Polymer dynamics in blended films of poly(3-hexylthiophene-2,5-diyl) (P3HT) and 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine ruthenium(II)carbonyl (RuOEP) are examined with 2D-IR vibrational echo spectroscopy (2D-IR VES). Solvent vapor annealing generates three unique structural states in the films for electrical and spectroscopic characterization. The field-effect hole mobilities of the unannealed films are unaffected by the presence of the RuOEP guest species, and comparable mobility increases are observed during the early stages of the annealing process. The FTIR spectrum of the CO symmetric stretch exhibits dramatic changes over the course of annealing, indicating that substantial changes occur in the surrounding chemical environments in the P3HT film. During the early annealing steps, the 2D-IR VES measurements indicate a loss of dynamics on the time scale of 400 fs, concurrent with annealing-induced hole mobility improvements. We cast these results in light of several recent theoretical studies that predict that structural dynamics can have a profound influence on charge carrier mobilities.

A longitudinal 1D- and 2D-IR spectroscopy study is presented in which the homogeneous and inhomogeneous contributions to the line shape of a surface-bound Si—H vibration are monitored in a silica sol–gel while it ages. The vibrational data are complemented by time-lapse 29Si NMR and rheological measurements leading up to the gel point. The silane stretching frequency evolves continuously over the equivalent of several days, tracking the increase in tertiary and quaternary functionalized silicon atoms. The 1D-IR peak shape of the mode is static up until gelation, but then broadens continuously as the gel ages. A frequency–frequency correlation function (FFCF) extracted from 2D-IR spectra reveals that the line shape changes stem from an increase in inhomogeneity while the homogeneous dynamics remain unaffected by changes in silica cross-linking.

Currently accepted hydrogenation mechanisms of Shvo’s catalyst include an activation step in which the inactive ruthenium dimer undergoes scission to form two different catalytic species. In this study, two-dimensional infrared spectroscopy (2D-IR) of the metal carbonyl vibrations of Shvo’s catalyst was used to monitor early reaction dynamics for the inactive and activated catalyst. Kinetic analysis of exchange peaks in the 2D-IR spectra demonstrate that thermally activated intramolecular proton exchange occurs on the ultrafast time scale. The results indicate an activation barrier for proton transfer of 2.1 kcal/mol and an upper limit for the dimer scission rate constant of 1.3 × 1011 s–1, which is well above the previously reported value. Deprotonation of the dimer leads to a pseudo stable species that remains dimeric at the ruthenium–hydride bridge for several hours. 2D-IR spectroscopy of this species shows that proton transfer is turned off, as expected. The data reveal new mechanistic details of the dynamic behavior of Shvo’s catalyst leading up to activation and introduce the feasibility of substrate binding to the dimeric form of the catalyst prior to scission.

Vibrational sum frequency generation (VSFG) spectroscopy was used to measure the interfacial spectra of fullerene thin films on dielectric substrates that are commonly used in IR spectroscopy. The VSFG spectra on SiO2 and CaF2 exhibit notably different intensities for the F1u and Ag vibrational modes. This difference is attributed to different interfacial surface charges and C60/surface interactions. DFT calculations were performed to model the influence of a unidirectional electrostatic perturbation on the IR and Raman activities. The VSFG activities were then calculated for comparison to the interfacial second-order susceptibilities obtained from multilayer interference fitting of the experimental spectra. We find that the negative surface charge of CaF2 substrates causes a larger perturbation of fullerene than native silica surfaces, which leads to a stronger influence on the VSFG spectra.

Optical interference effects can be a nuisance in spectroscopy, especially in nonlinear experiments in which multiple incoming and outgoing beams are present. Vibrational sum frequency generation is particularly susceptible to interference effects because it is often applied to planar, layered materials, driving many of its practitioners to great lengths to avoid signal generation from multiple interfaces. In this perspective, we take a positive view of this metaphorical “lemon” and demonstrate how optical interference can be used as a tool to extract subtle changes in interfacial vibrational spectra. Specifically, we use small frequency shifts at a buried interface in an organic field-effect transistor to determine the fractional charge per molecule during device operation. The transfer matrix approach to nonlinear signal modeling is general and readily applied to complex layered samples that are increasingly popular in modern studies. More importantly, we show that a failure to consider interference effects can lead to erroneous interpretations of nonlinear data.

Polarization multiplexed vibrational sum frequency generation (PM-VSFG) spectroscopy has been used to monitor the interfacial structure of polymer transistor interfaces in situ during thermal annealing treatments. The evolution of the field-effect carrier mobility is tracked simultaneously with the molecular orientation and ordering of poly(3-hexylthiophene) (P3HT) macromolecules on two different surface types. It is shown that fluorocarbon functionalized silica imparts very different molecular arrangements that avoid kinetic trapping during solution casting. In contrast, bare silica surfaces produce kinetically trapped polymer configurations that can be observed by PM-VSFG to reorient with thermal annealing. The interfacial results are compared to bulk structural changes in P3HT thin films as characterized by differential scanning calorimetry and linear spectroscopies. The electrical performances of these films are more closely correlated with interfacial parameters than the bulk properties of the polymer. In contrast with the bulk measurements, the PM-VSFG studies show that molecules at the organic/dielectric interface are actually less ordered after thermal annealing processes that render them with lower carrier mobilities.

Although the science behind the soda geyser demonstration is well known, describing the microscopic origins of this dramatic, sticky demonstration can be difficult. In this experiment, an apparatus was designed to contain the reaction, thereby allowing for quantitative analysis of the amount of CO2 released after dropping in various initiating objects. The exploratory studies were tested with a moderate sized group of 12–17 year old participants at a summer learning event, and their data confirmed that the difference in the surface area of the initiators was a primary factor for the release of dissolved gas. Additional studies were performed to relate the soda temperature to the amount of dissolved CO2. In this laboratory experiment, students gained a greater understanding of surface area and its effect on gas nucleation and bubble formation as well as gas solubility its temperature dependence. The experimental approach provided a dramatic yet contained format for students to form and test their own hypotheses about the chemical processes behind this popular classroom demonstration.Keywords: Elementary/Middle School Science; General Public; Hands-On Learning/Manipulatives; High School/Introductory Chemistry; Inquiry-Based/Discovery Learning; Laboratory Instruction; Precipitation/Solubility; Problem Solving/Decision Making; Surface Science; Water/Water Chemistry;

Porous sol–gel matrices were synthesized with IR-active Si–H vibrational chromophores integrated into the silica network. The Si–H vibrational mode was found to be highly accessible to solvents within the nanoscopic pores. Vibrational solvatochromism of the silane vibration was controlled largely by interactions between infiltrating solvents and the oxygen and hydroxide sites in the silica. Exchanging solvents in the silica matrices produced reversible solvatochromic shifts in some cases but led to irreversible shifts when strongly interacting solvents were tested, suggesting that a layer of solvent was not exchangeable. 2D-IR spectroscopy was used to monitor spectral diffusion and extract the homogeneous line widths of the Si–H mode for a range of infiltrating solvents as well as solvent-free aerogel samples. It was demonstrated that the silane vibration is sensitive to the nature of the infiltrating solvent, making these vibrationally active sol–gel films a general platform for solvent dynamics in nanoscopic confined volumes.

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. νCO 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.

The vibrational solvatochromism of bis(triphenylphosphine) iridium(I) carbonyl chloride (Vaska’s complex, VC) was investigated by FTIR spectroscopy. The carbonyl stretching frequency (νCO) was measured in 16 different organic solvents with a wide range of Lewis acidities for VC and its dioxygen (VC-O2), hydride (VC-H2), iodide (VC-I2), bromide (VC-Br2), and sulfide (VC-SX) adducts. The νCO of the VC-O2 complex was sensitive to the solvent electrophilicity, whereas minimal correlation was found for VC and the other adducts. The stretching frequency of the trans-O2 ligand on VC-O2 was measured to be anticorrelated with νCO, supporting a model in which this ligand indirectly affects the carbonyl frequency by modulating the extent of metal-to-CO back-bonding. The νCO 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 O2 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.

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.

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.

Vibrational sum frequency generation (VSFG) spectroscopy was used in conjunction with steady-state IR spectroscopy, atomic force microscopy (AFM), and spectroscopic ellipsometry to characterize organic semiconductor thin films that were vapor deposited on silica- and trimethoxy(octadecyl)silane (ODTMS)-functionalized silica surfaces. The growth of perylene derivative N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) was found to proceed differently on simple glass slides relative to that of native oxide on silicon and fused quartz slides. VSFG was applied to these samples to isolate structural changes that occurred specifically at the buried interface between the organic semiconductor and the silica dielectric upon thermal annealing. A model was introduced to globally fit the imide carbonyl symmetric and asymmetric interfacial spectra that included contributions from both inner and outer interfaces. The fits to the VSFG data and AFM topographic images revealed significant reordering at the outer interface on all substrates upon thermal annealing. Within the model, the spectroscopic data reported that the inner interfacial PTCDI-C8 monolayer reoriented to a more reclined phase on bare substrates after annealing but remained essentially unchanged on ODTMS monolayers. Electrical characterization of PTCDI-C8 field-effect transistors indicated that electron mobilities were higher on bare substrate devices but could be improved by a factor of 2 on both surface types by thermal annealing. The mobility effects were attributed to the annealing-driven coalescence of PTCDI-C8 grain boundaries. Consistent with previous structural reports, the molecular rearrangements of the first monolayer of PTCDI-C8 on bare substrates that were reported by VSFG spectroscopy had a noticeable impact on the device performance.

The impact of surface chemistry on interfacial molecular orientation is studied for polymeric organic field-effect transistor (oFET) assemblies using vibrational sum frequency generation (VSFG) spectroscopy. The carbon–carbon vibrational modes on the backbone of side-chain deuterated poly(3-hexylthiophene) (DP3HT) are demonstrated to be an excellent handle for molecular orientation. DP3HT is utilized to avoid overlap in this spectral region with alkyl CH bending vibrations. Raman and FTIR spectroscopies are used to characterize the vibrational spectra of the thin films, and band assignments are confirmed by DFT calculations. Organosilane self-assembled monolayers are used to prepare oFET dielectrics with a range of surface energies, as measured by their water contact angles. The surface chemistry is found to have a profound influence on the field-effect carrier mobilities with lower surface energies producing higher mobilities. Polarization selective VSFG spectroscopy is then used to determine the relative orientation of the C═C symmetric stretching mode with respect to the surface normal. High field-effect mobilities for DP3HT on low surface energy functionalized dielectrics are directly correlated with the relative orientations of this vibrational transition dipole moment as measured by VSFG. The connection between this nonlinear spectroscopic observable and molecular structure enables this approach to confirm that low surface energy dielectrics lead to edge-on orientation of DP3HT conjugated chains with consequently higher mobilities.

Vibrational sum frequency generation (VSFG) is used to characterize the buried polymer–dielectric interface in poly(triarylamine) (PTAA) organic field-effect transistors (oFETs) over a spectral range of more than 1100 cm–1. The FTIR and Raman spectra are presented for the neutral and chemically oxidized thin films of this polymer as a starting point for identifying the potential vibrational changes that are induced by doping. In the VSFG spectra collected as a function of applied gate bias, we find evidence for interfacial electric fields, polaronic absorbances, and a strong VSFG-active band that is a spectral signature of the interfacial molecules that are perturbed by electrical doping. In most cases, the unipolar electrical behavior of PTAA oFETs is directly correlated with intensity changes in this structural perturbation band regardless of the dielectric (SiO2) surface functionalization. However, in a few selected examples, the VSFG measurements demonstrate that accumulation of either holes or electrons can be achieved with this organic semiconductor, even in the absence of measurable source-drain currents. These results highlight the potential for PTAA to achieve ambipolar device operation and the power of nonlinear spectroscopy to provide the feedback needed to optimize this performance when electrical measurements cannot.

The solvation dynamics for deoxygenated and oxygenated Vaska’s complex, bis(triphenylphosphene) iridium(I) carbonyl chloride, (deoxy-VC and oxy-VC) were characterized using two-dimensional infrared (2D-IR) spectroscopy in d6-benzene, chloroform, and DMF. The iridium-bound carbonyl was used as a probe of the static and dynamic chemical environments in each solvent system. The linear IR spectra of the complexes were consistent with CO frequency modulation through d−π* backbonding interactions. The deoxy-VC center frequencies were insensitive to the solvent type, but those of oxy-VC were sensitive to the surrounding solvent, presumably due to the indirect influence of the dioxygen ligand on the carbonyl vibrational frequency. The vibrational lifetimes of the VC carbonyls were consistent with intramolecular relaxation through the metal d–π orbitals. 2D-IR spectra were analyzed using the inverse centerline slope (CLS) as a representative of the normalized frequency–frequency correlation function. Multiexponential fits to the CLS decays revealed solvation dynamics on several time scales, ranging from a few to tens of picoseconds, with a shift of the relative proportion of the slower dynamics for the oxygenated complexes. The measured dynamics were compared to previously determined oxidative addition rate constants to hypothesize the potential role of solvent shell fluctuations in the overall reaction rate.

Vibrational sum frequency generation (VSFG) spectroscopy is used to probe the polymer−silica interface of poly(3-hexylthiophene) (P3HT) organic field-effect transistors (oFETs) in situ during device operation. The VSFG spectra exhibit dramatic changes upon charge accumulation at the buried interface. Proper modeling of the data reveals that the changes in the spectroscopic features are almost exclusively due to changes in the amplitude and relative phase of the nonresonant signal, while the P3HT alkyl CH3 and CH2 vibrations remain unperturbed. We interpret the spectroscopic data in light of vibrationally resonant, electronically resonant, and electric field dependent enhancements that occur upon oxidative doping of the P3HT, as measured by visible to mid-IR spectroelectrochemistry. Notably, we observe electric field enhancement of the VSFG signals at both positive and negative gate biases despite unipolar current−voltage responses, which we attribute to the trapping of electrons at the dielectric interface.

Phase segregation of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine ruthenium(II)carbonyl (RuOEP) and regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) in thin films is investigated with infrared and UV−visible spectroscopies as well as transmission electron microscopy (TEM). The Fourier transform infrared (FTIR) spectrum of the ruthenium-bound CO symmetric stretching mode exhibits significant changes as these films are annealed in solvent vapors. The development of multiple inhomogeneously broadened microenvironments is observed, and UV−visible spectra and TEM support a model of homogeneous porphyrin distribution throughout the P3HT films that gradually becomes more heterogeneous as the P3HT and RuOEP molecules phase segregate. A complete model for the phase segregation process experienced by the embedded RuOEP is proposed to explain the collective experimental observations.

Polarization multiplexed vibrational sum frequency generation (PM-VSFG) spectroscopy has been used to monitor the interfacial structure of polymer transistor interfaces in situ during thermal annealing treatments. The evolution of the field-effect carrier mobility is tracked simultaneously with the molecular orientation and ordering of poly(3-hexylthiophene) (P3HT) macromolecules on two different surface types. It is shown that fluorocarbon functionalized silica imparts very different molecular arrangements that avoid kinetic trapping during solution casting. In contrast, bare silica surfaces produce kinetically trapped polymer configurations that can be observed by PM-VSFG to reorient with thermal annealing. The interfacial results are compared to bulk structural changes in P3HT thin films as characterized by differential scanning calorimetry and linear spectroscopies. The electrical performances of these films are more closely correlated with interfacial parameters than the bulk properties of the polymer. In contrast with the bulk measurements, the PM-VSFG studies show that molecules at the organic/dielectric interface are actually less ordered after thermal annealing processes that render them with lower carrier mobilities.