Side reaction doping

Alternatively, undesirable side reactions may lead to persistent cation radicals. Due to these side reactions doping (p-type) of the organic semiconductor may occur, leading to a higher conductivity, but lower luminescence efficiency (photo and electroluminescence). However, by chemical or thermal (post-bake step at 180 °C) treatment, complete dedoping is possible and the luminescence efficiency is fully recovered. Additionally, in some cases the cation radical is able to attack the oxetane through nucleophilic reaction and ultimately start the same chain reaction as above [33]. 

It is the sodium trithiocarbonate from this side reaction that gives the viscose dope its characteristic orange color. 

Chemoselective reduction of conjugated enones to allylic alcohols via hydrogen transfer from propan-2-ol over metal oxides is investigated in vapour phase conditions. The unique ability of Mgo to reduce exclusively carbonyl group is observed. However, because of the high basicity of MgO side reactions are present. It is shown that by doping the Mgo catalyst with HC1 a significant decrease of its basicity occurs and consequently side reactions are minimized. 

Typical oxidising dopants used include iodine, arsenic pentachloride, iron(III) chloride and NOPF6. A typical reductive dopant is sodium naphthalide. The main criteria is its ability to oxidise or reduce the polymer without lowering its stability or whether or not they are capable of initiating side reactions that inhibit the polymers ability to conduct electricity. An example of the latter is the doping of a conjugated polymer with bromine. Bromine it too powerful an oxidant and adds across the double bonds to form sp3 carbons. The same reaction may occur with NOPF, but at a lower rate. 

The latest results on imprinted chiral footprints [154] have shown that enantioselective catalysis (hydrolysis) does occur, and based on kinetic measurement the authors believe that this is due to an enantioselective mechanism. Kaiser and Andersson also chose aluminium doped silica as a polymeric material to obtain phenanthrene imprints and their work has been discussed earlier [52]. No selectivity towards the template was observed when imprinted silica was used as stationary phase. Only relative retention and capacity factors increased. Furthermore, even after careful extraction in a Soxhlet, the polymer still leaked phenanthrene. They also found that diazomethane yields a side reaction forming long alkyl chains. Finally they attempted to rej at the work of Morihara et al. [150-155]. but were not able to detect any selectivity using dibenzamide as the template and instead found that the template decomposes into at least five different products when adsorbed on the silica. Clearly further work is required on these systems. 

The decomposition of CdCOj is reversible and without side reactions [63]. The rate and activation energy are influenced by the gases present [64] especially carbon dioxide [65]. Mikhail et al. [66] measured a value of (96 kJ mol" ) which was very close to the dissociation enthalpy (99 kJ mol" ). Doping with Li"", Zn, Ca or Ba increased the rate of decomposition, but resulted in the production of curved Arrhenius plots in which the apparent value of E decreased with increasing temperature. These impurities may change the mechanism of decomposition of CdCOj and also cause variations in the surface areas and structures of the solid products. 

The //-doping of carbon during cathodic dehalogenation is a common side-reaction (see Tables 4.1, 4.2 and the text below). Kijima et al. [39,42,43] reduced diiodoacetylene to carbon at a platinum electrode in dimethylformamide media ... 

Kaspar et al. demonstrated the reduction of a,P-unsaturated ketones to allylic alcohols with /-PrOH in the gas phase over MgO as fixed bed catalyst at 250°C [7]. The MgO was formed in situ by heating Mg(OH)2 at 350°C in an air current during 4 hours. Regeneration of the catalyst was done in the same way. In a subsequent paper the chemoselective reduction of the carbonyl group of 4-hexen-3-one over various solid catalysts was reported [8]. MgO was found to show the highest chemoselectivity. However, as a result of its high basicity several side reactions were also observed. Doping of the MgO catalysts with HCl afforded solid catalysts with improved selectivity. 

The doping reaction is the same as that which occurs during the polymerization process, and the dedoping reaction is the reverse process. Many of the side reactions that can occur during polymerization can also be a problem during doping, particularly those involving overoxidation. 

Reactions on alumina surfaces. The water present in commerciaUy available aluminas is known to effect some undesired side reactions and has been used to some extent to effect desired transformations ie.g., selective hydrolysis, 2, 3601). Posner et ah reasoned that replacement of the water in alumina by alcohols, thiols, or amines should result in doped-aluminas useful for organic synthesis. In the general procedure the alumina is dehydrated by heating at 400° and 0.06 torr for 24 hr. and is then stirred with the dope in an inert solvent. 

The reason for having a solution of polymers in the doped state was to elucidate that molecular conformation in the active doped state, compared with the neutral one. Attempts to reach the goal were unsuccessful for a long time, probably because the presence of the species responsible for the conductivity, free radicals, radical ions, counter ions, participated in undesired side reactions, i.c. cross-linking which decreases the processability of the polymer. 

Table 2 classifies the main cases of doping, ignoring possible side reactions and the possi-bilily that the dopant may be completely insoluble. We shall consider some of the general principles, using lead chalcogenides as our example. It is known that p-type conduction is observed in lead chalcogenides when they are doped with alkali metals [26]. However, the direct... 

The reaction may be more complex than this, or side reactions are significant, since in electro-oxidation of CH3OH, depending on potential, pH, and electrode metal, some HCOOH and HCHO are known to be formed. These species are probably the cause of deactivation of methanol anodes which occurs at, e.g., Pt, and makes a direct methanol fuel cell at present impractical despite attempts at developing mixed-electrode materials, such as Sn-doped Ru or Ru-treated Pt. 

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