Reorientation-induced spectral diffusion of non-isotropic orientation distributions

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)Cu^IIN₃]BAr₄^F (1^PR3, ^R3P₃tren = tris[2-(phosphiniminato)ethyl]amine, BAr₄^F = 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 1^PR3 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.

Distinguishing steric and electrostatic molecular probe orientational ordering via their effects on reorientation-induced spectral diffusion

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.

An ultrafast vibrational study of dynamical heterogeneity in the protic ionic liquid ethyl-ammonium nitrate. I. Room temperature dynamics

Using ultrafast two-dimensional infrared spectroscopy (2D-IR), a vibrational probe (thiocyanate, SCN^-) was used to investigate the hydrogen bonding network of the protic ionic liquid ethyl-ammonium nitrate (EAN) in comparison to H₂O. The 2D-IR experiments were performed in both parallel (⟨ZZZZ⟩) and perpendicular (⟨ZZXX⟩) polarizations at room temperature. In EAN, the non-Gaussian lineshape in the FTIR spectrum of SCN^- suggests two sub-ensembles. Vibrational relaxation rates extracted from the 2D-IR spectra provide evidence of the dynamical differences between the two sub-ensembles. We support the interpretation of two sub-ensembles with response function simulations of two overlapping bands with different vibrational relaxation rates and, otherwise, similar dynamics. The measured rates for spectral diffusion depend on polarization, indicating reorientation-induced spectral diffusion (RISD). A model of restricted molecular rotation (wobbling in a cone) fully describes the observed spectral diffusion in EAN. In H₂O, both RISD and structural spectral diffusion contribute with similar timescales. This complete characterization of the dynamics at room temperature provides the basis for the temperature-dependent measurements in Paper II of this series.

Strong Propensity of Ionic Liquids in Their Aqueous Solutions for an Organic-Modified Metal Surface

Understanding ionic structure and electrostatic environments near a surface has both fundamental and practical value. In electrochemistry, especially when room temperature ionic liquids (ILs) are involved, the complex ionic structure near the interface is expected to crucially influence reactions. Here we report evidence that even in dilute aqueous solutions of several ILs, the ions aggregate near the surface in ways that are qualitatively different from simple electrolytes. We have used a vibrational probe molecule, 4-mercaptobenzonitrile (MBN), tethered to a metal surface to monitor the behavior of the ionic layers. The characteristic nitrile vibrational frequency of this molecule has distinct values in the presence of pure water (∼2232 cm^-1) and pure IL (for example, ∼2226 cm^-1 for ethylmethylimidazolium tetrafluoroborate, [EMIM][BF₄]). This difference reflects the local electrostatic field and the hydrogen-bonding variations between these two limiting cases. We tracked this frequency shift as a function of IL concentration in water all the way from pure water to pure IL. We report two important findings. First, only one nitrile peak is observed for the entire concentration range, indicating that at least on the length scale of the probe molecule water and ILs do not phase separate within the interface, and no heterogeneously distinct electrostatic environments are formed. Second, and more importantly, we find that even up to a significant mole fraction of bulk water (x ∼ 0.95), the nitrile frequency does not change from that indicative of a pure IL for [EMIM][BF₄], indicating preferential aggregation of the ions near the surface. Because this behavior is very similar to surfactants, we chose an imidazolium cation with a longer side chain which resulted in behavior expected from a surfactant, with a preferential layer of the ions on the surface even in dilute water solutions (x ∼ 0.995). This observation indicates that even those ILs that are not nominally categorized as surfactants have a strong tendency to aggregate at the surface. Because ILs serve as electrolytes in a range of electrochemical reactions, including those requiring water, our results are likely useful for mechanistic understanding and tuning of such reactions.

Reorientation-induced Stokes shifts caused by directional interactions in electronic spectroscopy: Fast dynamics of poly(methyl methacrylate)

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.

Ultrafast dynamics of ionic liquids in colloidal dispersion

Ionic liquid (IL)-surfactant complexes have significance both in applications and fundamental research, but their underlying dynamics are not well understood. We apply polarization-controlled two-dimensional infrared spectroscopy (2D-IR) to study the dynamics of [BMIM][SCN]/surfactant/solvent model systems. We examine the effect of the choice of surfactants and solvent, and the IL-to-surfactant ratio (W-value), with a detailed analysis of the orientation and structural dynamics of each system. Different surfactants create very different environments for the entrapped ILs, ranging from a semi-static micro-environment to a fluxional environment that evolves even faster than the bulk IL. The oil-phase also clearly affects the microscopic dynamics. The anisotropy decay for entrapped ILs completes within 10 ps, which is similar to free thiocyanate ion in water, while a significant reorientation-induced spectral diffusion (RISD) effect is observed. The entrapped ionic liquid are highly dynamic for all W-values, and no core-shell structure is observed. We hypothesize that, instead of an ionic liquid-reverse micelle (IL-RM), the microscopic structure of this system is small colloidal dispersions or pairs of IL and surfactants. A detailed analysis of the polarization-controlled 2D-IR spectra of AOT system reveals a potential ion-exchange mechanism.