Subash Chandra Sahoo

Helical Water Chain Mediated Proton Conductivity in Homochiral Metal–Organic Frameworks with Unprecedented Zeolitic unh-Topology

Four new homochiral metal-organic framework (MOF) isomers, [Zn(l-L(Cl))(Cl)](H(2)O)(2) (1), [Zn(l-L(Br))(Br)](H(2)O)(2) (2), [Zn(d-L(Cl))(Cl)](H(2)O)(2) (3), and [Zn(d-L(Br))(Br)](H(2)O)(2) (4) [L = 3-methyl-2-(pyridin-4-ylmethylamino)butanoic acid], have been synthesized by using a derivative of L-/D-valine and Zn(CH(3)COO)(2)·2H(2)O. A three-periodic lattice with a parallel 1D helical channel was formed along the crystallographic c-axis. Molecular rearrangement results in an unprecedented zeolitic unh-topology in 1-4. In each case, two lattice water molecules (one H-bonded to halogen atoms) form a secondary helical continuous water chain inside the molecular helix. MOFs 1 and 2 shows different water adsorption properties and hence different water affinity. The arrangement of water molecules inside the channel was monitored by variable-temperature single-crystal X-ray diffraction, which indicated that MOF 1 has a higher water holding capacity than MOF 2. In MOF 1, water escapes at 80 °C, while in 2 the same happens at a much lower temperature (∼40 °C). All the MOFs reported here shows reversible crystallization by readily reabsorbing moisture. In MOFs 1 and 2, the frameworks are stable after solvent removal, which is confirmed by a single-crystal to single-crystal transformation. MOFs 1 and 3 show high proton conductivity of 4.45 × 10(-5) and 4.42 × 10(-5) S cm(-1), respectively, while 2 and 4 shows zero proton conductivity. The above result is attributed to the fact that MOF 1 has a higher water holding capacity than MOF 2.

Dynamic Single Crystals: Kinematic Analysis of Photoinduced Crystal Jumping (The Photosalient Effect)†

Crystals on the move: If they are subjected to a strong light stimulus, crystals of the cobalt coordination compound [Co(NH3)5(NO2)]Cl(NO3) undergo sudden jumps and leap over distances 10(2)-10(5) times their own size to release the strain that accumulates in their interior. The first quantitative kinematic analysis of this phenomenon is reported. The observed effect could be employed for actuation on the macroscopic scale.

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.

Kinematic and mechanical profile of the self-actuation of thermosalient crystal twins of 1,2,4,5-tetrabromobenzene: a molecular crystalline analogue of a bimetallic strip.

A paradigm shift from hard to flexible, organic-based optoelectronics requires fast and reversible mechanical response from actuating materials that are used for conversion of heat or light into mechanical motion. As the limits in the response times of polymer-based actuating materials are reached, which are inherent to the less-than-optimal coupling between the light/heat and mechanical energy in them, a conceptually new approach to mechanical actuation is required to leapfrog the performance of organic actuators. Herein, we explore single crystals of 1,2,4,5-tetrabromobenzene (TBB) as actuating elements and establish relations between their kinematic profile and mechanical properties. Centimeter-size acicular crystals of TBB are the only naturally twinned crystals out of about a dozen known materials that exhibit the thermosalient effect-an extremely rare and visually impressive crystal locomotion. When taken over a phase transition, crystals of this material store mechanical strain and are rapidly self-actuated to sudden jumps to release the internal strain, leaping up to several centimeters. To establish the structural basis for this colossal crystal motility, we investigated the mechanical profile of the crystals from macroscale, in response to externally induced deformation under microscope, to nanoscale, by using nanoindentation. Kinematic analysis based on high-speed recordings of over 200 twinned TBB crystals exposed to directional or nondirectional heating unraveled that the crystal locomotion is a kinematically complex phenomenon that includes at least six kinematic effects. The nanoscale tests confirm the highly elastic nature, with an elastic deformation recovery (60%) that is far superior to those of molecular crystals reported earlier. This property appears to be critical for accumulation of stress required for crystal jumping. Twinned crystals of TBB exposed to moderate directional heating behave as all-organic analogue of a bimetallic strip, where the lattice misfit between the two crystal components drives reversible deformation of the crystal.

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.

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.

Three Point Chiral Recognition and Resolution of Amino Alcohols Through Well-Defined Interaction Inside a Metallocavity

Recognition and separation of one enantiomer over its mirror image is important because of the different biochemical activity shown by chiral compounds. For example, a-limonene and linalool have a different smell depending on the chirality and one enantiomer of the antidepressant drug methylphenidate is 13 times more potent than its isomer. Ideally, recognition of an enantiomer requires recognition of three out of four different groups around an sp-hybridised carbon centre. Recognition of the groups in biological receptors often uses non-covalent interactions as observed in the case of adrenaline receptors. The involvement of hydrogen bonds and other weak interactions in the recognition process has the advantage of easy recovery of the guest, but the lability of the guest makes the isolation of the host– guest complex more difficult. Recognition and separation of enantiomers by using synthesised hosts has been approached from diverse angles, including a few that use a rigid metal–organic framework (MOF). Although separation by using a chiral adsorbent as the stationary phase has progressed substantially, the understanding of recognition at the molecular level is limited to molecular modelling due to the difficulty in structural characterisation of large organic or MOF hosts. For example, Kim and co-workers used structurally characterised porous channels of a chiral metal complex as a host to enhance the stereoselectivity of an organic reaction, but structural characterisation with the adduct was not possible. A low molecular weight rigid host would, in principle, facilitate structural characterisation, but it is challenging to accommodate three different recognition sites within a small host. Chin and co-workers, by using a chiral Co complex as the host, showed the chiral interaction between the host and chiral guest in a set of structurally characterised covalently bonded host–guest complexes, but chiral separation was not possible because the interactions were weak due to the open nature of the cavity. In a continuation of our earlier attempts at synthesising a rigid chiral host by using metal complexes, we now present the first report on the synthesis and structural characterisation of a medium-sized chiral metallocavity and its host– guest complexes in which two different chiral amino alcohols were recognised through well-defined three point recognition resulting in chiral separation. We chose amino alcohols as the target guest because of their structural similarity to adrenaline and noradrenaline, a hormone and neurotransmitter, respectively, (Scheme 1) and amino alcohols have two different hydrogen-bonding capable groups. Thus, binding with the host can be enhanced by electrostatic attraction between the cationic ammonium form of the amino alcohol and the use of an anionic host.

Photosalient Effect: Dramatic conversion of light into mechanical motion

Materials showing mechanical response in presence of external stimuli are of relevance for the design of nanoscale actuating devices for a variety of small-scale applications including actuators, flexible electronics, artificial muscles, and others. In recent years, molecular actuators[1] (molecular rotor, elevator, etc.) and several macroscopic systems based on liquid-crystal elastomers, gels, and other polymers[2] have been developed. The most recent efforts are aimed at achieving rapid, reversible, maximum and fatigueless response with single crystals which display optimum coupling between light and the mechanical energy. When exposed to light, certain single crystals can jump up to thousands times their own size. The term “photosalient” was introduced recently to describe this phenomenon.[3] The photosalient effect in the motile crystals represents a direct and visually impressive demonstration of the conversion of light into mechanical motion through a photochemical reaction on a macroscopic scale, which sets the platform for the design of fast biomimetic and technomimetic actuating materials that can mimic animal motions, dynamics of macromolecules, or dynamic technical elements, in the future. In this presentation, we will describe the mechanical response from photosalient single crystals that undergo photoinduced linkage isomerization. To understand the mechanistic details, the mechanism of the process was studied by X-ray photodiffraction, kinematic analysis, IR spectroscopy and mechanical characterization. In contrast to many other solid-state transformations that involve nucleation and propagation of the reaction interface, in this system the reaction proceeds homogeneously whereupon solid solutions form without apparent phase separation.

Homochiral metal organic framework with unh-topology

Four new homochiral metal organic frameworks using amino acid derived links as a pure chiral precurser named, [Zn(l/d-ValPy)(X)](H2O)(1-4), [X= Clˉ(1,3), Brˉ(2,4)] have been synthesized from aqueous media via solvothermal route and characterized by singlecrystal X-ray diffraction. Structural analysis reveals that a 3D MOF with a 1D channel in the direction of c-axis were formed having a pore aparture of ~13Å. All the structures form a rare zeolitic topology i.e unh type, which is not reported so far in the literature. The guest water molecule sits inside the 1D channel in a helical fashion and can be removed without affecting the crystal sturucture as revealed from TGA analysis. The anions used in these reaction systems (either from metal or ligand source) play a crucial role in formation of 3D structure. The structures with desired topology forms only when the ligand source is either pure l or d but in from the racemic. Applications of these chiral MOFs towards catalytic or chiral separations under process in our lab. Oweing to the importance of chiral MOFs for various applications as shown by Web lin et.al and others1-3, we believe that the successful synthesis of these four chiral MOFs form a easy chiral source like amino acids, will open up new vistas in the search for useful applications.