Low-Temperature Vibrational Energy Transport via PEG Chains

Abstract

We used relaxation-assisted two-dimensional infrared spectroscopy to study the temperature dependence (10−295 K) of end-to-end energy transport across end-decorated PEG oligomers of various chain lengths. The excess energy was introduced by exciting the azido end-group stretching mode at 2100 cm −1 (tag); the transport was recorded by observing the asymmetric CO stretching mode of the succinimide ester end group at 1740 cm −1 . The overall transport involves diffusive steps at the end groups and a ballistic step through the PEG chain. We found that at lower temperatures the through-chain energy transport became faster, while the end-group diffusive transport time and the tag lifetime increase. The modeling of the transport using a quantum Liouville equation linked the observations to the reduction of decoherence rate and an increase of the mean-free-path for the vibrational wavepacket. The energy transport at the end groups slowed down at low temperatures due to the decreased number and efficiency of the anharmonic energy redistribution pathways.

Crucial effect of transverse vibrations on the transport through polymer chains

“…In that case the rate of decoherence is predicted to scale with the temperature as T 2 in agreement with earlier analysis [52], thus being more efficient compared to that for longitudinal phonons. This prediction is consistent with the decoherence rate temperature dependence used to interpret recent measurements of temperature dependent energy transport in PEG oligomers [28] where the propagation of a single optical phonon excited by the external infrared pulse and interacting with thermal molecular vibrations has been investigated.…”

“…Sec. V the results are compared with recent measurements of temperature dependent vibrational energy transport in PEG oligomers [28,72]. The paper is resumed by a brief conclusion.…”

“…For solid solvent and very low frequency modes the consideration should fail because of the molecular motion constraints imposed by the host solid. We hope that this limit is not reached in the experiments [28] discussed below in Sec. V; yet this assumption is the subject for the future verification.…”

“…The low temperature asymptotic behavior of decoherence rate will be used below in Sec. V for the analysis of vibrational energy transport in poly-ethylene glycol oligomers [28].…”

“…In periodic one-dimensional molecules the coherent ballistic transport can be naturally expected because of the delocalization of particle states at sufficiently low temperature where the thermal fluctuations are substantially frozen out. Indeed, it has been demonstrated even at room temperature that the vibrational energy can propagate in a coherent manner towards a long distances of tens of nanometers in various organic polymers [19,20,[25][26][27][28] dissolved in weakly polar solvents. However, at room temperature coherence is usually broken for vibrational energy transport in peptides dissolved in highly polar water [29][30][31] and for electron or electronic exciton transport due to very high extent of decoherence [9] and/or polar environment reorganization [32,33].…”