Abstract
Crystals of a family of six one-dimensional (1D) coordination polymers of cadmium(II) with cyanopyridines [[CdX2L2]n, where X = Cl, Br, or I and L = 3-cyanopyridine (3-CNpy) or 4-cyanopyridine (4-CNpy)] presented a variety of morphologies and mechanical responses with dominant two-dimensional (2D) anisotropic flexibility, which has not been previously reported. All mechanically adaptable crystals were 2D flexible and displayed a variety of direction-dependent responses; in addition to 2D isotropic flexibility observed for solely elastic materials, 2D anisotropic flexibility was noticed for both elastic and elastic → plastic crystals. The consequences of fine and controlled structural variations on mechanical behavior were additionally explored via microfocus single-crystal X-ray diffraction and complementary theoretical studies, revealing that the relative strength and direction of the hydrogen bonding interactions were the key parameters in delivering a specific mechanical response.
Highlights
Brittleness, a property typically associated with molecular crystalline solids, has long hampered the application of these highly ordered solid-state materials in advanced technologies.1 Relatively recently, it has been demonstrated that crystalline molecular materials, quite fragile, under certain circumstances may adapt to a variety of external stimuli while maintaining their integrity, which subsequently categorized them as exceptional candidates for application in emerging technologies and introduced the exploration of crystal flexibility to the forefront of solid-state research.Coordination polymers (CPs), in particular one-dimensional (1D) CPs, emerged as ideal model systems for exploring structural background and underlying principles that lead to flexible responsiveness and control of a targeted mechanical output of crystalline molecular solids
In the first report on the flexibility of 1D crystalline coordination polymers, we have shown that this particular class of materials is capable of displaying exceptional elasticity in response to an applied external mechanical pressure and that the extent of elastic responses could differ within a family of almost identical substances
In 1, 1D chains are arranged in a parallel fashion along the crystallographic c axis and antiparallel along the b axis, which is like the arrangement observed earlier for two other families of flexible coordination polymers.20−22 The C−H···N hydrogen bond was found to be the only dominant intermolecular interaction;23 it links the neighboring 1D chains from antiparallel layers and runs parallel to potential bending faces, i.e., crystal faces (011)/(011)/ (011)/(011) (Figure 2, left column, top)
Summary
Brittleness, a property typically associated with molecular crystalline solids, has long hampered the application of these highly ordered solid-state materials in advanced technologies. Relatively recently, it has been demonstrated that crystalline molecular materials, quite fragile, under certain circumstances may adapt to a variety of external stimuli (i.e., heat,− irradiation,− or pressure8−15) while maintaining their integrity, which subsequently categorized them as exceptional candidates for application in emerging technologies (i.e., optical waveguides, electrical conductors, and magnetic devices19) and introduced the exploration of crystal flexibility to the forefront of solid-state research. Arranged in antiparallel 2D layers as in 4, 1D building units from neighboring layers in 1 were more tilted to each other, which in turn resulted in developed crystal faces and an almost identical arrangement of molecular and intermolecular features parallel to bending faces, as well as in the directions of the application of force. Our findings suggest a different mechanism of elastic flexure for 1D materials (in comparison with the 0D material), they further point to the importance of concerted action of intermolecular interactions spreading in the two orthogonal directions (i.e., in regions A and B) for the crystal to display a flexible response and preserve the crystal integrity.
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