Jennifer E. Laaser

Adding a dimension to the infrared spectra of interfaces using heterodyne detected 2D sum-frequency generation (HD 2D SFG) spectroscopy

In the last ten years, two-dimensional infrared spectroscopy has become an important technique for studying molecular structures and dynamics. We report the implementation of heterodyne detected two-dimensional sum-frequency generation (HD 2D SFG) spectroscopy, which is the analog of 2D infrared (2D IR) spectroscopy, but is selective to noncentrosymmetric systems such as interfaces. We implement the technique using mid-IR pulse shaping, which enables rapid scanning, phase cycling, and automatic phasing. Absorptive spectra are obtained, that have the highest frequency resolution possible, from which we extract the rephasing and nonrephasing signals that are sometimes preferred. Using this technique, we measure the vibrational mode of CO adsorbed on a polycrystalline Pt surface. The 2D spectrum reveals a significant inhomogenous contribution to the spectral line shape, which is quantified by simulations. This observation indicates that the surface conformation and environment of CO molecules is more complicated than the simple “atop” configuration assumed in previous work. Our method can be straightforwardly incorporated into many existing SFG spectrometers. The technique enables one to quantify inhomogeneity, vibrational couplings, spectral diffusion, chemical exchange, and many other properties analogous to 2D IR spectroscopy, but specifically for interfaces.

Time-domain SFG spectroscopy using mid-IR pulse shaping: practical and intrinsic advantages.

Sum-frequency generation (SFG) spectroscopy is a ubiquitous tool in the surface sciences. It provides infrared transition frequencies and line shapes that probe the structure and environment of molecules at interfaces. In this article, we apply techniques learned from the multidimensional spectroscopy community to SFG spectroscopy. We implement balanced heterodyne detection to remove scatter and the local oscillator background. Heterodyning also separates the resonant and nonresonant signals by acquiring both the real and imaginary parts of the spectrum. We utilize mid-IR pulse shaping to control the phase and delay of the mid-IR pump pulse. Pulse shaping allows phase cycling for data collection in the rotating frame and additional background subtraction. We also demonstrate time-domain data collection, which is a Fourier transform technique, and has many advantages in signal throughput, frequency resolution, and line shape accuracy over existing frequency domain methods. To demonstrate time-domain SFG spectroscopy, we study an aryl isocyanide on gold, and find that the system has an inhomogeneous structural distribution, in agreement with computational results, but which was not resolved by previous frequency-domain SFG studies. The ability to rapidly and actively manipulate the mid-IR pulse in an SFG pules sequence makes possible new experiments and more accurate spectra.

Two-Dimensional Sum-Frequency Generation Reveals Structure and Dynamics of a Surface-Bound Peptide

Surface-bound polypeptides and proteins are increasingly used to functionalize inorganic interfaces such as electrodes, but their structural characterization is exceedingly difficult with standard technologies. In this paper, we report the first two-dimensional sum-frequency generation (2D SFG) spectra of a peptide monolayer, which are collected by adding a mid-IR pulse shaper to a standard femtosecond SFG spectrometer. On a gold surface, standard FTIR spectroscopy is inconclusive about the peptide structure because of solvation-induced frequency shifts, but the 2D line shapes, anharmonic shifts, and lifetimes obtained from 2D SFG reveal that the peptide is largely α-helical and upright. Random coil residues are also observed, which do not themselves appear in SFG spectra due to their isotropic structural distribution, but which still absorb infrared light and so can be detected by cross-peaks in 2D SFG spectra. We discuss these results in the context of peptide design. Because of the similar way in which the spectra are collected, these 2D SFG spectra can be directly compared to 2D IR spectra, thereby enabling structural interpretations of surface-bound peptides and biomolecules based on the well-studied structure/2D IR spectra relationships established from soluble proteins.

Extracting Structural Information from the Polarization Dependence of One- and Two-Dimensional Sum Frequency Generation Spectra

We present ways in which pulse sequences and polarizations can be used to extract structural information from one- and two-dimensional vibrational sum frequency generation (2D SFG) spectra. We derive analytic expressions for the polarization dependence of systems containing coupled vibrational modes, and we present simulated spectra to identify the features of different molecular geometries. We discuss several useful polarization combinations for suppressing strong diagonal peaks and emphasizing weaker cross-peaks. We investigate unique capabilities of 2D SFG spectra for obtaining structural information about SFG-inactive modes and for identifying coupled achiral chromophores. This work builds on techniques that have been developed for extracting structural information from 2D IR spectra. This paper discusses how to utilize these concepts in 2D SFG experiments to probe multioscillator systems at interfaces. The sample code for calculating polarization dependence of 1D and 2D SFG spectra is provided in the Supporting Information .

Photoexcitation Dynamics of Coupled Semiconducting Carbon Nanotube Thin Films

Carbon nanotubes are a promising means of capturing photons for use in solar cell devices. We time-resolved the photoexcitation dynamics of coupled, bandgap-selected, semiconducting carbon nanotubes in thin films tailored for photovoltaics. Using transient absorption spectroscopy and anisotropy measurements, we found that the photoexcitation evolves by two mechanisms with a fast and long-range component followed by a slow and short-range component. Within 300 fs of optical excitation, 20% of nanotubes transfer their photoexcitation over 5-10 nm into nearby nanotube fibers. After 3 ps, 70% of the photoexcitation resides on the smallest bandgap nanotubes. After this ultrafast process, the photoexcitation continues to transfer on a ~10 ps time scale but to predominantly aligned tubes. Ultimately the photoexcitation hops twice on average between fibers. These results are important for understanding the flow of energy and charge in coupled nanotube materials and light-harvesting devices.

Probing site-specific structural information of peptides at model membrane interface in situ

Isotope labeling is a powerful technique to probe detailed structures of biological molecules with a variety of analytical methods such as NMR and vibrational spectroscopies. It is important to obtain molecular structural information on biological molecules at interfaces such as cell membranes, but it is challenging to use the isotope labeling method to study interfacial biomolecules. Here, by individually (13)C═(16)O labeling ten residues of a peptide, Ovispirin-1, we have demonstrated for the first time that a site-specific environment of membrane associated peptide can be probed by the submonolayer surface sensitive sum frequency generation (SFG) vibrational spectroscopy in situ. With the peptide associated with a single lipid bilayer, the sinusoidal trend of the SFG line width and peak-center frequency suggests that the peptide is located at the interface beneath the lipid headgroup region. The constructive interferences between the isotope labeled peaks and the main peptide amide I peak contributed by the unlabeled components were used to determine the membrane orientation of the peptide. From the SFG spectral peak-center frequency, line width, and polarization dependence of the isotope labeled units, we deduced structural information on individual units of the peptide associated with a model cell membrane. We also performed molecular dynamics (MD) simulations to understand peptide-membrane interactions. The physical pictures described by simulation agree well with the SFG experimental result. This research demonstrates the feasibility and power of using isotope labeling SFG to probe molecular structures of interfacial biological molecules in situ in real time.

Dye aggregation identified by vibrational coupling using 2D IR spectroscopy

We report that a model dye, Re(CO)3(bypy)CO2H, aggregates into clusters on TiO2 nanoparticles regardless of our preparation conditions. Using two-dimensional infrared (2D IR) spectroscopy, we have identified characteristic frequencies of monomers, dimers, and trimers. A comparison of 2D IR spectra in solution versus those deposited on TiO2 shows that the propensity to dimerize in solution leads to higher dimer formation on TiO2, but that dimers are formed even if there are only monomers in solution. Aggregates cannot be washed off with standard protocols and are present even at submonolayer coverages. We observe cross peaks between aggregates of different sizes, primarily dimers and trimers, indicating that clusters consist of microdomains in close proximity. 2D IR spectroscopy is used to draw these conclusions from measurements of vibrational couplings, but if molecules are close enough to be vibrationally coupled, then they are also likely to be electronically coupled, which could alter charge transfer.

Survey of Ar-Tagged Predissociation and Vibrationally Mediated Photodetachment Spectroscopies of the Vinylidene Anion, C2H2-+

We report predissociation spectra of Ar-tagged C(2)H(2)(-) and C(2)D(2)(-) anions, and explore vibrationally mediated photodetachment from various vibrational levels of the bare C(2)H(2)(-) ion using velocity-map imaging. Intense photodetachment resonances are observed in the C-H stretching region that are strongly correlated with vibrational hot bands in the anion photoelectron spectra, indicating that one-color, resonant two-photon photodetachment (R2PD) is complicated by excitation of vibrationally excited states with autodetaching upper levels embedded in the continuum. Isolation of the R2PD spectrum was achieved using a two-color, IR-IR scheme in which vibrational excitation and photodetachment were carried out in two separate laser interaction regions.

Tuning Cationic Block Copolymer Micelle Size by pH and Ionic Strength

The formation, morphology, and pH and ionic strength responses of cationic block copolymer micelles in aqueous solutions have been examined in detail to provide insight into the future development of cationic micelles for complexation with polyanions such as DNA. Diblock polymers composed of a hydrophilic/cationic block of N,N-dimethylaminoethyl methacrylate (DMAEMA) and a hydrophobic/nonionic block of n-butyl methacrylate (BMA) were synthesized [denoted as DMAEMA-b-BMA (X-Y), where X = DMAEMA molecular weight and Y = molecular weight of BMA in kDa]. Four variants were created with block molecular weights of 14-13, 14-23, 27-14, 27-29 kDa and low dispersities less than 1.10. The amphiphilic polymers self-assembled in aqueous conditions into core-shell micelles that ranged in size from 25-80 nm. These cationic micelles were extensively characterized in terms of size and net charge in different buffers over a wide range of ionic strength (0.02-1 M) and pH (5-10) conditions. The micelle core is kinetically trapped, and the corona contracts with increasing pH and ionic strength, consistent with previous work on micelles with glassy polystyrene cores, indicating that the corona properties are independent of the dynamics of the micelle core. The contraction and extension of the corona scales with solution ionic strength and charge fraction of the amine groups. The aggregation numbers of the micelles were obtained by static light scattering, and the Rg/Rh ratios are close to that of a hard sphere. The zeta potentials of the micelles were positive up to two pH units above the corona pKa, suggesting that applications relying on micelle charge for stability should be viable over a wide range of solution conditions.

Interpolyelectrolyte Complexes of Polycationic Micelles and Linear Polyanions: Structural Stability and Temporal Evolution.

The complexation of poly(dimethylaminoethyl methacrylate)-block-poly(styrene) micelles with poly(styrenesulfonate) homopolymers was investigated in aqueous buffer at pH 4.5 as a function of ionic strength. The complexation process was monitored by turbidimetric titration, and the structure and stability of the complexes were assessed by dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryoTEM), and small-angle X-ray scattering. When complexes were formed by slow titration of one polyelectrolyte solution into the other, soluble complexes could be formed with either polyelectrolyte in excess as long as the mixture did not pass through the charge-neutral point. The initial complexes exhibited bimodal size distributions by DLS, with one population similar in size to or slightly smaller than the bare micelles, and the other significantly larger. The former correspond to individual micelles with complexed polyelectrolytes leading to a contracted corona; the latter reflect multimicelle aggregates that were directly observed by cryoTEM. At low ionic strength (e.g., 10 mM), these aggregates were stable on weeks-to-months time scales, but at high ionic strength (e.g., 500 mM), the aggregates rapidly annealed toward structures whose size and solubility depended on which polyelectrolyte was present in excess. These results are discussed in terms of the kinetics and thermodynamics of the polyelectrolyte complexation process and allow a detailed description of the interplay between kinetic and thermodynamic factors in this system. This work will inform design of polyelectrolyte complexes with tunable structure and stability for future applications.