Article pubs.acs.org/cm
Behavior of Binary Guests in a Porous Coordination Polymer Nobuhiro Yanai,† Takashi Uemura,*,† and Susumu Kitagawa*,†,‡ †
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ‡ Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. S Supporting Information *
ABSTRACT: Controlling the condensed state of multiple guests in nanoporous media is critical to many applications, but an understanding of this phenomenon in pores smaller than a few nanometers is still unavailable. In this work, we investigate the aggregation state of binary guests, poly(ethylene glycol) (PEG), and long-chain normal alkanes, in subnanometer channels of a porous coordination polymer (PCP) by monitoring their thermal transition behaviors. PEG and alkanes are immiscible in the bulk and their melting transitions are not affected by each other. Meanwhile, in the PCP nanochannels, the transition temperature and the heat of the binary−guest system were significantly different from when PEG or an alkane was individually included. This suggests the formation of microscopically segregated domain structures of PEG and alkane in the host crystal. The transition behaviors gradually varied by changing the introduction ratio between PEG and alkane, and thus the aggregation states of the two guests were successfully controlled by the simple variation of relative amounts. This methodology offers a promising route to control spatial configurations of multiple guest molecules in nanoporous matrices for advanced applications. KEYWORDS: porous coordination polymer, poly(ethylene glycol), alkane, binary guest mixture, nanoconfinement, thermal transition, DSC, segregated domain
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INTRODUCTION
How binary-guest mixtures behave in nanoconfined geometries is an emerging field of research in chemistry, physics, and materials science.1,2 The microphase separation of a guest mixture in a porous matrix affords a variety of structural patterns depending on the degree of confinement (pore size, shape, and connectivity) as well as the preference of the pore walls for one of the components of the mixture. This structure evolution has significant relevance to a wide range of applications. For example, it is essential to study phase behaviors of liquid mixtures in porous materials to understand and improve many technological processes (e.g., separation).3−5 Phase separation of polymers under confinement is also particularly important for fabrication of new polymer nanostructures, and the resulting nanosized domains can be used as templates for spatial control of metal nanoparticles.2,6 The important question remaining in this field is what happens if the pore size gets smaller, eventually to less than a few nanometers, which approaches the molecular scale. This problem is relevant to advanced applications, such as high performance in separation processes and precise nanofabrications. One can think of three possible scenarios for binary guest systems in such ultimately small spaces: the two guests are (1) accommodated in different crystals separately, (2) accommodated in the same crystal and segregated, or (3) accommodated in the same crystal and miscible on the molecular scale (Figure 1). © XXXX American Chemical Society
Figure 1. Three possible situations for binary guests in subnanometer pores: (a) accommodated in different crystals separately, (b) accommodated in the same crystal and segregated, or (c) accommodated in the same crystal and miscible on the molecular scale.
To address this question, we employed porous coordination polymers (PCPs) or metal−organic frameworks (MOFs), which have emerged as a unique class of hybrid nanoporous materials.7−11 The characteristic features of PCPs are highly regular channel structures, controllable channel sizes approximating molecular dimensions (
