Naba K. Nath

Mechanically Responsive Molecular Crystals

Pancě Naumov,*,† Stanislav Chizhik,‡,§ Manas K. Panda,† Naba K. Nath,† and Elena Boldyreva*,‡,§ †New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates ‡Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch of Russian Academy of Sciences, ul. Kutateladze, 18, Novosibirsk 630128, Russia Novosibirsk State University, ul. Pirogova, 2, Novosibirsk 630090, Russia

Thermally induced and photoinduced mechanical effects in molecular single crystals—a revival

The classical perception of single crystals of molecular materials as rigid and brittle entities has downsized the research interest in their mechanical effects that had been initiated and was active back in the 1980s. More recently, the modern analytical techniques for their mechanical, electron-microscopic, structural, spectroscopic and kinematic characterization have contributed to accumulate compelling evidence that under certain circumstances, even some seemingly rigid single crystals can deform, bend, twist, hop, wiggle or perform other ‘acrobatics’ that are atypical for non-soft matter. These examples contribute to a paradigm shift in our understanding of the elasticity of molecular crystals and also provide direct mechanistic insight into the structural perturbations at the limits of the susceptibility of ordered matter to internal and external mechanical forces. As the relevance of motility and reshaping of molecular crystals is being recognized by the crystal research community as a demonstration of a very basic concept—conversion of thermal or light energy into work—a new and exciting crystal chemistry around mechanically responsive single crystals rapidly unfolds.

Model for Photoinduced Bending of Slender Molecular Crystals

The growing realization that photoinduced bending of slender photoreactive single crystals is surprisingly common has inspired researchers to control crystal motility for actuation. However, new mechanically responsive crystals are reported at a greater rate than their quantitative photophysical characterization; a quantitative identification of measurable parameters and molecular-scale factors that determine the mechanical response has yet to be established. Herein, a simple mathematical description of the quasi-static and time-dependent photoinduced bending of macroscopic single crystals is provided. This kinetic model goes beyond the approximate treatment of a bending crystal as a simple composite bilayer. It includes alternative pathways for excited-state decay and provides a more accurate description of the bending by accounting for the spatial gradient in the product/reactant ratio. A new crystal form (space group P21/n) of the photoresponsive azo-dye Disperse Red 1 (DR1) is analyzed within the constraints of the aforementioned model. The crystal bending kinetics depends on intrinsic factors (crystal size) and external factors (excitation time, direction, and intensity).

Colossal positive and negative thermal expansion and thermosalient effect in a pentamorphic organometallic martensite

The thermosalient effect is an extremely rare propensity of certain crystalline solids for self-actuation by elastic deformation or by a ballistic event. Here we present direct evidence for the driving force behind this impressive crystal motility. Crystals of a prototypical thermosalient material, (phenylazophenyl)palladium hexafluoroacetylacetonate, can switch between five crystal structures (α-ε) that are related by four phase transitions including one thermosalient transition (α↔γ). The mechanical effect is driven by a uniaxial negative expansion that is compensated by unusually large positive axial expansion (260 × 10(-6)  K(-1)) with volumetric expansion coefficients (≈250 × 10(-6)  K(-1)) that are among the highest values reported in molecular solids thus far. The habit plane advances at ~10(4) times the rate observed with non-thermosalient transitions. This rapid expansion of the crystal following the phase switching is the driving force for occurrence of the thermosalient effect.

Biomimetic Crystalline Actuators: Structure–Kinematic Aspects of the Self-Actuation and Motility of Thermosalient Crystals

While self-actuation and motility are habitual for humans and nonsessile animals, they are hardly intuitive for simple, lifeless, homogeneous objects. Among mechanically responsive materials, the few accidentally discovered examples of crystals that when heated suddenly jump, propelling themselves to distances that can reach thousands of times their own size in less than 1 ms, provide the most impressive display of the conversion of heat into mechanical work. Such thermosalient crystals are biomimetic, nonpolymeric self-actuators par excellence. Yet, due to the exclusivity and incongruity of the phenomenon, as well as because of the unavailability of ready analytical methodology for its characterization, the reasons behind this colossal self-actuation remain unexplained. Aimed at unraveling the mechanistic aspects of the related processes, herein we establish the first systematic assessment of the interplay among the thermodynamic, kinematic, structural, and macroscopic factors driving the thermosalient phenomenon. The collective results are consistent with a latent but very rapid anisotropic unit cell deformation in a two-stage process that ultimately results in crystal explosion, separation of debris, or crystal reshaping. The structural perturbations point to a mechanism similar to phase transitions of the martensitic family.

Novel form V of tolbutamide and a high Z′ crystal structure of form III

Attempted co-crystallization of tolbutamide with p-phenylenediboronic acid and p-nitrophenol in EtOH afforded a modulated high Z′ crystal structure of the known form III of tolbutamide instead of the anticipated co-crystals. A novel form V of tolbutamide was crystallized when a trace of conc. HNO3 was added in MeOH solvent.

Isostructural polymorphs of triiodophloroglucinol and triiodoresorcinol

Triiodoresorcinol (TIR, 2,4,6-triiodoresorcinol) and triiodophloroglucinol (TIG, 2,4,6-triiodophloroglucinol) crystallized as orthorhombic (TIR-O and TIG-O in P212121) and monoclinic (TIR-M and TIG-M in P21/n) polymorphs mediated via inter-halogen I⋯I interactions. The orthorhombic polymorphs are isostructural and in turn similar to the crystal structure of 1,3,5-triiodobenzene (TIB). The isostructural monoclinic polymorphs are similar to the structure of 1,3,5-trifluoro-2,4,6-triiodobenzene (TIF). Triiodophenol (TIP) crystallized in a single orthorhombic form only. The monoclinic structures have tandem O–H⋯O hydrogen bonds in addition to inter-iodine interactions. The similarity of crystal structures was confirmed by the formation of 1 : 1 binary solid-solutions, TIP +  TIR-O and TIR + TIG-O, in orthorhombic space groupP212121. Dimorphs of TIR and TIG establish a structural link in the triiodobenzene series TIB (P212121) → TIP (P212121) → TIR (P212121 and P21/n) → TIG (P212121 and P21/n) → TIF (P21/n). The search for new polymorphs was initiated by isostructurality relationship in the series of compounds.

Surface and Bulk Effects in Photochemical Reactions and Photomechanical Effects in Dynamic Molecular Crystals.

The increasing number of reports on photomechanical effects in molecular crystals necessitates systematic studies to understand the intrinsic and external effectors that determine and have predictive power of their type and magnitude. Differential light absorption and product gradient between the surface and the bulk of the crystal are often invoked to qualitatively explain the mechanical response of crystals to light; however, the details on how this difference in photochemical response accounts for macroscopic effects such as surface modification, deformation, or disintegration of crystals are yet to be established. Using both bulk- and surface-sensitive analytical techniques, a rare instance of benzylidenefuranone crystals is studied here, and it is capable of several distinct types of photomechanical response including surface striation and delamination, photosalient effect (ballistic disintegration and motion), and photoinduced bending by dimerization. The results provide a holistic view on these effects and set the stage for the development of overarching theoretical models to describe the photomechanics in the ordered solid state.

Actuation based on thermo/photosalient effect: a biogenic smart hybrid driven by light and heat

Aimed at the design of efficient smart actuating materials, we have fabricated a self-actuating material that sets the platform for conceptually new, hybrid biocompatible actuators capable of dual mechanical response—by changes in temperature and by stimulation with weak ultraviolet or blue visible light. We demonstrate herein that microcrystallites of thermosalient and photosalient (leaping) solids can effectively utilize thermal or light energy and act as a robust and dynamically active “skeleton” to actuate sodium caseinate films as an elastic, flexible, biocompatible, natural protein matrix, similar to artificial muscle. The spectroscopic, kinematic and mechanical profiles of the new material are all consistent with a mechanism whereby the cooperative strains induced by reshaping and motions of the thermosalient crystals trigger macroscopic mechanical deformation of the matrix. The elastic medium absorbs the stress, thus providing reinforcing feedback to the brittle crystals. The hybrid material conveniently combines fast energy absorption and conversion within single crystals and elasticity of polymers and displays a remarkable improvement in the tensile properties relative to the non-doped caseinate. Being based on natural protein, this thermally and photoresponsive artificial muscle is also biologically compatible and environmentally benign.

Why is firefly oxyluciferin a notoriously labile substance

The chemistry of firefly bioluminescence is important for numerous applications in biochemistry and analytical chemistry. The emitter of this bioluminescent system, firefly oxyluciferin, is difficult to handle. The cause of its lability was clarified while its synthesis was reinvestigated. A side product was identified and characterized by NMR spectroscopy and X-ray crystallography. The reason for the lability of oxyluciferin is now ascribed to autodimerization of the coexisting enol and keto forms in a Mannich-type reaction.