Short Conjugation Length Research Articles

Finite field study on nonlinear optical properties of substituted ketene N, S-acetals

The nonlinear optical properties of title compounds calculated by AM1/finite field (AM1/FF) approach are discussed based on their crystal parameters. The origins of large molecular first hyperpolarizahilitiesβ and good transparency of the compounds are explained well through the aromatic character enhancement in the excited state and the short conjugation length in the ground state. Results show that the difference ofβ values obtained by two experimental methods (Solvatochromic and Hyper Rayleigh Scattering) is due to the molecular quasi-two-dimension character and the octupolar components’ contribution. The calculation has also demonstrated that the change ofγ xxx caused by different donor substituents is linearly related with the Hammett substituent constant σ+.

Structure and properties of polyaniline films prepared via electrochemical polymerization. I: Effect of pH in electrochemical polymerization media on the primary structure and acid dissociation constant of product polyaniline films

Electrochemical polymerization of aniline was conducted in constant current mode at different pH ranging from 0.2 to 3.7 in an ITO glass electrode-quartz cell assembly for in situ monitoring of the changes in potential, amount of charge passed through, and u.v.-visible absorption (u.v.-Vis) spectra during polymerization. u.v.-Vis spectra of product polyaniline (PAn) films were determined in different pH solutions to evaluate the apparent acid dissociation constants (p Ka) app from plots of log{ (Abs HAX − Abs obs ) (Abs obs − Abs A ) versus pH, where Abs obs, Abs HAX, and Abs A are the observed absorption strength and those of the fully protonated and deprotonated species, respectively, for a given wavelength light. In PAn films prepared at low pH (< 1.0), emeraldine form PAn chains with relatively long average conjugation length but broad distribution were produced, while in those prepared at high pH (> 1.5), PAn chains with short conjugation length were produced and later deteriorated as the polymerization proceeded, because the polymerization potential eventually exceeded a critical value (+ 0.8 V) beyond which degradation of produced PAn chains took place.

Reversible discoloration effects in the photoaging of poly(vinyl chloride)

AbstractIt is often observed that the photochemical degradation of titanium dioxide pigmented PVC leads to a latent discoloration that is revealed only during a further period of storage of the aged material in the dark. This effect is reversible, and photobleaching can be provoked by a new irradiation of the polymer. This behavior can be attributed to the formation of polyenic sequences with a short conjugation length, which present an absorption below 400 nm. The screen effect of the pigment protects these polyenes against photooxidation, which permits these polyenes to accumulate in the degraded polymer. In the absence of light, these polyenes can be thermally isomerized, so leading to isomer forms absorbing at longer wavelength above 400 nm. This absorption is responsible for a yellowing of the polymer. The isomerization is perfectly reversible and a further irradiation provokes the conversion to the isomer form absorbing below 400 nm, leading then to a photobleaching of the sample. These experiments can be repeated many times before any distortion occurs.

Large polaron hopping under high electric fields

Current voltage characteristics of organic light emitting diodes cannot be understood without the knowledge of the field dependent mobility in the organic material. This issue is addressed in the present paper within the framework of a large polaron hopping theory under high electric fields. We find that the hopping rate is very sensitive to the conjugation length. At short conjugation lengths the mobility is essentially activated as a function of the temperature and the field , like in small polaron hopping. At long conjugation lengths, it exhibits a Fowler Nordheim like dependence with the field, and a very weak temperature dependence at high fields.

Conjugation lengths in long polyene chains

Since the low energy absorption spectrum of a linear polyene is dominated by the strongly allowed 1 1A g → 1 1B u transition (S 0 → S 2), it can be reconstructed for any chain length if the 0-0 band excitation energy, the band profile, and intensity are known. This makes it possible to use information developed in high resolution experiments on well-defined linear polyenes to extract the distribution of effective conjugation lengths in an ensemble of long molecular chains from solution absorption spectra. A previously published analysis of absorption spectra of solutions of long polyenes showed that the longer the molecular chain, the more short conjugation lengths contribute to the conjugation length distribution. This is consistent with a simple statistical model which is reviewed here and used to calculate ensemble averages for powers of conjugation length.

Solvatochromism on regiochemically substituted poly(Hexylthiophenes)

Poly (3, 3′'-dihexylterthiophene) and poly (3′,4′-dihexylterthiophene) have been synthesized and studied with respect to their solvatochromic properties. While both polymers in chloroform solution have maximum absorption at approximately the same wavelength, they behave differently with respect to changes observed on their UV-visible spectra when the quality of the solvent is changed. A microscopic model for the solvation of both polymers has been developed based on semi-empirical quantum chemical calculations. The results are consistent with a high degree of conformational disorder in the 3′, 4′-substituted polymer even in the solid state, leading to chains of short conjugation length, and thus less sensitive to solvation effects. In 3,3′'-substituted polythiophene on the other hand, the substituents impose smaller torsional barriers, similar to the ones found in regioregular poly (3-hexylthiophene). A rod-to-coil like transition is thus expected to occur as a function of solvent quality.

Polyacetylene in Diamond Films Evidenced by Surface Enhanced Raman Scattering.

Trans-polyacetylene is evidenced by surface enhanced Raman scattering (SERS) in diamond films prepared by hot filament assisted chemical vapor deposition from C${\mathrm{H}}_{4}$ and ${\mathrm{H}}_{2}$. The analysis of the Raman spectra for different excitation energies shows that the deposited trans-polyacetylene is composed of chains exhibiting short conjugation lengths. Together with polyacetylene, diamond crystals of nanometric dimension are evidenced by both SERS and high resolution transmission electron microscopy.

Experimental determination of conjugation lengths in long polyene chains

Since the low energy absorption spectrum of a linear polyene is dominated by the strongly allowed 1 1Ag→1 1Bu transition (S0 to S2), it can be reconstructed for any chain length if the 0–0 band excitation energy, the band profile, and intensity are known. From this premise, this paper uses information developed in high resolution experiments on well-defined linear polyenes to extract the distribution of effective conjugation lengths from absorption spectra of solutions of long linear polyenes. These distributions are found to be dominated by short conjugation lengths.

Poly(alkyl thiophene-3-carboxylates). Synthesis and Characterization of Polythiophenes with a Carbonyl Group Directly Attached to the Ring

Polythiophenes with electron-withdrawing ester groups attached at the 3-position, namely poly(hexyl thiophene-3-carboxylate) (1a) and poly(octyl thiophene-3-carboxylate) (1b), have been synthesized using the Ullmann reaction with copper powder in DMF. Characterization of the polymers includes IR, 1 H and 13 C NMR, and UV-vis spectroscopy as well as TGA and molecular weight studies. The polymers also show red-orange and red fluorescence, respectively. The same polymers prepared using Ni(0) instead of Cu gave polymer with comparable molecular weights but higher polydispersity, which was not as uniform as shown by NMR spectroscopy and which had shorter conjugation lengths, as shown by blue-shifted UV-vis absorption and fluorescence maxima.

The effect of stereoregularity on the structure of poly(octylthiophene): An x-ray diffraction study

The structural properties of soluble Poly(3,3′-dioctyl-2,2′-bithiophene) (PDOBT), a stereoregular analogous of Poly(3-octylthiophene) (POT), have been studied by X-ray diffraction and UV-VIS absorption spectroscopy. The absorption spectra have been acquired in the temperature range 20–170°C. The results show that PDOBT is partial crystalline with the polymer chains packing in a parallel manner. The well defined head-to-head, tail-to-tail arrangement of th monomer units along the polymer chain leads to a crystalline structure differing considerably from the one of POT. Adjacent head-to-head coupled thiophene rings are twisted out of plane, leading to a short conjugation length as manifest by the absorption maximum, which is blueshifted relative to POT. In contrast to POT, PDOBT does not show any thermochromic effect. This agrees with our new structural model for PDOBT.

Camphorsulfonic Acid Fully Doped Polyaniline Emeraldine Salt: In situ Observation of Electronic and Conformational Changes Induced by Organic Vapors by an Ultraviolet/Visible/Near-Infrared Spectroscopic Method

It is shown that thin films of camphorsulphonic acid-doped polyaniline spun from chloroform or NMP have a coil-like conformation and shorter conjugation length while those films spun from m-cresol have an expanded coil-like conformation and longer conjugation length. The coil-like conformation can be irreversibly converted to the expanded coil-like conformation by exposure to m-cresol vapor at room temperature

Thermal and spectroscopic properties of the polydiacetylene derived from docosa‐10,12‐diyne‐1,22‐diylbis(phenylacetate)

AbstractPolydiacetylene (2) of varying molecular weight has been formed by UV‐ or 60Co γ‐irradiation of the diacetylene PhCH2CO2(CH2)9‐CCCC(CH2)9O2CCH2Ph (3), with conversion of 30−&gt;95%. Samples of (2) with differing physical properties have been studied, in the bulk phase, by CPMAS 13C NMR and VIS/UV spectroscopy and by DSC; complementary spectroscopic data on solutions of (2), including fluorescence studies, are also reported. Spectroscopic studies support the presence of an extended planar conjugated backbone in ‘as polymerised’ blue–black samples of (2), from which various forms of red‐coloured polymer, with shorter conjugation lengths, can be obtained. Studies of the thermal behaviour indicate the existence of several differing thermal transitions for these materials and the results are discussed.

Third-order nonlinear optical properties of polyazine derivatives

Experimental and theoretical results on third-order nonlinear optical susceptibility χ (3) and molecular hyperpolarizability (γ) of oligomeric and polymeric azines are reported for the first time. Molecular hyperpolarizabilities (γ) of well-defined conjugation length polyazines containing terminal electron-donating amino groups were calculated by employing the ab initio Coupled Perturbed Hartee-Fock with the 6-31G basis set. An azine monomer showed a molecular hyperpolarizability of the order of 14.7×10 −36 esu, which is the largest γ value ever reported for such a short π-electron conjugation length. Molecular hyperpolarizability increases significantly as the length of the π-electron delocalization increases. The γ values for the dimer, trimer, tetramer, and pentamer azine derivatives were 79.0×10 −36, 228.3×10 −36, 575.9×10 −36, and 903.0×10 −36 esu, respectively. Third-order nonlinear optical susceptibilities χ (3) of polyazine derivatives were determined by third-harmonic generation (THG) measurements. The χ (3)(-3ω; ω, ω, ω, ω) values of an azine monomer and a pentamer were 1.39×10 −13 and 15.3×10 −13 esu, respectively. On the other hand, the χ (3) of a polyazine possessing statistically alternating methyl and propyl substituents on the carbon atoms along the chain was found to be 8.0×10 −12 esu at 1.5 μm. The interesting third-order nonlinear properties of polyazines presumably originate from the multiple quantum-well structure of the main π-electron conjugated backbone.

Optical probes of polydiethynylsilanes

We have applied a variety of optical probe techniques for studying the ground-state properties, photoexcitations, and optical nonlinearities of the novel class π-conjugated polymers incorporating Si (or Ge), the polydiethynylsilanes (PDES): [(HCC) 2SiR 2] x , where R = alkyl, phenyl, etc. Our measurements imply that the PDES backbone structure consists of units of four-membered rings alternately connected to CH and SiR 2 units; thus it can be regarded as an almost degenerate ground state (AB) x with a band gap at 2 eV. The PDES are therefore the second class of conducting polymers that can support soliton excitations. Using the photomodulation spectroscopy technique, we have found in PDES photogeneration of charged solitons similar to that in trans-(CH) x , as predicted by theory. However, due to the short average conjugation length in PDES films, the soliton energy levels are split by end-effects confinement. We have also found at 2 eV χ (3) = 3 × 10 − 9 esu, which decays in a record time of 135 fs, followed by a slower decay component of 750 fs, without a long tail even at higher laser intensities.

Simple dependence of polarization properties of Raman spectrum of oriented trans-(CH) x

In this communication we report a experimental study of the sample dependence of polarization properties on oriented trans-(CH) x films. An analysis of these results is made in the framework of a conjugation length model. A significant increase of the depolarization ratio (YY/ZZ) (Z direction of the chains alignment, YZ plane of the chain) of the Raman lines for conjugation length smaller than 30 is observed. this result is explain by taking into account the properties of the electric dipole moments of short conjugation length segments.

Preparation and properties of highly conducting poly(arylene vinylenes)

Highly electroconductive poly(p-phenylene vinylene), PPV, films were successfully prepared by preventing a side reaction in pyrolysis of a soluble precursor polymer. The conductivity of PPV film reached 10 4 S/cm by H 2SO 4 doping because of decreasing sp 3 defects. The number of conjugated units in the new PPV films was estimated to be about 10 3 from the molecular weight dependence of the conductivity. Other poly(arylene vinylenes) were indicated to have shorter conjugation length than PPV. PPV fibers were also prepared from the soluble precursor polymer by a dry-spinning method. The stretched fiber of PPV showed a high conductivity of 2.7×10 4 S/cm by H 2SO 4 doping and high tensile strength of 200 kg/mm 2 as it was.

Structural determination of a semiconductive tetramer of aniline by IR, UV-visible, ESR, XPS and mass spectroscopy techniques

A tetramer of aniline was obtained by the chemical oxidation of aniline in sulphanilic acid. The analytical techniques of infrared, UV-visible and mass spectroscopy show formation of a relatively short conjugation length oligomer (tetraaniline). The electronic structure elucidated by X-ray photoelectron spectroscopy reveals that the tetraaniline skeleton contains both benzenoid, [-NH-C6H4-NH-], and quinoid, [-N=C6H4=N-], phenyl rings. In the oxidized state, the tetraaniline backbone containing alternate benzenoid and quinoid phenyl units structurally similar to emeraldine was not evident. Electron spin resonance and electrical conductivity data show localized charge carriers in the backbone. Experimental results predicting the molecular structure of tetraaniline are discussed.

In situ spectroscopic investigations of the growth, electrochemical cycling and overoxidation of polypyrrole in aqueous solution

In this paper, we present results from in situ ellipsometry and Fourier Transform Infra Red spectroscopy on the growth, electrochemical cycling and overoxidation of polypyrrole in 1M NaClO 4. The data show that the initial charge-carrying species are polarons. At higher potentials, bipolarons are also formed, and co-exist with the polarons; strongly suggesting that there is no appreciable energy gain in forming a bipolaron (with respect to two polarons). Both species have narrow, well-defined conjugation lengths, with bipolarons being ca 9 monomer units smaller than polarons. The oxidation of the polypyrrole results in the expulsion of solvated protons from the film, via the deprotonation of water; in addition, oxidation of the film is accompanied by a reversible 30% reduction in its thickness, a quite unexpected result in a view of the necessity for charge compensation through ingress of solvated anions. Ellipsometry shows that the major changes in L occur between −0.2 V and +0.2 V, ie at potentials associated with strong growth in bipolaron rather than polaron concentration. A model is postulated in which the major cause of the contraction is electrostriction, associated with the formation of bipolarons. That is, in turn, initiated by the generation of a critical concentration of polarons. The kinetically fast proton expulsion that takes place in the early part of the oxidation process is superseded at higher potentials by the thermodynamically-favoured influx of anions. Overoxidation of polypyrrole in aqueous solution results in the formation of pyrrolinones, with a range of short conjugation lengths, and CO 2 (from the complete oxidation of end-groups); as well as an influx of solvent into the film, which expands. Eventually, the pyrrolinone segments are also oxidized to CO 2, but the mechanical integrity of the polymer is maintained.

Conjugation length dependent transport in conducting polymers from a resistor network model

The effect of conjugation length (L) upon electronic conductivity components of conducting polymers is derived using a generalized resistor network model. Results are obtained for polymers which contain a statistical distribution of defects which limit conjugation, as well as for regular copolymers which have a fixed phase relationship between interruptions in conjugation on neighboring chains. The short-conjugation-length limits of the derived equations are identical with those previously obtained by evaluating molecular aspects of charge transfer. More specifically, when interchain transport fully limits both chain-direction conductivity (σ1) and an orthogonal conductivity (σ2), the calculated electrical anisotropy is σ1/σ2=L2/6d2F, where d is the interchain separation in a hopping direction, and the product d2F is the mean square average projection of the interchain vector on the electric field direction. The present analysis extends predictive capabilities over the entire range from short conjugation lengths to infinite conjugation lengths. For long conjugation lengths terminated by effectively infinite barrier defects, σ1/σ1(∞) and σ2/σ2(∞) are calculated from the parameters which define polymer structure and, for the former ratio, the ratio of infinite chain conductivities parallel [σ1(∞)] and orthogonal [σ2(∞)] to the chains. A general relationship, appropriate for a still wider range of conjugation lengths, is derived between σ1/σ1(∞) and [σ2/σ1(∞)]1/2(L/d)/F1/2, where the geometrical parameter F is of order unity in directions of high σ2. Using this relationship for a polymer of known structure, the chain-direction electrical conductivity in the infinite-chain limit can be derived from measurements of σ1, σ2, and average conjugation length. Good agreement is obtained between the calculated and observed dependence of conductivity upon conjugation length for available polymers, in which bulk conductivity is limited by interchain hopping.

Refined analysis of Raman excitation profiles for cis‐(CH)x and ‐(CD)x polymers

AbstractModel calculations are reported aimed at interpreting the Raman excitation profiles observed by Lichtmann et al. They refine earlier calculations by Siebrand and Zgierski based on displaced harmonic oscillators. A distribution of conjugation lengths is introduced and the dependence of the model parameters, in particular the resonant state energy, on this length is investigated. This, together with an allowance for vibrational frequency changes and non‐Condon effects, yields a much improved fit to the observed profiles. The results suggest a narrow distribution of relatively short conjugation lengths (⩽20 double bonds), contrary to most earlier reported distributions based on the dispersion of Raman bands, but comparable to a distribution based on isomerization kinetics.