Polymethine ions

Predict the wavelength (in nm) of the lowest-energy electronic transition in the following polymethine ion ... 

Although the simple FE MO method fares poorly for the conjugated polyenes, it works rather well for polymethine ions with the formula... 

Find the HMO energies for the polymethine ion (16.12) with k = 0. Use averages of the values in (16.67) to simplify the work, use symmetry orbitals. What value of j3 is required to fit the observed lowest-energy transition ... 

Photopolymerization. In many cases polymerization is initiated by ittadiation of a sensitizer with ultraviolet or visible light. The excited state of the sensitizer may dissociate directiy to form active free radicals, or it may first undergo a bimoleculat electron-transfer reaction, the products of which initiate polymerization (14). TriphenylaLkylborate salts of polymethines such as (23) ate photoinitiators of free-radical polymerization. The sensitivity of these salts throughout the entire visible spectral region is the result of an intra-ion pair electron-transfer reaction (101). 

Extraction and flotation separation of POMs as ion-pair with a bulky cation of Astrazone Violet (AV) with different organic solvents is investigated. Conditions for separation of dye excess from ion-pair PMo TiO j with polymethine dye AV ai e found. 

Arylmethane leuco dyes are converted into di- or triarylmethane dyes on oxidation. This class of dye precursors sometimes is referred to as leuco di- or triphenylmethane dyes, or di- or triphenylmethane leuco dyes. The use of the term di- or triarylmethane dyes can be misleading as the central carbon atom is a carbonium ion. Instead, the term di- and triarylmethine dye is recommended for this class as it correlates with the well-known polymethine dyes. Nevertheless, it has not been commonly used. 

The chemistry of the three most important chemical classes of organic colorants, the azo, carbonyl and phthalocyanine classes, has been dealt with individually in Chapters 3-5 respectively. In this chapter, the chemistry of a further five chemical classes which are of some importance for specific applications is discussed. These classes are the polymethines, arylcarbonium ion colorants, dioxazines, sulfur dyes and nitro dyes. A section of this chapter is devoted to each of these, each individual section contains a description of the principal structural features which characterise the particular colorant type, together with an outline of the chemistry of the main synthetic routes. There are many other chemical types of dyes and pigments that do not fall into the categories previously mentioned, but which are neglected in this text either because they are commercially of little importance or because they have been less extensively investigated. 

Fig. 3 (a) Typical polymethine and (b) cyanine molecular structures n is the number of methine groups and X are counter ions... 

Crowns with Polymethines Dyes. The ready synthesis, see Figure 3.18, of crown ethers attached to polymethines or hemicyanines has made these compounds a fertile area of research and application in metal ion sensing. AU of these polymethine derivatives are cation responsive e.g. (3.84b) forms complexes with Na, Mg, Ca, Sr and Ba with an increase in fluorescence intensity. 

In the benzidine-pyridine method [14,15], chlorine reacts with cyanide ions to form cyanogen chloride (CNCl). The product of the reaction of CNCl with pyridine (glutaconaldehyde) is condensed with primary amines to form polymethine dyes. 

The application of high molecular weight amines, trioctylamine (1,2-dichloroethane) [4], N-octylaniline (xylene) [5,6], diantipyrylmethane [4,7] and Cyanex 302 (toluene) [8] to separation of the elements has recently been reported. Extraction of ion pairs of anionic complexes of the analytes with the other counter ions, e.g. tiiphenylarsine oxide [9] or various dyes, e.g. Rhodamine B [10] and polymethine dyes [11], has recently been investigated. 

Various polymethine dyes (pinacyanol, pseudoisocyanine) are soluble in alcohol and acetone as well as water. In the first two solvents the spectrum does not change as the concentration is raised in these cases the dyes follow Beer s law. On the other hand if the solvent is water, important modifications take place in the spectrum (ScHEiBE et al ). These changes which are associated with a great rise of the viscosity, can best be explained by an association of the ions of the dye, first to double ions, later to polymolecular formations. 

In contrast, few reports concern dyes usable under red or near-infrared radiation (IR) laser lights in laminate. The use of the methylene blue/amine system, under the HeNe laser at 632 mn, has often been explored in the past [CAP 89]. Polymethines, thiazines, squarylium-triazine dyads have been proposed for the manufacture of HR and HOE. Some Dye/iodonium salt/silane PISs have also been mentioned but the efficiency in FRP, contrary to that attained in FRPCP, still remains relatively low under air CCRP and TEP, however, are feasible (e.g. [XIA 15] see section 1.3.12 and 1.3.13). A novel compoimd based on a dye-iodonium salt intra ion-pair (where the dye derives from a l,3-bis(dicyanomethylene) indane skeleton) works under a 635 nm light with an efficiency slightly lower than that of BM/amine [TEL 14]. 

Dye-borate systems were disclosed to initiate radical polymerization [P C 08, PAC 01, KAB 98] but the sensitivity significantly increases by addition of iodonium borates to a dye-borate system [SIM 09]. This combination avoids ion exchange in systems comprising a cationic polymethine dye and an iodonium ion because both comprise the same counter ion. Ion exchange can lead to undesired events (i.e. crystallization) affecting sensitivity of the lithographic material. Further studies on dye-borate systems discuss the efficiency of electron transfer from the borate to the acceptor excited [SCH 90]. Moreover, oxidation of the (anilino)acetic acid results in fast generation of the aminyl radical and the release of CO2 as shown in equation [7.5]. 

Pokrovskaya, K. 1. Levkoev, I. I. Natanson, S. V. Complex compounds of polymethine dyes with silver ions. I. The formation of silver ions of carbo- and polycarbocyanines. 7h. Fiz. Khim. 1956, 30, 161-171 Chem. Abstr. 1956, 50, 51973. 

Sulphur-containing polymethine dyes, including those incorporating thiazolidine and benzothiazole residues, exhibit certain anomalous chromophoric properties (e.g. abnormal vinylene shifts), which have been traced to an interannular non-bonded S-S interaction in the monomethine ion. The conclusion is supported both by a detailed examination of the... 

Hiickel pointed out that, on the basis of molecular orbital theory, monocyclic conjugated polymethines have filled shells of tt-electrons when the number of TT-electrons is An + 2, where n is an integer. These systems may be expected to be stable. The rule may be illustrated by reference to Fig. 2.1. If = 0, then a system with 27r-electrons should be stable. Such a situation is found in the cyclopropenyl positive ion, which has been isolated as the hexachloroanti-monate. For n = the prediction is that the cyclopentadienyl anion, benzene and the cycloheptatrienyl (tropylium) cation are stable. This is certainly in accord with experience. The stability of benzene is well known, the cydo-pentadienyl anion is readily formed by the action of potassium metal on cyclopentadiene, and the cycloheptatrienyl cation is one of the most stable carbonium ions known. Huckel s rule also predicts that some of the larger cyclic conjugated systems are stable, e.g. those with 10,14 and 18 rr-electrons. However, the situation is complicated by steric problems (see for example Garratt, 1971) and need not be considered further here. 

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