Лако-красочные материалы — производство

Since the discovery of the first synthetic dyes in the mid-19th century, chemists have been intrigued by the relationship between the colour of a dye and its molecular structure. Since these early days, the subject has been of special academic interest to those fascinated by the origin of colour in organic molecules. In addition, an understanding of colour and constitution relationships has always been of critical importance in the design of new dyes. In the very early days of synthetic colour chemistry little was known about the structures of organic molecules. However, following Kekule’s proposal concerning the structure of benzene in 1865, organic chemistry made significant and rapid progress as a science and, almost immediately, theories concerning the influence of organic struc­tures on the colour of molecules began to appear in the literature. One of the earliest observations of relevance was due to Graebe and Liebermann who, in 1867, noted that treatment of the dyes known at the time with reducing agents caused a rapid destruction of their colour. They con­cluded, with some justification, that the dyes were unsaturated com­pounds and that this unsaturation was destroyed by reduction.

Perhaps the most notable early contribution to the science of colour and constitution was due to Witt who, in 1876, proposed that dyes contain two types of group which are responsible for their colour. The first of these is referred to as the chromophore, which is defined as a group of atoms principally responsible for the colour of the dye. Secondly, there are the auxochromes, which he suggested were ‘salt-forming’ groups of atoms whose role, rather more loosely defined, was to provide an essen­tial ‘enhancement’ of the colour. This terminology is still used to a certain extent today to provide a simple explanation of colour, although Witt’s original suggestion that auxochromes were also essential for dyeing properties was quickly recognised as having less validity. A further no­table contribution was made by Hewitt and Mitchell who first proposed in 1907 that conjugation is essential for the colour of a dye molecule. In 1928, this concept was incorporated by Dilthey and Witzinger in their refinement of Witt’s theory of chromophores and auxochromes. They recognised that the chromophore is commonly an electron-withdrawing group, that auxochromes are usually electron-releasing groups and that they are linked to one another through a conjugated system. In essence, the concept of the donor-acceptor chromogen was born. Furthermore, it was observed that a bathochromic shift of the colour, i. e. a shift of the absorption band to longer wavelength, might be obtained by increasing the electron-withdrawing power of the chromophore, by increasing the electron-releasing power of the auxochromes and by extending the length of the conjugation.

The chromophore and auxochrome theory, which was first proposed more than 100 years ago, still retains some merit today as a simple method for explaining the origin of colour in dye molecules although it lacks rigorous theoretical justification. The most important chromo — phores, as defined in this way, are the azo (-N-N-), carbonyl (C — O), methine (-CH-) and nitro (NO2) groups. Commonly-encountered auxochromes, groups that normally increase the intensity of the colour and shift the absorption to longer wavelengths of light, include hydroxyl (OH) and amino (NR2) groups. The numerous examples of chemical structures which follow2 in later sections of this book will illustrate the many ways in which chromophores, auxochromes and conjugated aro­matic systems, together with other structural features designed to confer particular application properties, are incorporated into dye and pigment molecules. The concept may be applied to most chemical classes of dye, including azo, carbonyl, methine and nitro dyes, but for some classes which are not of the donor-acceptor type, for example the phthalocyanines, it is less appropriate. Nowadays, modern theories of chemical bonding, based on either the valence-bond or the molecular orbital approaches, are capable of providing a much more sophisticated account of colour and constitution relationships.