Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Ballistic Transport of Vibrational Energy through an Amide Group Bridging Alkyl Chains Published as part of The Journal of Physical Chemistry virtual special issue “Abraham Nitzan Festschrift”. Layla N. Qasim, E. Berk Atuk, Andrii O. Maksymov, Janarthanan Jayawickramarajah, Alexander L. Burin, and Igor V. Rubtsov* Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States
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ABSTRACT: Transport of vibrational energy via linear alkyl molecular chains can occur efficiently and with a high speed. This study addresses the question of how such transport is changed if an amide group is incorporated in the middle of such chain. A set of four compounds, Amn-4, was synthesized such that an amide group is connected to two alkyl chains. The alkyl chain on one side of the amide, featuring 4, 7, 11, or 15 CH2 units, is terminated by an azido group, while the alkyl chain on another side is of fixed length with four methylene groups terminated with a methyl ester group. The energy transport in Amn-4 in CD3CN solution, measured by relaxation-assisted two-dimensional infrared spectroscopy, was initiated and recorded using various combinations of tags and reporters, which included N3 and CO stretching modes of the end groups and amide-I and amide-II modes at the amide. It was found that the transport initiated by the amide-I mode in the alkyl chain attached to CO side of the amide proceeds with a constant speed of 4.2 Å/ps, supported by the CH2 rocking band of the chain. The end group-to-end group energy transport times for compounds with uneven alkyl chain length fragments appears to be additive. The transport from either end group of the molecule started as ballistic transport. The passage through the amide was found to be governed by intramolecular vibrational relaxation steps. After it passed the amide group, the transport was found to occur with constant but different speeds, dependent on the passage direction. The transport toward the ester was found to occur with the speed of 4.2 Å/ps, similar to that for the amide-I mode initiation and supported by the CH2 rocking band. The transport toward the azido group occurred with the speed of 8.0 Å/ps, which matches the speed supported by the CC stretching band. The results suggest that, after the CO group initiation, the excess energy reaches the amide group ballistically, redistributes at the amide, and reforms a wavepacket, which propagates further with a high speed of 8 Å/ps. This observation opens opportunities of controlling the energy transport process in molecules by affecting the alien group via specific interactions, including hydrogen bonding. density polyethylene fibers have shown to have very high thermal conductivity, and they are most effective in conducting heat along the fibers.13−16 Self-assembled monolayer junctions, featuring alkane oligomers between metal layers, have exhibited high thermal conductivity for alkane chains of up to 18 CH2 units.17,18 High thermal conductivity via acoustic modes was predicted for much longer alkane chains, on the scale of tens of nanometers in length.19 Other organic polymeric structures demonstrate a strong dependence of the thermal conductivity on the structure.4,20−24 A fundamental understanding of the relationship between molecular structure and conductivity is necessary for developing polymeric materials for molecular electronics.
1. INTRODUCTION The field of molecular electronics, conceptualized in the 1970s, has gained increasing interest in recent years.1 Continuous miniaturization of electronic circuits, as predicted by Moore’s law, is expected to reach the transistor densities of 1 × 1011/ cm2 by the year 2020, resulting in individual elements sizes of 10 nm,2 making them comparable to molecular sizes. Although the miniaturization of such electronics has great potential in device processing speeds, memory capacity, and sensing ability, there are many practical issues that make it difficult to harness their maximum potential. One such difficulty is the heat management of such densely packed environments; effective heat dissipation is critical to the performance of such devices.3−8 Lightweight organic structures are attractive for heat management, including heat dissipation, although pure organic materials are generally poor thermal conductors.9−12 However, there are exceptions, for example, ordered low© XXXX American Chemical Society
Received: November 30, 2018 Revised: January 2, 2019 Published: January 15, 2019 A
DOI: 10.1021/acs.jpcc.8b11570 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Several techniques have been developed to measure thermal conductivity of mesoscopic materials25−28 and there are a few methods suited to measuring thermal transport at the nanoscopic level. The latter rely on introducing some amount of energy into a portion of the molecule via exciting an electronic or vibrational transition with a short laser pulse, which relaxes rapidly and initiates energy redistribution and transport in the whole molecule. IR-pump−Raman-probe spectroscopy was used to track energy relaxation and transport on a molecular level, although only in relatively small molecules.29−31 Sum-frequency generation spectroscopy was used by the Dlott group to measure heat transport in selfassembled alkane monolayers attached to a gold surface in response to flash heating the gold to ∼1100 K. The authors found that the initial stage of the energy transport via alkyl chains is ballistic and occurs at a high speed of 9.5 Å/ps.32 It was suggested that optical bands of the alkane were involved in the energy transport.32,33 Pump−probe spectroscopy in the visible,34,35 mixed visible and mid-IR,36,37 and fully mid-IR38,39 spectral ranges were used to study energy transport in molecules to distances exceeding 10 Å. A high sensitivity of two-dimensional IR (2DIR) spectroscopy, specifically the dualfrequency relaxation-assisted 2DIR (RA 2DIR) approach, opened opportunities for interrogating energy transport in much larger molecules, exceeding 60 Å in length.40 Numerous molecular systems were interrogated using the RA 2DIR technique, including molecules with dissimilar functional groups along the molecular backbone41,42 and molecules with oligomeric backbones. The transport via several oligomeric chains, PEG,43 alkane,44,45 and perfluoroalkane,46,47 was found to occur ballistically, although to different maximal chain length.48 In addition, the energy transport speed was found to be dependent not only on the chain type but also on the way the energy is introduced into the chain. For example, when the transport via alkyl chain is introduced by exciting the νN3 mode at 2100 cm−1 on the azido moiety, the speed of 14.4 ± 2 Å/ps results, whereas a speed of 8.0 ± 0.3 Å/ps was obtained by exciting the CO of the carboxylic acid (1712 cm−1) or the asymmetric CO stretching mode of the NHS ester (1740 cm−1).44 The difference in speeds was associated with involvement of different optical bands of the chain, which support different group velocities. Whereas the energy transport via purely periodic oligomeric chains was studied in detail for a number of chains, the transport via chains with incorporated perturber, an alien group placed in the middle of the chain, has not been studied. It is not known how such perturber group will change the regime of energy transport across the whole compound and under what conditions the ballistic transport regime across the perturber can be achieved. It is also not known how to preserve high transport efficiency and if the interaction at the perturber moiety can modify the transport speed and efficiency. In this study an amide group was selected as a perturber linking two alkyls chains consisting of 4, 7, 11, and 15 CH2 units terminated by an azido group on one side and four CH2 units terminated by the methyl ester moiety on another side (Amn-4, n = 4, 7, 11, 15, Figure 1A). Various RA 2DIR measurements were performed for Amn-4 in CD3CN solution to interrogate the energy transport across the whole molecule and between the end groups and the amide (Figure 1B, arrows). A combination of theoretical analysis of the alkane chain bands and modeling of the intramolecular vibrational relaxation (IVR) pathways of the initially excited states
Figure 1. (A) Structure of Amn-4, n = 4, 7, 11, 15. The ovals represent different vibrational modes, which were used as tag or reporter modes: νN3 (blue), am-I (cyan), am-II (orange), and νC=O (magenta). (B) Solvent-subtracted linear infrared spectra of Amn-4 in CD3CN, n = 4, 7, 11. The arrows indicate the types of energy transport RA 2DIR measurements performed in this study.
permitted the identification of the energy transporting chain states. The experimental and computational methods used are described in Section 2. Section 3 describes the experimental data separately for each of the energy pathway studied (Figure 1B, arrows), which are then discussed and modeled in Section 4.
2. EXPERIMENTAL AND COMPUTATIONAL APPROACHES 2.1. 2DIR Measurements. The detailed description of a fully automated dual-frequency three-pulse echo 2DIR instrument with heterodyne detection is presented elsewhere.49 In brief, a Ti:sapphire laser producing 1.5 W power at 1 kHz repetition rate, 800 nm wavelength, and 80 fs pulse duration (Libra, Coherent) was used to pump a computer-controlled dual optical parametric amplifier (OPA, Palitra-duo, Quantronix). Two pairs of Signal and Idler pairs generated by OPA were directed to two computer-controlled difference frequency generation units (DFG; NIR Quantronix) to generate mid-IR pulses tunable in the frequency range from 500 to 5000 cm−1 and a pulse energy ranging from 1.0 to 10 μJ. The fully automated 2DIR instrument features the sensitivity of better than 1 × 10−4 cm−1 in measured anharmonicities, which is achieved by a combination of a closed-loop phase stabilization to better than 70 as, phase cycling, and spectral interferometry. The automatic frequency tuning from 800 to 4000 cm−1 is achieved by implementing a mid-IR beam direction stabilization schematic (
