Probing the structure of a rotaxane with two-dimensional infrared spectroscopy

Abstract

Femtosecond 2D-IR spectroscopy has been used to study the structure of a [2]rotaxane composed of a benzylic amide macrocycle that is mechanically interlocked onto a succinamide-based thread. Both the macrocycle and the thread contain carbonyl groups, and by determining the coupling between the stretching modes of these groups from the cross-peaks in the 2D-IR spectrum, the structure of the macrocycle-thread system has been probed. Our results demonstrate that 2D-IR spectroscopy can be used to observe structural changes in molecular devices on a picosecond time scale.

Fig. 1.

X-ray crystal structure of the rotaxane that was investigated in this study (6). For clarity, the carbon atoms in the thread are colored yellow, and only hydrogen atoms that are involved in hydrogen bonds are shown. The carbonyl groups are numbered, and the distance r and angle θ that have been determined from the 2D-IR spectrum are indicated.

Fig. 2.

Linear and 2D-IR spectra of the rotaxane shown in Fig. 1. (a) C═O-stretching region of the absorption spectrum of the rotaxane dissolved in CHCl₃ (solvent background is subtracted). Dashed line is the typical power spectrum of a pump pulse. (b) 2D spectrum of the rotaxane at a pump-probe delay of 1 ps for parallel polarizations of the pump and probe pulses, showing the transient absorption change Δα as a function of the pump and probe frequencies. Red indicates positive Δα; blue indicates negative Δα. The contour level spacing is 0.51 mOD. Five additional contours above and below Δα = 0 are drawn with a level spacing of 0.085 mOD. (c) The same as in b but for perpendicular polarizations of the pump and probe pulses. The contour level spacing is 0.19 mOD. Five additional contours above and below Δα = 0 are drawn with a level spacing of 0.032 mOD. The arrows indicate the most prominent cross-peaks.

Fig. 3.

Cross sections through the 2D spectra at ν_pump = 1,610 cm^-1. The spectrum for perpendicular pump and probe polarizations has been scaled to make the peaks at the diagonal equally large.

Fig. 4.

Calculated 1D and 2D spectra obtained by using the couplings given in Eq. 1 and the best-fit values given in Table 1. (a) Calculated linear absorption spectrum. (b) Calculated 2D spectrum of the rotaxane at a pump-probe delay of 1 ps for parallel polarizations of the pump and probe pulses. Red indicates positive Δα; blue indicates negative Δα. The contour level spacing is the same as in Fig. 2b. (c) The same as in b but for perpendicular polarizations of the pump and probe pulses. The contour level spacing is the same as in Fig. 2c.