Education & Certifications

  • Ph.D., Stanford University, Chemistry (2021)
  • M.S., University of Pennsylvania, Chemistry (2014)
  • B.A., University of Pennsylvania, Biochemistry (2014)

All Publications

  • Liquid Heterostructures: Generation of Liquid-Liquid Interfaces in Free-Flowing Liquid Sheets. Langmuir : the ACS journal of surfaces and colloids Hoffman, D. J., Bechtel, H. A., Huyke, D. A., Santiago, J. G., DePonte, D. P., Koralek, J. D. 2022


    Chemical reactions and biological processes are frequently governed by the structure and dynamics of the interface between two liquid phases, but these interfaces are often difficult to study due to the relative abundance of the bulk liquids. Here, we demonstrate a method for generating multilayer thin film stacks of liquids, which we call liquid heterostructures. These free-flowing layered liquid sheets are produced with a microfluidic nozzle that impinges two converging jets of one liquid onto opposite sides of a third jet of another liquid. The resulting sheet consists of two layers of the first liquid enveloping an inner layer of the second liquid. Infrared microscopy, white light reflectivity, and imaging ellipsometry measurements demonstrate that the buried liquid layer has a tunable thickness and displays well-defined liquid-liquid interfaces and that this inner layer can be only tens of nanometers thick. The demonstrated multilayer liquid sheets minimize the amount of bulk liquid relative to their buried interfaces, which makes them ideal targets for spectroscopy and scattering experiments.

    View details for DOI 10.1021/acs.langmuir.2c01724

    View details for PubMedID 36220141

  • Microfluidic liquid sheets as large-area targets for high repetition XFELs. Frontiers in molecular biosciences Hoffman, D. J., Van Driel, T. B., Kroll, T., Crissman, C. J., Ryland, E. S., Nelson, K. J., Cordones, A. A., Koralek, J. D., DePonte, D. P. 2022; 9: 1048932


    The high intensity of X-ray free electron lasers (XFELs) can damage solution-phase samples on every scale, ranging from the molecular or electronic structure of a sample to the macroscopic structure of a liquid microjet. By using a large surface area liquid sheet microjet as a sample target instead of a standard cylindrical microjet, the incident X-ray spot size can be increased such that the incident intensity falls below the damage threshold. This capability is becoming particularly important for high repetition rate XFELs, where destroying a target with each pulse would require prohibitively large volumes of sample. We present here a study of microfluidic liquid sheet dimensions as a function of liquid flow rate. Sheet lengths, widths and thickness gradients are shown for three styles of nozzles fabricated from isotropically etched glass. In-vacuum operation and sample recirculation using these nozzles is demonstrated. The effects of intense XFEL pulses on the structure of a liquid sheet are also briefly examined.

    View details for DOI 10.3389/fmolb.2022.1048932

    View details for PubMedID 36567947

  • Long Vibrational Lifetime R-Selenocyanate Probes for Ultrafast Infrared Spectroscopy: Properties and Synthesis. The journal of physical chemistry. B Fica-Contreras, S. M., Daniels, R., Yassin, O., Hoffman, D. J., Pan, J., Sotzing, G., Fayer, M. D. 2021


    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.

    View details for DOI 10.1021/acs.jpcb.1c04939

    View details for PubMedID 34339200

  • Distinguishing steric and electrostatic molecular probe orientational ordering via their effects on reorientation-induced spectral diffusion. The Journal of chemical physics Hoffman, D. J., Fica-Contreras, S. M., Pan, J., Fayer, M. D. 2021; 154 (24): 244104


    The theoretical framework for reorientation-induced spectral diffusion (RISD) describes the polarization dependence of spectral diffusion dynamics as measured with two-dimensional (2D) correlation spectroscopy and related techniques. Generally, RISD relates to the orientational dynamics of the molecular chromophore relative to local electric fields of the medium. The predictions of RISD have been shown to be very sensitive to both restricted orientational dynamics (generally arising from steric hindrance) and the distribution of local electric fields relative to the probe (electrostatic ordering). Here, a theory that combines the two effects is developed analytically and supported with numerical calculations. The combined effects can smoothly vary the polarization dependence of spectral diffusion from the purely steric case (least polarization dependence) to the purely electrostatic case (greatest polarization dependence). Analytic approximations of the modified RISD equations were also developed using the orientational dynamics of the molecular probe and two order parameters describing the degree of electrostatic ordering. It was found that frequency-dependent orientational dynamics are a possible consequence of the combined electrostatic and steric effects, providing a test for the applicability of this model to experimental systems. The modified RISD equations were then used to successfully describe the anomalous polarization-dependent spectral diffusion seen in 2D infrared spectroscopy in a polystyrene oligomer system that exhibits frequency-dependent orientational dynamics. The degree of polarization-dependent spectral diffusion enables the extent of electrostatic ordering in a chemical system to be quantified and distinguished from steric ordering.

    View details for DOI 10.1063/5.0053308

    View details for PubMedID 34241361

  • Free Volume Element Sizes and Dynamics in Polystyrene and Poly(methyl methacrylate) Measured with Ultrafast Infrared Spectroscopy. Journal of the American Chemical Society Fica-Contreras, S. M., Hoffman, D. J., Pan, J., Liang, C., Fayer, M. D. 2021


    The size, size distribution, dynamics, and electrostatic properties of free volume elements (FVEs) in polystyrene (PS) and poly(methyl methacrylate) (PMMA) were investigated using the Restricted Orientation Anisotropy Method (ROAM), an ultrafast infrared spectroscopic technique. The restricted orientational dynamics of a vibrational probe embedded in the polymer matrix provides detailed information on FVE sizes and their probability distribution. The probe's orientational dynamics vary as a function of its frequency within the inhomogeneously broadened vibrational absorption spectrum. By characterizing the degree of orientational restriction at different probe frequencies, FVE radii and their probability distribution were determined. PS has larger FVEs and a broader FVE size distribution than PMMA. The average FVE radii in PS and PMMA are 3.4 and 3.0 A, respectively. The FVE radius probability distribution shows that the PS distribution is non-Gaussian, with a tail to larger radii, whereas in PMMA, the distribution is closer to Gaussian. FVE structural dynamics, previously unavailable through other techniques, occur on a 150 ps time scale in both polymers. The dynamics involve FVE shape fluctuations which, on average, conserve the FVE size. FVE radii were associated with corresponding electric field strengths through the first-order vibrational Stark effect of the CN stretch of the vibrational probe, phenyl selenocyanate (PhSeCN). PMMA displayed unique measured FVE radii for each electric field strength. By contrast, PS showed that, while larger radii correspond to unique and relatively weak electric fields, the smallest measured radii map onto a broad distribution of strong electric fields.

    View details for DOI 10.1021/jacs.0c13397

    View details for PubMedID 33630576

  • Pulse-shaped chopping: Eliminating and characterizing heat effects in ultrafast infrared spectroscopy. The Journal of chemical physics Hoffman, D. J., Fica-Contreras, S. M., Pan, J., Fayer, M. D. 2020; 153 (20): 204201


    The infrared pulses used to generate nonlinear signals from a vibrational probe can cause heating via solvent absorption. Solvent absorption followed by rapid vibrational relaxation produces unwanted heat signals by creating spectral shifts of the solvent and probe absorptions. The signals are often isolated by "chopping," i.e., alternately blocking one of the incident pulses. This method is standard in pump-probe transient absorption experiments. As less heat is deposited into the sample when an incident pulse is blocked, the heat-induced spectral shifts give rise to artificial signals. Here, we demonstrate a new method that eliminates heat induced signals using pulse shaping to control pulse spectra. This method is useful if the absorption spectrum of the vibrational probe is narrow compared to the laser bandwidth. By using a pulse shaper to selectively eliminate only frequencies of light resonant with the probe absorption during the "off" shot, part of the pulse energy, and the resulting heat, is delivered to the solvent without generating the nonlinear signal. This partial heating reduces the difference heat signal between the on and off shots. The remaining solvent heat signal can be eliminated by reducing the wings of the on shot spectrum while still resonantly exciting the probe; the heat deposition from the on shot can be matched with that from the off shot, eliminating the solvent heat contribution to the signal. Modification of the pulse sequence makes it possible to measure only the heat signal, permitting the kinetics of heating to be studied.

    View details for DOI 10.1063/5.0031581

    View details for PubMedID 33261482

  • Amorphous polymer dynamics and free volume element size distributions from ultrafast IR spectroscopy. Proceedings of the National Academy of Sciences of the United States of America Hoffman, D. J., Fica-Contreras, S. M., Fayer, M. D. 2020


    A method for measuring the size and size probability distribution of free volume regions in polymeric materials using ultrafast infrared (IR) polarization-selective pump-probe experiments is presented. Measurements of the ultrafast dynamics of a vibrational probe (the CN stretch of phenyl selenocyanate) in poly(methyl methacrylate) show that the probe dynamics are highly confined. The degree of confinement was found to be both time-dependent and dependent on the vibrational frequency of the probe molecule. The experiments demonstrate that different vibrational frequencies correspond to distinct subensembles of probe molecules that have different dynamic properties determined by their local structural environments. By combining the degree of dynamical confinement with the molecular size of the probe molecule, the free volume element size probability distribution was determined and found to be in good agreement with the best established experimental measure of free volume. The relative probability of a free volume element size is determined by the amplitude of the nitrile absorption spectrum at the frequency of the measurement. The inhomogeneous broadening of the spectrum was linked to the vibrational Stark effect, which permits site selectivity. The observed dynamics at each frequency were then associated with a different size free volume element and distinct local electric field. The multiple timescales observed in the pump-probe experiments were connected to local structural fluctuations of the free volume elements.

    View details for DOI 10.1073/pnas.2003225117

    View details for PubMedID 32513742

  • CLS Next Gen: Accurate Frequency-Frequency Correlation Functions from Center Line Slope Analysis of 2D Correlation Spectra Using Artificial Neural Networks. The journal of physical chemistry. A Hoffman, D. J., Fayer, M. D. 2020


    The center line slope (CLS) observable has become a popular method for characterizing spectral diffusion dynamics in two-dimensional (2D) correlation spectroscopy because of its ease of implementation, robustness, and clear theoretical relationship to the frequency-frequency correlation function (FFCF). The FFCF relates the frequency fluctuations of an ensemble of chromophores to coupled bath modes of the chemical system and is used for comparison to molecular dynamics simulations and for calculating 2D spectra. While in the appropriate limits, the CLS can be shown to be the normalized FFCF, from which the full FFCF can be obtained, in practice the assumptions that relate the CLS to the normalized FFCF are frequently violated. These violations are due to the presence of homogeneous broadening and motional narrowing. The generalized problem of relating the CLS to the FFCF is reanalyzed by introducing a new set of dimensionless parameters for both the CLS and FFCF. A large data set was generated of CLS parameters derived from numerically modeled 2D line shapes with known FFCF parameters. This data set was used to train feedforward artificial neural networks that act as functions, which take the CLS parameters as inputs and return FFCF parameters. These neural networks were deployed in an algorithm that is able to quickly and accurately determine FFCF parameters from experimental CLS parameters and the fwhm of the absorption line shape. The method and necessary inputs to accurately obtain the FFCF from the CLS are presented.

    View details for DOI 10.1021/acs.jpca.0c04313

    View details for PubMedID 32551669

  • Reorientation-induced Stokes shifts caused by directional interactions in electronic spectroscopy: Fast dynamics of poly(methyl methacrylate). The Journal of chemical physics Thomaz, J. E., Kramer, P. L., Fica-Contreras, S. M., Hoffman, D. J., Fayer, M. D. 2019; 150 (19): 194201


    Dynamic Stokes shift measurements report on structural relaxation, driven by a dipole created in a chromophore by its excitation from the ground electronic state to the S1 state. Here, we demonstrate that it is also possible to have an additional contribution from orientational relaxation of the Stokes shift chromophore. This effect, called reorientation-induced Stokes shift (RISS), can be observed when the reorientation of the chromophore and the solvent structural relaxation occur on similar time scales. Through a vector interaction, the electronic transition of the chromophore couples to its environment. The orientational diffusive motions of the chromophores will have a slight bias toward reducing the transition energy (red shift) as do the solvent structural diffusive motions. RISS is manifested in the polarization-dependence of the fluorescence Stokes shift using coumarin 153 (C153) in poly(methyl methacrylate) (PMMA). A similar phenomenon, reorientation-induced spectral diffusion (RISD), has been observed and theoretically explicated in the context of two dimensional infrared (2D IR) experiments. Here, we generalize the existing RISD theory to include properties of electronic transitions that generally are not present in vibrational transitions. Expressions are derived that permit determination of the structural dynamics by accounting for the RISS contributions. Using these generalized equations, the structural dynamics of the medium can be measured for any system in which the directional interaction is well represented by a first order Stark effect and RISS or RISD is observed. The theoretical results are applied to the PMMA data, and the structural dynamics are obtained and discussed.

    View details for DOI 10.1063/1.5094806

    View details for PubMedID 31117782

  • Fast dynamics of a hydrogen-bonding glass forming liquid: Chemical exchange-induced spectral diffusion in 2D IR spectroscopy. The Journal of chemical physics Hoffman, D. J., Fica-Contreras, S. M., Fayer, M. D. 2019; 150 (12): 124507


    Polarization-selective Two Dimensional Infrared (2D IR) and IR pump-probe spectroscopies have been performed on the hydrogen bonding glass forming liquid 2-biphenylmethanol doped with the long-lived vibrational probe phenylselenocyanate over a wide range of temperatures. The spectral diffusion seen in the 2D spectra was found to have a large polarization dependence, in large excess of what is predicted by standard theory. This anomaly was explained by decomposing the 2D spectra into hydrogen-bonding and non-bonding components, which exchange through large-angle orientational motion. By adapting chemical exchange theories, parameters for the component peaks were then calculated by fitting the polarization-dependent spectral diffusion and the pump-probe anisotropy. A model of highly heterogeneous exchange and orientational dynamics was used to explain the observed time dependences as a function of temperature on fast time scales. The experimental observations, the kinetic modeling, and physical arguments lead to the determination of the times for interconversion of slow dynamics structural domains to fast dynamics structural domains in the supercooled liquid as a function of temperature. The slow to fast domain interconversion times range from 40 ps at 355 K to 5000 ps at 270 K.

    View details for PubMedID 30927894

  • Fast dynamics of a hydrogen-bonding glass forming liquid: Chemical exchange-induced spectral diffusion in 2D IR spectroscopy JOURNAL OF CHEMICAL PHYSICS Hoffman, D. J., Fica-Contreras, S. M., Fayer, M. D. 2019; 150 (12)

    View details for DOI 10.1063/1.5088499

    View details for Web of Science ID 000462914300030

  • Discontinuity in Fast Dynamics at the Glass Transition of ortho-Terphenyl JOURNAL OF PHYSICAL CHEMISTRY B Hoffman, D. J., Fayer, M. D. 2017; 121 (45): 10417–28


    The dynamics of the molecular glass former ortho-terphenyl through the glass transition were observed with two-dimensional infrared vibrational spectroscopy measurements of spectral diffusion using the small probe molecule phenylselenocyanate. Although the slow diffusive motions were not visible on the experimental time scale, a picosecond-scale exponential relaxation was observed at temperatures from above to well below the glass transition temperature. The characteristic time scale has a smooth temperature dependence from the liquid into the glass phase, but the range of vibrational frequencies the probe samples displayed a discontinuity at the glass transition temperature. Complementary pump-probe experiments associate the observed motion with density fluctuations. The key features of the dynamics are reproduced with a simple corrugated well potential energy surface model. In addition, the temperature dependence of the homogeneous vibrational dephasing was found to have a T2 functional form, where T is the absolute temperature.

    View details for PubMedID 29039665

  • Direct observation of dynamic crossover in fragile molecular glass formers with 2D IR vibrational echo spectroscopy JOURNAL OF CHEMICAL PHYSICS Hoffman, D. J., Sokolowsky, K. P., Fayer, M. D. 2017; 146 (12)


    The dynamics of supercooled liquids of the molecular glass formers benzophenone and ortho-terphenyl were investigated with 2D IR spectroscopy using long-lived vibrational probes. The long lifetimes of the probes enabled structural dynamics of the liquids to be studied from a few hundred femtoseconds to a nanosecond. 2D IR experiments measured spectraldiffusion of a vibrational probe, which reports on structural fluctuations of the liquid. Analysis of the 2D IR data provides the frequency-frequency correlation function (FFCF). Two vibrational probes were examined with equivalent results, demonstrating the observed liquid dynamics are not significantly influenced by the probe molecules. At higher temperatures, the FFCF is a biexponential decay. However, at mild supercooling, the biexponential decay is no longer sufficient, indicating a dynamic crossover. The crossover occurs at a temperature well above the mode-coupling theory critical temperature for the given liquid, indicating dynamic heterogeneity above the critical temperature. Examination of the low temperature data with lifetime density analysis shows that the change is best described as an additional, distinct relaxation that shows behavior consistent with a slow β-process.

    View details for DOI 10.1063/1.4978852

    View details for Web of Science ID 000397929300058

    View details for PubMedID 28388155

  • Critical Slowing of Density Fluctuations Approaching the Isotropic-Nematic Transition in Liquid Crystals: 2D IR Measurements and Mode Coupling Theory. journal of physical chemistry. B Sokolowsky, K. P., Bailey, H. E., Hoffman, D. J., Andersen, H. C., Fayer, M. D. 2016; 120 (28): 7003-7015


    Two-dimensional infrared (2D IR) data are presented for a vibrational probe in three nematogens: 4-cyano-4'-pentylbiphenyl, 4-cyano-4'-octylbiphenyl, and 4-(trans-4-amylcyclohexyl)-benzonitrile. The spectral diffusion time constants in all three liquids in the isotropic phase are proportional to [T*/(T - T*)](1/2), where T* is 0.5-1 K below the isotropic-nematic phase transition temperature (TNI). Rescaling to a reduced temperature shows that the decays of the frequency-frequency correlation function (FFCF) for all three nematogens fall on the same curve, suggesting a universal dynamic behavior of nematogens above TNI. Spectral diffusion is complete before significant orientational relaxation in the liquid, as measured by optically heterodyne detected-optical Kerr effect (OHD-OKE) spectroscopy, and before any significant orientational randomization of the probe measured by polarization selective IR pump-probe experiments. To interpret the OHD-OKE and FFCF data, we constructed a mode coupling theory (MCT) schematic model for the relationships among three correlation functions: ϕ1, a correlator for large wave vector density fluctuations; ϕ2, the orientational correlation function whose time derivative is the observable in the OHD-OKE experiment; and ϕ3, the FFCF for the 2D IR experiment. The equations for ϕ1 and ϕ2 match those in the previous MCT schematic model for nematogens, and ϕ3 is coupled to the first two correlators in a straightforward manner. Resulting models fit the data very well. Across liquid crystals, the temperature dependences of the coupling constants show consistent, nonmonotonic behavior. A remarkable change in coupling occurs at ∼5 K above TNI, precisely where the rate of spectral diffusion in 5CB was observed to deviate from that of a similar nonmesogenic liquid.

    View details for DOI 10.1021/acs.jpcb.6b04997

    View details for PubMedID 27363680

  • Quasi-rotating frame: accurate line shape determination with increased efficiency in noncollinear 2D optical spectroscopy JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS Kramer, P. L., Giammanco, C. H., Tamimi, A., Hoffman, D. J., Sokolowsky, K. P., Fayer, M. D. 2016; 33 (6): 1143-1156