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Dynamics of ultrafast internal conversion processes studied by femtosecond time-delayed photoelectron spectroscopy

Description: The authors have studied the dynamics of ultrafast internal conversion processes using femtosecond time-resolved photoionization and photoelectron spectroscopy. In hexatriene, following femtosecond pulse excitation at 250 nm, they use time-delayed photoionization to observe the formation and decay of an intermediate species on the subpicosecond time scale. With time-resolved photoelectron spectroscopy, the rapid evolution of vibrational excitation in this intermediate is observed, as electronic energy is converted to vibrational energy in the molecule. The photodynamics of cis and trans isomers of hexatriene are compared and found to be surprisingly different on the 2-3 psec time scale. These results are important for understanding the fundamental photochemical processes in linear polyenes, which have served as models for the active chromophores of many biological photosystems.
Date: August 1, 1995
Creator: Cyr, D.R. & Hayden, C.C.
Partner: UNT Libraries Government Documents Department

3-D structures and intrinsic defects in trans-polyacetylene

Description: We report first-principles local-density functional pseudopotential calculations of static and dynamic electronic structure properties of crystalline 3-D trans--(CH)/sub x/. We find a broken symmetry ground state of P2/sub 1//a symmetry with in-phase dimerizations. Using a Green's function technique, we show that the 3-D character of the electronic band edge states strongly suppresses self-trapping, destabilizing polarons and possibly bipolarons as well in perfectly ordered 3-D trans--(CH)/sub x/. 26 refs.
Date: January 1, 1989
Creator: Vogl, P. & Campbell, D.K.
Partner: UNT Libraries Government Documents Department

The workshop on conductive polymers: Final report

Description: Reports are made by groups on: polyacetylene, polyphenylene, polyaniline, and related systems; molecular, crystallographic, and defect structures in conducting polymers; heterocyclic polymers; synthesis of new and improved conducting polymers; future applications possibilities for conducting polymers; and challenges for improved understanding of properties. (DLC)
Date: January 1, 1985
Partner: UNT Libraries Government Documents Department

Theory of 3-D structure and intrinsic defects of trans-polyacetylene

Description: First-principles local-density functional pseudopotential calculations of static and dynamic electronic structure properties of crystalline 3-D trans-(CH)/sub x/ are reported. The P2/sub 1//a structure is predicted to have a lower energy than the P2/sub 1//n structure. A Green function calculation reveals that the 3-D character of the electronic band edge states destabilizes polarons and bipolarons and prevents self-trapping in perfectly ordered 3-D trans-(CH)/sub x/. 3 figs., 29 refs.
Date: January 1, 1988
Creator: Vogl, P.; Campbell, D.K. & Sankey, O.F.
Partner: UNT Libraries Government Documents Department

Controlled synthesis of polyenes by catalytic methods. Progress report, December 1, 1989--November 30, 1992

Description: A more direct approach to polyenes by the direct polymerization of acetylenes has been achieved. We were able to show that polymerization of acetylene itself can be controlled with a well- characterized alkylidene catalyst, but only if a base such as quinuclidine is present in order to slow down the rate of propagation relative to initiation. (Quinuclidine may also stabilize vinylalkylidene intermediates formed in the reaction). Unfortunately, ``living polyenes`` were no more stable than isolated polyenes, and so this approach had its limitations. Direct polymerization of acetylene by Mo(CH-t-Bu)(NAr)(O-t-Bu){sub 2} was more successful, but inherent polyene instability was still a problem. The most important result of the past grant period is the finding that dipropargyl derivatives (HC=CCH{sub 2}XCH{sub 2}C=CH; X = CH{sub 2}, C(CO{sub 2}R){sub 2}, SiR{sub 2}, etc.), which have been reported to be cyclopolymerized by various classical catalysts by as yet unknown mechanisms, are polymerized by Mo(CH-t-Bu)(NAr)[OCMe(CF{sub 3}){sub 2}]{sub 2} in dimethoxyethane. We speculate that intramolecular formation of a five-membered ring in the product of {alpha} addition is fast enough to yield another terminal alkylidene on the time scale of the polymerization reaction, while a six-membered ring is formed in a reaction involving a more reaction terminal alkylidene. Either intermediate alkylidene, but most likely the terminal alkylidene, could react with additional monomer to lead to growth of a chain having ``dangling`` triple bonds that eventually could be employed to form crosslinks.
Date: July 1, 1992
Creator: Schrock, R. R.
Partner: UNT Libraries Government Documents Department

Syntheses and Studies of Acetylenic Polymers

Description: Based on new diiodo aryl compounds a series of novel soluble polymers, poly(2,5-dialkoxy-1,4-phenyleneethynylene)s (PPE polymers) were synthesized using palladium-catalysis. The molecular weights (MW) range from 8,000 to 40,000. Properties such as absorption, fluorescence, and conductivity were studied. A PPE polymer with butoxy side chain exhibits a weak electrical conductivity ({sigma} = 10{sup {minus}3} S/cm) after doping with AsF{sub 5}. Absorption spectra in THF solution at room temperature (RT) show a maximum at 440 nm. However, absorption spectra of PPE polymers in the film state at (RT) show a maximum at 480 nm. PPE polymer-based light emitting diode (LED) devices have been prepared; greenish light from these LED devices can be observed. Poly(ethynylene-p-arylene-ethynylene-silylene)s were synthesized through the same palladium-catalyzed polymerization; MWs are between 6,000 and 82,000. Absorption and fluorescence were studied. Some of these polymers exhibit thermotropic liquid crystalline properties. In addition, nonlinear optical properties were briefly examined. Poly(silylene-ethynylene) homopolymers as well as alternating copolymers were synthesized through a novel palladium-catalyzed polymerization; MWs range from 56 {times} 10{sup 3} to 5.3 {times} 10{sup 3}. Thermal stability of these was also investigated; char yields range from 56 to 83%. One of these polymers exhibits thermotropic liquid crystalline properties.
Date: March 3, 1994
Creator: Ding, Yiwei
Partner: UNT Libraries Government Documents Department

Coulomb correlations and optical gap in polyacetylene

Description: A model including both electron-phonon coupling (as in the SSH Hamiltonian) and electron-electron interactions (on-site term U, nearest-neighbor term V) is treated within the variational scheme of Gutswiller. It is shown that for weak electron-phonon coupling the primary effect is a bond-order wave induced by electronic correlation, whereas the lattice dimerization is a secondary effect. Correspondingly the optical gap is mainly due to electronic correlation.
Date: January 1, 1986
Creator: Baeriswyl, D. & Maki, K.
Partner: UNT Libraries Government Documents Department

Condition Monitoring of a Thermally Aged HTPB/IPDI Elastomer by NMR CP Recovery Times

Description: A hydroxy-terminated polybutadiene (HTPB)/isophorone diisocyanate (IPDI) elastomer is commonly used as propellant binder material. The thermal degradation of the binder is believed to be an important parameter governing the performance of the propellant. The aging of these binders can be monitored by mechanical property measurements such as modulus or tensile elongation. These techniques, however, are not easily adapted to binder agents that are dispersed throughout a propellant. In this paper the authors investigated solid state NMR relaxation times as a means to predict the mechanical properties of the binder as a function of aging time. {sup 1}H spin-lattice and spin-spin relaxation times were found to be insensitive to the degree of thermal degradation of the elastomer. Apparently these relaxation times depend on localized motions that are only weakly correlated with mechanical properties. A strong correlation was found between the {sup 13}C cross-polarization (CP) NMR time constant, T{sub cp}, and the tensile elongation at break of the elastomer as a function of aging time. A ramped-amplitude CP experiment was shown to be less sensitive to imperfections in setting critical instrumental parameters for this mobile material.
Date: July 24, 2000
Creator: ASSINK,ROGER A.; LANG,DAVID & CELINA,MATHIAS C.
Partner: UNT Libraries Government Documents Department

Dynamics of H sub 2 elimination from unsaturated hydrocarbons

Description: State-of-the-art laser and molecular beam techniques are used to study the dynamics of H{sub 2} elimination from 1,4-cyclohexadiene and ethylene. Information on the transition state configurations and the dynamics of the dissociation processes for these reactions is reported. 152 refs., 54 figs., 12 tabs.
Date: February 1, 1991
Creator: Cromwell, E.F.
Partner: UNT Libraries Government Documents Department

Ultrafast spectroscopic studies of the photophysics of phenyl- substituted butadienes in liquids

Description: The transient absorption decay times of tetraphenylbutadiene (TPB) and tetraphenylmethylbutadiene (TPMB) are measured as a function of solvent viscosity and of probe wavelength. The TPB spectra suggest that after excitation, TPB relaxes to the bottom of the excited state well where it relaxes radiatively to the ground state surface. TPMB transient absorption spectra taken using different probe wavelengths decay on different timescales. 71 refs., 38 figs., 6 tabs.
Date: August 1, 1991
Creator: Hoff, R.L.
Partner: UNT Libraries Government Documents Department

Molecular and supramolecular orientation in conducting polymers

Description: Intrinsic anisotropy in electrical and optical properties of conducting polymers constitutes a unique aspect that derives ..pi..-electron delocalization along the polymer backbone and from the weak inter-chain interaction. To acquire such an intrinsic property, conducting polymers have to be oriented macroscopically and microscopically (at the chain level). A review of the various techniques, including stretch-alignment of the polymer and of precursor polymers, polymerization in ordered media, i.e., in a liquid crystal solvent, and synthesis of liquid crystalline conducting polymers will be given. 29 refs.
Date: January 1, 1987
Creator: Aldissi, M.
Partner: UNT Libraries Government Documents Department

Statics and adiabatic dynamics of nonlinear excitations in defected polyacetylene

Description: Within the Su, Schrieffer and Heeger model we have calculated, both analytically and numerically, the effects of model impurities on the electronic structure and lattice distortion of trans-polyacetylene. We find that the electron-phonon coupling may result in a fundamental alteration of the impurity level location. In simulated photoexcitation experiments on the defected system we find that, in addition to the kinks and breathers produced in the undefected system, ''trapped kinks'', excitons and polarons may also be produced. We suggest that the polarons produced in this novel way may be unusually stable and play an important role in hopping conduction mechanisms.
Date: January 1, 1985
Creator: Phillpot, S.R.; Baeriswyl, D.; Bishop, A.R. & Lomdahl, P.S.
Partner: UNT Libraries Government Documents Department

Production of super-smooth articles

Description: Super-smooth rounded or formed articles made of thermoplastic materials including various poly(methyl methacrylate) or acrylonitrile-butadiene-styrene copolymers are produced by immersing the articles into a bath, the composition of which is slowly changed with time. The starting composition of the bath is made up of at least one solvent for the polymer and a diluent made up of at least one nonsolvent for the polymer and optional materials which are soluble in the bath. The resulting extremely smooth articles are useful as mandrels for laser fusion and should be useful for a wide variety of other purposes, for example lenses.
Date: May 29, 1981
Creator: Duchane, D.V.
Partner: UNT Libraries Government Documents Department

[Synthetic and mechanistic investigation of olefin polymerization catalyzed by early transition metal compounds]

Description: During the second year we continued to prepare and characterize organoyttrium and organoscandium compounds for use as catalysts for polymerizing simple olefins and diolefins. Simple, one-component systems are being pursued, suitable for chain initiation, propagation, and termination studies. This document is divided into: dicarbollide derivatives of scandium as potential catalysts; design, synthesis, and characterization of the first isospecific [alpha] olefin polymerization catalysts; polymerization of [alpha] olefins and 1,5- hexadiene using organoscandium catalysts; and attempted preparations of diastereomeric Nb and Ta olefin/hydride and olefin/alkyl derivatives.
Date: January 1, 1993
Creator: Bercaw, J.E.
Partner: UNT Libraries Government Documents Department

Photochemistry of 1 and 2-(2-methylphenyl)-1,6-heptadiene. [4a-methyl-1,2,3,4,4a,9,10,10a-octahydrophenanthrene]

Description: In an attempt to synthesize partially saturated phenanthrene derivatives by an intramolecular Diels-Alder reaction between a photochemically produced o-xylylene (diene) and a tethered dienophile, it was found that 1 and 2 underwent a photochemically allowed (2 + 2) cycloaddition. Irradiation of 1 gave 6-(2-methylphenyl)bicyclo(3.2.0)heptane in 86% yield. Upon irradiation of 2, a benzvalene rearrangement of 2 first took place, producing the meta isomer 2-(3-methylphenyl)-1,6-heptadiene, followed by a (2 + 2) photocycloaddition giving 1-(3-methylphenyl)bicyclo(3.2.0)heptane in 15% yield. Direct irradiation of 2-(3-methylphenyl)-1,6-heptadiene gave the same bicyclo derivative as 2 in 34% yield. Examination of the fluorescence spectra of 1 and 2 in comparison with 1-(2-methylphenyl)propene and 2-(2-methylphenyl)-1-butene, respectively, has shown that 1 may be biased toward (2 + 2) cycloaddition where 2 is not biased toward (2 + 2) photocycloization. Attempts to produce 4a-methyl-1,2,3,4,4a,9,10,10a-octahydrophenanthrene by an intramolecular Diels-Alder reaction of the o-xylylene produced by irradiation of 3 will also be described.
Date: January 1, 1982
Creator: Barrows, R.D. & Hornback, J.M.
Partner: UNT Libraries Government Documents Department

Solitons and polarons in quasi-one dimensional conducting polymers and related materials

Description: In recent years it has become increasingly appreciated that fundamentally nonlinear excitations - solitons - play an essential role in an incredible variety of natural systems. These solitons, which frequently exhibit remarkable stability under interactions and perturbations, often dominate the transport, response, or structural properties of the systems in which they occur. In this article, we present an introduction to the solitons that occur in quasi-one-dimensional conducting polymers (synmetals) and related systems. The relevance of this subject to molecular electronic devices is twofold. First, many of these materials have molecular structures similar to possible prototype molecular switches. Second, to understand in detail how a molecular electronic device could work, it is essential to have a broad perspective on the nature of possible excitations in a variety of natural and synthetic molecular materials. 51 references.
Date: January 1, 1983
Creator: Campbell, D.K.
Partner: UNT Libraries Government Documents Department

Sensitization and quenching in the conversion of light energy into chemical energy. Progress report, February 1, 1980-January 31, 1981

Description: Extensive data from Stern-Volmer, Lamola-Hammond, and Ilenda-Daughenbaugh-Cristol quenching kinetics have now been accumulated on photosolvolysis in t-butyl alcohol for benzyl chloride and a number of meta and para substituted benzyl chlorides. Evidence for the existence of two triplet states, one relatively short-lived (tau 0-2 nsec) which gives solvolysis product and a second, relatively long-lived (tau 5-26 nsec), which does not give product, but instead is energy wasting, has been accumulated. The system, p-acetobenzyl chloride, has been investigated in detail. A method for quenching of singlet states for measurement of singlet lifetimes in the 100 picosecond to nanosecond range is being developed. Preliminary work on benzyl acetate photosolvolysis has been conducted. Some work on the goemetrical requirements for intra-molecular excitation transfer in bichromophoric molecules has been conducted. Several dienes related to norbornadiene have been prepared and preparative photoisomerizations to quadricyclene analogues have been carried out. Considerable attention has been given to certain di-..pi..-methane rearrangements, work on most of which is still in progress. One system, the ethyl ester of dibenzobarrelene-7-carboxylic acid, has been scrutinized in detail.
Date: September 1, 1980
Creator: Cristol, S.J.
Partner: UNT Libraries Government Documents Department

Femtosecond dynamics of fundamental reaction processes in liquids: Proton transfer, geminate recombination, isomerization and vibrational relaxation

Description: The fast excited state intramolecular proton transfer of 3-hydroxyflavone is measured and effects of external hydrogen-bonding interactions on the proton transfer are studied. The proton transfer takes place in {approximately}240 fsec in nonpolar environments, but becomes faster than instrumental resolution of 110 fsec in methanol solution. The dynamics following photodissociation of CH{sub 2}I{sub 2} and other small molecules provide the first direct observations of geminate recombination. The recombination of many different photodissociating species occurs on a {approximately}350 fsec time scale. Results show that recombination yields but not rates depend on the solvent environment and suggest that recombination kinetics are dominated by a single collision with surrounding solvent cage. Studies of sterically locked phenyl-substituted butadienes offer new insights into the electronic structure and isomerization behavior of conjugated polyenes. Data show no simple correlation between hinderance of specific large amplitude motions and signatures of isomerizative behavior such as viscosity dependent excited state lifetimes, implying that the isomerization does not provide a suitable for simple condensed phase reaction rate theories. The spectral dynamics of a photochromic spiropyran indicate that recombination, isomerization and vibrational relaxation all play important roles in photoreactivity of complex molecules. The interplay of these microscopic phenomena and their effect on macroscopic properties such as photochromism are discussed. All the results indicate that the initial steps of the photochromic reaction process occur extremely rapidly. Laser system and computer codes for data analysis are discussed.
Date: November 1, 1992
Creator: Schwartz, B. J.
Partner: UNT Libraries Government Documents Department

1. Mono([8]annulene)Uranium(4) half-sandwich complexes, 2. Novel syntheses of symmetrically substituted cyclooctatetetraenes

Description: A reproducible, high-yield synthesis of mono([8]annulene)uranium(4)dichloride (1) is reported, along with the X-ray crystal structural of the bis(pyridine) adduct. Metathesis reactions of the half-sandwich complex 1 with a variety of simple alkyl and alkoxy reagents failed to generate any isolable mono-ring complexes. Reactions of 1 with polydentate, delocalized anions did produce stable derivatives, including mono([8]annulene)uranium(4)bis(acetylacetonate) (4). An X-ray crystal structure of 4 is reported.
Date: October 1, 1991
Creator: Boussie, T. R.
Partner: UNT Libraries Government Documents Department

Controlled synthesis of polyenes by catalytic methods. Progress report, December 1, 1992--November 30, 1993

Description: Four studies are reported: living cyclopolymerization of diethyl dipropargylmalonate by Mo(CH-t-Bu)(NAr)[OCMe(CF{sub 3}){sub 2}]{sub 2} in dimethoxyethane, effect of chain length on conductivity of polyacetylene, nonlinear optical analysis of a series of triblock copolymers containing model polyenes, and synthesis of bifunctional hexafluoro-t-butoxide Mo species and their use as initiators in ROMP reactions.
Date: December 1, 1993
Creator: Schrock, R. R.
Partner: UNT Libraries Government Documents Department

Synthesis and chemistry of yttrium and lanthanide metal complexes

Description: The objective of this research project is to determine the special features of complexes of yttrium and the lanthanide metals which will allow the design and synthesis of materials with unique chemical, physical, and catalytic properties. Past studies of yttrium and lanthanide metal alkyl and hydride complexes stabilized by cyclopentadienyl co-ligands have shown that a substantial, often singular, organometallic chemistry is available via these metals. More extensive utilization of the chemical opportunities available through yttrium and the lanthanides would be possible, however, if stabilizing ancillary ligand systems less sensitive to oxidation and protonolysis than cyclopentadienides could be developed. Alkoxide ligands are attractive in this regard and our recent research had focused on alkoxides and the special opportunities they can provide to these metals. 6 refs., 10 figs.
Date: September 1, 1991
Creator: Evans, W.J.
Partner: UNT Libraries Government Documents Department

Organic photochemical storage of solar energy. Progress report, March 1, 1978-January 31, 1979

Description: Photosensitization mechanisms for driving energy storing reactions of readily available organic compounds have been studied. Aromatic sensitizers were used as complexing agents for a series of non-conjugated dienes which are capable of valence isomerization. Diene exciplexes, shown to be stabilized by electron donor-acceptor interaction, led to photoisomers with high chemical and quantum efficiency in two of the cases studied. For other systems, sensitizer-diene photoaddition reactions were observed. Exciplex isomerization of quadricyclanes and hexamethyldewarbenzene was studied. Factors determining the efficiency of rearrangement of excited complexes were examined including the role of exciplex excitation energy in controlling the novel adiabatic exciplex isomerization of hexamethyldewarbenzene. With triplet photosensitizers visible light (to 520 nm) was used to bring about an energy storing valence isomerization of a diester derivative of norbornadiene. High quantum yields (0.6) were measured at the longest wavelengths yet utilized for this type of isomerization. The quantum efficiency for isomerization using a very low energy triplet sensitizer was significantly enhanced at slightly elevated temperatures, suggesting that thermal energy (in amounts present in solar collector media) can be an aid in driving energy storing photo-reactions.
Date: February 1, 1979
Creator: Jones, G. II
Partner: UNT Libraries Government Documents Department

High pressure photophysics of organic molecules

Description: High pressure spectroscopic studies on several classes of organic compounds were made both in fluid solution (to 10 kbar) and in polymeric media (to 40 kbar). The first three studies were conducted in fluid solution and concern the effect of solvent viscosity on the nonradiative deactivation rates from electronically excited states. Pressure was utilized to attain high viscosities in organic solvents at room temperature. The primary experimental technique used was fluorescence emission spectroscopy. In the fourth and last study observations were made both in fluid solution and in plastic films. The focus of this study was the effect of pressure on the solvent-chromophore dispersion interaction in several polyenes and the concomitant changes in both the radiative and non-radiative rates from the excited states. Extensive use was made of fluorescence lifetime measurements and excitation spectra. 105 references.
Date: January 1, 1979
Creator: Brey, L. A.
Partner: UNT Libraries Government Documents Department