Dwaine O. Cowan

Electrical switching and memory phenomena in Cu‐TCNQ thin films

Stable and reproducible current‐controlled bistable electrical switching has been observed in polycrystalline organic semiconducting films. The effect has been observed in a lamellar structure with a film of microcrystalline Cu‐TCNQ between Cu and Al electrodes where the Cu‐TCNQ is grown on a Cu substrate via a spontaneous electrolysis technique. The switching effect is insensitive to moisture and is observed over a large temperature range. The current‐voltage characteristics reveal an abrupt decrease in impedance from 2 MΩ to less than 200 Ω at a field strength of 4×103 V/cm. The transition from a high‐ to low‐impedance state occurs with delay and switching times of approximately 15 and 10 nsec, respectively. Switching with high‐power dissipation yields a low‐impedance memory state which can be erased by application of a short current pulse. An interpretation of this behavior is based on the bulk properties of the mixed valence semiconductor Cu‐TCNQ.

Elements of organic photochemistry

1 Basic Photophysical and Photochemical Concepts.- 1.1. Introduction.- 1.2. Energy Distribution in the Excited Molecule.- 1.2a. Light Absorption.- 1.2b. Internal Conversion and Intersystem Crossing.- 1.2c. Fluorescence and Phosphorescence.- 1.3. Photochemical Kinetics: Concentrations, Rates, Yields, and Quantum Yields.- 1.4. Classification of Molecular Electronic Transitions and Excited States.- 1.4a. ? ? ?* Transitions.- 1.4b. n ? ?* and l? a? Transitions.- 1.4c. Intramolecular Charge-Transfer Transitions (CT).- Problems.- 2 Photochemical Techniques and the Photodimerization of Anthracene and Related Compounds.- 2.1. Absorption and Emission Spectra.- 2.1a. Transition Probability.- 2.1b. Polarization Spectra.- 2.1c. The Measurement of Fluorescence Spectra and Fluorescence Quantum Yields.- 2.1d. The Measurement of Fluorescence Lifetimes.- 2.2. The Photodimerization of Anthracene and Related Compounds.- 2.2a. Structural Aspects: The Effect of Substituents on the Photodimerization.- 2.2b. Preparative Photochemical Techniques.- 2.2c. Kinetic and Mechanistic Aspects of the Anthracene Photodimerization.- 2.3. The Anthracene Triplet State.- Problem.- References.- 3 Photochemical Techniques and the Photochemistry of Ketones.- 3.1. The Photoreduction of Aryl Ketones: Nature of the Excited State.- 3.2. Flash Photolysis.- 3.3. The Photoreduction of Aryl Ketones: Structural Aspects.- 3.4. The Photoreduction of Aryl Ketones: Secondary Reactions.- 3.5. The Photoreduction of Aryl Ketones: Synthetic Applications.- 3.6. The Photoreduction of Alkanones.- 3.7. Intramolecular Hydrogen Abstraction by Ketones (Type II Cleavage).- 3.7a. The Multiplicity of the Excited State.- 3.7b. Stereoelectronic Effects.- 3.7c. Substituent Effects.- 3.7d. Synthetic Applications.- 3.8. Hydrogen Abstraction by Groups Other Than the Carbonyl.- Problems.- References.- 4 The Photochemistry of Simple Carbonyl Compounds: Type I Cleavage and Oxetane Formation.- 4.1. Type I Cleavage.- 4.1a. The Nature of the Excited State: Part I.- 4.1b. Some Examples and Synthetic Applications of Type I Cleavage Reactions.- 4.1c. Type I Cleavage Reactions Resulting in Loss of Carbon Monoxide.- 4.1d. ?-Cleavage of Cyclopropyl Ketones.- 4.1e. The Nature of the Excited State: Part II.- 4.2. The Formation of Oxetanes from Carbonyls and Olefins.- 4.2a. Oxetane Formation from Olefins and Aryl Ketones and Aldehydes.- 4.2b. Synthetic Applications of Oxetane Formation.- 4.2c. Oxetane Formation from Olefins and Aliphatic Aldehydes and Ketones.- 4.2d. Perturbational Molecular Orbital Theory (PMO) Applied to Oxetane Formation.- Problems.- References.- 5 The Triplet State.- 5.1. Introduction.- 5.1a. The Identity of the Phosphorescent State as a Triplet.- 5.1b. The Definition and Properties of a Triplet State.- 5.2. Determination of Triplet Energy Levels.- 5.2a. Phosphorescence Spectroscopy.- 5.2b. Singlet ? Triplet Absorption Spectra.- 5.2c. Phosphorescence Excitation Spectroscopy.- 5.2d. Flash Photolysis.- 5.2e. Electron Excitation.- 5.2f. The Lowest Triplet Levels of Organic Molecules.- 5.3. Determination of the Efficiency of Intersystem Crossing.- 5.3a. Flash Photolysis.- 5.3b. Triplet-Sensitized Isomerization.- 5.3c. Photooxidation.- 5.3d. Delayed Fluorescence.- 5.3e. Electron Spin Resonance Spectroscopy.- 5.3f. Intersystem Crossing Quantum Yields of Organic Molecules.- 5.4. Determination of Triplet Lifetimes.- 5.4a. Flash Photolysis.- 5.4b. Luminescence Decay.- 5.4c. The Effect of Deuteration on Triplet Lifetime.- 5.4d. Triplet Lifetimes of Various Organic Molecules.- 5.5. Excited State Geometry.- 5.6. Spin-Orbit Coupling and Intersystem Crossing.- 5.6a. The Nature of Spin-Orbit Coupling.- 5.6b. Effect of Heavy Atoms on Intercombinational Transitions in Aromatic Compounds.- 5.6c. Effect of Heavy Atoms on Intercombinational Transitions in Carbonyl and Heterocyclic Compounds.- 5.6d. External Heavy-Atom Effects and Charge Transfer.- References.- 6 Electronic Energy Transfer.- 6.1. Excitation Transfer within a Chromophore System.- 6.1a. Internal Conversion and Intersystem Crossing Theory.- 6.1b. Radiationless Transitions: Phosphorescence Microwave Double Resonance.- 6.1c. Zero-Field Optically Detected Magnetic Resonance (ODMR).- 6.2. Theory of Excitation Transfer between Two Chromophores.- 6.2a. Radiative Transfer (Trivial Mechanism).- 6.2b. Resonance Transfer (Long-Range Transfer).- 6.2c. Energy Transfer via Exchange Interaction.- 6.2d. Exciton Transfer (Strong Coupling).- 6.3. Excitation Transfer between Two Chromophores.- 6.3a. Singlet-Singlet Energy Transfer (Forster Type).- 6.3b. Singlet-Singlet Energy Transfer via Collisions.- 6.3c. Intermolecular Triplet-Triplet Energy Transfer.- 6.3d. Application of Triplet-Triplet Energy Transfer.- 6.3e. Schenck Mechanism.- 6.3f. Intramolecular Triplet Energy Transfer.- 6.3g. Exciton Interaction.- 6.4. Exciplex Quenching.- References.- 7 Dienone and Enone Photochemistry.- 7.1. Dienone Photoreactions.- 7.2. Dienone to Cyclopropyl Ketone Formation.- 7.2a. 3-5 Bond Orders.- 7.2b. Zwitterionic vs. Diradical Intermediates.- 7.2c. Pivot vs. Slither Mechanism.- 7.3. Dienone to Hydroxy Ketone.- 7.4. Cyclopropyl Ketones.- 7.5. 2,4-Cyclohexadienones.- 7.6. Cyclohexenone Photorearrangements.- 7.6a. Aryl-Substituted Cyclohexenones.- 7.6b. Alkyl-Substituted Cyclohexenones.- Problems.- References.- 8 The Di-?-Methane Photorearrangement.- 8.1. Acyclic Di-?-Methane Photorearrangement.- 8.1a. Regiospecificity and Stereochemistry.- 8.1b. Substitution at the Central sp3 Carbon Atom and Di-?-Methane Reactivity.- 8.1c. Reaction Rate Constants.- 8.2. Aryl Di-?-Methane Photorearrangement.- 8.3. Bicyclic Di-?-Methane Photorearrangement.- 8.3a. Barrelene.- 8.3b. Benzobarrelene.- 8.3c. Naphthobarrelenes.- 8.3d. Anthrabarrelene.- 8.3e. Other Selected Examples.- 8.4. Oxa-di-?-Methane Rearrangement.- Problems.- References.- 9 Photochemical Cis-Trans and Valence Isomerization of Olefins.- 9.1. Introduction: Cis-Trans Isomerization of Stilbene.- 9.2. Potential Energy Diagrams.- 9.3. Photosensitized Stilbene Isomerization.- 9.4. Nonvertical Energy Transfer.- 9.5. Stilbene Isomerization via Direct Photolysis.- 9.5a. Vibrationally Excited Ground State.- 9.5b. Triplet State Mechanism.- 9.5c. Singlet State Mechanism.- 9.6. Substituted Stilbenes.- 9.7. Piperylene Photochemistry.- 9.8. Alkene Photoisomerization.- 9.9. Intramolecular Cycloaddition Reactions.- 9.9a. Theory.- 9.9b. Intramolecular (2 + 2) Cycloadditions and Cycloreversion Reactions.- 9.9c. Intramolecular (4 + 2) Photocycloaddition Reactions.- 9.10. Photoelectrocyclic Reactions.- 9.10a. Theory.- 9.10b. Examples of Electrocyclic Reactions.- Problems.- References.- 10 Photodimerization and Photocycloaddition Reactions Yielding Cyclobutanes.- 10.1. Photodimerization and Photocycloaddition Reactions of Olefins and Polyenes.- 10.1a. Photodimerization of Olefins and Polyenes.- 10.1b. Photocycloaddition Reactions of Olefins and Polyenes.- 10.2. Photodimerization and Photocycloaddition Reactions of Aromatic Compounds.- 10.3. Photodimerization and Photocycloaddition Reactions of ?,?-Unsaturated Carbonyls and Acid Derivatives.- 10.3a. Photodimerization of ?,?-Unsaturated Carbonyls and Acid Derivatives.- 10.3b. Photocycloaddition Reactions of ?,?-Unsaturated Carbonyls and Acid Derivatives.- 10.4. Dimerization in the Solid Phase.- References.- 11 Photoelimination, Photoaddition, and Photosubstitution.- 11.1. Photoelimination Reactions.- 11.1a. Photoelimination of Nitrogen.- 11.1b. Photoelimination of Nitric Oxide from Organic Nitrites.- 11.1c. Miscellaneous Photoeliminations.- 11.2. Photoaddition Reactions.- 11.2a. Photoaddition of Water, Alcohols, and Carboxylic Acids.- 11.2b. Miscellaneous Photoadditions.- 11.3. Photosubstitution Reactions.- References.- 12 An Introduction to Photobiology.- 12.1. Photosynthesis.- 12.1a. The Photosynthetic Apparatus.- 12.1b. A Mechanistic Model for Photosynthesis.- 12.2. The Photochemistry of Vision.- 12.2a. Anatomy of the Human Eye.- 12.2b. The Visual Pigments and the Chemistry of Vision.- 12.3. Phototaxis and Phototropism.- 12.4. Damage and Subsequent Repair by Light (Photoreactivation).- 12.4a. The Photochemistry of the Nucleic Acids.- 12.4b. Photoreactivation.- References.

Apparent giant conductivity peaks in an anisotropic medium: TTF-TCNQ

Abstract Measurements are presented of electrical potential values on single crystals of TTF-TCNQ which show an extraordinary apparent conductivity maximum near 70 K. The observed voltages in these anisotropic crystals are shown to be unusually sensitive to the points at which electrical contact is made. Our anomalous apparent conductivities can be understood in terms of the strong temperature dependence of the electrical anisotropy which is found, through measurements on several samples, to peak near 70 K.

Solution and Solid State Studies of Tetrafluoro-7,7,8,8-Tetracyano-p-Quinodimethane, TCNQF4. Evidence for Long-Range Amphoteric Intermolecular Interactions and Low-Dimensionality in the Solid State Structure

Solution and solid state studies of TCNQF4 are reported. The electron affinity of TCNQF4 has been derived from spectral observations of the visible-near IR charge-transfer band for the pyrene compl...

Crystal Structures for the Electron Donor Dibenzotetrathiafulavalene, DBTTF, and Its Mixed-stack Charge-transfer Salts with the Electron Acceptors 7,7,8,8-tetracyano-p-quinodimethane, TCNQ, and 2,5-difluoro-7,7,8,8-tetracyano-p-quinodimethane, 2,5-TCNQF2

Crystal structures for the electron donor DBTTF and its charge-transfer salts with the acceptors TCNQ and 2,5-TCNQF2 are reported. Crystal data for the three systems are as follows: (a) neutral DBT...

Semi-metallic behaviour of HMTSF-TCNQ at low temperatures under pressure☆

Abstract Some results of electrical resistivity (ϱ) and transverse magnetoresistance measurements on single crystals of the charge transfer compound HMTSF-TCNQ under pressure are reported. There is evidence that dϱ/d T remains positive over the whole temperature range under pressure, and that a T 2 law is obeyed from 0.19 to 2 K at 14 kbar. Together with a relatively large magnetoresistance this is an indication of semi-metallic behaviour.

A current-controlled electrically switched memory state in silver and copper-TCNQF4 radical-ion salts

Abstract An electric field is found to induce current-controlled memory switching in polycrystalline films of copper and silver complexed to the organic electron acceptor 2,3,5,6,-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF 4 ). The effect occurs in a two-terminal lamellar structure with the organic semi-conductor between two metal electrodes. Switching is reproducible and insensitive to moisture, light, and temperature. A memory state is created in these materials when an external field surpasses a threshold value. At this critical field the impedance of the material drops by more than four orders of magnitude. Typical impedance values in a 1.87-μm thick Cu-TCNQF 4 film for the high and low states are 1.6 × 10 6 and 60 ohms, respectively. Fast-pulse measurements made on these materials show the switching from the low to the high conductivity state occurs with a combined delay and switching time of less than 4 ns. The memory cannot be “erased” by subsequent voltage pulses, yet the memory is not permanent; its duration is controlled by sample thickness and the amount of energy used to invoke the state. General properties of metal-TCNQF 4 films are compared with the corresponding TCNQ salts.

Crystal structure of the radical-cation radical-anion salt from 2,2′-bi-1,3-dithiole and 7,7,8,8-tetracyanoquinodimethane

The three-dimensional structure of the radical-cation radical-anion salt from 2,2′-bi-1,3-dithiole (1) and 7,7,8,8-tetracyanoquinodimethane (2) has been determined by X-ray diffraction methods; the structure is composed of segregated columnar stacks of cations and anions.

Microwave conductivities of the organic conductors TTF-TCNQ and ATTF-TCNQ

Abstract The complex microwave conductivities of the organic salts TTF-TCNQ and ATTF-TCNQ show a metal-to-insulator transition near 60 K and distinguish these materials from the class of disordered one-dimensional conductors. The data show no evidence for the high-temperature superconducting fluctuations recently proposed by Heeger and co-workers.

Resistive and Magnetic Susceptibility Transitions in Superconducting (TMTSF)2C104

Abstract We report measurements of the ac magnetic susceptibility and dc resistive superconducting transitions in the organic superconductor (TMTSF)2C104. Inductive measurements show complete diamagnetic shielding below a broad transition and initial flux penetration at very low fields [Hc1(0) < 1 Oe]. The resistive transition is also broad, but occurs at a significantly higher temperature than the inductive transition, Tc = 1.0 K and 0.65 K respectively. Resistance measurements also show evidence of a phase transition in the vicinity of 24 K. Magnetic field induced transitions, measured both inductively and resistively, show marked anistropy both in magnitude and in breadth of the transition. Results suggest that (TMTSF)2C104 is a quasi ID or 2D superconductor at high temperatures and high magnetic fields and an anisotropic bulk superconductor at low temperatures and fields. Associated thermoelectric power measurements suggest that spin density waves coexist with the superconducting state.