanding process, we showed that a hybrid xTB/DFT-D approach could be considered in a subsequent study to push the limits of CSP beyond 200+ atoms and for cocrystals.

Ferroelectric Ionic Molecular Crystals with Significant Plasticity and a Low Melting Point: High Performance in Hot-Pressed Polycrystalline Plates and Melt-Grown Crystalline Sheets.

Among ferroelectric crystals based on small molecules, plastic/ferroelectric crystals are currently receiving particular attention because they can be used as bulk polycrystals. Herein, we show that an ionic molecular ferroelectric crystal, guanidinium tetrafluoroborate, exhibits significant malleability and multiaxial ferroelectricity despite the absence of a plastic crystal phase. Powder samples of this crystal can be processed into transparent bulk crystalline plates either by press-forming or by melt-growing. The plates show high ferroelectric performance and related properties, demonstrating the largest hitherto reported spontaneous polarization for bulk polycrystals of small-molecule-based ferroelectrics. Owing to the ready availability of large-scale materials and processability into various bulk crystalline forms, this ferroelectric crystal represents a highly promising functional material that will boost research on diverse applications as bulk crystals.

Macrocycle-Based Crystalline Supramolecular Assemblies Built with Intermolecular Charge-Transfer Interactions.

Synthetic macrocycles have served as principal tools for supramolecular chemistry, have greatly extended the scope of organic charge transfer (CT) complexes, and have proved to be of great practical value in the solid state during the past few years. In this Minireview, we summarize the research progress on the macrocycle-based crystalline supramolecular assemblies primarily driven by intermolecular CT interactions (a.k.a. macrocycle-based crystalline CT assemblies, MCCAs for short), which are classified by their donor-acceptor (D-A) constituent elements, including simplex macrocyclic hosts, heterogeneous macrocyclic hosts, and host-guest D-A pairs. Particular attention will be focused on their diverse functions and applications, as well as the underlying CT mechanisms from the perspective of crystal engineering. Finally, the remaining challenges and prospects are outlined.

Vibrational Circular Dichroism Spectroscopy of Chiral Molecular Crystals: Insights from Theory.

Chirality is a curious phenomenon that appears in various forms. While the concept of molecular (RS-)chirality is ubiquitous in chemistry, there are also more intricate forms of structural chirality. One of them is the enantiomorphism of crystals, especially molecular crystals, that describes the lack of mirror symmetry in the unit cell. Its relation to molecular chirality is not obvious, but still an open question, which can be addressed with chiroptical tools. Vibrational circular dichroism (VCD) denotes chiral infrared (IR) spectroscopy that is susceptible to both, the molecular as well as the intermolecular space by means of vibrational transitions. When carried out in the solid state, VCD delivers a very rich set of non-local contributions that are determined by crystal packing and collective motion. Since its discovery in the 1970s, VCD has become the method of choice for the determination of absolute configurations, but its applicability reaches beyond towards the study of different crystal forms and polymorphism. This brief review summarises the theoretical concepts of crystal chirality and how computations of solid-state VCD can shed light into the intimate connection of chiral structure and vibrational optical activity.

Giant Tunability of Charge Transport in 2D Inorganic Molecular Crystals by Pressure Engineering.

The unique intermolecular van der Waals force in emerging two-dimensional inorganic molecular crystals (2DIMCs) endows them with highly tunable structures and properties upon applying external stimuli. Using high pressure to modulate the intermolecular bonding, here we reveal the highly tunable charge transport behavior in 2DIMCs for the first time, from an insulator to a semiconductor. As pressure increases, 2D α-Sb2 O3 molecular crystal undergoes three isostructural transitions, and the intermolecular bonding enhances gradually, which results in a considerably decreased band gap by 25 % and a greatly enhanced charge transport. Impressively, the in situ resistivity measurement of the α-Sb2 O3 flake shows a sharp drop by 5 orders of magnitude in 0-3.2 GPa. This work sheds new light on the manipulation of charge transport in 2DIMCs and is of great significance for promoting the fundamental understanding and potential applications of 2DIMCs in advanced modern technologies.

Rigid-Flexible Hybrid Porous Molecular Crystals with Guest-Induced Reversible Crystallinity.

A weak CH/O hydrogen-bonded organic framework (HOF) with both rigidity and flexibility that could easily and reversibly switch from a non-crystalline to a crystalline phase was constructed. The specific solvent molecule acts as a "key" to control the crystallinity, while the highly rigid triangle macrocycle as the building block is the "lock". The introduction and removal of the "key" could influence the local flexibility of the whole framework and lead to switchable crystallinity. Furthermore, the obtained HOF exhibits excellent separation efficiency for benzene and cyclohexane (94.4 %).

Tuning the quasi-harmonic treatment of crystalline ionic liquids within the density functional theory.

Five ionic liquids are selected for benchmarking the performance of quasi-harmonic density functional theory (DFT) calculations of structural, phonon, and thermodynamic properties of their crystals. Data predicted by individual computational setups are sorted, establishing a distinct hierarchy among the first-principles approaches. PBE-D3 and B3LYP-D3 functionals are coupled with various plane wave and Gaussian-type orbital (GTO) basis sets. Propagation of the basis set superposition error and of the imperfections of both functionals into finite-temperature properties is discussed in detail. PBE-D3 together with a triple-zeta GTO basis set often yields the most accurate predictions of predicted molar volume and heat capacity with errors at 1% and 8%, respectively, representing the state-of-the-art for quasi-harmonic DFT calculations for crystalline ionic liquids. Fortuitous error cancellation between the basis-set superposition (overbinding) and PBE imperfection (overexpanding) strongly affects the overall accuracy, unlike the case of B3LYP/GTO calculations, impeding systematic convergence of the methodology towards higher accuracy.

Can a Finite Chain of Hydrogen Cyanide Molecules Model a Crystal?

When calculating structural or spectroscopic properties of molecular crystals, the question arises whether it is sufficient to simulate only a single molecule or a small molecular cluster or whether the simulation of the entire crystal is indispensable. In this work we juxtapose calculations on the high-pressure structural properties of the (periodic) HCN crystal and chains of HCN molecules of finite length. We find that, in most cases, the behavior of the crystal can be reproduced by computational methods simulating only around 15 molecules. The pressure-induced lengthening of the C-H bond in HCN found in calculations on both the periodic and finite material are explained in terms of orbital interaction. Our results pave the way for a more thorough understanding of high-pressure structural properties of materials and give incentives for the design of materials that expand under pressure. In addition, they shed light on the complementarity between calculations on periodic materials and systems of finite size.

Effect of pH on the Surface Layer of Molecular Crystals at the Solid-Liquid Interface.

Dissolution of solid matter into aqueous solution is one of the most challenging physicochemical aspects related to drug development. While influenced by several parameters, the effect of pH remains the most important one to be fully understood. The dissolution process is essentially controlled by activity at the surface of the molecular crystals, which is difficult to characterize experimentally. To address this, a combination of in situ atomic force microscopy (AFM) with molecular dynamics (MD) simulation is reported. AFM allows for direct visualization of the crystal surface of basic and acidic model compounds (carvedilol and ibuprofen) in contact with an aqueous medium with varying pH. A dramatic increase in surface mobility in the solid-liquid interface could be observed experimentally as a function of pH. The in situ AFM approach opens up for a more detailed understanding of the behavior of particulate matter in solution with importance at different levels, ranging from engineering aspects related to crystallization, and biological considerations related to bioavailability of the final drug product.

Prediction of Strong Transversal s(TE) Exciton-Polaritons in C60 Thin Crystalline Films.

If an exciton and a photon can change each other's properties, indicating that the regime of their strong bond is achieved, it usually happens in standard microcavity devices, where the large overlap between the 'confined' cavity photons and the 2D excitons enable the hybridization and the band gap opening in the parabolic photonic branch (as clear evidence of the strong exciton-photon coupling). Here, we show that the strong light-matter coupling can occur beyond the microcavity device setup, i.e., between the 'free' s(TE) photons and excitons. The s(TE) exciton-polariton is a polarization mode, which (contrary to the p(TM) mode) appears only as a coexistence of a photon and an exciton, i.e., it vanishes in the non-retarded limit (c→∞). We show that a thin fullerene C60 crystalline film (consisting of N C60 single layers) deposited on an Al2O3 dielectric surface supports strong evanescent s(TE)-polarized exciton-polariton. The calculated Rabi splitting is more than Ω=500 meV for N=10, with a tendency to increase with N, indicating a very strong photonic character of the exciton-polariton.

Isotropic Anhydrous Superprotonic Conductivity Cooperated with Installed Imidazolium Molecular Motions in a 3D Hydrogen-Bonded Phosphate Network.

Utilizing molecular motion is essential for the use of anhydrous superprotonic molecular proton conductors (σ beyond 10-4  S cm-1 ) as electrolytes in hydrogen fuel cells. However, molecular motion contributing to the improvement of intrinsic proton conduction has been limited and little clarified in relation to the proton conduction mechanism, limiting the development of material design guidelines. Here, a salt with a three-dimensional (3D) hydrogen-bonded (H-bonded) phosphate network with imidazolium cations installed inside was studied, whose components are known to exhibit molecular motions that contribute to proton conduction. Despite its anisotropic H-bonded network, the salt exhibits isotropic anhydrous superprotonic conductivity exceeding 10-3  S cm-1 at ≈351 K, which is the first example for organic molecular crystal. Variable-temperature X-ray structural analysis and solid-state 2 H NMR measurements revealed significant 3D molecular motion of imidazolium cations, which accelerate proton conduction via the 3D H-bonded phosphate network.

Regulating Crystal Packing by Terminal tert-Butylation for Enhanced Solid-State Emission and Efficacious Charge Transport in an Anthracene-Based Molecular Crystal.

Organic semiconductors with combinative high carrier mobility and efficient solid-state emission are full of challenges but urgently pursued for developing new emerging optoelectronics. Herein, by delicately regulating the crystal packing of an anthracene-based molecular crystal via terminal tert-butylation, we developed a superior high mobility emissive molecule, 2,6-di(6-tert-butylnaphthyl)anthracene (TBU-DNA). The unique "slipped herringbone" packing motif of TBU-DNA enables its appropriate exciton-exciton coupling and electron-phonon coupling, thus resulting in remarkably high solid-state emission (photoluminescence quantum yield, ΦF ≈74.9 %) and efficacious charge transport (carrier mobility, µ=5.0 cm2 V-1 s-1 ). Furthermore, OLETs based on TBU-DNA show an external quantum efficiency (EQE) of 1.8 %, which is among the highest EQE values for single component OLETs reported till now. This work presents a crystal engineering strategy via exquisite molecular design to realize high mobility emissive organic semiconductors.