Thermally Activated Transient Dipoles and Rotational Dynamics of Hydrogen-Bonded and Charge-Transferred Diazabicyclo [2.2.2]Octane Molecular Rotors

A confinement-regulated (H3C-NH3)+ ion as a smallest dual-wheel rotator showing bisected rotation dynamics

On the basis of variable-temperature single-crystal X-ray diffraction, rotational energy barrier analysis, variable-temperature/frequency dielectric response, and molecular dynamics simulations, here we report a new crystalline supramolecular rotor (CH3NH3)(18-crown-6)[CuCl3], in which the (H3C-NH3)+ ion functions as a smallest dual-wheel rotator showing bisected rotation dynamics, while the host 18-crown-6 macrocycle behaves as a stator that is not strictly stationary. This study also provides a helpful insight into the dynamics of ubiquitous -CH3/-NH3 groups confined in organic or organic-inorganic hybrid solids.

Modulation of Rotational Dynamics in Halogen-Bonded Cocrystalline Solids

Dynamic processes are responsible for the functionality of a range of materials, biomolecules, and catalysts. We report a detailed systematic study of the modulation of methyl rotational dynamics via the direct and the indirect influence of noncovalent halogen bonds. For this purpose, a novel series of cocrystalline architectures featuring halogen bonds (XB) to tetramethylpyrazine (TMP) is designed and prepared using gas-phase, solution, and solid-state mechanochemical methods. Single-crystal X-ray diffraction reveals the capacity of molecular bromine as well as weak chloro-XB donors to act as robust directional structure-directing elements. Methyl rotational barriers (Ea) measured using variable-temperature deuterium solid-state NMR range from 3.75 ± 0.04 kJ mol-1 in 1,3,5-trichloro-2,4,6-trifluorobenzene·TMP to 7.08 ± 0.15 kJ mol-1 in 1,4-dichlorotetrafluorobenzene·TMP. Ea data for a larger series of TMP cocrystals featuring chloro-, bromo-, and iodo-XB donors are shown to be governed by a combination of steric and electronic factors. The average number of carbon-carbon close contacts to the methyl group is found to be a key steric metric capable of rationalizing the observed trends within each of the Cl, Br, and I series. Differences between each series are accounted for by considering the strength of the σ-hole on the XB donor. One possible route to modulating dynamics is therefore via designer cocrystals of variable stoichiometry, maintaining the core chemical features of interest between a given donor and acceptor while simultaneously modifying the number of carbon close contacts affecting dynamics. These principles may provide design opportunities to modulate more complex geared or cascade dynamics involving larger functional groups.

Stress-Induced Inversion of Linear Dichroism by 4,4'-Bipyridine Rotation in a Superelastic Organic Single Crystal

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.

Multi-step phase transition crystal with dielectric constant bistability and temperature-dependent conductivity

[C4-bmim][Ni(mnt)2] (1) undergoes three-step phase transition with four phases before melting and has two-step dielectric constant bistability and temperature-dependent conducting properties.

Slip/Stick Viscosity Models of Nanoconfined Liquids: Solvent-Dependent Rotation in Metal-Organic Frameworks

Artificial molecular machines are expected to operate in environments where viscous forces impact molecules significantly. With that, it is well-known that solvent behaviors dramatically change upon confinement into limited spaces as compared to bulk solvents. In this study, we demonstrate the utility of an amphidynamic metal-organic framework with pillars consisting of ²H-labeled dialkynyltriptycene and dialkynylphenylene barrierless rotators that operate as NMR sensors for solvent viscosity. Using line-shape analysis of quadrupolar spin echo spectra we showed that solvents such as dimethylformamide, diethylformamide, 2-octanone, bromobenzene, o-dichlorobenzene, and benzonitrile slow down their Brownian rotational motion (10³-10⁶ s^-1) to values consistent with confined viscosity values (ca. 10⁰-10³ pa s) that are up to 10000 greater than those in the bulk. Magic angle spinning assisted ¹H T₂ measurements of included solvents revealed relaxation times of approximately 100-1000 ms over the explored temperature ranges, and MAS-assisted ¹H T₁ measurements of included solvents suggested a much lower activation energy for rotational dynamics as compared to those measured by the rotating pillars using ²H measurements. Finally, translational diffusion measurements of DMF using pulsed-field gradient methods revealed intermediate dynamics for the translational motion of the solvent molecules in MOFs.

Looking into the dynamics of molecular crystals of ibuprofen and terephthalic acid using 17 O and 2 H nuclear magnetic resonance analyses

Oxygen-17 and deuterium are two quadrupolar nuclei that are of interest for studying the structure and dynamics of materials by solid-state nuclear magnetic resonance (NMR). Here, 17 O and 2 H NMR analyses of crystalline ibuprofen and terephthalic acid are reported. First, improved 17 O-labelling protocols of these molecules are described using mechanochemistry. Then, dynamics occurring around the carboxylic groups of ibuprofen are studied considering variable temperature 17 O and 2 H NMR data, as well as computational modelling (including molecular dynamics simulations). More specifically, motions related to the concerted double proton jump and the 180° flip of the H-bonded (-COOH)2 unit in the crystal structure were looked into, and it was found that the merging of the C=O and C-OH 17 O resonances at high temperatures cannot be explained by the sole presence of one of these motions. Lastly, preliminary experiments were performed with a 2 H-17 O diplexer connected to the probe. Such configurations can allow, among others, 2 H and 17 O NMR spectra to be recorded at different temperatures without needing to tune or to change probe configurations. Overall, this work offers a few leads which could be of use in future studies of other materials using 17 O and 2 H NMR.

Pairing multicomponent stators with aromatic rotators for new emissive molecular rotors

We demonstrate here that the Ugi-Sonogashira protocol can be successfully used to obtain five new molecular rotors 10a-e with strong emission. They have been synthesized by combining multicomponent Ugi stators and several aromatic rotary components: phenylene, p-xylene, naphthalene and anthracene. The synthesized conjugated rotors are highly fluorescent (Φf = 0.39 to Φf = 0.10), and changes in their emission were observed upon variations of the surrounding media. Particularly, we found that they are sensitive to aggregation (THF/water) or high viscosity (methanol/glycerol) conditions. This work paves the way to develop new emissive rotors with exciting photophysical properties.

An Insight into Non-Covalent Interactions on the Bicyclo[1.1.1]pentane Scaffold

Bicyclo[1.1.1]pentane (BCP) is studied extensively as a bioisosteric component of drugs. Not found in nature, this molecular unit approximates the distance of a para-disubstituted benzene which is replaced in medicines as a method of improving treatments. Predicting interactions of these drugs with specific active sites requires knowledge of the non-covalent interactions engaged by this subunit. Structure determinations and computational analysis (Hirshfeld analysis, 2D fingerprint plots, DFT) of seven BCP derivatives chosen to probe specific and directional interactions. X-ray analysis revealed the presence of various non-covalent interactions including I ⋅⋅⋅ I, I ⋅⋅⋅ N, N-H ⋅⋅⋅ O, C-H ⋅⋅⋅ O, and H-C ⋅⋅⋅ H-C contacts. The preference of halogen bonding (I ⋅⋅⋅ I or I ⋅⋅⋅ N) in BCP 1-4 strictly depends upon the electronic nature and angle between bridgehead substituents. The transannular distance in co-crystals 2 and 4 was longer as compared to monomers 1 and 3. Stronger N-H ⋅⋅⋅ O and weaker C-H ⋅⋅⋅ O contacts were observed for BCP 5 while the O ⋅⋅⋅ H interaction was a prominent contact for BCP 6. The presence of 3D BCP units prevented the π ⋅⋅⋅ π stacking between phenyl rings in 3, 4, and 7. The BCP skeleton was often rotationally averaged, indicating fewer interactions compared to bridgehead functional groups. Using DFT analysis, geometries were optimized and molecular electrostatic potentials were calculated on the BCP surfaces. These interaction profiles may be useful for designing BCP analogs of drugs.

Solid-state NMR spectroscopy

Solid-state nuclear magnetic resonance (NMR) spectroscopy is an atomic-level method used to determine the chemical structure, three-dimensional structure, and dynamics of solids and semi-solids. This Primer summarizes the basic principles of NMR as applied to the wide range of solid systems. The fundamental nuclear spin interactions and the effects of magnetic fields and radiofrequency pulses on nuclear spins are the same as in liquid-state NMR. However, because of the anisotropy of the interactions in the solid state, the majority of high-resolution solid-state NMR spectra is measured under magic-angle spinning (MAS), which has profound effects on the types of radiofrequency pulse sequences required to extract structural and dynamical information. We describe the most common MAS NMR experiments and data analysis approaches for investigating biological macromolecules, organic materials, and inorganic solids. Continuing development of sensitivity-enhancement approaches, including 1H-detected fast MAS experiments, dynamic nuclear polarization, and experiments tailored to ultrahigh magnetic fields, is described. We highlight recent applications of solid-state NMR to biological and materials chemistry. The Primer ends with a discussion of current limitations of NMR to study solids, and points to future avenues of development to further enhance the capabilities of this sophisticated spectroscopy for new applications.

A multiaxial molecular ferroelectric with record high TC designed by intermolecular interaction modulation

Through precise and ingenious molecular modification, we successfully obtained a multiaxial ferroelectric, [FEtDabco]ZnI3 (N-fluoroethyl-N'-ZnI3-1,4-diazabicyclo[2.2.2]octonium), with a record high Tc (540 K) among molecular ferroelectrics, which is promising for application under extreme thermal conditions.

Tailoring the cavities of hydrogen-bonded amphidynamic crystals using weak contacts: towards faster molecular machines

This work describes the use of C–H⋯F–C contacts in the solid-state from the stator towards the rotator to fine-tune their internal motion, by constructing a set of interactions that generate close-fitting cavities in three supramolecular rotors 1–3I. The crystal structures of these rotors, determined by synchrotron radiation experiments at different temperatures, show the presence of such C–H⋯F–C contacts between extended carbazole stators featuring fluorinated phenyl rings and the 1,4-diazabicyclo[2.2.2]octane (DABCO) rotator. According to the ²H NMR results, using deuterated samples, and periodic density functional theory computations, the rotators experience fast angular displacements (preferentially 120° jumps) due to their low rotational activation energies (E_a = 0.8–2.0 kcal mol⁻¹). The higher rotational barrier for 1 (2.0 kcal mol⁻¹) is associated with a larger number of weak C–H⋯F–C contacts generated by the stators. This strategy offers the possibility to explore the correlation among weak intermolecular forces, cavity shape, and internal dynamics, which has strong implications in the design of future fine-tuned amphidynamic crystals.

This work describes the use of C–H⋯F–C contacts in the solid-state from the stator towards the rotator to fine-tune their internal motion, by constructing a set of interactions that generate close-fitting cavities in three supramolecular rotors 1–3I.

Dynamics of Hydrogen Bonding in Three-Component Nanorotors

The dynamics of hydrogen bonding do not only play an important role in many biochemical processes but also in Nature's multicomponent machines. Here, a three-component nanorotor is presented where both the self-assembly and rotational dynamics are guided by hydrogen bonding. In the rate-limiting step of the rotational exchange, two phenolic O-H-N,N(phenanthroline) hydrogen bonds are cleaved, a process that was followed by variable-temperature 1 H NMR spectroscopy. Activation data (ΔG≠ 298 =46.7 kJ mol-1 at 298 K, ΔH≠ =55.3 kJ mol-1 , and ΔS≠ =28.8 J mol-1  K-1 ) were determined, furnishing a rotational exchange frequency of k298 =40.0 kHz. Fully reversible disassembly/assembly of the nanorotor was achieved by addition of 5.0 equivalents of trifluoroacetic acid (TFA)/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) over three cycles.

Two-photon excited deep-red and near-infrared emissive organic co-crystals

Two-photon excited near-infrared fluorescence materials have garnered considerable attention because of their superior optical penetration, higher spatial resolution, and lower optical scattering compared with other optical materials. Herein, a convenient and efficient supramolecular approach is used to synthesize a two-photon excited near-infrared emissive co-crystalline material. A naphthalenediimide-based triangular macrocycle and coronene form selectively two co-crystals. The triangle-shaped co-crystal emits deep-red fluorescence, while the quadrangle-shaped co-crystal displays deep-red and near-infrared emission centered on 668 nm, which represents a 162 nm red-shift compared with its precursors. Benefiting from intermolecular charge transfer interactions, the two co-crystals possess higher calculated two-photon absorption cross-sections than those of their individual constituents. Their two-photon absorption bands reach into the NIR-II region of the electromagnetic spectrum. The quadrangle-shaped co-crystal constitutes a unique material that exhibits two-photon absorption and near-infrared emission simultaneously. This co-crystallization strategy holds considerable promise for the future design and synthesis of more advanced optical materials.

Hydrogen-Bonded Crystalline Molecular Machines with Ultrafast Rotation and Displacive Phase Transitions

Two new crystalline rotors 1 and 2 assembled through N-H⋅⋅⋅N hydrogen bonds by using halogenated carbazole as stators and 1,4-diaza[2.2.2]bicyclooctane (DABCO) as the rotator, are described. The dynamic characterization through ¹ H T₁ relaxometry experiments indicate very low rotational activation barriers (E_a ) of 0.67 kcal mol^-1 for 1 and 0.26 kcal mol^-1 for 2, indicating that DABCO can reach a THz frequency at room temperature in the latter. These E_a values are supported by solid-state density functional theory computations. Interestingly, both supramolecular rotors show a phase transition between 298 and 250 K, revealed by differential scanning calorimetry and single-crystal X-ray diffraction. The subtle changes in the crystalline environment of these rotors that can alter the motion of an almost barrierless DABCO are discussed here.

Doping of metal-free molecular perovskite with hexamethylenetetramine to create non-centrosymmetric defects

The metal-free perovskite (dabcoH₂²⁺)(NH₄)Br (d-Br) (dabco: 1,4-diazabicyclo[2.2.2]octane) was doped with non-centrosymmetric hexamethylenetetramine.