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1. Ultrafast Vibrational Dynamics of a Solute Correlates with Dynamics of the Solvent

   Crum, Vivian F.; Kiefer, Laura M.; Kubarych, K. J. (July 2021, Journal of chemical physics)

   null (Ed.)

   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. 

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2. Development of Interface-Specific Two-Dimensional Vibrational–Electronic (i2D-VE) Spectroscopy for Vibronic Couplings at Interfaces

   https://doi.org/10.3390/spectroscj3010001  

   Qian, Yuqin; Huang-Fu, Zhi-Chao; Brown, Jesse B; Rao, Yi (March 2025, Spectroscopy Journal)

   Bulk 2D electronic–vibrational (2D-EV) and 2D vibrational–electronic spectroscopies (2D-VE) were previously developed to correlate the electronic and vibrational degrees of freedom simultaneously, which allow for the study of couplings between electronic and vibrational transitions in photo-chemical systems. Such bulk-dominated methods have been used to extensively study molecular systems, providing unique information such as coherence sensitivity, molecular configurations, enhanced resolution, and correlated states and their dynamics. However, the analogy of interfacial 2D spectroscopy has fallen behind. Our recent work presented interface-specific 2D-EV spectroscopy (i2D-EV). In this work, we develop interface-specific two-dimensional vibrational–electronic spectroscopy (i2D-VE). The fourth-order spectroscopy is based on a Mach–Zehnder IR interferometer that accurately controls the time delay of an IR pump pulse pair for vibrational transitions, followed by broadband interface second-harmonic generation to probe electronic transitions. We demonstrate step-by-step how a fourth-order i2D-VE spectrum of AP3 molecules at the air/water interface was collected and analyzed. The line shape and signatures of i2D-VE peaks reveal solvent correlations and the spectral nature of vibronic couplings. Together, i2D-VE and i2D-EV spectroscopy provide coupling of different behaviors of the vibrational ground state or excited states with electronic states of molecules at interfaces and surfaces. The methodology presented here could also probe dynamic couplings of electronic and vibrational motions at interfaces and surfaces, extending the usefulness of the rich data that are obtained. 

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3. Magic-angle spinning NMR spectroscopy provides insight into the impact of small molecule uptake by G-quartet hydrogels

   https://doi.org/10.1039/D0MA00475H  

   Manjunatha Reddy, G. N.; Peters, Gretchen; Tatman, Ben; Rajan, Teena S.; Kock, Si Min; Zhang, Jing; Frenguelli, Bruno G.; Davis, Jeffery; Marsh, Andrew; Brown, Steven P. (January 2020, Materials Advances)

   Small molecule guests influence the functional properties of supramolecular hydrogels. Molecular-level understanding of such sol-gel compositions and structures is challenging due to the lack of long-range order and inherently heterogeneous sol-gel interface. In this study, insight into the uptake process of biologically relevant small molecules into guanosine-quartet(G4) borate hydrogels is obtained by gel-state magic-angle spinning (MAS) NMR spectroscopy. G4∙K + borate hydrogel can absorb up to 0.3 equivalent of cationic methylene blue (MB) without a significant disruption of the G4 fibrils that make up the gel, whereas the addition of over 0.3 equivalents of MB to the same gel leads to a gel-to-sol transition. The gel-to-sol transition process is characterized ex situ by analyzing and comparing the 1 H and 11 B MAS NMR spectra acquired before and after the MB uptake. In particular, 11 B isotropic chemical shifts and quadrupole interactions were determined by analyzing the 11 B MAS NMR spectra acquired at different magnetic fields, 11.7 T, 14.1 T and 20 T, which enable the different local bonding environments of borate anions in sol- and gel domains to be distinguished and identified. By comparison, uptake of heterocyclic molecules such as adenine, cytosine and 1-methylthymine into G4∙Na + borate hydrogels lead to stiff and clear gels while increasing the solubility of the nucleobases as compared to the solubility of the same compounds in water. G4∙Na + gel can uptake one equiv. of adenine with minimal disruption to the sol-gel framework, thus enhancing the adenine solubility up to an order of magnitude as compared to water. Combined multinuclear ( 1 H, 11 B and 23 Na) NMR spectroscopy analysis and vial inversion tests revealed that the nucleobases are embedded into pores of the sol phase rather than being closely interacting with the G-4 fibrils that make up the gel phase. These results indicate that G-4 hydrogels have potential applications as carrier systems for small molecules. Gel-state MAS NMR spectroscopy can be used to gain insight into host-guest interactions in complex heterogeneous sol-gel systems, which is often difficult to obtain from the conventional techniques such as X-ray scattering, electron microscopy and optical spectroscopy. 

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4. 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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5. Metal Ion-Induced Formation of Metallogels by Visible-Light-Responsive Phenylalanine-Functionalized Arylazopyrazole Ligands

   https://doi.org/10.3390/photochem3040026  

   Browning, Mikayla; Jefferson, Alexandra; Geter, Jazz; Ghebreyessus, Kesete (December 2023, Photochem)

   A visible-light-responsive arylazopyrazole-functionalized phenylalanine (4-MeS-AAP-NF) derived ligand was designed and synthesized, and it was found to form metallogels with reversible photo-responsive properties in mixed methanol/water (MeOH/H2O) solvents. The gelation behavior of the 4-MeS-AAP-NF ligand in the presence of different divalent metal ions in mixed methanol/water (MeOH/H2O) solvents at pH~11.60 was studied. It was found that the 4-MeS-AAP-NF ligand alone could not self-assemble to form any gels. However, in the presence of divalent metal ions, it readily formed the assembled metallogels in an alkaline aqueous/methanol solution with various morphologies. The results suggest that the gelation process was triggered by divalent metal ions. The presence of the AAP moiety in the gel matrix rendered the metallogel assemblies photo-responsive, and the reversible gel-to-sol phase transition was studied by UV-vis spectroscopy. The gels showed a slow, reversible visible-light-induced gel-to-sol phase transition under blue (λ = 405 nm) and then sol-to-gel transition by green light (λ = 530 nm) irradiation, resulting in the re-formation of the original gel state. The morphology and viscoelastic properties of the yellow–orange opaque metallogels were characterized by scanning electron microscopy (SEM) and rheological measurement, respectively. 

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