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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 photo-switchable 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.

This Frontier article highlights the advancement of [2+2] photocycloaddition reactions within coordination polymers to fine tune their diverse physical and chemical properties.

Explosive nitroaromatic compounds (epNACs) are a class of molecules responsible for massive loss of life in the form of terror attacks and health hazards. Discerning detection of 2,4,6-trinitrophenol (TNP) like mutagenic pollutant and epNACs using robust luminescent coordination polymers (CPs) has been always a potential research field for the sake of environmental health and homeland security. In this regard, one-dimensionalacetylenedicarboxylic acid and avp = 4-[2-(9-anthryl)vinyl]pyridine} have been synthesized using H 2 adc and avp ligands into aqueous solution of the nitrate salts of corresponding metal ion. Both of the polymeric compounds show extensive hydrogen bonding and π•••π stacking interactions to form three-dimensional (3D) structures. The presence of geometrically suitable proximal anthracene moieties (extended πconjugation) in both the compounds provides feasible interacting sites for incoming analytes that also explains their prompt sensing property. Interestingly, both the fluorescent sensing probes exhibit highly efficient ratiometric and selective sensing ability toward TNP with fast response over other comparative epNACs. A detailed investigation on the mechanistic pathways for the quenching response has been obtained by density functional theory calculation. The results show higher sensitivity of 2 with a limit of detection (LOD) of 0.45 μM as compared to 1 (LOD ∼ 0.51 μM). To the best of our knowledge, such hitherto unknown CPs acting as highly fluorescent sensors are not known in the literature. Here, strategically decorated anthracene moieties are congenial to geometrically accessible interacting sites for rapid and selective detection of an explosive nitro derivative as well as mutagenic pollutant TNP with higher sensitivity.

A Cd(ii)-based coordination compound, [CdI2(4-nvp)2] (1), has been synthesized using CdI2 and monodentate N-donor ligand 4-(1-naphthylvinyl)pyridine (4-nvp).

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A one-dimensional coordination polymer (1D CP), [Cd­(bpe)­(p-brba)2] n (1), has been synthesized by the slow diffusion method keeping the mixture of Cd­(NO3)2·6H2O and 1,2-bis­(4-pyridyl)­ethylene (bpe) ligand along with para-bromobenzoic acid (p-brba) in the dark. Interestingly, the compound 1 undergoes a single-crystal to single-crystal (SCSC) photochemical [2 + 2] cycloaddition reaction to generate dimerized 1D CP [Cd­(rctt-tpcb)1/2(p-brba)2] n (2) [rctt-tpcb = rctt-tetrakis­(4-pyridyl)­cyclobutane]. As a result, π···π stacking interactions among p-brba ligands of adjacent chains have been removed in the dimerized product. This structural transformation has a significant effect on the conductivity of the materials.

The two isostructural one-dimensional coordination polymers (1D CPs) [Cu­(5-nip)­(3-Clpy)2] n (1) and [Cu­(5-nip)­(3-Brpy)2] n (2) have been synthesized using a 5-nitroisophthalic acid (H25-nip) linker and the meta-substituted 3-chloropyridine (3-Clpy)/3-bromopyridine (3-Brpy) auxiliary ligands. The structural architectures and supramolecular interactions of the CPs have been investigated by single-crystal X-ray diffraction (SCXRD) and density functional theory (DFT) studies, respectively. The SCXRD study reveals that CPs 1 and 2 form a 1D double-chain structure with carboxylato-bridged cyclic secondary building units (SBUs). Interestingly, both CPs involve type I halogen···halogen (X···X) interactions combined with π···π stacking interactions to generate a three-dimensional (3D) supramolecular network. Moreover, both CPs exhibit interesting X···N π-hole interactions involving a nitro group as the electron acceptor. Both CPs show electrical conductivity in the semiconducting regime and behave as Schottky diodes. However, CP 1 has a higher electrical conductivity in comparison to CP 2.