Лако-красочные материалы — производство

2.5.1 Introduction

Methine dyes and polyene dyes are characterized by a chain of methine groups that forms a system of conjugated double bonds. The methine chain in polyene dyes is even-numbered and the terminal groups do not affect the excitation of electrons in the dye. The most important group of polyene dyes, the carotenoids, bear aliphatic or alicyclic groups at the end of the methine chain, and a very long conjugated chain is necessary to shift the excitation wavelength to the visible region. ^-Carotene contains a chain of 22 methine groups and absorbs at 450 and 478 nm.

Methine dyes are characterized by a chain of conjugated double bonds with an odd number of carbon atoms between the two terminal groups X and Y, which may be considered as being capable of charge exchange.

Till III p

X=C-<C=C).-Y «—» X-C=(C-C)„=yJ

One of the substituents X and Y is an electron acceptor and the other as an electron donor. The dyes can be cationic, anionic, or neutral.

Methine dyes have an extended system of conjugated double bonds. A large number of resonance structures may therefore be formulated, of which (1) and (2) are the most important, i. e., the electron density is lowest on the nitrogen atoms.

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N=C-(C=C),-N «—» N-C=(C-C)„=N I

1 2

In all the following formulas only one resonance structure is shown. To indi­cate that a я-electron system with many possible resonance structures is involved, the total charge of the molecule is given outside a square bracket.

In the following classification of methine dyes only nitrogen and oxygen are considered as charge-carrying terminal atoms, although other heteroatoms are possible. The charge-carrying terminal atoms may be joined directly to the methine chain, may be linked via an aromatic group to the methine chain, or may form part of a heterocyclic ring.

V /

N=C-(CH=CH).-N

. ^

Cyanine dyes (3) are the best known polymethine dyes. Nowadays, their commer­cial use is limited to sensitizing dyes for silver halide photography and as infrared absorbers for optical data storage and other (bio)imaging applications. However, derivatives of cyanine dyes provide important dyes for polyacrylonitrile.