Electronic polymers properties

Chemical appHcations of Mn ssbauer spectroscopy are broad (291—293) determination of electron configurations and assignment of oxidation states in stmctural chemistry polymer properties studies of surface chemistry, corrosion, and catalysis and metal-atom bonding in biochemical systems. There are also important appHcations to materials science and metallurgy (294,295) (see Surface and interface analysis). 

It would be preferable to incorporate both fluorescent and electron transport properties in the same material so as to dispense entirely with the need for electron-transport layers in LEDs. Raising the affinity of the polymer facilitates the use of metal electrodes other than calcium, thus avoiding the need to encapsulate the cathode. It has been shown computationally [76] that the presence of a cyano substituent on the aromatic ring or on the vinylene portion of PPV lowers both the HOMO and LUMO of the material. The barrier for electron injection in the material is lowered considerably as a result. However, the Wessling route is incompatible with strongly electron-withdrawing substituents, and an alternative synthetic route to this class of materials must be employed. The Knoevenagel condensation... 

Rotations around torsional barriers induce changes in chain conformation. For conjugated systems like polydiacetylenes, flow-induced changes in chain conformation can have a profound influence on the photon absorption and electronic conductivity properties of the material [73]. Flow-induced changes in molecular conformation form the basis for several technically important processes, the best known examples are the production of oriented fibers by gel spinning [74], the compatibility enhancement [75] and the shear-induced modification of polymer morphology [76]. 

The poly(cyclodiborazane)s containing a dithiafulvene unit in the backbone were also reported (scheme 25).38 This polymer showed an efficient extension of conjugation between the consecutive repeat units due to the strong electron donating property of the dithiafulvene unit. Before doping, this polymer exhibited an electrical... 

Although MEH-PPV 13 (at the time of discovery) was one of the most efficient soluble polymers for PLEDs application, its performance is not high enough for commercialization as LEP. One of the reasons is unbalanced hole-electron mobility in MEH-PPV (the mobility of holes is 100 times faster than the mobility of electrons) [133]. Copolymerization with other conjugated monomers, to some extent, can improve the electron-transporting properties and increase the EL performance. 

As we already mentioned, electron-transporting properties of PPV polymers can be adjusted by introduction of an oxadiazole moiety in the polymer structure. A variety of PPV copolymers... 

The tuning of electron injection and transport in PF has been undertaken by Shu s group [354], who introduced electron-deficient oxadiazole units as pendant groups in fluorene copolymer 257. The introduction of oxadiazole units into the PF can potentially improve the electron transport properties of the polymer, while their bulkiness can help to suppress aggregation effects (Chart 2.68). 

Excellent electron-transporting properties of quinoxaline (also demonstrated for noncon-jugated quinoxaline-containing polymer 588 [684] and quinoxaline-based polyether 589 [685]) resulted in a substantially decreased turn-on voltage of PPV/590 PLED (3.6 V), which is much lower than that of pure PPV in the same conditions (7 V). These diodes showed a maximum luminance of 710 cd/m2 (ca. 40 times brighter than the PPV diode at the same current density and voltage) [686]. 

Yamaguchi and coworkers [709] have prepared luminescent silole polymers 615a-e. The blue emission of the homopolymers 615a,b can be shifted into the red region by copolymerization with other conjugated units (but for the price of lowered PLQY). Although no device studies have been reported yet, excellent electron-transport properties are expected from such materials [710]. 

Introducing heteroaromatic moieties (mainly with N and, to a lesser extent, with O and S) in the backbone of the polymer or as a pendant group, can substantially modify the LUMO level of the materials, improving their electron-transport properties and facilitating electron injection in PLEDs, but the efficiencies still lag behind the other systems. 

Further, the electron-transport properties of the polymer 7 were enhanced by extending the separation between the redox center and backbone from a single Os—amino linkage to one that extends over 17 bonds. The goal was to provide mobility of the redox center independently of backbone motion, which is necessarily restricted by cross-linking. The mobility of the redox center can be characterized by an apparent diffusion coefficient, Z app- According to the relation proposed by Blauch and Saveant ... 

The electronic properties of n-conjugated polymers reflect well the basic electron-withdrawing or electron-donating properties of the components of the Ti-conjugated polymer [62]. In view of the electrochemical reduction potential, the thiophene unit and tetrathiafulvalene unit (Nos. 8 and 9 in Table 1) have a similar electronic effect in PAEs. It is reported that poly(arylenevinylene)s are also susceptible to electrochemical reduction [63, 64]. Due to the electron-accepting properties, PAEs are usually inert in electrochemical and chemical (e.g.,by I2 [54]) oxidation. 

A polymer containing anthracene, [Ru(bpy)3]2 , and [Os(bpy)3]2+ [all covalently linked to a 1 1 copolymer of styrene and m,p-(chlorome-thyl)styrene] has been prepared, and its emission spectrum and intramolecular electron-transfer properties have been studied (569). 

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