Photoisomerization of a rotaxane hydrogen bonding template: Light-induced acceleration of a large amplitude rotational motion

Abstract

Establishing methods for controlling aspects of large amplitude submolecular movements is a prerequisite for the development of artificial devices that function through rotary motion at the molecular level. Here we demonstrate that the rate of rotation of the interlocked components of fumaramide-derived [2]rotaxanes can be accelerated, by >6 orders of magnitude, by isomerizing them to the corresponding maleamide [2]rotaxanes by using light. molecular machines ͉ dynamics L arge amplitude internal rotations that resemble to some extent processes found in authentic machinery have recently inspired analogic molecular versions of gears (1), turnstiles (2), brakes (3), ratchets (4, 5), rotors (6), and unidirectional spinning motors (7-10) and are an inherent characteristic of many catenanes and rotaxanes (11-13). Establishing methods for controlling aspects of such movements is a prerequisite for the development of artificial devices that function through rotary motion at the molecular level. In this regard, we recently reported the unexpected discovery that the rate of rotation of the interlocked components of benzylic amide macrocyclecontaining nitrone and fumaramide [2]rotaxanes can be slowed (''dampened'') by 2-3 orders of magnitude by applying a modest (Ϸ1 V⅐cm Ϫ1) external oscillating electric field (14). Here we demonstrate that the rate of rotation of the interlocked components of the olefin-based rotaxanes can also be accelerated, by Ͼ6 orders of magnitude, using another broadly useful stimulus, light.Fumaramide threads template the assembly of benzylic amide macrocycles around them to form rotaxanes in high yields (15). This cheap and simple preparative procedure (suitable threads are prepared in a single step from fumaryl chloride and a bulky primary or secondary amine) is particularly efficient because the trans-olefin fixes the two hydrogen bond-accepting groups of the thread in an arrangement that is complementary to the geometry of the hydrogen bonddonating sites of the forming macrocycle. However, the feature of the fumaramide unit that makes it such an effective template also provides an opportunity to enforce a geometrical change in the thread after rotaxane formation, thus altering the nature and strength of the interactions between the interlocked components. Isomerization of the olefin from E-to Z-must necessarily disrupt the near-ideal hydrogen bonding motif between macrocycle and thread and therefore also change any internal dynamics governed by those interactions.To test this idea, the photochemical isomerization of three fumaramide-based threads (E-1-3) and rotaxanes (E-4-6) was investigated. The synthesis of rotaxanes E-4 and E-6 has been described (15), and E-5 was prepared in analogous fashion from the corresponding thread, E-2, isophthaloyl dichloride and p-xylylene diamine (Scheme 1).** Under the same reaction conditions the cis-olefin (maleamide) threads, Z-1-3, did not give detectable quantities of the corresponding Z-rotaxanes. Experimental ProceduresGeneral Method for the Photoisomeri...

Molecular, Supramolecular, and Macromolecular Motors and Artificial Muscles

“…These types of molecules can be designed to expand or to contract linearly in response to different stimuli, such as chemical, 18,79-83 electrochemical, [84][85][86][87][88] and optical stimuli. 20,21,[89][90][91][92] Depending on the design (vide infra), switchable rotaxanes are capable of contracting up to 67% of their initial (extended) length, [93][94][95] suggesting that artificial molecular muscles could be harnessed by tethering to a solid support to perform macroscale work. This section describes recent advances and future prospects for controlling the artificial molecular muscles on both single-molecule and microscopic levels and developing them into nanomechanical devices.…”

Controlling the Pirouetting Motion in Rotaxanes by Counterion Exchange

“…Moreover, it is even smaller compared to the barrier found on similar systems upon photoisomerization of the thread component (6.8 kcal mol -1 ). 12 The presence of a fumaryl group in the thread of compounds E- [5]- [7] gives the opportunity to study the dynamics of their isomerized Z counterparts upon UV light. Irradiation of E- [4] at 312 nm gives Z- [4] in 59%…”

Bimodal dynamics of mechanically constrained hydrogen bonds revealed by vibrational photon echoes

“…4,[15][16][17][18][19] Although the dynamics of catenane-and rotaxane-based systems have been extensively studied on the second and millisecond time scales, 16,20 the fundamental motions within their components, and the thermal fluctuations in the surrounding medium take place in the picosecond or subpicosecond time domain. In order to understand how such elementary motions of individual functional groups determine the global behavior of relatively massive (>500 D) mechanically linked fragments, it is necessary to have a "quantum tool" that provides detailed insight into both structure and dynamics on such time scales.…”