Energy Transport in PEG Oligomers: Contributions of Different Optical Bands

The transport of high-frequency vibrational energy in linear oligomer chains can be fast and efficient if specific conditions which permit ballistic transport are satisfied. These conditions include high delocalization and slow dephasing rate of chain states. We present new experimental results probing the energy transport in linear polyethylene glycol (PEG) oligomers of 0, 4, 8, and 12 PEG units terminated with IR-active end groups, N₃ and succinimide ester. The energy transport was initiated by vibrational excitation of one of the end groups and the energy arrival to another end group was detected using dual-frequency, two-dimensional infrared spectroscopy. In addition to end-group to end-group energy transport dynamics, the end-group-to-chain-state and chain-state-to-chain-state waiting-time dynamics are reported. The results show that despite rather short lifetimes for several IR-active chain states, the end-to-end energy transport occurs with a constant and rather high speed of 5.5 Å/ps, regardless of which end group initiated the transport (N₃ or asymmetric CO stretching mode of the succinimide), which contrasts previous reports for similarly terminated alkane chains where the transport was dependent on the way it was initiated. To understand the transport mechanism, the PEG chain dispersion relations were computed, indicating that while many chain bands can contribute to the transport, most of them have short lifetimes (≤1 ps) that cannot support a ballistic regime to distances exceeding that of PEG8. However, the states of a single rocking band, at about 800–850 cm^–1, feature longer lifetimes, permitting ballistic transport via this band for 50 Å at room temperature. Theoretical modeling, based on solving the quantum Liouville equation for a density matrix for a linear chain, was performed. The modeling indicates that under directed diffusion conditions, a switch between ballistic and diffusive transport regimes can occur without abrupt changes of the transport speed. The approaches developed in this study are applicable to other chain types, in particular, those involving heteroatoms in the backbone