Novel alkoxyanthracene donor and benzothiadiazole acceptor for organic thin film transistor and bulk heterojunction organic photovoltaic cells

Novel alkoxy anthracene (ODA)‐based polymeric semiconductors were designed for polymer solar cell applications. Alkoxyanthracene, which contains many π electrons and electron donating group, was easily synthesized. The copolymers, poly(alkoxy anthracene‐alt‐thiophene benzothiadiazole thiophene) poly(ODA‐TBT) and poly(alkoxy anthracene‐alt‐benzothiadiazole) poly(ODA‐BT), have been obtained by Suzuki coupling polymerization. Both polymers have ODA unit as a donor and benzothiadiazole as an acceptor. ODA‐TBT has thiophene linkages between ODA and benzothiadiazole. The optical, thermal, and electrochemical properties have been investigated by UV–visible absorption, thermal gravimetric analysis, differential scanning calorimetry, and CV. Organic thin‐film transistor using polymers showed that the hole mobility of poly(ODA‐alt‐TBT) was around 3.6 × 10^?3 cm²/Vs with on/off ratio of 9.91 × 10⁵ while that of poly(ODA‐alt‐BT) was around 1.21 × 10^?2 cm²/Vs with on/off ratio of 2.64 × 10⁶. Organic photovoltaic performance based on polymers were evaluated with a configuration of ITO/PEDOT:PSS/active layer/LiF/Al. Poly(ODA‐TBT) exhibits a short circuit current (J_sc) of 3.9 mA/cm² and power conversion efficiency (PCE) of 1.4%, and poly(ODA‐BT) exhibits the J_sc of 6.4 mA/cm² and PCE of 2.2%. The better device performance of poly(ODA‐BT) is attributed to its charge transfer ability and enhanced mobility and crystallinity although poly(ODA‐BT) does not have extended π‐conjugation due to twisted structure compared with poly(ODA‐TBT). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1306–1314     

Probing dynamics of complex molecular systems with ultrafast 2D IR vibrational echo spectroscopy

Ultrafast 2D IR vibrational echo spectroscopy is described and a number of experimental examples are given. Details of the experimental method including the pulse sequence, heterodyne detection, and determination of the absorptive component of the 2D spectrum are outlined. As an initial example, the 2D spectrum of the stretching mode of CO bound to the protein myoglobin (MbCO) is presented. The time dependence of the 2D spectrum of MbCO, which is caused by protein structural evolution, is presented and its relationship to the frequency-frequency correlation function is described and used to make protein structural assignments based on comparisons to molecular dynamics simulations. The 2D vibrational echo experiments on the protein horseradish peroxidase are presented. The time dependence of the 2D spectra of the enzyme in the free form and with a substrate bound at the active site are compared and used to examine the influence of substrate binding on the protein's structural dynamics. The application of 2D vibrational echo spectroscopy to the study of chemical exchange under thermal equilibrium conditions is described. 2D vibrational echo chemical exchange spectroscopy is applied to the study of formation and dissociation of organic solute-solvent complexes and to the isomerization around a carbon-carbon single bond of an ethane derivative.     

Accidental vibrational degeneracy in vibrational excited states observed with ultrafast two-dimensional IR vibrational echo spectroscopy

The coupling between the OD stretch v=2 level and benzene-ring modes in 2-methoxyphenol-OD (hydroxyl H replaced by D) is observed with ultrafast two-dimensional (2D) IR vibrational echo spectroscopy. Because of this coupling, the 1-2 transition peak in the 2D spectrum is split into a doublet with peaks of approximately equal amplitudes. Several molecules and solvents were used to study this phenomenon. Near-IR (NIR) spectroscopy measurements and density-functional theory calculations (B3LYP6-31+G(d,p) level) were also applied. Experimental results and calculations show that the OD stretch 1-2 transition is coupled to a combination band related to the benzene-ring motions. A simple quantum-mechanical model indicates that the combination band has a frequency of 5172 and 5176.5 cm(-1) in CCl4 and hexane, respectively. The transition between this combination band and the ground state is too weak to detect by NIR. The transition between this band and the OD stretch first excited state is also so weak that most of the intensity of the doublet comes from the oscillator strength produced by coupling to the OD stretch. The model gives the coupling strengths as 6.5 and 7 cm(-1) in CCl4 and hexane, respectively.     

Conformational dynamics and stability of HP35 studied with 2D IR vibrational echoes

Two-dimensional infrared (2D IR) vibrational echo spectroscopy was used to measure the fast dynamics of two variants of chicken villin headpiece 35 (HP35). The CN of cyanophenylalanine residues inserted in the hydrophobic core were used as a vibrational probe. Experiments were performed on both singly (HP35-P) and doubly CN-labeled peptide (HP35-P(2)) within the wild-type sequence, as well as on HP-35 containing a singly labeled cyanophenylalanine and two norleucine mutations (HP35-P NleNle). There is a remarkable similarity between the dynamics measured in singly and doubly CN-labeled HP35, demonstrating that the presence of an additional CN vibrational probe does not significantly alter the dynamics of the small peptide. The substitution of two lysine residues by norleucines markedly improves the stability of HP35 by replacing charged with nonpolar residues, stabilizing the hydrophobic core. The results of the 2D IR experiments reveal that the dynamics of HP35-P are significantly faster than those of HP35-P NleNle. These observations suggest that the slower structural fluctuations in the hydrophobic core, indicating a more tightly structured core, may be an important contributing factor to HP35-P NleNle's increased stability.     

Dynamics of proteins encapsulated in silica sol-gel glasses studied with IR vibrational echo spectroscopy

Spectrally resolved infrared stimulated vibrational echo spectroscopy is used to measure the fast dynamics of heme-bound CO in carbonmonoxy-myoglobin (MbCO) and -hemoglobin (HbCO) embedded in silica sol-gel glasses. On the time scale of approximately 100 fs to several picoseconds, the vibrational dephasing of the heme-bound CO is measurably slower for both MbCO and HbCO relative to that of aqueous protein solutions. The fast structural dynamics of MbCO, as sensed by the heme-bound CO, are influenced more by the sol-gel environment than those of HbCO. Longer time scale structural dynamics (tens of picoseconds), as measured by the extent of spectral diffusion, are the same for both proteins encapsulated in sol-gel glasses compared to that in aqueous solutions. A comparison of the sol-gel experimental results to viscosity-dependent vibrational echo data taken on various mixtures of water and fructose shows that the sol-gel-encapsulated MbCO exhibits dynamics that are the equivalent of the protein in a solution that is nearly 20 times more viscous than bulk water. In contrast, the HbCO dephasing in the sol-gel reflects only a 2-fold increase in viscosity. Attempts to alter the encapsulating pore size by varying the molar ratio of silane precursor to water (R value) used to prepare the sol-gel glasses were found to have no effect on the fast or steady-state spectroscopic results. The vibrational echo data are discussed in the context of solvent confinement and protein-pore wall interactions to provide insights into the influence of a confined environment on the fast structural dynamics experienced by a biomolecule.     

Spectrally- and time-resolved vibrational surface spectroscopy: ultrafast hydrogen-bonding dynamics at D2O/CaF2 interface

Time- and frequency-domain three-wave mixing spectroscopy (IR+visible sum frequency generation) is developed as the lowest-order nonlinear technique that is both surface selective and capable of measuring spectral evolution of vibrational coherences. Using 70 fs infrared and 40 fs visible pulses, we observe ultrafast spectral dynamics of the OD stretch of D2O at the CaF2 surface. Spectral shifts indicative of the hydrogen-bond network rearrangement occur on the 100 fs time scale, within the observation time window determined by the vibrational dephasing. By tuning the IR pulse wavelength to the blue or red side of the OD-stretch transition, we selectively monitor the dynamics of different subensembles in the distribution of the H-bond structures. The blue-side excitation (weaker H-bonding structures) shows monotonic decay and nu(OD) frequency shift to the red on a 100 fs time scale, which is better described by a Gaussian than an exponential frequency correlation function. In contrast, the red-side excitation (stronger H-bonding structures) results in a blue spectral shift and a recursion in the signal at 125+/-10 fs, indicating the presence of an underdamped intermolecular mode of interfacial water.     

Quantitative analysis of an organic thin film by XPS,AFM and FT‐IR

We report the characterization of Firpic (iridium(III)bis[4,6‐di‐fluorophenyl]‐pyridinato‐N,C²,]picolinate) organic thin film prepared by vacuum deposition to provide a systematic route to organic film quantification. To analyze the characteristics of thin Firpic films on a Si substrate, various techniques such as XPS, Fourier transform infra‐red (FT‐IR) spectrometer, and atomic force microscopy (AFM) are utilized. The Firpic films remain stable without surface morphological or compositional change during deposition and after exposure to X‐ray irradiation or atmospheric environment, for which qualities these films are believed to be an ideal platform as a pure organic thin film. The monotonic increases in FT‐IR and XPS intensities with film thickness are matching well with each other. In particular, from the XPS intensity analysis, the relative atomic sensitivity factors of the present system, electron attenuation length, and molecular density in the organic thin film can be evaluated. Copyright © 2011 John Wiley & Sons, Ltd.     

Protein dynamics in cytochrome P450 molecular recognition and substrate specificity using 2D IR vibrational echo spectroscopy

Cytochrome (cyt) P450s hydroxylate a variety of substrates that can differ widely in their chemical structure. The importance of these enzymes in drug metabolism and other biological processes has motivated the study of the factors that enable their activity on diverse classes of molecules. Protein dynamics have been implicated in cyt P450 substrate specificity. Here, 2D IR vibrational echo spectroscopy is employed to measure the dynamics of cyt P450(cam) from Pseudomonas putida on fast time scales using CO bound at the active site as a vibrational probe. The substrate-free enzyme and the enzyme bound to both its natural substrate, camphor, and a series of related substrates are investigated to explicate the role of dynamics in molecular recognition in cyt P450(cam) and to delineate how the motions may contribute to hydroxylation specificity. In substrate-free cyt P450(cam), three conformational states are populated, and the structural fluctuations within a conformational state are relatively slow. Substrate binding selectively stabilizes one conformational state, and the dynamics become faster. Correlations in the observed dynamics with the specificity of hydroxylation of the substrates, the binding affinity, and the substrates' molecular volume suggest that motions on the hundreds of picosecond time scale contribute to the variation in activity of cyt P450(cam) toward different substrates.     

Structural disorder of the CD3zeta transmembrane domain studied with 2D IR spectroscopy and molecular dynamics simulations

In a recently reported study [Mukherjee, et al. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 3528] we used 2D IR spectroscopy and 1-(13)C=(18)O isotope labeling to measure the vibrational dynamics of 11 amide I modes in the CD3zeta transmembrane domain. We found that the homogeneous line widths and population relaxation times were all nearly identical, but that the amount of inhomogeneous broadening correlated with the position of the amide group inside the membrane. In this study, we use molecular dynamics simulations to investigate the structural and dynamical origins of these experimental observations. We use two models to convert the simulations to frequency trajectories from which the mean frequencies, standard deviations, frequency correlation functions, and 2D IR spectra are calculated. Model 1 correlates the hydrogen-bond length to the amide I frequency, whereas model 2 uses an ab initio-based electrostatic model. We find that the structural distributions of the peptidic groups and their environment are reflected in the vibrational dynamics of the amide I modes. Environmental forces from the water and lipid headgroups partially denature the helices, shifting the infrared frequencies and creating larger inhomogeneous distributions for residues near the ends. The least inhomogeneously broadened residues are those located in the middle of the membrane where environmental electrostatic forces are weakest and the helices are most ordered. Comparison of the simulations to experiment confirms that the amide I modes near the C-terminal are larger than at the N-terminal because of the asymmetric structure of the peptide bundle in the membrane. The comparison also reveals that residues at a kink in the alpha-helices have broader line widths than more helical parts of the peptide because the peptide backbone at the kink exhibits a larger amount of structural disorder. Taken together, the simulations and experiments reveal that infrared line shapes are sensitive probes of membrane protein structural and environmental heterogeneity.     

Electronic coherences and vibrational wave-packets in single molecules studied with femtosecond phase-controlled spectroscopy

Employing femtosecond pulse-shaping techniques we investigate ultrafast, coherent and incoherent dynamics in single molecules at room temperature. In first experiments single molecules are excited into their purely electronic 0-0 transition by phase-locked double-pulse sequences with pulse durations of 75 fs and 20 nm spectral band width. Their femtosecond kinetics can then be understood in terms of a 2-level system and modelled with the optical Bloch equations. We find that we observe the coherence decay in single molecules, and the purely electronic dephasing times can be retrieved directly in the time domain. In addition, the Rabi-frequencies and thus the transition dipole moments of single molecules are determined from these data. Upon excitation of single molecules into a vibrational level of the electronically excited state also incoherent intra-molecular vibrational relaxation is recorded. Increasing the spectral band width of the excitation pulses to up to 120 nm (resulting in a transform-limited pulse width of 15 fs) coherent superpositions of excited state vibrational modes, i.e. vibrational wave packets, are excited. The wave-packet oscillations in the excited state potential energy surface are followed in time by a phase-controlled pump-probe scheme, which permits to record wave packet interference, and to determine the energies of vibrational modes and their coupling strengths to the electronic transition.     

Temperature dependent equilibrium native to unfolded protein dynamics and properties observed with IR absorption and 2D IR vibrational echo experiments

Dynamic and structural properties of carbonmonoxy (CO)-coordinated cytochrome c(552) from Hydrogenobacter thermophilus (Ht-M61A) at different temperatures under thermal equilibrium conditions were studied with infrared absorption spectroscopy and ultrafast two-dimensional infrared (2D IR) vibrational echo experiments using the heme-bound CO as the vibrational probe. Depending on the temperature, the stretching mode of CO shows two distinct bands corresponding to the native and unfolded proteins. As the temperature is increased from low temperature, a new absorption band for the unfolded protein grows in and the native band decreases in amplitude. Both the temperature-dependent circular dichroism and the IR absorption area ratio R(A)(T), defined as the ratio of the area under the unfolded band to the sum of the areas of the native and unfolded bands, suggest a two-state transition from the native to the unfolded protein. However, it is found that the absorption spectrum of the unfolded protein increases its inhomogeneous line width and the center frequency shifts as the temperature is increased. The changes in line width and center frequency demonstrate that the unfolding does not follow simple two-state behavior. The temperature-dependent 2D IR vibrational echo experiments show that the fast dynamics of the native protein are virtually temperature independent. In contrast, the fast dynamics of the unfolded protein are slower than those of the native protein, and the unfolded protein fast dynamics and at least a portion of the slower dynamics of the unfolded protein change significantly, becoming faster as the temperature is raised. The temperature dependence of the absorption spectrum and the changes in dynamics measured with the 2D IR experiments confirm that the unfolded ensemble of conformers continuously changes its nature as unfolding proceeds, in contrast to the native state, which displays a temperature-independent distribution of structures.     

Vibrational spectroscopy reveals electrostatic and electrochemical doping in organic thin film transistors gated with a polymer electrolyte dielectric

We apply attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy to directly probe active layers in organic thin film transistors (OTFTs). The OTFT studied uses the n-type organic semiconductor N-N'-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8) and a polymer electrolyte gate dielectric made from poly(ethylene oxide) and LiClO4. FTIR spectroscopy of the device shows signatures of anionic PTCDI-C8 species and broad polaron bands when the organic semiconductor layer is doped under positive gate bias (VG). There are two distinctive doping regions: a reversible and electrostatic doping region for VG 2 V. On the basis of intensity loss of vibrational peaks attributed to neutral PTCDI-C8, we obtain a charge carrier density of 2.9 x 10(14)/cm2 at VG=2 V; this charge injection density corresponds to the conversion of slightly more than one monolayer of PTCDI-C8 molecules into anions. At higher gate bias voltage, electrochemical doping involving the intercalation of Li+ into the organic semiconductor film can convert all PTCDI-C8 molecules in a 30-nm film into anionic species. For comparison, when a conventional gate dielectric (polystyrene) is used, the maximum charge carrier density achievable at VG=200 V is approximately 4.5 x 10(13)/cm2, which corresponds to the conversion of 18% of a monolayer of PTCDI-C8 molecules into anions.     

Hydrogen bond lifetimes and energetics for solute/solvent complexes studied with 2D-IR vibrational echo spectroscopy

Weak pi hydrogen-bonded solute/solvent complexes are studied with ultrafast two-dimensional infrared (2D-IR) vibrational echo chemical exchange spectroscopy, temperature-dependent IR absorption spectroscopy, and density functional theory calculations. Eight solute/solvent complexes composed of a number of phenol derivatives and various benzene derivatives are investigated. The complexes are formed between the phenol derivative (solute) in a mixed solvent of the benzene derivative and CCl4. The time dependence of the 2D-IR vibrational echo spectra of the phenol hydroxyl stretch is used to directly determine the dissociation and formation rates of the hydrogen-bonded complexes. The dissociation rates of the weak hydrogen bonds are found to be strongly correlated with their formation enthalpies. The correlation can be described with an equation similar to the Arrhenius equation. The results are discussed in terms of transition state theory.     

Anharmonic vibrational spectroscopy calculations with electronic structure potentials: comparison of MP2 and DFT for organic molecules

Density functional theory (DFT) technique is the most commonly used approach when it comes to computation of vibrational spectra of molecular species. In this study, we compare anharmonic spectra of several organic molecules such as allene, propyne, glycine, and imidazole, computed from ab initio MP2 potentials and DFT potentials based on commonly used BLYP and B3LYP functionals. Anharmonic spectra are obtained using the direct vibrational self-consistent field (VSCF) method and its correlation-corrected extension (CC-VSCF). The results of computations are compared with available experimental data. It is shown that the most accurate vibrational frequencies are obtained with the MP2 method, followed by the DFT/B3LYP method, while DFT/BLYP results are often unsatisfactory. Contribution to the Mark S. Gordon 65th Birthday Festschrift Issue.     

Detection of electronic and vibrational coherences in molecular systems by 2D electronic photon echo spectroscopy

Two-dimensional optical photon echo spectra are simulated for model systems which exhibit vibrational, electronic and a combination of electronic and vibrational coherent dynamics. The coherent motion manifests itself as periodic beatings of the spectrum cross-peak intensity with the population time. The intensity modulations are compared to evolution of the excited-state population and coordinate expectation value. The advantageous capabilities of the technique as well as possible difficulties in spectra interpretations are outlined. Possibilities for distinguishing electronic and vibrational coherences are discussed.     

Volume phase transition and structure of poly(N-isopropylacrylamide) microgels studied with 1H-NMR spectroscopy in D2O

Thermoresponsive colloidal microgels were prepared by polymerisation of N-isopropylacrylamide (NIPAM) with varying concentration of a cross-linking monomer, N,N-methylenebisacrylamide (MBA), in water with either 0.4 or 6.7 mM concentration of an anionic surfactant, sodium dodecylsulphate (SDS). Volume phase transitions of the prepared microgels were studied in D₂O by ¹H-NMR spectroscopy including the measurements of spin–lattice (T1) and spin–spin (T2) relaxation times for the protons of poly(N-isopropylacrylamide) (PNIPAM) at temperature range 22–50 °C. In addition, microcalorimetry, turbidometry, dynamic light scattering and electrophoretic mobility measurements were used to characterise the aqueous microgels. The results from the different characterisation methods indicated that PNIPAM microgels prepared in 6.7 mM SDS concentration are structurally different compared to their correspondences prepared in 0.4 mM concentration. Increasing MBA concentration in the microgel synthesis appears to increase the structural heterogeneity in both cases of SDS concentration. PNIPAM structures with significantly higher molecular mobilities at temperatures above 35 °C were observed in the microgels prepared in 0.4 mM SDS concentration, as indicated by the ¹H NMR relaxation times of different PNIPAM protons. We conclude that the high mobilities measured with NMR at elevated temperatures and also the clearly negative values of zeta potential are in connection to a fairly mobile surface layer with polyelectrolyte nature and a consequent high local lower critical solution temperature.     

Combined electronic structure/molecular dynamics approach for ultrafast infrared spectroscopy of dilute HOD in liquid H2O and D2O

We present a new approach that combines electronic structure methods and molecular dynamics simulations to investigate the infrared spectroscopy of condensed phase systems. This approach is applied to the OH stretch band of dilute HOD in liquid D2O and the OD stretch band of dilute HOD in liquid H2O for two commonly employed models of water, TIP4P and SPC/E. Ab initio OH and OD anharmonic transition frequencies are calculated for 100 HOD x (D2O)n and HOD x(H2O)n (n = 4-9) clusters randomly selected from liquid water simulations. A linear empirical relationship between the ab initio frequencies and the component of the electric field from the solvent along the bond of interest is developed. This relationship is used in a molecular dynamics simulation to compute frequency fluctuation time-correlation functions and infrared absorption line shapes. The normalized frequency fluctuation time-correlation functions are in good agreement with the results of previous theoretical approaches. Their long-time decay times are 0.5 ps for the TIP4P model and 0.9 ps for the SPC/E model, both of which appear to be somewhat too fast compared to recent experiments. The calculated line shapes are in good agreement with experiment, and improve upon the results of previous theoretical approaches. The methods presented are simple, and transferable to more complicated systems.     

Local hydrogen bonding dynamics and collective reorganization in water: ultrafast infrared spectroscopy of HOD/D(2)O

We present an investigation into hydrogen bonding dynamics and kinetics in water using femtosecond infrared spectroscopy of the OH stretching vibration of HOD in D(2)O. Infrared vibrational echo peak shift and polarization-selective pump-probe experiments were performed with mid-IR pulses short enough to capture all relevant dynamical processes. The experiments are self-consistently analyzed with a nonlinear response function expressed in terms of three dynamical parameters for the OH stretching vibration: the frequency correlation function, the lifetime, and the second Legendre polynomial dipole reorientation correlation function. It also accounts for vibrational-relaxation-induced excitation of intermolecular motion that appears as heating. The long time, picosecond behavior is consistent with previous work, but new dynamics are revealed on the sub-200 fs time scale. The frequency correlation function is characterized by a 50 fs decay and 180 fs beat associated with underdamped intermolecular vibrations of hydrogen bonding partners prior to 1.4 ps exponential relaxation. The reorientational correlation function observes a 50 fs librational decay prior to 3 ps diffusive reorientation. Both of these correlation functions compare favorably with the predictions from classical molecular dynamics simulations. The time-dependent behavior can be separated into short and long time scales by the 340 fs correlation time for OH frequency shifts. The fast time scales arise from dynamics that are mainly local: fluctuations in hydrogen bond distances and angles within relatively fixed intermolecular configurations. On time scales longer than the correlation time, dephasing and reorientations reflect collective reorganization of the liquid structure. Since the OH transition frequency and dipole are only weakly sensitive to these collective coordinates, this is a kinetic regime which gives an effective rate for exchange of intermolecular structures.     

Structure evolution in the PbO-ZrO2-TiO2 sol-gel system: Part II—Pyrolysis of acid and base-catalyzed bulk and thin film gels

The pyrolysis behavior of acid and base-catalyzed bulk and thin film Pb(Zr, Ti)O₃, or PZT, gels as well as their components have been studied using Thermo-Gravimetric Analysis (TGA), Differential Thermal Analysis (DTA), and Dynamic Mass Spectrometric Analysis (DMSA). The TGA/DTA data reflected the structural differences of the acid and base-catalyzed gels. Gels obtained using an acid catalyst were less cross-linked, denser, and more homogeneous than base-catalyzed gels. Based on the understanding of structure evolution in the silica system, the various events in the TGA/DTA studies of acid-catalyzed PZT gels were attributable to specific mechanisms. The DMSA showed that primarily water, 2-methoxyethanol, acetone, and carbon dioxide were evolved for both acid and base catalyzed PZT gels. The presence of the latter two volatiles was associated with the decomposition of the acetate group via the carbonate route. Acetate and carbonate groups were determined by Fourier Transform Intrared Spectroscopy (FTIR) to be present in the gel structure prior to pyrolysis. Differences in the synthesis of the prehydroiyzed solution were found to affect the amount of residual alkoxy groups, gel structure, pyrolysis behavior, and therefore, the resulting microstructures of sol-gel derived PZT thin films. Finally, some suggestions for improving the processing of sol-gel PZT thin films are given.