Correlating solvent dynamics and chemical reaction rates using binary solvent mixtures and two-dimensional infrared spectroscopy.

Two-dimensional infrared (2D-IR) spectroscopy was performed on Vaska's complex (VC) and its oxygen adduct (V C-O2) in binary solvent mixtures of chloroform or benzyl alcohol in d6-benzene. The second order rate constants for oxygenation were also measured in these solvent mixtures. The rate constant in chloroform mixtures is linear with mole fraction within the error of the measurements but changes nonlinearly in benzyl alcohol mixtures, displaying a preference for the alcohol over benzene. The rate constants were compared with FTIR spectra of the carbonyl ligand and the frequency-frequency correlation function of this mode determined by 2D-IR. The line shape broadening mechanisms of the linear spectra of the CO bound to VC and V C-O2 are similar to those previously reported for V C-I2. There is a particularly strong correlation between rate constants and homogeneous linewidths of the carbonyl vibration on the V C-O2 product state. Concurrently, the FTIR spectra and spectral diffusion observed by 2D-IR corroborate an increase in solvent heterogeneity around the product. We interpret these results in the context of the potential role of solvent dynamics in facilitating chemical reactivity.

Origins of spectral broadening in iodated Vaska's complex in binary solvent mixtures.

Linear absorption spectroscopy of the iridium-bound carbonyl on an iodated adduct of Vaska's complex has shown that the mean vibrational frequency is insensitive to solvation by a broad range of solvents, while the spectral line width changes significantly. The spectral broadening is more significant in chloroform than benzyl alcohol, which is puzzling considering that benzyl alcohol is more polar. In this study, 2D-IR spectroscopy was performed on this vibrational mode to dissect the linear line shape into its homogeneous and inhomogeneous contributions in binary solvent mixtures of either chloroform or benzyl alcohol in d6-benzene. The full frequency-frequency correlation function was determined, including the homogeneous line width and fast spectral diffusion. We find that the frequency fluctuation magnitudes show the most notable changes in chloroform mixtures, while the time constants for spectral diffusion change more dramatically in benzyl alcohol mixtures. Nonetheless, we conclude that the frequency fluctuation magnitudes in both solvent mixtures most clearly explain the differences in their linear line widths. The homogeneous contributions were found to either stay the same or decrease as the more polar solvent was added to d6-benzene, thereby implicating inhomogeneous dynamics as the dominant broadening mechanism.

Solvent-mediated vibrational energy relaxation from Vaska's complex adducts in binary solvent mixtures.

A vibrational pump-probe and FTIR study was performed on two different adducts of Vaska's complex in two different sets of binary solvent mixtures. The carbonyl vibrational mode in the oxygen adduct exhibits solvatochromic shifts of ~10 cm(-1) in either benzyl alcohol or chloroform relative to benzene-d6, whereas this vibration is nearly unchanged for the iodine adduct for the same three solvents. The width and center frequency of the carbonyl stretch for each adduct are compared to its vibrational lifetime in binary mixtures of benzene-d6 with either benzyl alcohol or chloroform. In neat solvents, the trends in line width, frequency, and vibrational lifetime are consistent for the two adducts, but complex relationships emerge when the trends in each property are compared as a function of mixed solvent composition. ν(CO) is more sensitive to the solvation environment around the trans ligand, whereas the line width and lifetime depend on the environment around the CO group itself. The carbonyl frequency and width vary nonlinearly across the two binary solvent series, indicating preferential solvation. In contrast, the vibrational lifetime changes linearly with solvent composition and is correlated with the mole fraction of chloroform but anticorrelated with the mole fraction of benzyl alcohol. The results are explained by differences in the densities of solvent modes that affect intermolecular relaxation of the carbonyl mode.

Vibrational solvatochromism in Vaska's complex adducts.

The vibrational solvatochromism of bis(triphenylphosphine) iridium(I) carbonyl chloride (Vaska's complex, VC) was investigated by FTIR spectroscopy. The carbonyl stretching frequency (ν(CO)) was measured in 16 different organic solvents with a wide range of Lewis acidities for VC and its dioxygen (VC-O(2)), hydride (VC-H(2)), iodide (VC-I(2)), bromide (VC-Br(2)), and sulfide (VC-S(X)) adducts. The ν(CO) of the VC-O(2) complex was sensitive to the solvent electrophilicity, whereas minimal correlation was found for VC and the other adducts. The stretching frequency of the trans-O(2) ligand on VC-O(2) was measured to be anticorrelated with ν(CO), supporting a model in which this ligand indirectly affects the carbonyl frequency by modulating the extent of metal-to-CO back-bonding. The ν(CO) values obtained from DFT calculations on VC adducts with solvent continua and explicit hydrogen bonds were used to aid the interpretations of the experimental results. The O(2) ligand is more susceptible to stronger specific solvent interactions and it binds in a fundamentally different mode from the monatomic ligands, providing a more direct communication channel with those metal d-orbitals that have the appropriate symmetry to back-bond into the carbonyl π*-orbital.

Static and dynamic structural memory in polyaniline thin films.

Steady-state UV-visible and FTIR spectroscopies were used to characterize the electronic and structural changes that occur in polyaniline (PANI) thin films over the course of a single deprotonation and reprotonation cycle. The dedoping from the emeraldine salt (PANI-ES) to the emeraldine base (PANI-EB) form was achieved by treatment with a weak base (ammonia gas), and the PANI-ES was recovered by exposure to humid air and then dry air. The spectroscopic changes were classified into two general categories: those in which the recovered sample features were intermediate to the initial PANI-ES and the deprotonated PANI-EB and those in which the recovered sample features changed monotonically from the starting PANI-ES toward a unique observable. Two-dimensional IR vibrational echo spectroscopy (2D-IR VES) was then used to demonstrate that ultrafast structural dynamics on the time scales of hundreds of femtoseconds to a few picoseconds could also be organized into these two categories. In contrast, it was found that the slower dynamics on the tens of ps time scale appear unperturbed by the dramatic structural changes of the dedoping-redoping cycle. We discuss the relevance of these dynamics to charge mobilities in the initial and final PANI-ES states and compare their behavior to the film electrical resistances over the course of the protonation cycle. We show that specific structural dynamics are correlated with changes in the film conductivities and that PANI films have a memory of not only the static molecular structures of the as-cast materials but also some of the dynamics that are inherent to those morphologies.

Ground-state structural dynamics in doped and undoped polyaniline films probed by two-dimensional infrared vibrational echo spectroscopy.

Two-dimensional infrared vibrational echo spectroscopy (2D-IR VES) provides information about the structural dynamics occurring on the ultrafast time scale, a temporal regime that is comparable to that of charge-hopping events in conducting polymer films. In this study, 2D-IR VES is used to study polyaniline (PANI) thin films in three states of varying conductivity: emeraldine base (PANI-EB, semiconducting), emeraldine salt (PANI-ES) doped with dinonylnaphthalene sulfonic acid (conductive), and PANI-ES doped with camphor sulfonic acid (highly conductive). UV-visible and FTIR spectroscopies were used to characterize the static electronic and structural differences between these materials, and then these results were compared to the dynamical results from 2D-IR VES. The electronic ground state ultrafast dynamics for the PANI-EB reveal very fast motions that are not present in either of the PANI-ES samples. Despite differences in conductivity, no significant dynamical differences are observed for the films prepared with the two dopants. We interpret these results in light of previous work on the structural ordering induced by doping with sulfonic acids and the possible correlations between charge carrier mobilities and low frequency structural dynamics.