Energy transport on a molecular level is investigated using dual-frequency relaxation-assisted two-dimensional infrared spectroscopy. The transport is initiated by vibrational excitation of a localized terminal mode and probed by influence of the excess energy on the frequency of another mode localized at another end. Very efficient energy transfer is found in molecular chains with repeating units. Excitation propagates with about a constant speed, suggesting a ballistic transport mechanism. Such transport regime was observed in compounds featuring two types of molecular backbones, polyethylene glycol (PEG) and perfluoroalkane; a faster speed was found via PEG chains where it is ca. 500 m/s (5 ?/ps).

Quantum dynamics of vibrational excitations is discussed using the model of 1D molecular chain with nearest neighbor interaction between identical molecular sites and two impurity sites in both the ends. Upon exciting one impurity site, its excess energy for relatively long for molecular scales time up to 100 ps is not redistributed uniformly among all other degrees of freedom. On the contrary an excitation propagates along the chain, is reflected from the chain ends, and quantum interference of forward and backward waves yields to recurrence cycles and echo phenomena. For a critical cycle number k_c, echo components of the neighboring cycles start to overlap, and eventually for k >> k_c the dynamics looks like chaotic one. The critical cycle number k_c depends on the coupling strength 0 < C < 1 of the impurity sites with its neighbors. kc achieves the maximum for C² ~ 1/2. The excitation passage is only slightly affected by decay of delocalized modes resulted from their interaction with the bath localized vibrations of chain cites.

The results are in qualitative agreement with the described above experimental data, and besides offer a way for loss-free energy transfer between separated in space reaction centers.