Heterovibrational Interactions, Cooperative Hydrogen Bonding, and Vibrational Energy Relaxation Pathways in a Rotaxane

Femtosecond two-color vibrational pump-probe spectroscopy is used to investigate the interaction between the NH- and CO-stretch vibrations in a rotaxane composed of a benzylic amide macrocycle mechanically locked onto a succinamide-based thread. From the transient absorption spectrum, we obtain the cross anharmonicities and cross-peak anisotropies arising from the NH(macrocycle)/CO(macrocycle) and NH(macrocycle)/CO(thread) interactions. The cross-peak anisotropies are used to determine angles between NH and CO bonds in the macrocycle and the thread, providing structural information with picosecond temporal resolution. The CO and NH groups that form the macrocycle-thread hydrogen bonds are found to interact much more strongly than the CO and NH groups in other molecular systems containing the same NH···OC hydrogen-bond motif. We attribute this enhancement of the NH/CO anharmonic interaction to a cooperative effect, by which the two ring-thread hydrogen bonds sharing a hydrogen-bond acceptor mutually amplify each other. The relaxation dynamics of the NH/CO cross peaks has also been investigated. Surprisingly, the NH/CO cross peak observed upon exciting the NH-stretch mode decays much more slowly than the corresponding diagonal NH-stretch peak. This can be explained by the presence of an intermediate state that becomes populated in the NH-stretch vibrational relaxation and that is coupled to the CO-stretch mode. Our results demonstrate that NH/CO heterovibrational 2D-IR spectroscopy is well suited to observe the elementary hydrogen-bond making and breaking steps involved in the motion of rotaxane-based molecular devices.