Photoinduced electron transfer (ET) in alkyne-linked donor-bridge-acceptor (DBA) compounds is strongly influenced by torsional flexibility, allowing control over ET without altering the donor-acceptor distance. Here, we investigate excited-state dynamics in Fc-C4-NAP, a DBA compound featuring a ferrocene (Fc) donor, a butadiyne bridge (C4), and a 1,8-naphthalimide (NAP) acceptor. Unlike analogues DBA compounds with fully organic planar donors, Fc-C4-NAP exhibits a complex excited-state manifold. Femtosecond transient absorption (TA) measurements in the visible and mid-IR regions found three characteristic relaxation times (0.3-0.5 ps, ∼2.6 ps, and 17-20 ps) following its excitation at 402 nm, which prepares NAP-centered excited states.TD-DFT computations indicate that the acceptor-based locally excited (LE) and the charge separated (CS) diabatic states are well coupled to the Fc states associated with d-states of Fe. This bridge-mediated coupling, estimated at 200-500 cm-1, is strong enough to induce significant mixing of the diabatic states, which also depends strongly on the torsional angle between the NAP and the C4-bonded cyclopentadienyl ring. The spectral changes observed in the TA experiments suggest that the fast component of 0.3-0.5 ps reflects the lifetime of the bright, dominantly NAP-centered state, which relaxes predominantly to the Fc-based states. The middle component of 2.6 ps could have multiple contributions, including relaxation of the nominal CS state, vibrational cooling, and solvation. The slow decay component of ca. 20 ps corresponds to the lifetime of the lowest-energy Fc states; two Fc states of similar energies but perpendicular polarizations. The complex nature of the eigenstates, unraveled by TD-DFT analysis, results in efficient competition of the energy transfer process to the Fc-based excited states with the CS process. These results highlight the key role played by diabatic state coupling, conformational dynamics, and Fc d-orbitals in shaping the ultrafast dynamics of Fc-based DBA systems, guiding the future design of photoactive materials for solar energy and molecular electronics applications.

The thermal conductivity of aligned polymer molecules can be exceptionally high along the alignment direction due to energy transport through strong covalent bonds. At the same time, it is highly sensitive to molecular conformation, varying by orders of magnitude as a result of gauche kinks. Here, we theoretically investigate phonon transport in kinked polymers by numerically evaluating thermal conductivity and interpreting the results in terms of phonon scattering from randomly distributed kinks. For strongly aligned polymers with restricted deviations from a linear backbone, we find that heat transport becomes superdiffusive at long lengths, with thermal conductivity scaling as κ ∝ L1/3. At shorter lengths, thermal conductivity exhibits non-monotonic behavior: it increases at very short scales due to ballistic transport of almost all phonons, then decreases at intermediate lengths due to the Anderson localization of most phonon modes. These results are consistent with experiments and molecular dynamics simulations, and they elucidate the microscopic mechanisms governing heat transport in polymers.

Thermal transport in long polymer molecules is commonly attributed to ballistic propagation of long-wavelength acoustic phonons, which act as Goldstone modes arising from translational symmetry, while the transport of other phonons is suppressed by Anderson localization. This mechanism leads to thermal conductivity that increases with molecular length. Consistent with this picture, strongly aligned polymers exhibit exceptionally high thermal conductivity, whereas poorly aligned polymers are orders of magnitude less conductive and function as thermal insulators. Here we show that this strong sensitivity to molecular alignment originates from phonon scattering by molecular kinks. Even in the long-wavelength limit, the kink scattering remains strong because kinks break translational symmetry both for longitudinal and transverse phonons. As a result, randomly oriented kinks cause a rapid decrease in thermal conductivity with increasing molecular length. These findings identify alignment control by means of kink engineering as a route for tuning thermal transport in polymers.

Using ultrafast dual-frequency two-dimensional infrared spectroscopy (DF-2DIR), we probed how the strong coupling of high-frequency molecular vibrations to surface lattice resonances of infrared antennas, which act as a photonic cavity, affects intramolecular vibrational relaxation (IVR). DF-2DIR allows one to probe the IVR pathways beyond the vibrational state subspace of the polaritons and reservoir states, which is typically accessed in conventional 2DIR experiments. We observed anharmonic coupling between lower polariton and high-frequency molecular vibrational modes not coupled to the cavity directly, which appeared in the strongly coupled system by virtue of the polariton's molecular component, and alternation of the rate of excitation energy excess transfer from the polariton to a distant molecular vibrational mode, which depended on the polariton transition frequency. These are in contrast with the weak coupling regime, where enhanced fields magnify molecular vibrational signals without affecting their dynamics. Our work demonstrates a promising experimental approach toward understanding of polariton chemistry phenomena.

We report on development of a vibrational probe (N3-C12H25, az12) suitable for reporting on the rigidity and fluidity of the interfacial region in lipid bilayers. Such probe can help assess critical biological functions of cell membranes, including membrane permeability. We demonstrated a high sensitivity of the az12 probe to the rigidity of the interfacial region, manifested in the sensitivity of the width of the N3 moiety asymmetric stretching mode absorption peak. The structural and dynamic changes associated with the gel-liquid-crystal phase transition (Lβ-Lα) for three different lipids dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), and egg sphingomyelin (SM) were studied using FTIR and two-dimensional infrared (2DIR) spectroscopies. In addition to the new az12 probe targeting the interfacial region, the N3-(CH2)11-CN probe (az11CN), recently developed to examine the hydrophobic region of bilayers, was also used, showing characteristic phase transitions for each bilayer at their characteristic phase transition temperatures. An order parameter, SN3, corrected for the difference in the polarities of the interfacial and hydrophobic regions, was constructed. It shows how the overall rigidity of the bilayer is divided between the interfacial and hydrophobic regions, emphasizing their correlations. 2DIR spectral diffusion data were acquired for az12 and az11CN probes in the bilayer at various temperatures, reporting on inhomogeneous and homogeneous line width contributions (rigidity) and correlation times (fluidity) for the interfacial and hydrophobic regions. The spectral diffusion data for the interfacial region for all three bilayer types show large static inhomogeneous contributions, which are absent in the hydrophobic region data. The spectral diffusion data revealed differences for different bilayers, most apparent for the interfacial region. A greater increase in a homogeneous line width for SM with temperature, compared to that for DPPC and DPPG, could be linked to an increase in water permeability to the interfacial region of SM at higher temperatures. We found that the az12 probe constitutes a powerful reporter to measure rigidity (exerted angular constraints) and fluidity (time responses of the environment) in the interfacial region of a bilayer.

Monoligated and bis-ligated CCC-NHC pincer Fe complexes with n-butyl substituents have been synthesized by the Zr metalation/transmetalation route. Both the direct metalation/transmetalation and transmetalation from the isolated ((CCCBu)-C-Bu-C-i-C-i)ZrNMe2Cl2, 3, yielded the octahedrally coordinated Fe(III) bis-ligated complex [((CCCBu)-C-Bu-C-i-C-i)(2)Fe]Cl, 2a. Transmetalation from in situ and isolated ((CCCBu)-C-Bu-C-i-C-i)ZrCl3, 5, in the presence of excess TMSCl and 1 equiv of the Fe source yielded the monoligated ((CCCBu)-C-Bu-C-i-C-i)FeCl2, 4. Conditions that convert [((CCCBu)-C-Bu-C-i-C-i)(2)Fe](+), 2, to ((CCCBu)-C-Bu-C-i-C-i)FeCl2, 4, complex have been found. Characterization included H-1 NMR, UV-visible, femtosecond transient absorption spectroscopies, TD-DFT computations, and mass spectroscopy along with X-ray crystallographic structure determinations.

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. The faster stage involves optical phonon multiple forward scattering due to absorption and emission of transverse acoustic phonons, i.e., collective bending modes with a quadratic spectrum; the transport remains ballistic and the speed can be altered. The subsequent slower stage involves phonon backscattering in multiphonon processes involving two or more acoustic phonons resulting in a switch to diffusive transport. If the initially excited optical phonon possesses a relatively small group velocity, then it is accelerated in the first stage due to its transitions to states propagating faster. This theoretical expectation is consistent with the recent measurements of optical phonon transport velocity in alkane chains, increasing with increasing the chain length.

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 N3-(CH2)n-COOMe type (denoted as aznME) were synthesized with n = 5, 10, 15, 19, 28, 37 and studied using relaxation-assisted two-dimensional infrared spectroscopy. The speed of the ballistic transport, initiated by the N3 tag excitation, increased ca. 3-fold for the longer chains (n = 19-37) compared to the shorter chains, from 14.7 to 48 Å/ps, in line with an earlier prediction (Nawagamuwage et al. 2021, J. Phys. Chem. B, 125, 7546). Modeling, based on solving numerically the Liouville equation, was capable of reproducing the experimental data only if three wavepackets are included, involving CH2 twisting (Tw), wagging (W), and rocking (Ro) chain bands. The approaches for designing molecular systems featuring a higher speed and efficiency of energy transport are discussed.

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 Lβ-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.

Controlling electron transfer (ET) processes in donor–bridge–acceptor (DBA) compounds by mid-IR excitation can enhance our understanding of the ET dynamics and may find practical applications in molecular sensing and molecular-scale electronics.

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.

Though local structures in ionic liquids are dominated by strong Coulomb forces, directional hydrogen bonds can also influence the physicochemical properties of imidazolium-based ionic liquids. In particular, the C-2 position of the imidazolium cation is acidic and can bind with suitable hydrogen bond acceptor sites of molecular solvents dissolved in imidazolium-based ionic liquids. In this report, we identify hydrogen-bonded microenvironments of the model ionic liquid, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate, and the changes that occur when molecular solvents are dissolved in it by using a C-D infrared reporter at the C-2 position of the cation. Our linear and nonlinear infrared experiments, along with computational studies, indicate that the molecular solvent dimethyl sulfoxide can form strong hydrogen-bonded dimers with the cation of the ionic liquid at the C-2 position. In contrast, acetone, which is also a hydrogen bond acceptor similar to dimethyl sulfoxide, does not show evidence of cation-solvent hydrogen-bonded conformers at the C-2 position. The outcome of our study on a broad scale strengthens the importance of cation-solute interactions in ionic liquids.

Ultrafast studies of molecular dynamics using femtosecond two-dimensional vibrational spectroscopy (2DIR) provide unique insights on molecular interactions. Recently, such studies were extended from bulk samples to molecules on surfaces of noble metals, including disordered and engineered nanostructures. The metal nanostructures provide field enhancement allowing sensitive measurements at interfaces, but they also alter the molecular dynamics. 2DIR measurements at interfaces reveal correlations between vibrational transitions and rates of the vibrational relaxation, dephasing, spectral diffusion, and resonant energy transfer. At the surfaces, molecular dynamics and interaction with the environment can be modified by geometrical constrains associated with tethering, by the interaction between the molecule and the substrate, or by the enhanced near-fields associated with the metal. Interestingly, in many cases, these anticipated effects were found to be absent or fade away already at the microscopic distances away from the surface. Herein, we provide an overview of recent experimental studies in the field.

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.

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, N-14(3-), and isotopically substituted, N-15(3-), 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 (14)N(3)Cn-a and (15)N(3)Cn-a. The transport was initiated by exciting the azido moiety stretching mode, the nu(N=N) tag, at 2100 cm(-1) ((14)N(3)Cn-a) or 2031 cm(-1) ((15)N(3)Cn-a). Opposite to the expectation, the ballistic transport speed was found to decrease upon N-14(3) -> N-15(3) 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 nu(N=N) + nu(N-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 nu(N=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 nu(N=N) initiation: The nu(N=N) mode relaxes efficiently into the twisting band states and low-frequency acoustic modes, and the nu(N-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 nu(N=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 nu(N=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 (14)N(3)Cn-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.

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 8.6 km/s, which exceeds the speed of sound in common metals (∼5 km/s) and polymers (∼2 km/s). Efficiencies of energy injection into the chain and transport along the chain are found to be very high and dependent on the extent of conjugation across the structure. By tuning the degree of conjugation via electronic doping of the chain, the transport speed and efficiency can be controlled. The study opens avenues for developing materials with controllable energy transport properties for heat management, schemes with efficient energy delivery to hard-to-reach regions, including transport against thermal gradients, and ways for initiating chemical reactions remotely.

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 106 and 108 raw enhancement factors, not adjusted to the amount of interrogated sample. In this perspective, we overview the advances in the development of nano-arrays of antenna-like nanostructures for mid-IR measurements and illustrate their use in linear and especially nonlinear two-dimensional IR approaches. We discuss how studies of the interaction mechanisms between light, plasmonic antennas, and molecular excitations benefit from the nonlinear two-dimensional time-resolved methods, which involve high-order scaling of the signal with the excitation field, high sensitivity to signal localization, and coherence of the excitation over a broad bandwidth. On the other hand, we demonstrate how studies of molecular structure and ultrafast dynamics by these advanced spectroscopic methods benefit from surface enhancement of signals by plasmonic antennas.

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)). Transient mid-IR absorption spectra of the C[triple bond, length as m-dash]C bridge stretching modes, transient spectra in the visible range, and TD-DFT calculations were used to decipher the ET mechanisms. We found that the electronic excited state energies and, especially, the transition dipoles (S0 → Sn) depend strongly on the dihedral angle (θ) between D and A and the frontier orbital symmetry, offering an opportunity to photo-select particular excited states with specific ranges of dihedral angles by exciting at chosen wavelengths. For example, excitation of D-C4-NAP at 400 nm predominantly prepares an S1 excited state in the planar conformations (θ ∼ 0) but selects an S2 state with θ ∼ 90°, indicating the dominant role of the molecular symmetry in the photophysics. Moreover, the symmetry of the frontier orbitals of such DBA compounds not only defines the photo-selection outcome, but also determines the rate of the S2 → S1 charge separation reaction. Unprecedented variation of the S2-S1 electronic coupling with θ by over four orders of magnitude results in slow ET at θ ca. 0° and 90° but extremely fast ET at θ of 20-60°. The unique features of high-symmetry alkyne bridged DBA structures enable frequency dependent ET rate selection and make this family of compounds promising targets for the vibrational excitation control of ET kinetics.

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. Evidence is provided for an amide solvation shell featuring two clearly distinguishable chloroform arrangements that undergo chemical exchange with a time scale of about 2 ps. Furthermore, the enthalpy of breaking the hydrogen bond is found to be 6-20 kJ mol(-1). Ab-initio computations support the findings of two distinct solvation shells formed by three chloroform molecules, where one thermally undergoes hydrogen-bond making and breaking.

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.