Paul Ewbank - Academia.edu (original) (raw)

Papers by Paul Ewbank

Research paper thumbnail of Preparation and Characterization of π-Stacking Quinodimethane Oligothiophenes. Predicting Semiconductor Behavior and Bandwidths from Crystal Structures and Molecular Orbital Calculations

Journal of the American Chemical Society, 2004

A series of new quinodimethane-substituted terthiophene and quaterthiophene oligomers has been in... more A series of new quinodimethane-substituted terthiophene and quaterthiophene oligomers has been investigated for comparison with a previously studied quinoid oligothiophene that has demonstrated high mobilities and ambipolar transport behavior in thin-film transistor devices. Each new quinoidal thiophene derivative shows a reversible one-electron oxidation between 0.85 and 1.32 V, a quasi-reversible one-electron second oxidation between 1.37 and 1.96 V, and a reversible two-electron reduction between -0.05 and -0.23 V. The solution UV-vis-NIR spectrum of each compound is dominated by an intense (epsilon congruent with 100 000 M(-1) cm(-1)) low energy pi-pi transition that has a lambda(max) ranging between 648 and 790 nm. All X-ray crystal structures exhibit very planar quinoidal backbones and short intermolecular pi-stacking distances (3.335-3.492 A). Structures exhibit a single pi-stacking distance with parallel cofacial stacking (sulfur atoms of equivalent rings pointed in the same direction) or with alternating distances and antiparallel cofacial stacking (sulfur atoms of equivalent rings pointed in the opposite direction). Examples of the layered and herringbone-packing motifs are observed for both the parallel and the antiparallel cofacial stacking. Analysis of the X-ray structures and molecular orbital calculations indicates that all of these compounds have one-dimensional electronic band structures as a result of the pi-stacking. For structures with a unique pi-stacking distance, a simple geometric overlap parameter calculated from the shape of the molecule and the slip from perfect registry in the pi-stack correlates well with the transfer integrals (t) calculated using molecular orbital theory. The calculated valence (633 meV) and conduction (834 meV) bandwidths for a quinoid quaterthiophene structure are similar to those calculated for the benchmark pentacene and indicate that both hole and electron mobilities could be significant.

Research paper thumbnail of Exploration of Ground and Excited Electronic States of Aromatic and Quinoid S,S -Dioxide Terthiophenes. Complementary Systems for Enhanced Electronic Organic Materials

Journal of the American Chemical Society, 2006

We analyze the electronic and molecular structures for the ground and excited electronic states o... more We analyze the electronic and molecular structures for the ground and excited electronic states of aromatic terthiophene (3T), the quinodimethane 3′,4′-dibutyl-5,5′′-bis(dicyanomethylene)-5,5′′-dihydro-2,2′:5′,2′′-terthiophene (3Q), and isologues with the middle ring S-oxidized (3TO2, 3QO2). These represent extremes of electron rich and deficient ground states, often exhibiting complementary properties. Oxidizing the central sulfur atom affects the molecular structure, electron affinity, and photophysical properties of both π systems. The consequences for 3T include de-aromatization of the central thiophene, red-shifting of the electronic absorption spectrum, and lowering of the reduction potential. The electron deficient quinoid 3QO2 shows an enhancement of electron affinity from reducing the electron-donor ability of sulfur, and a blue-shifting of its electronic absorption spectrum was seen. Fluorescence emission is quenched in the sulfonated terthiophene, and the contrary effect again would be expected upon sulfonation of a quinoid emitter. Raman vibrational spectroscopy, electrochemistry, and UV-vis and fluorescence spectroscopies are analyzed in conjunction with theoretical calculations.

Research paper thumbnail of Oligomeric Rods of Alkyl- and Hydridogallium Imides

Journal of the American Chemical Society, 2005

Reaction of [RGa(NMe2)2]2, where R ) Me, Et, Bu, and Hx, with ammonia at 150°C in an autoclave pr... more Reaction of [RGa(NMe2)2]2, where R ) Me, Et, Bu, and Hx, with ammonia at 150°C in an autoclave produced insoluble white powders formulated as oligomers of [RGaNH]n. The analogous reaction between NH3 and MeGa[N(SiMe3)2]2 at low temperature (<25°C) formed an isolable intermediate, )N(SiMe3)2]2, that was characterized using single-crystal X-ray diffraction. Infrared spectroscopy and X-ray diffraction of the oligomers were consistent with a rodlike structure comprised of six-membered, [RGaNH]3 rings stacked perpendicular to the long axis of the rod. The method of synthesis, formula, and diffraction results suggested a structural similarity between the alkyl, [RGaNH]n, and the previously reported hydride, [HGaNH]n. The structural and electronic properties of rods having the general formula H3-[(HXYH)3]nH3 (XY ) GaN, GeC; n ) 1-9) were investigated using density functional theory. Atomic electronegativity differences between the group 13/15 and 14/14 systems were found to play important roles in the geometrical structures of the two rods and also caused significant differences in the electronic structures. Energetically, it was found to be increasingly favorable to add additional cyclotrigallazane rings to the GaN rods, while for the GeC rods, there was a roughly constant energy cost associated with each additional ring. The electric dipole moments of the GaN rods increased substantially with length; in the GeC rods, charge separation occurred to a much smaller extent and had a polarization opposite to that found in GaN. In addition, increased dipole moments correlated with smaller electronic excitation energies, as predicted by time-dependent density functional theory. All of the powders exhibited luminescence in the visible spectrum at room temperature. Structure observed in the photoluminescence spectra of [HGaNH] n and [MeGaNH]n was interpreted as arising from rods of different length.

Research paper thumbnail of Conducting polymers, head-to-tail coupled regioregular polythiophenes, poly-(3-substituted) thiophene diol

Research paper thumbnail of N-methyl, N-(2,5-dibromo-3-thienylmethyl)butyramide data

Research paper thumbnail of Polythiophenes, block copolymers made therefrom, and methods of forming the same

ABSTRACT The present invention is directed, generally, to polythiophenes, and, more particularly,... more ABSTRACT The present invention is directed, generally, to polythiophenes, and, more particularly, to head-to-tail coupled regioregular polythiophenes, block copolymers made therefrom, and methods of forming the same.

Research paper thumbnail of N-(2,5-dibromo-3-thienylmethyl)methylamine hydrochloride data

Research paper thumbnail of Compound 2 data (N-Methyl-(2-bromothien-3-yl)methylamine)

Research paper thumbnail of Regioregular, head to tail coupled, amine functionalized polythiophenes

Research paper thumbnail of N-methyl-N-(2,5-dibromo-thien-3-ylmethyl) acetamide data

Research paper thumbnail of Compound 3 data

Research paper thumbnail of Compound 5a UV-Vis, protected and free

Research paper thumbnail of Compound 4 data

Research paper thumbnail of Polythiophenes, Block Copolymers Made Therefrom, And Methods Of Forming the Same

The present invention relates to polythiophenes, particularly regioregular head-to-tail poly (3-a... more The present invention relates to polythiophenes, particularly regioregular head-to-tail poly (3-alkylthiophenes)(HT-PATs), block copolymers made therefrom, and their methods of formation. The present invention provides HT-PATs with well-defined, specific ...

Research paper thumbnail of MECHANISTIC ASPECTS OF REGIOREGULARITY IN HEAD TO-TAIL COUPLED POLY(3-ALKYLTHIOPHES)

Research paper thumbnail of ChemInform Abstract: Reverse Selectivity in m-CPBA Oxidation of Oligothiophenes to Sulfones

…, Jan 1, 2008

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more 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 of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Research paper thumbnail of ChemInform Abstract: Reverse Selectivity in m-CPBA Oxidation of Oligothiophenes to Sulfones

…, Jan 1, 2008

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more 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 of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Research paper thumbnail of Introduction to Organic Thin Film Transistors and Design of N-Channel Organic Semiconductors

Chem. Mater, Jan 1, 2004

The development of new organic semiconductors with improved performance in organic thin film tran... more The development of new organic semiconductors with improved performance in organic thin film transistors (OTFTs) is a major challenge for materials chemists. There is a particular need to develop air-stable n-channel (electron-conducting) organic semiconductors with performance comparable to that of p-channel (hole-conducting) materials, for organic electronics to realize the benefits of complementary circuit design, i.e., the ability to switch transistors with either positive or negative gate voltages. There have been significant advancements in the past five years. In terms of standard OTFT metrics such as the field effect mobility (μFET) and on-to-off current ratio (ION/IOFF), n-channel OTFTs have achieved performance comparable both to that of n-channel amorphous silicon TFTs and to that of the best reported p-channel (hole-conducting) OTFTs; however, issues of device stability linger. This review provides a detailed introduction to OTFTs, summarizes recent progress in the development of new n-channel organic semiconductors, and discusses the critical properties that any prospective n-channel material must have. Methods important to semiconductor design such as electronic structure calculations and synthetic structural modifications are highlighted in a case study of the development of a new n-channel material based on a terthiophene modified with electron-withdrawing groups. The review concludes with a discussion of directions for future work in this area.

Research paper thumbnail of High Electron Mobility and Ambipolar Transport in Organic Thin‐Film Transistors Based on a π‐Stacking Quinoidal Terthiophene

Advanced …, Jan 1, 2003

Page 1. clear at the moment. It is possible that hydrodynamic flows at the expanding liquid±liqui... more Page 1. clear at the moment. It is possible that hydrodynamic flows at the expanding liquid±liquid interface result in assembly of the recrystallized calcite phase in the form of hollow cylinders, the hollow centers arising due to ...

Research paper thumbnail of Regioregular, head-to-tail coupled poly (3-alkylthiophenes) made easy by the GRIM method: Investigation of the reaction and the origin of regioselectivity

…, Jan 1, 2001

An investigation of the new synthetic method to synthesize regioregular, head-to-tail coupled pol... more An investigation of the new synthetic method to synthesize regioregular, head-to-tail coupled poly(3-alkylthiophenes) using magnesium-halogen exchange (Grignard metathesis) called the GRIM method is described. Treatment of 2,5-dibromo-3-alkylthiophenes with a variety of alkyl and vinyl Grignard reagents resulted in two metalated, regiochemical isomers, namely, 2-bromo-3-alkyl-5-bromomagnesiothiophene and 2-bromomagnesio-3-alkyl-5-bromothiophene in an 85:15 ratio. This ratio appears to be independent of reaction time, temperature, and Grignard reagent employed. Introduction of a catalytic amount of Ni(dppp)Cl 2 to this isomeric mixture afforded poly(3-alkylthiophene) that contained greater than 95% HT-HT couplings (typically 98% HT couplings were seen). The high degree of regioregularity found in the polymer can be explained by a combination of kinetic and thermodynamic effects arising from steric and electronic effects found in the catalytic reaction. A series of reaction investigations led to a general explanation of the origin of regioregularity in polythiophene polymerization reactions. These reactions included kinetic studies and competition experiments.

Research paper thumbnail of Preparation and Characterization of π-Stacking Quinodimethane Oligothiophenes. Predicting Semiconductor Behavior and Bandwidths from Crystal Structures and Molecular Orbital Calculations

Journal of the American Chemical Society, 2004

A series of new quinodimethane-substituted terthiophene and quaterthiophene oligomers has been in... more A series of new quinodimethane-substituted terthiophene and quaterthiophene oligomers has been investigated for comparison with a previously studied quinoid oligothiophene that has demonstrated high mobilities and ambipolar transport behavior in thin-film transistor devices. Each new quinoidal thiophene derivative shows a reversible one-electron oxidation between 0.85 and 1.32 V, a quasi-reversible one-electron second oxidation between 1.37 and 1.96 V, and a reversible two-electron reduction between -0.05 and -0.23 V. The solution UV-vis-NIR spectrum of each compound is dominated by an intense (epsilon congruent with 100 000 M(-1) cm(-1)) low energy pi-pi transition that has a lambda(max) ranging between 648 and 790 nm. All X-ray crystal structures exhibit very planar quinoidal backbones and short intermolecular pi-stacking distances (3.335-3.492 A). Structures exhibit a single pi-stacking distance with parallel cofacial stacking (sulfur atoms of equivalent rings pointed in the same direction) or with alternating distances and antiparallel cofacial stacking (sulfur atoms of equivalent rings pointed in the opposite direction). Examples of the layered and herringbone-packing motifs are observed for both the parallel and the antiparallel cofacial stacking. Analysis of the X-ray structures and molecular orbital calculations indicates that all of these compounds have one-dimensional electronic band structures as a result of the pi-stacking. For structures with a unique pi-stacking distance, a simple geometric overlap parameter calculated from the shape of the molecule and the slip from perfect registry in the pi-stack correlates well with the transfer integrals (t) calculated using molecular orbital theory. The calculated valence (633 meV) and conduction (834 meV) bandwidths for a quinoid quaterthiophene structure are similar to those calculated for the benchmark pentacene and indicate that both hole and electron mobilities could be significant.

Research paper thumbnail of Exploration of Ground and Excited Electronic States of Aromatic and Quinoid S,S -Dioxide Terthiophenes. Complementary Systems for Enhanced Electronic Organic Materials

Journal of the American Chemical Society, 2006

We analyze the electronic and molecular structures for the ground and excited electronic states o... more We analyze the electronic and molecular structures for the ground and excited electronic states of aromatic terthiophene (3T), the quinodimethane 3′,4′-dibutyl-5,5′′-bis(dicyanomethylene)-5,5′′-dihydro-2,2′:5′,2′′-terthiophene (3Q), and isologues with the middle ring S-oxidized (3TO2, 3QO2). These represent extremes of electron rich and deficient ground states, often exhibiting complementary properties. Oxidizing the central sulfur atom affects the molecular structure, electron affinity, and photophysical properties of both π systems. The consequences for 3T include de-aromatization of the central thiophene, red-shifting of the electronic absorption spectrum, and lowering of the reduction potential. The electron deficient quinoid 3QO2 shows an enhancement of electron affinity from reducing the electron-donor ability of sulfur, and a blue-shifting of its electronic absorption spectrum was seen. Fluorescence emission is quenched in the sulfonated terthiophene, and the contrary effect again would be expected upon sulfonation of a quinoid emitter. Raman vibrational spectroscopy, electrochemistry, and UV-vis and fluorescence spectroscopies are analyzed in conjunction with theoretical calculations.

Research paper thumbnail of Oligomeric Rods of Alkyl- and Hydridogallium Imides

Journal of the American Chemical Society, 2005

Reaction of [RGa(NMe2)2]2, where R ) Me, Et, Bu, and Hx, with ammonia at 150°C in an autoclave pr... more Reaction of [RGa(NMe2)2]2, where R ) Me, Et, Bu, and Hx, with ammonia at 150°C in an autoclave produced insoluble white powders formulated as oligomers of [RGaNH]n. The analogous reaction between NH3 and MeGa[N(SiMe3)2]2 at low temperature (<25°C) formed an isolable intermediate, )N(SiMe3)2]2, that was characterized using single-crystal X-ray diffraction. Infrared spectroscopy and X-ray diffraction of the oligomers were consistent with a rodlike structure comprised of six-membered, [RGaNH]3 rings stacked perpendicular to the long axis of the rod. The method of synthesis, formula, and diffraction results suggested a structural similarity between the alkyl, [RGaNH]n, and the previously reported hydride, [HGaNH]n. The structural and electronic properties of rods having the general formula H3-[(HXYH)3]nH3 (XY ) GaN, GeC; n ) 1-9) were investigated using density functional theory. Atomic electronegativity differences between the group 13/15 and 14/14 systems were found to play important roles in the geometrical structures of the two rods and also caused significant differences in the electronic structures. Energetically, it was found to be increasingly favorable to add additional cyclotrigallazane rings to the GaN rods, while for the GeC rods, there was a roughly constant energy cost associated with each additional ring. The electric dipole moments of the GaN rods increased substantially with length; in the GeC rods, charge separation occurred to a much smaller extent and had a polarization opposite to that found in GaN. In addition, increased dipole moments correlated with smaller electronic excitation energies, as predicted by time-dependent density functional theory. All of the powders exhibited luminescence in the visible spectrum at room temperature. Structure observed in the photoluminescence spectra of [HGaNH] n and [MeGaNH]n was interpreted as arising from rods of different length.

Research paper thumbnail of Conducting polymers, head-to-tail coupled regioregular polythiophenes, poly-(3-substituted) thiophene diol

Research paper thumbnail of N-methyl, N-(2,5-dibromo-3-thienylmethyl)butyramide data

Research paper thumbnail of Polythiophenes, block copolymers made therefrom, and methods of forming the same

ABSTRACT The present invention is directed, generally, to polythiophenes, and, more particularly,... more ABSTRACT The present invention is directed, generally, to polythiophenes, and, more particularly, to head-to-tail coupled regioregular polythiophenes, block copolymers made therefrom, and methods of forming the same.

Research paper thumbnail of N-(2,5-dibromo-3-thienylmethyl)methylamine hydrochloride data

Research paper thumbnail of Compound 2 data (N-Methyl-(2-bromothien-3-yl)methylamine)

Research paper thumbnail of Regioregular, head to tail coupled, amine functionalized polythiophenes

Research paper thumbnail of N-methyl-N-(2,5-dibromo-thien-3-ylmethyl) acetamide data

Research paper thumbnail of Compound 3 data

Research paper thumbnail of Compound 5a UV-Vis, protected and free

Research paper thumbnail of Compound 4 data

Research paper thumbnail of Polythiophenes, Block Copolymers Made Therefrom, And Methods Of Forming the Same

The present invention relates to polythiophenes, particularly regioregular head-to-tail poly (3-a... more The present invention relates to polythiophenes, particularly regioregular head-to-tail poly (3-alkylthiophenes)(HT-PATs), block copolymers made therefrom, and their methods of formation. The present invention provides HT-PATs with well-defined, specific ...

Research paper thumbnail of MECHANISTIC ASPECTS OF REGIOREGULARITY IN HEAD TO-TAIL COUPLED POLY(3-ALKYLTHIOPHES)

Research paper thumbnail of ChemInform Abstract: Reverse Selectivity in m-CPBA Oxidation of Oligothiophenes to Sulfones

…, Jan 1, 2008

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more 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 of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Research paper thumbnail of ChemInform Abstract: Reverse Selectivity in m-CPBA Oxidation of Oligothiophenes to Sulfones

…, Jan 1, 2008

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more 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 of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Research paper thumbnail of Introduction to Organic Thin Film Transistors and Design of N-Channel Organic Semiconductors

Chem. Mater, Jan 1, 2004

The development of new organic semiconductors with improved performance in organic thin film tran... more The development of new organic semiconductors with improved performance in organic thin film transistors (OTFTs) is a major challenge for materials chemists. There is a particular need to develop air-stable n-channel (electron-conducting) organic semiconductors with performance comparable to that of p-channel (hole-conducting) materials, for organic electronics to realize the benefits of complementary circuit design, i.e., the ability to switch transistors with either positive or negative gate voltages. There have been significant advancements in the past five years. In terms of standard OTFT metrics such as the field effect mobility (μFET) and on-to-off current ratio (ION/IOFF), n-channel OTFTs have achieved performance comparable both to that of n-channel amorphous silicon TFTs and to that of the best reported p-channel (hole-conducting) OTFTs; however, issues of device stability linger. This review provides a detailed introduction to OTFTs, summarizes recent progress in the development of new n-channel organic semiconductors, and discusses the critical properties that any prospective n-channel material must have. Methods important to semiconductor design such as electronic structure calculations and synthetic structural modifications are highlighted in a case study of the development of a new n-channel material based on a terthiophene modified with electron-withdrawing groups. The review concludes with a discussion of directions for future work in this area.

Research paper thumbnail of High Electron Mobility and Ambipolar Transport in Organic Thin‐Film Transistors Based on a π‐Stacking Quinoidal Terthiophene

Advanced …, Jan 1, 2003

Page 1. clear at the moment. It is possible that hydrodynamic flows at the expanding liquid±liqui... more Page 1. clear at the moment. It is possible that hydrodynamic flows at the expanding liquid±liquid interface result in assembly of the recrystallized calcite phase in the form of hollow cylinders, the hollow centers arising due to ...

Research paper thumbnail of Regioregular, head-to-tail coupled poly (3-alkylthiophenes) made easy by the GRIM method: Investigation of the reaction and the origin of regioselectivity

…, Jan 1, 2001

An investigation of the new synthetic method to synthesize regioregular, head-to-tail coupled pol... more An investigation of the new synthetic method to synthesize regioregular, head-to-tail coupled poly(3-alkylthiophenes) using magnesium-halogen exchange (Grignard metathesis) called the GRIM method is described. Treatment of 2,5-dibromo-3-alkylthiophenes with a variety of alkyl and vinyl Grignard reagents resulted in two metalated, regiochemical isomers, namely, 2-bromo-3-alkyl-5-bromomagnesiothiophene and 2-bromomagnesio-3-alkyl-5-bromothiophene in an 85:15 ratio. This ratio appears to be independent of reaction time, temperature, and Grignard reagent employed. Introduction of a catalytic amount of Ni(dppp)Cl 2 to this isomeric mixture afforded poly(3-alkylthiophene) that contained greater than 95% HT-HT couplings (typically 98% HT couplings were seen). The high degree of regioregularity found in the polymer can be explained by a combination of kinetic and thermodynamic effects arising from steric and electronic effects found in the catalytic reaction. A series of reaction investigations led to a general explanation of the origin of regioregularity in polythiophene polymerization reactions. These reactions included kinetic studies and competition experiments.