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

Abstract

Bimodal dynamics of mechanically constrained hydrogen bonds revealed by vibrational photon echoes Bodis, P.; Yeremenko, S.; Berna, J.; Buma, W.J.; Leigh, D.A.; Woutersen, S. General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We have investigated the dynamics of the hydrogen bonds that connect the components of a [2]rotaxane in solution. In this rotaxane, the amide groups in the benzylic-amide macrocycle and the succinamide thread are connected by four equivalent N−H· · ·O=C hydrogen bonds. The fluctuations of these hydrogen bonds are mirrored by the frequency fluctuations of the NH-stretch modes, which are probed by means of three-pulse photon-echo peak shift spectroscopy. The hydrogen-bond fluctuations occur on three different time scales, with time constants of 0.1, 0.6, and ≥200 ps. Comparing these three time scales to the ones found in liquid formamide, which contains the same hydrogenbonded amide motif but without mechanical constraints, we find that the faster two components, which are associated with small-amplitude fluctuations in the strength of the N−H· · ·O=C hydrogen bonds, are very similar in the liquid and the rotaxane. However, the third component, which is associated with the breaking and subsequent reformation of hydrogen bonds, is found to be much slower in the rotaxane than in the liquid. It can be concluded that the mechanical bonding in a rotaxane does not influence the amplitude and time scale of the small-amplitude fluctuations of the hydrogen bonds, but strongly slows down the complete dissociation of these hydrogen bonds. This is probably because in a rotaxane breaking of the macrocycle-axle contacts is severely hindered by the mechanical constraints. The hydrogen-bond dynamics in rotaxane-based molecular machines can therefore be regarded as liquidlike on a time scale 1 ps and less, but structurally frozen on longer (up to at least 200 ps) time scales.

Dethreading of Tetraalkylsuccinamide-Based [2]Rotaxanes for Preparing Benzylic Amide Macrocycles

“…Thread 3g was prepared from bis­(cyclohexylmethyl)­amine 1g (3.34 g, 16 mmol) and succinyl chloride 2 (616 mg, 4 mmol). White solid (1.40 g, 70%).…”

“…In order to establish the influence of the steric size − of the stoppers of succinamide-based rotaxanes − on the stability of their mechanical bond, we first prepared a series of surrogates, keeping the structure of the benzylic amide macrocycle constant and varying the alkyl substituents at the nitrogen atoms of the succinamide-based template. For this purpose, we assayed the five-component clipping reaction by using the tetraalkyl succinamides 3a – g , easily prepared from the corresponding secondary amines 1a – g and succinyl chloride ( 2 ), as templates and p -xylylenediamine and isophthaloyl chloride as precursors of the benzylic amide macrocycle (Table ).…”

“…White solid (1.40 g, 70%). The title compound showed identical spectroscopic data as those reported in ref (see the Supporting Information for a 1 H NMR spectrum).…”

Dampened circumrotation by CH⋯π interactions in hydrogen bonded [2]rotaxanes

“…Recently, making use of the well-established ability of the tetraalkyl succinamides to template the formation of a benzylic amide macrocycle by a five-component clipping reaction, some of us prepared a hydrogen-bonded [2]rotaxane for studying the intercomponent hydrogen-bond fluctuations by infrared ultrafast spectroscopy. 10 Herein we report that the incorporation of non-innocent stoppers, such as cyclohexylmethyl substituents, on a succinamide template 11 has a noticeable effect on the pirouetting motion of the benzylic amide macrocycle 12 through the establishment of non-covalent interactions, as shown by a dynamic 1 H NMR study carried out with a series of hydrogenbonded [2]rotaxanes (1a-e, Fig. 1) and evidenced by the molecular structure in the solid state of one of these species.…”

Rotaxanes and pseudorotaxanes with threads containing viologen units

“…Rotaxanes are a topic of enormous number of reports due to their valuable properties, interesting in the construction of molecular machines [21][22][23][24] and supramolecular polymers. 25,26 Pseudorotaxanes are also intensively studied, they are prototypes of molecular machines [27][28][29] and serve as building blocks for interlocked supramolecular assemblies, 30 some of them are promising in preparation of molecular sensors 31 and in drug delivery.…”