Polymethines electronic structure

Uncharged styryl (methine) disperse dyes were originally introduced to provide greenish yellow colours on cellulose acetate fibres. One such dye still in use is Cl Disperse Yellow 31 (6.226), which is made by condensing 4-(N-butyl-N-chloroethylamino)benzaldehyde with ethyl cyanoacetate. Suitable compounds for polyester usually contain the electron-accepting dicyanovinyl group, introduced with the aid of malononitrile. An increased molecular size leads to improved fastness to sublimation, as in the case of Cl Disperse Yellow 99 (6.227). A novel polymethine-type structure of great interest is present in Cl Disperse Blue 354 (6.228), which is claimed to be the most brilliant blue disperse dye currently available [85]. 

A considerable number of experiments have shown that symmetrical PMDs in the ground state have an all-trans configuration and are neady planar with practically equalized carbon—carbon bonds and slightly alternating valence angles within the polymethine chain (1,3,5,22,23). This is caused by some significant features of the PMD electron structure. 

In contrast to compounds of aromatic and polyene-like electronic structure, polymethines are conjugated chain molecules with equal bond lengths and charge alternation along the methine chain [18, 19], They exhibit the following common structural features ... 

Of particular interest are the intramolecularly ionic meropolymethine dyes (especially the merocyanines), whose electronic structure lies somewhere between that of polyenes and that of polymethines depending on the nature of X and X as well as on solvent polarity [20], These are systems in which an electron-donating group, D, is linked by a conjugated system, R, to an electron-accepting group, A. Their intermediate r-electronic structure can be described in terms of two mesomeric structures, D—R—A D —R—A , as, for example, this special vinylogous merocyanine dye ( = 0,1,2,...) ... 

Organic compounds with delocalized 7r-electron systems are leading candidates for nonlinear optical (NLO) materials, and interest in these materials has grown tremendously in the past decade [108-118]. Reliable structure-property relationships—where property here refers to first-order (linear) polarizability a, second-order polarizability and third-order polarizability y—are required for the rational design of optimized materials for photonic devices such as electro-optic modulators and all-optical switches. Here also, quantum-chemical calculations can contribute a great deal to the establishment of such relationships. In this section, we illustrate their usefulness in the description of the NLO response of donor-acceptor substituted polymethines, which are representative of an important class of organic NLO chromophores. We also show how much the nonlinear optical response depends on the interconnection between the geometric and electronic structures, as was the case of the properties discussed in the previous sections [ 119]. 

Polyene and polymethine dyes are two structurally related groups of dyes which contain as their essential structural feature one or more methine (-CH=) groups. Polyene dyes contain a series of conjugated double bonds, usually terminating in aliphatic or alicyclic groups. They owe their colour therefore simply to the presence of the conjugated system. In polymethine dyes, electron-donor and electron-acceptor groups terminate either end of the polymethine chain, so that they may be considered as typical donor-acceptor dyes. 

The structures of a number of neutral and anionic polymethine dyes are illustrated in Figure 6.3. There are many types of neutral poly-methines, utilising a wide range of electron donor and acceptor groups. For example, C. I. Disperse Yellow 99 (114) and C. I. Disperse Blue 354... 

A great variety of seemingly unrelated organic compounds have been demonstrated to transfer two electrons in a stepwise fashion, if they can be derived from the general structural types A, B or C. The intermediate oxidation level SEM thereby represents radical cations, radical anions or neutral radicals Their thermodynamic stability can be understood within a general theory of polymethines X—(CH)n 2—X containing Nil TT-electrones for which MO-LCAO calculations have been develope l... 

As shown by the NMR chemical shifts of negatively solvatochromic mero-polymethine dyes e.g. phenol blue), the electronic ground-state structure of these dyes changes from a polymethine-like state (b) in non-polar solvents to a polyene-like state (c) in polar solvents [50, 78]. 

The course of such reactions can therefore be predicted immediately from the rules for aromaticity in Section 3.14. All we have to do is to count the number of atoms in the ring and the total number of delocalized electrons. We can then tell at once if the transition state is isoconjugate with an even cyclic polyene, with an odd polymethine cation, or an odd polymethine anion. If the resulting structure is aromatic in the Hiickel series, ring opening (or... 

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