Jeffrey S. Hannam

Stiff, and sticky in the right places: Binding interactions in isolated mechanically interlocked molecules probed by mid-infrared spectroscopy

We report the results of high-resolution spectroscopic studies on isolated, jet-cooled [2]rotaxanes. Employing IR absorption spectroscopy, we show how these noncovalently bound, multicomponent molecular systems that so far have been out of reach from high-resolution techniques, can now be characterized at an unprecedented level. IR absorption spectra of prototypical hydrogen-bond assembled rotaxanes as well as their associated threads and macrocycles are shown to provide a direct view on the effects of interlocking the macrocycle and thread, and to offer a straightforward approach for the study of their structural and dynamical properties.

Controlled Hydrogen-Bond Breaking in a Rotaxane by Discrete Solvation

Controlling a molecular brake: Binding interactions between the thread and the macrocycle of a [2]rotaxane can be tuned in a quasi-continuous manner by adding hydrogen-bond-forming solvent molecules one at a time to an isolated [2]rotaxane molecule. Conformational changes that detach the thread from the macrocycle can be induced controllably, and the system resembles a molecular version of applying and releasing a brake.

Conformational Flexibility of a Rotaxane Thread Probed by Electronic Spectroscopy in Helium Nanodroplets

Ultrahigh-resolution spectroscopic studies have been performed to elucidate the conformational landscape of the succinamide-based thread 1 that is frequently employed in mechanically interlocked molecular assemblies. We show how dissolving single molecules into a helium nanodroplet enables us to resolve the broad absorption spectrum--which is normally observed--into the separate contributions of individual conformers that are populated under the employed experimental conditions. Excellent agreement is obtained with the results of molecular dynamics calculations. The absorption spectrum of each conformer reveals a splitting of the zero-phonon resonance that is different for each conformer and could thus serve as a spectral signature.

In trap fragmentation and optical characterization of rotaxanes

The first experiments on trapped rotaxanes are presented, combining collision induced fragmentation and in-trap laser spectroscopy. The intrinsic optical properties of three rotaxanes and their non-interlocked building blocks (thread and macrocycle) isolated in a quadrupolar ion trap are investigated. The excitation and relaxation processes under thermal activation as well as under photo-activation are addressed. The light and collision induced fragmentation pathways show that the degradation mechanisms occurring in the rotaxane are highly dependent on the nature of the thread. In the prospective of operating photoswitchable molecules, photo-activation is achieved in a controlled way by depositing photo-energy in the desired sub-unit of a mechanically interlocked structure.

Structural dynamics of rotaxanes studied by infrared photon echo spectroscopy

The structural dynamics of a rotaxane is investigated using infrared photon echo peak shift spectroscopy on the N-H stretch vibrational mode. The results demonstrate non-Markovian character of the dynamics of this vibrational mode and an oscillatory component related to the presence of low-frequency modes that are anharmonically coupled to the N-H stretch mode.