Being documented with only about a dozen serendipitous observations, the photosalient effect, where crystals leap when exposed to light, is considered a very rare phenomenon. Here, with a set of structurally related materials that undergo [2 + 2] photocycloaddition we present evidence that this effect is more common than it has been realized in the past, and we seek to establish correlations with the kinematics and the crystal structure toward rational design of photosalient materials. To that end, nine photoreactive complexes AgL2X2 (L = 4-styrylpyridine, 2′-fluoro-4-styrylpyridine, and 3′-fluoro-4-styrylpyridine, X = BF4 –, ClO4 – and NO3 –) were prepared. The [AgL2]+ cations in these structures pack by both head-to-head and head-to-tail alignment of the styrylpyridine ligands. Crystals of six out of the nine complexes were photosalient and popped, hopped, and/or leaped when exposed to UV light. It is concluded that the occurrence of the photosalient effect is determined not only by the nature of the ligand but also by the crystal packing which directs the magnitude, direction, and rate of volume expansion during the photoreaction.

Getting suitable crystals for single-crystal X-ray crystallographic analysis still remains an art. Obtaining single crystals of metal-organic frameworks (MOFs) containing organic polymers poses even greater challenges. Here we demonstrate the formation of a syndiotactic organic polymer ligand inside a MOF by quantitative [2+2] photopolymerization reaction in a single-crystal-to-single-crystal manner. The spacer ligands with trans,trans,trans-conformation in the pillared-layer MOF with guest water molecules in the channels, undergo pedal motion to trans,cis,trans-conformation prior to [2+2] photo-cycloaddition reaction and yield single crystals of MOF containing two-dimensional coordination polymers fused with the organic polymer ligands. We also show that the organic polymer in the single crystals can be depolymerized reversibly by cleaving the cyclobutane rings upon heating. These MOFs also show interesting photoluminescent properties and sensing of small organic molecules.

Solid-state [2 + 2] photocycloaddition reactivity, mechanical motion during cycloaddition reaction under UV light and naturally curved single crystals are reported in structurally similar mononuclear Zn(II) coordination complexes containing fluoro derivatives of 4-styryl pyridine ligands.

Although a plethora of metal complexes have been characterized, those having multifunctional properties are very rare. This article reports three isotypical complexes, namely [Cu(benzoate)L 2], where L = 4-styrylpyridine (4spy) (1), 2′-fluoro-4-styrylpyridine (2F-4spy) (2) and 3′-fluoro-4-styrylpyridine (3F-4spy) (3), which show photosalient behavior (photoinduced crystal mobility) while they undergo [2+2] cycloaddition. These crystals also exhibit anisotropic thermal expansion when heated from room temperature to 200°C. The overall thermal expansion of the crystals is impressive, with the largest volumetric thermal expansion coefficients for 1, 2 and 3 of 241.8, 233.1 and 285.7 × 10−6 K−1, respectively, values that are comparable to only a handful of other reported materials known to undergo colossal thermal expansion. As a result of the expansion, their single crystals occasionally move by rolling. Altogether, these materials exhibit unusual and hitherto untapped solid-state properties.

The degree of interpenetration is known to influence the gas sorption, catalytic, magnetic and nonlinear optical properties, chirality, and sensing of various molecules but not the solid-state [2 + 2] photocycloaddition reaction. In our previous studies of a solvothermal reaction using dimethylacetamide (DMA) as one of the solvents, a photoreactive 6-fold interpenetrated metal−organic framework with dia topology, [Zn(bpeb)(bdc)] (1) [bpeb = 1,4-bis[2-(4′-pyridyl)ethenyl]benzene; bdc = 1,4-benzenecarboxylate], was isolated. Because of the slip-stacked alignment of a dipyridyl ligand with two conjugated olefin bonds, the [2 + 2] cycloaddition reaction occurs under UV light leading to the formation of an organic polymer ligand fused with a coordination polymer, 2. On the contrary, under similar conditions when diethylformamide was used instead of DMA, a 5-fold interpenetrated structure, 3, with the same dia topology was obtained in this work. This has been found to be photostable as also predicted from the analysis of the solid state structure. All the solids show interesting photoluminescence properties, and the emissions were preferentially quenched by curcumin to make these materials as potentially useful sensing agents.