Temperature‐dependent vibrational relaxation in polyatomic liquids: Picosecond infrared pump–probe experiments

Understanding the Vibrational Structure and Ultrafast Dynamics of the Metal Carbonyl Precatalyst [Mn(ppy)(CO)4]

The solution phase structure, vibrational spectroscopy, and ultrafast relaxation dynamics of the precatalyst species [Mn(ppy)(CO)4] (1) in solution have been investigated using ultrafast two-dimensional infrared (2D-IR) spectroscopy. By comparing 2D-IR data with the results of anharmonic density functional theory (DFT) calculations, we establish an excellent agreement between measured and predicted inter-mode couplings of the carbonyl stretching vibrational modes of 1 that relates to the atomic displacements of axial and equatorial ligands in the modes and the nature of the molecular orbitals involved in M-CO bonding. Measurements of IR pump-probe spectra and 2D-IR spectra as a function of waiting time reveal the presence of ultrafast (few ps) intramolecular vibrational energy redistribution between carbonyl stretching modes prior to vibrational relaxation. The vibrational relaxation times of the CO-stretching modes of 1 are found to be relatively solvent-insensitive, suggestive of limited solvent-solute interactions in the ground electronic state. Overall, these data provide a detailed picture of the complex potential energy surface, bonding and vibrational dynamics of 1, establishing a fundamental basis for the next steps in understanding and modulating precatalyst behavior.

A second-order kinetic model for global analysis of vibrational polariton dynamics

The interaction between cavity photons and molecular vibrations leads to the formation of vibrational polaritons, which have demonstrated the ability to influence chemical reactivity and change material characteristics. Although ultrafast spectroscopy has been extensively applied to study vibrational polaritons, the nonlinear relationship between signal and quantum state population complicates the analysis of their kinetics. Here, we employ a second-order kinetic model and transform matrix method (TMM) to develop an effective model to capture the nonlinear relationship between the two-dimensional IR (or pump-probe) signal and excited state populations. We test this method on two types of kinetics: a sequential relaxation from the second to the first excited states of dark modes, and a Raman state relaxing into the first excited state. By globally fitting the simulated data, we demonstrate accurate extraction of relaxation rates and the ability to identify intermediate species by comparing the species spectra with theoretical ground truth, validating our method. This study demonstrates the efficacy of a second-order TMM approximation in capturing essential spectral features with up to 10% excited state population, simplifying global analysis and enabling straightforward extraction of kinetic parameters, thus empowering our methodology in understanding excited-state dynamics in polariton systems.

Time-resolved spectroscopic mapping of vibrational energy flow in proteins: Understanding thermal diffusion at the nanoscale

Vibrational energy exchange between various degrees of freedom is critical to barrier-crossing processes in proteins. Hemeproteins are well suited for studying vibrational energy exchange in proteins because the heme group is an efficient photothermal converter. The released energy by heme following photoexcitation shows migration in a protein moiety on a picosecond timescale, which is observed using time-resolved ultraviolet resonance Raman spectroscopy. The anti-Stokes ultraviolet resonance Raman intensity of a tryptophan residue is an excellent probe for the vibrational energy in proteins, allowing the mapping of energy flow with the spatial resolution of a single amino acid residue. This Perspective provides an overview of studies on vibrational energy flow in proteins, including future perspectives for both methodologies and applications.

Ultrafast vibrational dynamics of a solute correlates with dynamics of the solvent

Two-dimensional infrared (2D-IR) spectroscopy is used to measure the spectral dynamics of the metal carbonyl complex cyclopentadienyl manganese tricarbonyl (CMT) in a series of linear alkyl nitriles. 2D-IR spectroscopy provides direct readout of solvation dynamics through spectral diffusion, probing the decay of frequency correlation induced by fluctuations of the solvent environment. 2D-IR simultaneously monitors intramolecular vibrational energy redistribution (IVR) among excited vibrations, which can also be influenced by the solvent through the spectral density rather than the dynamical friction underlying solvation. Here, we report that the CMT vibrational probe reveals solvent dependences in both the spectral diffusion and the IVR time scales, where each slows with increased alkyl chain length. In order to assess the degree to which solute-solvent interactions can be correlated with bulk solvent properties, we compared our results with low-frequency dynamics obtained from optical Kerr effect (OKE) spectroscopy-performed by others-on the same nitrile solvent series. We find excellent correlation between our spectral diffusion results and the orientational dynamics time scales from OKE. We also find a correlation between our IVR time scales and the amplitudes of the low-frequency spectral densities evaluated at the 90-cm^-1 energy difference, corresponding to the gap between the two strong vibrational modes of the carbonyl probe. 2D-IR and OKE provide complementary perspectives on condensed phase dynamics, and these findings provide experimental evidence that at least at the level of dynamical correlations, some aspects of a solute vibrational dynamics can be inferred from properties of the solvent.

Long Vibrational Lifetime R-Selenocyanate Probes for Ultrafast Infrared Spectroscopy: Properties and Synthesis

Ultrafast infrared vibrational spectroscopy is widely used for the investigation of dynamics in systems from water to model membranes. Because the experimental observation window is limited to a few times the probe's vibrational lifetime, a frequent obstacle for the measurement of a broad time range is short molecular vibrational lifetimes (typically a few to tens of picoseconds). Five new long-lifetime aromatic selenocyanate vibrational probes have been synthesized and their vibrational properties characterized. These probes are compared to commercial phenyl selenocyanate. The vibrational lifetimes range between ∼400 and 500 ps in complex solvents, which are some of the longest room-temperature vibrational lifetimes reported to date. In contrast to vibrations that are long-lived in simple solvents such as CCl₄, but become much shorter in complex solvents, the probes discussed here have ∼400 ps lifetimes in complex solvents and even longer in simple solvents. One of them has a remarkable lifetime of 1235 ps in CCl₄. These probes have a range of molecular sizes and geometries that can make them useful for placement into different complex materials due to steric reasons, and some of them have functionalities that enable their synthetic incorporation into larger molecules, such as industrial polymers. We investigated the effect of a range of electron-donating and electron-withdrawing para-substituents on the vibrational properties of the CN stretch. The probes have a solvent-independent linear relationship to the Hammett substituent parameter when evaluated with respect to the CN vibrational frequency and the ipso ¹³C NMR chemical shift.

Unperturbed Detection of the Dynamic Structure in the Hydrophobic Core of Trp-Cage via Two-Dimensional Infrared Spectroscopy

The tyrosine ring mode is an intrinsic non-perturbing site-specific infrared reporter for conformational dynamics within protein systems. This transition is influenced by direct and indirect interactions associated with the electron-donating ability and the hydrophobicity of the surrounding molecules. Utilizing an intrinsic tyrosine moiety, two-dimensional infrared spectra of Trp-cage, often called the "hydrogen atom" of protein folding, were measured in the folded and denatured states to uncover the dynamics of the hydrophobic core. The vibrational lifetimes and the correlation decays of the tyrosine ring mode showed significant changes upon both temperature and chemical denaturation of the Trp-cage miniprotein, indicating important structural features of the hydrophobic core and its dynamics. The observed Trp6-Tyr3 interactions are in good agreement with the prior studies of the folded state, but they reach beyond the static structure. These stacking interactions and orientations fluctuate on the picosecond time scale as measured through the spectral dephasing within a dehydrated environment.

Polariton-Assisted Cooperativity of Molecules in Microcavities Monitored by Two-Dimensional Infrared Spectroscopy

Molecular polaritons created by the strong coupling between matter and field in microcavities enable the control of molecular dynamical processes and optical response. Multidimensional infrared spectroscopy is proposed for monitoring the polariton-assisted cooperative properties. The response of molecules to local fluctuations is incorporated and the full dynamics is monitored through the time- and frequency-resolved multidimensional signal. The cooperativity against solvent-induced disorder and its connection to the localization of the vibrational excitations are predicted. New insights are provided for recent 2DIR experiments on vibrational polaritons.

Solvent Quality Controls Macromolecular Structural Dynamics of a Dendrimeric Hydrogenase Model

We report a spectroscopic investigation of the ultrafast dynamics of the second-generation poly(aryl ether) dendritic hydrogenase model using two-dimensional infrared (2D-IR) spectroscopy to probe the metal carbonyl vibrations of the dendrimer and a reference small molecule, [Fe(μ-S)(CO)₃]₂. We find that the structural dynamics of the dendrimer are reflected in a slow phase of the spectral diffusion, which is absent from [Fe(μ-S)(CO)₃]_2, and we relate the slow phase to the quality of the solvent for poly(aryl ether) dendrimers. We observe a solvent-dependent modulation of the initial phase of vibrational relaxation of the carbonyl groups, which we attribute to an inhibition of solvent assistance in the intramolecular vibrational redistribution process for the dendrimer. There is also a clear solvent dependence of the vibrational frequencies of both the dendrimer and [Fe(μ-S)(CO)₃]₂. Our data represent the first 2D-IR study of a dendritic complex and provide insight into the solvent dependence of molecular conformation in solution and the ultrafast dynamics of moderately sized, conformationally mobile compounds.

Linear and Nonlinear Infrared Spectroscopies Reveal Detailed Solute-Solvent Dynamic Interactions of a Nitrosyl Ruthenium Complex in Solution

In this work, the solvation of a nitrosyl ruthenium complex, [(CH₃)₄N][RuCl₃(qn)(NO)] (with qn = deprotonated 8-hydroxyquinoline), which is a potential NO-releasing molecule in the bio-environment, was studied in two bio-friendly solvents, namely deuterated dimethyl sulfoxide (dDMSO) and water (D₂O). A blue-shifted NO stretching frequency was observed in water with respect to that in dDMSO, which was believed to be due to ligand-solvent hydrogen-bonding interactions, one N═O···D and particularly three Ru-Cl···D, that show competing effects on the NO bond length. The dynamic differences of the NO stretch in these two solvents were further revealed by transient pump-probe IR and two-dimensional IR results: faster vibrational relaxation and faster spectral diffusion (SD) were observed in D₂O, confirming stronger solvent-solute interaction and also faster solvent structural dynamics in D₂O than in DMSO. Further, a significant non-decaying residual in the SD dynamics was observed in D₂O but not in DMSO, suggesting the formation of a stable solvation shell in water due to strong multi-site ligand-solvent hydrogen-bonding interactions, which is in agreement with the observed blue-shifted NO stretching frequency. This work demonstrates that small solvent molecules such as water can form a relatively rigid solvation shell for certain transition metal complexes due to cooperative ligand-solvent interactions and show slower dynamics.

Rotational Dynamics of Solutes with Multiple Single Bond Axes Studied by Infrared Pump-Probe Spectroscopy

To investigate the relationship between the structural degrees of freedom around a vibrational probe and the rotational relaxation process of a solute in solution, we studied the anisotropy decays of three different N₃-derivatized amino acids in primary alcohol solutions. By performing polarization-controlled IR pump-probe measurements, we reveal that the anisotropy decays of the vibrational probe molecules in 1-alcohol solutions possess two decay components, at subpicosecond and picosecond time scales. On the basis of results showing that the fast relaxation component is insensitive to the vibrational probe molecule, we suggest that the anisotropy decay of the N₃ group on a subpicosecond time scale results from a local, small-amplitude fluctuation of the flexible vibrational probe, which does not depend on the details of its molecular structure. However, the slow relaxation component depends on the solute: with longer alkyl chains attached to the N₃ group, the anisotropy decay of the slow component is faster. Consequently, we conclude that the slow relaxation component corresponds to the reorientational motion of the N₃ group correlated with other intramolecular rotational motions (e.g., rotational motions of the neighboring alkyl chain). Our experimental results provide important insight into understanding the rotational dynamics of solutes with multiple single bond axes in solution.

Micellar and bicontinuous microemulsion structures show different solute–solvent interactions: a case study using ultrafast nonlinear infrared spectroscopy

Using aqueous and organic probes to simultaneously explore the structural dynamics of reverse micellar and bicontinuous microemulsion structures.

Picosecond difference-frequency-generation in orientation-patterned gallium phosphide

We report the generation of tunable high-repetition-rate picosecond radiation in the mid-infrared using the new quasi-phase-matched nonlinear material of orientation-patterned gallium phosphide (OP-GaP). The source is realized by single-pass difference-frequency-generation (DFG) between the output signal of a picosecond optical parametric oscillator (OPO) tunable across 1609-1637 nm with input pump pulses at 1064 nm in OP-GaP, resulting in tunable radiation across 3040-3132 nm. Using a 40-mm-long crystal, we have generated up to 57 mW of DFG average power at ~80 MHz repetition rate for a pump power of 5 W and signal power of 0.9 W, with >30 mW over >50% of the tuning range. The DFG source exhibits a passive power stability better than 3.2% rms over 1 hour in good spatial beam quality. To the best of our knowledge, this is the first picosecond frequency conversion source based on OP-GaP.

Unravelling the mechanisms of vibrational relaxation in solution

Time resolved vibrational cooling towards equilibrium in perfluorinated and chlorinated solvents provides detailed insights into the transfer of energy between solute and solvent molecules.

We present a systematic study of the mode-specific vibrational relaxation of NO₂ in six weakly-interacting solvents (perfluorohexane, perfluoromethylcyclohexane, perfluorodecalin, carbon tetrachloride, chloroform, and d-chloroform), chosen to elucidate the dominant energy transfer mechanisms in the solution phase. Broadband transient vibrational absorption spectroscopy has allowed us to extract quantum state-resolved relaxation dynamics of the two distinct NO₂ fragments produced from the 340 nm photolysis of N₂O₄ → NO₂(X) + NO₂(A) and their separate paths to thermal equilibrium. Distinct relaxation pathways are observed for the NO₂ bending and stretching modes, even at energies as high as 7000 cm^–1 above the potential minimum. Vibrational energy transfer is governed by different interaction mechanisms in the various solvent environments, and proceeds with timescales ranging from 20–1100 ps. NO₂ relaxation rates in the perfluorocarbon solvents are identical despite differences in acceptor mode state densities, infrared absorption cross sections, and local solvent structure. Vibrational energy is shown to be transferred to non-vibrational solvent degrees of freedom (V-T) through impulsive collisions with the perfluorocarbon molecules. Conversely, NO₂ relaxation in chlorinated solvents is reliant on vibrational resonances (V-V) while V-T energy transfer is inefficient and thermal excitation of the surrounding solvent molecules inhibits faster vibrational relaxation through direct complexation. Intramolecular vibrational redistribution allows the symmetric stretch of NO₂ to act as a gateway for antisymmetric stretch energy to exit the molecule. This study establishes an unprecedented level of detail for the cooling dynamics of a solvated small molecule, and provides a benchmark system for future theoretical studies of vibrational relaxation processes in solution.

Modified relaxation dynamics and coherent energy exchange in coupled vibration-cavity polaritons

Coupling vibrational transitions to resonant optical modes creates vibrational polaritons shifted from the uncoupled molecular resonances and provides a convenient way to modify the energetics of molecular vibrations. This approach is a viable method to explore controlling chemical reactivity. In this work, we report pump–probe infrared spectroscopy of the cavity-coupled C–O stretching band of W(CO)₆ and the direct measurement of the lifetime of a vibration-cavity polariton. The upper polariton relaxes 10 times more quickly than the uncoupled vibrational mode. Tuning the polariton energy changes the polariton transient spectra and relaxation times. We also observe quantum beats, so-called vacuum Rabi oscillations, between the upper and lower vibration-cavity polaritons. In addition to establishing that coupling to an optical cavity modifies the energy-transfer dynamics of the coupled molecules, this work points out the possibility of systematic and predictive modification of the excited-state kinetics of vibration-cavity polariton systems.

Vibration-cavity polaritons are mixed states produced by strong coupling between a vibrational mode and an optical cavity. Here, the authors show that these polaritons can coherently exchange energy and exhibit drastically altered transient spectra and dynamics compared to uncoupled vibrations.

Broadband mid-infrared pulses from potassium titanyl arsenate/zinc germanium phosphate optical parametric amplifier pumped by Tm, Ho-fiber-seeded Ho:YAG chirped-pulse amplifier

We present a concept of a white-light-seeded-cascaded mid-infrared (mid-IR) optical parametric amplifier (OPA) based on potassium titanyl arsenate and zinc germanium phosphate nonlinear optical crystals and producing 100-μJ level pulses centered at 5300 nm, with the spectrum supporting four-optical-cycle pulse duration. The OPA is pumped by 2090-nm master oscillator/power amplifier based on a Tm,Ho-fiber laser seeder and a Ho:YAG regenerative amplifier delivering 3.8-mJ sub-ps pulses at a repetition rate of 1 kHz. We validate that output parameters of the OPA are scalable by means of increasing the pulse energy, decreasing the pulse duration and redshifting the central wavelength.

Synthesis, structural characterization and theoretical approach of the tri(2-(2,6-dichlorophenyl)-1H-imidazo[4,5-f][1,10]phenanthroline) cobalt(II)

The crystal structure of a new coordination compound tri(2-(2,6-dichlorophenyl)-1H-imidazo[4,5-f][1,10]phenanthroline) Co(II) complex ([Co(dcpip)3]Cl2) was measured with X-ray diffraction measurements. The compound is crystallizes triclinic, Pī space group. The ligand, 2-(2,6-dichlorophenyl)-1H-imidazo[4,5-f][1,10]phenanthroline(dcpip), binds to Co(II) ions with a bis-dentate mode, and each Co(II) ion with a distorted octahedral coordination geometry. The calculated interaction energies of Co(II) with coordination atoms N are between 101.7-206.5 kJ/mol and 115.3-230.9 kJ/mol for B3LYP/6-31+G(∗) and PBE1PBE/6-31+G(∗) theoretical methods, respectively. The experimental Fourier transform infrared spectrum was assigned. The calculated IR based on B3LYP/6-31+G(∗) and PBE1PBE/6-31+G(∗) methods were performed and compared with experimental results. The UV-Vis experimental spectrum of [Co(dcpip)3]Cl2 was measured in methanol solution. The calculated electronic spectrum was performed with TD/B3LYP and TD/PBE1PBE methods with 6-31+G(∗) basis set. The first and second order hyperpolarizability for the compound was calculated. The calculated values of γtot are -1.5551344 × 10(-33) esu for B3LYP method and -1.3323259 × 10(-33) esu for PBE1PBE method. The nature bond orbital analysis and temperature dependence of the thermodynamic properties were calculated with the same methods.

Ultrafast Energy Transfer from Solvent to Solute Induced by Subpicosecond Highly Intense THz Pulses

The ultrafast energy transfer from an intense, subpicosecond THz pulse to bulk water at 300 K and density 1 g/cm(3) is simulated by ab initio molecular dynamics with explicit inclusion of the laser pulse. A 200 fs subcycle pulse of intensity 5 × 10(12) W/cm(2) corresponding to a peak field amplitude of 0.6 V/Å and achievable nowadays using optical rectification techniques results in a temperature jump from 300 K up to ∼1000 K within the first picosecond after the pulse. We discuss in detail the time-dependent structural changes caused by the THz pulse in the water medium and suggest possible ways to measure those changes by pump-probe experimental techniques. The ultrafast energy transfer from the energized water molecules to a solute molecule is studied on a test system, phenol. We find that phenol is, in the gas phase, insensitive to the THz pulse and only gains energy in solution via collisional energy transfer with the water molecules in its environment. The reason for this is found in the mode of interaction of the THz pulse with the aqueous medium. In short, water molecules respond mainly through their permanent dipole moments trying to orient themselves in the strong electric field of the pulse and disrupting their hydrogen-bonding structure. As compared with the water molecule, phenol has a smaller but still substantial permanent dipole moment. The moments of inertia of phenol are, however, too large for it to rotate in the short duration of the THz pulse. Therefore, the direct heating-up mechanism is mostly selective to the solvent molecules, whereas the solute heats up indirectly via collisions with its hot environment in about 1 to 2 ps.

β-Isocyanoalanine as an IR probe: comparison of vibrational dynamics between isonitrile and nitrile-derivatized IR probes

An infrared (IR) probe based on isonitrile (NC)-derivatized alanine 1 was synthesized and the vibrational properties of its NC stretching mode were investigated using FTIR and femtosecond IR pump–probe spectroscopy.

Vibrational Energy Relaxation of Thiocyanate Ions in Liquid-to-Supercritical Light and Heavy Water. A Fermi's Golden Rule Analysis

The vibrational relaxation dynamics following an ultrafast nitrile stretching (ν3) excitation of thiocyanate anions dissolved in light and heavy water have been studied over a wide temperature and density range corresponding to the aqueous liquid up to the supercritical phase. In both solvents, the relaxation of the ν3 = 1 state of the anion leads to a direct recovery of the vibrational ground state and involves the resonant transfer of the excess vibrational energy onto the solvent. In light water, the energy-accepting states are provided by the bending-librational combination band (νb + νL), while in heavy water, the relaxation is thermally assisted by virtual acceptor states derived from the stretching-librational/restricted translational hot band (νS - νL,T). The relaxation rate is found to strictly obey Fermi's Golden Rule when the density of resonant solvent states is estimated from the linear infrared spectra of the solute and the pure solvents.

Intermolecular vibrational energy transfers in liquids and solids

Resonant and nonresonant intermolecular vibrational energy transfers in liquids and solids are measured and elucidated using two competing mechanisms.

Dynamics in the isotropic phase of nematogens using 2D IR vibrational echo measurements on natural-abundance 13CN and extended lifetime probes

The long time scale orientational relaxation of nematogens in the isotropic phase is associated with the randomization of pseudonematic domains, which have a correlation length that grows as the isotropic-to-nematic phase transition temperature is approached from above. Here we begin to address the fast dynamics of the nematogen molecules within the domains using two-dimensional infrared (2D IR) vibrational echo experiments. The problems of performing ultrafast IR experiments in pure liquids are discussed, and solutions are presented. In addition, the issue of short vibrational lifetimes, which limit the ability of 2D IR experiments to examine dynamics over a wide range of times, is addressed. The experiments were performed on the nematogen 4-cyano-4'-pentylbiphenyl (5CB), with the CN stretch initially used as the vibrational probe. Although the CN stretch has a small transition dipole, because the sample is a pure liquid it is necessary to use an exceedingly thin sample to perform the experiments. The small sample volume leads to massive heating effects that distort the results. In addition, the high concentration in the pure liquid can result in vibrational excitation transfer that interferes with the measurements of structural dynamics, and the CN vibrational lifetime is very short (3.6 ps). These problems were overcome by performing the experiments on the natural-abundance (13)CN stretch (5(13)CB), which greatly reduced the absorbance, eliminating the heating problems; also, this stretch has a longer lifetime (7.9 ps). Experiments were also performed on benzonitrile, which showed that the heating problems associated with pure liquids are not unique to 5CB. Again, the problems were eliminated by conducting measurements on the (13)CN stretch, which has an even longer lifetime (20.2 ps) compared with the (12)CN stretch (5.6 ps). Finally, to extend the range of the dynamical measurements, 4-pentyl-4'-thiocyanobiphenyl (5SCB) was synthesized and studied as a dilute solute in 5CB. The CN stretch of 5SCB has a vibrational lifetime of 103 ps, which permits dynamical measurements to 200 ps, revealing the full range of fast structural dynamics in the isotropic phase of 5CB. It is shown that the 5SCB probe reports essentially the same dynamics as 5(13)CB on the short time scale that is observable with the 5(13)CB vibrational probe.

Fully absorptive 3D IR spectroscopy using a dual mid-infrared pulse shaper

This paper presents the implementation of 3D IR spectroscopy by adding a second pump beam to a two-beam 2D IR spectrometer. An independent mid-IR pulse shaper is used for each pump beam, which can be programmed to collect its corresponding dimension in either the frequency or time-domains. Due to the phase matching geometry employed here, absorptive 3D IR spectra are automatically obtained, since all four of the rephasing and non-rephasing signals necessary to generate absorptive spectra are collected simultaneously. Phase cycling is used to isolate the fifth-order from the third-order signals. The method is demonstrated on tungsten hexacarbonyl (W(CO)6) and dicarbonylacetylacetonato rhodium (I), for which the eigenstates are extracted up to the third excited state. Pulse shaping affords a high degree of control over 3D IR experiments by making possible mixed time- and frequency-domain experiments, fast data acquisition and straightforward implementation.

Determining equilibrium fluctuations using temperature-dependent 2D-IR

We demonstrate the capability of temperature-dependent 2D-IR to characterize sources of vibrational population transfer. In a model system of iron diene tricarbonyl "piano stool" complexes, this approach reveals symmetry breaking associated with equilibrium fluctuations and differentiates these from fluxional rearrangement. Tricarbonyl(1,3-butadiene)iron and tricarbonyl(1,5-cyclooctadiene)iron are shown to undergo intramolecular vibrational redistribution (IVR) coupled to the wagging motion of their carbonyl ligands. In the case of both molecules, these equilibrium fluctuations are distinguished from chemical exchange behaviors by their temperature dependence and arguments of molecular symmetry.

Dynamics in the interior of AOT lamellae investigated with two-dimensional infrared spectroscopy

The dynamics inside the organic regions of aerosol-OT (AOT)/water mixtures in the lamellar mesophase, bicontinuous cubic (BC) phase, and in an analogous molecule without the charged sulfonate headgroup are investigated by observing spectral diffusion, orientational relaxation and population relaxation using ultrafast two-dimensional infrared (2D IR) vibrational echo spectroscopy and IR pump-probe experiments on the asymmetric CO stretch of a vibrational probe, tungsten hexacarbonyl (W(CO)6). The water layer thickness between the bilayer planes in the lamellar phase was varied. For comparison, the dynamics of W(CO)6 in the normal liquid bis(2-ethylhexyl) succinate (EHS), which is analogous to AOT but has no charged sulfonate headgroup, were also studied. The 2D IR experiments measure spectral diffusion, which results from the structural evolution of the system. Spectral diffusion is quantified by the frequency-frequency correlation function (FFCF). In addition to a homogeneous component, the FFCFs are biexponential decays with fast and slow time components of ∼12.5 and ∼150 ps in the lamellar phase. Both components of the FFCF are independent of the number of water molecules per headgroup for the lamellae, but they slow somewhat in the BC phase. The dynamics in the ordered phases are in sharp contrast to the dynamics in EHS, which displays fast and slow components of the FFCF of 5 and 80 ps, respectively. As the hydration level of AOT increases, vibrational lifetime decreases, suggesting some change in the local environment of W(CO)6 with water content.