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Papers by Michael Winokur
Chemical Physics of Intercalation II, 1993
Conducting polymers are best known for their intriguing electronic and optical properties.1, 2 Wh... more Conducting polymers are best known for their intriguing electronic and optical properties.1, 2 What is perhaps not so well appreciated is that these materials also may be considered excellent candidates for investigations of intercalation in a “low dimensional” guest-host system. This opportunity results from the anisotropy intrinsic to conducting polymers in combination with their ability to form heavily-doped charge-transfer complexes.
Physical Review B, 1996
The short-range order within amorphous polyaniline ͑a-PANI͒ has been studied through use of a pai... more The short-range order within amorphous polyaniline ͑a-PANI͒ has been studied through use of a pairdistribution-function analysis of x-ray scattering data in combination with quantitative refinements of model structure functions. Both the emeraldine base and salt ͑ES͒ forms of a-PANI are less planar than their crystalline counterparts. Direct evidence for appreciable interchain hydrogen bonding is seen. A probable position of the dopant ions in hydrohalogenic-acid-doped ES has also been ascertained. The intrachain structure and the ion location are such that the effective-conjugation length is lowered and the chain-to-chain packing is frustrated. These characteristics hinder charge transport through the polymer matrix and prevent evolution towards a metallic state. ͓S0163-1829͑96͒50442-X͔
J Luminesc, 2007
We investigate localized exciton states in π-conjugated polymers with finite torsion. The localiz... more We investigate localized exciton states in π-conjugated polymers with finite torsion. The localized states are associated with a perturbed transfer integral for which the magnitude of cosine of the torsion angle exceeds the magnitude of the corresponding cosine of the unperturbed system. The localized state energy is calculated as a function of the ratio of the perturbed to unperturbed transfer integrals. Particular attention is paid to the optically active symmetric localized states, and the effective oscillator strength, or square of the absolute magnitude of the transition dipole moment, is calculated as a function of the energy. The relation of the theory to recent optical studies of poly(di-noctylfluorene) (PF8) is discussed.
Aps March Meeting Abstracts, Mar 1, 2006
Semi-crystalline films of various sigma-conjugated poly(di-n-alkylsilanes) (i.e., hexyl, octyl, d... more Semi-crystalline films of various sigma-conjugated poly(di-n-alkylsilanes) (i.e., hexyl, octyl, decyl and dodecyl) have been investigated using direct structural studies (i.e., x-ray diffraction and modelling) in combination with optical spectroscopy measurements (e.g., UV-vis absorption and photoluminescence). Thermal-quenching studies of these alkyl-substituted polysilanes are found to generate a complex sequence of structural phase transitions mediated by the formation of transient and metastable
Fundamental Materials Research
Physical Review B, 2003
X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to st... more X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to study the well-known order-disorder transition (ODT) to the β phase in poly(9,9-(di-n,noctyl)fluorene)) (PF8) thin film samples through combination of time-dependent and temperaturedependent measurements. The ODT is well described by a simple Avrami picture of one-dimensional nucleation and growth but crystallization, on cooling, proceeds only after molecular-level conformational relaxation to the so called β phase. Rapid thermal quenching is employed for PF8 studies of pure α phase samples while extended low-temperature annealing is used for improved β phase formation. Low temperature PL studies reveal sharp Franck-Condon type emission bands and, in the β phase, two distinguishable vibronic sub-bands with energies of approximately 199 and 158 meV at 25 K. This improved molecular level structural order leads to a more complete analysis of the higher-order vibronic bands. A net Huang-Rhys coupling parameter of just under 0.7 is typically observed but the relative contributions by the two distinguishable vibronic sub-bands exhibit an anomalous temperature dependence. The PL studies also identify strongly correlated behavior between the relative β phase 0-0 PL peak position and peak width. This relationship is modeled under the assumption that emission represents excitons in thermodynamic equilibrium from states at the bottom of a quasi-one-dimensional exciton band. The crystalline phase, as observed in annealed thin-film samples, has scattering peaks which are incompatible with a simple hexagonal packing of the PF8 chains.
Conductive Polymers and Plastics, 1999
The local structural ordering of polyaniline (PANI) as emeraldine (i.e., at 50% oxidation) in bot... more The local structural ordering of polyaniline (PANI) as emeraldine (i.e., at 50% oxidation) in both its base (EB) and salt (ES) forms has been studied using x-ray diffraction (XRD) techniques and these data reinforce the general belief that this local structure is extremely sensitive to the specific processing history. PANI powders precipitated as ES immediately following synthesis lead to reasonably crystalline materials whereas EB films cast from n-methyl pyrrolidine (NMP) solutions containing hydrogen bonding inhibitors are fully amorphous. Dedoping, by further treatments of ES with NH40H solutions, or doping, by immersion of EB in aqueous HF solutions, lead to further changes in the local molecular structure. Many of these structural phases bear a limited resemblance to the previously proposed structures for HCl-doped PANI powder and films [1,2]. The role of water hydration in the structural evolution is also examined and significant structural variations are found to occur in ES at radically different time scales.
The influence of thermal treatments on the photophysics in sigma-conjugated poly(di-n-alkylsilane... more The influence of thermal treatments on the photophysics in sigma-conjugated poly(di-n-alkylsilanes) (i.e., butyl, hexyl, octyl, decyl and dodecyl) have been investigated. In situ measurements of both thin film absorption and photoluminescence reveal numerous distinct structural conformers. The exact distribution of these conformations is sensitive to both thermal quenching and subsequent heating. Molecular modeling studies of large oligomers and ab initio calculations of representative tetramers and hexamers are used to identify energetically favorable chain conformations. Zero-energy calculations identify deep local minima corresponding to simple 7/3, 9/4 and 15/7 helices. Models of repeating dyads identify a 14/6 helix and other candidate low energy structures. These results are compared to the experimental data.
Physical Review B, 1990
At doping concentrations of approximately 55% per mole weight of sodium, a crystalline hexagonal ... more At doping concentrations of approximately 55% per mole weight of sodium, a crystalline hexagonal phase characterized by a quasi-one-dimensional channel structure in which sodium ions fill columns formed by three polymer chains is observed. At slightly higher doping levels a transformation to another less-ordered structural phase occurs. In this phase the alkali-metal columns are still identifiable, but the structural integrity of the host is diminished. Structural models for both doped phases are proposed
Chemical Physics of Intercalation II, 1993
Conducting polymers are best known for their intriguing electronic and optical properties.1, 2 Wh... more Conducting polymers are best known for their intriguing electronic and optical properties.1, 2 What is perhaps not so well appreciated is that these materials also may be considered excellent candidates for investigations of intercalation in a “low dimensional” guest-host system. This opportunity results from the anisotropy intrinsic to conducting polymers in combination with their ability to form heavily-doped charge-transfer complexes.
Physical Review B, 1996
The short-range order within amorphous polyaniline ͑a-PANI͒ has been studied through use of a pai... more The short-range order within amorphous polyaniline ͑a-PANI͒ has been studied through use of a pairdistribution-function analysis of x-ray scattering data in combination with quantitative refinements of model structure functions. Both the emeraldine base and salt ͑ES͒ forms of a-PANI are less planar than their crystalline counterparts. Direct evidence for appreciable interchain hydrogen bonding is seen. A probable position of the dopant ions in hydrohalogenic-acid-doped ES has also been ascertained. The intrachain structure and the ion location are such that the effective-conjugation length is lowered and the chain-to-chain packing is frustrated. These characteristics hinder charge transport through the polymer matrix and prevent evolution towards a metallic state. ͓S0163-1829͑96͒50442-X͔
J Luminesc, 2007
We investigate localized exciton states in π-conjugated polymers with finite torsion. The localiz... more We investigate localized exciton states in π-conjugated polymers with finite torsion. The localized states are associated with a perturbed transfer integral for which the magnitude of cosine of the torsion angle exceeds the magnitude of the corresponding cosine of the unperturbed system. The localized state energy is calculated as a function of the ratio of the perturbed to unperturbed transfer integrals. Particular attention is paid to the optically active symmetric localized states, and the effective oscillator strength, or square of the absolute magnitude of the transition dipole moment, is calculated as a function of the energy. The relation of the theory to recent optical studies of poly(di-noctylfluorene) (PF8) is discussed.
Aps March Meeting Abstracts, Mar 1, 2006
Semi-crystalline films of various sigma-conjugated poly(di-n-alkylsilanes) (i.e., hexyl, octyl, d... more Semi-crystalline films of various sigma-conjugated poly(di-n-alkylsilanes) (i.e., hexyl, octyl, decyl and dodecyl) have been investigated using direct structural studies (i.e., x-ray diffraction and modelling) in combination with optical spectroscopy measurements (e.g., UV-vis absorption and photoluminescence). Thermal-quenching studies of these alkyl-substituted polysilanes are found to generate a complex sequence of structural phase transitions mediated by the formation of transient and metastable
Fundamental Materials Research
Physical Review B, 2003
X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to st... more X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to study the well-known order-disorder transition (ODT) to the β phase in poly(9,9-(di-n,noctyl)fluorene)) (PF8) thin film samples through combination of time-dependent and temperaturedependent measurements. The ODT is well described by a simple Avrami picture of one-dimensional nucleation and growth but crystallization, on cooling, proceeds only after molecular-level conformational relaxation to the so called β phase. Rapid thermal quenching is employed for PF8 studies of pure α phase samples while extended low-temperature annealing is used for improved β phase formation. Low temperature PL studies reveal sharp Franck-Condon type emission bands and, in the β phase, two distinguishable vibronic sub-bands with energies of approximately 199 and 158 meV at 25 K. This improved molecular level structural order leads to a more complete analysis of the higher-order vibronic bands. A net Huang-Rhys coupling parameter of just under 0.7 is typically observed but the relative contributions by the two distinguishable vibronic sub-bands exhibit an anomalous temperature dependence. The PL studies also identify strongly correlated behavior between the relative β phase 0-0 PL peak position and peak width. This relationship is modeled under the assumption that emission represents excitons in thermodynamic equilibrium from states at the bottom of a quasi-one-dimensional exciton band. The crystalline phase, as observed in annealed thin-film samples, has scattering peaks which are incompatible with a simple hexagonal packing of the PF8 chains.
Conductive Polymers and Plastics, 1999
The local structural ordering of polyaniline (PANI) as emeraldine (i.e., at 50% oxidation) in bot... more The local structural ordering of polyaniline (PANI) as emeraldine (i.e., at 50% oxidation) in both its base (EB) and salt (ES) forms has been studied using x-ray diffraction (XRD) techniques and these data reinforce the general belief that this local structure is extremely sensitive to the specific processing history. PANI powders precipitated as ES immediately following synthesis lead to reasonably crystalline materials whereas EB films cast from n-methyl pyrrolidine (NMP) solutions containing hydrogen bonding inhibitors are fully amorphous. Dedoping, by further treatments of ES with NH40H solutions, or doping, by immersion of EB in aqueous HF solutions, lead to further changes in the local molecular structure. Many of these structural phases bear a limited resemblance to the previously proposed structures for HCl-doped PANI powder and films [1,2]. The role of water hydration in the structural evolution is also examined and significant structural variations are found to occur in ES at radically different time scales.
The influence of thermal treatments on the photophysics in sigma-conjugated poly(di-n-alkylsilane... more The influence of thermal treatments on the photophysics in sigma-conjugated poly(di-n-alkylsilanes) (i.e., butyl, hexyl, octyl, decyl and dodecyl) have been investigated. In situ measurements of both thin film absorption and photoluminescence reveal numerous distinct structural conformers. The exact distribution of these conformations is sensitive to both thermal quenching and subsequent heating. Molecular modeling studies of large oligomers and ab initio calculations of representative tetramers and hexamers are used to identify energetically favorable chain conformations. Zero-energy calculations identify deep local minima corresponding to simple 7/3, 9/4 and 15/7 helices. Models of repeating dyads identify a 14/6 helix and other candidate low energy structures. These results are compared to the experimental data.
Physical Review B, 1990
At doping concentrations of approximately 55% per mole weight of sodium, a crystalline hexagonal ... more At doping concentrations of approximately 55% per mole weight of sodium, a crystalline hexagonal phase characterized by a quasi-one-dimensional channel structure in which sodium ions fill columns formed by three polymer chains is observed. At slightly higher doping levels a transformation to another less-ordered structural phase occurs. In this phase the alkali-metal columns are still identifiable, but the structural integrity of the host is diminished. Structural models for both doped phases are proposed