Лако-красочные материалы — производство

2.3.3.1 Color

One of the main fascinations of indigo is that such a small molecule should be blue. The reason for indigo’s deep blue color was long unknown. Normally, exten­sive conjugation (e. g., phthalocyanines) and/or several powerful donor and accep­tor groups (e. g., azo and anthraquinone dyes) are required to produce blue dyes. Only with the intensive studies of Luttke et al. in the 1960s was any significant insight gained. After much controversy it was proven that the chromogen is the crossed conjugated system [6]. The deep indigo color is explicable in terms of the special arrangement of the atoms in the basic indigo chromophore and the high polarizability of the charge distribution, which is strongly influenced by the abil­ity of the molecule to form hydrogen bonds [3]. The deep shade is determined by the doubly cross-conjugated system of two donor and two acceptor groups, the conformation of the two carbonyl groups with respect to the central C= C double bond, and the nature of the heteroatom [4,6]. Thus the long-wave UV/Vis absorption band of indigo (610 nm in ethanol) is shifted hypsochromically as the heteroatom is replaced to give selenoindigo (562 nm), thioindigo (543 nm), and oxindigo (432 nm) [5].