Photomechanical properties in metal-organic crystals.

The emergence of materials that can effectively convert photon energy (light) into motion (mechanical work) and change their shapes on command is of great interest for their potential in the fabrication of devices (powered by light) that will revolutionize the technologies of optical actuators, smart medical devices, soft robotics, artificial muscles and flexible electronics. Recently, metal-organic crystals have emerged as desirable smart hybrid materials that can hop, split and jump. Thus, their incorporation into polymer host objects can control movement from molecules to millimetres, opening up a new world of light-switching smart materials. This feature article briefly summarizes the recent part of the fast-growing literature on photomechanical properties in metal-organic crystals, such as coordination compounds, coordination polymers (CPs), and metal-organic frameworks (MOFs). The article highlights the contributions of our group along with others in this area and aims to provide a consolidated idea of the engineering strategies and structure-property relationships of these hybrid materials for such rare phenomena with diverse potential applications.

Facile construction of an anthracene-decorated highly luminescent coordination polymer for the selective detection of explosive nitroaromatics and the mutagenic pollutant TNP.

Explosive nitroaromatic compounds (epNACs) are a group of chemicals that have caused significant human casualties through terrorist attacks and they also pose health risks. For the benefit of homeland security and environmental health, there is room for advancing research on the precise detection of epNACs. Coordination polymers (CPs) successfully serve this purpose because of their binding abilities and quenching capabilities. In this regard, a one-dimensional (1D) CP [Zn(bdc)(avp)2(H2O)]n (1; H2bdc = 1,4-benzenedicarboxylic acid and avp = 4-[2-(9-anthryl)vinyl]pyridine) was synthesized, which remarkably demonstrated extremely efficient ratiometric and selective sensing capacity toward epNACs and the mutagenic pollutant 2,4,6-trinitrophenol (TNP) with a quick response. Density functional theory (DFT) calculations provided a thorough analysis of the mechanistic routes behind the quenching reaction. Herein, geometrically accessible interaction sites were strategically decorated using anthracene moieties, allowing the quick and precise detection of explosive nitro derivatives and the carcinogenic pollutant TNP with increased sensitivity.

Exploitation of a 1D coordination polymer as a portable kit for an eye-catching fluorometric response towards sensing of trivalent cations.

The development and utilization of coordination polymers (CPs) have drawn interest for potential applications in different fields. Detection of metal ions in efficient and selective manners is an important field of research. It paves the way to protect human health by balancing toxic metal ions and biologically active metal ions in the atmosphere. In this regard, a new one-dimensional (1D) 4-(1-naphthylvinyl)pyridine (4-nvp) based CP [Cd(NCS)2(4-nvp)2]n (1) was synthesized and characterized structurally by single-crystal X-ray diffraction. Interestingly, this 1D CP underwent supramolecular aggregation via π⋯π stacking interactions, which specifically generated an environment for a potent "turn on" response in the presence of trivalent cations (Fe3+, Al3+, and Cr3+) in the nanomolar range but remained silent in the presence of other metal ions. Density functional theory (DFT) computations and X-ray photoelectron spectroscopy (XPS) were performed to establish the sensing phenomena. Fascinatingly, utilizing the sensitivity of 1 in an aqueous medium, a hands-on portable cotton swab kit was developed for instant identification of these three important trivalent metal cations.

Coordination polymers: a promising candidate for photo-responsive electronic device application.

The design and synthesis of electrically conductive coordination polymers (CPs) are of special interest due to their applications in the fabrication of many environmentally benign emerging technologies, such as molecular wires, photovoltaic cells, light emitting diodes (LEDs), field effect transistors (FETs) and Schottky barrier diodes (SBDs). Owing to their structural flexibility, easy functionality and adjustable energy levels, CPs are promising candidates for providing a better pathway for superior charge transport. Again, the utilization of visible light as an external stimulus to control and manoeuvre the electrical properties of the CPs is exceptionally motivating for the development of many optoelectronic devices, such as photodetectors, photo-switches, photodiodes and chemiresistive sensors. The applications of such materials in devices will solve questions regarding the energy crisis and environmental concerns. This study provides an overview of the recent advances in the development of photo-responsive CPs and the possibility of their application in developing optoelectronic devices. In this regard, a thorough literature survey was performed and the studies related to the fabrication of photosensitive conducting CPs for applications in optoelectronic devices are listed.

A dual-functional 2D coordination polymer exhibiting photomechanical and electrically conductive behaviours.

A photoactive two-dimensional coordination polymer (2D CP) [Zn2(4-spy)2(bdc)2]n (1) [4-spy = 4-styrylpyridine and H2bdc = 1,4-benzendicarboxylic acid] undergoes a photochemical [2 + 2] cycloaddition reaction upon UV irradiation. Interestingly, the crystals of 1 show different photomechanical effects, such as jumping, swelling, and splitting, during UV irradiation. In addition, the CP was employed for conductivity measurements before and after UV irradiation via current density-voltage characteristics and impedance spectroscopy, which suggest that they are semiconducting in nature and can be used as Schottky diodes. Thus, this work demonstrates the potential dual applications of a 2D CP based on photosalient and conductivity properties.

Regulating photosalient behavior in dynamic metal-organic crystals.

Dynamic photoactuating crystals have become a sensation due to their potential applications in developing smart medical devices, molecular machines, artificial muscles, flexible electronics actuators, probes and microrobots. Here we report the synthesis of two iso-structural metal-organic crystals, [Zn(4-ohbz)2(4-nvp)2] (1) and [Cd(4-ohbz)2(4-nvp)2] (2) {H4-ohbz = 4-hydroxy benzoic acid; 4-nvp = 4-(1-naphthylvinyl)pyridine} which undergo topochemical [2 + 2] cycloaddition under UV irradiation as well as sunlight to generate a dimerized product of discrete metal-complex [Zn(4-ohbz)2(rctt-4-pncb)] {rctt-4-pncb = 1,3-bis(4'-pyridyl)-2,4-bis(naphthyl)cyclobutane} (1') and one-dimensional coordination polymer (1D CP) [Cd(4-ohbz)2(rctt-4-pncb)] (2') respectively, in a single-crystal-to-single-crystal (SCSC) process. The Zn-based compound demonstrates photosalient behaviour, wherein crystals show jumping, splitting, rolling, and swelling upon UV irradiation. However, the Cd-based crystals do not show such behaviour maintaining the initial supramolecular packing and space group. Thus the photomechanical behaviour can be induced by choosing a suitable metal ion. The above findings are thoroughly validated by quantitative density functional theory (DFT) calculations which show that the Zn-based crystal shifts towards an orthorhombic structure to resolve the anisotropic UV-induced mechanical strain. Furthermore, the mechano-structure-property relationship has been established by complimentary nanoindentation measurements, which are in-line with the DFT-predicted single crystal values.

Cu(II)-Based Molecular Hexagons Forming Honeycomb-like Networks Exhibit High Electrical Conductivity.

Four new Cu(II)-based hexagonal complexes with the metallomacrocycle formulae [Cu6(5-nip)6(3-py)6(H2O)12] (1), [Cu6(5-nip)6(3-Clpy)6(H2O)12] (2), [Cu6(5-nip)6(3-Brpy)6(H2O)12] (3), and [Cu6(5-nip)6(3-Ipy)6(H2O)12] (4) have been synthesized using 5-nitroisophthalic acid (H25-nip) and pyridine (py)/3-halopyridine (3-Xpy; X = Cl, Br, and I) ligands. The structural features and supramolecular interactions of compounds 1-4 have been investigated using the single-crystal X-ray diffraction (SCXRD) technique. Interestingly, the hexagonal complexes undergo hydrogen bonding and π···π stacking interactions to form fascinating two-dimensional (2D) honeycomb-like structures. The synthesized complexes exhibit high electrical conductivity, arising from charge transport through space via π···π contacts. However, complexes containing 3-Brpy (3) and 3-Ipy (4) exhibit photosensitivity due to the presence of halogens with a larger size and lower ionization energy. The conductivity results are also in accordance with the theoretical prediction calculated by density functional theory (DFT) study.

Photomechanical effect in Zn(II) and Cd(II) 1D coordination polymers: photosalient to non-salient behaviour.

A Zn(II) based one-dimensional (1D) coordination polymer (CP), [Zn(cis-1,4-chdc)(4-nvp)] (1) {cis-1,4-H2chdc = cis-1,4-cyclohexanedicarboxylic acid and 4-nvp = 4-(1-naphthylvinyl)pyridine}, undergoes a solid-state photochemical [2+2] cycloaddition reaction, accompanied by mechanical motion, wherein crystals show swelling, jumping, splitting and bursting upon UV irradiation, whereas the analogous Cd(II) CP [Cd(cis-1,4-chdc)(4-nvp)] (2) does not show any such response under UV light, although it undergoes [2+2] photodimerization. The present study can certainly provide the fundamental understanding for designing smart photoactuating materials.

A Dual Functional 2D MOF Exhibiting Rare Photosalient Effect as well as Selective Pd(II) Sensing in Aqueous Medium.

A Zn(II) based two-dimensional metal-organic framework (2D MOF) [Zn2(suc)2(4-nvp)2] (1) [H2suc = succinic acid and 4-nvp = 4-(1-naphthylvinyl)pyridine] exhibits a "photosalient effect" under UV light as well as sunlight along with the release of a stereoselective cyclobutane ligand, 1,3-bis(4'-pyridyl)-2,4-bis(naphthyl)cyclobutane (rctt-4-pncb). Photolysis of in situ generated MOF in solution also leads to the formation of rctt-4-pncb crystals. Interestingly, compound 1 shows a high selectivity for Pd(II) sensing in aqueous medium.

Sunlight-Induced In Situ Isomerization of Both Ligands in a Mixed-Ligand Coordination Polymer: From Photosalient to Photoinert Crystals.

Reaction of Zn(NO3 )2 ⋅ 6H2 O, maleic acid (H2 mal) and trans-4-(1-naphthylvinyl)pyridine (trans-nvp) in the dark results in the formation of a one-dimensional coordination polymer (1D CP) [Zn(mal)(trans-nvp)] (1), which is photosalient in nature. The crystals of 1 pop violently under UV light and moderately in sunlight, and generate cyclobutane ligands. However, the same reaction mixture kept in visible light exhibits the rare example of in situ isomerization of both ligands: cis-trans transformation of maleate and trans-cis isomerization of the nvp ligands, and subsequent formation of another 1D CP [Zn(fum)(cis-nvp)2 (H2 O)2 ] (2, H2 fum=fumaric acid), which is found to be photoinert. Thus, altering the reaction condition from dark to visible light gives rise to photosalient to photoinert crystals.

Topochemical [2 + 2] Cycloaddition in a Two-Dimensional Metal-Organic Framework via SCSC Transformation Impacts Halogen···Halogen Interactions.

A photoactive two-dimensional metal-organic framework (2D MOF) [Zn(4-spy)(DCTP)]n (1) [where 4-spy = 4-styrylpyridine and H2DCTP = 2,5-dichloroterephthalic acid] undergoes photochemical [2 + 2] cycloaddition on UV irradiation to obtain three-dimensional (3D) MOF [Zn(rctt-4-ppcb)(DCTP)]n (2) [rctt-4-ppcb = 1,3-bis(4'-pyridyl)-2,4-bis(phenyl)cyclobutane] in a single-crystal to single-crystal (SCSC) manner. This structural transformation leads to stronger halogen···halogen interaction that is well-corroborated by density functional theory (DFT) calculations.

Mechanical Motion in Crystals Triggered by Solid State Photochemical [2+2] Cycloaddition Reaction.

Some special crystals respond to light by jumping, scattering or bursting just like popping of popcorn kernels on a hot surface. This rare phenomenon is called the photosalient (PS) effect. Molecular level control over the arrangement of light-responsive molecules in microscopic crystals for macroscale deformation or mechanical motion offers the possibility of using light to control smart material structures across the length scales. Photochemical [2+2] cycloaddition has recently emerged as a promising route to obtain photoswitchable structures and a wide variety of frameworks, but such reaction in crystals leading to macroscopic mechanical motion is relatively less explored. Study of chemistry of such novel soft crystals for the generation of smart materials is an imperative task. This minireview highlights recent advances in solid-state [2+2] cycloaddition in crystals to induce macroscale mechanical motion and thereby transduction of light into kinetic energy.

Sunlight assisted SCSC dimerization of a 1D coordination polymer impacts the selectivity of Pd(II) sensing in water.

A one-dimensional coordination polymer (1D CP) [Cd(4-nvp)2(5-ssa)]·(4-nvp) (1) [4-nvp = 4-(1-naphthylvinyl)pyridine and 5-ssa = 5-sulfosalicylic acid] undergoes topochemical [2+2] cycloaddition by sunlight irradiation to generate a two-dimensional (2D) CP [Cd(rctt-4-pncb)0.5(4-nvp)(5-ssa)]·(4-nvp) (1') [rctt-4-pncb = 1,3-bis(4'-pyridyl)-2,4-bis(naphthyl)cyclobutane] in a single-crystal to single-crystal manner. Interestingly, 1' can be reverted back to 1 by heating and both the CPs selectively recognize Pd2+ in aqueous medium; however, the limit of detection is improved after photodimerization.

Semiconducting properties of pyridyl appended linear dicarboxylate based coordination polymers: theoretical prediction via DFT study.

Coordination polymers (CPs) in recent times have emerged as active constituents in many semiconductor devices like light emitting diodes (LED), field effect transistors (FET), photovoltaic devices and Schottky barrier diodes. An intelligent choice of linkers, careful selection of metal ions and post synthetic modification (PSM) can provide a better pathway for charge transportation. However, a proper understanding of the charge transport mechanism in CPs is still inadequate due to the lack of considerable experimental and theoretical work. In this paper, we address the theoretical elucidation of semiconducting properties and a probable pathway for charge transportation in three of our previously published CPs using density functional theory (DFT). These results help us to recognize the orbitals that have major contributions in the formation of the valence band and also provide the most likely pathway for optimum electronic communication. In this regard, the role of hydrogen bonding and unpaired electrons of metal d-orbitals is also established.

Electrically conductive 1D coordination polymers: design strategies and controlling factors.

Due to the easy functionality and structural diversity of coordination polymers (CPs) coupled with superior thermal stability, many researchers have been prompted to explore the opportunity of introducing these hybrid materials as active components in various electronic devices, such as light emitting diodes (LED), solar cells, field effect transistors (FET), and Schottky barrier diodes (SBD). Therefore, the judicious selection of the structural components of CPs is directly related to their structure-property relationship and applications. One-dimensional (1D) CPs have recently emerged as excellent electrical conductors and are gaining enormous attention owing to their simple chain-like coordination arrays. In this article, we review the rational design strategies for synthesising 1D CPs and also point out the structural factors that affect the charge transport properties as well as the electrical conductivity of these materials.

Electrically conductive Cu(II)-based 1D coordination polymer with theoretical insight.

A nitro-functionalized Cu(ii)-based one-dimensional coordination polymer (1D CP) [Cu(nip)(4-phpy)2]n (1) (H2nip = 5-nitroisophthalic acid and 4-phpy = 4-phenylpyridine) was synthesized and characterized by elemental analysis, powder X-ray diffraction (PXRD) and single crystal X-ray diffraction (SCXRD). In the solid-state self-assembly of 1, two sets of weak intermolecular forces, CHπ interaction among the axially bound 4-phpy ligands and ππ interaction among bridging nip ligands from adjacent 1D coordination polymeric chains led to 3D supramolecular packing. Interestingly compound 1 exhibited electrical conductivity in the semiconducting regime and behaved as a Schottky barrier diode.

Impact of solid-state photochemical [2+2] cycloaddition on coordination polymers for diverse applications.

Photochemical [2+2] cycloaddition reactions have been developed as a promising route to generate photoswitchable structures and a wide variety of frameworks. There are numerous examples of discrete molecules that undergo cycloaddition reactions. However, photocycloaddition in coordination polymers (CPs) with potential applications is relatively less explored. Hence, the synthesis of such novel photoreactive CPs for diverse applications is a challenging task. This Frontier article highlights recent advances in the solid-state [2+2] cycloaddition of CPs and its impact on their applications.

The sunlight-driven photosalient effect of a 1D coordination polymer and the release of an elusive cyclobutane derivative.

A one-dimensional coordination polymer (1D CP) [Zn(glu)(4-nvp)] (1) [H2glu = glutaric acid and 4-nvp = 4-(1-naphthylvinyl)pyridine] with a paddle-wheel unit [Zn2(O2CC)4] exhibits the photosalient effect under UV as well as sunlight irradiation, resembling the popping of popcorn, along with the release of an elusive cyclobutane ligand.

Tuning of the para-position of pyridyl ligands impacts the electrical properties of a series of Cd(ii) ladder polymers.

Herein, we report the synthesis, structural characterization and electrical conductivity of three new one-dimensional coordination polymers (1D CPs), [Cd(nip)(4-Clpy)(CH3OH)] (1), [Cd(nip)(4-Brpy)(CH3OH)] (2) and [Cd(nip)(4-Phpy)(H2O)]·DMF·3H2O (3) (H2nip = 5-nitroisophthalic acid, 4-Clpy = 4-chloropyridine, 4-Brpy = 4-bromopyridine, 4-Phpy = 4-phenylpyridine and DMF = N,N-dimethylformamide). The conductivity of the compounds was successfully tuned via the modification of the para-position of the 4-Xpy (X = Cl, Br and Ph) ligands. The electrical analysis indicated that compound 3 has the highest conductivity (22.25 × 10-4 S m-1) under illumination followed by 2 (19.38 × 10-4 S m-1) and 1 (12.68 × 10-4 S m-1). The optical characterization and density functional theory (DFT) computation also support the observed progression of conductivity in the materials. Here, the highest conductivity of compound 3 is attributed to the better ππ stacking interactions among the 4-Phpy and nip ligands, and the consequently increased charge transport between adjacent chains.

Sunlight-Induced Topochemical Photodimerization and Switching of the Conductivity of a Metal-Organic Compound.

A metal-organic compound [Cd(quin)2(4-nvp)2] [1; Hquin = quinoline-2-carboxylic acid and 4-nvp = 4-(1-naphthylvinyl)pyridine] undergoes topochemical [2 + 2] cycloaddition by sunlight irradiation to generate a one-dimensional coordination polymer. This reaction is thermally reversible, and switching between two crystalline forms can be monitored by conductivity measurements.