Elucidating the Origins of a Range of Diverse Flexible Responses in Crystalline Coordination Polymers

Mateja Pisačić,Marijana Đaković,Željka Soldin,Ivan Kodrin,Ivana Biljan,Nina Popov

doi:10.1021/acs.chemmater.1c00539

Abstract

The mechanical adaptability of a family of six one-dimensional crystalline coordination polymers (CPs) of cadmium ([CdX2(3-X′py)2]n; 1: X = Br, X′ = Cl, 2: X = I, X′ = Cl, 3: X = I, X′ = Br, 4: X = Cl, X′ = I, 5: X = Br, X′ = I, and 6: X, X′ = I) to applied external force was examined, and a plethora of flexible responses was noticed. While two of the six CPs (4 and 6) were slightly elastic, the remaining four CPs (1–3 and 5) presented variable plastic deformation; three of these (1–3) displayed exceptional crystal flow, and one (2) demonstrated unprecedented ductility of crystalline metal–organic material. The feature was examined by theory and custom-designed experiments, and it was shown that specific and directional intermolecular interactions are not only the most influential structural feature in determining the type of mechanical responses (i.e., elastic vs plastic), with interlocking of adjacent molecules playing only a supportive role, but also an unavoidable tool for dialing-in a diversity of plastic responses in Cd(II) coordination polymers.

Highlights

The ability to adapt to external mechanical stimuli with precise and predictable responses is still relatively rare among molecular crystals.1−4 Endowing crystalline materials with such features enables a range of advanced practical applications in, e.g., flexible electronics5 and magnetic6 or optoelectronic devices
Having a closely related family of cadmium(II) coordination polymers that offers us a diverse data set of crystal responsiveness, we were able to rationalize the observed differences in mechanical adaptability against a backdrop of a variety of structural, morphological, and energy features
The exceptional crystal flow and ductility of our samples, which have hitherto not been reported for metal−organic crystalline substances, prompted us to elucidate the origin of such behavior

Summary

■ INTRODUCTION

The ability to adapt to external mechanical stimuli with precise and predictable responses is still relatively rare among molecular crystals.− Endowing crystalline materials with such features enables a range of advanced practical applications in, e.g., flexible electronics and magnetic or optoelectronic devices. Crystalline coordination polymers (CPs) are, due to their tunable magnetic and electronic properties, of particular interest in this context, but only a handful of such compounds have been found to display elastic or plastic− flexibility in response to external mechanical pressure. To elucidate the differences in directionality of flexible properties (viz., 1-D vs 2-D bending), a more detailed inspection of structural features parallel/orthogonal to two bending faces (indicated by blue double-arrows, Figure 3) was performed It revealed virtually the same arrangement of 1-D polymeric chains in both directions for 2-D (1−3 and 6) and a distinctly different one for 1-D bendable crystals (4−5). The Young’s moduli on the bent crystals were somewhat smaller than the ones determined on their initial shapes (1: 0.86 GPa, 2: 0.47 GPa, 3: 0.93 GPa, and 5: 2.11 GPa), indicating a slight ′′softening′′ of the crystals at the kink.31,c In addition, the crystal surfaces (orthogonal to the bending faces) were inspected with AFM both before and after the plastic deformation was induced (1−3 and 5) It showed that surfaces at the bent region became striated after bending (Figure 5), with distinctive ridges of thickness of approximately 0.2 μm running parallel to the bending face. This indicates spatial segregation of neighboring domains as a consequence of weakening of intermolecular interactions between the layers caused apparently by slippage of those layers over each other during bending

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES