Communications obviously a very stable state: the dye colloid can be heated or stored for more than half a year at room temperature without any flocculation.
The dispersion of 1a in water exhibits a deep red color (ruby-like; l max = 499.0 nm, e eff = 31 830 M ±1 cm ±1 ; color coordinates: x = 0.4087, y = 0.3078, Y = 50.38 for 2, normlight C, and UV-vis transmission T max = 0.1) and a moderately intense fluorescence (l max = 544 nm). The UV-vis spectra of 1a are shown in Figure 2. It is remarkable that the fluorescence excitation spectrum (l max = 465 (sh), 496, and 533 nm) differs from the absorption spectrum, which indicates that the major component is not the origin of fluorescence, but some minor component, which may be even a partly disturbed lattice of 1a or especially oriented molecules in the elementary cell. Fig. 2. e vs. l for UV-vis absorption (solid line) and fluorescence (dashed line) and fluorescence excitation (dash±dot) spectra of a dispersion of 1a in water. The dotted line is e vs. l for the absorption of 1a in chloroform (half of the coefficient of extinction).
The formation of fluorescent colloids of dyes is of general interest for scientific applications. Hydrophobic dyes may thus be dispersed in water to form colloids of solid dyes. However, the preparation of such colloidal dyes requires special techniques. Problems such as sedimentation and flocculation occur, which diminish long-term stability. This may be compensated to some extend by employing protective colloids. No such problems would be presented if the colloid were a very stable state itself, which makes the special value of dye 1a obvious. We are applying dye 1a as a tracer for the investigation of the movement of water in order to answer geochemical questions. This is an interesting alternative to dissolved dyes as tracer because there are no problems for dye 1a by adsorption in zeolites. The results of these investigations will be reported elsewhere.