Poly(p-phenylene vinylene)
Encyclopedia
Poly (PPV, or polyphenylene vinylene) is a conducting polymer of the rigid-rod polymer host family.
PPV is the only polymer of this type that has so far been successfully processed into a highly ordered crystalline thin film. PPV and its derivatives are conducting polymers of rigid-rod polymer family. They are the only conducting polymers that have been successfully processed in film with high levels of crystallinity. PPV is easily synthesized in good purity and high molecular weight. Although insoluble in water, its precursors can be manipulated in aqueous solution. The small optical band gap and its bright yellow fluorescence makes PPV a candidate in many electronic applications such as light-emitting diodes (LED) and photovoltaic devices. Moreover, PPV can be easily doped to form electrically conductive materials. Its physical and electronic properties can be altered by the inclusion of functional side groups.
between the bis(ylide) derived from an aromatic bisphosphonium salt and dialdehyde, especially 1,4-benzenedialdehyde.
Step growth coupling reactions
, such as this Wittig condensation, usually yield low molecular weight polymer with 5-10 repeat units. Incorporation of various side groups (alkyl, alkoxy, or phenyl) increases the solubility of the polymer and gives higher molecular weights.
An advantage of the step-polymerization approach is that ortho-, meta-, and para-xylylene linkages can be incorporated in the main chain. Copolymers of defined stereoregularity can also be easily made in this way.
PPV derivatives can be also produced via the Knoevenagel condensation
between a benzylic nitrile and an aromatic dialdehyde. Since this method produces many side reactions, such as hydrolysis of nitrile group, careful optimization of the reaction conditions was needed.
give reasonable molecular weights (3,000-10,000) when solubilizing groups present. However, this method requires one of the gaseous starting materials to be added in precise amounts .
A modification of the ROMP route to PPV used a silyl-substituted paracyclophane derivative. Transformation into PPV could be achieved by elimination of the silyloxy group followed by thermal treatment or treating the precursor polymer with acid. The advantage of this method is that polymers and block copolymers of well-defined molecular weight can be easily prepared.
PPV is a diamagnetic material and has a very low intrinsic electrical conductivity, on the order of 10-13 S/cm.[1] The electrical conductivity increases upon doping with iodine, ferric chloride, alkali metals, or acids. However, the stability of these doped materials is relatively low. In general, unaligned, unsubstituted PPV presents only moderate conductivity with doping, ranging from <<10-3 S/cm (I2 doped) to 100 S/cm (H2SO4-doped).[1] Draw ratios of up to 10 are possible. Alkoxy-substituted PPVs are generally easier to oxidize than the parent PPV and hence have much higher conductivities. Longer side chains lower the conductivity and hinder interchain hopping of charge carriers.
Polyphenylene vinylene is capable of electroluminescence
, leading to applications in polymer-based organic light emitting diodes. PPV was used as the emissive layer in the first polymer light-emitting diodes. Devices based on PPV emit yellow-green light, and derivatives of PPV obtained by substitution are often used when light of a different color is required. In presence of even a small amount of oxygen
, singlet oxygen
is formed during operation, by energy transfer from the excited polymer molecules to oxygen molecules. These oxygen radicals then attack the structure of the polymer, leading to its degradation. Special precautions therefore have to be kept during manufacturing of PPV in order to prevent oxygen contamination.
PPV is also used as an electron-donating material in organic solar cell
s. Although PPV-based devices suffer from poor absorption and photodegradation
, PPV and PPV derivatives (especialy MEH-PPV and MDMO-PPV) find frequent application in research cells.
PPV is the only polymer of this type that has so far been successfully processed into a highly ordered crystalline thin film. PPV and its derivatives are conducting polymers of rigid-rod polymer family. They are the only conducting polymers that have been successfully processed in film with high levels of crystallinity. PPV is easily synthesized in good purity and high molecular weight. Although insoluble in water, its precursors can be manipulated in aqueous solution. The small optical band gap and its bright yellow fluorescence makes PPV a candidate in many electronic applications such as light-emitting diodes (LED) and photovoltaic devices. Moreover, PPV can be easily doped to form electrically conductive materials. Its physical and electronic properties can be altered by the inclusion of functional side groups.
Step growth routes
PPV can be synthesized by Wittig-type couplingsWittig reaction
The Wittig reaction is a chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide to give an alkene and triphenylphosphine oxide....
between the bis(ylide) derived from an aromatic bisphosphonium salt and dialdehyde, especially 1,4-benzenedialdehyde.
Step growth coupling reactions
Step-growth polymerization
Step-growth polymerization refers to a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers, longer oligomers and eventually long chain polymers. Many naturally occurring and some synthetic polymers are produced by step-growth...
, such as this Wittig condensation, usually yield low molecular weight polymer with 5-10 repeat units. Incorporation of various side groups (alkyl, alkoxy, or phenyl) increases the solubility of the polymer and gives higher molecular weights.
An advantage of the step-polymerization approach is that ortho-, meta-, and para-xylylene linkages can be incorporated in the main chain. Copolymers of defined stereoregularity can also be easily made in this way.
PPV derivatives can be also produced via the Knoevenagel condensation
Knoevenagel condensation
The Knoevenagel condensation reaction is an organic reaction named after Emil Knoevenagel. It is a modification of the Aldol condensation.A Knoevenagel condensation is a nucleophilic addition of an active hydrogen compound to a carbonyl group followed by a dehydration reaction in which a molecule...
between a benzylic nitrile and an aromatic dialdehyde. Since this method produces many side reactions, such as hydrolysis of nitrile group, careful optimization of the reaction conditions was needed.
Heck coupling routes
The couplings of ethylene with a variety of aromatic dibromides via a Heck reactionHeck reaction
The Heck reaction is the chemical reaction of an unsaturated halide with an alkene and a base and palladium catalyst to form a substituted alkene. Together with the other palladium-catalyzed cross-coupling reactions, this reaction is of great importance, as it allows one to do substitution...
give reasonable molecular weights (3,000-10,000) when solubilizing groups present. However, this method requires one of the gaseous starting materials to be added in precise amounts .
Precursor Routes
A bicyclooctadiene compound has been coupled by ring-opening metathesis polymerization (ROMP) to give a precursor polymer of high molecular weight and soluble in organic solvents. This polymer can be deposited as thin films and converted thermally to PPV. Lower conversion temperatures could be employed with the presence of an amine catalyst.A modification of the ROMP route to PPV used a silyl-substituted paracyclophane derivative. Transformation into PPV could be achieved by elimination of the silyloxy group followed by thermal treatment or treating the precursor polymer with acid. The advantage of this method is that polymers and block copolymers of well-defined molecular weight can be easily prepared.
Structure and Properties
Highly oriented PPV films obtained by the soluble polymeric precursor route usually have P21 symmetry with a monoclinic unit cell containing two monomer units: c (chain axis) = 0.658, a = 0.790, b = 0.605 nm, and α (monoclinic angle) = 123o (Figure 1). The structural organization of PPV chains resembles that found in other highly oriented rigid-rod polymers, where the molecules are oriented along the fiber axis (often the stretching direction) but with partial axial translational disorder.PPV is a diamagnetic material and has a very low intrinsic electrical conductivity, on the order of 10-13 S/cm.[1] The electrical conductivity increases upon doping with iodine, ferric chloride, alkali metals, or acids. However, the stability of these doped materials is relatively low. In general, unaligned, unsubstituted PPV presents only moderate conductivity with doping, ranging from <<10-3 S/cm (I2 doped) to 100 S/cm (H2SO4-doped).[1] Draw ratios of up to 10 are possible. Alkoxy-substituted PPVs are generally easier to oxidize than the parent PPV and hence have much higher conductivities. Longer side chains lower the conductivity and hinder interchain hopping of charge carriers.
Applications
Due to its stability, processability, and electrical and optical properties, PPV has been considered for a wide variety of applications.[1] In 1989 the first polymer-based light emitting diode (LED) was discovered using PPV as the emissive layer. Polymers are speculated to have advantages over molecular materials in LEDs, such as ease of processing, reduced tendency for crystallization, and greater thermal and mechanical stability. Ever since the first breakthrough in 1989, a large number of PPV derivatives have been synthesized and used for LED applications. Although solid-state lasing has yet to be demonstrated in an organic LED, poly[2-methoxy-5-(2’-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) has been proven to be a promising laser dye due to its high fluorescence efficiency in solution.Polyphenylene vinylene is capable of electroluminescence
Electroluminescence
Electroluminescence is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field...
, leading to applications in polymer-based organic light emitting diodes. PPV was used as the emissive layer in the first polymer light-emitting diodes. Devices based on PPV emit yellow-green light, and derivatives of PPV obtained by substitution are often used when light of a different color is required. In presence of even a small amount of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, singlet oxygen
Singlet oxygen
Singlet oxygen is the common name used for the diamagnetic form of molecular oxygen , which is less stable than the normal triplet oxygen. Because of its unusual properties, singlet oxygen can persist for over an hour at room temperature, depending on the environment...
is formed during operation, by energy transfer from the excited polymer molecules to oxygen molecules. These oxygen radicals then attack the structure of the polymer, leading to its degradation. Special precautions therefore have to be kept during manufacturing of PPV in order to prevent oxygen contamination.
PPV is also used as an electron-donating material in organic solar cell
Organic solar cell
An organic photovoltaic cell is a photovoltaic cell that uses organic electronics--a branch of electronics that deals with conductive organic polymers or small organic molecules for light absorption and charge transport....
s. Although PPV-based devices suffer from poor absorption and photodegradation
Photodegradation
Photodegradation is degradation of a photodegradable molecule caused by the absorption of photons, particularly those wavelengths found in sunlight, such as infrared radiation, visible light, and ultraviolet light. However, other forms of electromagnetic radiation can cause photodegradation...
, PPV and PPV derivatives (especialy MEH-PPV and MDMO-PPV) find frequent application in research cells.