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

Abstract

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.

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

“…31 Briefly, infrared laser pulses with a duration of ∼90 fs, an energy of ∼10 μJ and a spectrum centered at 3400 cm −1 (FWHM ∼120 cm −1 ) are generated with a home-built BBO/KTP based white-light seeded optical parametric amplifier (OPA). The OPA is pumped by a Ti:Sapphire laser/amplifier system (Spectra Physics Hurricane).…”

“…In addition, the incorporation of these aliphatic stoppers improves the solubility of the interlocked molecule in halogenated solvents in comparison to previously investigated rotaxanes with aromatic stoppers. 23,31 All experiments are performed at room temperature on 10 mM solution of the [2]rotaxane in CDCl 3 kept between two CaF 2 windows using a 1 mm thick spacer.…”

Vibrational self-trapping in beta-sheet structures observed with femtosecond nonlinear infrared spectroscopy

“…Using a setup described previously, 19 we generate midinfrared pump and probe pulses with energies of 10 J and ϳ100 nJ, respectively, that are independently tunable from 2800 to 4000 cm −1 . The pulses have a duration of 150 fs, and bandwidths of ϳ80 cm −1 ͑pump͒ and ϳ200 cm −1 ͑probe͒.…”

Ultrafast Vibrational Dynamics of Membrane‐Bound Peptides at the Lipid Bilayer/Water Interface

“…To eliminate the influence on the membrane-bound H 2 Oo nt he excitation process and the signals of the amide Am ode,t he subphase solution of lipid bilayer was replaced by D 2 O. [23,24] Finally,the intermediate state (n*) relaxes to the "hot" ground state (n**), and then the intensity of the amide Ab and gradually recovers.F our-level vibrational model ( Figure 1C)h as been used to extract the vibrational lifetime (T 1 )a nd thermalization time constant (T th )ofthe vibrational dynamics in many condensed matters, including water, [11][12][13] peptides, [25,26] and others. Previous studies have shown that the residues in the core of lipid bilayer do not undergo hydrogen-deuterium exchange (HDX) of the amide proton in the 3-4 days.…”

“…An anharmonic shift of 137(AE 5) cm À1 is deduced. [25,26,29,30] However,t he NH/CO coupling is very important in physics and chemistry.I ts strength is directly associated with the molecular conformation. 120 cm À1 ).…”