1 research outputs found
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, N<sub>3</sub> 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 Ã…/ps, regardless of which end group
initiated the transport (N<sub>3</sub> or asymmetric Cî—»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<sup>–1</sup>, feature
longer lifetimes, permitting ballistic transport via this band for
50 Ã… 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