The hybrid inorganic–organic material [(CH₃)₂NH₂]₂NiCl₄ was reported to exhibit a remarkable thermochromism. [1] The color of this compound rapidly changes from deep red to deep blue upon heating at 383 K. Surprisingly, upon cooling to room temperature, the deep blue compound changes its color to dark, golden yellow. The so-produced yellow compound spontaneously transitions back to the starting deep red compound upon prolonged storage at ambient conditions. This color-change sequence can be cycled for a number of times without apparent degradation. Originally, it was believed that the color change originates from temperature-induced changes in the local geometry around the Ni⁺² cations in the structure. To shine light at these processes, we performed detailed studies using synchrotron X-ray powder diffraction, with diffraction data collected as a function of temperature. We discover that rather than undergoing thermochomic transitions, this compound is in fact a reacting system and the different color originate from different crystalline phases. The crystal structure and composition of these phases was solved and refined using the diffraction data. These structures were used to rationalize the color changes. This contribution emphasized the importance of powder X-ray diffraction, and crystallography in general, in the mechanistic studies of the stimuli-responsive crystalline compounds.

Molecules that change their colour, structure, and electronic properties in response to an external stimulus represent an emerging class of ‘smart’ material with potential applications in sensing, actuating and responsive technologies. The spin crossover (SCO) phenomenon leads to a redistribution of electrons within the d-orbitals of some transition metal complexes as a result of an external perturbation such as changes in temperature, pressure changes and light irradiation. The transition between high spin and low spin states involves a significant change in molecular volume and is often cooperative in crystalline materials, leading to dramatic changes in the optical, mechanical and magnetic properties.

We have demonstrated the use of mechanochemistry in the synthesis of SCO materials,¹ and have recently shown that they can be synthesised through contact of the reagents in the solid state without any applied mechanical force.² Recent work in our group has shown the significant promise of using supramolecular interactions to design new SCO materials with tunable thermally-responsive properties.³ This talk will focus on how various stimuli can affect the synthesis, structure and properties of these SCO materials in the solid state.

Rotors, motors and switches in the solid state find a favorable playground in porous materials, such as Metal Organic Frameworks (MOFs), thanks to their large free volume, which allows for fast dynamics. We fabricated MOFs with reorientable linkers and benchmark mobility also at very low temperature, to reduce the energy demand for motion-activation and light stimulus-response.

In particular, we have realized a fast molecular rotor in the solid state whose rotation speed approaches that of unhindered rotations in organic moieties even at very low temperatures (2 K). The rotors were hosted within the struts of a low-density porous crystalline MOF and energetically decoupled from their surroundings. A key point was the unusual crossed conformation adopted by the carboxylates around the pivotal bond on the rotor axle, generating geometrical frustration and very shallow wells along the circular trajectory. Continuos, unidirectional hyperfast rotation with an energy barrier of 6.2 cal/mol and a high frequency persistent for several turns is achieved (10 GHz below 2 K).[1]

Responsive porous switchable framework materials endowed with light-responsive overcrowded olefins, took advantage of both the quantitative photoisomerization in the solid state and the porosity of the framework to reversibly modulate the gas adsorption in response to light. [2]

Motors were inserted into metal-organic frameworks wherein two linkers with complementary absorption-emission properties were integrated into the same materials. Therefore, unidirectional motion was achieved by simple exposure to sun-light of the solid particles, which thus behave as autonomous nanodevices.[3]

MOF nanocrystals comprising high-Z linking nodes interacting with the ionizing radiation, arranged in an orderly fashion at a nanometric distance from diphenylanthracene ligand emitters showed ultrafast sensitization of the ligand fluorescence, thus supporting the development of new engineered scintillators.[4]

References

1. J. Perego, S. Bracco, M. Negroni, C. X. Bezuidenhout, G. Prando, P. Carretta, A. Comotti, P. Sozzani Nature Chem. 2020, 12, 845.

2. P. Sozzani, S. Bracco, S. J. Wezenberg, A. Comotti, B. L. Feringa et al. Nature Chem. 2020, 12, 595.

3. W. Danowski, F. Castiglioni, A. Comotti, B. L. Feringa et al. J. Am. Chem. Soc. 2020, 142, 9048.
4. J. Perego, F. Meinardi, S. Bracco, A. Comotti, A. Monguzzi et al. Nature Photonics 2021, doi 10.1038/s41566-021-00769-z.

Molecular crystals can be bent elastically by simultaneous expansion and contraction or plastically by delamination into slabs that glide along slip planes [1,2]. Here we describe a hitherto unreported mechanism of crystal bending in terephthalic acid crystal which undergoes pressure-induced phase transition upon bending where the two phases (form II and form I) coexist at ambient conditions. Scanning electron microscopy and microfocus XRD using synchrotron radiation provided direct evidence that upon bending, terephthalic acid crystals can undergo a mechanically induced phase transition without delamination and their overall crystal integrity is retained [3]. We report a distinctly different mechanism of plastic bending of molecular single crystals which have two phases and we provide the crystal structure of the bent section of such plastically bent crystal as direct evidence of the proposed mechanism. We also establish that this plastic deformation which effectively results in coexistence of two phases in the bent section of the crystal is the origin of unconventional properties such as shape-memory and self-restorative effects. Such plastically bent crystals act as bimorphs and their phase uniformity can be recovered thermally by taking the crystal over the phase transition temperature. This recovers the original straight shape and the crystal can be bent by a reverse thermal treatment, resulting in shape memory effects akin of those observed with some metal alloys and polymers. We anticipate that similar memory and restorative effects are common for other molecular crystals having metastable polymorphs.

[1] Ahmed, E., Karothu, D. P. & Naumov, P. (2018). Angew. Chem. Int. Ed. 57, 8837. [2] Naumov, P., Chizhik, S., Panda, M. K., Nath, N. K. & Boldyreva, E. (2015). Chem. Rev. 115, 12440. [3] Ahmed, E., Karothu, D. P., Warren, M. & Naumov. P (2019). Nat. Commun. 10, 3723.

Laser beam machining (LBM) of ceramics, polymers, or metals is usually performed using high-power femtosecond lasers (4–20 W). Using LBM, micro- or nano-sized patterns can be machined into surfaces of these materials to alter their properties for various applications. A drawback of such high-power techniques is the possibility of considerable chemical damage to the surface of the machined materials.

We now report the use of halogen bonding to generate new dye-based cocrystals with volatile cocrystal-forming molecules (coformers) that can be etched, cut, and punctured with micrometer-scale precision using low-powered laser beams (for example, between 0.5 and 20 mW).[1] This unique phenomenon, shown to be wavelength-tunable and power-dependent, can be utilized to machine molecular crystals by forming holes or cuts of controllable sizes. Using a microscope-guided low-power laser beam numerical control of this process can be achieved, enabling a variety of complex patterns to be inscribed onto the surface of molecular cocrystals. A mechanism is proposed with the volatile conformer acting as a leaving group, giving the ability to gently inscribe patterns using a low-power laser beam, without chemical decomposition of the cocrystals. This has not been previously reported in small molecule organic solids and appears to be a new emergent property achievable through crystal engineering by halogen bonding, opening a new type of materials to micrometer-scale shaping and machining applications.

[1] Borchers, T. H., Topić, F., Christopherson, J. -C., Bushuyev, O. S., Vainauskas, J., Titi, H. M., Friščić, T. & Barrett, C. J. (2021). ChemRxiv. https://doi.org/10.26434/chemrxiv.14398856.v1

Mechanical crystals are expected to be applicable for actuators and soft robots. Before the past decade, we have developed many mechanical crystals based on photoisomerization, and some based on phase transition and photothermal effect. In 2019, we have found a new kind of phase transitions, referred to as the photo-triggered phase transition. The photochromic crystal exhibiting a thermal, reversible single-crystal-to-single-crystal phase transition upon heating and cooling, transform to the identical phase upon light irradiation at temperatures lower than thermal phase transition temperature. A chiral salicylidnephenylethylamine [enol-(S)-1] crystal is known to undergo photoisomerization, and thermal phase transition. We have found that the enol-(S)-1 crystal exhibited the photo-triggered phase transition.

Upon heating, the enol-(S)-1 crystal in the α-phase (P2₁) transformed to the β-phase (P2₁2₁2₁) with the discontinuous β-angle change to 90° at 0 °C due to thermal phase transition from monoclinic to orthorhombic crystal system. Under UV light (365 nm) irradiation, the α-phase changed to the β-phase even at -30 °C. The mechanism was revealed that the photo-triggered phase transition is driven by the strain near the irradiated surface produced by the photoisomerization. A thick crystal in the α-phase deformed by the photo-triggered phase transition to the β-phase upon UV light irradiation; the surface temperature did not reach the thermal phase transition temperature. Furthermore, the thin plate-like crystal exhibited two-step bending motion by the photo-triggered phase transition and then the photoisomerization. Finally, by alternate irradiation of UV and visible light (488 nm) from the left, the plate-like crystal on the glass surface locomoted in the lower right direction. This finding leads to generalize the photo-triggered phase transition phenomenon and indicates that the photo-triggered phase transition enables to create various motions of crystals such as locomotion.