Effect of amphiphilic molecules upon chromatic transitions of polydiacetylene vesicles in aqueous solutions

Effect of amphiphilic molecules upon the chromatic transitions of polymerized 10,12-pentacosadiynoic acid (PCDA) vesicles in aqueous solutions was reported. The colorimetric response of polymerized PCDA vesicles for 1-pentanol is higher than that for ethanol due to more hydrophobic property of 1-pentanol. The colorimetric response of polymerized PCDA vesicles for sodium dodecyl sulfate (SDS) and Triton X-100 is lower than that for cetyltrimethylammonium bromide (CTAB). The strong ability of CTAB to induce chromatic transition of the vesicles is related to the positively charged headgroups of CTAB, which favors approach of CTAB to the negatively charged carboxylate groups at the vesicle surface. The insertion of alkyl chain of CTAB into the hydrophobic domain perturbs the conformation of the conjugated polymer backbone and induces color change of polydiacetylene vesicles. For a series of alkylamine hydrochloric salts, the longer the alkyl chain, the stronger the ability of alkylamine to induce chromatic transition of polydiacetylene vesicles.     

Isomeric transitions in size-selected methanol hexamers probed by OH-stretch spectroscopy

We have measured the isomeric transition between the energetically lowest lying isomers of S6 and C2-symmetry of (CH3OH)6. The clusters are size-selected by deflection in collisions with He, and the isomers are identified by their infrared spectra of the OH-stretching vibration. The measurements are carried out at three source temperatures 253, 300 and 373 K which correspond to the cluster temperatures 93, 106 and 135 K. The latter ones are estimated by a relaxation model that accounts for the cluster formation and the energy released by the condensation. The transition takes place at a cluster temperature of about 102 K which is in agreement with the Molecular Dynamics simulation of such a transition at about 117 K using a realistic model potential.     

Dynamics of phase transitions in a liquid crystal probed by Raman spectroscopy

Raman spectra of the Schiff 's base liquid crystalline compound 5O5, N-(4- n-pentyloxybenzylidene)-4'-n-pentylaniline, have been recorded as a function of temperature from 22 to 80°C in the 1140-1220 cm^-1 and 1550-1640 cm^-1 spectral regions. From careful deconvolution of the spectral features using Lorentzian profiles, precise values of peak positions, integrated intensities and linewidths of some selected Raman bands were obtained. The variations of the Raman spectral parameters with temperature are discussed in terms of changes in the molecular alignment and its effect on intra-/inter-molecular interactions at the Cr-G, G-SmF, SmF-SmC and SmA-N phase transitions. From a detailed study, it is inferred that the increased orientational/vibrational freedom of the alkyl chains, as well as the delocalization of the electron clouds, is responsible for the spectral anomalies at the Cr-G transition. Loss of positional ordering and twist around the -C₆H₄-N= bond takes place at the SmF-SmC transition. In the SmA-N transition, some evidence for the formation of cybotactic clusters was obtained.     

Dynamics of phase transitions in a liquid crystal probed by Raman spectroscopy

Raman spectra of the Schiff 's base liquid crystalline compound 5O5, N-(4- n-pentyloxybenzylidene)-4'-n-pentylaniline, have been recorded as a function of temperature from 22 to 80°C in the 1140-1220 cm^-1 and 1550-1640 cm^-1 spectral regions. From careful deconvolution of the spectral features using Lorentzian profiles, precise values of peak positions, integrated intensities and linewidths of some selected Raman bands were obtained. The variations of the Raman spectral parameters with temperature are discussed in terms of changes in the molecular alignment and its effect on intra-/inter-molecular interactions at the Cr-G, G-SmF, SmF-SmC and SmA-N phase transitions. From a detailed study, it is inferred that the increased orientational/vibrational freedom of the alkyl chains, as well as the delocalization of the electron clouds, is responsible for the spectral anomalies at the Cr-G transition. Loss of positional ordering and twist around the -C₆H₄-N= bond takes place at the SmF-SmC transition. In the SmA-N transition, some evidence for the formation of cybotactic clusters was obtained.     

Electronic structures of polydiacetylene backbones

Energies per unit cell for various polydiacetylene backbone sequences are determined. The unrestricted Hartree-Fock approach is used to obtain the lowest energy transitions.     

Assembly of polydiacetylene vesicles on solid substrates

A method to create assembly of polydiacetylene vesicles on solid substrates in a novel fashion is described. The formation of the assembly is based on electrostatic layer-by-layer deposition using negatively charged 10,12-pentacosadiynoic acid (PCDA) vesicles and polyelectrolyte polyethylenimine, or positively charged PCDA-2(')-aminoethylamide (PCDANH2) vesicles. This is an efficient method for preparing the chromatic sensor films of polydiacetylene vesicles after ultraviolet light irradiation. The strategy would be useful in the development of polydiacetylene-based chemosensors and biosensors.     

Excitonic quadrupole absorption in a polydiacetylene

Excitonic states in a polydiacetylene (polyTCDU) single crystal have been investigated using temperature-dependent optical absorption, Raman and polarization-dependent optical spectroscopy. The expected significance of a linear quadrupolar component in the excitonic transition has been experimentally demonstrated. Formation and detectability of such quadrupoles is a unique attribute of the symmetry, order, valence geometry and one-dimensionality of the polydiacetylene backbone.     

Phase transitions and relaxation processes in ethylene-vinyl acetate copolymers probed by fluorescence spectroscopy

The steady-state fluorescence of pyrene and anthracene are used to investigate the relaxation processes of several random ethylene-co-vinyl acetate copolymers, EVA, with defined comonomer compositions (EVA-9, EVA-18, EVA-25, EVA-33 and EVA-40). The temperature of the relaxation processes are compared with those of low-density polyethylene (LDPE) and poly(vinyl acetate) (PVAc). The polymer relaxation processes are assigned to T_g=300-310 K (glass transition temperature of the PVAc); T_α=270-300 K (relaxation processes of the ethylene units present in LDPE and EVA); T_g=220-250 K (glass transition of the LDPE and of the EVA); T_γ or T_β=160-190 K (relaxation processes of interfacial defects of methylenic chains of LDPE and rotation of the acetate group of the PVAc and the EVA); and T_γ=90-130 K (relaxation processes of small sequences of methylene units of LDPE and end groups of PVAc). An Arrhenius-type function was employed as an attempt to represent the experimental data of fluorescence intensity versus temperature above the γ-relaxation temperature. As obtained with other techniques, there is not a simple relationship between the polymer relaxation processes and the vinyl acetate content that can be explained by the morphology in these copolymers.     

Nature of the aqueous hydroxide ion probed by X-ray absorption spectroscopy

X-ray absorption spectra of aqueous 4 and 6 M potassium hydroxide solutions have been measured near the oxygen K edge. Upon addition of KOH to water, a new spectral feature (532.5 eV) emerges at energies well below the liquid water pre-edge feature (535 eV) and is attributed to OH- ions. In addition to spectral changes explicitly due to absorption by solvated OH- ions, calculated XA spectra indicate that first-solvation-shell water molecules exhibit an absorption spectrum that is unique from that of bulk liquid water. It is suggested that this spectral change results primarily from direct electronic perturbation of the unoccupied molecular orbitals of first-shell water molecules and only secondarily from geometric distortion of the local hydrogen bond network within the first hydration shell. Both the experimental and the calculated XA spectra indicate that the nature of the interaction between the OH- ion and the solvating water molecules is fundamentally different than the corresponding interactions of aqueous halide anions with respect to this direct orbital distortion. Analysis of the Mulliken charge populations suggests that the origin of this difference is a disparity in the charge asymmetry between the hydrogen atoms of the solvating water molecules. The charge asymmetry is induced both by electric field effects due to the presence of the anion and by charge transfer from the respective ions. The computational results also indicate that the OH- ion exists with a predominately "hyper-coordinated" solvation shell and that the OH- ion does not readily donate hydrogen bonds to the surrounding water molecules.     

Phase transitions and hindered rotation in dimethylacetylene at high pressures probed by Raman spectroscopy

We present Raman spectroscopy experiments in dimethylacetylene (DMA) using a sapphire anvil cell up to 4 GPa at room temperature. DMA presents phase transitions at 0.2 GPa (liquid to phase I) and 0.9 GPa, which have been characterized by changes in the Raman spectrum of the sample. At pressures above 2.6 GPa several bands split into two components, suggesting an additional phase transition. The Raman spectrum of the sample above 2.6 GPa is identical to that found for the monoclinic phase II (C2/m) at low temperatures, except for an additional splitting of the band assigned to the fourfold degenerated asymmetric methyl stretch. The global analysis of the Raman spectra suggests that the observed splitting is due to the loss of degeneracy of the methyl groups of the DMA molecule in phase II. According to the above interpretation, crystal phase II of DMA extends from 0.9 GPa to pressures close to 4 GPa. Between 0.9 and 2.6 GPa, the methyl groups of the DMA molecules rotate almost freely, but the rotation is hindered on further compression.     

Electronic structure of NPB and BCP molecules probed by x-ray emission spectroscopy

Soft x-ray absorption and emission spectroscopies have been employed to investigate the electronic structure and chemical bonding of two prototypical molecules, N,N(')-bis-(1-naphthyl)-N,N(')-diphenyl-1,1(')-biphenyl-4,4(')-diamine (NPB) and bathocuproine (BCP), which are frequently chosen because of their hole-transporting and hole-blocking properties, respectively. The resulting resonant C Kalpha x-ray emission spectra of these materials reveal different spectral features depending on the resonant excitation energy. According to the N absorption and emission spectra, the contribution of N atoms to the highest occupied and lowest unoccupied molecular orbitals is different for in NPB and in BCP. Detailed knowledge of these materials will allow tailoring charge transport properties of organic devices in order to develop high performance organic light-emitting diodes and photovoltaic cells.     

Electronic band structure of model polydiacetylene chains

Calculations are reported of the electronic band structure of model polydiacetylene extended chains, with the large side groups represented by H or CCH, using the extended Hückel linear combination of atomic orbitals approximation. Predicted band gap widths are in good agreement with the results of an earlierπ-band calculation, and considerably smaller than experimental values.     

Electronic absorption spectroscopy of alkaline earth metal uranate compounds

Summary Electronic absorption spectra have been measured by diffuse reflectance for MgUO_4–x, MgU₃O₉, CaUo)_4–x, Ca₂UO₅, Ca₃UO₆, Ca₂U₃O₁₁, CaU₂O₇, CaU₄O₁₂, SrUO₃, SrUO₄, SrUO_3.67, Sr₂U₃O₁₁, SrU₄O_12.8, Sr₂UO₅, Sr₃UO₆, BaUO₄, Ba₃UO₆, BaU₂O₇, Ba₂U₂O₇ and Ba₂U₃O₁₁.Measurements in the near i.r. region of the spectrum have identified transitions arising within the manifold of the 5f electronic levels which indicate the presence of uranium(V) in certain compounds. The portion of the spectrum between 20 000 and 30 000 cm^–1 is shown to contain charge-transfer transitions and, in certain instances, vibrational progressions which are characteristic of the symmetry of the UO₆ octahedron in the solid state structure.     

Environment-controlled interchromophore charge transfer transitions in dipeptides probed by UV Absorption and electronic circular dichroism spectroscopy

Charge transfer (CT) transitions between the C-terminal carboxylate and peptide group have been investigated for alanyl-X and X-alanine dipeptides by far-UV absorption and electronic circular dichroism (ECD) spectroscopy (where X represents different amino acid residues). The spectra used in the present study were obtained by subtracting the spectrum of the cationic species from that of the corresponding zwitterionic peptide spectrum. These spectra displayed three bands, e.g., band I between 44 and 50 kK (kK = 10(3) cm(-1)), band II at 53 kK, and band III above 55 kK, which were, respectively, assigned to a n(COO-) --> pi* CT transition, a pi(COO-) --> pi* CT transition, and a carboxylate pi --> pi* (NV1) transition, respectively By comparison of the intensity, bandwidth, and wavenumber position of band I of some of the investigated dipeptides, we found that positive charges on the N-terminal side chain (for X = K), and to a minor extent also the N-terminal proton, reduce its intensity. This can be understood in terms of attractive Coulomb interactions that stabilize the ground state over the charge transfer state. For alanylphenylalanine, we assigned band I to a n(COO-) --> pi* CT transition into the aromatic side chain, indicating that aromatic side chains interact electronically with the backbone. We also performed ECD measurements at different pH values (pH 1-6) for a selected subset of XA and AX peptides. By subtraction of the pH 1 spectrum from that observed at pH 6, the ECD spectrum of the CT transition was obtained. A titration curve of their spectra reveals a substantial dependence on the protonation state of the aspartic acid side chain of AD, which is absent in DA and AE. This most likely reflects a conformational transition of the C-terminus into a less extended state, though the involvement of a side chain --> peptide CT transition cannot be completely ruled out.     

Electronic and molecular structure of photoexcited [Ru(II)(bpy)3]2+ probed by picosecond X-ray absorption spectroscopy

L(2,3) X-ray absorption spectra of aqueous [Ru(II)(bpy)3]2+ have been recorded in its ground and excited states, 50 ps after short pulse laser excitation. Significant changes in both the XANES (X-ray Near-Edge Absorption Structure) and the EXAFS (Extended X-ray Absorption Fine Structure) regions of the excited state complex are detected. The XANES line shapes have been quantitatively simulated using a crystal field multiplet code in trigonal symmetry. In addition, spectral changes in the EXAFS region of both ground and excited states are analyzed in order to extract structural parameters of their corresponding molecular structures. We obtain a Ru-N bond contraction by approximately 0.03 angstroms in the excited-state complex, as compared to the ground-state compound. This contraction results from electrostatic and polarization contributions, limited by steric constraints on the bpy ligands.     

Redox-linked protein dynamics of cytochrome c probed by time-resolved surface enhanced infrared absorption spectroscopy

Time-resolved surface enhanced infrared absorption (SEIRA) spectroscopy is employed to analyse the dynamics of the protein structural changes coupled to the electron transfer process of immobilised cytochrome c (Cyt-c). Upon electrostatic binding of Cyt-c to Au electrodes coated with self-assembled monolayers (SAMs) of carboxyl-terminated thiols, cyclic voltammetric measurements demonstrate a reversible redox process with a redox potential that is similar to that of Cyt-c in solution, and a non-exponential distance-dependence of the electron transfer rate as observed previously (D. H. Murgida and P. Hildebrandt, Chem. Soc. Rev. 2008, 37, 937). On the basis of characteristic redox-state-sensitive amide I bands, the protein structural changes triggered by the electron transfer are monitored by rapid scan and step scan SEIRA spectroscopy in combination with the potential jump technique. Whereas the temporal evolution of the conjugate bands at 1693 and 1673 cm(-1) displays the same rate constants as electron transfer, the time-dependent changes of the 1660-cm(-1) band are slower by about a factor of 2. The study demonstrates that time-resolved SEIRA spectroscopy provides further information about the dynamics and mechanism of interfacial processes of redox proteins, thereby complementing the results obtained from other surface-sensitive techniques. In comparison with previous surface enhanced resonance Raman spectroscopic findings, the present results are discussed in terms of the local electric field strengths at the Au/SAM/Cyt-c interface.     

Electronic structure of transition metal-cysteine complexes from X-ray absorption spectroscopy

The electronic structures of HgII, NiII, CrIII, and MoV complexes with cysteine were investigated by sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy and density functional theory. The covalency in the metal-sulfur bond was determined by analyzing the intensities of the electric-dipole allowed pre-edge features appearing in the XANES spectra below the ionization threshold. Because of the well-defined structures of the selected cysteine complexes, the current work provides a reference set for further sulfur K-edge XAS studies of bioinorganic active sites with transition metal-sulfur bonds from cysteine residues as well as more complex coordination compounds with thiolate ligands.     

Relaxation dynamics in the excited states of a ketocyanine dye probed by femtosecond transient absorption spectroscopy

Relaxation dynamics of the excited singlet states of 2,5-bis-(N-methyl-N-1,3-propdienylaniline)-cyclopentanone (MPAC), a ketocyanine dye, have been investigated using steady-state absorption and emission as well as femtosecond time-resolved absorption spectroscopic techniques. Following photoexcitation using 400 nm light, the molecule is excited to the S₂ state, which is fluorescent in rigid matrices at 77 K. S₂ state is nearly non-fluorescent in solution and has a very short lifetime (0.5 ± 0.2 ps). In polar aprotic solvents, the S₁ state follows a complex multi-exponential relaxation dynamics consisting of torsional motion of the donor groups, solvent re-organization as well as photoisomerization processes. However, in alcoholic solvents, solvent re-organization via intermolecular hydrogen-bonding interaction is the only relaxation process observed in the S₁ state. In trifluoroethanol, a strong hydrogen bonding solvent, conversion of the non-hydrogen-bonded form, which is formed following photoexcitation, to the hydrogen-bonded complex has been clearly evident in the relaxation process of the S₁ state.     

DNA interaction probed by evanescent wave cavity ring-down absorption spectroscopy via functionalized gold nanoparticles

Evanescent wave cavity ring-down absorption spectroscopy (EW-CRDS) is employed to study interaction and binding kinetics of DNA strands by using gold nanoparticles (Au NPs) as sensitive reporters. These Au NPs are connected to target DNA of study that hybridizes with the complementary DNA fixed on the silica surface. By the absorbance of Au NPs, the interaction between two DNA strands may be examined to yield an adsorption equilibrium constant of 2.2 × 10¹⁰ M⁻¹ using Langmuir fit. The binding efficiency that is affected by ion concentration, buffer pH and temperature is also examined. This approach is then applied to the label-free detection of the DNA mutation diseases using the sandwich hybridization assay. For monitoring a gene associated with sickle-cell anemia, the detection limit and the adsorption equilibrium constant is determined to be 1.2 pM and (3.7 ± 0.8) × 10¹⁰ M⁻¹, distinct difference from the perfectly matched DNA sequence that yields the corresponding 0.5 pM and (1.1 ± 0.2) × 10¹¹ M⁻¹. The EW-CRDS method appears to have great potential for the investigation of the kinetics of a wide range of biological reactions.