Two stage decoherence of optical phonons in long oligomers.

Molecular vibrations are generally responsible for chemical energy transport and dissipation in molecular systems. This transport is fast and efficient if energy is transferred by optical phonons in periodic oligomers, but its efficiency is limited by decoherence emerging due to anharmonic interactions with acoustic phonons. Using a general theoretical model, we show that in the most common case of the optical phonon band being narrower than the acoustic bands, decoherence takes place in two stages.

Ballistic Energy Transport via Long Alkyl Chains: A New Initiation Mechanism.

In an effort to increase the speed and efficiency of ballistic energy transport via oligomeric chains, we performed measurements of the transport in compounds featuring long alkyl chains of up to 37 methylene units. Compounds of the N-(CH)-COOMe type (denoted as azME) were synthesized with = 5, 10, 15, 19, 28, 37 and studied using relaxation-assisted two-dimensional infrared spectroscopy. The speed of the ballistic transport, initiated by the N tag excitation, increased ca.

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.

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.

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.

Unidirectional coherent energy transport via conjugated oligo(p-phenylene) chains.

We discovered a way to funnel high-frequency vibrational quanta rapidly and unidirectionally over large distances using oligo(p-phenylene) chains. After mid-IR photon photoexcitation of a -COOH end group, the excess energy is injected efficiently into the chain, forming vibrational wavepackets that propagate freely along the chain. The transport delivers high-energy vibrational quanta with a range of transport speeds reaching 8.

Surface-enhanced ultrafast two-dimensional vibrational spectroscopy with engineered plasmonic nano-antennas.

Development of noble metal nanostructure substrates that provide strong near-field enhancements enables applications of linear and nonlinear infrared (IR) spectroscopies to study minute sample quantities, such as nanometer thick films and molecular monolayers. Large near-field enhancements of the electric fields used for spectroscopic interrogation of molecules at the nanostructure surface result in enhancement of the spectroscopic signatures. This enhancement scales with the nonlinear order of the method, providing particularly large signal gains for third- and fifth-order IR methods, reaching 10 and 10 raw enhancement factors, not adjusted to the amount of interrogated sample.

Proving and Probing the Presence of the Elusive C-H⋅⋅⋅O Hydrogen Bond in Liquid Solutions at Room Temperature.

Hydrogen bonds (H bonds) play a major role in defining the structure and properties of many substances, as well as phenomena and processes. Traditional H bonds are ubiquitous in nature, yet the demonstration of weak H bonds that occur between a highly polarized C-H group and an electron-rich oxygen atom, has proven elusive. Detailed here are linear and nonlinear IR spectroscopy experiments that reveal the presence of H bonds between the chloroform C-H group and an amide carbonyl oxygen atom in solution at room temperature.

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 (tag); the transport was recorded by observing the asymmetric C═O stretching mode of the succinimide ester end group at 1740 cm. The overall transport involves diffusive steps at the end groups and a ballistic step through the PEG chain.

Elongated conductive structures in detonation soot of high explosives.

High values of electrical conductivity are obtained at the detonation of condensed high explosives with the formula C H N O . Such values can be explained only in the framework of the contact conductivity hypothesis. In this case, the conductivity is provided by elongated highly-conductive structures which penetrate the whole space of the detonation wave.

Symmetry controlled photo-selection and charge separation in butadiyne-bridged donor-bridge-acceptor compounds.

Electron transfer (ET) in donor-bridge-acceptor (DBA) compounds depends strongly on the structural and electronic properties of the bridge. Among the bridges that support donor-acceptor conjugation, alkyne bridges have attractive and unique properties: they are compact, possess linear structure permitting access to high symmetry DBA molecules, and allow torsional motion of D and A, especially for longer bridges. We report conformation dependent electron transfer dynamics in a set of novel DBA compounds featuring butadiyne (C4) bridge, N-isopropyl-1,8-napthalimide (NAP) acceptors, and donors that span a range of reduction potentials (trimethyl silane (Si-C4-NAP), phenyl (Ph-C4-NAP), and dimethyl aniline (D-C4-NAP)).

Orientational Dependence of Cofacial Porphyrin-Quinone Electronic Interactions within the Strong Coupling Regime.

We examine the relative magnitudes of electronic coupling in two face-to-face rigid and diastereomeric (porphinato)zinc(II)-quinone (PZn-Q) assemblies, and , in which the six quinonyl carbon atoms lie in virtually identical arrangements relative to the PZn plane at sub-van der Waals donor-acceptor (D-A) interplanar separations. Steady-state and time-resolved transient optical data and computational studies show that minor differences in relative D-A cofacial orientation give rise to disparate magnitudes for both photoinduced charge separation (CS) and thermal charge recombination (CR). Time-dependent density functional theory (TDDFT) computations illuminate the nature of direct charge transfer states and the electronic structural factors that give rise to these differential s.

Unsymmetrical Bis-Alkynyl Complexes Based on Co(III)(cyclam): Synthesis, Ultrafast Charge Separation, and Analysis.

Donor-bridge-acceptor (D-B-A) systems with a polarizable bridge can afford rapid photoinduced electron transfer dynamics that may be susceptible to rate modulation by infrared excitation. We describe the synthesis, characterization, and electronic structure of a class of readily assembled D-B-A structures linked by a cobalt cyclam bridge. The reaction between [Co(cyclam)Cl]Cl and 4-ethynyl--isopropyl-1,8-naphthalimide (HCNAP) yields [Co(cyclam)(CNAP)Cl]Cl (), which reacts with LiCY at -78 °C to afford [Co(cyclam)(CNAP)(CD)]Cl with D as CH-4-NMe (), NAP (), Ph (), and CH-4-N(4-MeOPh) ().

Intense-field interaction regime with weak laser pulses and localized plasmonic enhancement: Reference-free demonstration by 3rd- and 5th-order infrared spectroscopies.

In bulk materials, intense field interaction is accompanied by undesired nonresonant processes. Plasmonic nanostructures localize enhanced fields exclusively in their vicinity. We report a 4-fold vibrational population inversion between all the excited and the ground states in the molecular monolayer on the surface of gold nanoantennas.

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.

How can infra-red excitation both accelerate and slow charge transfer in the same molecule?

A UV-IR-Vis 3-pulse study of infra-red induced changes to electron transfer (ET) rates in a donor-bridge-acceptor species finds that charge-separation rates are slowed, while charge-recombination rates are accelerated as a result of IR excitation during the reaction. We explore the underpinning mechanisms for this behavior, studying IR-induced changes to the donor-acceptor coupling, to the validity of the Condon approximation, and to the reaction coordinate distribution. We find that the dominant IR-induced rate effects in the species studied arise from changes to the density of states in the Marcus curve crossing region.

Author Correction: Electrical Conductivity Distribution in Detonating Benzotrifuroxane.

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Electrical Conductivity Distribution in Detonating Benzotrifuroxane.

Electrical conductivity profile behind the detonation front in the benzotrifuroxane (BTF) was measured using high-resolution technique. BTF is a peculiar high explosive which is completely hydrogen-free: its molecular formula is CNO. Results are compared with the conductivity distributions in detonating hexogen (RDX, CHNO) and triaminotrinitrobenzene based explosive (TATB, CHNO).

Radiative Enhancement of Linear and Third-Order Vibrational Excitations by an Array of Infrared Plasmonic Antennas.

Infrared gold antennas localize enhanced near fields close to the metal surface, when excited at the frequency of their plasmon resonance, and amplify vibrational signals from the nearby molecules. We study the dependence of the signal enhancement on the thickness of a polymer film containing vibrational chromophores, deposited on the antenna array, using linear (FTIR) and third-order femtosecond vibrational spectroscopy (transient absorption and 2DIR). Our results show that for a film thickness beyond only a few nanometers the near-field interaction is not sufficient to account for the magnitude of the observed signal, which nevertheless has a clear Fano line shape, suggesting a radiative origin of the molecule-plasmon interaction.

Expanding the Scope of Ligand Substitution from [M(SCPh] (M = Ni, Pd, Pt) To Afford New Heteroleptic Dithiolene Complexes.

The scope of direct substitution of the dithiolene ligand from [M(SCPh)] [M = Ni (1), Pd (2), Pt (3)] to produce heteroleptic species [M(SCPh)L] (n = 1, 2) has been broadened to include isonitriles and dithiooxamides in addition to phosphines and diimines. Collective observations regarding ligands that cleanly produce [M(SCPh)L], do not react at all, or lead to ill-defined decomposition identify soft σ donors as the ligand type capable of dithiolene substitution. Substitution of MeNC from [Ni(SCPh)(CNMe)] by L provides access to a variety of heteroleptic dithiolene complexes not accessible from 1.

Two-Dimensional Fano Lineshapes in Ultrafast Vibrational Spectroscopy of Thin Molecular Layers on Plasmonic Arrays.

Two-dimensional femtosecond infrared (2DIR) spectroscopy routinely provides insights into molecular structure and ultrafast dynamics in 1-100 μm thick bulk samples. Confinement of molecules to surfaces, gaps, crevices, and other topographic features, frequently encountered on the nanometer length scale, significantly alters their structure and dynamics, affecting physical and chemical properties. Amplification of 2DIR signals by the plasmon-enhanced fields around metal nanostructures can permit structural and dynamics measurements of the confined molecules.