Pressure-Driven Topological Transformations of Iodine Confined in One-Dimensional Channels

Abstract

The behavior of molecules and molecular chains confined in 1D nanochannels imposed by external interactions is a problem of fundamental interest. Here, we report structural manipulation of iodine confined inside zeolite (AFI) nanochannels by the application of high pressure. Structural transformations of the confined iodine under pressure have been unambiguously identified by polarized Raman spectroscopy combined with theoretical simulation. The length of the iodine chains and the orientation and intermolecular interaction of the confined iodine have been tuned at the molecular level by applied pressure. Almost all the confined iodine can be tuned into an axially oriented state upon compression, favoring the formation of long chains. The long iodine chains can be preserved to ambient pressure when released from intermediate pressures. ■ INTRODUCTIONStudies of the control and manipulation of atoms/molecules and their assemblies generate remarkable new insights into how physical and chemical systems function. They permit direct observation of molecular behavior that can be obscured by ensemble averaging and enables the study of important problems ranging from fundamental physics to the design of nanoscale electro-optical devices. In particular, much effort has been focused on the control of atomic/molecular chains due to their potential application as quantum wires. 1−9 By using simultaneous STM and TEM (transmission electron microscopy), gold nanowires composed of several atomic chains have been fabricated and show quantum conductance behavior. 2 Later, thinner nanowires have been fabricated in high vacuum with an electron beam thinning technique but the stability becomes lower with decreasing size and the length is still limited to a few nanometers (<6 nm). 3 On the other hand, filling materials into one-dimensional channels has been shown to be an efficient way to prepare atomic/molecular chains stable at ambient condition. 9−13 However, the atomic/molecular chains obtained usually show a mixture of different arrangements or have random orientations and are mixed with individual atoms/molecules in the channels due to size mismatch between the host channel and the filled species or because of inhomogeneous filling. 8−14 A typical example is that filling iodine into single wall carbon nanotubes (SWNTs) yields either helicoidal chains, polyiodides, discrete individual molecules, or new crystalline structures of iodine in the nanotube channels, depending on the tube diameter. 14−16 To obtain chains in a particular desired structural arrangement requires further manipulation. 17 For this purpose, understanding the transformation dynamics of the confined iodine imposed by external interactions becomes very important.High pressure serves as a powerful tuning parameter that has been used to tune the intermolecular interaction and structure of bulk materials. 18−23 In the confined environment, the configuration of the material is expected to be modified not only by the applied pressure but also by the evolution of the co...

Supramolecular Organization in Confined Nanospaces

“…Although a true polymerization did not occur, the distance between confined molecules gradually became comparable to the Br−Br intramolecular bond length with increasing pressures, suggesting that the dissociation of the bromine molecules and the formation of a Br n confined polymer should occur at even higher pressures – namely, above 24 GPa . Also iodine was found to exhibit a similar pressure‐driven self‐organizing behavior when confined into the same framework . Such pressure‐created (I 2 ) n chains showed an unexpected photoluminescence behavior under compression – which was absent in solid iodine, and was attributed to the unique 1D structure of the formed chains …”

“…Finally, inclusion in the MFI 3D‐channels resulted in a red‐brown color,, ascribed to stronger halogen interactions . Several studies have then focused on the organization of confined dihalogens, exploring not only the structure of the aggregates, but also how their organization is affected by size and concentration of the guests, structure and composition of the host,, solvation, temperature, and pressure . In general, molecules confined in small cages, like in SOD (Figure ), undergo rotational motion – which may be more frictioned for large guests, but is normally very fast .…”

“…The rotational barriers are greater in the AEL elliptical channels, featuring an alternation of wide and narrow regions (Figure ). Raman spectroscopy revealed that the confined dihalogens could orient either aligned or perpendicular to the channel axis, as the two orientations give rise to distinct Raman bands ,. The lower frequency band is due to wires of aligned molecules: indeed, the inter‐halogen interactions weaken the intramolecular bond, causing a decrease of its force constant .…”

Transformations of iodine species inside elliptical channels of AlPO4 -11 crystals at low temperature: a Raman study

“…Iodine species filled in SWCNTs were observed to form single, double, and triple helical iodine structures, as well as crystalline forms, depending on the diameters of SWCNTs . When confined in 1D round channels of AlPO 4 ‐5 (AFI), iodine molecules exist as individual molecules with different orientations, molecular chains, while those in the elliptical channels of AlPO 4 ‐11 (AEL) lose one external degree of freedom because of the quasi‐two‐dimensional confinement environment of AEL and exhibit different dynamic behaviors compared to those in AFI . Very recently, our research group introduced high pressure technology to explore the transformations of confined iodine in AFI and AEL (I@AFI and I@AEL) at room temperature .…”

“…When confined in 1D round channels of AlPO 4 ‐5 (AFI), iodine molecules exist as individual molecules with different orientations, molecular chains, while those in the elliptical channels of AlPO 4 ‐11 (AEL) lose one external degree of freedom because of the quasi‐two‐dimensional confinement environment of AEL and exhibit different dynamic behaviors compared to those in AFI . Very recently, our research group introduced high pressure technology to explore the transformations of confined iodine in AFI and AEL (I@AFI and I@AEL) at room temperature . It was found that the external pressure modified the confinement environment, drove an orientational transformation of individual iodine molecules from standing configuration to lying configuration, and increased the population of iodine chains.…”