352 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS • / •c--o /C----C•. --CH----N• •N----N• vinyl carbony] azomethine azo --N----O •NO• =•• •C•N nitroso nitro quinoid nitrile Figure 1. Chromophoric groups H.•C/CH2•cH2 Hc/CH2•cH2 Hc/CH2•cH Hc/CH•cH H2C•cH2/CH2 HC•cH2/CH2 HC•cH2/CH HC•cH2/CH2 cyclohexane cyclohexene 1,4-cyclohexadiene 1,3-cyclohexadiene X ..... = 165 m• (1) X .... = 182 m• (2) X .... -- 224 m• (3) X .... = 256 m• (4) Figure 2. Effect of increasing electron delocalization on absorption maximum mobile (delocalized) the electrons, the lower the energy required to raise them to excited higher energy levels, the higher the wavelength of the light absorbed, and, therefore, the deeper the color of the molecule. Historically, before science had progressed to its present level of knowledge and technology, electron delocalization was not so termed, but its relation to color was being discussed unknowingly, perhaps, in terms such as chromophores, chromogens, auxochromes, unsaturated centers, etc. It had been observed that colored organic molecules generally contained at least four of certain types of atomic groupings, and these groupings were termed chromophores, from the Greek "to bear color." This term is still in wide use today. It was further dis- covered that these chromophores contained atoms bound together by double or triple bonds, and hence the term "unsaturated centers" arose. A list of the more common chromophores is shown in Fig. 1. With the advent of new knowledge concerning the nature of the chemical bonds between atoms, it became possible to formulate a much more fundamental theory for discussing color in relation to structure, one which could accommodate the wide variety of structures which give rise to color. When one speaks of electron delocalization in unsaturated molecules, one is simply saying that one or two of the electrons associated with one of the multiple-bonded atoms are able to become associated with other atoms in the molecule as well. This is accomplished by an overlapping of the orbitals in which these electrons move.

RATIONALE OF DYES SYNTHESIS PROGRAM 353 NO2 NO2 NH2 benzene X ..... = 203 m• (5) (e = 7400) nitrobenzene p-nitroaniline X ..... = 268 rnt• (5) X ..... = 381 m• (5) (• = 7800) (• = •3,5oo) Figure 3. Effect of chromophores and auxochromes on benzene absorption To illustrate the effect of this ddocalization through orbital overlap on the wavelength of light absorbed by a molecule, the series of compounds shown in Fig. 2 can be considered. Cydohexane is composed entirely of localized sigma bonds with, therefore, no opportunity for overlap and electron ddocalization. In the gas phase, it exhibits an absorption maximum at 165 mu (in hexane, log e -- 3.0) (1). If one double bond corresponding to one vinyl chromophore is introduced to give cyclo- hexene, a bathochromic shift (i.e., to higher wavelengths) of the absorp- tion maximum to 182 mu (in hexane, log e = 3.88) (2) is obtained. In- troduction of a second double bond to give 1,4-cyclohexadiene again gives a bathochromic shift to an absorption maximum of 224 mu (in hexane, log • = 1.5) (3). If, however, this second double bond is in- troduced in a position alternate (conjugated) with the first double bond as in 1,3-cyclohexadiene, an even greater shift is obtained to a maximum of 265 m• (in hexane, log e = 3.9) (4). This increased absorption maxi- mum of the 1,3-compound over the 1,4-analog can be attributed to the overlap of the two conjugated double bond orbitals which is possible with the former but not the latter di-olefin. The organic dyes and pigments in actual practical use are essentially all aromatic or heterocyclic compounds (i.e., derivatives of benzene, naphthalene, pyridine, etc.). We know that benzene contains three conjugated vinyl chromophores and yet is still colorless. (Incidentally, a molecule such as benzene containing several chromophores but still lacking color is often termed a chromogen.) As illustrated in Fig. 3, the presence of additional groups is required to convert the simple benzene molecule into a colored entity. Introduction into benzene of a fourth chromophore, the nitro group, gives a significant bathochromic shift to the absorption maximum, yet still not sufficient to cause significant ab- sorption in the visible region of the spectrum. If one now places an amino group in the para-position to the nitro group, a significant in-