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.

Single crystals of coordination complexes showing mechanical motion under the influence of external stimuli are of great interest due to their applications. Especially light-driven systems such as photoactuators, switches, and...

Dynamic photoactuating crystals have become a sensation due to their potential applications in developing smart medical devices, molecular machines, artificial muscles, flexible electronics sensing and probes. 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 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) manner. The Zn-based compounds demonstrate photosalient behaviour, wherein crystals show jumping, splitting, rolling, and swelling upon UV irradiation. However, the Cd crystals do not show such behaviour maintaining the original unit cell. Thus the photomechanical behaviour can be induced by choosing a suitable metal ion. The above findings are well 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.

Solid-state structural transformation is an interesting methodology used to prepare various metal–organic frameworks (MOFs) that are challenging to prepare in direct synthetic procedures. On the other hand, solid-state [2 + 2] photoreactions are distinctive methodologies used for light-driven solid-state transformations. Meanwhile, most of these photoreactions explored are quantitative in nature, in addition to them being stereo-selective and regio-specific in manner. In this work, we successfully synthesized two photoreactive novel binuclear Zn(II) MOFs, [Zn2(spy)2(tdc)2] (1) and [Zn2(spy)4(tdc)2] (2) (where spy = 4-styrylpyridine and tdc = 2,5-thiophenedicarboxylate) with different secondary building units. Both MOFs are interdigitated in nature and are 2D and 1D frameworks, respectively. Both the compounds showed 100% and 50% photoreaction upon UV irradiation, as estimated from the structural analysis for 1 and 2, respectively. This light-driven transformation resulted in the formation of 3D, [Zn2(rctt-ppcb)(tdc)2] (3), and 2D, [Zn2(spy)2(rctt-ppcb)(tdc)2] (4) (where rctt = regio, cis, trans, trans; ppcb = 1,3-bis(4′-pyridyl)-2,4-bis(phenyl)cyclobutane), respectively. These solid-state structural transformations were observed as an interesting post-synthetic modification. Overall, we successfully transformed novel lower-dimensional frameworks into higher-dimensional materials using a solid-state [2 + 2] photocycloaddition reaction.