Chromophores insulated

Appreciable interaction between chromophores does not occur unless they are linked directly to each other, or forced into close proximity as a result of molecular stereochemical configuration. Interposition of a single methylene group, or meta orientation about an aromatic ring, is sufficient to insulate chromophores almost completely from each other. Certain combinations of functional groups afford chromophoric systems which give rise to characteristic absorption bands. 

Phosphazene polymers are inherently good electrical insulators unless side-group stmctures allow ionic conduction in the presence of salts. This insulating property forms the basis for appHcations as wire and cable jackets and coatings. Polyphosphazenes also exhibit excellent visible and uv radiation transparency when chromophoric substituents are absent. 

In this way, many green azo dyes have been made by combining separate conjugated systems in the same molecule, eg, one yellow and the other blue. The blocking or the insulating group prevents the electronic interaction of one chromophore system with the second. Chloramine Fast Scarlet 4BS... 

In the course of developing the Polacolor and SX-70 processes many insulated dye developers were synthesized and investigated. An extensive review of this work is available (21). The insulating linkage, chromophore, and developer moiety can each be varied. Substituents on the developer modify development and solubility characteristics substituents on the chromophore modify the spectral characteristics in terms of both color and tight stability. The attachment of two dyes to a single developer by amide linkage has also been described (22). 

The above two instances are sufficient to insulate chromophores from each other totally,... 

Chung [34] concluded that the semiconducting properties of a metal species influence discoloration. In contrast to metals belonging to the insulator group, metals belonging to the semiconductor group promote yellowing, perhaps due to catalysis of the polymerization of vinyl esters. The formation of chromophores is enhanced when the metal has a variable valency with a reduction potential near to zero. 

Intramolecular transfers between two ehromphores separated by insulating groups can lead to absorption by one chromophore and emission from the other. Complexes of rare earths specially of Eu+3, Sm+3. Gd+3 and Dy+3 emit line spectrum characteristic of the central metal ion when absorption takes place in the ligand moeity. 

An island dff-pff bond structure may explain why most polyphosphazenes are colorless rather than colored materials, and are insulators rather than electronic conductors. Exceptions do exist, but the exceptions are for polymers that have chromophores in the side groups or which bear electroactive side units. 

Although the anthracene units are coplanar, there is no electronic interaction between the chromophores in the crystal. Although such an arrangement makes this class of anthracene derivatives unusable for use in applications requiring efficient charge transport (organic transistors or solar cells), the insulation of the fluorescent chromophore induced by this method may make these materials useful as emitters in OLEDs. 

A uniform electric field distribution across the sample is extremely important for achieving device quality materials. Unfortunately, real chromophore materials do not always behave as uniform insulator materials. We have already demonstrated that ionic impurities can dramatically reduce the effective electric field felt by chromophores. The presence of spatially and temporally varying nonuniform space charge distributions leads to nonuniform poling fields. The resulting nonuniform chromophore order can lead to light scattering. 

In addition to the antennae effect, dendrimers are also effective insulators and exhibit the shell effect. In providing a dense shell around the incorporated chromophores, dendrimers effectively prevent aggregation which leads to non-emissive excimers and self-quenching that occurs when chromophores with small Stokes shifts are within short distances of one another. This shell effect allows for increased photoluminescence efficiency of the enclosed chromophore, which is important for optoelectronic devices. 

Bismuth, Bi, the 83rd element of the periodic table is the most metallic and the least abundant of the elements in the nitrogen family (group 15). It has an atomic mass of 208.980 and a ground state electronic configuration of [Xe] 4f 5d %s 6p. The bismuth atom usually utilizes the three 6p electrons in bond formation and retains the two 6s electrons as an inert pair, hence the oxidation state -1-3 exhibited by bismuth in the vast majority of its compounds. However, a variety of organobismuth compounds can contain the element in the -1-5 oxidation state. Coordination numbers are 2, 3,4, 5 and 6. Bismuth not only has metallic characteristics but also exhibits many properties similar to those of semiconductors and insulators. Consequently, it is often classified as a semi-metal or metalloid. Bismuth compounds are usually colorless unless the metal is bound to a chromophore. 

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