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A designed anthracene‐based monomer for topochemical Diels–Alder cycloaddition polymerization crystallized with head‐to‐tail arrangement of molecules, as revealed by single‐crystal X‐ray diffraction (SCXRD) analysis. The diene and dienophile units of adjacent monomer molecules are aligned at an average distance of 4.6 Å, suggesting a favorable crystalline arrangement for their intermolecular Diels–Alder cycloaddition reaction to form a linear polymer. Surprisingly, heating the monomer crystals at a temperature above 125 °C resulted in the formation of intramolecular Diels–Alder cycloadduct, which could be characterized by various spectroscopy and SCXRD analysis. Various time‐dependent studies such as NMR, PXRD, and DSC, studies established that the reaction followed topochemical pathway. Schmidt's topochemical postulates are generally used to predict the topochemical reactivity and product, by analyzing the crystal structure of the reactant. Though the crystal arrangement predicted polymerization, upon heating, the molecule avoided this pathway by undergoing a large rotation to form an intramolecular cycloadduct. Theoretical calculations supported the feasibility of the rotation, exploiting the flexibility of the molecule and voids present. These findings caution that the reliance on Schmidt's criteria for topochemical reactions may sometimes be misleading, especially in heat‐induced reactions. [ABSTRACT FROM AUTHOR]

Polymorphism plays a pivotal role in generating a range of crystalline materials with diverse photophysical and mechanical attributes, all originating from the same molecule. Here, we showcase two distinct polymorphs: green (GY) emissive and orange (OR) emissive crystals of 5′‐(4‐(diphenylamino)phenyl)‐[2,2′‐bithiophene]‐5‐carbaldehyde (TPA‐CHO). These polymorphs display differing optical characteristics, with GY exhibiting thermally activated delayed fluorescence (TADF) and OR showing room temperature phosphorescence (RTP). Additionally, both polymorphic crystals display mechanical flexibility and optical waveguiding capabilities. Leveraging the AFM‐tip‐based mechanophotonics technique, we position the GY optical waveguide at varying lengths perpendicular to the OR waveguide. This approach facilitates the exploration of the interplay between TADF and RTP phenomena by judiciously controlling the optical path length of crystal waveguides. Essentially, our approach provides a clear pathway for understanding and controlling the photophysical processes in organic molecular crystals, paving the way for advancements in polymorphic crystal‐based photonic circuit technologies. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Carborane compounds, known for their exceptional thermal stability and non‐toxic attributes, have garnered widespread utility in medicine, supramolecular design, coordination/organometallic chemistry, and others. Although there is considerable interest among chemists, the integration of suitable carborane molecules into ferroelectric materials remains a formidable challenge. In this study, we employ the quasi‐spherical design strategy to introduce functional groups at the boron vertices of the o‐carborane cage, aiming to reduce molecular symmetry. This approach led to the successful synthesis of the pioneering ferroelectric crystals composed of cage‐like carboranes: 9‐OH‐o‐carborane (1) and 9‐SH‐o‐carborane (2), which undergo above‐room ferroelectric phase transitions (Tc) at approximately 367 K and 347 K. Interestingly, 1 and 2 represent uniaxial and multiaxial ferroelectrics respectively, with 2 exhibiting six polar axes and as many as twelve equivalent polarization directions. As the pioneering instance of carborane ferroelectric crystals, this study introduces a novel structural archetype for molecular ferroelectrics, thereby providing fresh insights into the exploration of molecular ferroelectric crystals with promising applications. [ABSTRACT FROM AUTHOR]

The fabrication of materials that can switch between circularly polarized luminescence (CPL) signals is both essential and challenging. Here, two new halogen‐bonded fluorescent molecular photoswitches, namely, HB‐switch 1 and HB‐switch 2, containing α‐cyano‐substituted diarylethene compounds with different end groups were developed. Upon exposure to specific UV or visible light wavelengths, they exhibited controllable and reversible Z/E photoisomerization. When these switches were integrated into blue‐phase liquid crystals (BPLCs), the temperature range of BP significantly expanded. Notably, the BP system incorporating HB‐switch 1 exclusively achieved reversible polarization inversion of CPL signals under irradiation with specific UV/Visible light and during cooling/heating. The photo/thermal dual‐response behavior of the CPL signals can be attributed to the phase transition from a high‐symmetry 3D BP Icubic lattice to a low‐symmetry 1D helical superstructure induced by the Z/E photoisomerization of HB‐switch 1 and temperature changes. This study underscores the significance of employing halogen‐bond assembly strategies to design materials with switchable CPL signals, opening new possibilities for CPL‐active systems. [ABSTRACT FROM AUTHOR]

Studies of graphene show that robust chemical bonds such as covalent bonds with trigonal‐planar atoms afford layered atomic 2D crystals possessing unique properties. Although layered molecular crystals are of interest to diversify elements and structures of 2D materials, the structural diversity of molecules as well as weak intermolecular interactions inevitably makes the design to be one‐off and individual. We herein report a versatile method to assemble layered molecular crystals. By developing a D3‐symmetry host at vertices to form a honeycomb layer, a diverse range of layered 2D host–guest crystals were obtained. Substituents on the host, elements/structures of the guest, the stereochemistry of the host and types of intercalants were diversified, which should allow for 6×32×3×2 combinations for structural diversification. [ABSTRACT FROM AUTHOR]

The dense and ordered molecular arrangements endow dynamic molecular crystals with fast response, rapid energy conversion, low energy dissipation, and strong coupling between heat/light and mechanical energy. Most of the known dynamic crystals can only respond to a single stimulus, and materials that can respond to multiple stimuli are rare. Here, we report an organic crystalline material that can be bent plastically and is also thermosalient, as its crystals can move when they undergo a reversible phase transition. The crystals transmit light regardless of their shape or crystalline phase. The combination of light transduction and reversible thermomechanical deformation provides an opportunity to switch the waveguiding capability of the material in a narrow temperature range, which holds a tremendous potential for applications in heat‐averse electronic components, such as central processing units. Unlike existing electronics, the material we report here is completely organic and therefore much lighter, potentially reducing the overall weight of electronic circuits. [ABSTRACT FROM AUTHOR]

The molecular conformation, crystalline morphology, and properties of photochromic organic crystals can be controlled through photoirradiation, making them promising candidates for functional organic materials. However, photochromic porous molecular crystals with a networked framework structure are rare due to the difficulty in maintaining space that allows for photo‐induced molecular motion in the crystalline state. This study describes a photo‐responsive single crystal based on hydrogen‐bonded (H‐bonded) network of dihydrodimethylbenzo[e]pyrene derivative 4BDHP. A crystal composed of H‐bonded undulate layers, 4BDHP‐2, underwent photo‐isomerization in the crystalline state due to loose stacking of the layers. Particularly, enantio‐pure crystal (S,S)‐4BDHP‐2 allowed to reveal the structure of the photoisomerized crystal, in which the closed form (4BDHP) and open form (4CPD) were arranged alternately with keeping crystalline periodicity, although side reactions were also implied. The present proof‐of‐concept system of a photochromic framework that retains crystalline periodicity after photo‐isomerization may provide new light‐driven porous functional materials. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Molecular ultralong room‐temperature phosphorescence (RTP), exhibiting multiple stimuli‐responsive characteristics, has garnered considerable attention due to its potential applications in light‐emitting devices, sensors, and information safety. This work proposes the utilization of photochemical cascade processes (PCCPs) in molecular crystals to design a stepwise smart RTP switch. By harnessing the sequential dynamics of photo‐burst movement (induced by [2+2] photocycloaddition) and photochromism (induced by photogenerated radicals) in a bismuth (Bi)‐based metal‐organic halide (MOH), a continuous and photo‐responsive ultralong RTP can be achieved. Furthermore, utilizing the same Bi‐based MOH, diverse application demonstrations, such as multi‐mode anti‐counterfeiting and information encryption, can be easily implemented. This work thus not only serves as a proof‐of‐concept for the development of solid‐state PCCPs that integrate photosalient effect and photochromism with light‐chemical‐mechanical energy conversion, but also lays the groundwork for designing new Bi‐based MOHs with dynamically responsive ultralong RTP. [ABSTRACT FROM AUTHOR]

Polar molecular crystals, with their densely stacked polar nonlinear optical (NLO) active units, are favored for their large second harmonic generation (SHG) responses and birefringence. However, their potential for practical applications as Infrared (IR) NLO materials has historically been underappreciated due to the weak inter‐molecular interaction forces that may compromise their physicochemical properties. In this study, we propose molecular crystals with polar molecular cages as a treasure‐house for the development of superior IR NLO materials and a representative system, binary chalcogenide molecular crystals, composed of [P4Sn] (n=3–9) polar molecular cages, is introduced. These crystals may not only achieve wide band gap, large SHG response, and birefringence in a single structure, but also exhibit favorable physicochemical properties. We subsequently obtained a polar molecular crystal, α‐P4S5, which demonstrated exceptional IR optical properties, including a strong SHG response (1.1×AGS), wide band gap (3.02 eV), large birefringence (0.134@2050 nm), and a broad transmission range (0.41–14.7 μm). Moreover, it showed excellent water resistance and hardness. These findings highlight the potential of polar molecular crystals as a promising platform for the development of high‐performance IR NLO materials. [ABSTRACT FROM AUTHOR]

We designed a proline‐derived monomer with azide and alkene functional groups to enable topochemical ene‐azide cycloaddition (TEAC) polymerization. In its crystal, the monomer forms supramolecular helices along the 'a' axis through various non‐covalent interactions. Along the 'c' axis, the molecules arrange themselves head‐to‐tail in a wave‐like pattern, positioning the azide and alkene groups of adjacent molecules in close proximity and anti‐parallel orientation, complying with Schmidt's criteria for topochemical reaction. This prearranged configuration was expected to facilitate smooth topochemical polymerization, resulting in a 1,4‐triazoline‐linked polymer. Upon heating, the monomer underwent TEAC polymerization in a remarkable single‐crystal‐to‐single‐crystal fashion, but, to our surprise, it yielded an unexpected covalent helical polymer linked by 1,5‐disubstituted triazoline units. Remarkably, the crystal avoids the ready‐to‐react arrangement for polymerization, but connects monomer molecules within the supramolecular helix through the cycloaddition of azide and alkene groups, even though they are not in close proximity nor in the expected orientation. This unexpected path, involving a substantial 134° rotation of the alkene group, yields hitherto unknown 1,5‐disubstituted triazoline product regiospecifically. This study serves as a cautionary reminder that relying solely on topochemical postulates for predicting reactivity can sometimes be misleading. [ABSTRACT FROM AUTHOR]

Thermally triggered spatial symmetry breaking in traditional ferroelectrics has been extensively studied for manipulation of the ferroelectricity. However, photoinduced molecular orbital breaking, which is promising for optical control of ferroelectric polarization, has been rarely explored. Herein, for the first time, we synthesized a homochiral fulgide organic ferroelectric crystal (E)‐(R)‐3‐methyl‐3‐cyclohexylidene‐4‐(diphenylmethylene)dihydro‐2,5‐furandione (1), which exhibits both ferroelectricity and photoisomerization. Significantly, 1 shows a photoinduced reversible change in its molecular orbitals from the 3 π molecular orbitals in the open‐ring isomer to 2 π and 1 σ molecular orbitals in the closed‐ring isomer, which enables reversible ferroelectric domain switching by optical manipulation. To our knowledge, this is the first report revealing the manipulation of ferroelectric polarization in homochiral ferroelectric crystal by photoinduced breaking of molecular orbitals. This finding sheds light on the exploration of molecular orbital breaking in ferroelectrics for optical manipulation of ferroelectricity. [ABSTRACT FROM AUTHOR]

Precise synthesis of topologically predictable and discrete molecular crystals with permanent porosities remains a long‐term challenge. Here, we report the first successful synthesis of a series of 11 isoreticular multivariate hydrogen‐bonded organic frameworks (MTV‐HOFs) from pyrene‐based derivatives bearing −H, −CH3, −NH2 and −F groups achieved by a shape‐fitted, π–π stacking self‐assembly strategy. These MTV‐HOFs are single‐crystalline materials composed of tecton, as verified by single‐crystal diffraction, nuclear magnetic resonance (NMR) spectra, Raman spectra, water sorption isotherms and density functional theory (DFT) calculations. These MTV‐HOFs exhibit tunable hydrophobicity with water uptake starting from 50 to 80 % relative humidity, by adjusting the combinations and ratios of functional groups. As a proof of application, the resulting MTV‐HOFs were shown to be capable of capturing a mustard gas simulant, 2‐chloroethyl ethyl sulfide (CEES) from moisture. The location of different functional groups within the pores of the MTV‐HOFs leads to a synergistic effect, which resulted in a superior CEES/H2O selectivity (up to 94 %) compared to that of the HOFs with only pure component and enhanced breakthrough performance (up to 4000 min/g) when compared to benchmark MOF materials. This work is an important advance in the synthesis of MTV‐HOFs, and provides a platform for the development of porous molecular materials for numerous applications. [ABSTRACT FROM AUTHOR]

We report thermochromism in crystals of diphenyl diselenide (dpdSe) and diphenyl ditelluride (dpdTe), which is at variance with the commonly known mechanisms of thermochromism in molecular crystals. Variable temperature neutron diffraction studies indicated no conformational change, tautomerization or phase transition between 100 K and 295 K. High‐pressure crystallography studies indicated no associated piezochromism in dpdSe and dpdTe crystals. The evolution of the crystal structures and their electronic band structure with pressure and temperature reveal the contributions of intramolecular and intermolecular factors towards the origin of thermochromism—especially the intermolecular Se⋅⋅⋅Se and Te⋅⋅⋅Te chalcogen bonds and torsional modes of vibrations around the dynamic Se−Se and Te−Te bonds. Further, a co‐crystal of dpdSe with iodine (dpdSe‐I2) and an alloy crystal of dpdSe and dpdTe implied a predominantly intramolecular origin of the observed thermochromism associated with vibronic coupling. [ABSTRACT FROM AUTHOR]

Molecular crystals with the ability to transform light energy into macroscopic mechanical motions are a promising class of materials with potential applications in actuating and photonic devices. In regard to such materials, coordination polymers that exhibit dynamic photomechanical motion, associated with a phase transition, are unknown. Herein, we report an intriguing photoactive, one‐dimensional ZnII coordination polymer, 1, derived from 1,3,5‐tri‐4‐pyridyl‐1,2‐ethenylbenzene and 3,5‐difluorobenzoate. Single crystals of 1 under UV light irradiation exhibit rapid shrinking and bending, violent bursting‐jumping, splitting, and cracking behavior. Single‐crystal X‐ray diffraction analysis and 1H NMR spectroscopy reveal an unusual photoinduced phase transition involving a single‐crystal‐to‐single‐crystal [2+2] cycloaddition reaction that results in photomechanical responses. Interestingly, crystals of 1, which are triclinic with space group P1‾ ${P\bar{1}}$ , are transformed into a higher symmetry, monoclinic cell with space group C2/c. This process represents a rare example of symmetry enhancement upon photoirradiation. The photomechanical activity is likely due to the sudden release of stress associated with strained molecular geometries and significant solid‐state molecular movement arising from cleavage and formation of chemical bonds. A composite membrane fabricated from 1 and polyvinyl alcohol (PVA) also displays interesting photomechanical behavior under UV light illumination, indicating the material's potential as a photoactuator. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Zn metal anode suffers from dendrite growth and side reactions during cycling, significantly deteriorating the lifespan of aqueous Zn metal batteries. Herein, we introduced an ultrathin and ultra‐flat Sb2O3 molecular crystal layer to stabilize Zn anode. The in situ optical and atomic force microscopes observations show that such a 10 nm Sb2O3 thin layer could ensure uniform under‐layer Zn deposition with suppressed tip growth effect, while the traditional WO3 layer undergoes an uncontrolled up‐layer Zn deposition. The superior regulation capability is attributed to the good electronic‐blocking ability and low Zn affinity of the molecular crystal layer, free of dangling bonds. Electrochemical tests exhibit Sb2O3 layer can significantly improve the cycle life of Zn anode from 72 h to 2800 h, in contrast to the 900 h of much thicker WO3 even in 100 nm. This research opens up the application of inorganic molecular crystals as the interfacial layer of Zn anode. [ABSTRACT FROM AUTHOR]

Hydrogen‐bonded organic frameworks (HOFs) with low densities and high porosities are rare and challenging to design because most molecules have a strong energetic preference for close packing. Crystal structure prediction (CSP) can rank the crystal packings available to an organic molecule based on their relative lattice energies. This has become a powerful tool for the a priori design of porous molecular crystals. Previously, we combined CSP with structure‐property predictions to generate energy‐structure‐function (ESF) maps for a series of triptycene‐based molecules with quinoxaline groups. From these ESF maps, triptycene trisquinoxalinedione (TH5) was predicted to form a previously unknown low‐energy HOF (TH5‐A) with a remarkably low density of 0.374 g cm−3 and three‐dimensional (3D) pores. Here, we demonstrate the reliability of those ESF maps by discovering this TH5‐A polymorph experimentally. This material has a high accessible surface area of 3,284 m2 g−1, as measured by nitrogen adsorption, making it one of the most porous HOFs reported to date. [ABSTRACT FROM AUTHOR]

Pore‐shape fixing effects (PSFEs) in soft porous crystals are a relatively unexplored area of materials chemistry. We report the PSFE in the prototypical dynamic van der Waals solid p‐tert‐butylcalix[4]arene (TBC4). Starting with the high‐density guest‐free phase, two porous shape‐fixed phases were programmed using the stimuli of CO2 pressure and temperature. A suite of complementary in situ techniques, including variable‐pressure (VP) single‐crystal X‐ray diffraction, VP powder X‐ray diffraction, VP differential scanning calorimetry, volumetric sorption analysis, and attenuated total reflectance Fourier‐transform infrared spectroscopy was used to track dynamic guest‐induced transformations, providing molecular‐level insight into the PSFE. The interconversion between the two metastable phases is particle size dependent, making this the second example of the PSFE by crystal downsizing, and the first example involving a porous molecular crystal: larger particles undergo reversible transitions while smaller particles remain fixed in the metastable phase. A complete phase interconversion scheme was constructed for the material, thus allowing navigation of the phase interconversion landscape of TBC4 using the easily applied stimuli of CO2 pressure and thermal treatment. [ABSTRACT FROM AUTHOR]

Chiral ferroelectric crystals with intriguing features have attracted great interest and many with point or axial chirality based on the stereocarbon have been successively developed in recent years. However, ferroelectric crystals with stereogenic heteroatomic chirality have never been documented so far. Here, we discover and report a pair of enantiomeric stereogenic sulfur‐chiral single‐component organic ferroelectric crystals, Rs‐tert‐butanesulfinamide (Rs‐tBuSA) and Ss‐tert‐butanesulfinamide (Ss‐tBuSA) through the deep understanding of the chemical design of molecular ferroelectric crystals. Both enantiomers adopt chiral‐polar point group 2 (C2) and exhibit mirror‐image relationships. They undergo high‐temperature 432F2‐type plastic ferroelectric phase transition around 348 K. The ferroelectricity has been well confirmed by ferroelectric hysteresis loops and domains. Polarized light microscopy records the evolution of the ferroelastic domains, according with the fact that the 432F2‐type phase transition is both ferroelectric and ferroelastic. The very soft characteristics with low elastic modulus and hardness reveals their excellent mechanical flexibility. This finding indicates the first stereosulfur chiral molecular ferroelectric crystals, opening up new fertile ground for exploring molecular ferroelectric crystals with great application prospects. [ABSTRACT FROM AUTHOR]

In the quest for essential energy solutions towards an ecological friendly future, the transformation of visible light/solar energy into mechanical motions in metal‐free luminescent crystals offers a sustainable choice of smart materials for lightweight actuating, and all‐organic electronic devices. Such green energy‐triggered photodynamic motions with room temperature phosphorescence (RTP) emission in molecular crystals have not been reported yet. Here, we demonstrate three new stoichiometrically different Lewis acid‐base molecular organoboron crystals (PS1, PS2, and PS3), which exhibit rapid photosalient effects (ballistic splitting, moving, and jumping) under both ultraviolet (UV) and visible light associated with quantitative single‐crystal‐to‐single‐crystal (SCSC) [2+2] cycloaddition of preorganized olefins. Furthermore, these systems respond to sunlight and mobile (white) flashlight with a complete SCSC transformation in a relatively slow fashion. Remarkably, all PS1, PS2, and PS3 crystals display visible light‐promoted dynamic green RTP as their emission peaks promptly blue‐shift, due to instantaneous photomechanical effects. Time‐dependent structural mapping of intermediate photoproducts during fast SCSC [2+2] photoreaction, by X‐ray photodiffraction, reveals a rationale for the origin of these photodynamic motions associated with rapid topochemical transformations. The reported light‐driven behavior (mechanical motions, dynamic phosphorescence, and topochemical reactivity), is considered advantageous for the strategic design of stimuli‐responsive multi‐functional crystalline materials. [ABSTRACT FROM AUTHOR]

Multiple constituent coassembly is an emerging strategy to manipulate supramolecular chirality and chiroptical properties such as circularly polarized luminescence (CPL). However, the second or third constituent could not be removed from pristine self‐assembly. Here we developed a constitute‐removable chiral coassembly using sublimation that could realize coassembly with tunable supramolecular chirality, luminescence and CPL properties. Octafluoronapthalene (OFN) with small sublimation enthalpy formed coassemblies with perylene‐conjugated peptoids via arene‐perfluoroarene (AP) interaction that induced the emergence of macroscopic chirality and hypsochromic luminescence from yellow to green. Coassembly with OFN accelerated one‐dimensional growth and induced the emergence of macroscopic chirality and CPL. Despite the stability at ambient conditions, vacuum‐treatment triggered fast sublimation of OFN, which behaved as a sacrificial template. Physical removal of OFN retained the helical nanoarchitectures as well as the basic features of Cotton effects and CPL activities. X‐ray diffraction suggested the back‐fill consolidation occurred on the molecular voids by OFN removal that slightly varied the templated molecular arrangements. Sublimation of perfluorinated building units is green and efficient and non‐destructive, which is potentially applicable in constructing template‐directed chiroptical materials and devices. [ABSTRACT FROM AUTHOR]

We demonstrate an innovative technique to achieve organic 2D and 3D waveguides with peculiar shapes from an acicular, stimuli‐responsive molecular crystal, (2Z,2′Z)‐3,3′‐(anthracene‐9,10‐diyl)bis(2‐(3,5‐bis(trifluoromethyl)phenylacrylonitrile), Ant‐CF3. The greenish‐yellow fluorescent (FL) Ant‐CF3 molecular crystals exhibit laser power‐dependent permanent mechanical bending in 2D and 3D. Investigation of a single‐crystal using spatially‐resolved Raman/FL/electron microscopy, and theoretical calculations revealed photothermal (Z,E)/(E,E) isomerization‐assisted transition from crystalline to amorphous phase at the laser‐exposed regions. This phenomenon facilitates the dimension engineering of a 1D crystal waveguide into 2D waveguide on a substrate or a 3D waveguide in free space. The bends can be used as interconnection points to couple different optical elements. The presented technique has broader implications in organic photonics and other crystal‐related photonic technologies. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 %). [ABSTRACT FROM AUTHOR]

The molecular crystals that manifest unusual mechanical properties have attracted growing attention. Herein, we prepared an organic single crystal that shows bidirectional superelastic transformation in response to shear stress. Single‐crystal X‐ray diffractions revealed this crystal‐twinning related shape change is owed to a stress‐controlled 90° rotation of 4,4′‐bipyridine around the hydrogen bonds of a chiral organic trimer. As a consequence of the 90° shift in the aromatic plane, an interconversion of crystallographic a‐, b‐axes (a→b′ and b→a′) was detected. The molecular rotations changed the anisotropic absorption of linearly polarized light. Therefore, a stress‐induced inversion of linear dichroism spectra was demonstrated for the first time. This study reveals the superior mechanical flexibilities of single crystals can be realized by harnessing the molecular rotations and this superelastic crystal may find applications in optical switching and communications. [ABSTRACT FROM AUTHOR]

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‐Sb2O3 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‐Sb2O3 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Photomechanical molecular crystals are promising materials for photon‐powered artificial actuators. To interpret the photomechanical responses, the spatiotemporal distribution of photoproducts in crystals could be an important role in addition to molecular structures, molecular packings, illumination conditions, crystal morphology, crystal size, and so on. In this study, we have found that single crystals of 2,5‐distyrylpyrazine show a smooth single‐crystal‐to‐single‐crystal photomechanical expansion, and the photochemical reaction propagates from the edge to the center of the single crystal. We revealed that the surface effect (special reactivity at the crystal surface) in addition to the cooperative effect (the reaction is facilitated by neighboring molecules) is essential for the edge‐to‐center propagation of the photochemical reaction. Our results would provide a foundation for future studies of the photochemical reaction dynamics in photomechanical molecular crystals. [ABSTRACT FROM AUTHOR]

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 2H NMR measurements revealed significant 3D molecular motion of imidazolium cations, which accelerate proton conduction via the 3D H‐bonded phosphate network. [ABSTRACT FROM AUTHOR]

Recently, porous organic crystals (POC) based on macrocycles have shown exceptional sorption and separation properties. Yet, the impact of guest presence inside a macrocycle prior to adsorption has not been studied. Here we show that the inclusion of trimethoxybenzyl‐azaphosphatrane in the macrocycle cucurbit[8]uril (CB[8]) affords molecular porous host⋅guest crystals (PHGC‐1) with radically new properties. Unactivated hydrated PHGC‐1 adsorbed iodine spontaneously and selectively at room temperature and atmospheric pressure. The absence of (i) heat for material synthesis, (ii) moisture sensitivity, and (iii) energy‐intensive steps for pore activation are attractive attributes for decreasing the energy costs. 1H NMR and DOSY were instrumental for monitoring the H2O/I2 exchange. PHGC‐1 crystals are non‐centrosymmetric and I2‐doped crystals showed markedly different second harmonic generation (SHG), which suggests that iodine doping could be used to modulate the non‐linear optical properties of porous organic crystals. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Under cold pressure sp1/sp2‐to‐sp3 hybridization transformation has been exclusively observed in covalent or molecular crystals overwhelmingly above ≈10 GPa, and the approaches to lower the transition pressure are limited on external heat‐treatment and/or catalyzers. Herein we demonstrate that, by internal‐lattice stress‐transfer from ionic to covalent groups, the transformation can be significantly prompted, as shown in a crystal of LiBO2 under 2.85 GPa for the first case in ionic crystals. This unprecedentedly low transformation pressure is ascribed to the enhanced localized stress on covalent B−O frames transferred from ionic Li−O bonds in LiBO2, and accordingly the corresponding structural feature is summarized. This work provides an internal structural regulation strategy for pressure‐reduction of the s‐p orbital hybridization transformation and extends the sp1/sp2‐to‐sp3 transformation landscape from molecular and covalent compounds to ionic systems. [ABSTRACT FROM AUTHOR]

Integrating plasticity and elasticity in anisotropic molecular crystals is theoretically possible and is beneficial in enabling rich and complex deformations; however, it is much harder to implement in practice. Herein, we report a centimeter‐long organic crystal that is two‐dimensionally (2D) elastically bendable not only at room temperature but also at ultralow temperatures (−196 °C). The straight crystal can also be manually twisted and reconfigured to form arbitrary right‐handed or left‐handed helical structures. The integration of low‐temperature‐resistant (LTR) 2D elastic‐bending and reconfigurable plastic‐twisting deformations into one organic crystal expands the perspectives of the emerging crystal flexibility. Taking advantage of the unique multiple flexibility characteristics, spatial controllability of optical transmission for cryogenic applications and reusability of light‐polarization rotations have been implemented simultaneously in an organic crystal. [ABSTRACT FROM AUTHOR]

Circular polarized light is utilized in communication and display technologies and a major challenge is to develop systems that can be switched between left and right circular polarized luminescence with high degrees of polarization and enable multiple addressable stable states. Luminescent dyes in Liquid Crystal (LC) cholesteric phases are attractive systems to generate, amplify and modulate circularly polarized luminescence (CPL). In the present study, we employ light‐driven molecular motors as photo‐controlled chiral dopants in LCs to switch the handedness of the LC and the circular polarization of luminescence from an achiral dye embedded in the mesogenic material. Tuning of the color of the CPL and the retention time of the photoprogrammed helicity is demonstrated making use of a variety of motors and dyes. The flexibility offered by the design based on inherently chiral unidirectional rotary motors provides full control over CPL non‐invasively by light, opening possibilities for pixilated displays with externally addressable polarization. [ABSTRACT FROM AUTHOR]

The rapid development of information technology has resulted in a growing demand for low‐dimensional photonic materials. Organic semiconductor materials play an important role in various photonic devices due to their adjustable physicochemical properties, while individual organic crystals do not exhibit the desired performance due to the limitations of their simple structure. Branched organic crystals with inherent multichannel characteristics based on π‐conjugated molecules are favorable components in optoelectronics. However, the preparation of branched organic crystals still faces great challenges before they can be applied in integrated optoelectronic devices. In this Review, the development and representative examples of branched organic crystals in terms of molecular design, synthesis, and advanced applications are discussed. We also provide a summary and outlook for the direction of future research on branched organic crystals as excellent candidates in photonic integrated circuits. [ABSTRACT FROM AUTHOR]

Selective separation using porous adsorbents is an energy‐efficient alternative to traditional separation techniques. Stacked porous organic molecular frameworks (POMFs) capable of noncovalent π⋅⋅⋅π interactions are emerging as a new kind of adsorbents that facilitate green separation. Here we report a robust porous molecular crystal (TAPM‐1), which is stabilized by multiple intermolecular π⋅⋅⋅π interactions. With its long‐range π‐stacking, TAPM‐1 has excellent hydrophobicity, thermostability, recyclability, and high selectivity for aromatics over the corresponding cyclic aliphatics. This enables TAPM‐1 to serve as the stationary phase in the high‐resolution gas chromatographic separation of benzene and cyclohexane or toluene and methylcyclohexane. [ABSTRACT FROM AUTHOR]

Reshaping of elongated organic crystals that can be used as semiconductors, waveguides or soft robotic grippers by application of force or light is now a commonplace, however mechanical response of organic crystals to changes in humidity has not been accomplished yet. Here, we report a universal approach to instigating a humidity response into elastically bendable organic crystals that elicits controllable deformation with linear response to aerial humidity while retaining their physical integrity entirely intact. Hygroresponsive bilayer elements are designed by mechanically coupling a humidity‐responsive polymer with elastic molecular crystals that have been mechanically reinforced by a polymer coating. As an illustration of the application of these cladded crystalline actuators, they are tested as active optical transducers of visible light where the position of light output can be precisely controlled by variations in aerial humidity. Within a broader context, the approach described here provides access to a vast range of mechanically robust, lightweight hybrid hygroresponsive crystalline materials. [ABSTRACT FROM AUTHOR]

This review presents a critical and comprehensive overview of current experimental measurements of complete elastic constant tensors for molecular crystals. For a large fraction of these molecular crystals, detailed comparisons are made with elastic tensors obtained using the corrected small basis set Hartree–Fock method S‐HF‐3c, and these are shown to be competitive with many of those obtained from more sophisticated density functional theory plus dispersion (DFT‐D) approaches. These detailed comparisons between S‐HF‐3c, experimental and DFT‐D computed tensors make use of a novel rotation‐invariant spherical harmonic description of the Young's modulus, and identify outliers among sets of independent experimental results. The result is a curated database of experimental elastic tensors for molecular crystals, which we hope will stimulate more extensive use of elastic tensor information—experimental and computational—in studies aimed at correlating mechanical properties of molecular crystals with their underlying crystal structure. [ABSTRACT FROM AUTHOR]

Photomechanical molecular crystals that expand under illumination could potentially be used as photon‐powered actuators. In this study, we find that the use of high‐quality single crystals of 9‐methylanthracene (9MA) leads to more homogeneous reaction kinetics than that previously seen for polycrystalline samples, presumably due to a lower concentration of defects. Furthermore, simultaneous observation of absorbance and shape changes in single crystals revealed that the dimensional change mirrors the reaction progress, resulting in a smooth expansion of 7 % along the c‐axis that is linearly correlated with reaction progress. The same expansion dynamics are highly reproducible across different single crystal samples. Organic single crystals exhibit well‐defined linear expansions during 100 % photoconversion, suggesting that this class of solid‐state phase change material could be used for actuation. [ABSTRACT FROM AUTHOR]

The development of large pore single‐crystalline covalently linked organic frameworks is critical in revealing the detailed structure‐property relationship with substrates. One emergent approach is to photo‐crosslink hydrogen‐bonded molecular crystals. Introducing complementary hydrogen‐bonded carboxylic acid building blocks is promising to construct large pore networks, but these molecules often form interpenetrated networks or non‐porous solids. Herein, we introduced heteromeric carboxylic acid dimers to construct a non‐interpenetrated molecular crystal. Crosslinking this crystal precursor with dithiols afforded a large pore single‐crystalline hydrogen‐bonded crosslinked organic framework HCOF‐101. X‐ray diffraction analysis revealed HCOF‐101 as an interlayer connected hexagonal network, which possesses flexible linkages and large porous channels to host a hydrazone photoswitch. Multicycle Z/E‐isomerization of the hydrazone took place reversibly within HCOF‐101, showcasing the potential use of HCOF‐101 for optical information storage. [ABSTRACT FROM AUTHOR]

The ability to rationally design and predictably construct crystalline solids has been the hallmark of crystal engineering research. To date, numerous examples of multicomponent crystals comprising organic molecules have been reported. However, the crystal engineering of cocrystals comprising both organic and inorganic chemical units is still poorly understood and mostly unexplored. Here, we report a new diverse set of higher‐order cocrystals (HOCs) based on the structurally versatile—yet largely unexplored—phosph(V/V)azane heterosynthon building block. The novel ternary and quaternary cocrystals reported are held together by synergistic and orthogonal intermolecular interactions. Notably, the HOCs can be readily obtained either via sequential or one‐pot mechanochemical methods. Computational modelling methods reveal that the HOCs are thermodynamically driven to form and that their mechanical properties strongly depend on the composition and intermolecular forces in the crystal, offering untapped potential for optimizing material properties. [ABSTRACT FROM AUTHOR]

A porous molecular crystal (PMC) assembled by macrocyclic cyclotetrabenzoin acetate is an efficient adsorbent for CO2 separations. The 7.1×7.1 Å square pore of PMC and its ester C=O groups play important roles in improving its affinity for CO2 molecules. The benzene walls of macrocycle engage in an apparent [π⋅⋅⋅π] interaction with the molecule of CO2 at low pressure. In addition, the polar carbonyl groups pointing inward the square channels reduce the size of aperture to a 5.0×5.0 Å square, which offers kinetic selectivity for CO2 capture. The PMC features water tolerance and high structural stability under vacuum and various gas adsorption conditions, which are rare among intrinsically porous organic molecules. Most importantly, the moderate adsorbate‐adsorbent interaction allows the PMC to be readily regenerated, and therefore applied to pressure swing adsorption processes. The eluted N2 and CH4 are obtained with over 99.9 % and 99.8 % purity, respectively, and the separation performance is stable for 30 cycles. Coupled with its easy synthesis, cyclotetrabenzoin acetate is a promising adsorbent for CO2 separations from flue and natural gases. [ABSTRACT FROM AUTHOR]

Ultrabright fluorescent nanoparticles (NPs) hold great promise for demanding bioimaging applications. Recently, extremely bright molecular crystals of cationic fluorophores were obtained by hierarchical coassembly with cyanostar anion‐receptor complexes. These small‐molecule ionic isolation lattices (SMILES) ensure spatial and electronic isolation to prohibit aggregation quenching of dyes. We report a simple, one‐step supramolecular approach to formulate SMILES materials into NPs. Rhodamine‐based SMILES NPs stabilized by glycol amphiphiles show high fluorescence quantum yield (30 %) and brightness per volume (5000 M−1 cm−1/nm3) with 400 dye molecules packed into 16‐nm particles, corresponding to a particle absorption coefficient of 4×107 M−1 cm−1. UV excitation of the cyanostar component leads to higher brightness (>6000 M−1 cm−1/ nm3) by energy transfer to rhodamine emitters. Coated NPs stain cells and are thus promising for bioimaging. [ABSTRACT FROM AUTHOR]

Herein we report a linear ionic molecule that assembles into a supramolecular nano‐tunnel structure through synergy of trident‐type ionic interactions and π–π stacking interactions. The nano‐tunnel crystal exhibits anisotropic guest adsorption behavior. The material shows good thermal stability and undergoes multi‐stage single‐crystal‐to‐single‐crystal phase transformations to a nonporous structure on heating. The material exhibits a remarkable chemical stability under both acidic and basic conditions, which is rarely observed in supramolecular organic frameworks and is often related to structures with designed hydrogen‐bonding interactions. Because of the high polarity of the tunnels, this molecular crystal also shows a large CO2‐adsorption capacity while excluding other gases at ambient temperature, leading to high CO2/CH4 selectivity. Aggregation‐induced emission of the molecules gives the bulk crystals vapochromic properties. [ABSTRACT FROM AUTHOR]

The ability to exhibit life‐like oscillatory motion fueled by light represents a new capability for stimuli‐responsive materials. Although this capability has been demonstrated in soft materials like polymers, it has never been observed in molecular crystals, which are not generally regarded as dynamic objects. In this work, it is shown that molecular crystalline microwires composed of (Z)‐2‐(3‐(anthracen‐9‐yl)allylidene)malononitrile ((Z)‐DVAM) can be continuously actuated when exposed to a combination of ultraviolet and visible light. The photo‐induced motion mimics the oscillatory behavior of biological flagella and enables propagation of microwires across a surface and through liquids, with translational speeds up to 7 μm s−1. This is the first example of molecular crystals that show complex oscillatory behavior under continuous irradiation. A model that relates the rotation of the transition dipole moment between reversible E→Z photoisomerization to the microscopic torque can qualitatively reproduce how the rotational frequency depends on light intensity and polarization. [ABSTRACT FROM AUTHOR]

Long‐lived room temperature phosphorescence from organic molecular crystals attracts great attention. Persistent luminescence depends on the electronic properties of the molecular components, mainly π‐conjugated donor–acceptor (D‐A) chromophores, and their molecular packing. Here, a strategy is developed by designing two isomeric molecular phosphors incorporating and combining a bridge for σ‐conjugation between the D and A units and a structure‐directing unit for H‐bond‐directed supramolecular self‐assembly. Calculations highlight the critical role played by the two degrees of freedom of the σ‐conjugated bridge on the chromophore optical properties. The molecular crystals exhibit RTP quantum yields up to 20 % and lifetimes up to 520 ms. The crystal structures of the efficient phosphorescent materials establish the existence of an unprecedented well‐organization of the emitters into 2D rectangular columnar‐like supramolecular structure stabilized by intermolecular H‐bonding. [ABSTRACT FROM AUTHOR]

The coherence time of the 17‐electron, mixed sandwich complex [CpTi(cot)], (η8‐cyclooctatetraene)(η5‐cyclopentadienyl)titanium, reaches 34 μs at 4.5 K in a frozen deuterated toluene solution. This is a remarkable coherence time for a highly protonated molecule. The intramolecular distances between the Ti and H atoms provide a good compromise between instantaneous and spin diffusion sources of decoherence. Ab initio calculations at the molecular and crystal packing levels reveal that the characteristic low‐energy ring rotations of the sandwich framework do not yield a too detrimental spin‐lattice relaxation because of their small spin–phonon coupling. The volatility of [CpTi(cot)] and the accessibility of the semi‐occupied, non‐bonding dz2 orbital make this neutral compound an ideal candidate for single‐qubit addressing on surface and quantum sensing in combination with scanning probe microscopy. [ABSTRACT FROM AUTHOR]

The exploitation of new materials for adsorptive separation of industrially important hydrocarbons is of great importance in both scientific research and petrochemical industry. Nonporous adaptive crystals (NACs) as a robust class of synthetic materials have drawn much attention during the past five years for their superior performance in adsorption and separation. Pillararenes are the main family of macrocyclic arenes used for NACs construction, where the structure–function relationship has been intensively studied. In the past two years, some emerging types of synthetic macrocyclic arenes have been successfully brought into this research field, showing the gradual enrichment and diversification of NACs materials. This Minireview summarizes the recent advances of synthetic macrocycle‐based NACs, which are categorized by various practical applications in molecular separation. Besides, NACs‐based vapochromic supramolecular systems are also discussed. Finally, future perspectives and challenges of NACs are given. We envisage that this Minireview will be a useful and timely reference for those who are interested in new molecular and supramolecular crystals for storage and separation applications. [ABSTRACT FROM AUTHOR]