mid-IR Pump-probe Spectroscopy Research Articles

Probing Ligand Effects on the Ultrafast Dynamics of Copper Complexes via Midinfrared Pump-Probe and 2DIR Spectroscopies.

The effects of ligand structural variation on the ultrafast dynamics of a series of copper coordination complexes were investigated using polarization-dependent mid-IR pump-probe spectroscopy and two-dimensional infrared (2DIR) spectroscopy. The series consists of three copper complexes [(R3P3tren)CuIIN3]BAr4F (1PR3, R3P3tren = tris[2-(phosphiniminato)ethyl]amine, BAr4F = tetrakis(pentafluorophenyl)borate) where the number of methyl and phenyl groups in the PR3 ligand are systematically varied across the series (PR3 = PMe3, PMe2Ph, PMePh2). The asymmetric stretching mode of azide in the 1PR3 series is used as a vibrational probe of the small-molecule binding site. The results of the pump-probe measurements indicate that the vibrational energy of azide dissipates through intramolecular pathways and that the bulkier phenyl groups lead to an increase in the spatial restriction of the diffusive reorientation of bound azide. From 2DIR experiments, we characterize the spectral diffusion of the azide group and find that an increase in the number of phenyl groups maps to a broader inhomogeneous frequency distribution (Δ2). This indicates that an increase in the steric bulk of the secondary coordination sphere acts to create more distinct configurations in the local environment that are accessible to the azide group. This work demonstrates how ligand structural variation affects the ultrafast dynamics of a small molecular group bound to the metal center, which could provide insight into the structure-function relationship of the copper coordination complexes and transition-metal coordination complexes in general.

Quest To Demystify Water: Ideal Solution Behaviors Are Obtained by Adhering to the Equilibrium Mass Action Law.

The current belief that water solutions are not "ideal" is due to the assumption that the mass action law is not obeyed and activity coefficients are used to avoid violating an immutable law. The law is not violated when expressed by including waters of hydration as part of the reactants and as products in the formation of ion pairs by electrolytes. Mole fractions calculated by adhering to the law and recognizing effects of incomplete dissociation and amounts of water interacting strongly with solutes obey Raoult's law to high concentrations. Another belief is that, for a solution to be "ideal", there must be no volume or heat change on mixing the components. We demonstrate "ideality" despite volume changes and nonzero enthalpies of solution. Results with a variety of electrolytes correlate with residence times of waters near ions, viscosities, entropies of hydration, enthalpies of solution and dilution, ion ionization energies, Hofmeister series, and so forth. Older reports of "negative hydration", that had been considered unreliable, are now understood; the effect is due to formation of mobile, or "fast", water by strong hydrophobic associations. Our results are supported by findings from techniques perfected relatively recently: dielectric relaxation, mid-IR pump-probe spectroscopy, 2D-IR, atomic force microscopy, and so forth.

Effect of Osmolytes on the Conformational Behavior of a Macromolecule in a Cytoplasm-like Crowded Environment: A Femtosecond Mid-IR Pump-Probe Spectroscopy Study.

Osmolytes found endogenously in almost all living beings play an important role in regulating cell volume under harsh environment. Here, to address the longstanding questions about the underlying mechanism of osmolyte effects, we use femtosecond mid-IR pump-probe spectroscopy with two different IR probes that are the OD stretching mode of HDO and the azido stretching mode of azido-derivatized poly(ethylene glycol) dimethyl ether (PEGDME). Our experimental results show that protecting osmolytes bind strongly with water molecules and dehydrate polymer surface, which results in promoting intramolecular interactions of the polymer. By contrast, urea behaves like water molecules without significantly disrupting water H-bonding network and favors extended and random-coil segments of the polymer chain by directly participating in solvation of the polymer. Our findings highlight the importance of direct interaction between urea and macromolecule, while protecting osmolytes indirectly affect the macromolecule through enhancing the water-osmolyte interaction in a crowded environment, which is the case that is often encountered in real biological systems.

Role of Solvent Water in the Temperature-Induced Self-Assembly of a Triblock Copolymer.

Water-soluble triblock copolymers have received much attention in industrial applications and scientific fields. We here show that femtosecond mid-IR pump-probe spectroscopy is useful to study the role of water in the temperature-induced self-assembly of triblock copolymers. Our experimental results suggest two distinct subpopulations of water molecules: those that interact with other water molecules and those involved in the hydration of a triblock copolymer surface. We find that the vibrational dynamics of bulk-like water is not affected by either micellation or gelation of triblock copolymers. The increased population of water interacting with ether oxygen atoms of the copolymer during the unimer to micelle phase transition is important evidence for the entropic role of water in temperature-induced micelle formation at a low copolymer concentration. In contrast, at the critical gelation temperature and beyond, the population of surface-associated water molecules interacting with ether oxygen atoms decreases, which indicates important enthalpic control by water. The present study on the roles of water in the two different phase transitions of triblock copolymers sheds new light on the underlying mechanisms of temperature-induced self-aggregation behaviors of amphiphiles that are ubiquitous in nature.

Water Hydrogen-Bonding Network Structure and Dynamics at Phospholipid Multibilayer Surface: Femtosecond Mid-IR Pump-Probe Spectroscopy.

The water hydrogen-bonding network at a lipid bilayer surface is crucial to understanding membrane structures and its functional activities. With a phospholipid multibilayer mimicking a biological membrane, we study the temperature dependence of water hydrogen-bonding structure, distribution, and dynamics at a lipid multibilayer surface using femtosecond mid-IR pump-probe spectroscopy. We observe two distinguished vibrational lifetime components. The fast component (0.6 ps) is associated with water interacting with a phosphate part, whereas the slow component (1.9 ps) is with bulk-like choline-associated water. With increasing temperature, the vibrational lifetime of phosphate-associated water remains constant though its relative fraction dramatically increases. The OD stretch vibrational lifetime of choline-bound water slows down in a sigmoidal fashion with respect to temperature, indicating a noticeable change of the water environment upon the phase transition. The water structure and dynamics are thus shown to be in quantitative correlation with the structural change of liquid multibilayer upon the gel-to-liquid crystal phase transition.

Anomalous temperature dependence of the vibrational lifetime of the OD stretch vibration in ice and liquid water

The temperature dependence of the vibrational T1 lifetime of the OD stretch vibration of HDO in H2O ice was measured with femtosecond mid-IR pump-probe spectroscopy. We found an increase of T1 from 480 ± 40 fs at 25 K to 860 ± 60 fs at 265 K. These lifetimes are remarkably shorter than the vibrational lifetime of the OD stretch vibration of HDO in H2O in the liquid phase, which has a value of 1.7 ± 0.1 ps at 274 K and increases to 2.24 ± 0.09 at 343 K. The observed temperature dependence of T1 can be well explained from a relaxation mechanism in which the OD vibration relaxes via energy transfer to the bend-libration combination tones of H2O and HDO.

Reduced coupling of water molecules near the surface of reverse micelles

We report on vibrational dynamics of water near the surface of AOT reverse micelles studied by narrow-band excitation, mid-IR pump-probe spectroscopy. Evidence of OH-stretch frequency splitting into the symmetric and asymmetric modes is clearly observed for the interfacial H(2)O molecules. The polarization memory of interfacial waters is preserved over an exceptionally extended >10 ps timescale which is a factor of 100 longer than in bulk water. These observations point towards negligibly small intermolecular vibrational coupling between the water molecules as well as strongly reduced water rotational mobility within the interfacial water layer.