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1. Inhibition of vibrational energy flow within an aromatic scaffold via heavy atom effect

   https://doi.org/10.1063/5.0153760  

   Hassani, Majid; Mallon, Christopher J.; Monzy, Judith N.; Schmitz, Andrew J.; Brewer, Scott H.; Fenlon, Edward E.; Tucker, Matthew J. (June 2023, The Journal of Chemical Physics)

   The regulation of intramolecular vibrational energy redistribution (IVR) to influence energy flow within molecular scaffolds provides a way to steer fundamental processes of chemistry, such as chemical reactivity in proteins and design of molecular diodes. Using two-dimensional infrared (2D IR) spectroscopy, changes in the intensity of vibrational cross-peaks are often used to evaluate different energy transfer pathways present in small molecules. Previous 2D IR studies of para-azidobenzonitrile (PAB) demonstrated that several possible energy pathways from the N3 to the cyano-vibrational reporters were modulated by Fermi resonance, followed by energy relaxation into the solvent [Schmitz et al., J. Phys. Chem. A 123, 10571 (2019)]. In this work, the mechanisms of IVR were hindered via the introduction of a heavy atom, selenium, into the molecular scaffold. This effectively eliminated the energy transfer pathway and resulted in the dissipation of the energy into the bath and direct dipole–dipole coupling between the two vibrational reporters. Several structural variations of the aforementioned molecular scaffold were employed to assess how each interrupted the energy transfer pathways, and the evolution of 2D IR cross-peaks was measured to assess the changes in the energy flow. By eliminating the energy transfer pathways through isolation of specific vibrational transitions, through-space vibrational coupling between an azido (N3) and a selenocyanato (SeCN) probe is facilitated and observed for the first time. Thus, the rectification of this molecular circuitry is accomplished through the inhibition of energy flow using heavy atoms to suppress the anharmonic coupling and, instead, favor a vibrational coupling pathway. 

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2. High resolution two-dimensional infrared (HR-2DIR) spectroscopy of gas phase molecules

   https://doi.org/10.1063/5.0109084  

   Daniels, DeAunna A.; Wells, Thresa A.; Chen, Peter C. (November 2022, The Journal of Chemical Physics)

   Two-dimensional infrared (2DIR) spectroscopy has become an established method for generating vibrational spectra in condensed phase samples composed of mixtures that yield heavily congested infrared and Raman spectra. These condensed phase 2DIR spectrometers can provide very high temporal resolution (<1 ps), but the spectral resolution is generally insufficient for resolving rotational peaks in gas phase spectra. Conventional (1D) rovibrational spectra of gas phase molecules are often plagued by severe spectral congestion, even when the sample is not a mixture. Spectral congestion can obscure the patterns in rovibrational spectra that are needed to assign peaks in the spectra. A method for generating high resolution 2DIR spectra of gas phase molecules has now been developed and tested using methane as the sample. The 2D rovibrational patterns that are recorded resemble an asterisk with a center position that provides the frequencies of both of the two coupled vibrational levels. The ability to generate easily recognizable 2D rovibrational patterns, regardless of temperature, should make the technique useful for a wide range of applications that are otherwise difficult or impossible when using conventional 1D rovibrational spectroscopy. 

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3. Ultrafast 2DIR comparison of rotational energy transfer, isolated binary collision breakdown, and near critical fluctuations in Xe and SF ₆ solutions

   https://doi.org/10.1063/5.0118395  

   Rotondaro, Matthew C.; Jain, Arkash; Erramilli, Shyamsunder; Ziegler, Lawrence D. (November 2022, The Journal of Chemical Physics)

   The density dependence of rotational and vibrational energy relaxation (RER and VER) of the N 2 O ν 3 asymmetric stretch in dense gas and supercritical Xe and SF 6 solutions for near critical isotherms is measured by ultrafast 2DIR and infrared pump–probe spectroscopy. 2DIR analysis provides precise measurements of RER at all gas and supercritical solvent densities. An isolated binary collision (IBC) model is sufficient to describe RER for solvent densities ≤ ∼4M where rotational equilibrium is re-established in ∼1.5–2.5 collisions. N 2 O RER is ∼30% more efficient in SF 6 than in Xe due to additional relaxation pathways in SF 6 and electronic factor differences. 2DIR analysis revealed that N 2 O RER exhibits a critical slowing effect in SF 6 at near critical density ( ρ* ∼ 0.8) where the IBC model breaks down. This is attributable to the coupling of critical long-range density fluctuations to the local N 2 O free rotor environment. No such RER critical slowing is observed in Xe because IBC break down occurs much further from the Xe critical point. Many body interactions effectively shield N 2 O from these near critical Xe density fluctuations. The N 2 O ν 3 VER density dependence in SF 6 is different than that seen for RER, indicating a different coupling to the near critical environment than RER. N 2 O ν 3 VER is only about ∼7 times slower than RER in SF 6 . In contrast, almost no VER decay is observed in Xe over 200 ps. This VER solvent difference is due to a vibrationally resonant energy transfer pathway in SF 6 that is not possible for Xe. 

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4. Atomic-force-microscopy-based time-domain two-dimensional infrared nanospectroscopy

   https://doi.org/10.1038/s41565-024-01670-w  

   Xie, Qing; Zhang, Yu; Janzen, Eli; Edgar, James H; Xu, Xiaoji G (August 2024, Nature Nanotechnology)

   For decades, infrared (IR) spectroscopy has advanced on two distinct frontiers: enhancing spatial resolution and broadening spectroscopic information. Although atomic force microscopy (AFM)-based IR microscopy overcomes Abbe’s diffraction limit and reaches sub-10 nm spatial resolutions, time-domain two-dimensional IR spectroscopy (2DIR) provides insights into molecular structures, mode coupling and energy transfers. Here we bridge the boundary between these two techniques and develop AFM-2DIR nanospectroscopy. Our method offers the spatial precision of AFM in combination with the rich spectroscopic information provided by 2DIR. This approach mechanically detects the sample’s photothermal responses to a tip-enhanced femtosecond IR pulse sequence and extracts spatially resolved spectroscopic information via FFTs. In a proof-of-principle experiment, we elucidate the anharmonicity of a carbonyl vibrational mode. Further, leveraging the near-field photons’ high momenta from the tip enhancement for phase matching, we photothermally probe hyperbolic phonon polaritons in isotope-enriched h10BN. Our measurements unveil an energy transfer between phonon polaritons and phonons, as well as among different polariton modes, possibly aided by scattering at interfaces. The AFM-2DIR nanospectroscopy enables the in situ investigations of vibrational anharmonicity, coupling and energy transfers in heterogeneous materials and nanostructures, especially suitable for unravelling the relaxation process in two-dimensional materials at IR frequencies. 

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5. Revealing Lithium Ion Transport Mechanisms and Solvation Structures in Carbonate Electrolytes

   https://doi.org/10.1021/jacs.4c13423  

   Pan, Junkun; Charnay, Aaron P; Zheng, Weizhong; Fayer, Michael D (December 2024, Journal of the American Chemical Society)

   Optimizing lithium-ion battery (LIB) electrolytes is essential for high-current applications such as electric vehicles, yet experimental techniques to characterize the complex structural dynamics within these electrolytes are limited. These dynamics are responsible for Li+ transport. In this study, we used ultrafast infrared spectroscopy to measure chemical exchange, spectral diffusion, and solvation structures across a wide range of lithium concentrations in propylene carbonate-based LiTFSI (lithium bis(trifluoromethanesulfonimide) electrolytes, with the CN stretch of phenyl selenocyanate as the long-lived vibrational probe. Phenyl selenocyanate is shown to be an excellent dynamical surrogate for propylene carbonate in Li+ solvation clusters. A strong correlation between exchange times and ionic conductivity was observed. This correlation and other observations suggest structural diffusion as the primary transport mechanism rather than vehicular diffusion. Additionally, spectral diffusion observables measured by the probe were directly linked to the de-solvation dynamics of the Li+ clusters, as supported by density functional theory and molecular dynamics simulations. These findings provide detailed molecular-level insights into LIB electrolytes’ transport dynamics and solvation structures, offering rational design pathways to advanced electrolytes for next-generation LIBs. 

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