Probing the Hydrophobic Region of a Lipid Bilayer at Specific Depths Using Vibrational Spectroscopy.

A novel spectroscopic approach for studying the flexibility and mobility in the hydrophobic interior of lipid bilayers at specific depths is proposed. A set of test compounds featuring an azido moiety and a cyano or carboxylic acid moiety, connected by an alkyl chain of different lengths, was synthesized. FTIR data and molecular dynamics calculations indicated that the test compounds in a bilayer are oriented so that the cyano or carboxylic acid moiety is located in the lipid head-group region, while the azido group stays inside the bilayer at the depth determined by its alkyl chain length. We found that the asymmetric stretching mode of the azido group (νN3) can serve as a reporter of the membrane interior dynamics. FTIR and two-dimensional infrared (2DIR) studies were performed at different temperatures, ranging from 22 to 45 °C, covering the Lß-Lα phase transition temperature of dipalmitoylphosphatidylcholine (∼41 °C). The width of the νN3 peak was found to be very sensitive to the phase transition and to the temperature in general. We introduced an order parameter, SN3, which characterizes restrictions to motion inside the bilayer. 2DIR spectra of νN3 showed different extents of inhomogeneity at different depths in the bilayer, with the smallest inhomogeneity in the middle of the leaflet. The spectral diffusion dynamics of the N3 peak was found to be dependent on the depth of the N3 group location in the bilayer. The obtained results enhance our understanding of the bilayer dynamics and can be extended to investigate membranes with more complex compositions.

Maximum propagation speed and Cherenkov effect in optical phonon transport through periodic molecular chains.

Optical phonons serve as the fast and efficient carriers of energy across periodic polymers due to their delocalization, large group velocity because of covalent bonding, and large energy quantum compared to that for acoustic phonons as it was observed in a number of recent measurements in different oligomers. However, this transport is dramatically sensitive to anharmonic interactions, including the unavoidable interaction with acoustic phonons responsible for transport decoherence, suppressing ballistic transport at long distances. Here, we show that this decoherence is substantially suppressed if the group velocity of optical phonons is less than the sound velocity of acoustic phonons; otherwise, ballistic transport is substantially suppressed by a Cherenkov-like emission of acoustic phonons. This conclusion is justified considering energy and momentum conservation during phonon absorption or emission and supported by the numerical evaluation of the lifetimes of the optical phonons. It is also consistent with the recent experimental investigations of ballistic optical phonon transport in oligomers with the minor exception of relatively short oligophenylenes.

Experimentally revealing anomalously large dipoles in the dielectric of a quantum circuit.

Quantum two-level systems (TLSs) intrinsic to glasses induce decoherence in many modern quantum devices, such as superconducting qubits. Although the low-temperature physics of these TLSs is usually well-explained by a phenomenological standard tunneling model of independent TLSs, the nature of these TLSs, as well as their behavior out of equilibrium and at high energies above 1 K, remain inconclusive. Here we measure the non-equilibrium dielectric loss of TLSs in amorphous silicon using a superconducting resonator, where energies of TLSs are varied in time using a swept electric field. Our results show the existence of two distinct ensembles of TLSs, interacting weakly and strongly with phonons, where the latter also possesses anomalously large electric dipole moment. These results may shed new light on the low temperature characteristics of amorphous solids, and hold implications to experiments and applications in quantum devices using time-varying electric fields.

DNA Strand Displacement Driven by Host-Guest Interactions.

Base-pair-driven toehold-mediated strand displacement (BP-TMSD) is a fundamental concept employed for constructing DNA machines and networks with a gamut of applications─from theranostics to computational devices. To broaden the toolbox of dynamic DNA chemistry, herein, we introduce a synthetic surrogate termed host-guest-driven toehold-mediated strand displacement (HG-TMSD) that utilizes bioorthogonal, cucurbit[7]uril (CB[7]) interactions with guest-linked input sequences. Since control of the strand-displacement process is salient, we demonstrate how HG-TMSD can be finely modulated via changes to the structure of the input sequence (including synthetic guest head-group and/or linker length). Further, for a given input sequence, competing small-molecule guests can serve as effective regulators (with fine and coarse control) of HG-TMSD. To show integration into functional devices, we have incorporated HG-TMSD into machines that control enzyme activity and layered reactions that detect specific microRNA.

Tracking Energy Transfer across a Platinum Center.

Rigid, conjugated alkyne bridges serve as important components in various transition-metal complexes used for energy conversion, charge separation, sensing, and molecular electronics. Alkyne stretching modes have potential for modulating charge separation in donor-bridge-acceptor compounds. Understanding the rules of energy relaxation and energy transfer across the metal center in such compounds can help optimize their electron transfer switching properties. We used relaxation-assisted two-dimensional infrared spectroscopy to track energy transfer across metal centers in platinum complexes featuring a triazole-terminated alkyne ligand of two or six carbons, a perfluorophenyl ligand, and two tri(p-tolyl)phosphine ligands. Comprehensive analyses of waiting-time dynamics for numerous cross and diagonal peaks were performed, focusing on coherent oscillation, energy transfer, and cooling parameters. These observables augmented with density functional theory computations of vibrational frequencies and anharmonic force constants enabled identification of different functional groups of the compounds. Computations of vibrational relaxation pathways and mode couplings were performed, and two regimes of intramolecular energy redistribution are described. One involves energy transfer between ligands via high-frequency modes; the transfer is efficient only if the modes involved are delocalized over both ligands. The energy transport pathways between the ligands are identified. Another regime involves redistribution via low-frequency delocalized modes, which does not lead to interligand energy transport.

Competition of Several Energy-Transport Initiation Mechanisms Defines the Ballistic Transport Speed.

The ballistic regime of vibrational energy transport in oligomeric molecular chains occurs with a constant, often high, transport speed and high efficiency. Such a transport regime can be initiated by exciting a chain end group with a mid-infrared (IR) photon. To better understand the wavepacket formation process, two chemically identical end groups, azido groups with normal, 14N3-, and isotopically substituted, 15N3-, nitrogen atoms, were tested for wavepacket initiation in compounds with alkyl chains of n = 5, 10, and 15 methylene units terminated with a carboxylic acid (-a) group, denoted as 14N3Cn-a and 15N3Cn-a. The transport was initiated by exciting the azido moiety stretching mode, the νN≡N tag, at 2100 cm-1 (14N3Cn-a) or 2031 cm-1 (15N3Cn-a). Opposite to the expectation, the ballistic transport speed was found to decrease upon 14N3 → 15N3 isotope editing. Three mechanisms of the transport initiation of a vibrational wavepacket are described and analyzed. The first mechanism involves the direct formation of a wavepacket via excitation with IR photons of several strong Fermi resonances of the tag mode with the νN═N + νN-C combination state while each of the combination state components is mixed with delocalized chain states. The second mechanism relies on the vibrational relaxation of an end-group-localized tag into a mostly localized end-group state that is strongly coupled to multiple delocalized states of a chain band. Harmonic mixing of νN═N of the azido group with CH2 wagging states of the chain permits a wavepacket formation within a portion of the wagging band, suggesting a fast transport speed. The third mechanism involves the vibrational relaxation of an end-group-localized mode into chain states. Two such pathways were found for the νN≡N initiation: The νN═N mode relaxes efficiently into the twisting band states and low-frequency acoustic modes, and the νN-C mode relaxes into the rocking band states and low-frequency acoustic modes. The contributions of the three initiation mechanisms in the ballistic energy transport initiated by νN≡N tag are quantitatively evaluated and related to the experiment. We conclude that the third mechanism dominates the transport in alkane chains of 5-15 methylene units initiated with the νN≡N tag and the wavepacket generated predominantly at the CH2 twisting band. The isotope effect of the transport speed is attributed to a larger contribution of the faster wavepackets for 14N3Cn-a or to the different breadth of the wavepacket within the twisting band. The study offers a systematic description of different transport initiation mechanisms and discusses the requirements and features of each mechanism. Such analysis will be useful for designing novel materials for energy management.

Crucial effect of transverse vibrations on the transport through polymer chains.

The low temperature transport of electron, or vibrational or electronic exciton toward polymer chains, turns out to be dramatically sensitive to its interaction with transverse acoustic vibrations. We show that this interaction leads to a substantial polaron effect and decoherence, which are generally stronger than those associated with longitudinal vibrations. For site-dependent interactions, transverse phonons form subohmic bath leading to the quantum phase transition accompanied by full suppression of the transport at zero temperature and fast decoherence characterized by temperature dependent rate k2 ∝ T3/4 at low temperature, while k2 ∝ T2 for site-independent interactions. The latter dependence was used to interpret recent measurements of temperature dependent vibrational energy transport in polyethylene glycol oligomers.

Low-Temperature Vibrational Energy Transport via PEG Chains.

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.

Ballistic and diffusive vibrational energy transport in molecules.

Energy transport in molecules is essential for many areas of science and technology. Strong covalent bonds of a molecular backbone can facilitate the involvement of the molecule's high-frequency modes in energy transport, which, under certain conditions, makes the transport fast and efficient. We discuss such conditions and describe various transport regimes in molecules, including ballistic, diffusive, directed diffusion, and intermediate regime cases, in light of recently developed experimental and theoretical approaches.

Chaotic Dynamics in a Quantum Fermi-Pasta-Ulam Problem.

We investigate the emergence of chaotic dynamics in a quantum Fermi-Pasta-Ulam problem for anharmonic vibrations in atomic chains applying semi-quantitative analysis of resonant interactions complemented by exact diagonalization numerical studies. The crossover energy separating chaotic high energy phase and localized (integrable) low energy phase is estimated. It decreases inversely proportionally to the number of atoms until approaching the quantum regime where this dependence saturates. The chaotic behavior appears at lower energies in systems with free or fixed ends boundary conditions compared to periodic systems. The applications of the theory to realistic molecules are discussed.

Electronic torsional sound in linear atomic chains: Chemical energy transport at 1000 km/s.

We investigate entirely electronic torsional vibrational modes in linear cumulene chains. The carbon nuclei of a cumulene are positioned along the primary axis so that they can participate only in the transverse and longitudinal motions. However, the interatomic electronic clouds behave as a torsion spring with remarkable torsional stiffness. The collective dynamics of these clouds can be described in terms of electronic vibrational quanta, which we name torsitons. It is shown that the group velocity of the wavepacket of torsitons is much higher than the typical speed of sound, because of the small mass of participating electrons compared to the atomic mass. For the same reason, the maximum energy of the torsitons in cumulenes is as high as a few electronvolts, while the minimum possible energy is evaluated as a few hundred wavenumbers and this minimum is associated with asymmetry of zero point atomic vibrations. Theory predictions are consistent with the time-dependent density functional theory calculations. Molecular systems for experimental evaluation of the predictions are proposed.

Random-Defect Laser: Manipulating Lossy Two-Level Systems to Produce a Circuit with Coherent Gain.

We demonstrate a laser using material defects known for deleterious microwave absorption in quantum computing. These defects are two-level atomic tunneling systems (TSs), which are manipulated using a uniform swept dc electric field and two ac pump fields. The swept field changes the TS energies. TSs first pass through degeneracy with pump photons, which invert (excite) them with a high probability using rapid adiabatic passage. Population inversion is accomplished in spite of a broad distribution of TS parameters. Afterwards the TSs are brought to degeneracy with the resonator where they emit photons. The emission is found to be dependent on individual cavity-TS interactions, and the narrowing linewidth at increasing photon occupancy indicates stimulated emission. Characterization with a microwave probe shows a transition from ordinary defect loss to negligible microwave absorption, and ultimately to coherent amplification. Thus, instead of absorbing microwave energy, the TSs can be tuned to reduce loss and even amplify signals.

Ballistic Energy Transport in Oligomers.

The development of nanocomposite materials with desired heat management properties, including nanowires, layered semiconductor structures, and self-assembled monolayer (SAM) junctions, attracts broad interest. Such materials often involve polymeric/oligomeric components and can feature high or low thermal conductivity, depending on their design. For example, in SAM junctions made of alkane chains sandwiched between metal layers, the thermal conductivity can be very low, whereas the fibers of ordered polyethylene chains feature high thermal conductivity, exceeding that of many pure metals. The thermal conductivity of nanostructured materials is determined by the energy transport between and within each component of the material, which all need to be understood for optimizing the properties. For example, in the SAM junctions, the energy transport across the metal-chain interface as well as the transport through the chains both determine the overall heat conductivity, however, to separate these contributions is difficult. Recently developed relaxation-assisted two-dimensional infrared (RA 2DIR) spectroscopy is capable of studying energy transport in individual molecules in the time domain. The transport in a molecule is initiated by exciting an IR-active group (a tag); the method records the influence of the excess energy on another mode in the molecule (a reporter). The energy transport time can be measured for different reporters, and the transport speed through the molecule is evaluated. Various molecules were interrogated by RA 2DIR: in molecules without repeating units (disordered), the transport mechanism was expected and found to be diffusive. The transport via an oligomer backbone can potentially be ballistic, as the chain offers delocalized vibrational states. Indeed, the transport regime via three tested types of oligomers, alkanes, polyethyleneglycols, and perfluoroalkanes was found to be ballistic, whereas the transport within the end groups was diffusive. Interestingly, the transport speeds via these chains were different. Moreover, the transport speed was found to be dependent on the vibrational mode initiating the transport. For the difference in the transport speeds to be explained, the chain bands involved in the wavepacket formation were analyzed, and specific optical bands of the chain were identified as the energy transporters. For example, the transport initiated in alkanes by the stretching mode of the azido end group (2100 cm(-1)) occurs predominantly via the CH2 twisting and wagging chain bands, but the transport initiated by the C=O stretching modes of the carboxylic acid or succinimide ester end groups occurs via C-C stretching and CH2 rocking bands of the alkane chain. Direct formation of the wavepacket within the CH2 twisting and wagging chain bands occurs when the transport is initiated by the N═N stretching mode (1270 cm-1) of the azido end-group. The transport via optical chain bands in oligomers involves rather large vibrational quanta (700-1400 cm(-1)), resulting in efficient energy delivery to substantial distances. Achieved quantitative description of various energy transport steps in oligomers, including the specific contributions of different chain bands, can result in a better understanding of the transport steps in nanocomposite materials, including SAM junctions, and lead towards designing systems for molecular electronics with a controllable energy transport speed.

Room-temperature ballistic energy transport in molecules with repeating units.

In materials, energy can propagate by means of two limiting regimes: diffusive and ballistic. Ballistic energy transport can be fast and efficient and often occurs with a constant speed. Using two-dimensional infrared spectroscopy methods, we discovered ballistic energy transport via individual polyethylene chains with a remarkably high speed of 1440 m/s and the mean free path length of 14.6 Å in solution at room temperature. Whereas the transport via the chains occurs ballistically, the mechanism switches to diffusive with the effective transport speed of 130 m/s at the end-groups attached to the chains. A unifying model of the transport in molecules is presented with clear time separation and additivity among the transport along oligomeric fragments, which occurs ballistically, and the transport within the disordered fragments, occurring diffusively. The results open new avenues for making novel elements for molecular electronics, including ultrafast energy transporters, controlled chemical reactors, and sub-wavelength quantum nanoseparators.

Band-selective ballistic energy transport in alkane oligomers: toward controlling the transport speed.

Intramolecular transport of vibrational energy in two series of oligomers featuring alkane chains of various length was studied by relaxation-assisted two-dimensional infrared spectroscopy. The transport was initiated by exciting various end-group modes (tags) such as different modes of the azido (ν(N≡N) and ν(N═N)), carboxylic acid (ν(C═O)), and succinimide ester (νas(C═O)) with short mid-IR laser pulses. It is shown that the transport via alkane chains is ballistic and the transport speed is dependent on the type of the tag mode that initiates the transport. The transport speed of 8.0 Å/ps was observed when initiated by either ν(C═O) or νas(C═O). When initiated by ν(N≡N) and ν(N═N), the transport speed of 14.4 ± 2 and 11 ± 4 Å/ps was observed. Analysis of the vibrational relaxation channels of different tags, combined with the results for the group velocity evaluation, permits identification of the chain bands predominantly contributing to the transport for different cases of the transport initiation. For the transport initiated by ν(N≡N) the CH2 twisting and wagging chain bands were identified as the major energy transport channels. For the transport initiated by ν(C═O), the C-C stretching and CH2 rocking chain bands served as major energy transporters. The transport initiated by ν(N═N) results in direct formation of the wave packet within the CH2 twisting and wagging chain bands. These developments can aid in designing molecular systems featuring faster and more controllable energy transport in molecules.

Communication: fast transport and relaxation of vibrational energy in polymer chains.

We investigate ballistic vibrational energy transport through optical phonon band in oligomeric chains in the presence of decoherence. An exact solution is obtained for the excitation density in the space-time representation in the continuous limit and this solution is used to characterize the energy transport time and intensity. Three transport mechanisms are identified such as ballistic, diffusive, and directed diffusive regimes, occurring at different distances and time scales. The crossover between the two diffusive regimes is continuous, while the switch between the ballistic and diffusive mechanisms occurs in a discontinuous manner in accord with the recent experimental results on energy transport in perfluoroalkanes.

Temperature dependence of the ballistic energy transport in perfluoroalkanes.

Temperature dependence of intramolecular energy transport in perfluoroalkane oligomers with a chain length of 3-11 carbon atoms terminated by a carboxylic acid moiety on one end and a -CF2H group on another end was studied in solution experimentally and theoretically. Experiments were performed using a dual-frequency relaxation-assisted two-dimensional infrared spectroscopy method. The energy transport was initiated by exciting the C═O stretching mode of the acid and recorded by measuring a cross-peak amplitude between the C═O stretching and the C-H bending modes as a function of the waiting time between the excitation and probing. An efficient transport regime with a mean free path of 16.4 ± 2 Å is observed at 35 °C. The energy transport speed decreases at elevated temperatures, indicating a switch from the ballistic transport regime to diffusive. The modeling of the energy transport involving both ballistic and diffusive mechanisms is performed. It explains the temperature dependence of the energy transport speed and confirms a switch of the transport regime from ballistic at lower temperatures to diffusive at higher temperatures.

Theoretical study of internal vibrational relaxation and energy transport in polyatomic molecules.

We attempted to theoretically characterize internal vibrational relaxation and energy relaxation pathways due to anharmonicity in polyatomic molecules. Energy transport dynamics have been modeled based on a generalization of Marcus electron transfer theory. Modifications have been made to our previously developed theory in order to improve the description of internal vibrational dynamics. We applied our method to several molecules studied experimentally by relaxation-assisted two-dimensional infrared spectroscopy (RA 2DIR). The theoretical predictions were found to be consistent with the majority of the experimental data.

Universal dielectric loss in glass from simultaneous bias and microwave fields.

We derive the ac dielectric loss in glasses due to resonant processes created by two-level systems and a swept electric field bias. It is shown that at sufficiently large ac fields and bias sweep rates, the nonequilibrium loss tangent created by the two fields approaches a universal maximum determined by the bare linear dielectric permittivity. In addition, this nonequilibrium loss tangent is derived for a range of bias sweep rates and ac amplitudes. A predicted loss function can be understood in a Landau-Zener theory and used to extract the two-level system density, dipole moment, and relaxation rate.

Structure dependent energy transport: relaxation-assisted 2DIR measurements and theoretical studies.

Vibrational energy relaxation and transport in a molecule that is far from thermal equilibrium can affect its chemical reactivity. Understanding the energy transport dynamics in such molecules is also important for measuring molecular structural constraints via relaxation-assisted two-dimensional infrared (RA 2DIR) spectroscopy. In this paper we investigated vibrational relaxation and energy transport in the ortho, meta, and para isomers of acetylbenzonitrile (AcPhCN) originated from excitation of the CN stretching mode. The amplitude of the cross-peak among the CN and CO stretching modes served as an indicator for the energy transport from the CN group toward the CO group. A surprisingly large difference is observed in both the lifetimes of the CN mode and in the energy transport rates for the three isomers. The anharmonic DFT calculations and energy transport modeling performed to understand the origin of the differences and to identify the main cross-peak contributors in these isomers described well the majority of the experimental results including mode excited-state lifetimes and the energy transport dynamics. The strong dependence of the energy transport on molecular structure found in this work could be useful for recognizing different isomers of various compounds via RA 2DIR spectroscopy.