Electronic Transitions (electronic + transition)

Distribution by Scientific Domains
Distribution within Chemistry

Selected Abstracts

4- and 4,5-Substituted N -Methoxythiazole-2(3H)-thiones , Preparation,UV/Vis Spectra, and Assignment of Electronic Transitions in Comparison to N -Methoxypyridine-2(1H)-thione Using Time-Dependent Density Functional Theory Calculations

Jens Hartung
Abstract Experimentally observed absorptions in UV/Vis spectra of N -methoxy-4-methylthiazole-2(3H)-thione, N -methoxy-5-(p -methoxyphenyl)-4-methylthiazole-2(3H)-thione, N -methoxypyridine-2(1H)-thione, and selected N -hydroxy derivatives thereof have been assigned to ,,,*-type transitions as dominating character, using the results from ab initio calculations [time-dependent density functional theory (TD)RI-BLYP/TZVPP]. Theory further predicts that electronic excitations in N -methoxythiazole-2(3H)-thiones on one side and N -meth-oxypyridine-2(1H)-thione on the other side differ significantly with respect to character and statistical weight of contributing transitions. These effects originate predominantly from contributions of the endocyclic sulfur atom onto orbital energies and shapes in thiazole-2(3H)-thiones, and may be intensified by substituents such as a p -methoxyphenyl group located in position 5. Since the majority of the calculated spectral differences between thiazole- and pyridinethiones refers to excitations of low intensity, the findings from the present study correlate with two important experimental facts: (i) Apart from minor shifts in the exact spectral location of UV/Vis absorptions, electronic spectra of N -hydroxy- or N -methoxy-substituted pyridine-2(1H)-thiones and thiazole-2(3H)-thiones are surprisingly similar in shape. (ii) N -alkoxypyridine-2(1H)-thiones and N -alkoxythiazole-2(3H)-thiones liberate upon UV/Vis excitation oxygen-centered radicals with a comparable efficiency. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

Electronic Transitions in [Re6S8X6]4- (X: Cl, Br, I): Results from Time-Dependent Density Functional Theory and Solid-State Calculations.

CHEMINFORM, Issue 50 2006
Lindsay E. Roy
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source]

Highly Efficient Visible-Light-Induced Photocleavage of DNA by a Ruthenium-Substituted Fluorinated Porphyrin

Michelle Cunningham
Abstract A new porphyrin, meso -5-(pentafluorophenyl)-10,15,20-tris(4-pyridyl)porphyrin, has been synthesized. Coordination of two [Ru(bipy)2Cl]+ moieties (where bipy = 2,2,-bipyridine) to the pyridyl nitrogen atoms in the 10,15-positions gives the target complex. Electronic transitions associated with the ruthenium,porphyrin include an intense Soret band and four less intense Q-bands in the visible region of the spectrum. An intense ,,,* transition in the UV region associated with the bipyridyl groups and a metal-to-ligand charge-transfer (MLCT) band appearing as a shoulder to the Soret band are also observed. Electrochemical properties associated with the complex include a redox couple in the cathodic region with E1/2 = ,0.84 V vs. Ag/AgCl attributed to the porphyrin and a redox couple in the anodic region at E1/2 = 0.79 V vs. Ag/AgCl due to the RuIII/II couple. DNA titrations and ethidium bromide displacement experiments indicate the ruthenium porphyrin interacts with DNA potentially through a partial intercalation mechanism. Irradiation of aqueous solutions of the ruthenated complex and supercoiled DNA at a 100:1 base pair/complex ratio with visible light above 400 nm indicates that the complex causes double-strand breaks of the DNA.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Optical and electrical studies on spray deposited ZnO thin films

P. P. Sahay
Abstract ZnO thin films were prepared by spray pyrolytic decomposition of zinc acetate onto a glass substrate. These films were analyzed for the optical and electrical properties. Optical studies show that in these films the electronic transition is of the direct transition type. The optical energy gap for the films of different thicknesses is estimated to be in the range 2.98 , 3.09 eV. Electrical studies indicate that the films exhibit thermally activated electronic conduction and the activation energies are found to be dependent on the film thickness. The complex impedance measurements were carried out over a wide range of frequencies at room temperature (300 K). All the impedance spectra contain only a single arc, but the arc has a non-zero intersection with the real axis in the high frequency region. Also, the arc has its centre lying below with the real axis which indicates the multirelaxation behavior of the films. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Intramolecular electronic communication in a dimethylaminoazobenzene,fullerene C60 dyad: An experimental and TD-DFT study

K. Senthil Kumar
Abstract An electronically push,pull type dimethylaminoazobenzene,fullerene C60 hybrid was designed and synthesized by tailoring N,N -dimethylaniline as an electron donating auxochrome that intensified charge density on the ,-azonitrogen, and on N -methylfulleropyrrolidine (NMFP) as an electron acceptor at the 4 and 4, positions of the azobenzene moiety, respectively. The absorption and charge transfer behavior of the hybrid donor-bridge-acceptor dyad were studied experimentally and by performing TD-DFT calculations. The TD-DFT predicted charge transfer interactions of the dyad ranging from 747 to 601 nm were experimentally observed in the UV-vis spectra at 721 nm in toluene and dichloromethane. A 149 mV anodic shift in the first reduction potential of the NN group of the dyad in comparison with the model aminoazobenzene derivative further supported the phenomenon. Analysis of the charge transfer band through the orbital picture revealed charge displacement from the n(NN) (nonbonding) and , (NN) type orbitals centered on the donor part to the purely fullerene centered LUMOs and LUMO+n orbitals, delocalized over the entire molecule. The imposed electronic perturbations on the aminoazobenzene moiety upon coupling it with C60 were analyzed by comparing the TD-DFT predicted and experimentally observed electronic transition energies of the dyad with the model compounds, NMFP and (E)-N,N -dimethyl-4-(p-tolyldiazenyl)aniline (AZNME). The n(NN) , ,*(NN) and ,(NN) , ,*(NN) transitions of the dyad were bathochromically shifted with a significant charge transfer character. The shifting of ,(NN) , ,*(NN) excitation energy closer to the n , ,*(NN) in comparison with the model aminoazobenzene emphasized the predominant existence of charge separated quinonoid-like ground state electronic structure. Increasing solvent polarity introduced hyperchromic effect in the ,(NN) , ,*(NN) electronic transition at the expense of transitions involved with benzenic states, and the extent of intensity borrowing was quantified adopting the Gaussian deconvolution method. On a comparative scale, the predicted excitation energies were in reasonable agreement with the observed values, demonstrating the efficiency of TD-DFT in predicting the localized and the charge transfer nature of transitions involved with large electronically asymmetric molecules with HOMO and LUMO centered on different parts of the molecular framework. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [source]

Vibrational analysis of Ni(II)- and Cu(II)-octamethylchlorin by polarized resonance Raman and Fourier transform infrared spectroscopy

Robert J. Lipski
We measured the polarized resonance Raman spectra of Cu(II)-2,2,7,8,12,13,17,18-octamethylchlorin in CS2 at various excitation wavenumbers in a spectral region covering the Qy, Qx and Bx optical absorption bands. Additionally, we measured the FTIR-Raman spectrum of the highly overcrowded spectral region between 1300 and 1450 cm,1. The spectral decomposition was carried out by a self-consistent global fit to all spectra obtained. The thus identified Raman and IR lines were assigned by comparison with the resonance Raman spectra of Cu(II)-octaethylporphyrin, by utilizing their depolarization ratio dispersions and by a normal mode analysis. The latter was based on a modified transferable molecular mechanics force field of Ni(II)-octaethylporphyrin [E. Unger, M. Beck, R.J. Lipski, W. Dreybrodt, C.J. Medforth, K.M. Smith and R. Schweitzer-Stenner, J. Phys. Chem. B103, 10229 (1999)]. A comparison of normal mode patterns obtained for Cu(II)-octamethylchlorin and Cu(II)-octaethylporphyrin revealed that some modes are significantly distorted by the reduction of the pyrrole ring, in accordance with results which Boldt et al. reported earlier for Ni(II)-octaethylchlorin [N.J. Boldt, F.J. Donohoe, R.R. Birge and D.F. Bocian, J. Am. Chem. Soc.109, 2284 (1987)]. In contrast to conclusions drawn from this study, however, the results of our vibrational analysis and several further lines of evidence suggest that the normal modes of corresponding chlorines and porphyrins are still comparable, because they display contributions from the same local coordinates. Thus, the classical normal mode classification developed for metalloporphyrins is also applicable to metallochlorins. Finally, we performed a preliminary analysis of the absorption spectrum and the resonance excitation profiles and depolarization ratio dispersions of some Raman lines. The results show that the electronic properties of Cu(II)-octamethylchlorin can still be described in terms of Gouterman's four orbital model [M. Gouterman, J. Chem. Phys.30, 1139 (1959)]. In regions of the Q bands, Raman scattering of A1 modes is determined by interferences between Franck, Condon coupling and interstate Herzberg, Teller coupling between Qx(Qy) and Bx(By) states. The B2 modes are resonance enhanced by Herzberg, Teller coupling between Qx and Qy and between Qx(Qy) and By(Bx). Franck, Condon coupling of A1 modes with large contributions from C,Cm stretching vibrations is comparatively strong for Qx. This is interpreted as reflecting the expansion of the chlorin macrocycle by an electronic transition into this excited state. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Benzo[b]phosphole-Containing ,-Electron Systems: Synthesis Based on an Intramolecular trans -Halophosphanylation and Some Insights into Their Properties

Aiko Fukazawa Dr.
Abstract The intramolecular trans -halophosphanylation of 2-(aminophosphanyl)phenylacetylenes mediated by PBr3 followed by the oxidation with H2O2, produces 3-bromobenzo[b]phosphole oxide derivatives. This cyclization is also used for the synthesis of a 3-iodo derivative by conducting the reaction in the presence of LiI. Based on this synthetic method, various benzophosphole-containing ,-conjugated compounds, including a phosphoryl and methylene-bridged stilbene 10, 2,3,6,7-tetraphenylbenzo[1,2- b:4,5- b,]diphosphole- P,P, -dioxides 11, and their phosphine sulfide derivatives 12, are synthesized. The study of the structure,property relationships in a series of the bridged stilbenes, including a bis(methylene)-bridged stilbene 10, and a bis(phosphoryl)-bridged stilbene, reveals that as the contribution of the phosphoryl groups increased, the absorption and emission maxima substantially shift to longer wavelengths. The intrinsic substituent effects of the phosphoryl group in this skeleton are to decrease the oscillator strength of the electronic transition and thus decrease the radiative decay rate constants from the singlet excited state. Nevertheless, these compounds maintain high fluorescence quantum yields (,F>0.8) owing to the significantly retarded nonradiative decay process. In the study of the benzodiphosphole derivatives 11 and 12, their cyclic voltammetry revealed that both of the phosphoryl and phosphine sulfide derivatives have low reduction potentials (,1.7 to ,1.8,V vs ferrocene/ferrocenium couple) with the high reversibility of the redox waves. These compounds also showed high thermal stabilities with the high glass transition temperatures of 147,159,°C, indicative of their potential utilities as amorphous materials. [source]

Alkali Metal Doped Organic Molecules on Insulators: Charge Impact on the Optical Properties

Thomas Dienel
Doping-induced absorption changes of organic molecules on an insulating solid are reported. The charge transfer between alkali metal atoms and individual molecules on a surface leads to new electronic transitions identified with optical absorption spectroscopy. Progressive doping allows the discrimination of neutral, monoanionic and dianionic molecules in the solid state through examination of the spectra and rate equation modeling. [source]

Revealing the Electron,Phonon Coupling in a Conjugated Polymer by Single-Molecule Spectroscopy,

R. Hildner
Electron,phonon coupling in a ,-conjugated polymer is revealed by single-molecule spectroscopy in combination with statistical pattern recognition techniques. The technique allows to reveal the phonon-side band in the spectra of methyl-substituted ladder-type poly(para-phenylene) (see figure). For this polymer a weak electron,phonon coupling strength is found at low temperatures. The distribution of the phonon frequencies provides strong evidence that the low-energy vibrational modes, which couple to the electronic transitions, stem from vibrations of the host matrix. [source]

Electron invariants and excited state structural analysis for electronic transitions within CIS, RPA, and TDDFT models

A. V. Luzanov
Abstract We revisit the interpretative scheme (Luzanov et al., Theor Exp Chem 1974, 10, 354) of singly excited configuration interaction (CIS) model given earlier at semiempirical level. Detailed computations and spectral (natural orbital) treatment of the CIS density matrices of various types are presented. The corresponding hole-particle densities and related excitation localization indices are described. All the quantities are extended to the excited states calculated in the random phase approximation and closely related time-dependent density functional theory (TDDFT). The localization indices and charge transfer numbers which are invoked to describe interfragment interactions provide a basis for our scheme which is referred to as the excited state structural analysis for electronic transitions. The proposed analysis is exemplified by various moderate and large-size conjugated molecules treated within ab initio TDDFT and the Parizer,Parr,Pople approximation. Finally, we propose a possible generalization to the electronic transitions between CIS-like states followed by applications to singlet organic biradicals treated within the ,-electron spin-flip CIS. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source]

The electronic and optical properties of oligo(trans -1,2-di(2-thienyl)-1,3-butadiene): A theoretical study

Nei Marçal
Abstract In the present work we investigated the theoretical electronic structure of poly(trans -1,4-di(2-thienyl)-1,3-butadiene) (PTB) and determined the optical properties of its neutral and doped oligomers. Geometrical optimizations were at the semiempirical level by using the Austin method 1 (AM1). The band structure of , electrons regarding to the neutral PTB polymer was obtained by using a tight-binding Hamiltonian. The densities of electronic states (DOS) for neutral and doped copolymers were calculated by using the negative factor counting technique. The spatial charge distribution of the oligomeric chain was also analyzed. The energy of the electronic transitions and their associated oscillator strength values were calculated for the neutral, double, and single charged oligomers to determine the UV,vis absorption spectra. The calculations were performed using the intermediate neglect of differential overlap Hamiltonian in combination with the single configuration-interaction technique in order to include correlation effects. The band gap obtained in the PTB was about 2.101 eV for the optics absorption and 1.73 eV for the DOS. The bipolaron states appear in the gap, about 0.57 eV and 0.48 eV below and above the conduction and valence bands, respectively. When the dopants concentration is increased the DOS showed that the energy gap tends to vanish, which may lead to semiconductor,metal transition. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source]

Computational studies of electron-transfer processes in old yellow enzyme

Ginger M. Chateauneuf
Abstract Old Yellow Enzyme (OYE) is a flavoenzyme that was first isolated from brewer's bottom yeast. Homologues have been identified in other strains of yeast, bacteria, and plants. In plants, the OYE homologue functions enzymatically in the synthesis of plant hormones, but the biological function of OYE in yeast is still unknown. Flavin mononucleotide (FMN) is the cofactor that is noncovalently bound in the enzyme. OYE binds several phenolic ligands that serve as models for reactive biological substrates. These complexes have broad long-wavelength absorption bands, which have been ascribed to charge-transfer interactions, with the phenolate anion acting as the electron donor and the FMN as the acceptor [Abramovitz, A. S.; Massey, V. J Bio Chem 1976, 251, 5327,5336]. The computational characterization of these electronic transitions in the active site will help in understanding the biological processes in the enzyme. It was found that at several levels of computational methods, and through computationally mutating relevant amino acids, a charge-transfer process is occurring. This result agrees with previous experimental work and is consistent with all ultraviolet,visible spectrophotometric data. The preliminary results for the computational studies of these electron-transfer processes will be presented. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source]

Photoabsorption processes in nitrous oxide and formaldehyde

I. Martín
Abstract Absorption oscillator strengths and photoionization cross sections for electronic transitions involving Rydberg states that are relevant to the photochemistry of N2O and H2CO are reported. These compounds have been found to play an important role in the evolution of Earth's upper atmosphere. However, the difficulties encoutered in both laboratory measurements and theoretical calculations on the photoabsorption of these compounds are responsible for the scarcity of data in the literature. The present calculations have been performed with the molecular-adapted quantum defect orbital (MQDO) method, of which the adequacy for this type of studies has recently been assessed. A comparative analysis of the photoabsorption intensities in the molecules and their constituting atoms has enabled us to predict the variation of the extent of atomic character of the molecular Rydberg orbitals with the degree of excitation. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source]

Structural, electronic, bonding, magnetic, and optical properties of bimetallic [RunAum]0/+ (n + m , 3) clusters

Dimitrios N. Garbounis
Abstract The structural, electronic, bonding, magnetic, and optical properties of bimetallic [RunAum]0/+ (n + m , 3; n, m = 0,3) clusters were computed in the framework of the density functional theory (DFT) and time-dependent DFT (TD-DFT) using the full-range PBE0 non local hybrid GGA functional combined with the Def2-QZVPP basis sets. Several low-lying states have been investigated and the stability of the ground state spinomers was estimated with respect to all possible fragmentation schemes. Molecular orbital and population analysis schemes along with computed electronic parameters illustrated the details of the bonding mechanisms in the [RunAum]0/+ clusters. The TD-DFT computed UV-visible absorption spectra of the bimetallic clusters have been fully analyzed and compared to those of pure gold and ruthenium clusters. Assignments of all principal electronic transitions are given and interpreted in terms of contribution from specific molecular orbital excitations. © 2010 Wiley Periodicals, Inc. J Comput Chem 2010 [source]

Study of electronic spectra of free-base porphin and Mg-porphin: Comprehensive comparison of variety of ab initio, DFT, and semiempirical methods

Abstract SAC (symmetry adapted cluster)/SAC-CI and CASPT2 (multiconfigurational second-order perturbation theory) electron excitation spectra of free-base porphin and magnesium-porphin were determined using basis set functions augmented by both the polarization and diffuse functions,6-31+G(d). Such basis is recommended for correct description of the spectra because diffuse functions play fundamental roles in the formation of Rydberg MOs. The obtained results indicated that already the lowest roots in Au, B1u, B2g, and B3g irreducible representations display Rydberg character. The calculated spectra are in a good agreement with both experimental and recently calculated electronic transitions. It is concluded that the SAC/SAC-CI level spectral lines are significantly affected by configuration selection when energy thresholds 5.0 × 10,6 and 5.0 × 10,7 a.u. are used for the determination of ground and excited state properties. © 2004 Wiley Periodicals, Inc. J Comput Chem 26: 294,303, 2005 [source]

TDDFT investigation on nucleic acid bases: Comparison with experiments and standard approach

M.K. Shukla
Abstract A comprehensive theoretical study of electronic transitions of canonical nucleic acid bases, namely guanine, adenine, cytosine, uracil, and thymine, was performed. Ground state geometries were optimized at the MP2/6-311G(d,p) level. The nature of respective potential energy surfaces was determined using the harmonic vibrational frequency analysis. The MP2 optimized geometries were used to compute electronic vertical singlet transition energies at the time-dependent density functional theory (TDDFT) level using the B3LYP functional. The 6-311++G(d,p), 6-311(2+,2+)G(d,p), 6-311(3+,3+)G(df,pd), and 6-311(5+,5+)G(df,pd) basis sets were used for the transition energy calculations. Computed transition energies were found in good agreement with the corresponding experimental data. However, in higher transitions, the Rydberg contaminations were also obtained. The existence of ,,* type Rydberg transition was found near the lowest singlet ,,* state of all bases, which may be responsible for the ultrafast deactivation process in nucleic acid bases. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 768,778, 2004 [source]

A DFT/TD-DFT study for the ground and excited states of peramine and some pyrrolopyrazinone compounds

Abstract Peramine, a heterocyclic natural molecule, reveals two main, different in nature, electronic absorption bands. Theoretical calculations at the TD-B3LYP/6,311++G(d,p) level of theory show that the electronic excitations are connected predominantly with ,,,,,* and charge-transfer (CT) transitions. Excitation of electrons from the pyrrolopyrazinone ring to the side chain plays a role in creating the CT transition. The character and energy of the first 30 singlet,singlet electronic transitions have also been investigated for the most stable conformation of peramine. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Raman scattering of acoustical modes of silicon nanoparticles embedded in silica matrix,

M. Ivanda
Abstract The Raman scattering from acoustical phonons of silicon quantum dots in glass matrix was investigated. Two peaks that correspond to symmetric and quadrupolar spheroidal vibrations were found. A model calculation for in- and off-resonance scattering conditions was used, which considered the homogeneous broadening due to interaction with matrix and the inhomogeneous broadening due to particle size distribution. A strong dependence of the light-to-vibration coupling coefficient on the particles size was needed for fitting the Raman data. This result suggests that resonance with electronic transitions of the silicon nanoparticles is important for excitation at 514.5 nm. The size distribution obtained from the Raman data is in agreement with the results of high-resolution transmission electron microscopy. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Ultraviolet Absorption Spectra of Substituted Phenols: A Computational Study,

Lei Zhang
ABSTRACT Vertical excitation energies for electronic transitions from the ground state to the first two excited states of phenol, mono- and disubstituted methoxyphenols and methyl-substituted phenols have been characterized with the Time-Dependent Density Functional Theory (TD-DFT), the Complete Active Space Self-Consistent Field method (CASSCF) and the Coupled Cluster with Single and Double Excitations Equation-of-Motion approach (CCSD-EOM) to simulate and interpret experimental ultraviolet absorption spectra. While CASSCF excitation energies for the first two transitions either are grossly overestimated or exhibit a weak correlation with experimental data, both TD-DFT and CCSD-EOM perform very well, reproducing the spectral shifts of both the primary band and secondary band observed upon substitution. The conformational dependence of the calculated excitation energies is generally smaller than the shifts caused by substitution. [source]

Pulsed electrically detected magnetic resonance in organic semiconductors

C. Boehme
Abstract Carbon-based materials have an intrinsically weak spin,orbit coupling which imposes spin selection rules on many electronic transitions. The spin degree of freedom of electrons and nuclei can therefore play a crucial role in the electronic and optical properties of these materials. Spin-selection rules can be studied via magnetic resonance techniques such as electron,spin resonance and optically detected magnetic resonance as well as electrically detected magnetic resonance (EDMR). The latter has progressed in recent years to a degree where the observation of coherent spin motion via current detection has become possible, providing experimental access to many new insights into the role that paramagnetic centers play for conductivity and photoconductivity. While mostly applied to inorganic semiconductor materials such as silicon, this new, often called pulsed-(p) EDMR spectroscopy, has much potential for organic (carbon-based) semiconductors. In this study, progress on the development of pEDMR spectroscopy on carbon-based materials is reviewed. Insights into materials properties that can be gained from pEDMR experiments are explained and limitations are discussed. Experimental data on radiative polaron-pair recombination in poly[2-methoxy-5-(20-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) organic light emitting diodes (OLEDs) are shown, revealing that under operating conditions the driving current of the device can be modulated by spin-Rabi nutation of the polaron spin within the charge carrier pairs. From this experimental data it becomes clear that for polaron pairs, the precursor states during exciton formation, exchange interaction is not the predominant influence on the observed pEDMR spectra. [source]

FEMO modelling of optical properties of natural biopolymers chitin and chitosan

G. Luna-Bárcenas
Abstract Chitin and its derivative chitosan are widely used in the food, cosmetic, agricultural and biomedical industries because of their physicochemical and biocompatible properties. In this work, we studied thin films of both materials prepared by a solvent-cast method. The optical properties were investigated in the UV-VIS and infrared (IR) spectral regions; both biopolymers exhibit similar absorption bands characterized by two regions at 250,500 nm and at 850,1300 nm. Chitin and chitosan have practically the same parameters of a crystalline structure. We assume that these features (similarity of the structure and of the optical properties) are not just the coincidence but reflect the essential relation between the crystalline structure symmetry and the electronic transitions. The later was modeled on the basis of the quantum mechanical Free Electron Molecular Orbital approximation, using the new type, mirror-like boundary conditions. The calculations made on the basis of the known parameters of crystalline structure give reasonable agreement with the experimental spectra without any adjustable parameters. The difference between the optical properties of the two materials in the IR region reflects the difference in their molecular structure and composition. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Supramolecular order following binding of the dichroic birefringent sulfonic dye Ponceau SS to collagen fibers

BIOPOLYMERS, Issue 3 2005
B. C. Vidal
Abstract The optical anisotropies (linear dichroism or LD and birefringence) of crystalline aggregates of the sulfonic azo-dye Ponceau SS and of dye complexed with chicken tendon collagen fibers were investigated in order to assess their polarizing properties and similarity to liquid crystals. In some experiments, the staining was preceded by treatment with picric acid. Crystalline fibrous aggregates of the dye had a negative LD, and their electronic transitions were oriented perpendicular to the filamentary structures. The binding of Ponceau SS molecules to the collagen fibers altered the LD signal, with variations in the fiber orientation affecting the resulting dichroic ratios. The long axis of the rod-like dye molecule was assumed to be bound in register, parallel to the collagen fiber. Picric acid did not affect the oriented binding of the azo dye to collagen fibers. There were differences in the optical anisotropy of Ponceau SS-stained tendons from 21-day-old and 41-day-old chickens, indicating that Ponceau SS was able to distinguish between different ordered states of macromolecular aggregation in chicken tendon collagen fibers. In the presence of dichroic rod-like azo-dye molecules such as Ponceau SS, collagen also formed structures with a much higher degree of orientation. The presence of LD in the Ponceau SS-collagen complex even in unpolarized light indicated that this complex can act as a polarizer. © 2005 Wiley Periodicals, Inc. Biopolymers 78: 121,128, 2005 This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com [source]

How Does the Trans,Cis Photoisomerization of Azobenzene Take Place in Organic Solvents?

CHEMPHYSCHEM, Issue 5 2010
Giustiniano Tiberio Dr.
Abstract The trans,cis photoisomerization of azobenzene-containing materials is key to a number of photomechanical applications, but the actual conversion mechanism in condensed phases is still largely unknown. Herein, we study the isomerization in a vacuum and in various solvents via a modified molecular dynamics simulation adopting an ab initio torsion,inversion force field in the ground and excited states, while allowing for electronic transitions and a stochastic decay to the fundamental state. We determine the trans,cis photoisomerization quantum yield and decay times in various solvents (n -hexane, anisole, toluene, ethanol, and ethylene glycol), and obtain results comparable with experimental ones where available. A profound difference between the isomerization mechanism in vacuum and in solution is found, with the often neglected mixed torsional,inversion pathway being the most important in solvents. [source]

Single-Molecule Spectroscopy on a Ladder-Type Conjugated Polymer: Electron,Phonon Coupling and Spectral Diffusion

CHEMPHYSCHEM, Issue 14 2009
Richard Hildner Dr.
Abstract We employ low-temperature single-molecule spectroscopy combined with pattern recognition techniques for data analysis on a methyl-substituted ladder-type poly(para -phenylene) (MeLPPP) to investigate the electron,phonon coupling to low-energy vibrational modes as well as the origin of the strong spectral diffusion processes observed for this conjugated polymer. The results indicate weak electron,phonon coupling to low-frequency vibrations of the surrounding matrix of the chromophores, and that low-energy intrachain vibrations of the conjugated backbone do not couple to the electronic transitions of MeLPPP at low temperatures. Furthermore, these findings suggest that the main line-broadening mechanism of the zero-phonon lines of MeLPPP is fast, unresolved spectral diffusion, which arises from conformational fluctuations of the side groups attached to the MeLPPP backbone as well as of the surrounding host material. [source]

A novel spectroscopic probe for molecular chirality

CHIRALITY, Issue 3 2006
Na Ji
Abstract Recent advances in developing sum frequency generation (SFG) as a novel spectroscopic probe for molecular chirality are reviewed. The basic principle underlying the technique is briefly described, in comparison with circular dichroism (CD). The significantly better sensitivity of the technique than CD is pointed out, and the reason is discussed. Bi-naphthol (BN) and amino acids are used as representatives for two different types of chiral molecules; the measured chirality in their electronic transitions can be understood by two different molecular models, respectively, that are extensions of models developed earlier for CD. Optically active or chiral SFG from vibrational transitions are weaker, but with the help of electronic-vibrational double resonance, the vibrational spectrum of a monolayer of BN has been obtained. Generally, optically active SFG is sufficiently sensitive to be employed to probe in-situ chirality of chiral monolayers and thin films. © 2006 Wiley-Liss, Inc. Chirality [source]