Molecular Shuttling of a Compact and Rigid H‐Shaped [2]Rotaxane

Abstract

Bistable [2]rotaxanes (molecular shuttles) are mechanically interlocked molecules (MIMs) that can be switched between different translational co-conformations in response to an external stimulus. [1] Chemical, electrochemical, and photochemical inputs have been used to control the actions of these systems in solution and some very efficient and sophisticated examples are known, including those that occupy nonequilibrium states and function using a ratchet-type mechanism. [2] Since most [2]rotaxane molecular shuttles are large, flexible molecules with multiple functional groups they operate with efficiency in solution where the interlocked components enjoy a high degree of structural and dynamic freedom. The size and flexibility of the molecular structure are rarely a problem, [3] however, these attributes make them much less practical for highly condensed phases where the size, positional orientation and degree of aggregation of the MIM components are crucial to optimum switching efficiency. [4] To incorporate efficient molecular switches into materials and devices with a high density of functional components, a compact structure with a rigid axle and short, linear track for translational motion would be most desirable. [5] We report herein a compact MIM-based switch with welldefined geometry, rigid backbone, and efficient switching characteristics for future application in highly dense media. Highlights include 1) the development of a new recognition template for [2]pseudorotaxane formation built around a 24membered crown ether wheel and a benzimidazolium axle with extended aromatic substituents, 2) a high yielding, modular synthesis of [2]rotaxane molecular shuttles with a compact and rigid, H-shaped molecular structure, 3) access to three distinct molecular states and corresponding molecular shuttling rates through acid-base chemistry, and 4) control of the rate of molecular shuttling through lithium ion "ferrying" between neutral binding sites. [6] The interaction of dibenzo[24]crown-8 (DB24C8) with the imidazolium (K a = 8 m À1 ) [7] or phenylbenzimidazolium, [1-H] + (K a = 5.0 10 1 m À1 ) cation is weak but this can be drastically increased by adding aromatic groups to the 4-and 7-positions of the benzimidazolium unit. The benzimidazolium salt [2-H][BF 4 ] was synthesized by reacting 4,7-dibromo-2-phenyl-1H-benzimidazole [8] with 2 equivalents of 4-fluorophenylboronic acid under Suzuki coupling conditions followed by protonation with HBF 4 (see the Supporting Information). The 1 H NMR spectrum of a solution comprised of equimolar amounts of [2-H] + and DB24C8 (1.0 10 À3 m, CD 3 CN, 298 K) shows the efficient formation of [2]pseudorotaxane [2-H&DB24C8] + (Figure 1). The association constant of 1.0 10 3 m À1 is two orders of magnitude larger than for simple imidazolium or benzimidazolium derivatives. Significant shifts to higher frequency were observed for the NH (+ 0.73 ppm) and proton a resonances (+ 0.37 ppm) indicating hydrogen-bonding interactions while shifts to lower frequency for peaks b and c on th...

Stimuli-Responsive Host–Guest Systems Based on the Recognition of Cryptands by Organic Guests

“…Host–guest systems play a significant role in the development of advanced supramolecular materials because of their good selectivity, high efficiency, and responsiveness. As the first-generation of hosts in host–guest chemistry, crown ethers have been widely used in various areas such as artificial molecular machines, − drug delivery materials, and supramolecular polymers. − …”

Integrating the Mechanical Bond into Metal-Organic Frameworks

“…It should be noted that a lengthy [2]rotaxane has been mounted inside the large cavities of NU-1000 via SALI and functions as a redox active molecular switch. 31 An alternative strategy is to use a design that positions the track of the molecular shuttle between the linker struts-i.e., on the crossbar of an H-shaped linker [33][34][35][36] (Figure 3C). This design was first used by Loeb, Schurko, and co-workers to make UWDM-4 and install a thermally driven, degenerate molecular shuttle inside a MOF with Zn 4 O nodes.…”

A twist in the non-slanted H-mers to control π-conjugation in 2-dimensions and optical properties

“…Out of these, the benzimidazole (the fused imidazole) containing H-mers are synthesized in one step via dialdehyde and diamine condensation reaction, which is relatively easy to synthesize unlike the other fused heterocycle based H-mers. 24,25,28 The non-slanted H-mers represent the repeat units of 2D-p-conjugated materials. 29,30 Therefore, in addition to extended p-conjugation between the strands, these H-mers are also appropriate to grow into polymeric structures due to the presence of mirror-plane symmetry compared to the slanted-H-mers (Scheme 1).…”

“…29,30 Therefore, in addition to extended p-conjugation between the strands, these H-mers are also appropriate to grow into polymeric structures due to the presence of mirror-plane symmetry compared to the slanted-H-mers (Scheme 1). Even though the fused imidazole containing H-mers have the potential to generate interesting materials for optical and electronic applications, most of the studies are focused on their self-assembled monolayers on solid substrates 23,24,31,32 and use in molecular rotors 25,28 as axles. To fill in this knowledge gap, herein we have synthesized three H-mers to understand the impact of twist between the strands and rungs as well as the electronic nature of the rung on the H-mer optical, sensing, and photophysical properties.…”