Formation and Characterization of Crystalline Molecular Arrays of Gas Molecules in a 1-Dimensional Ultramicropore of a Porous Copper Coordination Polymer Ryo Kitaura, ²,# Ryotaro Matsuda, ² Yoshiki Kubota, ‡ Susumu Kitagawa,* ,² Masaki Takata, § Tatsuo C. Kobayashi, ⊥ and Megumi Suzuki ⊥ Department of Synthetic Chemistry and Biological Chemistry, Kyoto UniVersity, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan, Department of Natural Science, Osaka Women’s UniVersity, Sakai, Osaka 590-0035, Japan, JASRI-the SPring-8, Koto 1-1, Hyogo 679-5198, Japan, and Department of Physics Faculty of Science, Okayama UniVersity Tsushima-naka, Okayama 700-8530, Japan ReceiVed: August 8, 2005; In Final Form: October 9, 2005 Molecules and atoms confined in a nanospace may have properties distinctly different from those of the bulk fluid, owing to the formation of a specific molecular array characteristic of nanospace. In situ synchrotron powder X-ray diffraction measurements have been used to observe confined guest molecules such as N 2 ,O 2 , Ar, and CH 4 in the well-regulated ultramicropore of a copper coordination polymer, 1 ([Cu 2 (pzdc) 2 pyz]: pzdc ) 2,3-pyrazinedicarboxylate and pyz ) pyrazine). The obtained crystal structures indicate that guest molecules are confined in a linear fashion to form crystalline-like regular ordered arrays, in contrast to the situation in the gas and liquid state, even at temperatures above the boiling point, and the ordered arrays are characteristic of the kind of gas molecule and the geometrical and potential properties of the ultramicropore of 1. Introduction Molecules adsorbed in microporous materials tend to form a specific molecular assembly subjected to their restricted porous geometry and the adsorption enhancement effect of multiple attractive interactions exerted from the confronting and neigh- boring pore walls; this is characteristic of the nanometer-sized space of a micropore (pore size < 2 nm). This adsorption enhancement effect has been widely studied not only for its industrial application with gas storage materials and heteroge- neous catalysts, but also for the unique properties of confined molecules that are distinctly different from those of the bulk fluid. For example, it has been reported that molecules confined in a micropore show an abnormal freezing and melting behavior, high Second-Harmonic Generation (SHG) activity, and specific magnetic properties. 1-5 Therefore, confinement of molecules in a nanometer-sized space would lead not only to finding novel phenomena, but also to providing novel magnetic, electric, and photophysical materials, which we call a “nanospace labora- tory”. 6 To develop a nanospace laboratory, a better understand- ing of the adsorption phenomena and detailed structural information of confined guest molecules is indispensable. Although a great deal of useful and interesting information is available about adsorption and confinement phenomena of guest molecules in microporous materialssincluding molecular simu- lations such as molecular dynamics and Monte Carlo simula- tions, thermodynamic studies such as sorption isotherms and heat measurements, nuclear magnetic resonance measurements, and in situ X-ray small-angle scattering measurementssdetails of the ordered state of confined molecules remain elusive. 7-14 Consequently, we have tried to perform a direct observation of the guest molecules confined in microporous materials using in situ synchrotron X-ray diffraction. Recently, many crystalline porous coordination polymers that are constructed from metal ions and bridging ligands have been synthesized and studied for their adsorption performance with various gas molecules. 9,15-25 Porous coordination polymers have advantages over the conventional microporous materials such as activated carbon and zeolites: (1) well-regulated and des- ignable nanochannels, and (2) a variety of nanochannels ranging from one- to three-dimensional types. 6,26-30 These features are of key importance in performing direct observation by X-ray crystallography of various molecular assemblies confined in the nanochannel. 31-33 Thus, our strategy for observing regular assemblies of guest molecules is to utilize the well-regulated channels of porous coordination polymers. Figure 1 shows the * Address correspondence to this author. Phone: 81-75-383-2733. Fax: 81-75-383-2732. E-mail: kitagawa@sbchem.kyoto-u.ac.jp. ² Kyoto University. # Current Address: Department of Chemistry, Nagoya University, furo- cho, Nagoya 464-8602, Japan. ‡ Osaka Women’s University. § JASRI-the SPring-8. ⊥ Okayama University. Figure 1. Crystal structure of {[Cu2(pzdc)2pyz]‚2H2O}n (1 ⊃ H2O) along the a axis. Green, blue, white, gray, and red balls and sticks represent the Cu, nitrogen, hydrogen, carbon, and oxygen atoms, respectively. Solvated water molecules are omitted for clarity. 23378 J. Phys. Chem. B 2005, 109, 23378-23385 10.1021/jp054411j CCC: $30.25 © 2005 American Chemical Society Published on Web 11/16/2005