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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]

The racemization of chiral organic compounds is a common chemical phenomenon. However, it often poses configurational‐stability issues to the application of this class of compounds. Achieving chiral organic compounds without the risk of racemization is fascinating, but it is challenging due to a lack of strategies. Here, we reveal the cove‐regions bridging strategy for achieving persistently chiral multi‐helicenes (incapable of racemization), based on the synthesized proof‐of‐concept double hetero[4]helicenes featuring macrocycle structures with a small 3D cavity. Additionally, we demonstrate that the strategy is also effective in tuning the electronic structures of multi‐helicenes, resulting in a conversion from luminescence silence into thermally activated delayed fluorescence (TADF) for the present system. Furthermore, red circularly polarized TADF based on small double [4]helicene systems is achieved for the first time using this strategy. The disclosed cove‐regions bridging strategy provides an opportunity to modulate the electronic structures and luminescent properties of multi‐helicenes without concern for racemization, thus significantly enhancing the structural and property diversity of multi‐helicenes for various applications. [ABSTRACT FROM AUTHOR]

Polymorphism arising from different conformations or packing arrangements in crystal lattices would manipulate physical properties, including morphology, electrical conductivity, emission, and nonlinear optical properties. However, the photochemistry tuned by polymorphism is seldom reported. In this work, photochemical reactive isomers of (E)-2-(1H-benzo[d]imidazol-2-yl)-3-(4-fluorophenyl)acrylonitrile (E-CN-BIFPh) and (Z)-3-(1H-benzo[d]imidazol-2-yl)-2-(4-fluorophenyl)acrylonitrile (Z-BIFPh-CN) in which a cyano group is located on different positions in conjugated skeletons were synthesized. Interestingly, E-CN-BIFPh crystallized into sheet-like structures emitting orange-red fluorescence (OR-phase) and block-like structures showing green fluorescence (G-phase). Besides, cuboid-like crystals with green fluorescence (G-phase) and needle-like crystals emitting blue light (B-phase) were formed from Z-BIFPh-CN. It is interesting that [2+2] cycloaddition took place only in the OR-phase crystal of E-CN-BIFPh and the B-phase crystal of Z-BIFPh-CN , which powered the molecular crystals to exhibit obvious photoactuation. In the case of the G-phase crystals of E-CN-BIFPh and Z-BIFPh-CN , the geometric parameters for "olefin pair" deviated from Schmidt's criteria, and hence, no photodimerization occurred. Notably, the OR-phase crystal of E-CN-BIFPh exhibited high-contrast mechanofluorochromism (MFC) with a red-shift of 115 nm in emission. The phase transition of the G-phase crystals of Z-BIFPh-CN upon heating at 105 °C led to a change in the emission color from green to blue, showing thermochromism. Such polymorphism crystallization-controlled multi-stimuli-responsive molecular crystals might be employed as elements in artificial muscles, smart microrobots, wearable systems and sensors. [ABSTRACT FROM AUTHOR]

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters with narrow emission spectra have garnered significant attention in future organic light‐emitting diode (OLED) displays. However, current C=O/N‐embedded MR‐TADF systems still lack satisfactory performance in terms of electroluminescence bandwidths and external quantum efficiencies (EQEs). In this study, a C=O/N‐embedded green MR‐TADF emitter, featuring two acridone units incorporated in a sterically protected 11‐ring fused core skeleton, is successfully synthesized through finely controlling the reaction selectivity. The superior combination of multiple intramolecular fusion and steric wrapping strategies in the design of the emitter not only imparts an extremely narrow emission spectrum and a high fluorescence quantum yield to the emitter but also mitigates aggregation‐induced spectral broadening and fluorescence quenching. Therefore, the emitter exhibits leading green OLED performance among C=O/N‐based MR‐TADF systems, achieving an EQE of up to 37.2 %, a full width at half maximum of merely 0.11 eV (24 nm), and a Commission Internationale de l′Éclairage coordinate of (0.20, 0.73). This study marks a significant advance in the realization of ideal C=O/N‐based MR‐TADF emitters and holds profound implications for the design and synthesis of other MR‐TADF systems. [ABSTRACT FROM AUTHOR]

Inspired by sophisticated and intelligent biological systems, various smart actuations triggered by physical or chemical stimuli have been developed. Photomechanical materials that can convert light energy into mechanical work have attracted significant attention given that light, as clean energy, enables controllable, remote and precise manipulation without electromagnetic disturbance. Compared with polymers and hydrogels, photomechanical molecular crystals with a higher Young's modulus may exhibit rapid and significant movement via efficient energy conversion, which is closely related to their crystal structures. Over the past few decades, the halogen effect has received increasing attention in crystal engineering due to the modulation of intermolecular interactions, such as halogen bonds (X⋯X) and hydrogen bonds (C–H⋯X). In this review, we summarize the photomechanical effects of crystalline materials in which halogen effects play a key role in molecular arrangements, fueled by the photochemical reactions of cis–trans isomerization, [2+2] cycloaddition, [4+4] cycloaddition, and electrocyclic reactions. Furthermore, their possible applications in soft robots, biomimetic systems, artificial muscles, biomedicine and microfluidics, and some challenges are also highlighted. [ABSTRACT FROM AUTHOR]

Generally, the potential reactive "olefin pairs" in the molecular crystals satisfying Schmidt's criteria could undergo topological [2+2] cycloaddition. In this study, another factor that affects the photodimerization reactivity of chalcone analogues was found. The cyclic chalcone analogues of (E)‐2‐(2,4‐dichlorobenzylidene)‐2,3‐dihydro‐1H‐inden‐1‐one (BIO), (E)‐2‐(naphthalen‐2‐ylmethylene)‐2,3‐dihydro‐1H‐inden‐1‐one (NIO), (Z)‐2‐(2,4‐dichlorobenzylidene)benzofuran‐3(2H)‐one (BFO), and (Z)‐2‐(2,4‐dichlorobenzylidene)benzo[b]thiophen‐3(2H)‐one (BTO) have been synthesized. While the geometrical parameters for the molecular packing of the above four compounds did not exceed Schmidt's criteria, [2+2] cycloaddition did not occur in the crystals of BIO and BTO. The single crystal structures and Hirshfeld surface analyses revealed that interactions of C=O⋅⋅⋅H (CH2) existed between adjacent molecules in the crystal of BIO. Therefore, the carbonyl and methylene groups linked with one carbon atom in carbon‐carbon double bond were tightly confined in the lattice, acting as a tweezer to inhibit free movement of the double bond and suppressing [2+2] cycloaddition. In the crystal of BTO, similar interactions of Cl⋅⋅⋅S and C=O⋅⋅⋅H (C6H4) prevented free movement of the double bond. In contrast, the intermolecular interaction of C=O⋅⋅⋅H only exists around the carbonyl group in the crystals of BFO and NIO, leaving the C=C double bonds to move freely and allowing the occurrence of [2+2] cycloaddition. Driven by photodimerization, the needle‐like crystals of BFO and NIO displayed evident photo‐induced bending behavior. This work demonstrates that the intermolecular interactions around carbon‐carbon double bond affect the [2+2] cycloaddition reactivity except for Schmidt's criteria. These findings provide valuable insights into the design of photomechanical molecular crystalline materials. [ABSTRACT FROM AUTHOR]

Organic crystals that are capable to deform (reversibly or irreversibly) similar to polymer materials have been widely reported over the past ten years. However, most of the reported organic crystals can only be elastically bent within a narrow temperature range, and reports on their thermal behavior that would encourage applications of these energy‐transducing elements in extreme conditions are not readily available. This work designs a linear and flat π‐conjugated molecule with double intramolecular hydrogen bonds that prevent thermally induced conformational distortions. The molecule assembles as a rigid building block into centimeter‐sized wide organic crystals that can be elastically bent over a temperature range spanning close to 400 °C, from −196 to ≈200 °C. The emission wavelength of the crystals is also temperature dependent, and can be continually tuned from 547 to 577 nm upon heating from 20 to ≈200 °C. This inspires the design of a lightweight, organic, elastic optical waveguide where the output energy is controlled by the operating temperature. The wide range of crystal flexibility expands the range of conditions for application of organic crystals as optical waveguides. [ABSTRACT FROM AUTHOR]

Herein, we have synthesized three new phenylquinolinylacrylonitrile derivatives. It is found that ((Z)-2-phenyl-3-(quinolin-4-yl)acrylonitrile) (1) and ((Z)-2-(4-fluorophenyl)-3-(quinolin-4-yl)acrylonitrile) (2) can form organogels in polar solvents, and compound 2 shows good gelation ability in acetone/H2O (v/v = 1/1.3) with a low critical gelation concentration (CGC) of 0.11 wt%. Interestingly, the free standing organogel (FSOG) with remarkable mechanical strength can be formed from gelator 2 based on its strong self-assembling capability. The storage modulus of FSOG was measured to be up to the 104 Pa level at a low concentration (0.74 wt%), which is much higher than those of most LMWGs. The FSOG exhibited reversible external force-induced bending without breaking and we could cut it arbitrarily. Immersing the FSOG in water for several hours affords the free-standing hydrogel (FSHG). Since a dense film was formed on the surface of FSHG, the mechanical strength of FSHG was further improved compared with that of FSOG. When exposed to 365 nm light, the freshly prepared organogel would be collapsed into a suspension because 2 underwent the tandem reactions of isomerization, electrocyclization and oxidative aromatization to yield 3-fluorobenzo[i]phenanthridine-12-carbonitrile (4) to destroy the intermolecular interactions among the molecules of 2. In addition, due to the aggregation-induced emission of compounds 1 and 2, the gained xerogels exhibited bright fluorescence. For instance, the blue fluorescence of the xerogel film of 2 could be switched off by TFA (trifluoroacetic acid) vapor with a response time of 0.38 s and a detection limit of 0.43 ppm. [ABSTRACT FROM AUTHOR]

Three groups of chalcone derivatives and their analogues involving halogen atoms (X=F, Cl, Br) have been synthesized. Firstly, the nearly planar acyclic chalcone derivatives were inclined to undergo photo‐induced stereospecific [2+2] cycloaddition, which triggered the crystals to exhibit macroscopic motions of bending or cracking. In particular, the single‐crystal‐to‐single‐crystal transformation happened upon UV irradiation of the crystals, which was helpful for the understanding photomechanical effects. Cyclic 3,4‐dihydronaphthalene‐based chalcone analogues possess a more twisted conformation, and they tend to undergo trans‐cis isomerization. No photomechanical effect was observed for the crystals of the cyclic chalcone analogues due to the lower isomerization rate. The twist degree of chroman‐based molecules was in between of the first two, [2+2] cycloaddition and trans‐cis isomerization simultaneously took place in crystals. Photo‐induced bending and twisting were observed for the crystals of chroman‐based chalcone analogues. Therefore, the differences in molecular dihedral angles in α,β‐unsaturated ketones were responsible for their photochemical characters and in turn to tune the photomechanical effects. In this work, a bridge between the molecular structures and solid‐state photochemical reactions triggered photomechanical crystals is built. [ABSTRACT FROM AUTHOR]