: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

There is often a trade-off between mechanical properties (modulus and toughness) and dynamic self-healing. Here we report the design and synthesis of a polymer containing thermodynamically stable whilst kinetically labile coordination complex to address this conundrum. The Zn-Hbimcp (Hbimcp = 2,6-bis((imino)methyl)-4-chlorophenol) coordination bond used in this work has a relatively large association constant (2.2 × 1011) but also undergoes fast and reversible intra- and inter-molecular ligand exchange processes. The as-prepared Zn(Hbimcp)2-PDMS polymer is highly stretchable (up to 2400% strain) with a high toughness of 29.3 MJ m−3, and can autonomously self-heal at room temperature. Control experiments showed that the optimal combination of its bond strength and bond dynamics is responsible for the material’s mechanical toughness and self-healing property. This molecular design concept points out a promising direction for the preparation of self-healing polymers with excellent mechanical properties. We further show this type of polymer can be potentially used as energy absorbing material. There is often a trade-off between mechanical properties (modulus and toughness) and dynamic self-healing in materials. Here the authors design and synthesize a polymer containing thermodynamically stable whilst kinetically labile coordination complexes to address this conundrum.

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Thermodynamically stable whilst kinetically labile coordination bonds lead to strong and tough self-healing polymers

Achieving a desirable combination of solid-like properties and fast self-healing is a great challenge due to slow diffusion dynamics. In this work, we describe a design concept that utilizes weak but abundant coordination bonds to achieve this objective. The designed PDMS polymer, crosslinked by abundant Zn(II)-carboxylate interactions, is very strong and rigid at room temperature. As the coordination equilibrium is sensitive to temperature, the mechanical strength of this polymer rapidly and reversibly changes upon heating or cooling. The soft–rigid switching ability σ, defined as G’max /G’min, can reach 8000 when ΔT = 100 °C. Based on these features, this polymer not only exhibits fast thermal-healing properties, but is also advantageous for various applications such as in orthopedic immobilization, conductive composites/adhesives, and 3D printing. Combining solid-like properties with fast self-healing is a great challenge due to slow diffusion dynamics. Here the authors demonstrate a rigid and healable material by using weak but abundant coordination bonds to crosslink a PDMS polymer.

A rigid and healable polymer cross-linked by weak but abundant Zn(II)-carboxylate interactions

The anticipated shift in the focal point of interest of solid-state chemists, crystal engineers, and crystallographers from structure to properties to function parallels the need to apply our accumulated understanding of the intricacies of crystal structure to explaining the related properties, with the ultimate goal of harnessing that knowledge in applications that require soft, lightweight, or biocompatible organic solids. In these developments, the adaptive molecular crystals warrant particular attention as an alternative choice of materials for light, flexible, and environmentally benign devices, primarily memories, capacitors, sensors, and actuators. Some of the outstanding requirements for the application of these dynamic materials as high-efficiency energy-storage devices are strongly induced polarization, a high switching field, and narrow hysteresis in the case of reversible dynamic processes. However, having been studied almost exclusively by chemists, molecular crystals still lack the appropriate investigations that reliably evaluate their reproducibility, scalability, and actuating performance, and some important drawbacks have diverted the interest of engineers from these materials in applications. United under the umbrella term crystal adaptronics, the recent research efforts aim to realistically assess the appositeness of dynamic crystals for applications that require fast, reversible, and continuous operation over prolonged periods of time. With the aim of highlighting the most recent developments, this Perspective discusses their assets and pitfalls. It also provides some hints on the likely future developments that capitalize on the untapped, sequestered potential of this distinct materials class for applications.

The Rise of the Dynamic Crystals

A strategy is outlined for the design of hand-twisted helical crystals. The starting point in the exercise is the one-dimensional (1D) plastic crystal, 1,4-dibromobenzene, which is then changed to a 1D elastic crystal, exemplified by 4-bromophenyl 4′-chlorobenzoate, by introduction of a molecular synthon −O–CO– in lieu of the supramolecular synthon Br···Br in the precursor. The 1D elastic crystals are next modified to two-dimensional (2D) elastic crystals, of the type 4-bromophenyl 4′-nitrobenzoate where the halogen bonding and C–H···O hydrogen bonding are well-matched. Finally, varying the interaction strengths in these 2D elastic crystals gives plastic crystals with two pairs of bendable faces but without slip planes. Typical examples are 4-chlorophenyl and 4-bromophenyl 4′-nitrobenzoate. This type of 2D plasticity represents a new type of bendable crystals in which plastic behavior is seen with a fair degree of isotropic character in the crystal packing. The presence of two sets of bendable faces, gene...

Crystal Engineering of Hand-Twisted Helical Crystals.

One of the most inevitable limitations of any material that is exposed to mechanical impact is that they are inexorably prone to mechanical damage, such as cracking, denting, gouging, or wearing. To confront this challenge, the field of polymers has developed materials that are capable of autonomous self-healing and recover their macroscopic integrity similar to biological organisms. However, the study of this phenomenon has mostly remained within the soft materials community and has not been explored by solid-state organic chemists. The first evidence of self-healing in a molecular crystal is now presented using crystals of dipyrazolethiuram disulfide. The crystals were mildly compressed and the degree of healing was found to be 6.7 %. These findings show that the self-healing properties can be extended beyond mesophasic materials and applied towards the realm of ordered solid-state compounds.

Self-Healing Molecular Crystals

Mechanically compliant organic crystals are the foundation of the development of future flexible, light-weight single-crystal electronics, and this requires reversibly deformable crystalline organic materials with permanent magnetism. Here, we report and characterize the first instance of a plastically bendable single crystal of a permanent organic radical, 4-(4′-cyano-2′,3′,4′,5′-tetrafluorophenyl)-1,2,3,5-dithiadiazolyl. The weak interactions between the radicals render single crystals of the β phase of this material exceedingly soft, and the S–N interactions facilitate plastic bending. EPR imaging of a bent single crystal reveals the effect of deformation on the three-dimensional spin density of the crystal. The unusual mechanical compliance of this material opens prospects for exploration into flexible crystals of other stable organic radicals towards the development of flexible light-weight organic magnetoresistance devices based on weak, non-hydrogen-bonded interactions in molecular crystals.

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Mechanically compliant single crystals of a stable organic radical

The photochemical conversion of 1,8a-dihydroazulene-1,1-dicarbonitrile (DHA) to vinylheptafulvene (VHF) is a positive T-type photoswitch that is well understood in solution, but has not been explored in the solid state. Upon excitation with UV light, DHA is converted into VHF in the solid state, with a distinct color change from yellow to deep-red, and retention of crystallinity. The structure of the ring-opened product was assigned to syn-VHF using variable-temperature infrared spectroscopy, and determined by X-ray photodiffraction in a crystal enriched with the product by two-photon excitation. A radical pathway becomes an observable photoreaction channel at low temperatures, and includes a strongly colored, short-lived diradical intermediate.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/chem.201804677

Reversible Multicolor Photochromism of Dihydroazulene Crystals

A photoinduced analogue of the thermal Orton rearrangement reaction by which an N-chlorine atom from a side amino group is transferred to a phenyl ring was studied in the solid state. Contrary to the mixture of products obtained in solution, in the N-chloro-N-acetylaminobenzene crystals the photoreaction proceeds with complete preservation of crystallinity, affording selectively and quantitatively the para isomer of chloroacetanilide. Study of the reaction mechanism by in situ steady-state photodiffraction, a combination of photoexcitation by UV light and single-crystal X-ray diffraction analysis, provided evidence for creation of N-acetyl-N-phenylaminyl (AcPhN•) radical as a metastable reaction intermediate. The structure of the aminyl radical produced in 9.2% yield from the major disordered component in the statically 85.6:14.4 disordered crystal was directly observed for the first time. The unprecedented stability of the radical is prescribed to the solid-state cage effect, the reactive center of the r...

Direct observation of aminyl radical intermediate during single-crystal to single-crystal photoinduced Orton rearrangement.

We report a solid-state photochemical rearrangement reaction by which aromatic N-chloroamides exposed to UV light or sunlight are rapidly and efficiently converted to chloroaromatic amides. The course, the intermediate (nascent chlorine vs dichlorine) and the outcome of the reaction depend on the excitation (exposure time, wavelength, and intensity) and on inherent structural factors (the directing role of the substituents and, as demonstrated by the different reactivity of two polymorphs of N-chlorobenzanilide, the supramolecular structure). The photolysis of the chloroamides provides facile photochemical access to arylamidyl radicals as intermediates, which in the absence of strong hydrogen bond donors are stabilized in the reactant crystals by C–H/N–Cl···π interactions, thus, providing insight into their structure and chemistry. Thorough theoretical modeling of the factors determinant to the stability and the nature of the spin-hosting orbital evidenced that although the trans-Π|| state (Np spin) of th...

Photoinduced Rearrangement of Aromatic N-Chloroamides to Chloroaromatic Amides in the Solid State: Inverted ΠN–ΣN Occupational Stability of Amidyl Radicals

Hideyuki Hara

Papers

Self-Healing Molecular Crystals

Mechanically compliant single crystals of a stable organic radical

Reversible Multicolor Photochromism of Dihydroazulene Crystals

Direct observation of aminyl radical intermediate during single-crystal to single-crystal photoinduced Orton rearrangement.

Photoinduced Rearrangement of Aromatic N-Chloroamides to Chloroaromatic Amides in the Solid State: Inverted ΠN–ΣN Occupational Stability of Amidyl Radicals