Infrared Study of Intercomponent Interactions in a Switchable Hydrogen‐Bonded Rotaxane

Abstract

The macrocycle in rotaxane 1 is preferentially hydrogen bonded to the succinamide station in the neutral form, but can be moved to the naphthalimide station by one-electron reduction of the latter. The hydrogen bonding between the amide NH groups of the macrocycle and the C double bond O groups in the binding stations in the thread was studied with IR spectroscopy in different solvents in both states. In addition, the solvent effect on the vibrational frequencies was analyzed; a correlation with the solvent acceptor number (AN) was observed. The conformational switching upon reduction could be detected by monitoring the hydrogen-bond-induced shifts of the nu(CO) frequencies of the C double bond O groups of the succinamide and the reduced naphthalimide stations. The macrocycle was found to shield the encapsulated station from the solvent: wavenumbers of nu(CO) bands of the C double bond O groups residing inside the macrocycle cavity remain unaffected by the solvent polarity.

Successive Translocation of the Rings in a [3]Rotaxane

“…Such folded conformations were also detected for the previously studied thread which contained succinamide and naphthalimide stations. [29] The relative intensities of the unperturbed bands and the red-shifted bands show that the extended conformations are predominant. The radical anions of the rotaxanes, 2…”

“…. [29,37] The strong band at 1677 cm -1 can be assigned to the unperturbed (CO)succ mode. The contribution from C=O stretching in intramolecularly hydrogen bonded conformations appears on the red side of the main band.…”

“…In the case of the naphthalimide rotaxane, the corresponding band was observed at a lower frequency of 1660 cm -1 . [29] Since shuttling takes place, one expects a negative peak at 1667 cm -1 from the C=O stretching in the macrocycle which was originally hydrogen bonded to the succinamide station. The dip between the bands at 1678 and 1659 cm -1 in the difference spectrum in Figure 4 is due to this contribution.…”

“…In previous studies in our laboratories, we have investigated the hydrogen bonding interactions between the macrocycle and the binding stations of rotaxanes that are switchable by electron transfer. [28][29][30][31][32][33] Infrared spectroscopy has proven to be a powerful tool in these studies, because intercomponent interactions lead to specific spectroscopic signatures. [29,[34][35][36][37][38] In this paper, the switching in [2] and [3]rotaxanes 2 and 3 (Scheme 1) containing a pyromellitimide (pmi) station is investigated by means of IR spectroelectrochemistry.…”

“…The hydrogen bonding between the macrocycle and the reduced aromatic imide station is evaluated in terms of acceptor strength and spatial arrangement of the hydrogen bond accepting C=O groups. The binding of the macrocycle to the pmi station will be compared to the binding to the 1,8-naphthalimide [28,29,36,37] and naphthalene diimides [32] in the corresponding rotaxanes studied before.…”