5. Y. Zhang, Z. Li, P. Kim, L. Zhang, C. Zhou, ACS Nano 6, 126–132 (2012). 6. S. Choubak, M. Biron, P. L. Levesque, R. Martel, P. Desjardins, J. Phys. Chem. Lett. 4, 1100–1103 (2013). 7. J. D. Plummer et al., Silicon VLSI Technology: Fundamentals, Practices and Modeling (Prentice Hall, Upper Saddle River, NJ, 2000). 8. M. Ohring, Materials Science of Thin Films (Academic Press, Waltham, MA, ed. 2, 2002). 9. M. P. Levendorf et al., Nature 488, 627–632 (2012). 10. Z. Liu et al., Nat. Nanotechnol. 8, 119–124 (2013). 11. S. M. Kim et al., Nano Lett. 13, 933–941 (2013). 12. Y. B. Gao et al., Nano Lett. 13, 3439–3443 (2013). 13. P. Sutter, R. Cortes, J. Lahiri, E. Sutter, Nano Lett. 12, 4869–4874 (2012). 14. J. Tian et al., Nano Lett. 12, 3893–3899 (2012). 15. C. Oshima, A. Nagashima, J. Phys. Condens. Matter 9, 1–20 (1997). 16. Y. Zhang et al., Nat. Phys. 4, 627–630 (2008). 17. S. Joshi et al., Nano Lett. 12, 5821–5828 (2012). 18. J. M. Wofford, S. Nie, K. F. McCarty, N. C. Bartelt, O. D. Dubon, Nano Lett. 10, 4890–4896 (2010). 19. J. M. Pruneda, Phys. Rev. B 81, 161409 (2010). 20. S. Bhowmick, A. K. Singh, B. I. Yakobson, J. Phys. Chem. C 115, 9889–9893 (2011). 21. J. Wintterlin, M.-L. Bocquet, Surf. Sci. 603, 1841–1852 (2009). 22. X. Li et al., Science 324, 1312–1314 (2009). 23. K. K. Kim et al., Nano Lett. 12, 161–166 (2012). 24. S. Najmaei et al., Nat. Mater. 12, 754–759 (2013). 25. A. M. van der Zande et al., Nat. Mater. 12, 554–561 (2013). Acknowledgments: This work was partially supported by NSF (ECCS-1231808), the Defense Advanced Research Projects Agency (approved for public release; distribution is unlimited), and the National Secretariat of Higher Education, Science, Technology and Innovation of Ecuador (SENESCYT). A portion of this research was conducted at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Work at Sandia was supported by the Office of Basic Energy Sciences, Division of Materials and Engineering Sciences, U.S. Department of Energy, under contract DE-AC04-94AL85000. We thank R. M. Feenstra for discussions on LEEM. Authors declare no conflicts of interest. Supplementary Materials www.sciencemag.org/content/343/6167/163/suppl/DC1 Materials and Methods Supplementary Text Figs. S1 to S11 Reference (26) 18 September 2013; accepted 3 December 2013 10.1126/science.1246137 Self-Accelerating CO Sorption in a Soft Nanoporous Crystal Hiroshi Sato, 1,2 Wataru Kosaka, 1,2 Ryotaro Matsuda, 1,2 * Akihiro Hori, 2 Yuh Hijikata, 3 Rodion V. Belosludov, 4 Shigeyoshi Sakaki, 3 Masaki Takata, 2,5 Susumu Kitagawa 1,2,6 * Carbon monoxide (CO) produced in many large-scale industrial oxidation processes is difficult to separate from nitrogen (N 2 ), and afterward, CO is further oxidized to carbon dioxide. Here, we report a soft nanoporous crystalline material that selectively adsorbs CO with adaptable pores, and we present crystallographic evidence that CO molecules can coordinate with copper(II) ions. The unprecedented high selectivity was achieved by the synergetic effect of the local interaction between CO and accessible metal sites and a global transformation of the framework. This transformable crystalline material realized the separation of CO from mixtures with N 2 , a gas that is the most competitive to CO. The dynamic and efficient molecular trapping and releasing system is reminiscent of sophisticated biological systems such as heme proteins. C arbon monoxide (CO) is a central re- source for carbon-based chemical products such as polymer fibers, plastics, and med- icines (1–4). Although huge amounts of CO are produced in industrial processes such as steel manufacturing, CO is mixed with other gases (such as unburned N 2 from air). This exhaust gas cannot be used as a carbon resource and is burned to produce a huge amount of CO 2 . For effective separation, the selective uptake and ready release of CO are inseparable; however, these objectives often conflict because the trade-off for the in- crease in interaction with CO is some loss of ease in subsequent CO release. To date, CO separation has been limited to processes that strongly chemi- sorb CO on transition metal ions such as mono- valent Cu + (5), but high temperatures are required to release CO. A synergistic system that makes use of a weak local interaction and subsequent global structural change could overcome this limitation in a manner similar to allosteric effects in the binding and release of O 2 by heme pro- teins (6). Despite their crystalline form, porous coordi- nation polymers (PCPs) or metal-organic frame- works (7–10) can provide a nanometer-sized soft space that is transformable in response to guest accommodation (11–17). This feature encour- aged us to create a synergy system in a PCP solid to achieve the ultimate separation of mixed gases. Herein, we report on a PCP that has specific but weak CO adsorption and recognition sites and a reversibly transformable framework. Weak ad- sorption of CO on the Cu 2+ site induces marked global structural changes in a positive cooperative manner, which produces additional space and al- lows further adsorption of CO as so-called reversible self-accelerating CO adsorption (fig. S1 and movie S1) ( 18). As a result, this crystalline porous com- pound achieved unprecedented highly effective trapping of CO from a gas mixture with N 2 . We prepared the PCP 1 composed of 5- azidoisophthalate (aip) (19) and divalent Cu 2+ ions. In the as-synthesized crystal of PCP 1 {[Cu(aip)(H 2 O)](solvent) n , (where n is the num- ber of solvent molecules)}, the Cu 2+ and aip ligands form Cu 2+ paddle-wheel units, the axial positions of which are occupied by water mole- cules (Fig. 1A). An infinite kagomé-type two- dimensional (2D) sheet structure (20, 21) is formed through the connection between the paddle-wheel units by the aip ligands (Fig. 1B). The 2D sheets are stacked with a separation distance of 6.7 Å, creating two types of 1D in- finite channels with cross-section sizes of 9 × 9Å 2 (channel L; hexagonal larger channel) and 4×4Å 2 (channel S; triangular smaller chan- nel), respectively, along the c axis (Fig. 1, B to D). The total solvent-accessible volume was estimated to be 38% (766 Å 3 ) of the unit cell volume. The contributions by channels L and S to the total solvent-accessible volume were estimated as 65% (498 Å 3 ) and 35% (268 Å 3 ), respectively. Thermogravimetric analysis (fig. S2) (18) in- dicated that the guest molecules were easily re- moved by heating at 80°C to give a dried PCP 2 [Cu(aip)] that is thermally stable up to ~180°C. We determined the crystal structure of the dried PCP 2 by Rietveld analysis using synchrotron powder x-ray diffraction data (fig. S3) (18). The removal of water molecules from the axial site of the Cu 2+ paddle-wheel unit caused one of the carboxylate oxygen atoms in the adjacent layer to coordinate to this site to form the paddle-wheel chains along the c axis (Fig. 1, E and H, and fig. S5) (18). We observed marked changes in the shape and volume of channels upon the removal of the guest molecules (Fig. 1, F and G). The total solvent-accessible volume was reduced from 38% of the unit cell to 25%. In the structure of the dried PCP 2, the carboxylates twisted against the Cu-Cu axis, and the aromatic planes further inclined to the pores to make the channels nar- rower than those of the as-synthesized PCP 1. We noted distinct shape changes in channel S. Chan- nel S was squeezed at the neck, causing a struc- tural change from a bellows-like shape (Fig. 1C) to a garlic-like shape (Fig. 1G) (each pore is almost separated). We expected that access of 1 Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Kyoto 615-8510, Japan. 2 RIKEN SPring-8 Center, Hyogo 679-5148, Japan. 3 Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103, Japan. 4 Institute for Mate- rials Research, Tohoku University, Sendai 980-8577, Japan. 5 Japan Synchrotron Radiation Research Institute/SPring-8, Hyogo 679-5198, Japan. 6 Department of Synthetic Chemistry and Bio- logical Chemistry, Graduate School of Engineering, Kyoto Uni- versity, Kyoto 615-8510, Japan. *Corresponding author. E-mail: rmatsuda@icems.kyoto-u.ac.jp (R.M.); kitagawa@icems.kyoto-u.ac.jp (S.K.) www.sciencemag.org SCIENCE VOL 343 10 JANUARY 2014 167 REPORTS